Lithium batteries celebrated their 50^th anniversary in 2022, but they achieve only 25% of their theoretical energy density. Graphitic carbon and the layered NMC oxides, Li[NiMnCoAl]O₂ dominate the anode and cathode respectively. The carbon in the anode must go for the highest energy density applications, as it takes up half the cell volume. Can we go back to lithium metal, the holy grail anode? The present status and possible approaches will be discussed. For the cathode, most of the cobalt must be eliminated, and eventually the nickel, but there will be trade-offs between cost, reactivity, energy density and lifetime. In addition, the morphology of electrode materials has a major impact on reactivity and I will discuss these challenges. Alternatives to Li-NMC cells will also be discussed, including vanadium based electrodes. We thank the US Department of Energy Battery500 Consortium for their support of this research.

NASA/JPL missions have visited every planet in our Solar System. Over the last twenty years, five rovers have explored Mars in coordination with a number of orbiters. Samples have been brought back from a comet tail and the solar wind. Saturn and its satellites have been studied extensively with Cassini. Planets around neighboring stars were discovered. Many new insights in our planet’s environment have been acquired. The speaker will describe, from first-hand experience, the excitement and impact of these discoveries and the challenges and plans for the next 20 years.

JAXA has proposed a brand-new concept for satellites in Low Earth Orbit (LEO). The Super Low Altitude Test Satellite (SLATS), also known as TSUBAME, is the first Earth observation satellite to occupy a super-low orbit or Very Low Earth Orbit (VLEO) below 300 km. SLATS successfully completed its operation on October 1, 2019. This mission demonstrated some key technologies for VLEO satellites. In terms of space material design, atomic oxygen (AO) protective technology will be widely used to improve the reliability of VLEO as well as LEO satellites. Among the AO protective technologies is the creation of materials that are strong against AO. Research on ground AO evaluation technologies has also been conducted, with several institutes running simulated AO irradiation test facilities. Moreover, some of the proposed computer simulation methods can simulate the degradation mechanism that is attacked by AO. One potential challenge concerns the accuracy of neutral atmosphere models in the VLEO region. This final presentation discusses the future direction of AO-resistant technology.

Lean satellites utilize non-traditional, risk-taking development and management approaches with the aim to provide the satellite value to the customer by realizing the satellite mission at low-cost and in short time. The risk tolerance makes the satellite size small. Many so-called pico/nano/micro/small satellites fall in this category. Lean satellites extensively use commercial-off-the-shelf components made for non-space use. In the past 20 years, lean satellites cumulated their flight heritage in Low Earth Orbit (LEO). Commercial lean satellite manufacturers have flourished and they now provide reliable satellite buses to their customers. Based on the successful flight records, lean satellites are now being considered for missions beyond LEO, such as the Moon or asteroids. This presentation will discuss how lean satellites can survive in the harsh environment.

Radiation effects are of major concern for proper function of electronic devices in harsh environments as space, avionics, or accelerators, where high radiation fields exist. The radiation effects in electronic devices are manifested in two main ways, namely total accumulated dose and single event effects (SEE). Accurate measurement of accumulated doses and of particle fluxes in space is important for refining the space environment models, in order to improve our prediction of the radiation damage in orbit, and to optimize radiation mitigation of electronic components. Systems operating in an ionizing radiation field may accumulate high total ionizing dose (TID), which leads to parametric degradation and even functional failure. We have thoroughly investigated the effects of TID on the proper function of a Radiation Sensitive Field Effect Transistor (RADFET) as a radiation dosimeter, and on the evolution of different types of charge traps in the RADFET subjected to ionizing radiation. Single particles impinging on electronic devices may cause catastrophic SEE effects, or non-destructive yet disrupting SEE effects, such as single event upset (SEU). The occurrence of SEU in a SRAM-based FPGA device has been studied with different particle sources and under various operation voltages. The results demonstrate the effects of the voltage and of the nature of the impinging particle on the SEU sensitivity of each of the memory logic states. Recently we have harnessed our knowledge of radiation hardness assurance of electrical components, and developed a scientific payload for TAUSAT-1, a LEO nanosatellite aimed at studying the space environment. Among other detectors, the payload consisted of a RADFET TID detector and a particle sensor based on SEE detection in the SRAM-based FPGA. TAUSAT-1 has accumulated valuable radiation data during its 14 months mission at 400 km altitude, which contributed to the validation of our current radiation models.

In recent years, terahertz (THz) communications have been intensively studied as enabling technologies towards the 6th generation (6G) mobile communication systems, to meet the anticipated demand for the data rate of over 100 Gbit/s. Photonics- as well as electronics-based systems have been developed mostly in the sub-THz region from 100 GHz to 300 GHz (D-band: 110 GHz~170 GHz, H-band: 170 GHz~260 GHz, and J-band: 220 GHz~325 GHz). Particularly, the photonics-based system is to be more suitable not only for achieving higher data rates but also for combining fiber-optic networks and wireless networks seamlessly. This talk reviews a recent progress of photonics-based THz communications research, and presents our latest challenges, which include key devices such as lasers, photodiodes, and detectors, and their system demonstration towards 1-Tbit/s wireless communications.

Terahertz (THz) topological photonics provides a unique platform for 6G communication, with the prospect of massive bandwidth and data rate capacity in an on-chip configuration. Emerging data-intensive applications rely on broadband (tens of GHz) THz devices, which include waveguides, filters, and interconnects. Topological bandgap engineering in the inversion-symmetry broken Valley-Hall Photonic Crystal (VPC), where edge states exist at the domain walls, provides robust transport of THz signals. In this talk, I will present frequency division multiplexing technique through a topological VPC chip, which is one of the important requirement for full-duplex high-speed communication at THz frequencies.

Periodically driven systems are ubiquitously found in both classical and quantum regimes. In the field of photonics, these Floquet systems have begun to provide insight into how time periodicity can extend the concept of spatially periodic photonic crystals and metamaterials to the time domain. However, despite the necessity arising from the presence of nonreciprocal coupling between states in a photonic temporal crystal, a unified non-Hermitian band structure description remains elusive. We experimentally reveal the unique Bloch-Floquet and non-Bloch band structures of a photonic temporal crystal emulated in the microwave regime with a one-dimensional array of time-periodically driven resonators. These non-Hermitian band structures are shown to be two measurable distinct subsets of complex eigenfrequency surfaces of the photonic temporal crystal defined in complex momentum space.

THz emission technique, as an excellent tool to study the nonlinear optical response of materials, has been proved to be sensitive to the nontrivial quantum geometry of topological materials, exampled as topological insulators, Weyl semimetals, Dirac semimetals, etc. In our work, THz emission from centrosymmetric Dirac semimetal PtSe₂ is observed, which is from the ultrafast photocurrent generated by fs-laser pump. With controlling the pump polarization state, sample azimuthal angle, incident angle, and detection polarization, we identify the photocurrent mechanism as photon drag effect. In addition, the THz emission efficiency of PtSe₂ is obtained as two orders of magnitude larger than that of the standard THz-generating nonlinear crystal ZnTe. In our observations, the pump photon momentum breaks the symmetry of the system, which activates the hidden giant second-order optical nonlinearity related to the topological nontrivial band structure of PtSe₂. Our work demonstrates a possible method to optically manipulate the nonlinearity of topological materials in ultrafast time scale.

Terahertz technology is one of the emerging fields of research for wireless communications, spectroscopy, and imaging. The detection method of the terahertz waveforms is, however, still limited because of the extremely fast oscillation period that could not be resolved with conventional electronic devices such as an oscilloscope. To overcome this difficulty, we implement single-shot terahertz time-domain spectroscopy using an ultrashort pulsed laser and a reflective echelon mirror. By combining with the phase-offset electro-optic sampling method, we realized the single-shot detection of the terahertz waveform with the sensitivity down to 1 V/cm. We used a Ti:sapphire regenerative amplifier with a 10 Hz repetition rate, 0.6 mJ pulse energy, 800 nm center wavelength, and 100 fs pulse duration. The laser was shined on a reflective echelon mirror with a step width and size of 5 um and 20 um, respectively. The echelon has 800 steps, which corresponds to the time window of 26 ps, and time resolution of 32 fs. The diffracted probe pulses are then focused on an electro-optic (EO) crystal ((110)-cut ZnTe crystal with 2-mm thickness) and the transmitted pulse are imaged with a digital camera with 16-bit resolution. To increase the sensitivity, we placed crossed Nicol polarizers after the echelon and placed the EO crystal and a slightly phase-shifted quarter waveplate between them. As a demonstration, we measured the terahertz waveform generated from an impact avalanche and transit time (IMPATT) diode with 10 mW output at 0.15 THz. By focusing the output to the EO crystal, we observed clear sinusoidal modulation of the transmitted probe pulses. The modulation depth has a peak-to-peak amplitude of 1 V/cm with a 0.15 THz frequency. The results demonstrate the promising capability of the single-shot system for measuring terahertz radiations from various devices and their ultrafast dynamics.

Thermoelectric materials harvest heat energy and are able to convert it into electrical power. This conversion efficiency is quantified by the figure-of-merit, zT = (α²σ/κ)T. Ternary materials of the Cu-In-Te family of compounds have shown great potential for thermoelectric applications owing to their layered crystal structure and rich defect chemistry. The thermoelectric properties of the ternary Cu-In-Te system were studied by Sb doping and Cu deficiency. It is observed that point defects, such as vacancies and anti-sites play an important role in both the electronic and thermal properties of these ternary compounds. Our experimental results show that the Cu deficiency and Sb doping improves the thermoelectric performance of CuInTe₂, where the Power factor, α²σ ~ 1mW/mK² and low thermal conductivity (κ ~ 1 W/m-K) are achieved to reach zT > 0.6 at 650 K.

Ag₂Se is an intriguing material for room temperature energy harvesting. Herein, we report the fabrication of zigzag Ag₂Se nanorod arrays by Glancing angle deposition technique (GLAD) followed by simple selenization in a two-zone furnace. The thermoelectric performance was investigated by varying the number of zigzag arms. The novel Ag₂Se zigzag nanorod arrays with four arms shows an excellent zT= 1.42 ± 0.15 and power factor of 3202.21 ± 105.23 µW/m-K², respectively at 300 K. The superior thermoelectric performance of four arm zigzag Ag₂Se nanorod arrays compared to one arm Ag₂Se nanorods could be ascribed to the unique nanocolumnar architecture that not only offers a preferential path for carrier transport but also enhanced the scattering of phonons at the tilted rough boundaries and interfaces. The synergetic dependence between the tilt structure and the thermoelectric properties opens a new avenue to fabricate scalable nanostructure thin films for practical applications in next-generation thermoelectric devices.

Inorganic thermoelectrics have progressed in leaps and bounds in the recent years. This is largely driven by the advancements in both physical understanding coupled with structural properties. In particular, p-type AgSbTe₂ has recently emerged as one of the best thermoelectric materials for low and medium temperature applications. Nevertheless, it suffers from longstanding stability and inconsistency problems, which results in n-type Ag₂Te precipitates and drastic deterioration in performance. In this work, we trace the origin of the variability of thermolectric properties of AgSbTe₂ in literatures to the cooling rate during synthesis. Furthermore, we demonstrate a non-equilibrium annealing strategy to achieve consistent properties. Ultimately, a peak zT of 1.15 at 623 K was achieved for optimally annealed and quenched pristine AgSbTe₂. Importantly, in the absence of dopant to stabilize the AgSbTe₂ phase, we propose limiting its application to around room temperature for cooling, and above 633 K for waste heat harvesting.

Thermoelectric (TE) materials and generators (TEGs) can power electronics by converting heat into electricity. Flexible TEGs can be the solution for wearable electronics power supply, which can be widely used in communication, health monitoring etc. The discovery of silver chalcogenides Ag₂Q (Q = S, Se, Te) based ductile semiconductors opens a new way to develop inorganic flexible TEGs. In this work, by tuning the ratios of S/Se/Te, we outlined a compositional region in which the Ag₂Q materials exhibit a cubic structure, large plasticity and high thermoelectric performance. Then, we selected a typical composition and studied in detail its transport and mechanical properties. At room temperature, the material shows a high zT = 0.47, good ductility (strains being 10% in tensile, 20% in three-point bending and over 50% in compression) and shape conformability. The material is then roll-processed to films with a series of thickness; the variation of microstructure and transport properties with the thickness is studied. Finally, flexible thin films (< 50 μm) were fabricated and its application in flexible temperature monitoring was explored.

Chemically synthesized metal halide perovskite nanocrystals have recently emerged as a new class of efficient light emitting materials which are particularly interesting for development of light-emitting diodes (LEDs). Stability of perovskite-based LEDs is still an issue [1], which can be partially mitigated by proper interface design, such as the use of inter-layer amine terminated carbon dots [2]. As for many other nanocrystals, proper surface passivation is a key to ensure high colloidal stability and processability of perovskites; this can be achieved by employment of multi-amine chelating ligands [3]. The use of the lead-based metal halide perovskites is sometimes considered an issue because of the toxicity concerns related to the lead component. To avoid lead in perovskites, co-doping of cerium and bismuth [4], as well as tellurium and bismuth [5] into lead-free double perovskite Cs₂AgInCl₆ nanocrystals is a useful strategy resulting in their improved photoluminescence efficiency. 1. Kong et al. Stability of Perovskite Light-Emitting Diodes: Existing Issues and Mitigation Strategies Related to both Material and Device Aspects. Adv. Mater. 2022, 34, 2205217. 2. Dong et al. Amine-Terminated Carbon Dots Linking Hole Transport Layer and Vertically Oriented Quasi-2D Perovskites through Hydrogen Bonds Enable Efficient LEDs. ACS Nano 2022, 16, 9679-9690. 3. Zeng et al. Surface Stabilization of Colloidal Perovskite Nanocrystals via Multi-Amine Chelating Ligands. ACS Energy Lett. 2022, 7, 1963-1970. 4. Wang et al. Co-Doping of Cerium and Bismuth into Lead-Free Double Perovskite Cs₂AgInCl₆ Nanocrystals Results in Improved Photoluminescence Efficiency. ACS Nanosci. Au 2022, 2, 93-101. 5. Wang et al. Co-Doping of Tellurium with Bismuth Enhances Stability and Photoluminescence Quantum Yield of Cs₂AgInCl₆ Double Perovskite Nanocrystals. Nanoscale 2022, 14, 15691-15700.

In nanomaterials with a large surface area, the surface structure is considered to have a significant effect on stability and physical properties. In order to investigate this relationship in detail, we have been studying materials called magic-size clusters, the size of which is well defined by the number of constituent atoms. We have analyzed the detailed structure and kinetics of cysteine in CdSe magic-size clusters synthesized using cysteine as a ligand, but the structure of the CdSe itself is still unresolved. Here, we report the structure around the surface of the CdSe core by using solid-state DNP NMR. ¹¹¹Cd-{¹⁵N} J-HMQC revealed a surface Cd site coordinated with N for the first time, which was considered as Cd coordinated by S and two Se from the chemical shift. The other surface Cd peak observed in a CP/MAS ¹¹¹Cd spectrum was assigned to Cd coordinated by S and three Se by comparison with the J-HMQC spectrum. The CSA parameters of N-coordinated Cd was obtained, which are consistent with the assignment due to the chemical shift.

In this work, we propose and show advanced heterostructures of colloidal quantum wells (CQWs), also known as nanoplatelets (NPLs), and their utilization for photonic applications, specifically for light-emitting diodes. The anisotropic shape and tight quantum confinement along the vertical direction provide NPLs with many unique thickness-dependent optical characteristics, including narrow photoluminescence (PL), high quantum yield (QY), giant oscillator strength, giant modal gain coefficient, and large absorption cross-section. Although previous research reported successful applications of type-I NPL-LEDs with high device performance, the use of type-II NPLs for LEDs has been barely investigated due to the obtained poor device performance even with alloyed type-II NPLs. In this talk, we will present the development of CdSe/CdTe/CdSe core/crown/crown (multi-crowned) type-II NPLs and their emerging optical properties, which will be highlighted and compared with the traditional core/crown counterparts. Unlike traditional type-II NPLs such as CdSe/CdTe, CdTe/CdSe, and CdSe/CdSe_xTe_1-x core/crown heterostructures, this advanced heterostructure reaps the benefits of having two type-II transition channels, resulting in high quantum yield (QY) and longer fluorescence lifetime. The hypothesis of the multiple type-II exciton recombination pathways will further be discussed with the simulations of electron and hole wavefunctions for both multi-crowned and core/crown NPLs in comparison. In addition, NPL-LEDs based on core/crown and multi-crowned NPLs will be shown to exhibit the remarkable performance of multi-crowned NPLs in terms of EL performance. The findings of this study suggest that multi-crowned NPLs enable high-performance solution-processed LEDs, which may pave the path for future NPL-based display and lighting technologies.

Colloidal quantum well light emitting diodes (CQW-LEDs) are highly promising for the new-generation displays and lighting technology, because they can exhibit high efficiency, good color purity, low power consumption, fast response, ultrathin thickness, ultralight weight, flexibility, and other excellent properties. Although the exploration of CQWs in LEDs is impressive, the performance of CQW-LEDs lags far behind compared with other types of LEDs (e.g., organic LEDs, colloidal quantum-dot LEDs, and perovskite LEDs). Herein, in order to further improve the performance of CQW-LEDs, different methods are used to control the distribution of charge and exciton in CQW-LEDs. The influence of charge injection and transport, morphology, material composition, device engineering, and other factors on the performance is deeply clarified. A series of high-performance CQW-LEDs are developed, and the external quantum efficiency of CQW-LEDs is close to the theoretical limit of 20%. Finally, flexible CQW-LEDs and white CQW-LEDs have been demonstrated.

In this work, we report an ITO/ ZnO CQDs/ PQT-12/ Ag structure based inorganic/organic hybrid heterojunction UV-Visible photodetector. Here, poly (3,3’’’-dialkylquarterthiophene) (PQT-12), a p-type organic semiconducting polymer, is used to act the hole transport layer (HTL) cum active material for the visible region, whereas an n-type ZnO colloidal quantum dots (CQDs) layer is used to act as the electron transport layer (ETL) cum active material for UV- region. The ZnO CQDs with an average size of ~2.1 nm have been synthesized using the hot injection method under nitrogen environment. The indium doped tin oxide (ITO) coated glass is used as substrate and silver (Ag) is used for the anode contact. The thin film of ZnO CQDs) is deposited on the ITO coated glass substrate via spin coating method followed by annealing at 450° C for 30 min. The sol-gel method is used to obtain the PQT-12 film on ZnO CQDs layer. PQT-12 sol-gel is first prepared by dissolving it in the dichlorobenzene solution of 10 mg/mL. The spin-coated PQT-12 film is then annealed at 100° C under nitrogen environment. Finally, Ag metal dots of 1 mm diameter (80 nm thickness) were deposited on the top of PQT-12 layer for the anode contacts. The photo response of the proposed device is investigated using monochromatic light with wavelengths varying from 350 to 700 nm. The maximum responsivity (R), detectivity (D*) and external quantum efficiency (EQE) are obtained as ~28.93 A/W, ~8.82 × 10¹² cmHz^1/2/W and 7004 % at 375 nm wavelength. The values of R, D* and EQE are obtained as ~ 5.11 A/W, ~ 2.12 × 10¹² cmHz^1/2/W and 1184 % in the visible region at 535 nm under 1 V reverse bias voltage.

Quantum emitters based on excitons in 2D materials and their heterostructures have great potential for quantum photonics applications. Methods of electrical and mechanical controlling excitons and their quantum emission are highly desirable. This talk will discuss using electromechanical methods including acoustic waves and strain engineering to control, transport, and modulate excitons and quantum emitters. We show in 2D systems, excitons can be transported by acoustic waves beyond the diffusion limit and the internal phonon states are heralded by single-photon emissions from 2D quantum emitters.

The reduced symmetry in strong spin-orbit coupling materials such as transition metal ditellurides (TMDTs) gives rise to non-trivial topology, unique spin texture, and large charge-to-spin conversion efficiencies [1,2]. In the first part if the talk I will discuss the planar Hall effect in 1T’-MoTe2 as well as using this material for spin readout in a van der Waals heterostructure. As an all-electrical scheme to generate, detect and manipulate spin current, the spin Hall effect (SHE) has been heavily investigated as a primary route towards next-generation spintronic devices. Herein, by constructing SHE devices using van der Waals (vdW) heterostructures, we achieved nonlocal spin readout signal of 150 mΩ and local spin readout signal of 6.7 Ω, which exceed the state-of-the-art by at least a factor of ~100 and ~20, respectively. The record-high spin readout signal is due collectively to suppressed spin dephasing channels at the vdW interfaces, long spin diffusion and large charge-spin interconversion in semimetal MoTe₂. In the second part of the talk, I will discuss the nonlinear Hall effect (NLHE) in MoTe₂ as a function of its symmetry and thickness. NLHE produces a second harmonic Hall voltage that varies quadratically with a perpendicular current under time-reversal symmetric conditions, thus they can be useful as RF rectifier. We observed large in-plane nonlinear Hall (NLH) effect for the bilayer and trilayer T_d phase MoTe₂ under time reversal-symmetric conditions, while these vanish for thicker layers. Our work highlights the importance of thickness-dependent Berry curvature effects in TMDTs that are underscored by the ability to grow thickness-precise layers. References: [1] Song, P. Kian Ping Loh* et al. Nat. Mater. 19, 292–298 (2020). [2] Ma, T., Chen, H., Yananose, K. Kian Ping Loh* et al. Nat Commun 13, 5465 (2022). 

Two-dimensional (2D) semiconducting materials, such as MoS₂, PdSe₂, phosphorene, etc., hold great potential for many important applications, such as in nanoelectronics, thermoelectric conversion and solar energy harvesting. In this talk, we first report our work on the development of multi-terminal MoS₂-based memtransistor for neuromorphic computing. By combining first-principles calculations and theoretical modelling, we investigate the structures and energetics of intrinsic point defects in MoS₂, their evolution and reaction under thermal and electric fields, and their effects on the electrical properties, such as electrical conductivity and Schottky barrier. We further examine the origin of synaptic behavior in the MoS₂-based multi-terminal memtransistor and the performance, such as, long-term potentiation, long-term depression, etc. We then report our realization of an aerosol‐jet‐printed Ag/MoS₂/Ag memristor capable of storing and processing data on flexible substrates. This memristor is realized in a crossbar structure by developing a scalable and low-temperature printing technique utilizing a functional MoS₂ ink platform. Finally, we report the development of a low voltage memristor array based on an ultrathin PdSeO_x/PdSe₂ heterostructure, which shows a remarkable uniform switching with low set and reset voltage variability. Our studies show that these nanodevices are capable of efficiently mimicking many interesting behaviors of biological synapses, demonstrating their potential to enable energy‐efficient neuromorphic computing beyond von Neumann architecture.

The two-dimensional kagome lattice has attracted tremendous attention because it can host a variety of exotic electronic physics, such as quantum spin liquids, flat-bands, various topological states. The recently discovered quasi-layered kagome superconductor CsV₃Sb₅ with topological band structures offer new opportunities to study superconductivity (SC), anomalous Hall effect, charge density wave (CDW), nematicity and so on. The origins of CDW, unconventional SC, and their correlation with different electronic states in this kagome system are of great significance, but so far, are still under debate. Here, we report the synthesis of high-quality CsV₃Sb₅ crystals and the observation of unconventional SC and pair density wave using STM/STS. Furtherly, we have successfully prepared, for the first time, the Ti-substituted CsV₃Sb₅ crystals with directly substitute for V atoms in the kagome layers. CsV_3-xTi_xSb₅ shows two distinct SC phases along with the evolution of intertwining electronic states upon substitution. Moreover, we have obtained thin CsV₃Sb₅ and CsV_3-xTi_xSb₅ nanosheets with different thickness by mechanical exfoliation and studied the thickness-dependent evolution of SC, CDW, magnetoresistance, and quantum oscillations. These findings open up a way to synthesise a new family of quasi-layered kagome superconducting materials, and further representing a new platform for modulating the different correlated electronic states and superconducting pairing in kagome superconductors. References: [1] H. Chen, H.T. Yang, C. M. Shen, H.-J. Gao et al., Roton pair density wave in a strong-coupling kagome superconductor, Nature 599, 222 (2021). [2] C. Broyles, H. T. Yang, H.-J. Gao, S. Ran, et al., Effect of the Interlayer Ordering on the Fermi Surface of Kagome Superconductor CsV₃Sb₅ Revealed by Quantum Oscillations, Phys. Rev. Lett. 129, 157001 (2022). [3] H. T. Yang, H. Chen, H.-J. Gao, et al., Titanium doped kagome superconductor CsV_3−xTi_xSb₅ and two distinct phases, Sci. Bull. 67, 2176 (2022).

Two-dimensional (2D) transition metal dichalcogenides (TMDs) have attracted tremendous attentions in recent years due to their intriguing physical properties and promising applications in electronics and optoelectronics. The construction of 2D heterostructures and superlattices can further enrich the physical properties of 2D semiconductors for new applications. However, lateral superlattices with characteristic width smaller than 5 nm, which are needed for quantum confinement effects and quantum-well applications, cannot be obtained via conventional edge epitaxy growth. In this presentation, I will present our recent results on controlled fabrication of sub-2-nm quantum-well arrays in semiconductor monolayers driven by periodic strain fields. Such periodic strain fields can be provided by misfit dislocation arrays in a lattice-mismatched lateral heterointerface (e.g. sulfide and selenide lateral interfaces) or by grain boundaries in TMD monolayers. I will also show that large-scale 60° GBs, often known as mirror twin boundaries (MTBs), with strings of 4-membered rings separated by individual 8-membered rings, can be introduced into TMD monolayers with controllable density by careful control over the nucleation and growth process during CVD growth. These sub-2-nm quantum well arrays and MTBs can serve as novel one-dimensional electronic systems in 2D semiconductors, opening up new opportunities for exploring novel quantum states and applications.

Development of biocompatible nanocarriers that can transport anti-tumor drugs in the body represent a major challenge of precision medicine. For any systemically administered drug, the transport to the site of interest is inhibited by various physiological barriers, which reduces or even blocks the therapeutic efficiency of molecular drugs. Therefore, advanced drug-delivery systems are needed to overcome biological barriers. In this context, hollow silica (SiO₂) nanoparticles functionalized with receptor-targeting ligands are promising drug-carriers to transport higher amounts of therapeutic payloads and to reduce any undesired off-site effects. Moreover, hollow nanoparticles can incorporate more than one drug enabling theranostic and thera-regenerative approaches. This talk will discuss the potential benefits of inorganic nanoparticles towards precision drug delivery.

Nano porous materials such as zeolites and nanostructured composite materials are becoming more important in the fields of catalysis. The understanding of the surface structure characteristics and the precise analysis of the local arrangement of elements in these material systems are indispensable for the deep understanding of the structure-performance relationships, which, however, depends on the availability of sophisticated analysis techniques. At present, representative methods for observing the structure and composition of nanostructures are transmission electron microscopy (TEM), atomic force microscopy (AFM), and scanning electron microscopy (SEM). TEM can obtain local composition and crystal structure information at atomic-level as projection information, but it is difficult to selectively obtain surface information. AFM can obtain surface structure information at the atomic level, especially the accurate height information, however it is difficult to observe structures at very different heights. Compared to these, SEM has a large depth of focus and can obtain surface morphology information with secondary electron images, composition information with EDS method, and crystal orientation information using a backscattered electron detector even for samples with height differences, so it is expected to be applied to zeolites with surface unevenness. However, SEM has issues with an insufficient spatial resolution, electron charging and severe damage by electron beams for observations of zeolite. In order to solve these problems and obtain surface information that governs the properties, development of a high-resolution low incident voltage SEM is desirable. On the other hand, in the low incident voltage SEM, the effects of objective lens aberration and chromatic aberration become significant, and the electron probe diameter increases, resulting in a decrease in spatial resolution. For this reason, we have been working to improve the spatial resolution of the SEM under low incident voltage conditions.

We describe wet chemistry approaches, solution based synthesis and hydrothermal processing of various nanomaterials, primarily Quantum Dots (QDs). By varying the size, shape and composition of the QDs we are able to optimize their bandgap and optoelectronic properties. The QDs are then used as building blocks to fabricate three types of solar technologies: (i) Quantum Dot Solar Cells [1,2]; (ii) Quantum Dot Photoelectrochemical cells for Hydrogen Production [3–11]; (iii) Luminescent Solar Concentrators [12–15] and in optical nanothermometers [16–18]. References: [1] Adv. Func. Mater. 27, 1701468 (2017); [2] Nano Energy 55, 377 (2019); [3] Nano Energy 31, 441 (2017); [4] Appl. Cat. B 250, 234 (2019); [5] Appl. Cat. B 264, 118526 (2020); [6] J. Mater. Chem. A 8, 20698 (2020); [7] Nano Energy 79, 105416 (2021); [8] Appl. Cat. B 280, 119402 (2021); [9] Nano Energy 81, 105626 (2021); [10] Chem. Eng. J. 429, 132425 (2022); [11] Nano Energy 100, 107524 (2022); [12] Adv. En. Mater. 6, 1501913 (2016); Nano Energy 37, 214 (2017); [13] Nano Energy 44, 378 (2018); [14] Nano Energy 50, 756 (2018); [15] J. Mater. Chem. A 8, 1787 (2020); [16] ACS Phot. 6, 2479 (2019); [17] ACS Phot. 6, 2421 (2019); [18] Small 16, 2000804 (2020).

Amine-functionalized cellulose nanocrystal materials were synthesized, characterized, and evaluated for the remediation of pesticide contaminants from organic and aqueous media. The results demonstrated their ability to degrade malathion up to 100% degradation of the compound into detectable lower molecular weight by-products. A poly(ethylenimine) cellulose nanocrystal (CNC-PEI) material was also capable of degrading aqueous solutions of malathion, deltamethrin, and permethrin with 100%, 95%, and 78% degradation, respectively. The reusability of the CNC-PEI was confirmed. Thus, these materials can potentially serve as a new and sustainable remediation technique based on their ability to effectively degrade various pesticides. 

Electrochemical route for generation of high quality H₂ fuel with zero carbon footprints is a promising way to deal with energy crisis associated with the extensive use of depleting fossil fuels. In water electrolysis, the sluggish kinetics for oxygen evolution reaction (OER) is improved by the usage of noble metal electrocatalysts. However, the scarcity, and high cost impede their usage in the long run. To overcome these limitations, transition metal phosphide catalysts are considered as viable alternatives owing to their stability in wide pH range, high conductivity, ease in availability and low toxicity. Herein, we will discuss about the performance of metal phosphide heterostructures (FeP-CoP) derived from simple physical mixing of FeP and CoP towards OER. The mixed metal phosphides (FeP-CoP) showed a huge increase in OER performance as compared to the individual metal phosphides. It shows very low overpotential, high current density (1.37 A cm^-2), improved mass activity (18987 A g^-1), and high stability (200 h). This high performance originates from the effective formation of heterointerfaces and charge transfer between different metal sites on physical mixing. It is evident from our experimental analyses that the metal phosphides get converted into metal oxides, and oxyhydroxides during OER. Theoretical analysis further reveals that the oxygenated surface formed at the interface reduces the energy barrier for final potential determining step. This leads to optimal adsorption energies of the intermediates, hence improvement in OER performance in alkaline conditions. Reference: Bhutani, D.; Maity, S.; Chaturvedi, S.; Chalapathi, D.; Waghmare, U. V.; Narayana, C.; Prabhakaran, V. C.; Muthusamy, E., Heterostructure from heteromixture: unusual OER activity of FeP and CoP nanostructures on physical mixing, Journal of Materials Chemistry A, 2022, 10 (42), 22354-22362.

Agricultural production is entering a new era, aiming for improved sustainability, increased crop yield, and least impact to the environment and ecosystem. The specific productivity needs to be increased while preserving land fertility and minimising losses to pathogens and pests movement and impact in response to climate change. The conventional synthetic agrochemicals are effective but hazardous to the environment and ecosystems. Introduction of nanotechnology will afford effective and sustainable alternatives of agrochemicals for the upcoming revolution of agricultural production. The nanoparticles will play an essential role in the new generation of topically sprayed agrochemicals. Particularly, nanoparticles can act as the active ingredients of long-lasting foliar fertilisers to supplement essential micronutrients, prompting the crop growth and reproduction. The biomolecules, that are eco-friendly and effective against pathogens and pests but too vulnerable to be practically adopted, can be protected by nanoparticles from the degrading environmental factors to acquire commercially feasible formulations. Another potential function of nanoparticles will be the “structural additives” in the agrochemical formulation to enhance the efficiency of application and consequently reduce the dosage and relevant costs. The standard commercial practices of agricultural industries need to be clearly identified and incorporated into the current research and development of agricultural nanotechnology, considering the positions of agriculture in the society structure and the supply chain. To advance the development of nanoparticle-involved agrochemicals and stimulate discussion on this relatively new field of functional nanomaterials, we will review a selection of nanoparticle applications in agriculture and propose our opinions on the prospective of agricultural nanotechnology.

It is well known that additive manufacturing (AM) of metallic components offers a number of technological advantages. Additionally, alloys made with the AM techniques such as laser powder bed fusion (LPBF) have substantially finer microstructures (due to rapid solidification) and distinct mesoscale features. A synergetic interplay between these micro- and meso-structural features leads to high strength – toughness combinations. Additionally, the ‘bottom up’ approach of building components—line-by-line and layer-by-layer with in-situ alloying capability—enables microstructural design of new alloys. Implications of these in terms of possible directions for designing AM alloys with high mechanical performance will be discussed.

With the rapid development of laser powder bed fusion (LPBF) technique, the high strength aluminum alloys fabrication has attracted growing attentions. The laser focus shift is a key factor affecting the solidification significantly during the laser processing. Therefore, in this study, an Al6xxx alloy is fabricated by LPBF using different laser focus shift. Due to the different energy distribution varied by the focus shift, the melt pool characteristics, defects, microstructures, and the mechanical properties all demonstrate an asymmetrical evolution. As the laser focus shift changes from -5 mm to +5 mm, the melt pool changes from conductive mode first to keyhole mode and then back to conductive mode. Accordingly, the grain distribution changes along with the melt pool. Although all the samples consist of ultrafine grains and coarse grains, the ratio for the ultrafine grains decreases first and then increases with the changes in laser focus shift. Due to the changes in the grain size distribution and the defect evolution, the mechanical properties are also as a function of the defocusing distance. Our work states a novel method to effectively control the microstructure and mechanical property regime of the high strength aluminum alloys.

Discrete Element Method was applied to model the powder deposition process to characterize the effect from the powder properties and working conditions. The powder properties include the powder material property, size distribution, surface condition and shape, while the working conditions comprise the facility-based process of the powder flow, such as pouring, rotation, blade sliding, etc. Generally, a highly dense packing derives from mono-size, smoothed surface and simple-shape powders, and active flow working conditions. Due to the complexity of input parameters, optimized combination of powder property and working conditions are required in order to achieve desired powder packing qualities.

