Beng Kang Tay

Niobium oxide dihalide films show a second-harmonic generation (SHG) efficiency scaling with their thickness, owing to their weak interlayer electronic coupling, but their use in integrated nonlinear photonics has been limited by the lack of robust electrical tuneability. Here we integrate ferroelectric NbOBr₂, of thicknesses ranging from 20 nm to 40 nm, with a silicon nitride microring resonator to realize electrically reconfigurable SHG on a photonic chip. NbOBr₂ combines strong second-order susceptibility from intrinsic inversion-symmetry breaking with electrically switchable in-plane ferroelectric polarisation. By modulating the Schottky barrier at the NbOBr₂-metal interface, ferroelectric switching controls carrier injection, free carrier absorption and dispersion, thereby tuning the cavity-enhanced nonlinear response. The device achieves an SHG conversion efficiency of 158% W^-1 under continuous-wave excitation and a 53% electrical modulation depth. The polarisation states can also be programmed over multiple levels and retained after switching, enabling non-volatile control of the optical response. Using the fundamental and frequency-converted outputs as complementary channels, we further demonstrate dual-channel edge detection.

The advent of filtered cathodic vacuum arc (FCVA) technology has revolutionized the deposition of high-performance multifunctional coatings by combining dense microstructures with minimal defect formation. Unlike conventional physical vapor deposition and chemical vapor deposition processes, FCVA operates effectively at low temperatures while simultaneously addressing two long-standing challenges: macroparticle contamination and high-thermal stress on substrates. These advantages enable its successful application across diverse industrial domains, including semiconductor device fabrication, aerospace engineering, biomedical implants, and flexible electronic systems. This review provides a comprehensive overview of the fundamental principles, technological evolution, and material systems enabled by FCVA. Attention is given to plasma filtration architectures and their influence on ion transport, macroparticle suppression, and their relationship to film growth. Recent developments in substrate biasing strategies, reactive plasma control, and real-time plasma diagnostics are discussed as key approaches for regulating stress, microstructure, and coating uniformity. In addition, the integration of FCVA with other plasma-based deposition techniques is examined as a route to extending compositional flexibility and functional performance. By enabling the design of tailored multifunctional coatings, FCVA is poised to accelerate the deployment of next-generation technologies ranging from temperature-sensitive electronic devices to lightweight aerospace structures and biocompatible medical systems. The paper concludes with a forward-looking discussion on scalability, cost considerations, and sustainable processing strategies, outlining the conditions under which FCVA may remain competitive within the broader landscape of advanced coating technologies.

Tin diselenide (SnSe₂) is a two-dimensional (2D) semiconductor with a high electron affinity (∼5.2 eV) and inherent n⁺-type conductivity. While conventional chemical vapor deposition (CVD) has been investigated for the growth of SnSe₂, the domain sizes are typically limited to ∼50 μm. This limitation arises primarily from difficulties in precise control of the thermodynamics of nucleation and growth along with the kinetics of vapor-phase transport. To address this issue, we employed flux-assisted growth (FAG) to facilitate liquid-phase precursor transport via a molten flux. As a result, we achieved crystalline SnSe₂ flakes with significantly enlarged domain sizes exceeding 300 μm. The synthesized SnSe₂ exhibits a well-ordered 1T-phase lattice structure, a high Hall mobility of 262.5 cm²·V^-1·s^-1, and an electron concentration of 8.23 × 10¹³ cm^-2, confirming its degenerate n-type nature. Furthermore, the SnSe₂-based devices show reproducible photoresponse cycles governed by trap-mediated processes yielding a multitime scale fading memory. Our findings provide insights into the feasibility of flux-grown SnSe₂ for p-n⁺ van der Waals tunneling heterostructures and in-sensor reservoir computing.

To meet the rapid advancements in artificial intelligence and information technology, neuromorphic computing systems have attracted significant attention as promising alternatives to the conventional von Neumann paradigm. Two-dimensional (2D) ferroelectric materials feature atomic-scale thickness, moderate bandgaps, and compatibility with van der Waals heterostructures. These properties offer unique opportunities to overcome the scaling limitations and functional constraints of conventional ferroelectrics, enabling the development of next-generation low-power and highly integrated electronic and neuromorphic systems. In this review, we systematically summarize the fundamental mechanisms and synthesis strategies of 2D ferroelectricity, including bottom-up synthesized techniques and top-down exfoliation approaches, and discuss intrinsic and extrinsic methods for tailoring polarization to optimize device performance. We focus on representative 2D ferroelectric device platforms, such as ferroelectric field-effect transistors, ferroelectric tunnel junctions, ferroelectric semiconductor FETs, negative capacitance FETs, and ferroelectric photovoltaics, and examine their potential for in-memory computing, in-sensor computing, and neuromorphic logic. Finally, we highlight the unique advantages of 2D ferroelectrics in unifying sensing, memory, and processing, and analyzing the remaining challenges in scalable fabrication, environmental stability, and CMOS-compatible integration. We also outline future opportunities, including hybrid light–matter computing, reservoir computing, and three-dimensional ferroelectric architectures, emphasizing the importance of cross-disciplinary collaboration in realizing energy-efficient, brain-inspired electronic systems.

