Anisotropic nonlinear optical two-dimensional (2D) materials hold great potential for advancing photonics and optoelectronics applications due to their coexistence of strong nonlinear optical response and high nonlinear anisotropy ratio, such as crystal orientation identification, bio-microscopy, optical switching. This work focused on the unique layer-dependent symmetry breaking and strong second harmonic generation (SHG) anisotropy based on palladium phosphide sulfide (PdPS), a pentagonal 2D semiconductor with Cairo tiling. PdPS exhibits even-layer inversion symmetry breaking and highly tunable SHG anisotropy, contrast to odd-layer inversion symmetry. Notably, the SHG anisotropy ratio reaches up to 32.7 for six layers (< 5 nm) PdPS. This ratio is the highest in such thin 2D materials to the best of our knowledge. Furthermore, its optical properties can be tuned easily through layer number and microdevice configuration. As layer number of PdPS decreases from six layers to two layers, the anisotropic ratio drops drastically from 32.7 to 0.89. In addition, the overall SHG signal can be increased by 10 times when integrated with a photonic crystal device. These findings suggest that PdPS holds great promise for use in polarization-sensitive and layer-engineered nonlinear photonic applications like photodetectors, polarized lasers, polarized light emitting diodes and reflective polarizer.

Understanding the relationship between CO₂ reduction reaction (CO₂RR) performance and surface terminations of MXenes is crucial for designing effective electrocatalysts. This study explores the impact of common terminations on Mo₂CT_x using a computational hydrogen electrode (CHE) model integrated with a pseudo-microkinetic model (pseudo-MM). Unlike traditional CHE methods, CHE/pseudo-MM considers the energy differences of all steps, providing a comprehensive view of CO₂RR mechanisms while reducing computational cost generated from calculating transitional state. The electrolyte is considered as acetonitrile with 1-ethyl-3-methylimidazolium tetra-fluoroborate (EMIMBF₄) to inhibit the generation of hydrogen. Theoretical predictions reveal surface terminations dictate the selectivity of C₁ products, whose proton is provided by EMIMBF₄. The selectivity for fully –F, –O– and –OH–terminated Mo₂CT_x surfaces varies with the applied potential, as confirmed by experiments. Electrochemical CO₂RR in acetonitrile with EMIMBF₄ electrolyte confirms these predictions, showing that CH₄ outperforms CO and gradually becomes the dominant product as the applied potential increases. These findings demonstrate the qualitative accuracy of the proposed CHE/pseudo-MM for predicting CO₂RR selectivity, particularly for gaseous products, over Mo₂CT_x systems.

Two-dimensional (2D) transition metal nitrides (TMNs) have garnered significant attention in fields such as energy storage and nanoelectronics due to their unique electrical properties, high chemical stability, and excellent mechanical strength. In polycrystalline 2D TMNs films, grain boundaries (GBs) are inevitable structural defects that could play a crucial role in determining the material's properties. Developing rapid optical visualization methods is essential for obtaining large-scale information on the distribution of GBs. However, the rapid visualization of GBs in 2D TMNs, as well as the impact of GBs on the material's electrical properties, has never been previously reported. In this study, we demonstrate the growth of monolayer tungsten nitride crystals on SiO₂/Si substrates by chemical vapor deposition (CVD). High-resolution transmission electron microscopy reveals the presence of GBs at the junctions of twisted grains. A wet-etch process utilizing buffered oxide etchant (BOE) enables rapid and effective visualization of these GBs with optical microscopy. By analyzing grains with different twist angles, we find that GBs at specific angles demonstrate increased stability during etching. Electrical measurements revealed that tilted GBs hinder electrical transport, with GBs of a 62° twist angle showing sheet conductance nearly half that within the monolayer grain. This work not only provides insights into GBs in monolayer tungsten nitride but also lays the groundwork for exploring GBs-related properties in other 2D TMNs.

Flexible electronics is the research field with interdisciplinary crossing and integration. It shows the promising advantages of novel device configurations, low-cost and low-power consumption due to their flexible and soft characteristics. Atomic layered two-dimensional (2D) materials especially transition metal dichalcogenides, have triggered great interest in ultra-thin 2D flexible electronic devices and optoelectronic devices because of their direct and tunable bandgaps, excellent electrical, optical, mechanical, and thermal properties. This review aims to provide the recent progress in 2D TMDs and their applications in flexible electronics. The fundamental electrical properties and mechanical properties of materials, flexible device configurations, and their performance in transistors, sensors, and photodetectors are thoroughly discussed. At last, some perspectives are given on the open challenges and prospects for 2D TMDs flexible electronic devices and new device opportunities.