High-entropy alloy and multi-metal oxide materials exhibit exotic electrochemical material properties which are not found in their single-element compositions. The thin film coating of these materials on electrode substrates by conventional methods poses a variety of challenges. Herein we report the femtosecond laser direct writing approach for in-situ synthesis and deposition of high entropy alloy and oxide materials on electrodes. A solution-based precursor ink is deposited and dried on the substrate and illuminated by a femtosecond laser. During the illumination, dried precursor ink is transformed to high entropy alloy/oxides and is simultaneously bonded to the substrate. The formulation of precursor ink for laser processing is universally applicable to a large family of oxides and alloys. The process is conducted at room temperature and in an open atmosphere. To demonstrate, a large family of 57 MMOs and alloys are synthesized from a group of 13 elements. As a proof of concept, Ni_0.24Co_0.23Cu_0.24Fe_0.15Cr_0.14 high entropy alloy synthesized on stainless-steel foil by FsLDW is used for the oxygen evolution reaction, which achieves a current density of 10 mA cm⁻² at a significantly low overpotential of 213 mV.

Laser-cladding (LC) is a direct-energy deposition (DED) method that can be applied for additive manufacturing (AM) and protective coating of metal alloys, which has the advantage of being able to manufacture complex structural components. It also has the capability for rapid prototyping of advanced alloys via in-situ monolithically and gradually varying the elemental compositions in three-dimensional configurations. We have studied LC of stainless and Ni-based superalloys on low-alloy steel substrates. Structural and crystallographic characterization along with corrosion testing were employed for evaluating the performance with and without introducing post-LC cold-working process. In this presentation, we will be discussing the recent progress in LC process in connection with a few cases that has been developed in our group.

Metal halide perovskites (MHPs) are emerging as promising candidates for next-generation light emitters for vivid display applications because of their high color purity (FWHM ~ 20 nm) and low process cost. Although a lot of strategies have been reported, electroluminescence efficiency and stability of MHP still lag behind existing light-emitting diodes (LED). In this talk, our recent strategies for efficient, bright, and stable perovskite LEDs will be delivered. First, for achieving efficient electroluminescent devices, we reported a comprehensive material strategy for suppression of defect formation in colloidal perovskite nanocrystal (PNC). Doping of guanidinium (GA⁺) into formamidinium lead bromide (FAPbBr₃) PNCs leads to smaller PNCs with stronger charge confinement.¹ Furthermore, a PNCs surface-stabilizing bromine-based small molecule, 1,3,5-tris(bromomethyl)-2,4,6-triethylbenzene (TBTB), was applied as a halide vacancy healing agent.¹ We also demonstrated large-area applications by developing a modified-bar coating method to fabricate large-area devices with high efficiency similar to that of small-area devices made by the spin-coating method.² Last, we achieved simultaneously efficient, bright, and stable perovskite LEDs by developing an in situ core/shell PNC structure. By splitting large 3D perovskite crystals into PNCs and surrounding them with small organic ligands, significant improvement in both efficiency and lifetime could be achieved with both excellent charge transport and charge confinement.³ 1. Kim, Y.-H.et al. Comprehensive defect suppression in perovskite nanocrystals for high-efficiency light-emitting diodes. Nat. Photonics 15, 148–155 (2021). 2. Kim, Y.-H.et al. Exploiting the full advantages of colloidal perovskite nanocrystals for large-area efficient light-emitting diodes. Nat. Nanotechnol. 17, 590–597 (2022). 3. Kim, J. S.et al. Ultra-bright, efficient and stable perovskite light-emitting diodes. Nature 611, 688–694 (2022).

I will give a quick overview of recent progress and future prospects of perovskite tandem solar cells reported in the literature. I will then talk about some of the research activities in my group. They include i) low bandgap perovskites, ii) high bandgap perovskites, iii) perovskite-perovskite tandem, iv) perovskite-Si tandem cell monolithic integration strategy, v) perovskite-Si tandem via SAM-HTM engineering, and vi) 3-junction tandem.

Colloidal lead halide perovskite (LHP) nanocrystals (NCs) are of immense interest as classical and quantum light sources. LHP NCs form by sub-second fast and hence hard-to-control ionic metathesis reactions, which severely limits the access to size-uniform and shape-regular NCs. We show that a synthesis path comprising an intricate equilibrium between the precursor (TOPO-PbBr₂ complex) and the PbBr₃^- solute for the NC nucleation may circumvent this challenge [1]. This results in a scalable, room-temperature synthesis of monodisperse and isolable CsPbBr₃ as well as FAPbBr₃ (FA= formamidinium) and MAPbBr₃ (MA=methylammonium) NCs. NCs of all these compositions exhibit up to four excitonic transitions in their absorption spectra, and the size-dependent confinement energy for all transitions is independent of the A-site cation. We then discuss the size-dependent single-photon emission across the LHP NC compositions. High single-photon purity (from a cavity-free, nonresonantly excited single 6.6 nm CsPbl₃ NCs showcases the great potential of CsPbX₃ NCs as room-temperature highly pure single-photon sources for quantum technologies [2]. In another study, we address the linewidth of the single-photon emission from perovskite NCs at room temperature. Particularly, we demonstrate that emission line-broadening in these quantum dots is primarily governed by the coupling of excitons to low energy surface phonons. Mild adjustments of the surface chemical composition allow for attaining much smaller emission linewidths of 35-65 meV (vs. initial values of 70-120 meV) [3]. NC self-assembly into long-range ordered superlattices is a versatile platform for materials engineering, particularly for attaining collective phenomena with perovskite NCs, such as superfluorescence [4, 5]. 1. Quinten Akkerman et al. Science, 2022, 377, 1406-1412 2. Chenglial Zhu et al. Nano Lett. 2022, 22, 3751-3760 3. Gabriele Raino et al. Nat. Commun., 2022, 13, 2587 4. lhor Cherniukh et al. Nature, 2021, 593, 535-542 5. lhor Cherniukh et al. ACS Nano, 2022, 16, 5, 7210-7232.

The boundaries of materials with non-trivial band topology host electronic states that are robust against external perturbations. When the topological phase transition is driven by spin-orbit interactions, topological states are spin-polarized and their spin can be addressed by electrical currents, making them highly attractive for spintronics. In the last decade, a number of topological materials have been discovered. Once their topological states characterized, a natural step for tuning their properties and applying them in devices is to search for manipulation strategies. Here I will present two different strategies to manipulate different type of topological states. In the first example, I will show how the magnetic interaction with topological surface states of chalcogenide-based 3D topological insulators can be finely tuned using metalorganic molecules. By coordinating the same magnetic ion with different ligands, we can span from the strong interaction regime, where the TSS is quenched in the first quintuple layer [1], to the weakly interacting regime where both the pristine TSS and the ion’s magnetic moment are preserved [2].In the second example, we will use a surface hosting a monoatomic step superlattice as a template to grow nanostructured α-Bismuthene [3]. The passivation with the surface atoms makes it possible to grow a single monolayer of α-Bismuthene, in contrast to the black phosphorous-like bilayers that are achieved in thicker films. The growth of the film improves the quality of the step superlattice, similar to that found for the BiAg₂ Rashba surface alloy [4]. This single Bismuthine layer also has edge localized states, that appear aligned in periodic arrays due to the built-in step superlattice. [1] M. Caputo et al., Nano Letters, 16(6), 3409 (2017). [2] M. G. Cuxart et al., ACS Nano, 14(5), 6285 (2020).[3] K. García et al., in preparation.[4] J. E. Ortega et al., submitted.

Two-dimensional materials, particularly at the 2D limit, exhibit versatile properties different from their bulk counterparts, and can find great application potentials in various fields such as electronics, catalysis, energy etc. The van der Waals interactions, even though weak, plays a vital role in the epitaxy of these materials. In this talk, I will present how we employed vdW interactions to epitaxy new 2D materials and to engineer the stacked heter-bilayers. The 1T phase of transition metal dichalcogenides (TMDs) that usually exhibit rich physical phenomena. For example, we successfully grew 1T-TiSe₂/1T-TiTe₂ bilayers with controllable twist angle through sophisticated epitaxy control. We discovered a moiré enhanced charge density wave state at small twist angles. This is the first demonstration of epitaxy and controllable twist angle in the 1T-phase of TMD bilayers.

Emergent quantum phenomena in two-dimensional van der Waal (vdW) magnets are largely governed by the interplay between the exchange and Coulomb interactions. The ability to tune the Coulomb interaction in such strongly correlated materials enables the precise control of spin-correlated flat-band states, bandgap (E_g) and unconventional magnetism, all of which are crucial for next-generation spintronics and magnonics applications. Here, we demonstrate a giant gate-tunable renormalization of spin-correlated flat-band states and bandgap in magnetic chromium tribromide (CrBr₃) monolayers grown on graphene. Our gate-dependent scanning tunneling spectroscopy (STS) studies reveal that the inter-flat-band spacing and bandgap of CrBr₃ can be continuously tuned by 120 meV and 240 meV respectively via electrostatic injection of carriers into the hybrid CrBr₃/graphene system, equivalent to the modulation of the Cr on-site Coulomb repulsion energy by 500 meV. This can be attributed to the self-screening of CrBr₃ arising from the gate-induced carriers injected into CrBr₃, which dominates over the opposite trend from the remote screening of the graphene substrate. Precise tuning of the spin-correlated flat-band states and bandgap in 2D magnets via electrostatic modulation of Coulomb interactions not only provides new strategies for optimizing the spin transport channels but also may exert a crucial influence on the exchange energy and spin-wave gap, which could raise the critical temperature for magnetic order.

Twisted multilayer materials composed of two-dimensional semiconducting monolayers, such as transition metal dichalcogenides, exhibit many surprising properties. In my talk, I will demonstrate how materials modelling can be used to gain insights into the complex interplay of electrons, excitons and phonons in these systems. In particular, I will demonstrate how electronic and optical properties of these materials can be tuned by an applied electric field which influences the delicate competition between electrons originating from different monolayer valleys. I will also show how excitons that are momentum-indirect can be activated and finally how excitons can "surf" on special moire vibrations known as phasons. 

Neural networks based on memristive devices have shown potential in substantially improving throughput and energy efficiency for machine learning and artificial intelligence, especially in edge applications. Because training a neural network model from scratch is very costly in terms of hardware resources, time, and energy, it is impractical to do it individually on billions of memristive neural networks distributed at the edge. A practical approach would be to download the synaptic weights obtained from the cloud training and program them directly into memristors for the commercialization of edge applications. Some post-tuning in memristor conductance to adapt local situations may follow afterward or during applications. Therefore, a critical requirement on memristors for neural network applications is a high-precision programming ability to guarantee uniform and accurate performance across a massive number of memristive networks. That translates into the requirement of many distinguishable conductance levels on each memristive device, not just lab-made devices but more importantly, devices fabricated in foundries. Analog memristors with many conductance states also benefit other applications, such as neural network training, scientific computing, and even mortal computing. Here we report over 2048 conductance levels, the largest number among all types of memories ever reported, achieved with memristors in fully integrated chips with 256x256 memristor arrays monolithically integrated on CMOS circuits in a standard foundry.¹ We have unearthed the underlying physics that previously limited the number of achievable conductance levels in memristors and developed electrical operation protocols to circumvent such limitations. These results reveal insights into the fundamental understanding of the microscopic picture of memristive switching and provide approaches to enable high-precision memristors for various applications. 1. M. Rao, et al., J. Joshua Yang, “Thousands of conductance levels in memristors monolithically integrated on CMOS”, Nature (accepted, 2023).

In this talk I will describe electrochemical neuromorphic device that switche at record-low energy (<0.1 fJ projected, <10 pJ measured) and voltage (< 1mV, measured), displays >500 distinct, non-volatile conductance states within a ~1 V operating range. Furthermore, it achieves record classification accuracy when implemented in neural network simulations. Our organic neuromorphic device works by combining ionic (protonic) and electronic conduction and is essentially similar to a concentration battery. Our synapses display outstanding speed (<20 ns) and endurance achieving over 10⁹ switching events with very little degradation all the way to high temperature (up to 120°C). Having developed these devices for a few years, we have recently started working with collaborators on demonstrating what they can do. As a result, I will describe two demonstrations of learning using our devices. In the first one, a crossbar array of synapses “learns” to perform basic logic operations. In the second one, a simple electronic circuit that incorporates the synapse “learns” when to turn right and when to turn left in a predetermined maze. In the last demonstration, we take advantage of the inherent compatibility of our polymers with living matter. We show that a chemical signal, such as dopamine secreted from cells can be used to generate electronic updates to the device, as a first step towards integrated brain-machine interfaces.

Artificial synapses are an important building block for neuromorphic computing, a field that seeks to replicate the function of neurons in the brain and improve the efficiency of computers beyond the Von Neumann architecture. These devices hold the promise to reproduce fundamental brain functions such as learning and memory, and could potentially lead to the development of more efficient and intelligent computing systems. In this work, we demonstrate for the first time the properties of long-term potentiation, short-term potentiation, and synaptic depression using light as a stimulus and persistent photoconductivity as a measure in a Cu2ZnSnSe₄/CdS/ZnO-based photovoltaic device artificial synapse. These devices exhibit wavelength selectivity, with different wavelengths of light leading to distinct responses. The response to light can also be tuned by adjusting the oxygen content in the ZnO layer. Illumination in short bursts exhibits a short-term plasticity behavior with a memory effect similar to an RC integrator, which is important for digital computing. Longer illumination leads to a potentiation persisting for up to 4 days. Interestingly, illumination beyond 1200 nm leads to both short-term potentiation and long-term depression, a response that has not been previously observed in electronic or optoelectronic synapses. Conversly, UV illumination below 500 nm nm is particularly efficient at producing both a long term and short-term potentiation, hinting at the charging of trap states at the vicinity of the p-n interface. These findings provide insight into the defect trapping mechanisms of artificial synapses and represent a new opportunity for ultra-low energy computing. The results of this study could potentially lead to the development of more efficient and intelligent computing systems, and further research in this area could unlock even more exciting potential applications.

In the edge AI information processing, oxide-based memristors are expanding their functionalities from data storing as non-volatile memories to data processing as AI accelerators. Among various presumable applications of the memristors, device implementation of the reservoir computing, which is a machine learning algorism suitable for low-power processing of time-series data (TSD), is attracting considerable attention. In this case, the memristor is called a physical reservoir device (PRD). The hysteretic and non-linear response of the memristor can extract features from TSD and the smaller neural network (NN) and simpler weight update rule can be used compared to the conventional deep NN. Here, we evaluated the TaOx memristor performance as PRD in the electrocardiogram (ECG) data classification task. The memristor used in this study has a staking structure of TiN(TE)/Ta/TaOx-L/TaOx-H/TiN(BE). TaOx-L and TaOx-H are the Ta oxide layers prepared by a reactive sputtering. The resistivity of TaOx-H is much higher than TaOx-L. TE and BE denote the top and bottom electrodes. The device size is 300 nm in diameter. Three types of ECG signals were repeatedly input to the TaOx memristor as the voltage signals after adjusting the voltage amplitude to fit the operating voltage conditions of the device. Each time the ECG signal was input, only 5 output current values (OCVs) were measured and they were used as the feature values in the subsequent data classification process using NN. We also measured OCVs from a resistor as a control experiment. The OCVs from the TaOx memristor increased the learning accuracy of NN more rapidly (under the smaller epoch number) compared to those from the resistor. Moreover, the inference accuracy was also higher in the case of the TaOx memristor. The adoptability of TaOx memristors to an ECG data classification as PRD was successfully demonstrated.

Compared with various artificial neural network models, reservoir computing (RC) is developed to process complex spatiotemporal data with the advantage of low computing cost. As a brain-inspired neuromorphic computing algorithm, reservoir computing (RC) can project features from temporal inputs into a high-dimensional space and be further processed by a smaller neural network. Recently, dynamic memristors have shown promising potential for the hardware implementation of RC systems. In this study, we fabricate a double-oxide IGZO/TaO_x memristor with the volatility characteristic and the self-rectified I-V curves. Furthermore, the intrinsic short-term memory can be demonstrated through the IGZO/TaO_x memristor by voltage pulse stimuli which presents internal dynamics of the memristor. This is the key feature for effectively extracting and analyzing the temporal input data in RC. The critical characteristic parameter of memristors that influences the performance of RC systems, the decay time constant, is also identified and discussed in this work. Afterwards, we verify the nonlinear current response of the memristor device can be well-controlled by the pulse amplitude and width. These results indicate that the device can filter postsynaptic current based on various stimulations and integrate postsynaptic signals in a nonlinear manner. Finally, based on the intrinsic dynamic behavior, our bilayer device can distinguish inputs with various temporal features. In summary, this work demonstrates that a memristor-based reservoir can be utilized to recognize sequential data and further, the tunable decay time constants are adopted to recognize the sequential data with the different frequencies.

Vanadium dioxide (VO₂) is widely studied for its prominent insulator-metal transition (IMT) near room temperature, with potential applications in novel memory devices and brain-inspired neuromorphic computing. In the previous work of our group, Rana et al. observed multiple intermediate stable resistive states between the insulating and metallic states in VO₂ films by tailored temperature sweeps. [1] The existence of these intermediate resistive states is particularly attractive for reconfigurable electronic circuitry. In this work, we fabricated planar single-bridge devices from VO₂ thin films. Under voltage/current sweeps, Joule heating in the device triggers the IMT leading to volatile resistive switching behaviour. Furthermore, intermediate steps can happen during the reset when tuning the voltage under a high compliance current. This unique measurement can allow multistate memory within one VO₂-based memory cell (in our demonstration 3 bits per cell) and reliable multilevel operation. [2]In order to exploit these intermediate states, we also fabricated devices with parallel double VO₂ bridges with varying bridge dimensions or bridge-to-bridge distances. In the double-bridge devices, we obtain a higher degree of control for the intermediate states and the switching behavior depends ultimately on heat dissipation effects. This principle can readily be extended to more parallel bridges and complex network configurations. To develop optimum device designs, we carried out nanoscale thermal mapping of in-operando devices using Scanning Thermal Microscopy (SThM), which gives us a straightforward indication of the current distribution among the bridges. [1] A. Rana, C. Li, G. Koster, and H. Hilgenkamp, Sci. Rep. 10, 2 (2020).[2] X. Gao, C. M. M. Rosário, and H. Hilgenkamp, AIP Adv. 12, 015218 (2022). 

The concept of detection limits and how they are formulated were established in Lloyd Currie's 1968 paper, which has been widely adopted by many international standards and regulations, including the 1975 International Union of Pure and Applied Chemistry (IUPAC). In X-ray detection with advanced materials like perovskite, the counts produced by the detector for dose measurement are the digitized electric current data, rather than the time-accumulated radiation decay events. The X-ray quanta emitted by the thermionic emission based X-ray source and the charge carrier fluctuation within perovskite are not decay events in their nature, but it is recognized that Currie's formulation is still applicable to counting practices for non-nuclear decay. However, it can be difficult for many to correctly measure and interpret X-ray detection limits using the definitions recommended by IUPAC. In this paper, we will present a method of measuring detection limits through dark current counting and propose an equation that converges to Currie's formulation under certain conditions. To measure X-ray sensitivity, a dosimeter is typically used to measure the dose rate of the X-ray source. Additionally, it can be problematic when the sensitive volume is larger than the X-ray beam, which requires volume correction or finding a calibrated dosimeter with a sensitivity volume smaller than the X-ray beam coverage.

Scintillation is a physical process where scintillators emit UV or visible lights upon the absorption of high-energy radiation. One of the basic requirements for excellent scintillators is that the density of charge trapping centers should be as low as possible, which otherwise gives rise to persistent luminescence through slow release of trapped carriers. Phosphors emitting visible and near-infrared persistent luminescence have been explored extensively owing to their unusual properties and commercial interest in their applications in glow-in-the-dark paints, optical information storage, and in vivo bioimaging. However, developing persistent phosphor with ultraviolet C (200~280 nm) afterglow remains challenging. In this talk, I will demonstrate our recent advances on the design and synthesis of X-ray charged UVC persistent phosphors. I will show that the judicious choice of wide-bandgap fluoride perovskites with Pr³⁺ dopants leads to the discovery of the first UVC persistent phosphor. Following this, I will show how theoretical calculations enable us to rationally boost the afterglow intensity of UVC persistent phosphors via theory-guided defect tuning, and will demonstrate that the combination of theoretical calculations with experiments not only leads to the finding of non-rare-earth phosphor with a UVC afterglow duration more than 2 h but also offers a much clearer afterglow mechanism. Finally, I will present the perspective on the possible application of UVC persistent phosphors.

In this talk, I will report about our efforts in implementing existing materials or developing new ones for X-ray detector applications. In particular, I will talk about our progress on:- The growth of non-perovskite or delta-phase of CsPbI₃ microwiers for stable and high-resolution X-ray detection;¹- The optimized crystallizaton of large-area single crystals for Megavoltage X-ray detectors used in cancer treatment;²- And our ongoing efforts for robotic-growth of conventional perovskites for flat-panel X-ray detectors,³ as well as designing pi-pi interactions in novel materials for low-dose X-ray detection. [1] S. Kundu, D. Richtsmeier, A. Hart, V. Yeddu, Z. Song, G. Niuc, D. T. Gangadharana, E. Dennis, J. Tang, O. Voznyy, M. Bazalova-Carter, M. I. Saidaminov. Orthorhombic Non-perovskite CsPbI3 Microwires for Stable High-Resolution X-ray Detectors. Advanced Optical Materials, 2022, 10, 2200516. [2] S. Kundu, J. O’Connell, A. Hart, M. Bazalova-Carter, M. I. Saidaminov. Halide Perovskites for Direct Conversion Megavoltage X-ray Detectors. Advanced Electronic Materials, 2022, https://doi.org/10.1002/aelm.202200640. [3] Y. Haruta, M. Kawakami, Y. Nakano, S. Kundu, S. Wada, T. Ikenoue, M. Miyake, T. Hirato, M. I. Saidaminov. Scalable Fabrication of Metal Halide Perovskites for Direct X-ray Flat-Panel Detectors: A Perspective. Chemistry of Materials 2022, 34, 12, 5323.

Halide perovskite semiconductors for direct X- and gamma-ray detection have currently attracted enormous attentions due to the bright prospects in various scenarios, such as medical imaging and nuclear nonproliferation in homeland security. Halide perovskites featuring excellent charge transport properties and low cost in preparation may offer a competitive opportunity compared to the conventional room-temperature semiconductors. As previously evidenced the hole carriers in perovskite semiconductors have seemingly better transport properties than electrons carriers. The unipolar sensing strategy could eliminate the such challenge induced by the electron trapping issue. However, the development of unipolar detectors for perovskite semiconductors is still at an early stage where substantial efforts are requested upon the device optimization. Here, our progresses on the unipolar perovskite detectors were reported with the configuration of pixelated and virtual Frisch grid type aiming at their deployment for the high energy resolution gamma-ray spectroscopy. The thickness of single-crystal detectors varied from ~3 to 10 mm which were grown by melt method. In contrast to the ambipolar configuration, the unipolar design as indicated adequately restrained signal induction region of the hole carrier, which in turn eliminated the depth of interaction dependency between the signal amplitude partially. The relationship of the carrier drift time and the signal amplitude in various detector configuration were analyzed to estimate the charge transport properties of hole carrier. The energy resolution was determined based on the signal amplitude analysis. The issues in achieving high energy resolution by unipolar perovskite semiconductors were also analyzed. These results shall be of interest in the applications of high performance room temperature gamma-ray detectors.

The advent of solid-state batteries has spawned a recent increase in interest in lithium conducting solid electrolytes. However, many open questions remain when trying to optimize electrolytes and understand solid state battery chemistries. In this presentation, we will explore the current focus of halide-based ionic conductors in solid state batteries and discuss stability limitations in solid state battery cells at the anode as well as the cathode composites. In a second part, we show the influence of Si type anode materials on the effective transport and behavior of solid-state batteries. Finally, we will discuss that it is not only important to find fast ionic conductors but that for an effective thermal battery management the thermal transport properties of solid ionic conductors need to be explored and understood. Here we will show the diffusive thermal transport nature of solid electrolytes and their different scaling relations that put in question the assumption of Bruggeman transport in solid state batteries.

Ternary halide compounds have attracted great attention over the past 5 years as they are considered as solid-state electrolytes for Li-ion batteries. However, their sensitivity against oxygen and moisture limits their application. One of the most promising ternary halides is Li₃InCl₆ (LIC) which can be prepared by either following a water-based synthesis route or by taking advantage of a solvent-free mechanochemical approach; both routes yield compounds with a rather high conductivity in the mScm^-1 range. We assume that such fast Li⁺ transport is connected to the layer structure of LIC enabling rapid 2D diffusive processes. The aim of our study is to prove and to characterize low-dimensional Li⁺ diffusion in polycrystalline LIC via NMR and broadband conductivity spectroscopy. Here, our LIC sample, prepared by the water-based synthesis protocol, possesses an ionic bulk conductivity of 0.5 mScm^-1 at 20 °C. Importantly, the dispersive regimes of the conductivity isotherms follow Jonscher’s power law with an exponent κ of approximately 0.5. Indeed, κ = 0.5 is expected for 2D Li⁺ transport. The idea of dealing with a low-dimensional diffusion process is underpinned by ⁷Li NMR spin-lattice relaxation measurements. The NMR rates pass through an asymmetric rate peak that is characteristic for 2D (long-range) diffusion pathways. While ionic transport is characterized by activation energies of 0.44(1) eV (−100 °C to −30 °C) and 0.40(1) eV (T > 0 °C), respectively, local ion dynamics are subjected to barriers as low as 0.11(1) eV. At 57 °C, the Li⁺ jump rate, as directly obtained by NMR, amounts to 7.3 x 10⁸ s⁻¹ revealing extremely fast exchange processes that are comparable to those in argyrodite-type compounds.

Li-rich compounds with the antiperovskite structure have recently attracted great attention for use as solid electrolytes in all-solid-state batteries. The large variety of crystal systems, structure derivatives, and ionic substitutions in this class of materials is worth investigating to find novel candidates with potentially exceptional solid electrolyte properties. In this work, we performed high-throughput DFT calculations in the thousand-scale material space of anion-site-substituted compositions of parent structures Li₃XZ, Li₄XZ₂, and Li₇X₂Z₃, where X = {O, S, Se, Te} and Z = {F, Cl, Br, I} and screened for thermodynamically (meta)stable new compounds. Multiple electrolyte-property criteria were then employed as objective functions for the Bayesian-driven optimization search of promising candidate solid electrolytes: electrochemical window upper bound (DFT electronic band gap energy), chemical stability (DFT bulk reaction energy vs. H₂O), and ionic conductivity (by AIMD approach). Results on successfully synthesized compounds and their measured properties will also be discussed. Aside from the generated large-scale DFT dataset of potential novel materials, this study also provides potentially useful physical/chemical insights for multi-objective solid electrolyte design. (Jalem et al., Chem. Mater. 2021, 33, 15, 5859–5871; Jalem et al., In preparation).

Solid electrolytes (SEs) are key components of all-solid-state batteries (ASSBs), which promise higher energy density and faster charging, along with safer operation compared to commercial Li ion batteries. While ionic conductivity is arguably the most important performance indicator of a solid electrolyte, there are a number of intrinsic and extrinsic variables that have profound impact on the ionic transport properties of a solid, including polymorphism, microstructure, and pressure effects. As a consequence, new research lines have been emerging which focus on interfacial engineering, defect and microstructure design, and questions such as cycling stability or the role of stack pressure on battery performance increasingly enter the focus of ASSB design. In this talk, we will discuss the development and optimization of lithium and sodium thiophosphate and halide SEs and highlight the importance of the synthesis procedure (conventional solid-state synthesis vs mechanochemical ball-milling), phase engineering, and cation or stacking fault disorder on the ionic transport properties. Furthermore, we demonstrate the effect of stacking and pelletizing pressure as a powerful tool to influence the microstructure and, hence, ionic conductivity of mechanically soft thiophosphates such as tetragonal Li₇SiPS₈. Using Heckel analysis for granular powder compression reveals distinct pressure regimes which differently impact the Li ion conductivity of LGPS-type SEs. For two samples with different particle distributions, our multiscale experimental and theoretical study captures both atomistic and microstructural effects of pressure at different compression stages, thus emphasizing the importance of microstructure, particle size, and pressure control in polycrystalline SEs.

The utilization of sulfur as an earth-abundant and cost-effective material for energy storage technology has been a longstanding challenge for over 60 years, but it has not yet become commercially viable. However, the recent development of solid-state batteries has brought sulfur active materials back into the spotlight, as they can eliminate the notorious shuttle effect caused by reaction intermediates dissolved in liquid electrolytes. While solid-state lithium-sulfur batteries have the potential to address some of the issues associated with traditional lithium-ion batteries, they also present their own unique challenges. Specifically, the ionically and electronically insulating active materials require the formation of composites with solid electrolytes and electron-conductive additives to ensure sufficient ion and electron supply at the triple-phase boundary. However, this compositing process can make the transport pathways for charge carriers very tortuous, and requires careful optimization to achieve the maximum attainable energy density. Additionally, the requirement for a high interfacial area density can lead to the pronounced degradation of the solid electrolytes. The formation of less conductive interphases further decreases the overall transport in the composites. In this study, we aimed to understand the control factors of transport within composites. The Li-argyrodite was utilized as a model electrolyte. The effects of various factors on ion transport, including e.g. the type and shape of carbon used as an electron conductive additive and its surface modification, were examined. In addition, we investigated the influence of voids in the composites using applied pressure as a variable. We also assessed the correlation between the decomposition behavior under voltage application and the aforementioned parameters. Overall, the developments of a cathode composite design principle facilitating fast ion transport within composites and even more conductive electrolytes are necessary for further developments in solid-state lithium-sulfur batteries.

We present a first principles study of phase stability among structures in the Li₂S and P₂S₅ binary system, which encompasses the superionic conducting b-Li₃PS₄, a-Li₃PS₄, Li₇P₃S₁₁, and HT-Li₇PS₆ phases. These all exhibit high room temperature Li conductivity of greater than 0.1 mS/cm, and thus show great promise as electrolytes for Li solid state batteries.^1,2,3,4 These structures have all been shown to be metastable at room temperature, as special synthesis procedures such as nanosizing and ball milling are often used to stabilize b-Li₃PS₄ and Li₇P₃S₁₁.^1,3 Furthermore, a-Li₃PS₄ has not been experimentally stabilized at room temperature.² These challenges in synthesis motivates a first principles thermodynamic study on the phases in this tie-line, which could guide further experimental efforts to realize and optimize these materials. To model phase stability in this system, we treat configurational entropy contributions with density functional theory (DFT) calculations and the cluster expansion method, and vibrational contributions with harmonic phonon calculations. Analysis of DFT relaxations clarifies the nature of Li sublattices in b-Li₃PS₄ and HT-Li₇PS₆. New ground state orderings are proposed for Li₇P₃S₁₁, a-Li₃PS₄, HT-Li₇PS₆, and LT-Li₇PS₆. Phase transitions among the Li₃PS₄ and Li₇PS₆ polymorphs are predicted from first principles, and transition temperatures show reasonable agreement with experiment. Configurational and vibrational sources of entropy are separately examined and quantified. Our theoretical work provides clear thermodynamic understanding which can guide further experimental efforts in stabilizing the superionic conducting phases in the Li₂S-P₂S₅ binary system. [1] Liu, Z.; Liang, C. et al. J Am Chem Soc 2013, 135 (3), 975–978.[2] Kaup, K.; Nazar, L. et al. Mater Chem A 2020, 8 (25), 12446–12456. [3] Yamane, H.; Tatsumisago, M. et al. Solid State Ionics 2007, 178 (15–18), 1163–1167. [4] Ziolkowska, D. A.; Wang, H. et al. ACS Appl Mater Inter 2019, 11 (6), 6015–6021.

All-solid-state Sodium (Na) ion batteries are widely regarded as promising candidates for a reliable stationary energy storage grid, where it uses a solid electrolyte as the ion conducting matrix to shuttle Na ions within the electrochemical cell. Not only does solid electrolytes have enhanced safety aspects compared to some of their liquid counterpart, it also serves as physical separators, thereby reducing the number of components in the battery for simpler assembly. Together with the higher abundance and accessibility of Na, Na-ion batteries are more economical to produce. The higher safety features and lower production cost of all-solid-state Na ion batteries resulted in a surge in research activities in search of solid electrolytes for battery applications. Among these promising solid electrolytes, the Na SuperIonic CONductor, or NaSICON, exhibits superior ionic conductivity, good mechanical and thermal stability, enabling safe battery operations to address growing energy demands. NaSICON with composition Na3.4Zr2Si2.4P0.6O12 (N3.4ZSP) has shown to be highly conductive due to the optimal Na to vacancy ratio, as well as the high Silicon (Si) content. Upon further doping with ions of higher oxidation state such as Nb5+/Ta5+ on the Zr4+ site, we could further increase the Si content to improve the ionic conductivity of the NaSICON solid electrolyte. Using variable-temperature electrochemical impedance spectroscopy, we observed that Nb-doped N3.4ZSP showed high ionic conductivity, up to 4.3 mS cm-1 in total ionic conductivity at room temperature. The effect of dopants in the NaSICON structure will be probed using Raman spectroscopy and validated using computation, which will address the whereabouts of the dopants in the bulk and the microstructure of these NaSICON materials and provide useful insights to the science behind doping in NaSICON.

Hybrid perovskite solar cells have witnessed great successes recently while their instability is a big hurdle for practical application. Herein we discuss our recent progress in addressing the stability of perovskite solar cells, including introduction of capping layers to improve the stability against moisture and heat, and perovskite size engineering to suppress phase segregation. In particular, quantum dots (QDs) have the advantages of quantum confinement effect, defect-tolerant nature, and processability for flexible devices. We discuss a new surface ligand engineering strategy in designing new hybrid perovskite QDs with controllable compositions and sizes The QDs have been used as building blocks in quantum dot solar cells delivering a certified record efficiency of 16.6% with excellent long-term operation stability. By using QDs as light absorbing materials, the QD based photocatalysts also exhibited good stable performance in photocatalytic hydrogen production. The combination of perovskite QDs with Metal-Organic Framework (MOF) materials to form new hybrid composites led to ultrastable photoluminescent property for > 10,000 hours. The integration of perovskite solar cells and rechargeable batteries have led to a single module type rechargeable solar batteries with an overall storable solar energy conversion efficiency of >12%.

Production of green hydrogen via high-efficiency water splitting by using sustainable energy sources is one of the main strategies toward carbon neutrality. Several new types of hydrogen production strategies, such as piezo-catalytic and pyro-catalytic water splitting, have emerged in recent years, where the spontaneous polarizations of ferroelectric materials have been utilized for hydrogen generation by harvesting mechanical vibration or thermal energies from the surrounding environment. In this talk, I will introduce on our recent work in piezo- and pyro-catalytic hydrogen production with a focus on (1) our proposed band theory of spontaneous polarization driven water splitting, and (2) the strategy to efficiently accelerate the pyro-catalytic water splitting. Finally, a future perspective will be given on the spontaneous polarization driven piezo- and pyro-catalytic reactions including, but not limited to the deeper understanding of the catalytic mechanism, rational design of the nanostructured ferroelectric materials, and potential applications.

