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.
The considerable thermal expansion of halide perovskites is one of the challenges to device stability, yet the physical origin and modulation strategy remain unclear. Herein, we report first-principles calculations of the thermal properties of halide perovskites at 300 K using oxides as a reference. We found that the large thermal expansion of halide perovskites can mainly be attributed to their low bulk modulus and volumetric heat capacity because of the soft crystal lattice, whereas composition-dependent anharmonicity emerges as the most important factor in determining thermal expansion with the same structure. We discovered that thermal expansion of halide perovskites can be decreased by weakening the B−X bond to promote the octahedral anharmonicity. We further proposed an effective thermal expansion coefficient descriptor of halide perovskites with a Pearson correlation coefficient of nearly −80%. Our findings provide insights into the underlying mechanisms and chemical trends in the thermal expansion behavior of halide perovskites.
The easy transferability and feasible epitaxial growth of two‐dimensional semiconductors pave a promising way to further tune their optoelectronic properties by constructing van der Waals heterostructures. The authors herein performed a high‐throughput first‐principles study of electronic structure and optical properties of the layer‐by‐layer stacking heterostructing superlattices of two‐dimensional transition metal dichalcogenides. The indirect‐to‐direct band‐gap transition or even semiconductor‐to‐metal transition can be realized by varying component compositions of superlattices. The results provide useful guidance for engineering optoelectronic properties by forming heterostructures and superlattices in two‐dimensional semiconductors. (DOI: https://doi.org/10.1002/inf2.12155)
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