•Prediction of few-layer arsenic trichalcogenides with broad and tunable band-gaps.•Strain-induced indirect to direct band-gap transition in these layered compounds.•Low exfoliation energy renders them attractive for artificial hetrostructures.•Promising candidates for applications in photocatalysis and optoelectronics.Since the discovery of graphene, two-dimensional (2D) layered nanomaterials have been receiving continuous attention owing to their extraordinary properties and promising applications in nanoelectronics. However, many 2D nanomaterials are gapless or possess a small band-gap (≤2 eV), which greatly restricts their applications. Here, by means of ab initio calculations and molecular dynamics simulcations, we report a class of emerging 2D semiconductors, mono- and few-layer arsenic trichalcogenides (As2S3 and As2Se3), with a broad band-gap range from 2.06 eV to 3.18 eV, which can be manipulated by the number of layers or external strains. Interestingly, under moderate tensile strain, the nanolayers undergo a transition from indirect to direct band-gap semiconductors. More importantly, these 2D semiconductors exhibit suitable band-edge alignment and desirable optical absorption, suggesting their potential applications for photocatalysis and optoelectronics. Thanks to the small exfoliation energies, these 2D layered materials could be fabricated from experiments feasibly and serve as promising candidates in constructing van der Waals heterostructures for future nanoelectronics.Download high-res image (562KB)Download full-size image

Hydrogen and oxygen play an important role in the hydrogen embrittlement and oxidation of novel Co-based alloys with γ/γ′ microstructure. In this study, the adsorption of hydrogen and oxygen atoms on the FCC-Co(111) surface and their diffusion behavior from the surface into the sub-layers and bulk have been investigated by means of first-principles calculations. It is observed that hydrogen and oxygen atoms prefer to adsorb on the fcc and hcp (threefold hollow) sites, respectively. The hydrogen atom can penetrate into the first and second sub-layers energetically, while it is not feasible for the oxygen atom as diffusion from the surface into the first sub-layer is more difficult. It is found that the calculated diffusion coefficients of hydrogen are in good agreement with the available experimental data. Finally, we briefly discuss the changes in total magnetic moment along the Oct–Tet–Oct diffusion path and the associated electronic structures. The present work is helpful to provide comprehensive guidance for the development and applications of novel Co-based alloys.

Development of novel van der Waals (vdW) heterostructures from various two-dimensional (2D) materials shows unprecedented possibilities by combining the advantageous properties of their building layers. In particular, transforming the vdW heterostructures from type-I to type-II is of great interest and importance to achieve efficient charge separation in photocatalytic, photovoltaic, and optoelectronic devices. In this work, by means of ab initio calculations, we have systematically investigated the electronic structures, optical properties, and mechanical properties of MXene/Blue Phosphorene (BlueP) vdW heterostructures under various deformations. We highlight that, under strain, the type-I heterostructures can be transformed to type-II with their conduction band minimum (CBM) and valence band maximum (VBM) separated in different layers. Interestingly, the locations of the CBM or VBM in MXene/BlueP vdW heterostructures can also be reversed by compressive or tensile strain between the building layers, which indicates that either layer can be utilized as an electron donor or acceptor by varying its deformation conditions. Meanwhile, this compressive (tensile) strain can also induce a red (blue) shift in the optical absorption spectra of MXene/BlueP vdW heterostructures. Finally, our results on the mechanical flexibility and deformation mechanism of MXene/BlueP vdW heterostructures suggest their great long-term stability as well as promising applications in flexible devices. We believe that our findings will open a new way for the modulation and development of vdW heterostructures in flexible optical/electronic devices.