Healthcare is one of the main global challenges recognized by UN. Bioelectronics is the field wherein the marriage between biology and electronics enables unique solutions for the healthcare community. However, current electronic components are mostly non-biodegradable and not compatible with the soft human tissues. The aim of my research is to overcome these challenges through i) development of novel electronic material library using polymer-inorganic material systems and to create novel healthcare solutions. Through green chemistry we have developed novel biodegradable, biocompatible and bioresorbable electronic materials based on polymers. Printed electronics is the new emerging fabrication technique that allows electronic components, circuits and devices to be put on a desired surface using nanoparticle inks. The synthesized materials are converted into printable inks to fabricate soft and flexible devices. I will showcase some of the case studies on the application of the materials and flexible devices.

There is boom in the energy requirement due to the ever increasing population and modern human lifestyle. Energy harvesting is the most propitious approach to ameliorate the global energy challenge. Energy harvesting can be done in many ways depending upon the amount, source and the type of the energy being converted. There are various technologies that can be employed to harness energy such as piezoelectric, thermoelectric, electromagnetic, and photovoltaic. Several piezoelectric materials have been investigated such as PZT, AlN, ZnO, KNN, PVDF etc. for energy harvesting. Recent demand for flexible electronics requires integration of KNN with the lead free one. Polymer based piezoelectric materials provide high flexibility and ease of fabrication. PVDF is one such material which is flexible as well as shows some piezoelectricity. So, the materials having high piezoelectric coefficients can be incorporated with the PVDF matrix to enhance the energy harvesting efficiency along with flexibility and better mechanical strength. In the present work, KNN-PVDF composites have been prepared with different concentrations of KNN to study their structural, electrical and ferroelectric properties. KNN-PVDF composite with 10 % KNN concentration show dielectric constant of 37 with low leakage current of the order of 10^-11 A at an applied voltage of 5V with promising ferroelectric property having P_max = 0.26 µC/cm² at 1 kHz. The result shows the potential application of KNN-PVDF composite for mechanical energy harvesters.

Aqueous multi-valence metal ions battery is a new type of energy storage system with low cost, safe and eco-friendly merits. Due to the highest theoretical capacity and abundant reserves of metal aluminum, the secondary aqueous aluminum ion battery (AAIB) is an ideal choice for energy storage system. However, the severe passivation and hydrogen evolution induced self-corrosion prefer to occur on the surface of metal aluminum anode. These side reactions result in the inferior stability, which leads to difficulty in its practical requirements. In view of the above issue, the reporter obtained the Zn-Al alloy anode by a simple deposition process of Al³⁺ onto Zn foil substrate. Through the in-situ electrochemical activation of Mn-rich MnO, the Al_xMnO₂ cathode was synthesized to incorporate a two-electron reaction. A new AAIB system is assembled by using Al_xMnO₂ cathode, zinc substrate supported Zn-Al alloy anode and an Al(OTF)₃ aqueous electrolyte. The architected cell delivers a record-high discharge voltage plateau near 1.6 V and specific capacity of 460 mAh g^-1 for over 80 cycles. Moreover, the reporter also engineered the amorphous aluminum (a-Al) interfacial layer by the lithium-ion alloying/de-alloying processes. Unveiled by experimental and theoretical investigations, the amorphous structure greatly lowers Al nucleation energy barrier, which forces Al deposition competitive with electron-stealing hydrogen evolution reaction (HER). Simultaneously, the inhibited HER mitigates the passivation, promoting interfacial ions transfer kinetics and enabling steady aluminum plating/stripping for 800 h in the symmetric cell. The resultant multiple full cells using Al@a-Al anodes deliver approximately 0.6 V increase in discharge voltage plateau compared to that of bare Al-based cell, which far outperforms all reported aqueous AMBs. 

Generation of clean energy and its storage is an important issue of current time. To meet challenge of global warming and finite nature of fossil fuels there is need to develop renewable energy sources as well as efficient energy storage and conversion system. Development of energy storage technology can use efficiency and supply systems by storing the energy when it is in excess and then it at a time of high demand. Research in material development as well as engineering improvements need to be continued to create efficient energy storage systems. Out of two efficient way to store energy, battery and capacitor, later has advantage of charging time, energy density, power density, and cycle life over battery. In present work, in situ electrochemical polarization of an online solution containing multi-walled carbon nanotube was used to prepare highly porous CNT composite films. The electrochemical capacitance performances of these films were investigated with cyclic voltammetry and ac impedence spectroscopy in with a three-electrode system. It was found that CNT films show much higher specific capacitance, better cyclic stability and more promising for applications in supercapacitors than a pure PANI film electrode. Compared with pure PANI film, the CNT composite film was highly porous film.

Sunlight-driven water splitting using particulate photocatalysts has been attracting growing interest, because such systems can be spread over large areas by potentially inexpensive processes [1]. In fact, a solar hydrogen production system based on 100-m² arrayed photocatalytic water splitting panels and an oxyhydrogen gas-separation module was built, and its performance and system characteristics including safety issues were reported recently [2]. Nevertheless, it is essential to radically improve the solar-to-hydrogen energy conversion efficiency (STH) of particulate photocatalysts and develop suitable reaction systems. In my talk, recent progress in photocatalytic materials and reaction systems will be presented. The author’s group has studied various semiconductor materials as photocatalysts for water splitting. Recently, the apparent quantum yield (AQY) of SrTiO₃ has been improved to more than 90% at 365 nm, equivalent to an internal quantum efficiency of almost unity, by refining the preparation of the photocatalyst and cocatalysts [3]. This observation means that particulate photocatalysts can drive the endergonic overall water splitting reaction with almost no recombination loss. For practical solar hydrogen production, however, it is essential to develop photocatalysts that are active under visible light. Ta₃N₅ [4], Y₂Ti₂O₅S₂ [5], TaON [6], and BaTaO₂N [7] were reported to be active in photocatalytic overall water splitting via one-step excitation under visible light. In these achievements, the synthesis of well-crystallized semiconductor particles and the loading of composite cocatalysts were important for promoting the water splitting reaction while suppressing backward reactions. [1] Hisatomi et al., Nat. Catal. 2, 387 (2019). [2] Nishiyama et al., Nature 598, 304 (2021). [3] Takata et al., Nature 581, 411 (2020). [4] Wang et al., Nat. Catal. 1, 756 (2018). [5] Wang et al., Nat. Mater. 18, 827 (2019). [6] Xiao et al., Angew. Chem. Int. Ed. 134, e202116573 (2022). [7] Li et al., ACS Catal. 12, 10179 (2022).

Photocatalysis with the use of inexhaustible sunlight and nanomaterials has been regarded as a promising technology to generate clean and sustainable energy fuels. Polymeric g-C₃N₄ has been widely studied for its amazing visible-light responsive behavior and the coupling of hybrid materials is essentially significance to promote the redox capability of g-C₃N₄. Among the myriad of nanomaterials, noble-metal-free catalysts such as Ni₂P, Ti₃C₂ and Zn_xCd_1-xS have been widely explored in photocatalysis owing to their good conductivity, cost-effective and robust nature. Heterojunction construction appears as the auspicious way to amplify the superior use of these noble-metal-free nanomaterials. In our previous work, the optimal loading of 3.5 wt% 0D Ni₂P cocatalyst on g-C₃N₄ presented a remarkable H₂ production rate of 474.7 μmol g⁻¹ h⁻¹ and an apparent quantum yield (AQY) of 3.2% at 435 nm. Moreover, 2D Ti₃C₂Tx was integrated with crystalline g-C₃N₄ to form a 3D aerogel structure. The optimized Ti₃C₂Tx/g-C₃N₄ hybrid exhibited outstanding H₂ evolution integrated with selective benzyl alcohol oxidation reaction under visible light irradiation, recording a high activity of 1377.46 μmol g^-1 h^-1 and 33.46 μmol toward hydrogen and benzaldehyde formation, respectively, outperforming other CN samples. Most recently, our defective Zn_xCd_1-xS was also coupled with g-C₃N₄ toward the photoreforming of plastic to remarkable H₂ evolution reaction of 944.8 μmol g^-1 h^-1 with an apparent quantum yield of 1.39% at 420 nm, recording 54.9 and 1.53-fold better than pristine ZnCdS and g-C₃N₄, respectively, ascribed to the extended light absorption edge and bolstered charge transfer. Hence, these works present an important insight into uncovering the immense potential of noble-metal-free catalysts as effective charge trapping sites in the hybrid system for multifunctional applications not only limited to water splitting and alcohol oxidation, but also plastic photoreforming.

Photoreforming is a process that harnesses the redox ability of photocatalysts upon illumination, to simultaneously drive the reduction of H⁺ into hydrogen gas and oxidation of organic compounds. Significant effort has been devoted to improving the photocatalytic hydrogen evolution efficiency over the past few decades, while substantially less focus has been directed towards the oxidation reactions. More recently, the realization of the potential for simultaneous hydrogen production with value-added organics has inspired researchers to use photooxidation pathways to tune the selectivity of oxidized products. As a distinct benefit, the less energetically demanding organic reforming is highly favorable when compared to the slow kinetics of oxygen evolution which negates the need for expensive and/or harmful hole scavengers. To achieve an efficient and economically viable photoreforming process, the selection and design of an appropriate inorganic photocatalyst is essential. Herein, different strategies were used to improve the selectivity of photoreforming of organic waste into high-value and desirable chemicals, including defect engineering, surface functionalisation, co-catalyst loading and photocatalyst doping of oxide, nitride and sulfide-based photocatalysts.

Oxygen evolution reaction (OER) as a half oxidation reaction of water dissociation hinders the overall redox reaction efficiency due to its thermodynamic and kinetic barriers. Iodide oxidation reaction (IOR) characterized by low thermodynamic potential and fast reaction kinetics is a prominent alternative to the OER. In this study, we propose a molybdenum disulfide (MoS₂) electrocatalyst for an efficient and ultrastable anode driving IOR. MoS₂ nanosheets uniformly coated on porous carbon fiber paper by employing atomic layer deposition reveal an iodide oxidation current density of 10 mA cm^–2 at a potential of 0.63 V compared to the reversible hydrogen electrode, which is 1.53 V smaller than that demanded for OER to deliver an identical current density. The smaller anodic potential applied to the MoS₂-based electrode during IOR catalysis prevents degradation of the active sites on the surface of MoS₂, allowing for superior stability of 100 h compared to inferior durability under high oxidative potential in OER. Subsequently, we design a two-electrode device comprising MoS₂ anode for IOR and commercial Pt/C catalyst cathode for hydrogen evolution reaction. Furthermore, the photovoltaic–electrochemical hydrogen production system composed of this electrolyzer and a single perovskite photovoltaic cell shows a notable current density of 21 mA cm^–2 at 1 sun under unbiased condition.

The semiconductor-based conversion of solar energy to H₂ by water splitting, which can directly store the energy in molecular bonds, is currently an attractive method for green and sustainable energy production. Antimony selenide has recently gained popularity for its favourable properties such as optimal band gap, high absorption coefficient and cost-effectiveness as water splitting material. However, one of the main limitations of this material is its photovoltage deficit. In an effort to further improve the promising Sb₂Se₃ thin films for PEC water-splitting in a low-cost manner, simple and low-temperature treatments were explored. The FTO/Ti/Au/Sb₂Se₃ semiconductor is treated with ammonium sulphide as an etching solution followed by Copper(II) chloride as surface passivation treatment which collectively increased the onset potential from 0.14 V to 0.28 V vs reversible hydrogen electrode (RHE) and the photocurrent from 13 mA cm⁻² to 23 mA cm⁻² at 0 vs RHE as compared to the untreated Sb₂Se₃ films. From SEM and XPS studies, it is clear that the etching treatment induces a surface modification and removes the Sb₂O₃ layer, which is formed during the synthesis process. CuCl₂ further enhances the performance due to surface passivation, improving charge separation at the interface; this is supported by DFT-MD calculations carried out on the 001 surface of Sb₂Se₃ before TiO₂ deposition.

In transition-metal-oxide heterostructures, the anomalous Hall effect (AHE) is a powerful tool for detecting the magnetic state and revealing intriguing interfacial magnetic orderings. However, achieving a larger AHE at room temperature in oxide heterostructures is still challenging due to the dilemma of mutually strong spin-orbit coupling and magnetic exchange interactions. Here, we exploit the Ru doping-enhanced AHE in La_2/3Sr_1/3Mn_1-xRu_xO₃ epitaxial films. As the B-site Ru doping level increases up to 20%, the anomalous Hall resistivity at room temperature can be enhanced from nΩ∙cm to μΩ∙cm scale. Ru doping leads to strong competition between ferromagnetic double-exchange interaction and antiferromagnetic super-exchange interaction. The resultant spin frustration and spin-glass state facilitate a strong skew-scattering process, thus significantly enhancing the extrinsic AHE. We also explored the possible skyrmionic magnetism and magnetotransport in the anisotropic strained LSMRO films, in which the highly tunable magnetic anisotropy and pounced anomalous Hall effect play critical roles. Our findings could pave a feasible approach for boosting the controllability and reliability of oxide-based spintronic devices.

Antiferromagnetic spintronics is one of the leading candidates for next-generation electronics. Among abundant antiferromagnets, noncollinear antiferromagnets are promising for achieving practical applications due to coexisting ferromagnetic and antiferromagnetic merits. In this talk, I would like to present the emerging noncollinear antiferromagnetic spintronics from fundamental physics to device applications. Current challenges and future research directions for this field would also be envisioned.

Traditionally, it is widely accepted that the lattice flexibility of an epitaxial layer is restricted by its growth template and bulk counterpart. In this report, we demonstrate that the interface-engineered (La,Ca)MnO₃ layer exhibits an anomalous lattice tilting, featured by the inter-axis angle α that exceeds the range of lattice flexibility mentioned above. By increasing the adjacent CaRuO₃ layer thickness, the (La,Ca)MnO₃ layer shows the decreasing α down to 89.25^o, which is out of the flexible range between 93.82^o (from the CaRuO₃ template) and 89.86^o [from the (La,Ca)MnO₃ bulk]. The resulted antiparallel lattice-tilting makes the (La,Ca)MnO₃/CaRuO₃ interface similar to a crystal twinning plane, to lower the interfacial energy raised by the structural discontinuity. Also, a monotonic reduction of magnetic coercivity (from 205 to 70 Oe) is observed on decreasing α (from 90^o to 89.25^o) in (La,Ca)MnO₃ layers, providing an additional approach to tunable magnetic properties without changing the epitaxial strain. Our results not only present a new lattice-engineering strategy of using the interface similar to a crystal-twinning plane in designing heterostructures, but also reveal the application of such strategy for tunable magnetic properties beyond the epitaxial strain.

Correlated topological phases (CTPs) with interplay between topology and electronic correlations have attracted tremendous interest in condensed matter physics. Therein, correlated Weyl semimetals (WSMs) are rare in nature and, thus, have so far been less investigated experimentally. In particular, the experimental realization of the interacting WSM state with logarithmic Fermi velocity renormalization has not been achieved yet. Here, experimental evidence of a correlated magnetic WSM state with logarithmic renormalization in strained pyrochlore iridate Pr₂Ir₂O₇ (PIO) which is a paramagnetic Luttinger semimetal in bulk, is reported. Benefitting from epitaxial strain, “bulk-absent” all-in–all-out antiferromagnetic ordering can be stabilized in PIO film, which breaks time reversal symmetry and leads to a magnetic WSM state. With further analysis of the experimental data and renormalization group calculations, an interacting Weyl liquid state with logarithmically renormalized Fermi velocity, similar to that in graphene, is found, dressed by long-range Coulomb interactions. This work highlights the interplay of strain, magnetism, and topology with electronic correlations, and paves the way for strain-engineering of CTPs in pyrochlore iridates.

In the last few decades, the quaternary and ternary transition-metal chalcogenides, having low crystal symmetry, have become a rich playground for independently organized investigations of magnetic exchange and electronic transport interactions in magnetic semiconductors. Among the known quaternary transition-metal chalcogenides, the transition-metal chalcogenide family AB₂X₄; (A = Fe, Mn; B = Sb, Bi, and X = S, Se) exhibits diverse crystal structures, where the A-site cation connectivity ranges from 1 to 3 D, depending on B-site cations, and anions. Among these compounds, MnSb₂Se₄ crystallizes in a monoclinic space group C2/m and orders antiferromagnetically below T_N = 22.5 K. In addition, careful analysis of the x-ray diffraction revealed the presence of antisite disorder (∼ 19 %) between Mn and Sb sites. As we know that antisite-disorder can be a significant parameter in determining the ground state of a magnetic material. In this talk, I’ll be talking about the impact of antisite disorder on the magnetic ground state of MnSb₂Se₄. We have prepared MnSb₂Se₄ samples with different antisite disorders by controlling the cooling rate of the furnace. The sample S1 with the lower disorder (∼ 28 %) shows quasi-one-dimensional magnetism and exhibits type-II multiferroicity below 22.5 K [1]. The sample S2 with the higher disorder (∼ 40 %) shows a cluster-glass state as supported by various DC and AC magnetization measurements. A striking observation is the presence of colossal magnetoresistance in sample S2 as it was absent in S1. Interestingly, the origin of colossal magnetoresistance is not a double exchange mechanism as heterovalency has been ruled out by electric spin resonance (ESR) measurements. The presence of colossal magnetoresistance can be attributed to competition between various magnetic states which adds MnSb₂Se₄ to the list of a handful of materials that do not belong to the manganite family, but they still show negative colossal magnetoresistance.

The concept of critical coupling lies at the heart of efficient light harvesting with an optical resonator. In my talk I will explore two principal avenues of utilizing critical coupling in nanophotonics for efficient light collection. Firstly, highly disordered colloidal assemblies allow broadband light harvesting which can be tuned via inducing lifetime changes for specific modes. This concept is materials-independent and allows in its simplest form for an elegant way of colour generation. Secondly, quasi- bound states in the continuum allow very fine control over radiative lifetime. I will present various example applications, from photocatalysis to control over ligh/matter interactions in two-dimensional materials. A variety of new geometries such as radial and heigh-induced quasi- bound states in the continuum will be presented.

Nanoscale 3D printing using two-photon polymerization lithography (TPL) enables the fabrication of complex arbitrary structures, ranging from sub-wavelength resonant structures for structural colors, to achromatic metalenses. In this talk, we will discuss recent progress in our efforts to achieve higher degrees of functionalities, e.g. colorful vortex beam generation, using combinations of structural colors and microlenses.

The ultimate goal of the inverse design approach in nanophotonics is to explore novel optical phenomena and achieve the utmost device performance with minimal computational cost. However, efficiently exploring a large number of geometrical parameters to search for these extrema remains a challenging task for both data-driven and conventional optimization algorithms. Here we report an efficient design approach that uses machine learning as a black box optimizer for free-form nanophotonic structures with desired colours. In contrast to previous reports that use machine learning to achieve mapping between the design and the output space, we develop a “learn while exploring” search algorithm that learns to bifurcate the design space progressively into good and bad regions. Such simple rules allow for efficient learning of the design space landscape. We experimentally verify the optimal design for red colour made of amorphous silicon on a quartz substrate with a record-high saturation and reflectivity (i.e., CIE coordinates: 0.655, 0.33 and reflectivity of >80%). Moreover, the colour performance is independent of polarisation over a large viewing angle (> 60°). This is an outstanding result given the silicon nanostructure with a thickness of only 120nm, and no anti-reflection coating is applied. The algorithm converges under 300 function calls in a 13-dimensional design space, outperforming both conventional algorithms and machine learning in terms of data efficiency and convergence speed. Moreover, it allows us to achieve generalized broadband multipolar interaction, which has not been reported previously. This work greatly expands the toolbox for designing nanophotonic structures with desired spectral responses that can find broad applications in flat optics, optical sensing, and spectroscopy.

Nanoimprint lithography (NIL) is a well-established approach to texture a surface with micro and nano-structures. It is alternative to other etching methods such as photolithography, electron-lithography and etching. It can be applied to a large variety of materials and is extremely appealing for its high performances associated with its ease of implementation and low costs. The first work on NIL was developed in the 1990s, when 25 nm patterns were reported by etching in a silicon mold and transferring it in a polymeric resist. Since then, the most common materials for NIL have been organic polymers that can be framed in regular patterns up to ~m² with roll-to-plate and roll-to-roll methods. A similar approach [1] can be used to frame resonant photonic structures made of fully-inorganic metal-oxides (MOx, such as SiO₂, TiO₂, ZrO₂) forming ordered [2,3,4] and disordered [5] (hyperuniform) metasurfaces. In this presentation we will show that MOx-NIL can provide: 1) large-scale nanostructures printed up to 200 mm wafers; 2) a tunable refractive index ranging from ~1.2 (for porous silica) to 2.7 (for dense titania) and transparent from NUV to NIR frequency; 3) structures with a footprint ranging in between 50 nm to a few um; 4) large aspect-ratio up to ~6 (height/FWHM); 5) integration of light emitters [6]. The application of these structures are countless and range from strucutral color, sensing, meta-lenses, light trapping, light emission etc.
[1] Modaresialm et al. Chemistry of Materials 33, 5464 (2021)
[2] Modaresialm et al., ACS Applied Materials & Interfaces 13, 53021 (2021)[3] Checcucci et al., Advanced Optical Materials 7, 1801406 (2019)[4] Garcia et al., ACS applied materials & interfaces 13, 47860 (2021)[5] Chehadi et al ACS Applied Materials & Interfaces 13, 37761 (2022)
[6] Chehadi et al., Advanced Optical Materials 10, 2201618 (2022).

Non-benzenoid polycyclic aromatic hydrocarbons (PAHs) and nanographenes have received a lot of attentions because of their unique optical, electronic, and magnetic properties, but their synthesis remains challenging. The non-hexagon rings (pentagons or heptagons) can be regarded as defects in these PAHs and may introduce special properties, however the structure-property relationships are unclear. In this presentation, I will report non-benzenoid PAHs with 5/7 and 5/7/5 membered rings. Apart from discussions of their fundamental structural features and relevant absorption and redox properties, I will present our exploration on their semiconducting properties. The results reveal that they show p-type semiconducting property with hole mobility up to 1.27 cm² V^-1 s^-1, which is among the highest for the non-benzenoid PAHs up to date. Additionally, I will introduce the analogues of non-benzenoid PAHs with N/S atoms.

The chemistry of non-alternant isomers of pyrene has been established by many pioneers. Their unique electron configurations and molecular orbital characteristics are derived from topological differences of the π-electron network, which is produced by replacing hexagons of alternant hydrocarbons with a pentagon and heptagon pair. Among the possible seven isomers of pyrene, bis-periazulene (cyclohepta[def]fluorene) remains elusive despite many synthetic efforts. Previous theoretical studies have offered an intriguing hint that bis-periazulene has a triplet open-shell ground state. Presumably, the high reactivity of bis-periazulene due to an m-quinoidal subunit has made isolation and characterization inaccessible. In this work, details of the synthesis and physical properties of the triaryl derivatives of bis-periazulene are described. The triaryl derivatives, in which three ortho-disubstituted aromatic groups were used to kinetically stabilize the reactive sites, were realized via an eight-step synthesis from the fluorene derivatives. The synthesized triaryl derivatives were successfully isolated in their crystalline forms and characterized by X-ray analysis. Contrary to previous theoretical predictions, the determined molecular geometry and the observed magnetic behavior exhibited singlet ground states. Notably, our study demonstrated that bis-periazulene contained three aspects of π-conjugation: peripheral, charge-separated, and open-shell π-conjugations. The double peri-benzoannulation into an azulene core provides fascinating electronic features.

On-surface synthesis has attracted attentions as effective method for the evaluation of electronic structure of acenes and graphene nanoribbons. We alreadry reported the on-surface synthesis of heptacene and nonacene [1]. Heteroatom substitution in acenes allows tailoring of their remarkable electronic properties, expected to include spin-polarization and magnetism for larger members of the acene family. Here, we present a strategy for the on-surface synthesis of undecacene analogs substituted with four nitrogen atoms on an Au(111) substrate, by employing specifically designed diethano-bridged precursors.[2] By comparing experimental features of scanning probe microscopy with ab initio simulations, we demonstrate that the ground state of the synthesized tetraazaundecacene has considerable open-shell character on Au(111). [1] J. I. Urgel, S. Mishra, H. Hayashi, J. Wilhelm, C. A. Pignedoli, M. D. Giovannantonio, M. Yamashita, N. Hieda, P. Ruffieux, H. Yamada*, R. Fasel*, Nat. Commun. 2019, 10, 861.[2] K. Eimre*, J. I. Urgel*, H. Hayashi, M. D. Giovannantonio, P. Ruffieux, S. Sato, S. Ohtomo, Y. S. Chan, N. Aratani, D. Passerone, O. Gröning, H. Yamada*, R. Fasel*, C. A. Pignedoli*, Nat. Commun. 2022, 13, 511.

Although aromaticity in 2D π-conjugated polycyclic molecules has been intensively studied, aromaticity in 3D fully π-conjugated cages remains largely unexplored mainly due to the synthetic challenges. Herein, we report the synthesis of two fully π-conjugated cages (1 and 2) showing global aromaticity or antiaromaticity. Cage 1 has an open-cage structure, consisting of two isomeric trimers and an additional macrocycle across four dimethylmethylene-bridged triphenylamine (DTPA) units. The detailed study reveals that: 1) its dication (1²⁺×2SbF₆^-) displays bicyclic (anti)aromaticity with one macrocycle being aromatic (38π) and another macrocycle being antiaromatic (28π); 2) its tetracation (1⁴⁺×4SbF₆^-) exhibits dominant 2D Hückel antiaromaticity in one of the macrocycles (36π). Cage 2 contains two DTPA units and three quinoidal bithiophene arms. The detailed study reveals that: (1) its dication (2²⁺×2SbF₆^-) has singlet ground state and is 3D globally aromatic, with individual macrocycles being Hückel aromatic (38π); (2) its tetracation (2⁴⁺×4SbF₆^-) has triplet ground state and is also 3D globally aromatic, with individual macrocycles being Baird aromatic (36π). Aromaticity in the asymmetric fully conjugated cages is complex and the (anti)aromaticity of individual macrocycles could interfere with each other. In this case, the π-electrons could prefer to mainly delocalize on one or some of its macrocycles, which may satisfy different aromatic rules. While in symmetric fully conjugated cages, π-electrons are forced to delocalize uniformly on all macrocycles, leading to 3D global aromaticity or antiaromaticity. Therefore, highly symmetric fully conjugated cages are still required to attain true 3D aromaticity. However, this work still brings new insights into the close correlation between 3D global aromaticity and 2D Hückel/Baird aromaticity. References: Wu, S. et. al, Angew. Chem. Int. Ed. 2022, 61 (9), e202115571. Wu, S. et. al, J. Am. Chem. Soc. 2022, 144 (50), 23158-23167.

Resistive-switching-based phase change memory (PCM) devices are of interest for computing with reduced power, non-volatililty and fast response times. Here, we study switching processes in individual Ag-In-Sb-Te (AIST) PCM devices in the TEM by using short (< 50 ns) current pulses. The switching layer is thin (40 nm) compared to the total thickness of the device (300 nm, including 150 nm of SiN_x and protective capping layers). In a typical experiment, an AIST cell is cycled between a low resistance state (LRS) and a high resistance state (HRS). A 50 ns square pulse is initially used to RESET the cell to the HRS. 100 ns triangular pulses are then used to SET the cell to the LRS. In the pristine LRS, the cell is crystalline. Upon switching to the HRS, an amorphous region forms. A HRS with reduced resistance is formed after one “sweep” pulse, with a decreased amorphous region and some crystallites. After a second SET pulse, an LRS state is formed with a resistance that is higher than for the pristine state and a crystalline conducting channel on one side of the bridge. After another RESET pulse, an HRS is formed with an increased amorphous region. Finally, an almost fully crystallized state is formed in the bridge, with one small amorphous region and a resistance that is slightly higher than in the pristine state. Results obtained using STEM diffraction to better identify the amorphous and crystalline regions in the device will be presented.

Non-precious metals such as Fe, Co, and Mn are potential candidate materials for electrodes owing to their low cost and high natural abundance. Fe-based materials have been explored as next generation electrode materials for energy storage and conversion due to the lack of dendritic growth in alkaline electrolyte systems. However, a critical problem of Fe is its low stability in harsh conditions such as strong acidic or alkaline media during liquid-phase (electro)chemical reactions. Tremendous effort has been made on improving Fe electrode through additives and electrode design, yet the failure mechanisms of Fe electrodes, such as dissolution, passivation, and self-discharge, are poorly understood. Further improving the performance of the Fe electrode requires a good understanding of Fe electrode stability under typical electrochemical conditions to understand the interplay of kinetic and thermodynamic phenomena such as initial electrode microstructure, electrolyte composition and cycling regime.
Here, we explore the possibility of using operando liquid cell electron microscopy with high spatial and temporal resolution as a direct probe to examine the stability of the Fe electrode in aqueous alkaline electrolyte under potential cycling conditions. Liquid cell TEM has predominantly been used in neutral or acidic conditions; thus, we first assess the stability of the Fe electrode in alkaline electrolyte under electron beam irradiation with no bias. We further investigate the behavior of the Fe electrode in different alkaline electrolyte composition under cyclic voltammetry conditions. Our real-time imaging shows that the electrochemical processes on Fe electrode during potential cycling are intrinsically complex and fascinating and can be related to stability predictions from the Pourbaix diagram. Exploring the possibility of non-precious metals as electrode materials potentially opens new opportunities for solving a range of problems relating to battery and electrocatalyst design.

Copper-based catalysts generally exhibit high selectivity toward valuable C₂₊ products during the electrochemical CO₂ reduction reaction (CO₂RR). However, the origin of this selectivity and the influence of precursor materials on it are not fully understood due to the elusive catalyst evolution during CO₂RR. We combine operando X-ray diffraction (XRD) and operando Raman spectroscopy to monitor the structural and compositional evolution of three oxidized Cu precursors (Cu₂(OH)₂CO₃, Cu(OH)₂, CuO) during CO₂RR. The results indicate that the three precursors are completely reduced to Cu(0) with similar grain sizes (11 nm) despite their different electroreduction kinetics when delivering their maximum C₂₊ Faradaic efficiencies. The high grain boundary density associated with the small grain size explains their significantly enhanced C₂₊ selectivity (70%) compared to bulk Cu (13%). Operando Raman spectroscopy reveal the lack of co-occurrence of CO₂RR intermediates and Cu₂(OH)₂CO₃, Cu(OH)₂, CuO, or Cu₂O, ruling out the involvement of these oxidized Cu species in CO₂RR. Most interestingly, operando XRD indicates that the Cu nanocrystals derived from Cu(OH)₂ and Cu₂(OH)₂CO₃ exhibite 0.43%~0.55% tensile strains during the in situ precursor electroreduction, which could not be detected by conventional ex situ XRD. In contrast, Cu nanocrystals derived from CuO are hardly strained. DFT calculations suggest that the tensile strain in Cu lattice is conducive to promoting the overall CO₂RR selectivity, which is consistent with experimental observations. Therefore, the excellent CO₂RR performance of some derived Cu catalysts is attributed to the combined effect of the small grain size and lattice strain, both originating from the in situ precursor electroreduction. The findings, for the first time, establish correlations between Cu precursors, lattice strains, and catalytic behaviors, demonstrating the unique ability of operando characterization techniques in probing catalyst evolution during electrochemical processes.

During the current global energy crisis, research into novel and improved energy storage devices is more important than ever. Solid-state batteries (SSB) have long been predicted as the next step in the green energy revolution, with the promise of making batteries that are safer and more energy packed per volume compared to existing liquid Li-ion batteries. While this is the theory, SSBs are yet to take the world by storm due to degradation issues believed to arise at the electrode-electrolyte interface. Describing and understanding this degradation process is, therefore, key to extending the battery cyclability and increasing its coulombic efficiency to create durable and efficient battery devices for a better tomorrow. While electron microscopy techniques are excellent for high-resolution imaging of micro and nanostructures, many lithium based battery materials are difficult to handle and observe due to air and beam sensitivity. For this purpose Li_1.5Al_0.5Ge_1.5(PO₄)₃ (LAGP) is a good option. This oxide solid electrolyte has a good air and electron beam stability [1]. Here we present in situ electron microscopy imaging of a full stack SSB during cycling. Combined with advanced focused-ion beam lamella preparation techniques, we have prepared a full stack solid-state micro battery, which then has been mounted on a dual heating and biasing chip and electrically connected for in situ cycling. This allows us to directly study changes in the battery microstructure, specifically at the solid-electrolyte-electrode interfaces, and correlate observations with battery degradation after multiple cycles. Chemical changes can also be seen from energy dispersive X-ray spectroscopy maps of the cycled and pristine samples. [1] S. Cretu et al. The Impact of Inter-grain Phases on the Ionic Conductivity of LAGP Solid Electrolyte Prepared by Spark Plasma Sintering (eprint arXiv:2211.06129, November 2022).

Real-time monitoring of electrochemical reactions necessitates probing of complex interfacial phenomena. Collectively, the dynamic nature of the material surface and the modifications of the chemical environment around it take place when in contact with the fluidic electrolyte or/and upon application of potential leading to gaseous products. Thus, extensive efforts are made to characterizing the evolution of solid-liquid-gas interfacial processes under realistic operating conditions. Within the possible techniques, transmission electron microscopy (TEM) has the advantage that it can be tuned to provide real-time information of morphological, structural, and chemical information down to the sub-nanometer resolution depending on the choice of the recorded signals. Herein, I will discuss the insights that can be gained from electrochemical liquid phase TEM (ec-LPTEM) for various systems including the evolution of metallic nanocatalysts during the first stages of CO₂ reduction reaction, the corrosion of the current collector on the cathode side of lithium-ion batteries, and the wettability of oxide-based oxygen-evolving catalysts.

Organic conjugated molecules, known as dyes, which can absorb and emit light, are potential candidates for quantum computing due to their unique optical properties. When dyes are aggregated, they exhibit exciton delocalization and coherence features importantly at ambient temperature. These novel features can overcome exiting quantum computing challenges, such as extremely low operating temperature, noise, and humidity conditions. The development of such applications requires a candidate with high extinction coefficient, high transition dipole moment, good aggregation ability, and high exciton exchange energy. Dye aggregate networks via deoxyribonucleic acid (DNA) templating show a potential to meet those criteria. DNA nanotechnology provides scaffolding upon which dyes attach in an aqueous environment. To better control the process and optimize the properties, we applied machine learning-driven multiscale modeling techniques to identify candidate dyes and reveal their dye aggregate-DNA interactions and the dye orientations. Those structural features were found to have a strong impact on the resultant performance of the DNA-templated dye aggregates. The computational results were validated with experiments.