Pentagonal 2D materials like penta-palladium-phosphorus-sulfur (PdPS) are renowned for their intriguing physical properties stemming from their unique Cairo pentagonal tiling topology. In this study, we explored the enhancement of thermoelectric properties in exfoliated penta-PdPS layers through lithium intercalated penta-PdPS (Li_xPdPS), derived from bulk crystals produced via chemical vapor transport (CVT). We present a systematic examination of the effects of Li⁺ intercalation on the in-plane electrical conductivity and thermoelectric power factor across various stages of intercalation (2.5, 10.5, and 20.5 cycles) and over a temperature range from 20 to 380 K. Our findings reveal that both the Seebeck coefficient (|S|) and electrical conductivity (σ) are optimally tuned at intermediate Li⁺ intercalation stages. At 10.5 cycles, |S| increases by 50% and σ by 1.6 times relative to pristine PdPS, yielding a peak power factor (PF) of 80 μW m^-1 K^-2 at 380 K, over 3-fold higher than the pristine value (24 μW m^-1 K^-2). Thicker Li_xPdPS flakes further enhance PF by accommodating more Li⁺ ions, which raise carrier concentration and promote energy filtering. These improvements arise from pseudorecrystallization of grain boundaries and chemical doping, effectively decoupling σ and |S|. Controlled Li⁺ intercalation thus offers a powerful means to modulate electronic and lattice structures, enabling high-efficiency thermoelectric performance in 2D penta-PdPS and related layered materials.

Robust solid lubrication systems are crucial to the reliability, efficiency, and lifetime of advanced mechanical assemblies operating in demanding environments. Carbon nanotubes (CNTs) have attracted extensive attention as emerging solid lubricants due to their exceptional mechanical strength, high aspect ratio, thermal stability, and intrinsic self-lubricating capability. However, the coupled relationship between mechanical reinforcement and the tribological response in CNT-based coatings remains insufficiently understood, limiting their optimized deployment across metal, polymer, and ceramic substrates. This review provides a comprehensive assessment of CNT solid lubricants with a particular emphasis on the synergistic interactions among structural characteristics, mechanical behavior, and frictional performance. The discussion encompasses the roles of CNT geometry, chirality, dispersion state, defect density, and interfacial bonding in governing deformation, crack propagation, load transfer, lubrication modes, and wear resistance across different composite systems. Furthermore, advances in in situ scanning electron microscopy (SEM) and transmission electron microscopy (TEM) are summarized, highlighting their ability to reveal real-time deformation, interfacial evolution, nanoscale contact mechanics, superlubricity transitions, and fracture mechanisms. The review compares the dominant strengthening and lubrication mechanisms observed in different material platforms, including grain refinement, crack bridging, Hall-Petch strengthening, Orowan looping, interlayer sliding, rolling dynamics, interfacial filling, and transfer film formation. Finally, key challenges and outlooks are outlined, including the need for controlled interfacial modulation, scalable dispersion strategies, stimulus-responsive lubrication, and multiscale mechanistic integration under extreme operating conditions. This review establishes a cohesive framework linking CNT structures, mechanical properties, tribological functionality, and advanced characterization, providing guidance for the rational design of next-generation CNT-based solid lubrication systems.

Van der Waals materials of the MB₂T₄ family (M = transition metal or rare-earth metal, B = Bi or Sb, T = Te, Se, or S) have attracted wide interest for their exotic topological and magnetic properties, as well as potential spintronic applications. However, the direct growth of 2D ternary MB₂T₄ remains challenging due to multiple competing phases and complex atomic arrangements. Here, we report a flux-assisted, phase-controlled growth strategy to directly grow six distinct 2D MB₂T₄ crystals. Using MB₂T₄ bulk powder as precursor, its dissolution into the cosolvent ensures stoichiometric control, while thermodynamic-kinetic equilibrium suppresses phase separation. Taking MnSb₂Te₄ as an example, we obtained highly ordered septuple layers, with superconducting quantum interference device (SQUID) and reflective magnetic circular dichroism (RMCD) measurements confirming layer-dependent ferromagnetism and Curie temperature (T_c) ranging from 12.3 to 33.7 K. This strategy provides an effective route for synthesizing complex layered crystals and offers versatile platforms for advancing spintronic applications.

Optical phase modulators are critical components in integrated photonics, but conventional designs suffer from a trade-off between modulation efficiency and optical loss. Two-dimensional materials like graphene offer strong electro-optic effects, yet their high optical absorption at telecom wavelengths leads to significant insertion losses. Although monolayer transition metal dichalcogenides (TMDs) provide exceptional telecom-band transparency for low-loss electro-refractive response, their practical implementation in phase modulators requires top electrodes to enable vertical electric field tuning, which typically introduces parasitic absorption. Here, we address this challenge by developing hybrid tungsten oxyselenide/graphene (TOS/Gr) electrodes that minimize optical loss while enabling efficient phase modulation in TMD-based devices. The UV-ozone-converted TOS (from WSe ₂ ) acts as a heavy p-type dopant for graphene, making the graphene transparent in the NIR region while enhancing its conductivity. Our complete device integrates a hybrid TOS/graphene transparent electrode with a hexagonal boron nitride dielectric spacer and monolayer WS ₂ electro-optic material on a SiN microring platform. This achieves a high modulation efficiency of 0.202 V·cm while maintaining an exceptionally low extinction ratio change of just 0.08 dB, demonstrating superior performance compared to modulators employing conventional electrodes. Our breakthrough in near-lossless phase modulation opens new possibilities for energy-efficient optical communications, photonic computing, and fault-tolerant quantum networks.