Exploring new two-dimensional (2D) crystals attracts great interest in the materials community due to their potential intriguing properties. Here, we report a new family of 2D transition metal borides (labeled as MBenes) that can be produced by selectively etching the A layer from a family of layered transition metal borides (MAB phases). The emerged MBenes are demonstrated to possess great stability with isotropic and ultrahigh Young's modulus. Meanwhile, our results show that 2D Mo2B2 and Fe2B2 MBenes are metallic with excellent electronic conductivity, which are highly desirable for applications in Li-ion batteries (LIB) and electrocatalysis. Furthermore, 2D Mo2B2 and Fe2B2 are confirmed to have an omnidirectional small diffusion energy barrier and high storage capacity for Li atoms, which highlight MBenes as appealing electrode materials for LIBs. Moreover, 2D Fe2B2 MBene also exhibits superior catalytic activity for the hydrogen evolution reaction (HER) with hydrogen adsorption Gibbs free energy close to the optimal value (0 eV), indicating its promising application as an electrocatalyst for hydrogen evolution. Considering the large number of possible MAB phases, more MBenes with attractive applications are anticipated theoretically and/or experimentally in the near future.

Identifying suitable photocatalysts for photocatalytic water splitting to produce hydrogen fuel via sunlight is an arduous task by the traditional trial-and-error method. Thanks to the progress of density functional theory, one can nowadays accelerate the process of finding candidate photocatalysts. In this work, by ab initio calculations, we investigated 48 two-dimensional (2D) transition metal carbides also referred to as MXenes to understand their photocatalytic properties. Our results highlight 2D Zr2CO2 and Hf2CO2 as the candidate single photocatalysts for possible high efficiency photocatalytic water splitting. A significant property of 2D Zr2CO2 and Hf2CO2 is that they exhibit unexpectedly high and directionally anisotropic carrier mobility, which may effectively facilitate the migration and separation of photogenerated electron–hole pairs. Meanwhile, these two MXenes also exhibit very good optical absorption performance in the wavelength ranging approximately from 300 to 500 nm. The stability of 2D Zr2CO2 and Hf2CO2 in liquid water is expected to be good based on ab initio molecular dynamics simulations. Finally, the adsorption and decomposition of water molecules on the 2D Zr2CO2 surface and the subsequent formation process of hydrogen were studied, which contributes to the unravelling of the micro-mechanism of photocatalytic hydrogen production on MXenes. Our findings will open a new way to facilitate the discovery and application of MXenes for photocatalytic water splitting.

Sb2Te3 exhibits outstanding performance among the candidate materials for phase-change memory; nevertheless, its low electrical resistivity and thermal stability hinder its practical application. Hence, numerous studies have been carried out to search suitable dopants to improve the performance; however, the explored dopants always cause phase separation and thus drastically reduce the reliability of phase-change memory. In this work, on the basis of ab initio calculations, we have identified yttrium (Y) as an optimal dopant for Sb2Te3, which overcomes the phase separation problem and significantly increases the resistivity of crystalline state by at least double that of Sb2Te3. The good phase stability of crystalline Y-doped Sb2Te3 (YST) is attributed to the same crystal structure between Y2Te3 and Sb2Te3 as well as their tiny lattice mismatch of only ∼1.1%. The significant increase in resistivity of c-YST is understood by our findings that Y can dramatically increase the carrier’s effective mass by regulating the band structure and can also reduce the intrinsic carrier density by suppressing the formation of SbTe antisite defects. Y doping can also improve the thermal stability of amorphous YST based on our ab initio molecular dynamics simulations, which is attributed to the stronger interactions between Y and Te than that of Sb and Te.Keywords: electrical resistivity; phase-change material; Sb2Te3; thermal stability; Y doping

The solidification behaviors of a Mo-rich Ni3Al based single crystal superalloy at different withdrawal rates were investigated. Experimental analysis and thermodynamic calculation (by JMatPro) both indicated that the high Al and Mo contents in residual liquid promoted the precipitations of γ′ and Mo-rich phases at the last stage of solidification. Mo-rich phase in the cellular microstructure was only α-Mo phase, while two types of Mo-rich phase—α-Mo and γ/NiMo eutectic phases existed in the dendritic microstructures. Solid back diffusion during solidification led to the volume fraction of interdendritic precipitation initially increasing and then decreasing with increasing withdrawal rate.

A modified Ostwald ripening theory was used to evaluate the coarsening behavior of γ′ phases in Ni based single crystal superalloys during thermal exposure. After introducing a dimensionless factor correlated with volume fraction of γ′ phase into the classical Lifshitz–Slyozov–Wagner (LSW) coarsening theory, this model was thus developed by coupling formulaic interfacial energy and diffusion coefficient (including activation energy and pre-exponential factor) as function of alloy composition. The coarsening rate coefficients and radius sizes of γ′ phases were then presented, which show good agreement with the corresponding experimental results.