The research paradigms of materials science have recently evolved into data science as the fourth paradigm in addition to experiment, theory, and computation. Combining various research paradigms can form a synergetic multi-paradigm approach to address the challenging issues of materials design. In this talk we introduce the data-driven multi-paradigm materials design for developing the advanced alloys by combining high-throughput computation (HTC), high-throughput experiment (HTE), and data analysis (machine learning, ML), aiming to accelerate materials design and development at lower cost. Two case studies are introduced as follows: (1) Computation & Machine Learning: In this work, we first carried out high-throughput FP calculations systematically on the stability and mechanical properties of several thousand alloying configurations in Nb-based superalloy, considering more than ten alloying element substitutions at multiple nonequivalent sites. Then the ML models were developed based on the FP computational data (HTC-ML). Specifically, we designed a “Center Environment” (CE) feature model by combining elemental properties and local composition and structure information, predicting both the substitution energy and local geometry of alloying elements in superalloy. (2) Experiment & Machine Learning: Conventional trail-and-error experiment development approaches rely heavily on the intuition and experience of researchers and are limited by the low efficiency of single sample experiment mode. We developed a ML aided HTE approach (HTE-ML) to optimize both the composition and processing parameters of multicomponent alloys. The HTE approach conduct experiments at a batch sample mode featuring mutli-station, automation, and parallelization, more efficient than the conventional single sample experiment mode. To further accelerate the development process, we designed and conducted only the fractional HTEs guided by the ML prediction in multi-step iterative experiments.

DFT study of complex nanostructures are usually very difficult partly because the DFT calculations of large systems are very expensive. We have been developing a large-scale DFT code CONQUEST to overcome this size problem of DFT calculations. The code was recently released under an open-source MIT license [1]. It uses local orbital and linear-scaling methods, and has high efficiency on massively parallel computers [2]. Using the code, we can treat very large and complex systems, containing tens of thousands, hundreds of thousands or even millions of atoms. We will demonstrate CONQUEST can calculate the atomic positions of complex nano-scale materials observed in experiments and can clarify the unique electronic properties originated from the complex structures. CONQUEST is powerful also for the structure modeling of disordered materials. However, it is often difficult to analyze the calculated results since the structures do not have simple structural orders in many cases. We recently proposed a new method [3] based on unsupervised machine learning techniques to analyze the local atomic structures observed in large-scale DFT-MD simulations. It will be shown that this method has many advantages to find the characteristic structural properties in complex and disordered systems. This work has been done in collaboration with D. R. Bowler (UCL, UK), A. Nakata, M. Tamura, J. Lin, A. Lu (NIMS, Japan), L. Truflandier (U. Bordeaux, France), M. Matsuda, Y. Futamura and T. Sakurai (U. Tsukuba, Japan). (1) https://ordern.github.io(2) J. Chem. Phys. 152: 164112 (2020).(3) Phys. Rev. B 105, 075107 (2022).

Machine learning (ML) can accelerate materials research & discovery in several ways, often in complimentary approaches. We will discuss a few directions where we can leverage ML in the context of atomistic modeling for designing new materials and their integrated use in devices and chemical reactors. The central framework in the Amsterdam Modeling Suite (AMS) enables the exploration of potential energy surfaces (PESs), mechanical, and electronic properties at several levels of theory. The central AMS driver supports advanced PES explorations, molecular dynamics (MD) and Grand Canonical Monte Carlo (GCMC). On-the-fly machine learned potentials such as FLARE[1] and universal graph neural network potentials such as M3GNet[2] can immediately be used for simulations such as chemical vapor deposition with the molecule gun in AMS. The ParAMS module furthermore provides a comprehensive framework to build training data and optimize machine learned potentials, as well as ReaxFF and DFTB parameters. With different levels of electronic structure methods available in AMS, we are exploring ML methods to predict properties more efficiently for molecular materials. Examples for OLED applications include training DFTB transfer integrals on DFT data, TDDFT(B) luminescence and excitonic properties on accurate qsGW+BSE calculations, and predicting novel molecules that have the desired optical and electronic properties yielding the best OLED device performance in multiscale simulations. [3]For catalysis, we can accelerate the multiscale workflow[4] through employing machine learning for reaction exploration, and by building surrogate models for a faster integration between kinetic Monte Carlo and Computational Fluid Dynamics. We will briefly discuss future directions relevant to battery materials and polymers where we can harness the powerful combination of atomistic modeling with ML. [1] J. Vandermause et al. npj Computational Materials 6, 20 (2020)
[2] C. Chen, S. Ong, Arxiv https://arxiv.org/abs/2202.02450 (2022)
[3] https://www.scm.com/oled-workflow
[4] https://www.scm.com/reaxpro

Reliable and dynamic control of magnetic properties in technologically relevant magnetic materials is at the heart of a variety of emerging practical applications in spintronics. Gate voltage-controlled ionic diffusion in magnetic devices has shown to provide non-volatile control of perpendicular magnetic anisotropy (PMA), the Dzyaloshinskii Moriya interaction (DMI), as well as the velocity and pinning of magnetic domain walls, opening a solid path towards novel multifunctional spintronics devices. In amorphous CoFeB/HfO2 we observe that electric fields induce the migration of mobile oxygen-rich ionic species present in HfO2 across the CoFeB/HfO2 interface which can define different magneto-ionic regimes in the CoFeB films: under-oxidised (in-plane magnetisation), optimally-oxidised (PMA) and over-oxidised (in-plane magnetisation). The gate voltage can therefore induce a spin-reorientation transition between magneto-ionic states with in-plane anisotropy and PMA, accompanied by changes in DMI and domain wall velocity. In this system reversibility is only observed between the optimally oxidised and over-oxidised states. In CoFeB/Pt/MgO/HfO2, the MgO layer is polycrystalline and a reversible voltage induced motion of oxygen species is observed, with an accompanying non-volatile spin-reorientation transition from in-plane to perpendicular anisotropy. Moreover, no traces of oxidation or loss of magnetic moment are found and the crystallinity of the MgO layer is conserved after several +/- gate voltage cycles. This behaviour is understood in terms of the contribution from Pt at the interface and of the crystallinity of the MgO layer, which can provide ionic motion channels through grain boundaries.
Our studies show the complexity of the magneto-ionic mechanisms and the strong influence of surface composition and structure on the observed effects on the magnetic properties. It shows also that oxygen magneto-ionics can function both in oxidising and non-oxidasing conditions. Interface engineering is therefore the key to the design of efficient spintronics devices with magneto-ionic functionalities.

The realization of chiral spin configurations of static magnetization with topological characteristics, such as skyrmions and domain walls, is of immense interest for applications in energy-efficient and scalable computing. Chiral spin textures are stabilized in multilayer thin films via the interplay of conventional, symmetric exchange interaction (A) with the interfacial Dzyaloshinskii-Moriya interaction (DMI) [1]. It is therefore imperative to accurately quantify both these interactions and elucidate their microscopic origin in chiral multilayers. However, such quantitative studies are extremely challenging in the ultrathin limit (∼ 1 nm) of magnetic films. Here, we quantify the symmetric exchange interaction across a series of ultrathin films using Brillouin light scattering (BLS) spectroscopy, T-dependent magnetometry, microscopy, and DFT calculations [2]. Contrary to expectations, we find that the BLS-measured values are ~ 2-5 times larger than magnetometry-measured values. DFT calculations suggest the possible source of discrepancy being the anharmonicity of the spin wave dispersion, and differences in the probed wavevectors between techniques. We find that the prevailing interpretation of symmetric exchange as a “constant” may not apply for ultrathin materials. Next, we examine the evolution and inter-dependencies of symmetric and asymmetric exchange (DMI) across a range of Co/Pt multilayers with varying chirality and fixed thickness. Contrary to literature, which suggests a linear relationship between the two, we find that they evolve conversely, i.e., one increases, while the other decreases. We elucidate the microscopic origin of this anomalous relationship using XMCD spectroscopy, which establishes that the orbital contribution to magnetism, and its evolution with chirality, plays a crucial role. These results establish a microscopic framework to measure and tailor exchange interactions in ultrathin magnetic films.
References [1] A. Soumyanarayanan et al. Nature Materials 16, 898 (2017). [2] T. Böttcher, et al. Phys. Rev. B (2023).

Electrical control of magnetic materials has been of paramount interest in spintronics research, and many interesting phenomena have been revealed, leading to various opportunities of applications. Non-collinear antiferromagnet with chiral-spin structure is an attractive system showing intriguing properties that were believed to be inherent to ferromagnets such as the anomalous Hall effect [1]. Here I discuss physics and functionalities of Mn₃Sn, a representative room-temperature noncollinear antiferromagnetic system. First, I will show an epitaxial thin-film growth technique that is necessary to explore the device functionalities [2] and show basic transport and magneto-optical properties [3,4]. Then, I will show a chiral-spin rotation induced by spin-orbit torque under electric current application [5] and discuss its opportunities for unconventional devices. This study is partly supported by JSPS Kakenhi 19H05622, MEXT X-NICS JPJ011438, and RIEC Cooperative Research Projects. [1] S. Nakatsuji et al., Nature 527, 212 (2015). [2] J.-Y. Yoon et al., Appl. Phys. Express 13, 013001 (2019). [3] J.-Y. Yoon et al. AIP Adv. 11, 065318 (2021). [4] Y. Takeuchi et al., Nature Materials 20, 1364 (2021). [5] T. Uchimura et al., 120, 172405 (2022).

In conventional magnetic materials, the Hall transport of electrons is typically dominated by the “anomalous” term, which is proportional to the magnetization. Chiral magnets hosting topological spin textures such as skyrmions are expected to have an additional “topological” contribution to the Hall effect due to their emergent Berry phase [1]. For anisotropic chiral magnets, including chiral multilayers, it is common to interpret the residual (post-anomalous) Hall signal as having topological origin [2], and as direct evidence for topological spin textures [3], which has generated considerable debate in the community [4]. Here, we report the observation of a large residual Hall signal across both chiral and achiral multilayers. By performing bespoke concurrent optical magnetometry and electrical characterization, complemented with Lorentz transmission electron microscopy measurements on a series of samples with systematically varying magnetic interactions, we investigate the relationship between the domain structure and their electrical response. While the residual Hall signal is found to be large in the regime of isolated disconnected domains, its topological origin is questioned by its ubiquity across achiral samples. Instead, our theoretical calculations suggest significant contributions from non-destructive accumulation of anomalous Hall contributions of isolated multi-domains. These results warrant a reinterpretation of the anomalous Hall effect in multi-domain magnets. [1] Nagaosa, N., Tokura, Y. Topological properties and dynamics of magnetic skyrmions. Nature Nanotech8, 899–911 (2013) [2] Soumyanarayanan, A., Raju, M., Gonzalez Oyarce, A. et al. Tunable room-temperature magnetic skyrmions in Ir/Fe/Co/Pt multilayers. Nature Mater 16, 898–904 (2017). [3] Raju, M., Yagil, A., Soumyanarayanan, A. et al. The evolution of skyrmions in Ir/Fe/Co/Pt multilayers and their topological Hall signature. Nat Commun 10, 696 (2019) [4] Kimbell, G., Kim, C., Wu, W. et al. Challenges in identifying chiral spin textures via the topological Hall effect. Commun Mater 3, 19 (2022). 

Artificial spin ice (ASI) structures [1] are geometrically frustrated magnetic metamaterials which have attracted much attention with emergent magnetic monopoles, phase transitions, collective switching dynamics, and potential for application in memory devices and neuromorphic computing [2]. The connectivity of ASI structures has a strong impact on their dynamics by altering the magnetization at their vertices [3], with disconnected ASI dominated by demagnetizing fields and dipolar interactions from neighboring elements, leading to a curling of the magnetization, while connected ASI must accommodate an additional exchange field, leading to the formation of domain walls. In this work, we use micromagnetic simulations to understand the different magnetic microstates and magnetization reversal dynamics of artificial square ice with varying connectivity, in particular exploring an intermediate system where connectivity is reduced by introducing small gaps at the vertices. We present the key differences in terms of vertex fractions and their variation with the connectivity of the spin ice lattice. Our results suggest that novel magnetic microstates and high energy vertices may be engineered in ASI systems by altering the lattice connectivity, with implications for the electrical transport properties of connected ASI [4] and for the application of ASI systems as functional elements for neuromorphic computing. References: [1] Skjærvø et al. Advances in artificial spin ice. Nat Rev Phys 2, 13–28 (2020). [2] Gartside et al. Reconfigurable training and reservoir computing in an artificial spin-vortex ice via spin-wave fingerprinting. Nat. Nanotechnol. 17, 460–469 (2022). [3] Chaurasiya et al. Comparison of Spin-Wave Modes in Connected and Disconnected Artificial Spin Ice Nanostructures Using Brillouin Light Scattering. ACS Nano 2021 15 (7), 11734-11742[4] Gia-Wei Chern. Magnetotransport in Artificial Kagome Spin Ice. Phys. Rev. Applied 8, 064006 (2017)

We present efforts at Sandia National Laboratory’s Ion Beam Laboratory (IBL) towards the development of deterministic defect center fabrication in wide bandgap substrates highlighting the fabrication of defect centers in diamond and silicon carbide. This talk will address the role of focused ion beam implantation combining localized implantation of single defect centers, in-situ counting using a variety of novel ion sources with <50 nm spatial resolution and in-situ photoluminescence (PL). The IBL operates seven focused ion beam (FIB) systems that range in ion energy from less than 1 keV to greater than 70 MeV, with ion species from protons (H) to lead (Pb) over a range of spot sizes from nm to mm. Here we will concentrate on the development of liquid metal alloy ion sources (LMAIS) for our mass filtered FIB systems combining high spatial resolution with CAD based patterning to enable the formation of arbitrary patterned implantation. These novel ion source allow for the fabrication of new defect centers in a wide range of substrates. This high spatial resolution implantation is combined with in-situ counting and in-situ PL to develop a pathway towards deterministic defect center formation. The in-situ counting is accomplished using Ion Beam Induced Charge Collection (IBIC) allowing for the explicit control the number of implanted ions beating the limitation of Poisson Statistics encountered using ion implantation. Additional, as the yield of the defect center formation is typically 1-5%, depending on the ion species, implantation energy and defect center, we have developed an in-situ PL setup to directly observe the optical activation of the defect centers. Sandia National Laboratories is a multimission laboratory managed and operated by National Technology & Engineering Solutions of Sandia, LLC, a wholly owned subsidiary of Honeywell International Inc., for the U.S. Department of Energy’s National Nuclear Security Administration under contract DE-NA0003525.

The desire to control and functionalise the properties of materials on the nanoscale is an ever-increasing demand being placed on researchers in order to deliver technologies with new and enhanced capabilities. To achieve this requirement the Platform for Nanoscale Advanced Materials Engineering (P-NAME) facility has been established based upon a new generation of focused ion beam systems for nanoscale implantation. The facility combines an ion column with high spatial resolution (<20 nm) for implantation, with a co-incident scanning electron microscope enabling accurate and non-destructive sample localisation. Critically, P-NAME is equipped to deliver deterministic single-ion doping through to ultra-high (>10¹⁹ ions/cm²) doses. This enables the facility to deliver bespoke doping for application in quantum (e.g. qubits), photonic (e.g. optically active centres) and electronic/spintronic technologies. The performance of the P-NAME facility has been validated and will be outlined within the presented work. This includes the delivery of a wide range of ion species (B, Si, Mn, Co, Cu, Ge, In, Sn, Sb, Nd, Er, Au and Bi) and their isotopic selectivity demonstrating the ability to clearly resolve key isotopes (e.g. ²⁸Si, ²⁹Si and ³⁰Si). Detailed analysis using time-of-flight and nano-secondary ion mass spectroscopy will be provided alongside high-resolution transmission electron microscope studies of samples pre- and post-processing, following ion implantation (e.g. rapid thermal annealing). Examples of systems doped using the facility will be provided including high-dose ⁷⁰Ge, ⁶³Cu, ⁶⁵Cu, ¹⁴²Nd, ¹⁴⁴Nd, and ¹⁴⁶Nd into Si, and optically active Er doping of non-linear systems. The ability to direct write using the P-NAME facility to deliver arrays of bespoke doping patterns will be shown. The facility is continuing to develop additional capability, supported by the EPSRC Programme Grant Nanoscale Advanced Materials Engineering, which will be presented alongside the above.

Color centers in semiconductors are promising qubit candidates for applications in quantum sensing and quantum communication. Color centers often form when dopants are introduced into the host crystal matrix combined with energetic radiation and thermal annealing. Quantum information science (QIS) applications benefit from color centers that can be formed reliably of high quality and this poses new challenges and opportunities for material processing including with ion beams and plasmas. This talk will be focused on our recent work to synthesize high-quality color centers using terawatt to petawatt laser-driven ion beams to implant various elements including boron, titanium and carbon into silicon. Time-resolved current measurements with 100 TW class laser shots show a very intense plasma expansion pulse of low energy ions (<1 keV) that trails the pulse of high energy ions from target normal sheath acceleration. Color centers, including qubit candidates such as W, G, and C-centers in silicon, form directly under these conditions of intense (dual)-ion pulse irradiation and laser-ion doping. Color center synthesis with the laser-ion plasma-driven approach will be compared to results from use of other processing methods (including conventional ion implantation, thermal annealing, exposure to H2 plasmas, etc.). Finally, the most recent results on fs laser pulse irradiation enabling the deterministic formation and passivation of high-quality color centers in silicon will be presented. We outline directions towards scalable integration of color centers in silicon for applications in QIS.

Fast ion beams, focused to the nanoscale, have numerous applications in science and industry. The quest for smaller spatial resolutions is one of the ongoing key technical developments. However, spatial resolutions below 10 nm have not been demonstrated, because of problems with the focusing technology, and also because it is difficult to robustly confirm such small spot sizes. Here we propose near-axis scanning transmission ion microscopy using molecular ions to address these challenges and demonstrate a H₂⁺ molecular beam with a resolution of 6.0×10 nm². Using the near-axis scanning transmission ion microscopy, some preliminary results are presented on fundamental physics, nanoimaging, nanopatterning, 2D materials and thin film applications.

Single photon emitters (SPE) are fundamental building blocks for future quantum technology applications. However, many approaches lack the required spatial placement accuracy and Si technology compatibility required for many of the envisioned applications. Here, we present a method to place single or few SPEs emitting in the telecom O-band¹. The successful integration of these telecom quantum emitters into photonic structures such as micro-resonators, nanopillars and photonic crystals with sub-micrometer precision paves the way toward a monolithic, all-silicon-based semiconductor-superconductor quantum circuit for which this work lays the foundations. To achieve our goal, we employ home built AuSi liquid metal alloy ion sources (LMAIS) and an Orsay Physics CANION M31Z+ focused ion beam (FIB). Silicon-on-insulator substrates from different fabrication methods have been irradiated with Si⁺⁺ 40 keV ions in a spot pattern of 6 to 500 ions per spot. For the analysis and confirmation of the fabrication of true SPEs a home build photoluminescence setup has been used. G-centers formed by the combination of two carbon atoms and a silicon atom with a zero phonon lines (ZPL) at 1278 nm have been created in carbon rich SOI wafers. In ultra clean SOI wafers W-centers, a tri-interstitial Si complex has been created with a ZPL at 1218 nm. The achieved lateral SPE placement accuracy is below 50 nm in both cases and the success rate of SPE formation is more than 50%. Finally, we give an overview on possible other applications and give an outlook on future projects and instrumentation developments. ¹Hollenbach, M., Klingner, N., Jagtap, N.S. et al. Wafer-scale nanofabrication of telecom single-photon emitters in silicon. Nature Communications 13, 7683 (2022). https://doi.org/10.1038/s41467-022-35051-5.

Quantum technologies benefit from atom-scale control of both the quantum bit and its local environment. In semiconductor materials, defects and dopants house localized electrons where the spin degree of freedom can be initialized and coherently controlled with a combination of optical and microwave signals. These semiconductor spin-qubits can be embedded in devices where their optical and spin properties can be enhanced by controlling both their electronic and nuclear-spin environment. Organometallic molecules provide an analogous platform for localized, coherent quantum states where both the qubit and its environment can be further modified via chemical synthesis. We have demonstrated optical addressability and coherent microwave control of the electron-spin ground-state of organometallic molecules containing a central chromium ion [1]. Moreover, the spin-optical interface of these molecular qubits can be tuned by modifying the structure and symmetry of the ligand field [1,2]. Atomistic modification of not only the qubit itself, but also its host environment can further be used to protect the qubit from magnetic field noise, for example due to the fluctuating electron or nuclear spin bath, yielding a five-fold improvement in spin coherence times by inducing noise-insensitive clock transitions [3]. This scalable approach to engineering quantum systems from the bottom up opens new avenues for tailoring synthetic qubits toward device-scale integration. [1] S.L. Bayliss*, D.W. Laorenza* et al. Science 370, 1309 (2020). [2] D. W. Laorenza et al. JACS 143,50 (2021). [3] S.L. Bayliss*, P. Deb*, D.W. Laorenza* et al. PRX 12, 031028 (2022).

Rare-earth ions in solid-state hosts exhibit low homogeneous broadening and long spin coherence at cryogenic temperatures thus making them a promising candidate for optical quantum memories, optical-microwave transducers and single dopant-based devices such as single photon emitters operating at the telecommunication wavelengths. Here, we present an overview of the spin and optical properties of Er ensembles in Si accessed via resonant photoluminescence excitation (PLE) spectroscopy. Samples were positioned directly on top of tailor-fabricated superconducting single photon detectors, placed in a dilution refrigerator unit and resonantly excited using fiber optics. Our method provided high collection efficiency and allowed spectral measurements of low Er density samples. The observed Er PLE spectra were strongly affected by the presence of co-dopants such as O, B or P reflecting their influence on the formation of optically active Er sites and corresponding Er optical transition energies. Higher O densities resulted in especially rich PLE spectra suggesting that a variety of Er-O complexes formed due to O doping. Lowering the O concentration, we achieved PLE spectra dominated by a single Er site in P doped samples. By lowering the Er concentration from high (10¹⁸ cm^-3) to low densities (10¹⁶ cm^-3), we were able to increase Er spin lifetimes from ~0.1 s to ~30 s. Long spin relaxation times allowed us to identify PLE spectra of new Er sites in Si. In both concentration regimes, we observed sub-MHz transient spectral holes suggesting long optical coherence times. Furthermore, we observed sub-MHz spin transitions in both natural and isotopically purified Si and achieved spin coherence times above 1 ms in isotopically purified Si. Narrow optical linewidths and long spin lifetimes show that Er in Si is an excellent candidate for future quantum information and communication applications.

The silicon vacancy (SiV) centre in diamond has emerged as a promising candidate for quantum network nodes, due to its stability that allows it to be integrated in diamond nanostructures. Notably, it is very sensitive to strain, allowing its emission wavelength and coherence times to be tuned using strain. The SiV has also been theoretically shown to have one of the largest strain susceptibility for a solid state qubit, making it attractive for interface with phonons in strain fields. In this talk, we propose a spin-phonon system in diamond that enables strong interaction between an SiV spin and phonons in strain fields of diamond nanostructures. By virtue of localising strain fields in nanostructures, large single-phonon displacements are possible. When combined with the spin strain susceptibility ~100 THz/strain, single-phonon coupling rates of ~1 MHz to SiV spin can be reached for GHz phonon modes. With phonon damping rates of < 100 kHz already demonstrated for GHz phonon modes in diamond nanostructures at 4 K temperature, strong coupling between SiV spin and single phonons can be reached. Our proposed system also features optical readout via near-field optomechanical interactions with a telecom optical cavity, with the optical cavity being in direct contact with an adiabatic fibre taper. This enables probing and excitation of phonon modes with low optical insertion losses. The telecom operating wavelength allows the system, acting as a node, to be integrated with existing fibre network infrastructure, paving the way for large-scale networking of quantum network nodes.

Recently the negatively charged boron vacancies in hexagonal boron nitride (hBN) have been shown as spin defects that have great potential in quantum sensing. However, so far the sensitivity is limited by either photoluminescence (PL) brightness or the optically detected magnetic resonance (ODMR) contrast, and linewidth. In this work, we demonstrate the generation of these spin defects using high energy helium ion beams and perform ODMR measurements with different laser and microwave powers. The spin defects generated by high energy helium ions exhibit a high PL brightness and ODMR contrast while keeping a small linewidth, hence a good sensitivity. By comparing different fluences of helium irradiations, we determine an optimal fluence which is sufficient in creating spin defects without damaging the overall crystal lattice structure. With this optimal fluence, we can obtain a high signal-to-noise ratio ODMR spectrum with an accurate measurement of zero field splitting frequency, and a best sensitivity. Moreover, with a focused beam, we can deterministically create such spin defects with nanometer precision. Hexagonal boron nitride (hBN) has been a centre of interest due to its ability to host several bright quantum emitters at room temperature. However, the identification of the observed emitters remains challenging due to spectral variability as well as the lack of atomic defect structure information. In this work, we report two new blue quantum emitters with zero phonon line (ZPL) centred around 460 nm and 490 nm in hBN powders. We further demonstrate that the new emissions can be created by high temperature annealing or high energy ion irradiation in exfoliated hBN flakes. Our results not only discover a new group of blue quantum emissions in hBN, but also provide an insight on the physical origin of the emissions by correlating the emission wavelength with local atomic structures in hBN.

Color centers in bulk semiconductors or 2D materials hosts have become an important platform for quantum information science (QIS) with applications ranging from quantum sensing to quantum communication and computing. There are only a few known “quantum defects” (e.g., the NV center in diamond or the divacancy in SiC) and none of them combine all the required properties. More specifically, a quantum defect combining high brightness in a technologically relevant wavelength, and spin coherence in a host that is easy to process and nanofabricate is yet to be found. We will report on our high-throughput computational screening efforts to identify new quantum defects to be used as spin-photon interfaces. We will focus on two important hosts: silicon and WS2. We will highlight the technical challenges with computing defects properties to a very large scale and the more general question of the true electronic structure requirements for a high-performance quantum defect. We will finally report on our first computational findings and current experimental efforts and discuss the dissemination of our results to the QIS community.

Piezoelectric actuators are indispensable over a wide range of industries for their fast response and
precise displacement. High electric field induced piezoelectric strains have been actively studied in lead-free ceramics via defect engineering concept1-5. We show that a giant strain (1.05%) and a large-signal piezoelectric strain coefficient (2100 pm/V) are achieved in strontium (Sr)–doped (K,Na)NbO3 lead-free piezoceramics, being synthesized by the conventional solid-state reaction method without any post treatment. This material may provide a lead-free alternative with a simple composition for piezoelectric actuators. The underlying mechanism responsible for the giant electrostrain is the coupling of the defect dipoles with ferroelectric domains, which is done by tailoring the V_(K/Na)'-V_O defect dipoles and microstructure, thus providing a paradigm for the design of giant-strain piezoelectric materials4. In addition, we also propose a strategy incorporating the morphotropic phase boundary (MPB) concept based on the defect chemistry, forming a defect-engineered MPB system and achieving a giant strain of 1.12% in lead-free Bi0.5Na0.5TiO3 (BNT)–based ceramics. The large asymmetrical strain is mainly attributed to two factors: The defect dipoles along crystallographic [001] direction destroy the long-range ordering of the ferroelectric and activate a reversible phase transition while promoting polarization rotation when the dipoles are aligned along the applied electric field5. Considering the electric field induced high piezoelectric strain, good fatigue resistance, and thermal stability, the defect engineered KNN– based and BNT–based ceramics are expected to be great potential lead-free alternatives for broad temperature range and high-displacement piezoelectric actuator applications.
1. X. Ren, Nat. Mater. 3 (2004) 91-94.
2. Z. Zhao, et al., Acta Materialia, 200 (2020) 35-41.
3. W. Feng, et al., Nature Communications 13 (2022) 5086
4. H. F. Geng, et al., Science 378 (2022) 1125-1130.
5. H. J. Luo, et al., Science Advances, 9 (2023), ade7078.

Microstructures in ferroelectric materials are generally characterised by a patchwork of domains (regions in which electrical dipoles are locally aligned) separated by domain walls (interfaces between domains). Under applied fields, domain patterns change. A necessary consequence is that domain walls must move, be newly formed or be annihilated. Domain walls are therefore both mobile and ephemeral in nature. Importantly, while most ferroelectrics are inherently electrically insulating, domain walls can be semiconducting, metallic and even superconducting. Under these circumstances, domain walls represent 2D electrical sheet structures which can be created, moved and destroyed dynamically, all within an insulating ferroelectric supporting matrix. This has led to the possibility of domain walls being used in completely new forms of transient nanoscale circuitry, which can exist in one moment, for one purpose, only to be wiped away and completely reconfigured in the next moment, to allow for a different circuit function, making the ferroelectric into an extremely Smart Material.
In this talk, I will summarise some recent research done in my group in Queen's University Belfast, on the manipulation of conducting domain walls in ion-sliced single crystal lithium niobate thin films. By controlling domain wall injection and highly localised variations in the magnitude of the polarisation discontinuity across the walls, we have been able to use domain wall-based circuits to realise memristors, artificial synapses, p-n junction-based diodes, rectifiers and logic gates, all of which will be discussed.

The continuing need for improved performance and reduced power requirements of electronic components, for example for wireless sensor networks, has prompted renewed interest in the development of advanced piezoelectric and pyroelectric sensors which can also be coupled with harvesting technologies capable of capturing energy from ambient vibrations and heat. This work provides an overview of piezoelectric materials for sensing, with the closely related sub-classes of pyroelectrics and ferroelectrics [1,2]. The particular advantages of exploiting porosity in these fascinating materials is emphasised, including how the pore structure can be tailored to optimise the dielectric and ferroelectric properties of these materials [3,4]. Examples of modeling and manufacture of porous materials and sensors are discussed, including SONAR applications and hydrostatic behaviour. The potential of novel porous, composites, and sandwich structures are also briefly described.[1] C. R. Bowen, H. A. Kim, P. M. Weaver and S. Dunn, Piezoelectric and ferroelectric materials and structures for energy harvesting applications, Energy and Environmental Science, 7, 25-44 (2014)[2] CR Bowen, J Taylor, E LeBoulbar, D Zabek, A Chauhan, R Vaish, Pyroelectric materials and devices for energy harvesting applications, Energy & Environmental Science 7 (12), 3836-3856 (2014)[3] Y. Zhang, J. Roscow, R. Lewis, H. Khanbareh, V. Yu Topolov, M. Xie, C R Bowen Understanding the effect of porosity on the polarisation-field response of ferroelectric materials, 2018, Acta Materialia, 154, 100-112 (2018)[4] M Yan, Z Xiao, J Ye, X Yuan, Z Li, C Bowen, Y Zhang, D Zhang, Porous ferroelectric materials for energy technologies: current status and future perspectives, Energy & Environmental Science, 14, 6158-6190 (2021)This work is supported by UKRI Frontier Research Guarantee on “Processing of Smart Porous Electro-Ceramic Transducers - ProSPECT”, project No. EP/X023265/1.

[00l] grain-oriented 0.95K_0.5Bi_0.5TiO₃-0.05BiAlO₃ (KBT5B) ceramics were prepared via the reactive template grain growth and tape casting technique. High aspect ratio KBT5B powders were prepared using plate-shape Bi₄Ti₃O₁₂ particle as the template. The effects of grain morphology modification and texturing on the structural, dielectric, and piezoelectric properties of textured KBT5B were systematically investigated. The average plate size of ~ 6.5 μm with a thickness of ~ 350 nm was observed in the KBT5B sample. A Lotgering factor (f) of ~ 81% along the [00l] crystallographic direction was achieved in the textured KBT5B sample. Two anomalies observed in the temperature-dependent dielectric behavior of poled textured KBT5B sample signified additional poling-induced phase transition. A lower value of room temperature RT dielectric constant (e_r) ~ 200 (at 1 MHz) and a higher piezoelectric charge coefficient (d₃₃) ~ 142 pC/N were obtained in the poled-textured KBT5B ceramic compared to that of ceramic with randomly-oriented grains. Consequently, the piezoelectric voltage coefficient (g₃₃) estimated for the textured ceramic showed a 650 % increment (g₃₃ ~ 80×10^-3 Vm/N) over the randomly oriented KBT5B sample (g₃₃ ~ 12.5×10^-3 Vm/N). Further, a piezo device fabricated using poled textured KBT5B ceramic for energy harvesting showed an output voltage of ~ 7 V with a current response of ~ 2 μA under normal finger-tapping motion.

Polyvinylidene fluoride (PVDF), a semicrystalline polymer, is considered as one of the excellent material due to its electroactive behaviour that gives rise to the piezo-, pyro- and ferro-electric properties.^1,2 Therefore, it is possible to utilize this polymer in pressure, temperature and memory-based devices that includes sensors and energy harvester applications. In order to achieve electroactive properties in PVDF, the initial phase transformation is essential key to achieve and studied through infrared spectroscopy. In general, the acquired techniques to nucleate the electroactive phase in PVDF are bi-axial stretching and hot-pressing but it restricts the applications due to inhomogeneous film formation. However, it also needs electrical poling in later stages after film formation in order to aligned molecular dipoles in a particular direction to eventually utilize for the piezo-and pyro-electric applications. In contrast, here we have demonstrated a potential approach in which, in-situ poling and electroactive phase nucleation in PVDF film, is possible to achieve in a single step process. To study the different electroactive phase nucleation of PVDF, various combination of temperatures, solvents and electric field strengths are optimized under corona poling. The obtained results indicate that in-situ poling requires lesser electric field as compared to later external poling, avoiding the electrical breakdown of the PVDF film. Further, we have demonstrated the potential applications of these in-situ prepared films as a self-powered energy harvesters and sensors for real life applications.³ References: 1) A. J. Lovinger, Science 220, 1115 (1983). 2) H. Ohigashi, J. Appl. Phys. 47, 949 (1976). 3) V. Gupta, A. Babu, S.K. Ghosh, Z. Mallick, H.K. Mishra, D. Saini, D. Mandal, Appl. Phys. Lett. 119, 252902, (2021).

It is well-known that many insects such as butterflies and beetles owe their striking colouration to the microscopic structure, rather than the inherent optical properties, of the relevant materials. Unlike the use of dyes and pigments used in conventional colouration of consumer and other products, structural colours do not fade with time and exposure to light, and can be created with a minimal set of materials potentially simplifying aspects of the manufacturing process and recycling at the end of life of the product. Given that the colouration can be produced without absorption, they also present an ultracompact approach to creating filters for use in colour cameras and other applications. Nanoimprint technology provides a scalable and versatile approach to generating surface colours by manipulating the size, shape and arrangement of nanoscale features on nanostructured surfaces. These can be tailored to produce a characteristic surface colouration and corresponding absorption and/or transmission through the device. In this presentation, the development of a polarisation-tunable ‘plasmonic’ pixel will be presented along with the use of nanoimprint lithography to rapidly generate devices at scale. Harnessing the dynamics of the resist flow during the nanoimprint process to generate multilevel features from a simple binary mold will be highlighted. Finally, extension of this work to colourimetric refractive index sensing will be discussed.