Two-dimensional (2D) ferromagnets with out-of-plane (OOP) magnetic anisotropy are potential candidates for realizing the next-generation memory devices with ultra-low power consumption and high storage density. However, a scalable approach to synthesize 2D magnets with OOP anisotropy directly on the complimentary metal-oxide semiconductor (CMOS) compatible substrates has not yet been mainly explored, which hinders the practical application of 2D magnets. This work demonstrates a cascaded space confined chemical vapor deposition (CS-CVD) technique to synthesize 2D Fe_xGeTe₂ ferromagnets. The weight fraction of iron (Fe) in the precursor controls the phase purity of the as-grown Fe_xGeTe₂. As a result, high-quality Fe₃GeTe₂ and Fe₅GeTe₂ flakes have been grown selectively using the CS-CVD technique. Curie temperature (T_C) of the as-grown Fe_xGeTe₂ can be up to ~ 280 K, nearly room temperature. The thickness and temperature-dependent magnetic studies on the Fe₅GeTe₂ reveal a 2D Ising to 3D XY behavior. Also, Terahertz spectroscopy experiments on Fe₅GeTe₂ display the highest conductivity among other Fe_xGeTe₂ 2D magnets. The results of this work indicate a scalable pathway for the direct growth and integration of 2D ternary magnets on CMOS-based substrates to develop spintronic memory devices.

The catalysis of Au thin film could be improved by fabrication of array structures in large area. In this work, nanoimprint lithography has been developed to fabricate flexible Au micro-array (MA) electrodes with ~ 100% coverage. Advanced electron microscopy characterisations have directly visualised the atomic-scale three-dimensional (3D) nanostructures with a maximum depth of 6 atomic layers. In-situ observation unveils the crystal growth in the form of twinning. High double layer capacitance brings about large number of active sites on the Au thin film and has a logarithmic relationship with mesh grade. Electrochemistry testing shows that the Au MAs perform much better ethanol oxidation reaction than the planar sample; MAs with higher mesh grade have a greater active site utilisation ratio (ASUR), which is important to build electrochemical double layer for efficient charge transfer. Further improvement on ASUR is expected for greater electrocatalytic performance and potential application in direct ethanol fuel cell.

Layered MoTe₂ has shown great promises for optoelectronics and energy-storage applications due to its exceptional optical and electrochemical properties. To date, considerable efforts have been devoted to fabricating layered MoTe₂ with lateral orientation by means of mechanical/chemical exfoliation and chemical vapor deposition (CVD) methods. As compared to its horizontal counterparts, vertically aligned MoTe₂ with higher density of active edge sites is expected to possess unique optoelectronic and electrochemical properties, while which has not been reported yet. In this work, we report a versatile and scalable CVD growth of vertically aligned MoTe₂ with length of up to ~ 7.5 µm on Mo foil. Remarkably, the dominant phase of the vertically aligned MoTe₂ can be tuned from 2H to 1T’ by increasing the growth temperature from 630 to 780 °C. Owing to the weak interaction between the as-grown MoTe₂ and Mo foil, the as-grown MoTe₂ can be easily detached from the Mo foil. This in turn enabled economic reuse of the Mo foil for multiple growth. Moreover, the vertical growth of the MoTe₂ is proposed to be caused by the internal strain generated during tellurization of Mo foil. Furthermore, the as-grown MoTe₂ can also be directly dispersed in solvent to produce high-quality MoTe₂ nanosheets. The versatility of this growth strategy was further demonstrated by fabricating other vertically aligned transition metal chalcogenides (TMDs) such as TaTe₂ and MoSe₂. Hence, this work paves the path towards achieving unique TMDs structures to enable high-performance optoelectronic and electrochemical devices.

A topologically mediated synthesis of porous boron nitride aerogel has been experimentally and theoretically investigated for carbon dioxide (CO₂) uptake. Replacement of the carbon atoms in a precursor aerogel of graphene oxide and carbon nanotubes was achieved using an elemental substitution reaction, to obtain a boron and nitrogen framework. The newly prepared BN aerogel possessed a specific surface area of 716.56 m²/g and exhibited an unprecedented twentyfold increase in CO₂ uptake over N₂, adsorbing 100 cc/g at 273 K and 80 cc/g in ambient conditions, as verified by adsorption isotherms via the multipoint Brunauer-Emmett-Teller (BET) method. Density functional theory calculations were performed to give hints on the mechanism of such high selectivity of CO₂ over N₂ adsorption in BN aerogel, which may be due to the interaction between the intrinsic polar nature of B-N bonds and the high quadrupole moment of CO₂ over N₂.