•Solidification paths of five Ni–Al–Ta model single crystal superalloys were studied.•Liquidus temperature first decreased and then increased with decreasing Al content.•Microsegregation behavior was studied by experiment and simulation methods.•The microsegregation of Al had obvious influence on last stage solidification path.The non-equilibrium solidification behaviors of five Ni–Al–Ta ternary model single crystal alloys with different Al contents were investigated by experimental analysis and theoretical calculation (by JMatPro) in this study. These model alloys respectively represented the γ׳ phase with various volume fractions (100%, 75%, 50%, 25% and 0%) at 900 °C. It was found that with decreasing Al content, liquidus temperature of experimental alloys first decreased and then increased. Meanwhile, the solidification range showed a continued downward trend. In addition, with decreasing Al content, the primary phases of non-equilibrium solidified model alloys gradually transformed from γ׳ phase to γ phase, and the area fraction of which first decreased and then increased. Moreover, the interdendritic/intercellular precipitation of model alloys changed from β phase (for 100% γ׳) to (γ+γ׳)Eutectic (for 75% γ׳), (γ+γ׳)Eutectic+γ׳ (for 50% γ׳ and 25% γ׳) and none interdendritic precipitation (for 0% γ׳), and the last stage non-equilibrium solidification sequence of model alloys was determined by the nominal Al content and different microsegregation behaviors of Al element.

Development of novel van der Waals (vdW) heterostructures from various two-dimensional (2D) materials shows unprecedented possibilities by combining the advantageous properties of their building layers. In particular, transforming the vdW heterostructures from type-I to type-II is of great interest and importance to achieve efficient charge separation in photocatalytic, photovoltaic, and optoelectronic devices. In this work, by means of ab initio calculations, we have systematically investigated the electronic structures, optical properties, and mechanical properties of MXene/Blue Phosphorene (BlueP) vdW heterostructures under various deformations. We highlight that, under strain, the type-I heterostructures can be transformed to type-II with their conduction band minimum (CBM) and valence band maximum (VBM) separated in different layers. Interestingly, the locations of the CBM or VBM in MXene/BlueP vdW heterostructures can also be reversed by compressive or tensile strain between the building layers, which indicates that either layer can be utilized as an electron donor or acceptor by varying its deformation conditions. Meanwhile, this compressive (tensile) strain can also induce a red (blue) shift in the optical absorption spectra of MXene/BlueP vdW heterostructures. Finally, our results on the mechanical flexibility and deformation mechanism of MXene/BlueP vdW heterostructures suggest their great long-term stability as well as promising applications in flexible devices. We believe that our findings will open a new way for the modulation and development of vdW heterostructures in flexible optical/electronic devices.

Identifying suitable photocatalysts for photocatalytic water splitting to produce hydrogen fuel via sunlight is an arduous task by the traditional trial-and-error method. Thanks to the progress of density functional theory, one can nowadays accelerate the process of finding candidate photocatalysts. In this work, by ab initio calculations, we investigated 48 two-dimensional (2D) transition metal carbides also referred to as MXenes to understand their photocatalytic properties. Our results highlight 2D Zr2CO2 and Hf2CO2 as the candidate single photocatalysts for possible high efficiency photocatalytic water splitting. A significant property of 2D Zr2CO2 and Hf2CO2 is that they exhibit unexpectedly high and directionally anisotropic carrier mobility, which may effectively facilitate the migration and separation of photogenerated electron–hole pairs. Meanwhile, these two MXenes also exhibit very good optical absorption performance in the wavelength ranging approximately from 300 to 500 nm. The stability of 2D Zr2CO2 and Hf2CO2 in liquid water is expected to be good based on ab initio molecular dynamics simulations. Finally, the adsorption and decomposition of water molecules on the 2D Zr2CO2 surface and the subsequent formation process of hydrogen were studied, which contributes to the unravelling of the micro-mechanism of photocatalytic hydrogen production on MXenes. Our findings will open a new way to facilitate the discovery and application of MXenes for photocatalytic water splitting.