In the science fiction comedy “Back to the future” from 1985, the main character, Marty McFly, is accidentally sent thirty years into the past. Going back almost 30 years, by 1995, 70 nm was seen as the physical limit of photolithography. NIL soon demonstrated, as a parallel, low-cost method, 10 nm resolution. Today we see EUV lithography being widely adapted for chip manufacturing. The question is whether NIL that was long seen as the main competitor, can be seen as a failure or a success? Where is NIL now? If we could go back 30 years, what would we have done differently? Today, NIL is alive, used in a plethora of application areas, and even seen in semiconductor chip high volume manufacturing. This is mainly because NIL was diverse from the beginning, already starting with variants of thermal to UV-assisted imprint, using a variety of materials, tools, and developing a range of hybrid processing techniques. When throughput was seen as a bottleneck, NIL was enlarging areas to square meters, developing step&repeat schemes, reaching out to roll-to-roll techniques and developing heatable stamps with sub-seconds processes. NIL is present as a viable technology that is more a toolbox than a single process. Its advantage is that various tool providers offer services to customers, that feature shapes range from high aspect ratio structures for biomimetics to slanted gratings with undercuts for augmented reality glasses that both rigid stamps with good antisticking properties and soft stamps that enable conformal imprint over non-flat topography are in use. And, finally, that its nanometer resolution capability is not seen as the sole criteria for a process called NIL. NIL continues to be an old story in modern times, as a mass fabrication technology in the age of industrialization, with a future that reaches beyond 30 years.

Surface of a medical device determines if the material will be accepted or rejected by the human body. There are various ways by which the surface of the material can be modified, and we would broadly classify them into two, chemical modification and physical modification. For chemical modification biologics such as growth factors or drugs can be incorporated into materials. While as physical modification involves use of techniques such as sand blasting and acid leaching. It has been shown in literature that mere physical modification of biomaterials can lead to effects that were previously attributed only to growth factors and drugs. This opened up a much lucrative approach to modify the physical properties of the material as it would lead to much less rigorous regulatory approval processes thereby a short transit time to clinical translation. However, techniques such as sand blasting are not very reproducible and may vary depending upon a lot of factors. The advent of photolithography and nanoimprint techniques has made it reliable to replicate surface topographies at a nanometre scale resolution. We have been extensively using these techniques for over the last decade and have translated some of the breakthroughs into products for patient care. I will be sharing the tale of our journey and we have exploited nanoimprinting for our implantable materials during my talk.

Augmented reality displays, metalenses, and 3D sensors require a viable fabrication pathway for high performance planar optics. Waveguides and metalenses are commonly used in AR displays and 3D sensors, respectively. Both rely on a high refractive index (RI) contrast for high performance and efficiency. Metalenses are comprised of high aspect ratio, high refractive index posts that direct light to the center of the focal spot, depending on their position in the metalens and the pillar diameter. Pillar symmetry and positioning provide design control over polarization, amplitude and phase. Current approaches to all-inorganic metalens fabrication require subtractive processing and lengthy sequences of time, materials and cost intensive processing steps. Direct nanoimprint lithography using nanoparticle dispersion inks provides a cost effective alternative without sacrificing performance. In particular, high refractive index metal oxide nanocrystals, such as anatase titanium dioxide, are ideal materials for planar optics as they are optically transparent and mechanically stable. Herein, we extend our work on direct nanoimprint lithography for visible wavelength metalenses, where we fabricated of 400 um diameter metalenses with numerical apertures of 0.2 by performing 15 imprints in 30 minutes with a single stamp using TiO₂ nanoparticle dispersion inks. We further demonstrate the use of large area masters for full-wafer fabrication of metalenses and designs that improve metalens efficiencies. We employ ALD post deposition treatment that enables tuning of the refractive index from 1.9 to 2.1 for improved performance. The focusing efficiencies of an array of as-imprinted 4 mm metalenses (smallest dimension ~ 80 nm, highest aspect ratio ~ 10) were 61% on average but the post-treatments with a few cycles of ALD increased the average efficiency up to 75% for the best-performing lens. We also demonstrate the exceptional optical and material stabilities of the all-inorganic imprinted materials.

Recent years have witnessed the fast growth of fluorogens with aggregation-induced emission characteristics (AIEgens) in biomedical research. The weak emission of AIEgens as molecular species and their bright luminescence as nanoscopic aggregates distinguish them from conventional organic luminophores and inorganic nanoparticles, making them wonderful candidates for many high-tech applications. In this talk, I summarize our recent AIE work in the development of new fluorescent bioprobes for biosensing and imaging. The AIE dot probes with different formulations and surface functionalities show advanced features over quantum dots and small molecule dyes in noninvasive cancer cell detection, long-term cell tracing, and vascular imaging. In addition, our recent discovery that AIEgens with high brightness and efficient reactive oxygen species generation in the aggregate state further expanded their applications to image-guided cancer surgery and therapy. By combing the accurate prediction of material performance via first-principle calculations and Bayesian optimization-based active learning, a self-improving discovery system was realized for high-performance photosensitizers, which significantly accelerated our materials innovation for biomedical research.

The atomic oxygen (AO) presence in low earth orbit (LEO) environment is the principal factor damaging the spacecraft's external surface due to severe oxidizing effects, capable to degrade the thermal, mechanical, and optical properties of exposed materials. Therefore, accurate determination of the AO flux provides important information for mission lifetime assessment. Monitoring sensitive systems (such as optics) exposure to AO in real-time will enable online prediction of its remaining lifespan. Moreover, AO monitoring can be used to detect changes in solar activity in real time, thus providing data of great importance to “space weather” forecasts. In this work, we present a simple, compact, and cost-effective solid-state based device with high sensitivity to AO. This electronic sensor is based on two key semiconductor components that exhibit unique electrical properties when assembled together: diamond substrate and transition-metal oxide (TMO) coating. The TMO serves as an electron acceptor, promoting hole conductivity on the diamond surface through a process of transfer doping. When exposed to oxygen, the TMO goes through a process of redox, thereby changing its band structure, and reducing the hole concentration in the diamond surface. This process results in an increase in the diamond surface resistivity that can be monitored in real-time. The results of flux and fluence measurements obtained during ground-based exposure to various AO sources are very promising. The change in the diamond resistance, while exposed, was measured in situ and compared to the LEO equivalent AO fluence, calculated from polyimide mass loss measurement taken concurrently using a QCM. The results show a linear response of the increase in diamond resistance as a function of fluence as measured up to 1x10²⁰ O-atoms/cm². Our work demonstrates the potential of this device to enable real-time AO flux monitoring capability, taking advantage of the robustness and favorable diamond-TMO electronic properties.

Understanding the oxidation and nitridation mechanisms of carbon and carbon-composite materials is key to the reliable design of heat shields for hypersonic flight through air. We have thus conducted molecular beam experiments – both molecular beam-surface scattering and exposures in a new table-top shock tunnel – to gain an understanding of the high temperature oxidation and nitridation of model carbon materials. The experiments were performed with both pulsed and continuous molecular beams of O or N atoms and, in some cases, mixed beams containing O and N atoms. The reactive scattering dynamics of O on various carbon surfaces (e.g., HOPG, vitreous carbon, carbon-fiber preform) suggest that the oxidation mechanisms on all sp² types of carbon are similar but that surface morphology influences the relative importance of the individual mechanisms. All products exhibited the dynamical characteristics of thermal desorption. The efficiencies of the gas-surface interactions, both reactive and non-reactive, were quantified as a function of surface temperature. In addition to reacting with carbon to produce CO₂ (minor product) and CO (major product), oxygen atoms may recombine on the surface to produce O₂ with an efficiency that is somewhat lower than that to produce CO. Nitrogen atoms may recombine on the surface to produce N₂ or react to produce CN. The recombination efficiency of N atoms is generally more than an order of magnitude higher than the reaction efficiency to produce CN. Even a small percentage of N atoms in the presence of O atoms can increase the reactivity of O atoms on a carbon surface by more than 50%. The data from the molecular beam-surface scattering experiments has been used to develop an air-carbon ablation model, and this model has been tested in studies of carbon ablation phenomena with the table-top shock tunnel.

Aliena is a Singapore-based company that provides Hall thrusters and associated components as technological enablers for satellites intending to execute emerging operations in space. The devices produced have to be reliable and robust owing to the harsh environments of space as well as interfacing with the thruster – which is in essence an ion accelerator. Aliena is also providing their systems for a small satellite mission (ELITE) that aims to operate a remote sensing satellite at very-low-Earth-orbits (VLEO), where limits are further pushed due to the aggressive atmospheric environments at VLEO. This paper would give a broad technological overview of the developments at Aliena giving rise to the flight of the MUlti-Stage Ignition Compact (MUSIC) Hall thruster, which operates nominally at 80 W and delivers significant thrust-to-power ratio while maintaining a high specific impulse for drag compensation and lifetime extension of missions at VLEO. This work would highlight the design considerations for the development of such a device for the ELITE mission and highlight results from environmental qualification tests for the system and components leading up to launch. Additionally, highlights from the campaigns leading up to an in-orbit verification of the system onboard the 12U satellite mission (ORB-12 STRIDER) will also be provided. MUSIC is coupled with an in-house developed low-current hollow cathode in its current flight configuration, which features a novel design for thermal management. Additive manufacturing is also employed in the thruster unit for optimized performance in a small form factor. Finally, this paper would highlight ongoing work on advanced nanomaterial-based neutralizers which aim to provide rapid ignition capabilities to satellites at power ranges from 20 – 80 W. Such systems aim to open up new domains for satellites to operate in space at VLEO, and unleashes the full potential of small satellite missions through flight at lower altitudes.

Multipactor is an electron avalanche-like discharge occurring in radiofrequency (RF) components operating under certain vacuum conditions and with high-power RF electromagnetic fields. This phenomenon occurs when free electrons impact a surface with such an energy that secondary electrons are excited and emitted from the surface; the electron density then increase if more electrons are emitted than the number of incident electrons. This growth can lead to one or several electrical discharges which can have several negative effects, from degrading the component performance to breaking the RF transmission. The current trend is to increase the RF power in payload equipment for satisfying the continuous growth of needs in terms of data rate and users. Devices propagating RF high power waves, like filters, diplexers, multiplexers, … are located at the Tx output of the payload, and are particularly sensitive to multipactor effect. Such complex components require coupling irises, slots or aperture, which are made with small gaps that separates the metallic or dielectric surfaces. The radical way to avoid the multipactor effect is to oversize these geometrical parts, which inevitably leads to a strong enlargement of the whole RF filter structure. Another potential approach to increase the multipactor threshold is to control the surface properties of the RF device material (usually silver). This can be accomplished by depositing a carbon nanolayer on silver to modify the electron emission yield (EEY), without altering the electrical conductivity. Several techniques can be used for carbon deposition, including HIPIMS and FCVA. In this work, carbon deposition is applied to an L-band filter, using coaxial stepped impedance resonators (SIR) topology. Such resonators have convenient degrees of freedom to control broadband frequency behavior, quality factor, as well as all the geometrical dimensions, which is convenient to study the effect of carbon layers on multipactor power threshold.

We innovate a unique antiferromagnetic-ferromagnetic (IrMn₃|CoFeB) heterostructure and demonstrate that it can efficiently generate THz radiation without any external magnetic field. We assign it to the exchange bias or interfacial exchange coupling effect and enhanced anisotropy. By precisely balancing the exchange bias effect and the enhanced THz radiation efficiency, an optimized 5.6-nm-thick IrMn₃|CoFeB|W tri-layer heterostructure is successfully realized, yielding an intensity surpassing that of Pt|CoFeB|W. Moreover, the intensity of THz emission is further boosted by togethering the tri-layer sample and bi-layer sample. Besides, the THz polarization may be flexibly controlled by rotating the sample azimuthal angle, manifesting sophisticated active THz field manipulation capability. With 4-inch large-size samples, we also generate strong-field THz waves from this magnetic-field free spintronic emitters. The field-free coherent THz emission we demonstrate here shines light on the development of spintronic THz optoelectronic devices.

Polarization control of terahertz (THz) waves is crucial for improving signal modulation and developing efficient on-chip THz devices. Thus, leveraging the degree of freedom to generate chiral THz waves is envisioned to increase the link capacity for next-generation 6G communication; however, controlling the chirality across the entire THz bandwidth has remained a challenge. This talk presents a method for achieving magnetic-field-controlled linear-to-circular polarized THz emission from synthetic anti-ferromagnetic(anti-FM) heterostructures coupled through Ruderman–Kittel–Kasuya–Yosida interactions across a Ruthenium(Ru) interlayer. Femtosecond photoexcitation of the heterostructure(FM₁/Ru/FM₂) generates orthogonal vectors of linear-polarized spin current, which produce transverse THz pulses upon spin relaxation at the two FM/Ru interfaces. The relaxation asymmetry between the collinear and transverse spins at the FM/Ru interfaces introduces a phase shift between the emitted THz pulses, spanning the entire bandwidth. The method exploits asymmetric spin-to-charge conversion in the spintronic heterostructures and offers advanced chiral THz sources for future communications.

Two dimensional magnetic materials have played an important role in many fields based on their excellent properties, especially in the spintronics. Their combination provides an critical opportunity for the development of a new spintronic devices，especially working in terahertz band. The experiment has been successfully realized in Fe₃GeTe₂/Bi₂Te₃ heterostructures. However, this process has a critical drawback is that it requires an external magnetic field.In this work, we use superlattice to generateterahertz spin currents without magnetic field at room temperature. There is a contact free ultrafast detection of the spin currents and room temperature magnetism by using terahertz emission spectroscopy. The transmitted THz radiation from (Fe₃GeTe₂/CrSb）superlattice is recorded simultaneously with the surface, yielding picosecond time scale dynamic information. The THz emission time-domain signal of the superlattice sample is 10 times greater than that of the CS-only film (4 nm) sample, whilst that of the FGT-only film sample is barely detectable. These findings indicate that the predominant THz emission from the superlattice is not the consequence of CS-only or FGT-only films. To determine the emission mechanism, we vary the sample's azimuth, laser polarization, and rotate the sample by 180°, measuring the THz radiation waveform. These outcomes to be properties of THz spintronics emission. Nevertheless, the radiation has two paradoxes against the THz spintronics emission: The Curie temperature of the superlattice sample is approximately 200 K, Where do spin waves originate? The other is that the superlattice has anisotropic magnetic properties and cannot generate in-plane spin current. By applying magnetic field and cooling the sample, we found that femtosecond laser pulse can excite the ultrafast magnetization of FGT at room temperature, and the magnetic moment of Fe₃GeTe₂ is offset under the strong coupling of CrSb. Therefore, we have proved the powerful magnetic detection ability of terahertz emission spectrum.

Terahertz radiation has many potentially useful applications. Graphene, in turn, is a promising material for creating a highly efficient ultrafast THz photodetector [1]. Bilayer graphene is of particular interest because of the possibility to electrically tune the band gap value in the range from 0 meV to more than 100 meV [2]. However, the net terahertz photoresponse in bilayer graphene depending on the band gap value has not yet been studied. In this work, we studied the photoresponse on a pn junctions in gapped bilayer graphene under 0.13 THz irradiation. The pn junction was electrically induced in a graphene-based field-effect transistor using three gates – one bottom and two top gates, biased independently. The distance between top gates was about 150nm. The dominating detection mechanisms in our structure were resistive self-mixing at room temperature and the photothermoelectric effect with a contribution of photoresponse at tunnel junctions at low temperature. We have shown that both photocurrent and photovoltage increased dramatically with an increase in the graphene band gap value. The estimated photoresponsivity and noise equivalent power of our THz detector at 25K is about 50kV/W and 36fW/√Hz respectively [3]. [1] Sebastián Castilla et al., Nano Lett. 2019, 19, 2765−2773. [2] Zhang, Y. et al. Nature 2009, 459, 820–823. [3] Titova et.al. arxiv preprint, arXiv:2212.05352.

Infrared detectors do not rival with visible (VIS) detectors in terms of sensitivity, cost-effectiveness, and integration, motivating new approaches to perform IR detection with VIS detectors. The photomechanical detector is an uncooled IR detection technology, where each pixel is typically a bi-material microcantilever. The IR radiation drives the temperature rise of microcantilevers, which in turn parametrically couples the microcantilevers themself to a bending motion, resulting in intensity modulation of VIS light so as to detect the IR signal with VIS detectors. Moreover, for materials selection of microcantilevers, key functional considerations are thermal sensitivity and time constant, which are proportional to the coefficient of thermal expansion (CTE) mismatch and heat capacity of the two materials, respectively. Carbon nanotubes (CNT) is thermal contraction axially and the CTE value is -11×10^-6 K^-1 around room temperature. Combining axial thermal contraction of CNT with thermal expansion of metals, such as gold, may yield bi-material pixels with great thermomechanical sensitivity. Meanwhile, the surprisingly low heat capacity nature of the CNTs inherently renders it a rapid thermal response. Here, we experimentally demonstrate such an optomechanical IR detector based on super-aligned CNT. The results indicate that the temperature sensitivity of FPA is increased by 1.3 times and the response time is reduced by half due to the application of CNT.

As one of the most commonly used terahertz (THz) sources, high-performance photoconductive THz sources are one of the driving forces for the development of THz science and technologies. However, the weak THz power hinders its application in THz imaging and spectroscopy applications. To fully control the performance of photoconductive THz sources, we set our sights on the booming field of metamaterial with a high degree of design freedom and flexible manipulation capabilities. Here, we proposed and demonstrated some optimized gallium arsenide (GaAs) -based photoconductive THz sources by nano-scale metamaterials which are directly etched on the optical pump range. These structures reduce the reflection of the GaAs substrate to the pump laser light (λ= 780 nm) and localize the photocarriers in the electric field region, benefiting the transient photocurrents and thus the THz power. Compared with the reference, our devices realize significant THz power enhancement in the whole frequency range. Interestingly, the enhancement is poorly related to the terahertz frequency. In addition, the THz emission enhancement of the nanoscale arrays favors a low-power pump. This work not only performs a multi-discipline study jointing nano-metamaterials and THz sources but also promotes the R&D of practical THz systems based on simple emitters.

The challenges of discovering high performance thermoelectrics are fundamentally related to the contraindicated properties required for high thermoelectric performance, specifically low thermal conductivity, high electronic conductivity and high thermopower. This talk discusses electronic and vibrational structures favorable for thermoelectric performance. These can be used to screen materials to identify promising ones. For the electronic structure the screens include both dispersions at the band edge and bonding characteristics that favor low thermal conductivity. Special electronic structure features particularly cross-gap hybridization leading to high dielectric constant provide a route to high electronic conductivity. Turning to the thermal conductivity vibrational features, a generalization of the rattling concept is presented that allows one to find materials that are likely to have very low thermal conductivity based on phonon dispersions.

The existence of a net thermal and electrical current of conductive thermal waves is demonstrated even in the absence of a mean temperature gradient. This effect, which we call thermoelectric shuttling, is generated by the temperature dependence of the thermal and electrical conductivities of materials excited with a thermal excitation periodically modulated in time. We show that this modulation gives rise to a thermal current superimposed on the one generated by the mean temperature gradient, which enhances heat transport when the thermal conductivity increases with temperature. By contrast, if the thermal conductivity decreases as temperature increases, the thermal-wave heat current inverts its direction and reduces the total heat flux. Similar results are obtained for the electrical current. The reported shutting effect is sensitive to the amplitude of the periodic thermal excitation, which can facilitate its observation and application to harvest energy from the temperature variations of the environment.

High-temperature thermal transport is fundamentally important for a multitude of energy conversion and storage processes, such as thermoelectric, power generation (e.g., gas turbines), thermochemical, solar-thermal, thermophotovoltaic, and thermal energy storage. Heat transfer physics at high temperature is also markedly different from the counterparts at room- and low- temperatures, including the higher anharmonity of lattice dynamics resulting in stronger phonon-phonon scattering and the more prominent or even dominant role of radiation heat transfer. On the other hand, high temperature brings tremendous challenges on the materials, especially on their thermal and chemical stability. Our group (Thermal Energy Materials and Physics, or TEMP) at UCSD has been working on several fronts related to high temperature thermal transport materials and physics over the past few years, including the development of high-temperature materials and devices for engineering applications in solar energy harvesting, thermal insulation, thermal storage, heat exchangers, selective emitters, etc., as well as basic investigations of conduction-radiation coupled phenomena in nanostructures at high temperature. In this talk, we will discuss the general features, opportunities, and challenges in high-temperature thermal transport research as well as selected examples of our work in this area.

The resonant excitation of metallic nanostructures is fascinating due to their ability to control light at the nanoscale. Resonant excitation of metallic nanostructures refers to the process of exciting the plasmonic resonance of a metallic nanostructure by illuminating it with light at a specific wavelength. In thermoplasmonic applications, resonant excitation of metallic nanostructures can be used to enhance the absorption of light and convert it into heat. In this paper, we investigate the effect of a seed layer in a bilayer nanostructure geometry across the mid infrared region of 9-12 µm. Based on the thickness of the seed layer, a variation in absorbance intensity and wavelength is observed for a constant nanodisk diameter. A red shift in peak absorbance wavelength for a gold nanodisk, λ_Au= 10.53 µm to Au/Cr nanodisk, λ_Au/Cr=10.62 µm is observed with a variation in the Cr layer thickness. In comparison to the control (polar SiO₂ substrate), the peak absorbance wavelength was 9.68 µm for a fixed nanodisk diameter. Such models could benefit a further understanding of coupled surface plasmon-phonon polaritons in multilayer models to enhance or tune mid infrared absorption for thermoplasmonic applications. The optimization of the unit cell and supercell geometries were further considered to attain a wider absorption band for polar dielectric-metal multilayer nanostructures about 10 um. The plasmonic effect refers to the phenomenon in which the electromagnetic energy is concentrated and enhanced in metallic nanostructures, leading to an increase in localized temperature that can act as a resource for energy conversion. The ability to engineer nanoscale hybrid-metallic dielectric structures allows the development of new devices to study the exciting physics of thermoplasmonics for solar and thermal energy harvesting, as well as mid-IR imaging applications.

Colloidal quantum dots/perovskites based light-emitting diodes (QLEDs/PeLEDs) have attracted extensive attention for lighting and display applications because of their excellent advantages, such as narrow emission bandwidth, color tunability, high brightness and low-cost fabrication techniques. The external quantum efficiencies (EQEs) for QLEDs/PeLEDs are catching up with those for commercial organic light emitting diodes (OLEDs), thanks to the quantum efficiency improvement, the device configuration optimization as well as the deep understanding of electroluminescence principle. Despite the rapid advance in device performance has been achieved, the operational lifetime of these devices still cannot meet the requirements for practical applications. In this report, we will present our latest advances in improving performance and stability of high color-purity QLEDs/PeLEDs. References: (1) Xuyong Yang* et al., Efficient Tandem Quantum-Dot LEDs Enabled by An Inorganic Semiconductor-Metal-Dielectric Interconnecting Layer Stack, Advanced Materials, 2022, 34, 2108150. (2) Xuyong Yang* et al., Stability of Perovskite Light-Emitting Diodes: Existing Issues and Mitigation Strategies Related to both Material and Device Aspects, Advanced Materials, 2022, 34, 2205217. (3) Xuyong Yang* et al., Smoothing the energy transfer pathway in quasi-2D perovskite films using methanesulfonate leads to highly efficient light-emitting devices, Nature Communications, 2021, 12, 1246. (4) Xuyong Yang*, A Multi-functional Molecular Modifier Enabling Efficient Large-Area Perovskite Light-Emitting Diodes, Joule, 2020, 4, 1977. (5) Xuyong Yang* et al., Trifluoroacetate induced small-grained CsPbBr₃ perovskite films result in efficient and stable light‐emitting devices, Nature Communications, 2019, 10, 665.

Tungsten oxide nanomaterials are of interest for chromogenic device applications. Despite rapid progress in electrochromic devices based on tungsten oxide even to the extent of commercialized functional devices, the underlying mechanisms of visible light induced photochromism in hydrated tungsten oxides has attracted the attention of the research community. Visible light induced photochromism involves reversible color change in the photochromic-active material under irradiation of visible light of wavelength >400 nm or sunlight. Tungsten oxide in its hybrid form with organic moieties, has shown interesting photochromic properties under UV-irradiation. However, visible light induced photochromism and the role of structural water in pure tungsten oxide hydrates has not been explored in detail. In this work, we present the first results on one pot synthesis of pure hydrated WO₃ nanoplatelets prepared by colloidal chemistry route starting from tungsten metallic powder. The obtained hydrated WO₃ were heat treated at 100 C to 500 C. They were characterized for structural (XRD, HR-TEM), vibrational (Raman and IR) and optical properties (spectroscopic ellipsometer). TEM measurements revealed the nanoplatelet features with high degree of crystallization for samples prepared at 100 C and above. The thickness of the platelets will be reported based on AFM studies. A topo-tactic phase transition is observed from hexagonal phase to monoclinic phase of WO₃ accompanied by dehydration. The temperature dependent de-hydration was monitored by following the water molecule fingerprint in IR spectrum of the prepared samples. The photochromism is strongly related to the oxidation states of tungsten and they are studied by XPS. Spectroscopic ellipsometer measurements of the optical conductivity of the samples before and after visible light irradiation. A quantitative model based on small polaron absorption will be applied to explain the experimentally observed optical conductivity thus quantifying the visible light induced photochromism.

Two-dimensional hybrid-organic-inorganic lead bromide perovskites (2D-PVK) have shown great potentials as scintillators, exhibiting features like high light yields, fast decay time, negligible afterglow and environmental stability. Here we present the scintillation properties of 2D-PVKs, and in particular butylammonium lead bromide (BA₂PbBr₄) and phenetylammonium lead bromide (PEA₂PbBr₄), which feature room temperature light yields of 40,000 and 11,000, respectively and near-constant scintillation emission for a wide range of temperatures. To further enhance the scintillation properties of 2D-PVKs we explored ion doping, using two different alkali metal cations, lithium (Li⁺) and rubidium (Rb⁺). We demonstrate here that introducing Li- or Rb-ions into the perovskite structure leads to a significant improvement of the light yield by over 70% for Li-doping and 60% got Rb-doping. The introduction of dopants also leads to a faster scintillation decay times for the 2D-PVK, with fast components under 5 ns. Li-doping also is shown to lead to an improvement of the energy resolution of 2D-PVK crystals, with a record of 7.7% at 662 keV for the Li-doped PEA₂PbBr₄. In addition, we observe that introducing a large ion such a Rb+, leads to an expansion of the crystal lattices the perovskites, leading in turn to a narrowing of the bandgaps, down to 22 %. Finally, we demonstrate that doped 2D-PVKs can be used for a wide range of radiation detection and very fast applications, and imaging applications as well.

Quantum dots (QDs) of 2D materials have attracted great interest in applications on the field of optoelectronics, electrocatalysis, biological sensors due to their extraordinary physical, chemical, and optical properties. MoS₂ QDs have more attention on optoelectronics and hydrogen evolution reaction study due to quantum confinement effects, nonlinear optical (NLO) properties, direct bandgap, etc. Herein, we report comprehensive photophysical and femtosecond (fs) NLO properties of MoS₂ colloidal QDs prepared using femtosecond laser ablation for optoelectronic applications. We prepared four samples using two solvents (ethanol and DI water) with ablating times of 20 minutes and 10 minutes named as Et10, Et20, DW10 and DW20 QDs resulting in mean diameters of (2.06-4.40) nm. From excitation-dependent photoluminescence studies, the Et-based QDs showing no shift in emission peaks but red shift in DW-based QDs providing information about the defect and localized states act as emitting centers and polydispersity nature. Surface-related recombination studies were performed using time-resolved emission spectra (TRES), with decay times increased from 2.45 to 5.95 ns with increasing emission wavelength in DW QDs. To explore the NLO properties of QDs, we are reporting, for the first-time, fs ablated MoS₂ colloidal QDs using Z-scan method with fs laser pulses. QDs prepared using Et20 minutes showed saturable absorption (SA) and SA followed by reverse saturable absorption (RSA) with increasing the intensity from lower to higher with two-photon absorption coefficients (2PA) of (7.01-9.31)×10^-11 cm/W. We have observed that other QDs exhibited RSA behaviour with 3PA coefficients of (4.19-5.31)×10^-4 cm³/GW². We have extracted the nonlinear refractive index and second hyperpolarizability with values (0.98-1.21)×10^-15 cm²/W, (7.05-9.78)×10^-31 esu, respectively. From the photophysical and NLO studies including NLO parameters, optical limiting, and figures of merit, these QDs can be used as multicolor-imaging, optical switching, optical limiting applications, optoelectronic applications.

Long searched but only now discovered two-dimensional (2D) magnets are one of the select group of materials that retain or impart strongly spin correlated properties at the limit of atomic layer thickness. In this presentation I will discuss how different layered compounds (e.g. CrX₃ (X=F, Cl, Br, I), MnPS₃, Fe_5-xGeTe₂,CrGeTe₃) can provide new playgrounds for applications and fundamental exploration of spin correlations involving quantum-effects, topological spin-excitations and higher-order exchange interactions. In particular, I will show how vdW magnets do not require any magnetic anisotropy to stabilise magnetism and demonstrate the null applicability of the Mermin-Wagner theorem for 2D magnetism in practical applications. Moreover, some recent results of ultrafast laser excitations on different vdW heterostructures will be shown towards all-optical control of magnetic properties.

Atomic defects in semiconductors can act as optically addressable luminescence centers that can serve as a building block for solid-state quantum technology. Unlike color centers in insulators where optical transitions mainly involve atom-like defect states, defect emission lines in semiconductors are often attributed to bound exciton (BX) complexes, which are many-body particles retaining some free exciton character. Monolayer transition metal dichalcogenides are expected to host a variety of BX complexes in the presence of atomic defects. These complexes are not only rich in physics, reflecting the unique properties of the host crystal, but also electrically tunable, making them attractive for quantum photonic devices. However, despite the common observation of defect-mediated emission in TMDs, the structural and physical origin of BXs remains elusive, preventing strategic quantum defect engineering. I will first discuss controlled in-situ and ex-situ generation of atomic defects in the dilute limit where quantum effects are expected [1-3]. I will then discuss determination of the many-body nature of BXs through electro- and magneto-optical spectroscopy [3,4]. Finally, I will discuss our observation of single atomic defect conductivity which allows rapid quantification of selected impurities in the dilute limit (<10¹⁰ cm^-2) in ambient condition [5]. [1] Loh et al. “Impurity-induced emission in Re-doped WS₂ monolayers” Nano Lett. 21, 5293 (2021). [2] Zhang et al. “Optically Active Chalcogen Vacancies in Monolayer Semiconductors” Adv. Opt. Mater. 10, 2201350 (2022). [3] Loh et al. “Dilute acceptor-bound exctions in monolayer semiconductor” Under review. [4] Chen et al. “Gate-tunable bound exciton manifolds in monolayer MoSe₂” Under review. [5] Nam et al. “Single atomic point defect conductivity for dilute impurities imaging in 2D semiconductors” Under review.

The breaking of inversion symmetry and the strong spin−orbit coupling in monolayer TMDCs induce a conduction band spin splitting of about a few tens of millielectronvolts, giving rise to the so-called spin-allowed, bright and spin-forbidden, dark excitons. Their transition dipole orientations are orthogonal to each other, and the latter has a lifetime significantly longer than the former due to the spin-flip. Such unique characteristics of dark excitons have great potential in implementing coherent two-level systems for quantum information processing and Bose−Einstein condensation. However, optical selection rules dictate that only spin-preserved electronic transitions are optically active, making these dark excitons optically inactive at room temperature. In this talk, I will first discuss how to employ plasmon-exciton coupling spectroscopy to probe the presence of dark excitons in monolayer MoS₂ and WS₂ by examining their temperature-dependent bright-exciton-plasmon coupling strength with single Au@Ag core-shell nanocuboida (ACS Photonics 2019, 6(2), 411-421). Following this, I will illustrate the important role of dark excitons in the temperature-dependent photoluminescence intensity of monolayer WS₂ and show that the temperature-dependent emission energy can be well described with the Varshni formula (Nanoscale Horizons 2019, 4(4), 969-974). Finally, I will show that coupling the spin-forbidden dark excitons to a metal nanoparticle-on-mirror cavity leads to plasmon-induced resonant emission with signal intensity comparable to that of the spin-allowed bright excitons (Nano Letters 2022, 22(5), 1915-1921). A three-state quantum model combined with full-wave electrodynamic calculations reveals that the radiative decay rate of the dark excitons can be enhanced by nearly 6 orders of magnitude through the Purcell effect, therefore compensating its intrinsic nature of weak radiation. Our nanocavity approach provides a useful paradigm for understanding the room-temperature dynamics of dark excitons, potentially paving the road for employing dark exciton in quantum computing and nanoscale optoelectronics.

Twisted hetero bilayers of transition metal dichalcogenides (TMDCs) provide an excellent platform to explore artificially created superlattice structures, known as moiré patterns. Periodic variation of interlayer distance, coupled with a local strain-induced reconstruction in such superlattices, gives rise to the moiré potential envelope, the depth and period dependent on the twist angle. However, dynamic tuning of moiré potential will be useful at a given twist angle and has not been explored. This work shows harmonic to anharmonic switching of the moiré potential well in WS2/WSe2 hetero-bilayer by exploiting exciton-exciton dipolar repulsion through tuning the incident optical power. We create a twisted hetero bilayer of WS2/WSe2 with a twist angle of about 59 degrees. We observe three equally spaced inter-layer exciton (ILE) resonances (namely X0, X1, and X2) with varying degrees of localization, suggesting the harmonic nature of the moiré potential well. However, with increasing optical power, we observe a varying degree of blue shift for different ILE emissions due to varying dipolar repulsions, which can be directly correlated with the degree of localization. The strongly localized ILE (X0) blueshifts rapidly, whereas less-localized ILE emissions (X1 and X2) show negligible blueshift. This, in turn, gives rise to the unequal separation of ILE emissions at higher power, indicating induced anharmonicity. We also observe a transition from mono-exponential to a bi-exponential decay along with the appearance of a fast component in the exciton decay at higher power for X2. We attribute this to the degeneracy lifting at the higher energy exciton resulting from dipolar repulsion-induced anharmonicity. In contrast, the decay of X0 remains mono-exponential at all the power values as the ground state does not have degeneracy. Such dynamic tuning of moiré potential is intriguing for new experiments and moiré-related device applications.

