The indirect‐to‐direct band‐gap transition in transition metal dichalcogenides (TMDCs) from bulk to monolayer, accompanying with other unique properties of two‐dimensional materials, has endowed them great potential in optoelectronic devices. The easy transferability and feasible epitaxial growth pave a promising way to further tune the optical properties by constructing van der Waals heterostructures. Here, we performed a systematic high‐throughput first‐principles study of electronic structure and optical properties of the layer‐by‐layer stacking TMDCs heterostructing superlattices, with the configuration space of [(MX2)n(M′X′2)10−n] (M/M′ = Cr, Mo, W; X/X′ = S, Se, Te; n = 0‐10). Our calculations involving long‐range dispersive interaction show that the indirect‐to‐direct band‐gap transition or even semiconductor‐to‐metal transition can be realized by changing component compositions of superlattices. Further analysis indicates that the indirect‐to‐direct band‐gap transition can be ascribed to the in‐plane strain induced by lattice mismatch. The semiconductor‐to‐metal transition may be attributed to the band offset among different components that is modified by the in‐plane strain. The superlattices with direct band‐gap show quite weak band‐gap optical transition because of the spacial separation of the electronic states involved. In general, the layers stacking‐order of superlattices results in a small up to 0.2 eV band gap fluctuation because of the built‐in potential. Our results provide useful guidance for engineering band structure and optical properties in TMDCs heterostructing superlattices.
Currently, a major obstacle restricting the commercial application of halide perovskites is their low thermodynamic stability. Herein, inspired by the high-stability high-entropy alloys, we theoretically investigated a variety of multielement double-perovskite alloys. First-principles calculations show that the entropy contribution to Gibbs free energy, which offsets the positive enthalpy contribution by up to 35 meV/f.u., can significantly enhance the material stability of double-perovskite alloys. We found that the electronic properties of bandgaps (1.04–2.21 eV) and carrier effective masses (0.34 to greater than 2 m 0) of the multielement double-perovskite alloys can be tuned over a wide range. Meanwhile, the parity-forbidden condition of optical transitions in the Cs2AgInCl6 perovskite can be broken because of the lower symmetry of the configurational disorder, leading to enhanced transition intensity. This work demonstrates a promising strategy by utilizing the alloy entropic effect to further improve the material stability and optoelectronic performance of halide perovskites.
At present, the main gas-sensing mechanism of oxidized MXene (Ti 3 C 2 T x ) is commonly regarded as Schottky barrier modulation, but the influence of surface defects generated by oxidation is ignored and ambiguous. Herein, oxidized Ti 3 C 2 T x crumpled spheres (MS) are obtained, accompanying numerous surface defects through thermal oxidation of MS synthesized by ultrasonic spray pyrolysis technology and gas-sensing properties of oxidized MS with Ti 3 C 2 T x /TiO 2 crumpled spheres (MT-10-1) without new surface defects are compared. It is demonstrated that the significant improvement of the gas-sensing properties of oxidized MS is due to the introduction of Ti atom defects rather than Ti 3 C 2 T x /TiO 2 heterojunction in-situ generated by oxidation. First-principles density functional theory calculations show that Ti atom vacancy can greatly improve the adsorption ability of Ti 3 C 2 T x to gases (especially for NO 2 ). Subsequently, with the facile oxidability, Ti 3 C 2 T x is utilized as a reductant to assist the reduction of graphene oxide, and Ti 3 C 2 T x /TiO 2 /rGO crumpled spheres are subtly designed and successfully synthesized for further enhancing the gassensing performance. The MG-2-1 sensor achieves a low detection limit of NO 2 (10 ppb), great NO 2 selectivity, and high NO 2 response. The clarification of the gas-sensing mechanism of oxidized Ti 3 C 2 T x and the utilization of oxidation of Ti 3 C 2 T x provide a new idea for the application of MXenes.
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