Among the broad-range spectrum, near-infrared (NIR) plasmonic is of particular importance for telecommunication, energy harvesting, and sensing et al. To fully utilize the potential of NIR plasmonic in modern electronics, the plasmon sources have to be compatible with electrical control. However, bulk NIR plasmonics is generally not directly controllable with electrostatics because of the strong screening effect and high carrier concentration required to support NIR plasmon. Here we overcome this constraint by taking advantage of the enhanced electrostatics of atomically thin materials. Using the strong gating capability of ionic gel, we observed plasmonic resonance in NIR range that can be modulated electrostatically over a range of ~ 400 cm^-1 in few-layered NbSe₂ gratings. The layered nature of NbSe₂ provides extra degree of freedom to modulate resonance frequency with thickness, due to its thickness dependent carrier density with different number of layers. While most layered metals are very sensitive when thinned down to few layers, the 2D plasmonic shows remarkable stability under ambient condition once covered by monolayer h-BN. Our study identifies 2D layered metals as plasmonic sources to extend the electrostatic modulation of plasmonic to the technologically important NIR range.

In this lecture, it is demonstrated that single nm size CeO₂ can be synthesized by the precise control of reaction time in the range around several 10 msec in supercritical hydrothermal synthesis. Rapid heating of metal salt solution to the supercritical state was achieved by mixing with preheated supercritical water flow, and the quick quench has been done by mixing with cold water and a cooling water jacket. This enabled the precise control of reaction time by changing a flow rate and the length of the reactor between the two mixing points. In the range of several 10 msec, significant particle growth due to the collision of particles from 1 nm to several nm was observed. The formed nano particles are of single crystals, but have significant strains, which leads to form the significant oxygen deficiency. EELS analysis indicated that with decreasing size, Ce valence was changed from tetra-valence to tri-valence. These structural changes gave rise to the extremely high oxygen transfer and oxygen storage capacity (two orders of magnitude higher) at lower temperatures ranging from 150-350 C (several hundred degrees for bulk materials). By using the extremely small CeO₂ nanoparticles as a catalyst, low temperature reforming of heavy hydrocarbons (lignin or bitumen) to produce lighter oils and hydrogen was successful.

Our society is facing an increasing challenge on various issues. Among them, energy and environment are listed as the top two that need to be solved immediately, where catalysis plays a vital role. It has been recognized that “Green Hydrogen” from electrocatalytic water splitting may be “Our Future” due to the global goal of net-zero carbon emission by 2050. The sluggish kinetics of the oxygen evolution reaction (OER) and Hydrogen evolution (HER) is the bottleneck that limits the efficiency of water splitting, which stimulates researchers to design suitable catalysts. Therefore, the design of catalysis has been attracting increasing attention for hydrogen production because of its green, easily adaptable, and scale-up nature. There has been tremendous interest in recent years to discover, and demonstrate unique properties and applications of 2D materials. The most direct and efficient strategy in the design and synthesis of new materials is to change the structural units, which would lead to a paradigm shift. Our research group focuses on developing multiple Heterointerface engineered 2D materials as active and durable water-splitting catalysts. In this talk, we will briefly discuss the research advances made in this emerging field by our group on the design and advanced characterization techniques of novel electrocatalysts including (1) the fabrication of electrocatalysts for water splitting, (2) the surface reconstruction and phase transition of electrocatalyst, (3) the role of multi-phase heterostructure on the high catalytic activity, (4) the in-situ characterization for the identification of the active site. Our results demonstrated that multimetal compound systems were catalytically active for water splitting in electrochemical energy devices, which may find practical application in hydrogen-energy technologies.

Advances in materials design and discovery have been made in the last few years with machine learning combined with large databases on materials properties. The creation of generative large language models such as ChatGPT is now bound to revolutionize knowledge discovery in materials sciences, which will go well beyond the current achievements in materials discovery. In this lecture, a discussion will be presented of the technologies involving high-throughput experiments and computer simulations exploited in materials discovery, in addition to the proposal of novel approaches to mine scientific literature in materials. The latter approaches encompass natural language processing and network science with which one may obtain the landscape of research on given topics or even scientific journals, and identify materials and processing conditions for targeted applications. For instance, with such methods one may determine the most impactful topics in materials sciences are associated with energy-related materials and organic electronics. Furthermore, tools based on large language models and other artificial intelligence methods permit the development of computer-assisted diagnosis systems for personalized medicine, precise agriculture and different types of automated surveillance. This will be achieved via deep learning to leverage multimodal data from distinct sources, e.g. text, scientific data, images and videos.

Life is powered by multi-enzymatic tandem processes with unrivaled catalytic efficiencies owing to biological reactor properties such as compartmentalization, nano-confinement, and out-of-equilibrium dynamics. Various efforts have been made to develop a bioinspired self-assembled system.¹ However, most of designed synthetic systems operate in an equilibrium fashion, whereas Nature shows autonomous out-of-equilibrium behavior. Herein, we developed a pH clock-mediated transient assembled nanozyme based on supramolecular coordination chemistry mimicking multi-enzymatic activities.² Nanomaterials that can mimic natural enzymes are known as “nanozymes.”³ The intrinsic drawbacks of natural enzymes, such as high cost, ease of denaturation, difficulties in recycling, and laborious preparation, significantly limit their practical applications; as a result, improved functional enzyme mimic (nanozyme) gained tremendous attention. The developed nanozyme exhibits effective laccase-like catalytic activity with a lower K_M value than that of the natural enzyme. Additionally, the nanozyme showed greater stability in harsh conditions, whereas natural enzymes undergo loss of activity due to denaturation in such severe environments. The system also demonstrates effective peroxidase-like activity by mimicking NADH peroxidase. Nanozymes that can tune their catalytic activity in response to external fuels provides functionally essential platforms resembling non-equilibrium natural systems. Interestingly, the active nanozyme displays dynamic laccase-mimetic activity in response to pH change, which has rarely been explored. The self-assembled system can switch between different states by a simple acid/base trigger which can “turn on” and “turn off” its catalytic activity accordingly. Altogether, a flexible nanozyme system with multiple enzymatic activities can be designed using this novel platform that will open many avenues for the development of enzyme equivalents with out-of-equilibrium “life-like” properties. References: 1. Sirong Li et al Nat Commun.,2022, 13, 8272. Manju Solra et al ACS Appl. Mater. Interfaces2022, 14, 40, 45096–451093.Dongdong Wang et al Acc. Chem. Res.2020, 53, 7, 1389–1400.

Modern day research focusses on the development of greener protocols to design biologically relevant molecules with minimal waste generation and reusable catalysts. Till date, mostly metal (Ir, Ru, Pd etc.) catalysts have been used for synthetic transformations. These often pose problems associated with lack of recyclability, product contamination with the catalyst and expensive synthetic routes for catalyst generation. Nanomaterials serve as dynamic, sustainable, economic and potential substitutes with a higher surface to volume ratio in lower dimensions, which can enhance the number of catalytically active sites. Despite the aforesaid advantages, these have rarely been used in catalysis. Keeping in mind the tremendous potential that nanomaterials harbour, we have demonstrated their usage for various photo-mediated organic transformations such as cross-coupling, rearrangement reactions and cyclisation. In each case, the nanomaterials used could be derived from earth-abundant precursors via facile, scalable methods. These included various nanosheets and quantum dots from transition metal dichalcogenides (TMDs) and naturally occurring amino acids. The TMD composition and activity could be correlated for imine synthesis. This was subsequently applied for C-C/C-P coupling in the same pot as a hydrogen evolution reaction, wherein stoichiometric quantities of amine form a quaternary salt terminally, which could be coupled with relevant nucleophiles to generate anti-HIV drug moieties. Following this, we also applied the QDs to devise benzimidazoles via concomitant C-C/C-N bond formation followed by cyclisation. Finally, we demonstrated their use in the Newman Kwart rearrangement which is a key step for the conversion of phenols to thiophenols. In each case, the catalyst retained its activity in subsequent runs. Our strategies essentially show that the correct choice of a nanomaterial can catalyse an organic transformation to furnish the desired product, possibly photothermally. The field harbours tremendous potential, particularly for the design of chiral nanomaterials for the synthesis of bioactive molecules.

Adding second-phase particles to alloys, thereby generating a metal-matrix composite (MMC), can have a beneficial influence on (high-temperature) strength, wear resistance and density of alloys. As powder-based methods, many AM processes such as laser powder bed fusion (LPBF) are well-suited to synthesize this class of materials. In this presentation, I will summarize various attempts to generate MMCs: Ex-situ, i.e. by adding ceramic particles to the metal powder before AM, and in-situ, i.e. by reactions in the liquid state during AM. In the ex-situ approach, the challenges include the flowability of the blended powder feedstock, a homogeneous distribution of particles without excessive agglomeration, and the question whether particles react during the existence time of the melt pool. In the in-situ approach, I will present MMCs that are the result of metal-gas reactions during LPBF. A sufficient reactivity of the metal feedstock with the active gas atmosphere is required, while ensuring a stable and safe AM process. The examples presented range from Al alloys to HEAs and steels.

Powder characterisation is integral to powder metallurgy processes, including various powder-bed additive manufacturing processes. Typical powder feedstock properties of interest include the powder size distribution, powder morphology, and elemental composition; all this information provides critical insights into the feedstock’s influence on the material properties of the fabricated parts. Laser diffraction is one of the most established standard techniques to gauge particle size distribution (PSD) for AM process monitoring and control, offering a statistically significant PSD of the feedstock after a relatively short test. Nevertheless, one of the challenging tasks for conducting laser diffraction tests is to determine the refractive index (RI) of the powder that is being analysed; (Mie) scattering detectors can under or over-estimate the particle size if the wrong RI is chosen. This is especially tricky when mixed feedstock with different materials (and hence different RI) or smaller particle sizes are examined. Therefore, complementary techniques that involve extinction detectors have also been utilised to determine PSD and morphology based on the “shadow” cast by the particles. In this work, we expand on the use of Single Particle Optical Sizing (SPOS; combination of extinction and scattering detectors) to progress towards a wet powder characterisation technique based on an additional variable, time. By incorporating a suitable time-lapse analysis of the PSD, an additional layer of information can be obtained. The SPOS of the AM powders allows for better quantitative results (particles/mg) of both the coarse particles as well as the fine particles in the PSD, yielding much tighter process/parameter control for AM.This is especially useful for mixed feedstocks whereby the densities of the individual powder materials are relatively different. Importantly, this provides a supplementary pathway for information on the PSD of mixed feedstock, as we progress towards an era of greater interest in multi-material 3D and even 4D printing.

As a promising additive manufacturing (AM), binder jetting has a wide range of potential industrial applications and feasible material systems. Binder jetting can also address the rapid melting-solidification issues compared to other AM processes. The packing density of the powder bed during the binder jetting process affects post-processing steps and final component properties. This work focuses on the effects of powder spreading process on the powder bed quality for binder jetting using the discrete element method (DEM). The developed DEM model not only considers the powder spreading by the roller but also the powder recoating by the hopper which has not been widely studied in the literature. The relative density of loose powder within the printed density cups was measured for comparison with DEM simulation results. The simulation results are expected to reveal the effects of spreading process parameters, such as recoating speed, hopper oscillation speed, and hopper oscillation amplitude, on the powder bed quality.

The Ordered Powder Lithography (OPL) is a new non-laser powder based additive manufacturing technology. The OPL method uses metal powder and an inert second powder, called the negative powder, which supports the shape of the metal powder during the printing process. After printing is completed, the samples are sintered. The need for infrastructure is substantially reduced as no shielding gas or high voltage electricity is used. This technique is much faster than conventional selective laser melting or electron beam melting. In addition, it is anticipated that this production method will not introduce significant mechanical anisotropy. In this study Ti-6Al-4V is used to investigate the possibilities for producing parts with the OPL using different sintering approaches. The samples are characterized by means of hardness, density, and CT. An overview of the boundary conditions of this technology will be given. The capabilities of this new technology will be shown using a generic part.

Additive manufacturing is modelled on different scales. The limitation in computational power limits the possibility to couple these scales. The typical used approaches can be based on macro, meso or micro scales. There are some variations in use of these notations. The macro scale approach is appropriate for design of geometry of the component to be produced. The modelling of the whole component does in many cases require exclusion of the effect of the AM process on details of geometry and material microstructure and properties – at least when the powder bed process is used. Fluid flow or discrete powder models are examples of meso scale modelling. The current talk will focus on techniques that stem from Computational Welding Mechanics (CWM). Coupled microstructure and property models will be described and shown for Alloy 718 [1-3]. The microstructural features are ranging from solutes, precipitates to grain structure that are important for the plastic properties of the material. The mechanism-based plasticity model has long-range and short-range components. The former utilizes the density of immobile dislocations for the so-called strain hardening. This choice of model alleviates the coupling to microstructural features. The use of this kind of models for simulation of the AM process will be shown. References Fisk, M., J.C. Ion, and L.E. Lindgren, Flow stress model for IN718 accounting for evolution of strengthening precipitates during thermal treatment. Computational Materials Science, 2014. 82(10): p. 531-539. Moretti, M.A., L.-E. Lindgren, and P. Åkerström, Physics-Based Flow Stress Model for Alloy 718. Metallurgical and Materials Transactions A, 2022. 3. Fisk, M. and A. Lundbäck, Simulation and validation of repair welding and heat treatment of an alloy 718 plate. Finite Elements in Analysis and Design, 2012. 58(7): p. 66-73.

Additive manufacturing (AM), or 3D printing, breaks new ground to materials engineering and property tailoring to meet specific performance requirements, facilitating the development of innovative components. However, the benefits of the AM technology come at a price. Complex physics of AM closely linked with numerous process parameters affects the final microstructure and properties of a produced component and makes the manufacturing process challenging to control. Even small variations of process parameters or scan strategy can trigger unexpected and previously unexplored effects and unwanted defects developing in a final product. Integrated process-structure-property (PSP) modelling represents an attractive tool to further advance our understanding of the fundamental relationships between the production process and parts’ mechanical properties and to enhance our prediction capabilities in this space, with the overarching aim of materials design and reverse engineering for the required performance of a part to be manufactured. This talk presents the results of process-microstructure-property modelling for alloys fabricated by laser powder bed fusion (LPBF) AM. Specifically, the finite difference method is used for melt-pool-scale thermal simulations of the LPBF process. The thermal histories obtained represent an input for microstructure simulations using cellular automata. The microstructural data, including the grain morphology and crystallographic orientations, is considered explicitly in microstructure-based finite element simulations in terms of anisotropic elasticity and crystal plasticity. Effects of some process parameters on the microstructure and mechanical behaviour of additively manufactured materials are analysed. O. Zinovieva and A. Zinoviev acknowledge the UNSW Canberra start-up grant [grant number PS66347], UNSW ResTech AWS Cloud Scheme, and UNSW Resource Allocation Scheme [project mz70]. V. Romanova and R. Balokhonov acknowledge the Government research assignment for ISPMS SB RAS, project FWRW-2021–0002.

Semiconductor materials based on metal halide perovskites have demonstrated great success in the area of solar cells and other optoelectrical devices such as light emitting diodes. Nevertheless, the materials are notorious for their poor stability under environmental factors that are closely relevant to the device operational condition such as humidity, temperature, irradiation and electrical field. This have restricted the application of the materials in practice. Therefore, in the past decade, significant research efforts have been made to combat the stability issue of perovskite materials. In my talk, I will present our study of increasing the stability of metal halide perovskite materials using different strategies such as doping engineering or coating engineering. In particular, I will discuss the role of certain dopants in increasing the structural stability of perovskite materials, leading to increased performance and stability in optoelectronic devices including solar cells and light emitting diodes. I will also demonstrate our research of using coating strategy to synthesize stable metal halide perovskite nanocrystals, which have been successfully used as catalysts for the chemical reaction of CO2 reduction, forming valuable fuels with excellent selectivity and stability.

A major challenge in metal-halide perovskites is to effectively replace lead by less toxic elements. Tin appears the only reasonable candidate, due to the similar structural and electronic properties of these two elements. A major difference, however, is the stability of oxidized Sn(IV) phases, which are not stable in lead-halides. Stabilizing tin perovskites is thus a major challenge, with defect activity likely representing the key to achieve such a goal. Here we present results of advanced modelling studies on the defect mediated degradation pathways of prototypical compounds. We show how Sn-vacancies are central in promoting both material p-doping and formation of Sn(IV) phases. Interestingly, while p-doping dominates in the bulk, Sn oxidation is only favoured at surfaces or grain boundaries. Thus achieving uniform thin films coupled with proper surface passivation strategies represent a pathway towards more stable tin-based devices. Surprisingly, THPs have also received a large attention because of their superior stability in water environment compared to their lead counterparts. We further unveil the key factors determining the stability of THPs in water finding that the presence of amorphous phases at the water/perovskite interface is crucial in preventing material degradation. Moreover, the reactivity of THPs towards photocatalytic hydrogen production at the perovskite/water interface is investigated. Our results highlight that the occurrence of electron polarons at the surface of THPs is paramount in determining the efficiency of the reaction. The stabilization of localized electrons stems from the energy of the conduction band edge and from the peculiar tin-perovskites defect chemistry, largely centered on tin. Band edge tuning is governed by the interplay between the A-site cation and nature of the halogen, thus fine-tuning the energy levels can be achieved by varying the chemical composition, providing a successful strategy to boost the photo-reactivity of these materials.

Bandgap tunability of lead mixed-halide perovskites makes them promising candidates for various applications in optoelectronics since they exhibit sharp optical absorption onsets despite the presence of disorder from halide alloying. Here we use localization landscape theory to reveal that the static disorder due to compositional alloying for Iodide:Bromide perovskite contributes at most 3 meV to the Urbach energy. Our modelling reveals that the reason for this small contribution is due to the small effective masses in perovskites, resulting in a natural length scale of around 20nm for the “effective confining potential” for electrons and holes, with short range potential fluctuations smoothed out. The increase in Urbach energy across the compositional range agrees well with our optical absorption measurements. We model systems of sizes up to 80 nm in three dimensions, allowing us to explore halide segregation, accurately reproducing the experimentally observed absorption spectra and demonstrating the scope of our method to model electronic structures on large length scales. Our results suggest that we should look beyond static contribution and focus on the dynamic temperature dependent contribution to the Urbach energy.

Grain size and grain boundaries have an important influence on the performance of perovskite solar cells. This includes the microstructure of the perovskite material itself as well as of materials in other critical layers, such as titania. Because of the intrinsically large-scale nature of microstructure, its modeling is difficult. Elements of microstructure, such as grain boundaries or other defects, are typically postulated based on chemical intuition, which biases the analysis of their effects. Experimental insight into the real distributions of grain boundaries and other non-equilibrium structures in bulk is also difficult. We will present our studies of possibilities of more natural computational generation of elements of microstructure with molecular dynamics, beginning from pristine crystals. On the examples of methylammonium lead iodide and TiO2, we show that such generation is more difficult in these materials than in monoelemental semiconductors and metals. We identify time-temperature schedules which mimic possible experimental schedules (albeit in different time frames) and result in grainy structures. We highlight the role of the force field model and deficiencies of existing forcefields for this type of simulation. We also show how one can accelerate grain generation by seeding while largely preserving the idea of natural formation of grain boundaries.

Recently, quasi-2D Ruddlesden‒Popper perovskites (2D-RPPs) have attracted extensive research interest in the field of perovskite light-emitting diodes (PeLEDs) owing to their excellent optoelectronic properties. The 2D-RPPs are derived from conventional 3D perovskite with ABX₃ crystal structure by introducing bulky spacer cation A′. The chemical formula of 2D-RPPs is expressed as A′₂A_n−1B_nX_3n+1 (n = 1, 2, ..., ∞), where n is the number of inorganic octahedra layers sandwiched between spacer cations. Although 2D-RPPs-based PeLEDs have progressed rapidly in terms of performance, it is still challenging to achieve blue-emissive and color-pure PeLEDs since conventional fabrication processes induce spatial segregation of spacer cations, consequently generating multiple perovskite phases (i.e., various n values). With the mixed perovskite phases inside the film, the energy funneling from the perovskite phase with large bandgaps (i.e., low-n phase) to the small bandgap (i.e., high-n phase) occurs, hindering the deep-blue emission. Therefore, a novel strategy capable of precisely controlling the phase evolution of the 2D-RPPs during crystallization is required. Herein, we demonstrate the high-efficiency deep-blue PeLEDs based on phase-pure 2D-RPPs enabled by the rapid crystallization method. When the as-spin-coated perovskite precursor film was submerged into the hot antisolvent bath, immediate crystallization occurred due to the rapid extraction of precursor solvent by antisolvent. Thanks to the extremely fast crystallization kinetics, the organic spacer cations could be uniformly distributed, successfully yielding phase-pure n = 2 2D-RPP crystals. Moreover, when the rapid crystallization method was utilized, randomly oriented perovskite crystals were acquired, facilitating the movement of charge carriers inside the perovskite film. Owing to the enhanced charge transport property, the maximum EQE of deep-blue PeLEDs reached 0.63% with an emission wavelength centered at 437 nm. Prolonged stability of the unencapsulated PeLEDs was also confirmed with negligibly varying electroluminescence spectra during 5 min of operation.

Antisolvent treatment is paramount in the fabrication of high-efficiency perovskite optoelectronic devices as it affords a high crystallization rate critical for the formation of pin holes-free perovskite films. Although the antisolvent choice determines the domain distribution of quasi-two-dimensional (2D) perovskite, and hence the emission wavelength (blue vs green), as well as its light emission efficiency, few studies have examined it in detail. Herein, the crystallisation dynamics and resulting optoelectronic properties of PBA (phenyl-butyl-ammonium) based quasi 2D perovskites (A’₂A_m-1Pb_mX_3m+1), which are commonly employed to create blue emissive films, are scrutinised for the first time through in-situ photoluminescence (PL) measurements during film formation. The m domain distribution can be tailored by selecting antisolvents with various solubility of PBA cation. Antisolvents with higher PBA solubility promote the formation of smaller bandgap films due to larger m domains and vice versa. This study effectively reveals a route to tailor quasi-2D perovskite optoelectronic properties via antisolvent engineering. Fine-tuning the optoelectronic properties can be done by blending two antisolvents with contrasting PBA cations solubility. By doing so, a blue emissive light emitting diode with emission wavelength ranging from 471 to 509 nm can be fabricated with external quantum efficiency of 2.9% at 471nm.

Elucidating the origin of spin-polarized electronic states of 2D atomic layer crystals (ALCs) formed on solid surfaces is one of the hottest topics in both fundamental science and applications. So far, the Rashba-Bychkov (RB) effect, which arises from the combination of potential gradient induced by broken inversion symmetry and spin-orbit coupling, was believed to be the main origin of the spin-polarized states, though the spin textures of most ALCs are different from that expected by the ideal RB effect. In this talk, I will first introduce that various spin textures, ranging from the RB-type to those that cannot be explained based on the origins proposed so far, can be simply induced by the orbital angular momentum. And then, I will show how the solid surface can reveal the hidden spins of a bilayer ALC. This talk aims to provide an overview on the insights gained on the spin-polarized electronic states of ALCs and to point out opportunities for exploring exotic physical properties when combining spin and other physics, such as superconductivity, because the two ALCs, which will be presented in this talk, become superconducting at low temperature. Furthermore, the results presented in this talk will also open a new avenue to realize future spintronics-based quantum devices.

A closely spaced but electronically isolated electronic double-layer system is fascinating to study interlayer quasiparticle interactions and to reveal intriguing interlayer correlated states. Recent progresses in the development of graphene and other two-dimensional (2D) electronic systems have sparked renewed interest in the field of strong interlayer interactions and corresponding novel quantum phases. In particular, the highly tunable electronic properties of constituent 2D layers, together with the accessibility of ultra-small interlayer separation, enable the investigation of the drag effect in previously inaccessible strong-coupling regimes. In this talk, I will present our recent work on the interlayer drag experiments in several graphene-based electronic double-layer systems, including: 1) Revealing a fingerprint feature of drag effect between massless and massive fermions in heterostructures consisting of monolayer graphene and bilayer graphene separated by hBN spacer [1]. 2) Discovery of a new type of quantum interference effect in inter-layer Coulomb drag, with the interference pathway comprising different carrier diffusion paths across the two constituent graphene layers [2]. 3) Demonstration of a giant and highly-tunable drag effect between graphene and superconducting LaAlO₃/SrTiO₃ heterointerface, and a brand-new Josephson-Coulomb drag mechanism is proposed to account for such effect, rooting in the interactions between the substantially enhanced dynamical quantum fluctuations of the superconducting phases in Josephson junction arrays and the normal electrons [3]. Our findings establish a novel platform, i.e., electronic double-layer systems, to exploit novel inter-layer quantum effects, and offer unforeseen opportunities for new-principle electronic devices.
References:
[1] L. Zhu et al., Nano Lett. 20, 1396 (2020).
[2] L. Zhu et al., Submitted.
[3] R. Tao et al., Nature Phys. (2023). https://doi.org/10.1038/s41567-022-01902-7.

Selective C-H bond activation is one of the most challenging topics for organic reactions. The difficulties arise not only from the high C-H bond dissociation enthalpies but also the existence of multiple equivalent/quasi-equivalent reaction sites in organic molecules. Here, we successfully achieve the selective activation of four quasi-equivalent C-H bonds in a specially designed nitrogen-containing polycyclic hydrocarbon (N-PH), which is confirmed by sequential-annealing experiments of N-PH/Ag(100) using scanning tunneling microscopy and non-contact atomic force microscopy. Further annealing leads to the formation of N-doped graphene nanoribbons with partial corannulene motifs, realized by the C-H bond activation process. Density functional theory calculations reveal that the adsorption of N-PH on Ag(100) differentiates the activity of the four ortho C(sp³) atoms in the N-heterocycles into two groups, which leads to a selective dehydrogenation. Our work provides a route of designing precursor molecules with ortho C(sp³) atom in an N-heterocycle to realize surface-induced selective dehydrogenation in quasi-equivalent sites.

In this study, we used nitrogen-based self-assembled monolayers (SAMs) with three different triazole derivative molecules (Benzotriazole, 1-Dodecyl-3,5-diamino-1,2,4-triazole, and Guanosine) to deposit on the copper. In comparison to the traditional SAM head group, nitrogen-based SAMs can only deposit on copper but not on the low-dielectric substrate to achieve selective deposition. According to electrochemical impedance spectrum film characterization, its selectivity could reach 90%, 85%, and 78%, respectively. The enhanced selectivity could be due to the compact film on the copper, as determined by in situ X-ray photoelectron spectroscopy characterization. In addition, we used molecule simulation with the density functional theory (DFT) method to demonstrate that this selectivity is related to the SAM molecule property. This passivation technology enables selective deposition on substrates with nanoscale precision, enabling bottom-up material fabrication for various semiconductor applications.

Two-dimensional (2D) semiconductors such as molybdenum disulfide (MoS₂) could potentially replace silicon in future electronic devices. However, the low carrier mobility in the 2D MoS₂ at room temperature and its inferior interface with high-k dielectrics, remain critical challenges for high-performance electronics. In this talk, we show that by introducing rippled lattice structure in 2D MoS₂, a record-high carrier mobility can be achieved at room temperature, due to the increased intrinsic dielectric constant and much suppressed phonon scattering. For the interface between conventional high-k dielectrics and 2D MoS₂, we find that hydrogenation is a desired approach to passivate the dangling bonds and improve the interface properties, in which the hydrogenation can selectively occur at high-k dielectrics such as Si₃N₄ and HfO₂, and do not affect the 2D semiconductor MoS₂. Finally, we report a data-driven approach to accelerate the development of various promising inorganic molecular crystals as the high-performance high-k dielectrics for 2D MoS₂ based electronic devices. These results deepen the understanding of the carrier mobility in 2D semiconductors and their interface with high-k dielectrics, and could be useful for developing a broad range of high-performance 2D electronic and optoelectronic devices.

Neuromorphic computing, as a brain-inspired technology, provides us with a novel system architecture and computing mode for developing high-level intelligent chips. Also, it is expected to solve the “unsustainable development” issue of current AI hardware in terms of energy consumption and computing power. Memristors, as emerging devices, can emulate the related functions of neurons or synapses according to different time dynamics, providing a novel physical basis for building high-efficient neuromorphic chips. In this talk, I will focus on our recent progress on memristor-based neuron devices, the neuron circuits implementation, and their system applications. First, I will discuss emerging neuron devices and development states, and introduce our research progress on NbO_x-based neuron devices. Then, I will present our research on building artificial neurons with NbO_x-based neuron devices and describe their applications in neuromorphic systems. At last, to meet the requirement of edge intelligent sensing application, we also carried out research on neuron devices for sensing signal processing, providing a promising solution for implementing neuromorphic sensing systems. Finally, the prospects and challenges are given from the view of the device and system applications.

With the advantages of high parallelism, flexibility, and energy efficiency, the brain-inspired computing paradigm has received a lot of attention in recent years. Artificial synapses and neurons are the fundamental building blocks of brain-inspired computing systems and play a critical role in the implementation of system functions. Therefore, the development of artificial synapses and neurons with simple structures and rich functions is one of the research priorities of brain-Inspired electronics. The functional realization of biological neural systems depends mainly on ion transportation processes. The artificial synapses and neurons based on conventional CMOS technology have a low degree of structural bionic and cannot effectively emulate various ion transportation processes. As a result, conventional CMOS synapses and neurons often have complex structures and high hardware overheads, making it difficult to meet the demand for integration density of brain-like computing systems. Emerging nanoionic devices such as memristors and ion-gated transistors, which are rich in internal ion transportation processes, have unique advantages in realizing neuromorphic computing functionalities. In this paper, we reviewed our recent work on nanoionic synaptic devices and neuronal devices, highlighting their potential for emerging brain-inspired electronics.

The brain-inspired neuromorphic computing system has attracted interest because of its advantages of high efficiency, good tolerance, and the ability to implement cognitive functions. The hardware of neuromorphic computation is founded on the emulation of a biological synapse with inherent learning functions. Metal oxide based memristors are widely used to mimic the synapse owing to its functional resemblance to the biological counterpart. The faithful replication of synaptic functions is one vital routine in achieving a high-fidelity memristor-based neuromorphic computation. In this report, we would like to introduce our recent works on the memristor based artificial synapses. For instance, the Bienenstock-Cooper-Munro (BCM) learning rule, as a typical case of spikerate-dependent plasticity, is mimicked using a generalized triplet-spike-timing-dependent plasticity scheme in a WO_x memristive synapse [1]. We also developed a new type of moisture-powered memristor, having a structure of W/WO_x/oxygen-plasma-treated amorphous-carbon (OAC)/Pt [2]. Exploration of optoelectronic memristors with the capability to combine sensing and processing functions is required to promote development of efficient neuromorphic vision. The authors develop a plasmonic optoelectronic memristor that relies on the effects of localized surface plasmon resonance (LSPR) and optical excitation in an Ag–TiO₂ nanocomposite film. [3] Fully light-induced synaptic plasticity under visible and ultraviolet light stimulations is demonstrated, which enables the functional combination of visual sensing and low-level image pre-processing (including contrast enhancement and noise reduction) in a single device. References:[1] Z.Q. Wang and T.Zeng et al. Nature Comm., 2020, 11:1510. [2] Y. Tao, Z.Q. Wang, et al. Nano Energy, 2020, 71, 104628.[3] X. Y. Shan, Z.Q. Wang, et al. Adv. Sci. 2022, 9, 2104632.

The visual perception system is the most important system for human learning since it receives over 80% of the learning information from the outside world. With the exponential growth of artificial intelligence technology, there is a pressing need for high-energy and area-efficiency visual perception systems capable of processing efficiently the received natural information. Currently, memristors with their elaborate dynamics, excellent scalability, and information (e.g., visual, pressure, sound, etc.) perception ability exhibit tremendous potential for the application of visual perception. Here, we propose a fully memristor-based artificial visual perception nervous system (AVPNS) which consists of an optoelectronic memristor, a threshold switching (TS) memristor and an electrochemical actuator. We use a photoelectric and a TS memristor to implement the synapse and leaky integrate-and-fire (LIF) neuron functions, respectively. With the proposed AVPNS we successfully demonstrate the biological image perception, integration and fire. Furthermore, AVPNS can simulate the eye muscle contraction and reproducing the self-protection response of closing eyes when the human eyes are injured by intense light. Light absorption and charge carrier extraction are advantages of optoelectronic memristors. The system achieves a fast response speed and a large response current of up to 40 µs and 0.8 µA. Artificial vision systems offer a potential technique for bionanotechnology, particularly in the domain of artificial intelligence simulation of biosensor systems.

The brain-inspired neuromorphic computing system has attracted great attention as innovative technology owing to its ability to perform intelligent and energy-efficient computation. Analog-type memristors with gradual and linear conductance modulation are considered to be leading candidates for bio-realistic synapse emulation in hardware implementation of neuromorphic computing. However, the nonlinear weight updating property of analog-type memristors makes it difficult to be trained efficiently in a neuromorphic system due to the uncontrollable filamentary morphology. In this work, we report the engineering and optimization of the conductance linearity in Hf_0.5Zr_0.5O₂ (HZO) based analog-type memristive synapse by insertion of a Cu nanoparticle (NP) layer. When Cu NP is added to the HZO film under bias voltage, the Cu²⁺ ions are expected to replace Zr⁴⁺ ions because the ionic radius of a Cu²⁺ ion (0.73 Å) is similar to that of a Zr⁴⁺ ion (0.72 Å). Moreover, a large number of oxygen vacancies (Vo²⁺) are expected to be formed and accumulated around the Cu NPs to maintain charge neutrality in the HZO film, which guide conductive filaments (CFs) to grow along Cu NPs due to the electric field localization effect of Cu NPs. Diverse synaptic functions, including excitatory postsynaptic current (EPSC), paired-pulse facilitation (PPF) and spike-timing-dependent plasticity (STDP), are faithfully emulated. In addition, taking advantage of good-linearity conductance modulation capacity of the HZO based memristor, decimal operation and high-accuracy artificial neural network (ANN) are successfully realized. This work provides a new approach to develop high-linearity and bio-realistic artificial synapses for high-accuracy neuromorphic computing systems.

Conductive filaments (CFs) play a critical role in the mechanism of resistive random-access memory (ReRAM) devices. However, in situ detection and visualization of the precise location of CFs are still key challenges. Herein, we demonstrate for the first time, the use of an organic π-conjugated molecule with phase transformation behavior for the positioning and visualizing of the CFs in the ReRAM devices. The π- conjugated molecule (called TT molecule) has the capacity to transform its molecular shape between twisted and planner states under varying temperatures, which can be translated into corresponding adaptations in crystallization, aggregation behavior, and optical properties. Hence, based on localized Joule heating generated within filament regions, the π- conjugated molecule exhibited reversible optical transformation and thus reflecting the location of the underlying CF. Customized patterns of CFs were induced and observed by the π-conjugated molecule layer, which confirmed the hypothesis. Additionally, statistical studies on filaments distribution were conducted to study the effect of device sizes and bottom electrode heights, which serves to enhance the understanding of switching behavior and their variability at device level. Therefore, this phase transformation molecule and such approach have great potential in aiding the development of ReRAM technology.

Lead-free perovskite (LFP) scintillators are a rapidly developing area of research, with potential applications in medical X-ray imaging and gamma ray detection. The stochiometric composition of LFP scintillators can be varied in many ways, however these materials all generally benefit from broad ‘white’ spectral emission, high light yields, and a large Stoke’s shift. Here we present our latest results on two examples of In-based double LFP scintillators, which have the form A₂MInX₆ where A is an alkali metal of either Cs or Rb; M is Na, K or Ag, or a mixture; and X is a halide. We have studied the bismuth-doped material Cs₂Ag_xNa_1−xBi_yIn_1−yCl₆ (referred to as CANBIC), using a mechanical synthesis method to produce large masses of both bulk crystals and nanoparticles. This material gives an intense broad ‘white’ emission centred at ~590 nm, and we will discuss the radioluminescent light yields and the relatively slow (microsecond) luminescent decay times observed in this material. High resolution TEM imaging is also used to investigate the atomic composition and structure. We will also present results from an alternative class of Rb-based LFP scintillator, Rb₂AgX₃, where X is either Br or Cl. These materials also offer high light yields, with a broad ‘white’ emission centred at 515nm and 580nm, respectively. The Rb-based LFP scintillators show a fast radiative recombination from self-trapped excitons, with typical decay times of 10ns. The performance of these two classes of LFP scintillators, and their suitability for X-ray imaging applications, will be compared.

X-ray imaging scintillators are very essential for many different technologies that impact our daily lives, including medical radiography, computed tomography, food industry, high energy physics and security screening because of their capability to efficiently convert high-energy ionizing irradiation into low-energy visible photons. Organic and organometallic have emerged as potential materials for X-ray imaging technologies due to their low toxicity, high stability, good mechanical flexibility, and low-cost fabrication. Nevertheless, their low X-ray absorption cross-section and inefficient exciton utilization efficiency greatly limit their potential application and their possible future commercialization. However, these drawbacks can potentially be overcome by appropriate energy funneling designs. With this unique approach, X-ray sensitivity and imaging resolution of organic and organometallic scintillators can be substantially improved by an efficient energy transfer from appropriate X-ray sensitizers. In this talk, I will summarize the recent progress in high-performance energy transfer-based scintillators using a variety of X-ray sensitizers including ceramic, perovskite, organometallic, organic emitters and metal organic frameworks. In detail, possible energy transfer processes between different X-ray sensitizers and organic/organometallic scintillation materials will be presented and discussed, providing an outlook on the potential of other energy transfer mechanisms and materials systems as new alternatives for the fabrication of high-performance scintillators beyond perovskites and ceramics. Moreover, an in-depth analysis of the corresponding energy transfer mechanisms that could serve as a profound reference in understanding the effect of intermolecular interactions on the efficiency of energy transfer through various steady-state and ultrafast time-resolved spectroscopy, as well as theoretical calculations will also be presented and discussed.

Development X-ray imaging with low dose, high resolution and high dynamic range is of great significance in both industrial and medical applications. We first established a physical picture of X-ray excited photophysics and studied the differences and similarities among X-ray, visible light, and electrical excitations. Based on it, we proposed a design principle for highly emissive and fast X-ray scintillators, and particularly emphasized two promising mechanism: self-trapped excition emission and thermally activated delayed folurescence. The bright triplet state is also one of the important reasons why many metal halide scintillators can obtain high efficiency. We have developed a series of bismuth based and copper based metal halide scintillators, developed a preparation process to reduce optical scattering, enhance light transmission and irradiation stability, and obtained 20lp/mm X-ray imaging resolution and stable light output under long-term irradiation. Finally, a new stacked X-ray detector structure is proposed, which can realize multi energy spectrum and large array X-ray imaging. This is a new function that the current flat X-ray imaging system does not have, and can greatly enhance the material resolution of flat X-ray imaging.

Three-dimensional metal halide perovskites single crystals have been considered as promising candidates for next-generation X-ray detectors due to the advantages of high carrier collection efficiency, unique defect-tolerant nature and large X-ray absorption cross section. In particular, the detection performance of X-ray detectors based on perovskite single crystals designed with methylammonium (MA) as A-site cation has been continuously breakthroughs. Although the stability of the device can be improved by advanced packaging processes, the volatile MA cations still pose stability hazards and limit the long-term reliability of perovskite X-ray detectors. To improve the stability, it is attractive to shift from MA to caesium (Cs) and formamidinium (FA). Recently, the mixed-cation/halide Cs_xFA_1-xPb(I_1-yBr_y)₃ with optimal tolerance coefficient and structural stability helps enhance stability, but mixed cations and halides bring new challenges to the dynamic control of uniform crystal growth, which usually causes the presence of crystal imperfections, including dislocations and vacancies. Here we address these challenges and develop high-performance perovskite X-ray detectors with excellent stability. We make use of the strong bonding between guanidinium (GA) and iodide, and decrease the halide vacancies by incorporating GA into stable Cs-FA iodide-based perovskites. The detrimental tensile strain caused by oversized GA incorporation leads to unfavourable relaxation via Pb vacancies and hence inhomogeneous stress. We introduce a low concentration of strontium (Sr) to the B site to increase the energetic cost associated with the formation of Pb vacancies, prohibiting this unfavourable channel for strain relaxation. With synergistic strain engineering on both A and B sites, our stable MA-free perovskite single crystals show excellent optoelectronic properties, enabling a high-performance X-ray detector with ultralow LoD of 7.09 nGy_air s^-1 and high sensitivity of (2.6 ± 0.1) × 10⁴ μC Gy_air^-1 cm^-2 at 1 V cm^-1 with over 180 days stability.

The rapidly expanding market for lithium-ion batteries for portable electronics, electric vehicles, and grid storage has generated much interest to increase the energy density and lower the cost. In this regard, reducing or eliminating altogether cobalt in layered oxide cathodes by increasing the nickel content has become an intensive focus in recent years. Nickel offers a higher charge storage capacity than cobalt as the Ni^3+/4+:3d band barely touches the top of the O^2-:2p band unlike the Co^3+/4+:3d band that overlaps with the top of the O^2-:2p band. However, cathodes with Ni contents > 80% are encountered with severe capacity fade and safety concerns. This presentation focuses on an in-depth assessment of the degradation mechanisms and pathways of high-nickel layered oxide cathodes with Ni contents of > 80 %, particularly the end member LiNiO₂ with 100% Ni. The cathodes with secondary particle sizes of ~ 12 microns are synthesized through a coprecipitation of the hydroxide precursors with a tank reactor, followed by calcining at optimal temperatures under oxygen gas flow. The synthesized cathodes are evaluated in coin cells and pouch full cells with both graphite and lithium-metal anodes with different electrolytes. The cathodes and anodes before and after thousands of deep cycles are characterized with a suite of analytical techniques, such as X-ray diffraction, X-ray photoelectron spectroscopy, time-of-flight – secondary ion mass spectrometry, and high-resolution transmission electron microscopy to delineate the dominant degradation mechanisms, particularly crack vs. surface reactivity. Furthermore, the effect of cathode composition and electrolyte on gas evolution is evaluated with online electrochemical mass spectrometry.

LiCoO₂ (LCO) has a layered rocksalt structure with a space group of R-3m and is normally synthesized via conventional solid-state reaction via a high-temperature calcination process ≥800 °C. The LCO also has another polymorph, having a spinel framework with a space group of Fd-3m and synthesized at lower temperature ≈ 400 °C. Therefore, the layered LCO is traditionally called a high-temperature (HT-) phase, while the spinel LCO is referred to as a low-temperature (LT-) phase. However, a first principles studies predicted that the spinel LCO is a metastable phase at all the temperature ranges. Here we developed a new synthesis procedure to obtain a highly crystalline layered LCO at a low temperature of ≤300 °C. We traced the structural evolution of the precursor materials during the synthesis process to understand the reaction pathway and the crystal growth process. The observed reaction pathway suggests that the water molecules accelerate the crystal growth of the layered LCO in the molten hydroxides. The kinetically enhanced crystal growth of the layered LCO typically completes within 30 minutes, at 300 °C. Our work here experimentally proves that the layered LCO is a thermodynamically stable phase even at low temperatures, as predicted in the theoretical studies.

The spinel LiNi_0.5Mn_1.5O₄ (LNMO) is one of the most promising candidates as a cobalt-free positive electrode material. Its theoretical capacity of 147 mAh.g^-1 obtained by oxidation of Ni²⁺ to Ni⁴⁺ at an average potential of approximately 4.7 V vs. Li/Li⁺ leads to a high energy density of 650 Wh/kg. However, several characteristics affect this material's performance and stability, such as the stoichiometry and the morphology of the primary particles. Additionally, Ni and Mn can occupy either a single crystallographic site or two different ones, forming two polymorphs with cubic symmetry called disordered and ordered structures, respectively. Even though averaged information can be obtained about the extent of the transition metal order by diffraction or spectroscopy techniques, the local information on the process of ordering within the crystal and its homogeneity at the particle scale are still unknown. In our study, we used molten salt synthesis to control LNMO particles' morphology (platelets versus octahedra) and thickness by changing the annealing conditions while keeping the transition metal disorder in the crystal structure. Moreover, a post-annealing step allowed us to tailor the extent of ordering between Ni and Mn while maintaining the thin platelet morphology. Indeed, various Li environments were observed by neutron powder diffraction and spectroscopies (Raman and nuclear magnetic resonance (NMR)), whereas a single Li site is expected for a fully ordered LNMO structure. These samples being transparent to electrons due to their morphology were analyzed by scanning transmission electron microscopy coupled with precession (ASTAR System - Nanomegas). We collected electron diffraction data for each pixel of the acquired images (4D-STEM). This technique gave in-depth information on the homogeneity of the transition metal order at the particle scale, with a spatial resolution of 10 nm, and allowed to propose the mechanism involved throughout the post-annealing process.

The resource pressure on the Li-ion industry requires urgent attention to new cathode materials that do not use Ni or Co. In a departure from classic well-ordered compounds, compounds with random cation configurations were recently shown to display very high reversible Li-storage capacity, and can be synthesized with almost any metal in the periodic table. In this presentation, I will explore the space between fully disordered and fully ordered compounds. Partially disordered compounds are materials with long-range order, but with a very high level if disorder introduced, either by a modified synthesis approach or by chemo-mechanical ball milling. We have identified several such partially disordered materials for which the reversible capacity and rate capability is significantly increased through disorder. Partially disordered spinel (PDS) is a Mn-based oxy-fluoride spinel with significant 16c/16d disorder and an excess of cations. At a critical level of disorder, the typical 3V plateau that reflects the two-phase region present in regular spinel, can be removed and replaced by a solid-solution region. This removes the inhomogeneous strains present in a normal spinel and enables stable cycling in the 3V regime, resulting in a material with specific energy > 1000Wh/kg and excellent rate capability. I will discuss the microscopic mechanism by which the two-phase region turns into a solid solution and the atomistic origin of the higher rate capability. A similar concept can be used to improve the electrochemical performance of layered compounds which have been previously overlooked as cathode materials. I will show that by applying the proper amount of cation disorder, Cr-Mn-based layered materials can be turned into high rate, high energy density cathode materials.

In order to investigate the effect of the particle morphology on the fast-charging characteristics of high-nickel layered-cathode materials, single-crystalline and secondary-particle type cathode with different primary/secondary particle sizes were synthesized via careful microstructure design. The effect of crystalline and particle size on the fast-charging performance was identified by comparing the charging characteristics at various current rates. Also, the effect of rapid charging on the cycle-life performance of the high-nickel cathode materials was confirmed through repeated fast-charging experiment. The size of the primary particle (crystalline size) has a greater effect on the rapid-charging performance than the entire particle size. In addition, to suppress the performance degradation of high-nickel cathode during fast charging, it is important to suppress the increase in the surface area through micro-crack suppression, and it is also necessary to precisely control the size of the primary particle for efficient lithium diffusion during fast charging.

Mitigating oxygen release from cathode active materials is critically important for the realization of high-energy-density and robust batteries. However, fundamental knowledge of oxygen release was not well understood so far. The purpose of this study is to demonstrate the effectiveness of conventional evaluation techniques for oxygen defect formation for the direct investigation of oxygen release from cathode active materials. For this purpose, oxygen release of Li(Ni,Co,Mn)O₂ is evaluated by thermogravimetry and coulometric titration which enable us to investigate oxygen defect formation in the target cathode active material, and changes of crystal structure and electronic structure due to oxygen release are evaluated by X-ray diffraction and X-ray absorption spectroscopy (XAS). The oxygen composition of Li(Ni,Co,Mn)O_2-δ can reach 1.9 under low P(O₂) condition from the stoichiometric oxygen composition, meaning that Li(Ni,Co,Mn)O₂ can release lattice oxygen until 5 mol% of oxygen loss without reduction decomposition. XAS revealed that selective reduction of high-valent Ni like Ni³⁺ is proceeded in the initial stage of oxygen release, followed by the selective reduction of Co³⁺, and Mn reduction is negligibly small. The observed oxygen release behavior is analyzed by defect chemistry and thermodynamics. Through defect chemical analysis and charge compensation during oxygen release, it was revealed that high-valent Ni drastically destabilizes lattice oxygen of Li(Ni,Co,Mn)O₂.

Open surface droplet microfluidic platform has been demonstrated to be a promising technology in the applications of analytical chemistry, clinical diagnosis, epidemiology, environment monitor, etc. The key success of this technology relies on precise and flexible droplet manipulation. In recent, the use of light-fluid interaction for droplet manipulation has received extensive attention owing to its unique features of contactless and flexible control, high reconfigurability, easy parallel manipulation, high-spatiotemporal resolution, and rapid response. One of the light-fluid interactions is the localized photothermal effect, whose incorporation into open surface droplet microfluidics creates the localized photothermal effect based open surface droplet manipulation platform. In this platform, the droplet evaporation induced by the localized photothermal effect along with the accompanied interfacial behaviors can realize various functions. Therefore, this presentation will summarize recent progress on the droplet evaporation and interfacial phenomena induced by the photothermal effect of a focused laser beam in our group. The presented results are expected to be useful for future applications in the open surface droplet microfluidics based on the localized photothermal effect.

Decorating single atoms of transition metals on MXenes to enhance the electrocatalytic properties of the resulting composites is a useful strategy for developing efficient electrocatalysts, and the mechanisms behind this enhancement are under intense scrutiny. We anchored Co single atoms onto several commonly used MXene substrates (V₂CT_x, Nb₂CT_x and Ti₃C₂T_x) and systematically studied the electrocatalytic behavior and the mechanisms of oxygen and hydrogen evolution reactions (OER and HER, respectively) of the resulting composites. Co@V₂CT_x composite displays an OER overpotential of 242 mV and an HER overpotential of 35 mV at 10 mA cm^-2 in 1.0 M KOH electrolyte, which is much lower than for Co@Nb₂CT_x and Co@Ti₃C₂T_x, making it comparable to the commercial noble metal Pt/C and RuO₂/C electrocatalysts. The experimental and theoretical results point out that the enhanced bifunctional catalytic performance of Co@V₂CT_x benefits from the stronger hybridization between Co 3d and surface terminated O 2p orbitals which optimized the electronic structure of Co single atoms in the composite. This, in turn, results in lowering the OER and HER energy barriers and acceleration of the catalytic kinetics in case of the Co@V₂CT_x composite. The advantage of Co@V₂CT_x was further validated by its high overall water splitting performance (1.60 V to deliver 10 mA cm^-2). Our study sheds light on the origins of the catalytic activity of single transition metals atoms on MXene substrates, and provides guidelines for designing efficient bifunctional MXene-based electrocatalysts. Reference: Zhao, X. Zheng, Q. Lu, Y. Li, F. Xiao, B. Tang, S. Wang, D. Y. W. Yu, A. L. Rogach. Electrocatalytic Enhancement Mechanism of Cobalt Single Atoms Anchored on Different MXene Substrates in Oxygen and Hydrogen Evolution Reactions. EcoMat. 2023, 5, e12293.

Developing structure-sensitive electrocatalysts with Pt-rich surfaces consisting of oxophilic metals (Rh) and conductive (carbon) support is vital to boost the electro-oxidation of alcohols. Pt₃Rh-Co₃O₄ ternary alloy nanoparticles of ~2–3 nm were uniformly distributed on carbon (Pt₃Rh-Co₃O₄/C) thru one-pot chemical reduction method. The chemical inertness and oxophilicity of Rh and the abundant oxygen defects of Co₃O₄ improved the kinetics of the methanol oxidation reaction (MOR), ethanol oxidation reaction (EOR), oxygen evolution reaction (OER) and hydrogen evolution reaction (HER) in Pt₃Rh-Co₃O₄/C by promoting the oxidative removal of surface adsorbed species. Pt₃Rh-Co₃O₄/C exhibits high intrinsic activity for both MOR (6.8 mA/cm²_Pt) and EOR (3.17 mA/cm²_Pt) due to strong electronic, ligand, and bifunctional effects. After 7000 potential cycles (PCs), it retains 81% (MOR) and 84% (EOR) of its initial value, indicating extended stability. In the case of OER and HER, the Pt₃Rh-Co₃O₄ has low overpotential of 290 and 55 mV at 10 mA/cm² respectively, which is relatively much lesser than Rh-free catalyst and commercial Pt/C (for HER) and IrO₂ (for OER) catalysts. The durability of the catalysts shows slight shift in the overpotential after 5000 PCs for both OER (+21 mV) and HER (-5 mV) signifies the extended performance in acidic medium. Compared to other Pt-based benchmark catalysts, Pt₃Rh-Co₃O₄/C demonstrates higher CO tolerance, synergistic effect, and good electron conductivity results in extended activity, and stability, corroborate as a potential and competing electrocatalyst for alcohol electro-oxidation and water-splitting reactions.

As the atmospheric carbon dioxide (CO₂) level keeps hitting the new record, humanity is facing an ever-daunting challenge to efficiently mitigate CO₂ from the atmosphere. Though electrochemical CO₂ reduction presents a promising pathway to convert CO₂ to valuable fuels and chemicals, the general lack of suitable electrocatalysts with high activity and selectivity severely constrains this approach. Herein, we investigate a novel class of electrocatalysts, the quasi-Cu-mers, in which the CuN₄ rather than Cu atom itself serve as the basic building block. We first synthesized the respective quasi-Cu-monomers, -dimers and -trimers hosted in a graphene-like substrate and then performed both experimental characterization and density functional theory (DFT) calculations to examine their atomic structures, evaluate their electrocatalytical performance and understand their underlying mechanisms. Our experimental results show that the quasi-Cu-trimers not only outperform the quasi-Cu-dimer and quasi-Cu-monomer when catalyzing CO₂ to CO, they also show a superior selectivity against the competing hydrogen evolution reaction (HER). Our DFT calculations not only support the experimental observations, but also reveal the volcano curve and the physical origin for the qausi-Cu-trimer superiority. The present work thus presents a new strategy in design of high-performance electrocatalysts with high activity and selectivity.

Considerable progress has been made in ligand engineering on electronic structures of gold nanoclusters (NCs) for effective electrocatalytic carbon dioxides reduction reaction (CO₂RR). Still, a molecular-level understanding of ligand-mediated microenvironment near the catalytic interface remains intensively elusive. Herein, we present an atomically precise bi-ligand capped Au₂₅ NC as a model and collocate the number and position in the motif of the involved ligand containing the pyrimidine ring. Such a ligand capable of absorption on CO₂ molecules endows gold NCs with a high Faradaic efficiency of 98.6% and Turnover frequency (TOF) with 39 s^-1 for CO production at -0.9 V. The combined experimental measurements and theoretical study reveal the pyrimidine-containing ligand endows gold NCs with the capability of CO₂ adsorption near the active sites, which accelerates the kinetics of CO₂RR. Such ligand introduction creates enzyme-like electroactive pockets on cluster surfaces for effective electrocatalysis. More broadly, this work provides new insight into refined surface chemistry modification on tuning the electrocatalytic microenvironment.

There has been ongoing extensive research on designing and fabricating the low-cost, robust, and highly efficient catalysts for electrocatalysis.[1] In order to improve the intrinsic activity of electrocatalysts, interface engineering has been recognized as a highly effective strategy.[2] However, there is a few works has been reported on defining the number of interfaces for conducting quantitative/semi-quantitative analysis, which is important to investigate the structure-property relationship and thus provide theoretical guidance. In this regard, we will present our recent efforts on the controlled fabrication of nanohybrid catalysts with rich heterointerfaces via a facile selective ion-exchange route, which was realized by utilizing different physicochemical properties of various cations/anions and reasonable post-treatments. Particularly, the number of interfaces can be fined-tuned by adopting different reaction conditions, so that the quantitative/semi-quantitative analysis can be conducted based on the experimental investigations, characterizations and theoretical calculations.[3] The synergy arising from interfacial electron redistribution and multiple active sites substantially helps optimize the surface binding energy of the different intermediates and contribute to the overall activity. As a result, the nanohybrid with rich interfaces exhibit excellent electrocatalytic performance, with high activity, fast kinetics, and long-term durability. This interface engineering strategy can be expanded to advance other interfacial structures, which is believed to afford inspiration in the field of interface engineering for high-performance catalysts. [1] Li Y, Wang H, Priest C, Li S, Xu P, Wu G. Adv. Mater. 2021, 33, 2000381. [2] Li X, Kou Z, Wang J. Small Methods 2021, 5, 2001010. [3] Li X, Kou Z, Xi S, Zang W, Yang T, Zhang L, Wang, J. Nano Energy, 2020, 78, 105230.

Inorganic lead halide perovskite nanocrystals (NCs) are an exciting class of luminescent materials with high defect tolerance and broad spectral tunability, but such NCs are vulnerable to degradation under ambient conditions. A class of modular zwitterion-functionalized isopropyl methacrylate polymers designed to stabilize a wide variety of perovskite NCs of different compositions are presented, which also enable processing in green solvents. Specifically, we report polymers in which the zwitterion spacing is tuned to accommodate the different lattice parameters of CsPb(Cl_1–xBr_x)₃ and CsPbI₃ NCs, and we report partially fluorinated polymers prepared to accommodate the needs of infrared-emitting NCs. We show that as-synthesized CsPbBr₃, CsPbI₃, and Yb³⁺:CsPbCl₃ NCs are easily transferred into these zwitterionic polymers via a simple ligand-exchange procedure. These NC/polymer composites were then cast into thin films that showed substantially improved photoluminescence (PL) and stability compared with more conventional NC/polymer films. Specifically, CsPbBr₃ and CsPbI₃ NCs in films of their appropriately designed polymers had PL quantum yields of ∼90% and ∼80%, respectively. PL quantum yields decreased under continuous illumination but self-healed completely after dark storage. We also found that all the NC compositions studied here maintain their PL quantum yields in NC/polymer composite films even after 1 year of ambient storage. These encouraging results demonstrate the utility of such modular zwitterion-functionalized polymers for hosting specific perovskite NCs, potentially opening avenues for robust new photonic applications of this important class of NCs.

The quasi-Fermi level splitting energy (qFLS, photovoltage) of photocatalysts and photoelectrodes corresponds to the free energy content available for conversion into electrical power and solar fuels. Standard electrochemical scans cannot provide this information, due to the lack of a direct electrical connection to the solid-liquid interface. Here we show that surface photovoltage (SPV) spectroscopy observes the qFLS in a semi-contactless way. For the measurement, the semiconductor films (BiVO₄, CuGa₃Se₅) or wafers (n-GaP or p-GaP) are immersed in aqueous electrolytes and placed underneath a vibrating Kelvin probe. We find that for fast redox couples (hydrogen peroxide, sulfite) the SPV signal under illumination agrees well with the photovoltage obtained separately with open circuit voltage and photoelectrochemical scans. This confirms that the SPV signal equals the qFLS in these cases. The ability of measuring the qFLS of solid-liquid junctions with SPV will be helpful for the identification of semiconductor-liquid junctions with optimized solar energy conversion properties.

Recently, lead halide perovskites have attracted significant attention as promising absorber materials for photoelectrochemical (PEC) solar water splitting. However, charge-accumulation-induced ion migration at the interface causes perovskite degradation and efficiency loss. To suppress the charge accumulation and improve the PEC performance of the perovskite photoanode, we propose a simple interface engineering by decorating the SnO₂/perovskite interface with a mixture of polyethylenimine ethoxylated (PEIE) and chlorobenzenesulfonic acid (CBSA). The PEIE and CBSA acts as the dipole layer and conductive passivator, respectively. The mixed CBSA+PEIE treatment effectively passivates the oxygen vacancies in SnO₂ and adjusts the band alignment between SnO₂ and the perovskite. The synergistic effects of the mixture treatment facilitate an effective carrier extraction at the SnO₂/perovskite interface, enhance the PEC performance, and improve the stability of the device. The resulting interface-engineered perovskite photoanode exhibits an onset potential of 0.5 V versus the reversible hydrogen electrode (RHE), and a saturated photocurrent density of 20 mA cm^-2 at 1.23 V_RHE with stable operation.

Over the last decade, perovskite solar cell (PSC) has emerged as a promising future-generation solar cell technology with a power conversion efficiency (PCE) approaching 26%. However, the long-term stability of PSC remains a barrier to its commercialization. Different classes of defect passivators have been investigated to improve the stability of the PSC. Among them, Lewis base-based passivators are particularly effective owing to their ability to passivate various Pb²⁺ and Pb^0-related defects. Due to their higher softness, sulfur-based materials have been found to be especially effective for the passivation of interfacial defects. However, a limited amount of work has been carried out in the domain of inorganic sulfur-based interface passivators. This work explores the interface passivation and stability-enhancing ability of different sulfur-based inorganic hole transport layers (HTL) in the inverted architecture PSC. In our work, we have investigated several different sulfur-based hole transport layers. We observe that all of our sulfur-based HTLs provide better stability than NiO-based control devices. Among several different sulfur-based HTLs also, a significant difference in the device stability was observed, indicating a possible contribution from the cation and the crystal structure of the sulfide compound. The T₈₀ lifetime (time at which 80% of the initial efficiency is retained) of the device with our most optimized sulfide-based HTL was found to be more than 650 hours (unencapsulated device at 40% RH and 20 °C). In contrast, the control NiO-based device had a T₈₀ value of less than 200 hours. This stability difference can be correlated to the slowing down of the PbI₂ formation inside the perovskite matrix. We believe this improvement is due to the better quality interface formation owing to the interaction between Pb and the chalcogenide ions in the HTL-PSC interface.

Using low cost and environmentally friendly materials for solar energy production is a leading approach to combat the current energy crisis. FeS₂ in the pyrite phase has been studied intensively for solar cell applications in the late 80s and beginning of 90s, but the FeS₂ solar cells have never exceeded a power conversion efficiency greater than 3%. We have adopted a novel approach for this rediscovered material using FeS₂ monograin powder crystals as absorber layer in the monograin layer (MGL) solar cell. The MGL solar cell accommodates a monolayer of nearly unisize semiconductor powder crystals as the absorber. The MGL solar cell technology has not been applied for FeS₂ before and offers unique advantages. In the developed MGL technique n-type FeS₂ crystals in contact with p-type nickel oxide buffer layer are used. High quality FeS₂ microcrystals were synthesized from FeS and S in the liquid phase of potassium iodide as flux in sealed quartz ampoules. The synthesis technique used enables to control precisely the synthesis-growth conditions (temperature and sulfur vapor pressure) and therefore also the bulk and surface composition – the most critical property of this material. X-ray diffraction and Raman analyses confirmed the formation of pyrite FeS₂ crystals of very high crystallinity (sharp and narrow Raman peaks at 343, 379, 430 cm^-1). SEM studies revealed crystallites of uniform shape with shiny facets. Near-stoichiometric bulk composition of crystals was determined by energy dispersive and wavelength-dispersive X-ray spectroscopy and surface composition by XPS. Impurities incorporating from KI flux were determined by ICP-MS method. With the aim of analyzing the defect structure of pyrite, the results of excitation power and temperature dependent photoluminescence studies will be presented for the first time. The electrical properties of pyrite will be measured in the photoelectrochemical cell and the pyrite/nickel oxide heterojunction solar cells.

In the last decade, perovskite-based solar cells have dramatically increased in efficiency, while the performance of silicon-based solar cells has stagnated as their efficiency become close to the theoretical limit. However, the hybrid organic-inorganic perovskites (HOIP) which enable the rapid improvement of photovoltaic efficiency suffer from a lack of long-term stability. To overcome the instability which may originate from their organic components, the development of full-inorganic perovskites with comparable performance could be the solution for next-generation solar-cell materials. In this work, we explore all-known ABX3 inorganic compounds from structure databases to search for novel solar harvesting materials. From the inspiration of perovskite structure which has various advantages as a photovoltaic material, we screened and classify the materials which have octahedral-connecting features. Following a sequential screening scheme, we investigate the bandgaps, optical absorbance, and effective masses of screened perovskite-inspired crystal structures. As a result, we suggest several new candidate materials for fully-inorganic photovoltaic absorbers and analyze the specific properties depending on the various structural types that we classified. We expect that this work will provide useful information for discovering and designing new promising materials from the expanded material pool.

In state-of-the-art Perovskite solar cells achieved over 25% power conversion efficiencies through an improvement of charge transport layers by efficient carrier transport without non-radiative recombination. Effective management of charge carriers significantly impacts the fill factor and open-circuit voltage, two critical parameters that determine device efficiency. An ideal electron transport layer (ETL) should possess complete and conformal coverage, as well as optimal band alignment that facilitates efficient extraction of electrons while blocking hole transfer to the ETL. Additionally, low electrical series resistance and minimal defect density in charge transport layers are essential to prevent detrimental interface recombination. To produce high-quality thin films with these ideal characteristics, charge transport layers are deposited using techniques such as sputter and pulsed laser deposition (PLD). These methods offer the advantage of achieving stoichiometric control and the addition of doping materials, overcoming the challenge of solution processing, required to optimize the properties of metal oxide transport layers. In this study, titanium oxynitride (TiO_xN_y) using the PLD method to tailor the carrier density, electron mobility, and conductivity, thereby enhancing the electron transport characteristics from the light absorber layer in solar cells. Controlled substrate temperature and oxygen partial pressure during the deposition facilitated the achievement of optimal thin-film characteristics, including high optical-transparency and high electrical-conductivity. The deposition of the optimal characteristic is achieved by sequentially changing the oxygen partial pressure during the process and grading the nitrogen atomic ratio from the electrode towards the perovskite materials. Titanium oxynitride thin-films with a graded nitrogen atomic ratio and varying oxygen partial pressure, and were characterized using such as atomic force microscopy, X-ray photoelectron spectroscopy, and Hall measurement. The deposited thin-films exhibit high crystallinity, uniformity, and excellent electrical performance, which contributed to their outstanding electron transport functionality in solar cell devices, resulting in an improved power conversion efficiency.

Nanocomposite films hold great promise for multifunctional devices by integrating different functionalities within a single film. The microstructure of the precipitate/secondary phase is an essential element in designing composites’ properties. The interphase strain between the matrix and secondary phase is responsible for strain-mediated functionalities, such as magnetoelectric coupling and ferroelectricity. However, a quantitative microstructure-dependent interphase strain characterization has been scarcely studied. Here, it is demonstrated that the PbTiO₃(PTO)/PbO composite system can be prepared in nano-spherical and nanocolumnar configurations by tuning the misfit strain, confirmed by a three-dimensional reconstructive microscopy technique. With the atomic resolution quantitative microscopy with a depth resolution of a few nanometers, it is discovered that the strained region in PTO is much larger and more uniform in nanocolumnar compared to nano-spherical composites, resulting in much enhanced ferroelectric properties. The interphase strain between PbO and PTO in the nanocolumnar structure leads to a giant c/a ratio of 1.20 (bulk value of 1.06), accompanied by a Ti polarization displacement of 0.48 Å and an effective ferroelectric polarization of 241.7 µC cm^-2, three times compared to the bulk value. The quantitative atomic-scale strain and polarization analysis on the interphase strain provides an important guideline for designing ferroelectric nanocomposites.
[1] M. S. Li et al., Small, 2203201 (2023).
[2] M. S. Li et al., Advanced Functional Materials, 29, 1906655 (2019) 

With the demand of high-performance electronic devices in the future information technology, people are looking for alternative quantum materials to push forward the Moore’s law. Contrary to the traditional semiconductors or 2D materials, complex oxides exhibit abundant quantum phenomena or functionalities due to the coexistence and competition of electron, spin, lattice orbital degrees of freedom. For these materials, reduction of energy cost is usually compensated by the shrinkage of the thickness or dimension. Under this circumstance, the surface structures and states could be crucial to the whole properties and functionalities. However, direct observation of surface quantum states in complex oxides is still challenging. In this presentation, I will share with you our efforts on controlling the surface lattice structures at atomic resolution, and therefore characterizing the selective surface orbital orders in dielectric oxide and emergent electronic states in ferroelectric oxide. I hope our study may provide a little insight on building emergent surface states for future high-efficient energy conversion and electronic devices with a low energy consumption.

The design and fabrication of novel quantum devices in which exotic phenomena arise from moiré physics has sparked a new race of conceptualization and creation of artificial lattice structures. This interest is further extended to the research on thin-film transition metal oxides, with the goal of synthesizing twisted layers of perovskite oxides concurrently revealing moiré landscapes. By utilizing a sacrificial-layer-based approach, we show that such high-quality twisted bi-layer oxide nanomembrane structures can be achieved. We observe atomic-scale distinct moiré patterns directly formed with different twist angles, and the symmetry-inequivalent nanomembranes can be stacked together to constitute new complex moiré configurations. This study paves the way to the construction of higher-order artificial oxide heterostructures based on different materials/symmetries and provides the materials foundation for investigating moiré-related electronic effects in an expanded selection of twisted oxide thin films.

Two-dimensional (2D) semiconductors, such as MoS₂, WS₂, and WSe₂, are promising channel materials for next-generation ultrascaled transistors due to their atomic thicknesses, inert surfaces, and high mobilities. Integration of high-k dielectric layers with 2D semiconductors is a crucial step to fulfil this promise. However, the growth of conventional high-k materials, including HfO₂ and Al₂O₃, on 2D semiconductors faces many challenges. Since the surface of 2D semiconductors is dangling-bond-free, preprocessing, such as surface functionalization or seeding layer growth is required. These additional steps not only degrade the capacitance of the dielectric layers, but also undermine the electronic properties of 2D semiconductors. Recently, we have developed a mechanical approach that exploits van der Waals (vdW) forces to integrate single-crystal strontium titanate (SrTiO₃) with 2D semiconductors. This approach adopted the freestanding oxide technology where crystalline oxides grown on dissolvable, lattice-matched substrates are released and transferred by vdW adhesion. Based on this approach, we achieve high-performance n-type MoS₂ transistors with an on/off ratio exceeding 10⁸ and a subthreshold swing of as low as 66 mV dec⁻¹. Combined with the p-type WSe₂ transistors, we demonstrate low-power complementary metal-oxide-semiconductor (CMOS) inverter circuits with high voltage gains. These results not only proves the feasibility of using single-crystal high-k perovskite oxides as the dielectric material for 2D transistors, but also provide a route to a range of oxide-2D material heterostructures with novel functionalities.

Scaling on-chip interconnects poses unprecedented challenges as node technology advances. Interconnect materials such as copper, when used at smaller sizes, have greater effective resistivity, thus dissipating more heat. Graphene, a quantum material with high thermal conductivity, mobility, and elasticity, is an ideal material for interconnects. The zero-band gap and zero backscattering for normal incidence in graphene limit its ability to exhibit strong confinement for waveguide operationality [1]. Here, we propose a way for localizing carriers in graphene by generating a very strong on-chip magnetic field. In graphene, applied non-uniform strain produces strong pseudo potential and pseudo magnetic gauge fields attributed to the change in bond length [2]. The pseudo magnetic field (PMF) results in the formation of the pseudo–Landau Levels and increases the local density of states in the deformed region. A wave-guided medium is created according to the strain profile by localizing the carriers in the deformed region [3]. We show the wave-guided medium created using the gaussian flat-top and in-plane strain profile confines carriers within the two barriers following the slopes of strain profile. The modulation of confinement-width is achieved using strain value and profile parameters. We show that strain-deformed graphene can be used as a waveguide and it also provides a low dissipation path for high mobility carriers making it suitable for high-frequency applications. The proposed model can also be used for valleytronics applications [4]. The numerical simulation of graphene waveguides is carried out based on the tight binding method and scattering matrix approach [5]. References: 1. Katsnelson, et al. Nature physics 2.9 (2006): 620-625. 2. Guinea, et Nature Physics 6.1 (2010): 30-33.3.Wu, Yong, et al. Nano letters 18.1 (2018): 64-69. 4.S Tapar, B Muralidharan - Bulletin of the American Physical Society, 20225.Groth, et New Journal of Physics 16.6 (2014): 063065.

The nitrogen-vacancy (NV) center is a defect in which two adjacent sites in diamond’s tetrahedral lattice of carbon atoms are replaced. One site contains a nitrogen atom instead of carbon while the other is vacant. In its negatively charged state, the NV center gains an extra electron from the lattice, forming a ground state spin system which can be polarized with 532-nm light, even at room temperature. One of the spin states fluoresces much more brightly than the others so that fluorescence can be used for spin-state readout. At the same time, the NV’s electron spin states are sensitive to magnetic and electric fields. These properties make NVs attractive both as a scalable platform for quantum information systems and for high sensitivity electromagnetic field quantum sensors. An integrated optics platform in diamond is essential for both quantum information systems and quantum sensing, where the NV is used as an optically detecting atomic probe. This is because of the ultimate stability and integration provided by monolithic waveguides, in addition to the potential for enhanced optical interaction with NVs. For the first time, 3D laser microprocessing is combined with ion implantation nanofabrication to exploit the advantages of both techniques to achieve integrated high quality quantum emitters and buried optical waveguides in diamond. Ion implantation is used to form NV quantum emitters at nanometric depths at the end facets of laser written optical waveguides. This hybrid fabrication scheme enables development of 2D quantum sensing arrays, facilitating spatially and temporally correlated magnetometry. This method is also applied to implant SiV, another promising quantum emitter, in laser written photonic circuits, to engineer light at the single photon level, which could enable next generation quantum computation systems in diamond.

Plasmonic nanoparticle lattices can support hybrid photon-plasmon excitations—surface lattice resonances—that exhibit both deep-subwavelength light confinement and strong far-field scattering. Single-layer nanoparticle lattices are a powerful platform that have facilitated nanoscale lasing, strong coupling, Bose-Einstein condensation, enhanced single-photon emission, reconfigurable lensing, photo-electrocatalysis, and thermal regulation of smart materials. To date, there has been limited work on stacked plasmonic materials. This talk will describe the preparation and properties of stacked bilayer and multilayer plasmonic lattices. First, we will describe how these architectures can generate mixed color and white-light lasing emission. Next, we will discuss how superimposing two or more of these periodic lattices can result in moiré superlattices. Using dye molecules as local dipole emitters to excite and probe the optical modes, we revealed moiré effects via ultra-long range coupling in plasmonic lattices.

Mie-resonant nanoantennas made of high-index dielectric materials or semiconductors have emerged as a versatile platform for advanced nanophotonic devices [1]. On the other hand, halide perovskites have emerged as promising solution-processible semiconductor materials for many applications from photovoltaics and optoelectronics to nanophotonics. Indeed, optical properties of halide perovskites suggest many novel opportunities for designing advanced nanophotonic devices due to their low-cost fabrication and simplicity of their integration with other types of nanostructures, relatively high values of the refractive index (especially at low temperatures), as well as strong and tunable excitons at room temperatures. Moreover, the perovskites possess extremely high optical gain at room temperature paving the way to miniaturization of lasers. However, smart nanophotonic designs for lasing properties optimization and efficient emission out-coupling become highly important at the subwavelength scale. This talk provides an overview of our recent results in the study of optical effects observed in Mie-resonant perovskite nanostructures. Namely, we start from results on Mie resonances in perovskite nanoparticles [2], where their spectrally tunable coupling with excitons results in reconfigurable Fano resonances [3]. Then, we discuss how the Mie resonances can be employed for perovskite nanolasers [4]. Finally, we discuss the role of substrate for further compactization of perovskite lasers [5] as well as for emission out-coupling [6]. References: [1] AI Kuznetsov, AE Miroshnichenko, ML Brongersma, YS Kivshar, and B Luk’yanchuk. Science 354(6314), aag2472 (2016) [2] E Tiguntseva, GP Zograf, FE Komissarenko, et al. Nano Letters 18 (2), 1185-1190 (2018) [3] EY Tiguntseva, DG Baranov, AP Pushkarev, et al. Nano Letters 18 (9), 5522-5529 (2018) [4] E Tiguntseva, K Koshelev, A. Furasova, et al. ACS Nano 14(7), 8149-8156 (2020) [5] M Masharin, D Khmelevskaya, VI Kondratiev, et al. (2022, submitted) [6] KR Safronov, AA Popkova, DI Markina, et al. Laser & Photonics Reviews 2100728 (2022).

Presently, there is an intense global research effort to use photonics and photons, the building blocks of light, to replace or complement electronics and electrons in a wide range of multidisciplinary technological fields. For example, in telecommunications, photons can enable unbreakable encryption, increase speed and bandwidth, and reduce energy consumption. In the automotive industry, light detection and ranging sensors (LIDAR) are becoming the industry standard for self-driving technologies. In healthcare, a range of optical sensors has been developed and integrated into billions of wearable devices (e.g., Apple Watch, Fitbit, etc.). In consumer electronics, augmented/holographic displays are a core, enabling technology behind the metaverse revolution. With this new wave of emerging applications, there is an urgent need to develop advanced light sources that are highly efficient, compact, and multifunctional with tunable wavelengths throughout the visible and near-infrared spectrum. In the last decade, researchers in the field have focused on the most promising candidate on offer – optical metasurfaces. Made of artificial subwavelength nanostructures, metasurfaces can manipulate the phase, amplitude, polarisation, and frequency of photons to achieve unprecedented functionalities and miniaturisation not found in conventional optics. In this talk, I will summarise our recent efforts at the Novel Light Source group, IMRE, on the development of metasurface-based light-emitting devices. Specifically, I will introduce our concept of using TiO2 metasurface to enhance colour down-conversion efficiency for microLED display technology. In addition, I will also show that, by tailoring exotic photonic concepts such as the bound state in the continuum, light emitting devices with novel functionalities (i.e., multi-beam lasing, polaritonic emission, single photon strong coupling) can be realised.

Colloidal semiconductor nanocrystals are promising active materials for solution-processable optoelectronics and compact light sources. They are synthesized at low costs, have a wide frequency tuning range, and may be integrated readily with a variety of platforms. So far, room-temperature optically pumped lasing from colloidal nanoplatelets (NPLs) have been reported, via bound-state-in-the-continuum (BIC) modes of novel nanoantenna cavities (femtosecond), and whispering gallery modes in microspheres (nanosecond). For practical applications, optically pumped continuous-wave (CW) lasing or electrically pumped lasing would be more favored. However, longer excitation duration results in rapid degradation of materials due to heating and eventually failure of efficient emission before reaching the lasing regime. Here, we report efficient room-temperature nanosecond lasing in CdSe/CdZnS core-shell NPLs via a high-order BIC mode in a metasurface resonator supporting dual resonances that enhance both absorption and emission of the NPLs. Thermal management in the optical cavity is also studied, aiming for lasing with longer excitation duration, on the path towards CW operation.

Recent advances in the synthesis of (hetero)helicenes and their long homologues have given new stimuli to use these inherently chiral 3D aromatic systems as functional molecules in enantioselective catalysis, molecular recognition, self-assembly, surface science, chiral materials and other fields of science. To illustrate that, two studies will be presented: (a) Synthesis of enantiopure extremely long (hetero)helicenes by multiple intramolecular diastereoselective [2+2+2] cycloisomerisation of centrally chiral aromatic oligoalkynes,¹ and measurement of their single molecule electrical conductance by the STM break-junction method,² and (b) helically chiral π-electron macrocycles and their self-assembly into well-ordered 2D molecular crystals observed by ambient AFM on HOPG corroborated by MD simulations.³ The transition of the formally antiaromatic to aromatic π-electron macrocycle upon adsorption will be discussed. References: [1] Stará, I. G.; Starý, I. Acc. Chem. Res. 2020, 53, 144. [2] Nejedlý, J.; Šámal, M.; Rybáček, J.; Gay Sánchez, I.; Houska, V.; Warzecha, T.; Vacek, J.; Sieger, L.; Buděšínský, M.; Bednárová, L.; Fiedler, P.; Císařová, I.; Starý, I.; Stará, I. G. J. Org. Chem. 2020, 85, 248. [3] Houska, V.; Ukraintsev, E.; Vacek, J.; Rybáček, J.; Bednárová, L.; Pohl, R.; Stará, I. G.; Rezek, B.; Starý, I. Nanoscale 2022, 14, DOI: 10.1039/D2NR04209F. Acknowledgments: Supported by the Czech Science Foundation (Reg. No. 20-23566S) and IOCB CAS (RVO: 61388963).

Our research group has focused on the development of novel synthetic methods for heteroatom-containing π-functional molecules with unique structures and innovative properties. In this presentation, I would like to present our recent achievements on the synthesis and properties of various nitrogen-embedded polycyclic aromatic molecules enabled by 1,3-dipolar cycloaddition reactions using polycyclic aromatic azomethine ylides. In 2015, we reported a benzene-fused azacorannulene as the first example of heteroatom-embedded corannulenes. In the following research, we are expanding the variety of azacorannulene derivatives. One direction is to add more nitrogen atoms into the azacorannulene skeleton to form multiazacorannulenes, and the other is to insert another element into the azacorannulene skeleton to form azahomocorannulenes. The structures, properties and applications of the obtained azacorannulene family will be introduced and discussed.

In this talk, I will present several strategies to prepare covalent organic frameworks as well as their optical/electronic applications.

Conjugated macrocycles provide an exciting playing field for the discovery of effects and properties that cannot usually be attained. In contrast to other organic molecules, they can sustain global aromatic or antiaromatic ring currents, which offers exciting opportunities for fundamental and applied research. Utilizing such ring currents occurring in paracyclophanetetraene (PCT), we have presented a molecular design concept that addresses the issues of degradation and unsatisfying long-term cycling performance of organic electrode materials by switching between local and global aromaticity upon redox reactions.^1,2,3 However, for a broader use of PCT as a (sub)structure in organic electronic materials, a suitable reactive building block for the synthesis of such materials was missing. Considering the high reactivity and widespread use of aromatic carboxylic anhydrides in the synthesis of organic materials, we therefore aimed to develop a building block that features PCT as a substructure as well as carboxylic anhydride groups for a facile functionalisation. The molecule resulting from this design process, squarephaneic tetraanhydride (SqTA), will be presented in this talk.⁴ The building block can be obtained in a three-step synthesis and functionalised efficiently to give materials with highly interesting properties. Considering the short synthesis and the unique properties of SqTA and the materials obtained from its further conversion, we expect widespread use of SqTA in the synthesis of organic materials. [1] Eder, Yoo, Nogala, Pletzer, Santana Bonilla, White, Jelfs, Heeney, Choi, Glöcklhofer, Angew. Chem., Int. Ed. 2020, 59, 12958. DOI: 10.1002/anie.202003386 [2] Rimmele, Nogala, Seif-Eddine, Roessler, Heeney, Plasser, Glöcklhofer, Org. Chem. Front. 8 (2021) 4730. DOI: 10.1039/D1QO00901J [3] Pletzer, Plasser, Rimmele, Heeney, Glöcklhofer, Open Res. Europe 1 (2021) 111. DOI: 10.12688/openreseurope.13723.2 [4] Eder, Ding, Thornton, Sammut, White, Plasser, Stephens, Heeney, Mezzavilla, Glöcklhofer, Angew. Chem., Int. Ed. 61 (2022) e202212623. DOI: 10.1002/anie.202212623.

It is of great interest to study the electronic structures, physical properties and chemical reactivities of π-conjugated molecules with [4n+2] π-electrons perimeters and containing antiaromatic-subunits, as the latter can play a significant role in the properties of such molecules. As so far, example of this kind of molecules is very rare, only dicyclopenta[a,e]pentalene. Herein, we report the synthesis and properties of derivatives of fused indacene dimers, substituted s-indaceno[2, 1-a]-s-indacene (6) and as-indaceno[3, 2-b]-as-indacene (7). The fusion of two antiaromatic subunits, s-/as-indacene, could get a polycyclic hydrocarbon based diradicaloids with 22π electrons perimeter. Variable-temperature ¹H NMR/ESR measurements, electronic absorption spectra, and theoretical calculations confirm that both molecules display an open-shell singlet ground state. However, they show totally different electronic structure and (anti)aromaticity natures. In compound 6, antiaromatic pentalene and s-indacene subunits show dominant contribution to the overall properties, while 22π electrons delocalized on the periphery of the backbone in 7. Compound 7 shows larger HOMO-LUMO energy gap, but smaller singlet-triplet energy gap than 6, which can be explained by their quinoidal structures. 

Semicrystalline polymers are used daily in various forms, e.g., fibers, films, and bottles. Their excellent properties, such as high mechanical and thermal properties, are governed by hierarchical structures composed of 10–20 nm thick lamellar crystals. Nano-diffraction imaging (NDI), a novel imaging technique based on scanning transmission electron microscopy (STEM), uses a nanometer-size electron beam to scan across a specimen. Note that NDI may be alternatively called 4D-STEM. The electron diffraction (ED) pattern at each position is recorded onto a two-dimensional (2D) pixelated detector. In this study, NDI was used to image the nanoscale spatial distribution and orientation of lamellar crystals of polyethylene (PE), one of the most popular and electron-beam-sensitive semicrystalline polymers, without any pretreatment (e.g., RuO₄ staining). Moreover, the chain-tilting angles in lamellar crystals, closely related to the thermodynamical stability of polymer crystals and remained a controversial and open question for over 40 years, can be directly measured using NDI.

The evolution of shipping fuels is a major driver in meeting the Paris Climate Agreement, initially through the introduction of the new reduced 0.5% sulphur (S) in Heavy Fuel Oil (HFO) marine standard as of January 2020, and then the eventual transitioning to proposed alternatives such as hydrogen and ammonia. Each of these fuels presents new and undefined challenges in the design of lubricants and dispersants, with a thorough understanding of the fuel lubricant contamination from combustion products being of paramount importance. For marine vessels consuming HFO, asphaltene combustion products which aggregate/deposit in the piston undercrown regions of the engine are of major concern; these materials affect key performance parameters such as engine efficiency and emission control, and if left unchecked, can result in catastrophic mid-ocean cylinder explosions. Asphaltenes are amorphous/disordered and heterogeneous organic materials that can host transition metal elements in key parts of their aromatic structural composition. These materials can be notoriously difficult to analyse accurately by single characterisation technique approaches. This study uses a unique solvent extraction strategy in combination with SAX/WAX diffraction, ¹H/¹³C solid state MAS NMR, EPR and IR/Raman approaches to understand differences in the nature of asphaltenes derived from HFO combustion before and after the implementation of the 0.5% S HFO standard. This approach enables comparisons to the overall structural motifs and reactive functionalities characterising the bulk and interfacially active asphaltene components derived from HFO combustion to be identified. This work demonstrates that significant speciation changes to the asphaltene deposits occurs from the use of 0.5% S HFO, including marked reductions in the S, N and O heteroatom contents, the elimination of most V-supporting porphyrin structures and a complete chemical transformation of the interfacially active faction that now assumes an increased saturate character.

Hybrid lead halide perovskites are next-generation semiconductors for applications in solar energy, light-emitting, lasing, and sensing. Besides impressive cost-effectiveness, the tunability of the optoelectronic properties is an important advantage of this class of materials. Tailoring properties can be achieved by changing the lead halide composition, dimensionality, or the nature of the organic component. In conventional low-dimensional perovskites, the organic cation plays two main roles: an insulation interlayer to create natural quantum well/rod/dot systems and a hydrophobic protector against moisture. However, aimed molecular engineering of the organic molecule can tune the properties and stability beyond these roles. In this talk, we demonstrate several examples of how a right choice of an organic cation helps achieve various functionalities of the perovskites. Choosing a long and flexible aliphatic hexadecylammonium (HDA) cation yielded a 2D perovskite with an unusual high pressure response. Under moderate pressure of 3 GPa, (HAD)₂PbI₄ perovskite formed a micro-domain structure with various band gap junctions. In contrast, a short rigid bi-cation imidazoliumethylammonium forced the inorganic part to form corrugated layers, which resulted in in-plane anisotropy of the optical properties. Halogenation of the organic cation, e.i. phenylethylammonium (PEA), was proven to increase the stability and efficiency of solar cells. We demonstrated the halogen type and its position in the phenyl ring to determine the conformation of the XPEA cation and to affect the band gap and stability of the material. Utilizing coper-organic complexes as the organic layer was another way to improve stability due to the formation of additional bonds. The copper orbitals appeared to modify the band structure directly. Using semiconducting organic cations as an interlayer is a new promising direction of 2D perovskite research. Such perovskites, being easy to synthesize and scale, are natural atomically-thin heterostructures and open great opportunities for fundamental studies and applications.

The treatment and prevention of infectious diseases is a global challenge that is becoming increasingly urgent to solve with the worldwide rise of bacteria with resistance to conventional antibiotics. The engineering of antimicrobial materials as an alternative to traditional antibiotics is an interdisciplinary field incorporating chemistry, materials engineering, biological sciences, and medicine concepts. Supramolecular biomaterials have played an essential role in developing antimicrobial materials that mimic nature’s antimicrobial materials in terms of chemistry, structures, and physical properties. However, there are still many challenges in the supramolecular design of biomaterials with tunable physical and biological properties that combine broad antimicrobial activity with self-regulated passive targeting of the sites of infection. Our research focuses on developing smart antimicrobial materials with tunable properties that target the specific environments of bacteria-infected tissues. We utilize nature’s antimicrobial peptides (AMPs) that are part of the innate immune system of all mammals and plants. Integrating AMPs into precision-engineered nanostructures with different nano-fabrication techniques allowed the engineering of effective biomimetic antimicrobial nanomaterials. We have developed materials that enable the modulation of their physical properties to kill bacteria on demand while at the same time promoting wound healing. In operando small angle X-ray scattering and diffraction techniques, cryogenic transmission electron microscopy and biological evaluation studies showed that the extraordinary antimicrobial properties of these materials originate from their pH-triggered nanostructure transformations. The in-vitro studies are further supported by in-vivo studies using a mouse wound model. In this presentation, I will outline our recent work on developing antimicrobial materials as low-viscous dispersions and adhesive coatings for surface engineering, along with their clinical applications.

Charge transport (CT) across a two-dimensional molecular assembly can involve competitive pathways. Using custom-designed self-assembled monolayers on gold substrate we probe (i) the competition of intramolecular and intermolecular pathways in a molecular assembly and (ii) competition of different intramolecular pathways within a single molecule. For this purpose, we apply so-called core-hole-clock approach in the framework of resonant Auger electron spectroscopy, allowing the measurement of the characteristic CT time from the terminal tail group of the assembled molecules to the substrate. The excitation of the tail group, starting the CT process, occurs by narrow-band synchrotron radiation. We show that the intramolecular CT is generally preferable and so-called matrix effects play a negligible role for this process, strongly favoring the through-bond CT model. In the case of availability of several alternative pathways within a single molecule, a pathway with the highest conductance becomes highly dominant, while other, less conductive pathways contribute only minorly to the entire CT.

The pursuit of highly efficient gene delivery systems in plants is very important for genetic engineering. Traditional delivery methods such as agrobacterium-mediated transformation, polymer particle-based delivery, biolistic particle bombardment or gene gun delivery and electroporation have been widely used. However, these methods have their limitations due to genotype dependence and other drawbacks, e.g. non-site specific delivery. Nanotechnology has emerged as a promising strategy to deliver genes into specific subcellular compartments in plants. Currently, the knowledge about how nanovectors translocate in plant tissues and into plant cells is limited. Although there are some studies on the effect of nanovector size, shape and charge on their uptake by plants, it is still unclear how the surface functionality of nanovectors affects their uptake path (how nanovectors enter into plant leaf, via cuticle, stomata, or other paths), translocation (apoplast vs symplast, via vascular or non-vascular path, internalization vs non-internalization) and absorption and whether the nanovector undergoes any transformation in the plants. Thus, there is a great need to develop newer gene delivery vectors or methods.  In this study, we designed and synthesized different sizes of gold nanovectors (8 – 200 nm) using a solution-based method, capped them with different ligands via a ligand exchange method and finally loaded them with RNA and DNA in nanovectors. After foliar application, we monitored their uptake, translocation and absorption as well as the availability of the gene to the plants. We aim to determine whether (1) our nanovector can succeed to deliver genes into plants and (2) the generation of genome stable transgenic plants. We hope to provide a new routine and critical parameters for genome engineering in plants.

Zeolites are best known as molecular sieves in catalytic cracking. Their other useful properties include cation exchange and water uptake, both of which are crucial for plant growth [1]. Plant growth studies using zeolites as substrates or additives has been done [2 – 3] but they have not been studied in detail according to their structure. This means that there is a large unexplored space within zeolite materials to be used as plant growth substrates. I will present results from a preliminary study done in conjunction with an industrial partner, Panasonic Factory Solutions Asia Pacific (PFSAP), Singapore. In their indoor farm, one third of operational costs comes from industry standard single-use peat. Zeolite was compared against the peat and has shown been able to grow 4 cycles at comparable yields as single use peat. This shows that natural unoptimized zeolite can already replace peat as a reusable growing substrate, due to a proposed mechanism of uptake, storage and release of important nutrient ions and water for use by plant growth. More research is ongoing to study the zeolites as plant growth substrate and how it can be controlled to be a superior, reusable, and cheap plant growing substrate. The preliminary work is being written and is set to be published.

In this talk, data-driven modeling of composite structures is presented. Specifically, machine learning (ML), a subdivision of artificial intelligence (AI), is implemented to study the mechanical behavior of composite adhesive single-lap joints (SLJs) subjected to tensile loading. The experimental data for training and testing the ML models are compiled from peer-reviewed journal papers from various research groups to eliminate bias and increase the diversity within the dataset. The dataset consists of eight continuous SLJ input parameters, which are used to predict the SLJ damage mode and failure strength. Regression and classification models are built using deep neural networks (DNN) and random forests (RF). Finite element (FE) modeling is conducted, and the numerical performance is compared with the accuracy of the regression ML models. Results show that ML models can predict strength with high accuracy. Furthermore, both DNN and RF classify damage modes accurately without the need for complex failure criteria, which cannot be typically achieved using traditional FE methods. This study utilizes ML algorithms to gain a deeper understanding of structure-property-performance relationships, leading to better designs of composite adhesive joints.

The high-pressure phase of titanium dioxide can have potential as a new class of superhard material due to its high hardness and shear modulus. The stability and superhard material properties of the high-pressure phase are easily constrained by external temperature and pressure conditions, which are closely related to its martensitic phase transformation behaviour. Molecular dynamics offers new opportunities to study dynamical processes on large spatial and temporal scales, but the classical force field with its poor portability and low accuracy hinders its application to structural phase transformations. Here, we obtain an accurate TiO₂ interatomic interaction potential based on deep machine learning methods trained on first-principles (FP) data. This machine learning potential is more accurate than the empirical force field Matsui-Akaogi (MA) potential in predicting the structural properties of various TiO₂ crystalline phases, such as equation of state, elastic constants, phonon spectra and structural phase transition barrier, with an accuracy comparable to that of the density functional theory (DFT) approach. Finally, the molecular dynamics simulations are used to reveal the pressure-driven martensitic phase transition path of the bulk columbite phase to the baddeleyite phase dynamically from the nanoscale. The simulations show that the phase transition barrier at 300 K and a pressure of about 37 GPa is 141.6 meV/TiO₂, which is easily overcome by thermal fluctuation. This is in qualitative agreement with the experimental phase transition conditions of 500 ℃ and 18 GPa. During the phase transition, crystal shearing and volume compression occur simultaneously, and the Ti and O atoms phase transformation to baddeleyite phase in a diffusion-free manner through collective ordering and simultaneous movement. Our study can provide a theoretical basis for solving the high-pressure phase instability problem.

High-entropy alloys (HEAs) are composed of multiple principal elements, leading to extreme chemical disorder in the lattice. Most of the outstanding functional and structural performance in HEAs relates to their diffusion properties under the rough potential energy landscape (PEL) induced by chemical disorder. Due to the highly rugged and multi-dimensional nature of PEL, it is challenging to describe how the diffusion process is controlled by the PEL in HEAs. Here we develop machine learning (ML) models to accurately represent the local atomic environment dependence of PEL in HEAs. By combining the ML model with the kinetic Monte Carlo (kMC) method, we reveal that self-diffusion in HEAs is predominantly governed by the PEL roughness, as characterized by the elemental-specific site energies and migration barriers. We further developed an analytical model that can be used to evaluate diffusion properties in HEAs as long as the elemental-specific energy landscapes are available.

We introduce a query-and-learn active learning to search for stable crystal structure, clarifying the stabilization mechanism of SmFe₁₂-based compounds which exhibits prominent magnetic properties. The proposed method aims to (i) accurately estimate formation energies with limited first-principles calculation data, (ii) visually monitor the progress of the structure search process, (iii) extract correlations between structures and formation energies, and (iv) recommend the most beneficial candidates of SmFe₁₂-substituted structures for the subsequent first-principles calculations. 3307 structures of SmFe_12−α−β are created by substituting X, Y elements–Mo, Zn, Co, Cu, Ti, Al, Ga–with α + β < 4 into iron sites of the original SmFe₁₂ structures. Using the optimized structures and formation energies obtained from the first-principles calculations after each active learning cycle, we construct an embedded two-dimensional space to rationally visualize the set of all the calculated and non calculated structures for monitoring the progress of the search. The optimal model attained a prediction error for the formation energy of 1.25 × 10⁻² (eV/atom) using one-sixth of the training data, and the recall rate of potentially stable structures was nearly 4 times faster than the random search. The formation energy landscape visualized using the embedding representation revealed that the substitutions of Al and Ga have the highest potential to stabilize the SmFe₁₂ structure. Finally, the correlations between the distortion in coordination number and the corresponding formation energy are revealed.

The understanding of ion behaviors under external electric fields is crucial for the development of energy-related devices using ion transport. While first-principles calculations within density functional theory (DFT) have been widely employed to investigate the ion transport phenomena, the huge computational costs hinder their applications to many issues, especially phenomena under external electric fields. Thus, simulation methods for accurate prediction of physical quantities with low computational costs are necessary. In this study, we developed a neural network-based model to predict Born effective charge tensors from given atomic structures. By combining forces due to an applied uniform electric field, expressed as a product of the Born effective charge and the electric field, and forces evaluated by a neural network potential (NNP) method [1], we built a scheme to simulate ion dynamics under external electric fields. Here we demonstrate the validity of our scheme taking amorphous Li₃PO₄ as an example [2].We constructed both the Born effective charge predictor and NNP using DFT calculation data for various atomic configurations. Using the constructed models, we performed molecular dynamics simulations under the uniform electric field of 0.1 V/Å. The mean square displacement values showed that the Li ion motions were enhanced along the direction of the electric field, which seems physically reasonable. In addition, we found that the Li ion motions perpendicular to the electric field were also grown because of the non-negligible external forces arisen from the off-diagonal components of the Born effective charges. We expect that microscopic processes of various phenomena caused by eternal electric fields can be investigated by our scheme. [1] J. Behler and M. Parrinello, Phys. Rev. Lett. 98, 146401 (2007).[2] R. Otsuka, M. Hara, K. Shimizu, and S. Watanabe, in preparation.

The 2D layered lead-halide organic-inorganic perovskite material has recently been the focus of investigation by many research groups because of its extraordinary stability and optoelectronic properties, making them the ideal candidate for many optoelectronic applications. The microstructure of 2D perovskite materials, in particular, the layer distribution of 2D perovskite materials subjected to different organic spacers or processing temperatures are pivotal to the performance of materials; however, owing to both the limitations in experimental characterization tools as well as first-principle calculations, comprehensive insights into the microstructures of 2D perovskite materials is still lacking. In this work, a quantum accurate, machine learning-enabled force-field based on the Spectral Neighbor Analysis Potential (SNAP) scheme from the training set based on carefully selected ab-initio molecular dynamics trajectories including both BA and PEA organic spacers were trained. The trained SNAP potential could predict the energies of the reference structures with high fidelity and can be successfully extrapolated to outside the original training sets. In addition, the trained model is robust for long-time-scale molecular dynamics simulations. A series of hybrid Monte Carlo simulations based on the trained energy model were performed to study the layer distribution of 2D perovskite material to examine the impacts of organic spacers and processing temperatures. Our large-scale atomistic simulations indicate non-uniform layer distribution for both spacers, which is in good agreement with experimental results. Hence, the present study demonstrates that utilizing a machine learning-enabled energy model is a promising approach to extracting the microscale details of complex perovskite materials.

The development of novel solid state ionic conductors is critically important for the designs of better electrochemical batteries and fuel cells, in particular solid oxide fuel cells. Computational prescreening and selection of such materials can help discover novel ionic conductors but is also challenging due to the high cost of electronic structure calculations which would be needed to compute the properties of interest such as material's stability and ion diffusion barrier or rate. This is made more difficult in the presence of multiple possible diffusion paths. The bond valence (BV) approach is attractive for rapid prescreening among multiple compositions and structures, but the simplicity of the approximation can make the results unviable. We explore the possibility of enhancing the accuracy of the bond valence approach by adapting the parameters of the approximation to the chemical composition. Specifically, ｗe model the screeening factor - an important parameter of the BV approximation - as a function of descriptors of the chemical composition. We use linear and neural network models and show, on the examples of perovskite type oxides which have been proposed as promising solid state ionic conductors, that this can noticeably improve the ability of the BV approximation to model structures, in particular new, putative crystal structures whose structural parameters are yet unknown. On the other hand, relative insertion site energies still require improvements. We also relate the errors of the BV model to the degree of ionicity of the bonding. 

Two-dimensional (2D) van der Waals (vdW) magnets are emerging candidates for ultralow-power and ultra-compact device applications. However, most of the discovered 2D materials require cryogenic temperatures or special protections to function. Therefore, it becomes increasingly important to have control over their unique atomic-level magnetism at room temperature, at which most of the devices operate. Recently, we have discovered tunable room temperature ferromagnetism in atomically thin transition metal dichalcogenides (TMDs), including metallic monolayers of VSe₂ [1] and semiconducting monolayers of V-doped WX₂ (X=S, Se) [2,3], that have the potential to transform the fields of 2D vdW spintronics, opto-spin-caloritronics, and valleytronics. In this talk, I will demonstrate light-tunable ferromagnetism in 2D-TMD based magnetic semiconductors [4] and how this unique property can be exploited to boost “spin to charge” conversion via the spin Seebeck effect (SSE) [5,6] and propose a new strategy for optically controlled SSE with the intent to establish the research thrust of “Opto-Spin-Caloritronics.” 

Spintronics involves manipulating and controlling the spin degree of freedom for novel storage and computing applications [1]. The major challenge is the efficient generation and detection of pure spin current. Traditionally, nonmagnetic materials have been utilized for spin-to-charge conversion. Recently, the focus has shifted to novel quantum materials such as two-dimensional (2D) materials and non-collinear antiferromagnets (AFM) for efficient spin-to-charge conversion. These quantum materials offer several advantages like giant spin-charge conversion, unconventional spin-orbit torques (SOTs), and gate tunable spin-charge conversion. Considering the above advantages, we have investigated 2D transition metal dichalcogenides (TMDs) and non-collinear AFM Mn₃Sn for spin-to-charge conversion in detail. First, we report on the room temperature observation of a large spin-to-charge conversion arising from the interface of Ni₈₀Fe₂₀ (Py) and four distinct large-area (∼5 × 2 mm²) monolayer (ML) TMDs, namely, MoS₂, MoSe₂, WS₂, and WSe₂ [2]. We show that both spin mixing conductance and the Rashba efficiency parameter (λ_IREE) scale with the spin-orbit coupling strength of the ML TMD layers. The λ_IREE parameter is found to range between -0.54 and -0.76 nm for the four ML TMDs, demonstrating a large spin-to-charge conversion. Through Mn₃Sn thickness-dependent ISHE measurements, we found a large and negative spin Hall conductivity, which can be explained by a shift of the Fermi level caused by a slight excess of Mn in our films [3]. Our findings demonstrate novel techniques for engineering spin-to-charge conversion using quantum materials for functional spintronic devices. [1] I. Žutić et al., Rev. Mod. Phys. 76, (2004) 323. [2] H. Bangar et al., ACS Appl. Mater. Interfaces 14 (2022) 41598. [3] H. Bangar et al., Adv Quantum Technol. 6, 2200115 (2022).

Interstitial anionic electrons (IAEs) mainly determine the physicochemical properties of the two-dimensional(2D) electrides that readily donate electrons. IAEs are expected to be formed in various chemical reactions due to their exotic properties such as low work function and magnetism. The presence of interstitial quasi-atomic electrons (IQEs) sites and hydrogen anions formation in the electride is unique factors. However, an experimental investigation of hydrogenation of the magnetic 2D electrides was hardly reported. Here, we report the reversible hydrogenation of ferromagnetic [Gd₂C]^2+.2e^- electride, which is 2D electride, induces the reversible magnetic and structural transformation, consistent with theoretical calculations. According to the relative ratio of IQE and hydrogen anions concentration, the crystal structure and magnetic phase of hydrogenation electride [Gd₂C]^2+.2e^- exhibited the coupled transition from the ferromagnetic layer structure of the R3-m space group to the canted antiferromagnetic layer structure of the P3-1m space group, and it also showed a reverse transition upon dehydrogenation reversibly. Our findings provide a new avenue for crystal structure and variation of magnetic phase through hydrogenation of electride, which have promising applications in future electronic and spintronic devices.