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The modulation of exciton energy and state density of layer-structured transition metal dichalcogenides (TMDs) is required for diverse optoelectronic device applications. Here, the spontaneous inversion of exciton state population in monolayer MoS2 is observed by turning the pump light power. The excitons prefer to exist in low energy state under low pump power, but reverse under high pump power. To discuss the mechanism in depth, we propose a semiclassical model by combining the rate equation and photo−exciton interaction. Considering the modifying of exciton−exciton annihilation, the spontaneous inversion of exciton state population is phenomenologically described.

2D BiCuSeO is an intrinsic p-type degenerate semiconductor due to its self-doping effect, which possesses great potential to fabricate high-performance 2D-2D tunnel field-effect transistors (TFETs). However, the controllable synthesis of multinary 2D materials by chemical vapor deposition (CVD) is still a challenge due to the restriction of thermodynamics. Here, the CVD synthesis of quaternary 2D BiCuSeO nanosheets is realized. As-grown BiCuSeO nanosheets with thickness down to ≈6.1 nm (≈7 layers) and domain size of ≈277 µm show excellent ambient stability. Intrinsic p-type degeneracy of BiCuSeO, capable of maintaining even in a few layers, is comprehensively unveiled. By varying the thicknesses and temperatures, the carrier concentration of BiCuSeO nanosheets can be adjusted in the range of 10

Constructing heterojunction is a promising approach to enhance the piezo-photocatalytic performance of ZnO nanostructures. In this study, ZnO/ZnS heterojunction nanoarrays have been synthesized by facile vulcanizing surface of wurtzite ZnO nanorods into blende ZnS nanoparticles through hydrothermal method. Both the synthesized wurtzite ZnO nanorods and blende ZnS nanoparticles are piezoelectric semiconductors with wide band gaps. For degrading methyl blue solution under UV irradiation and ultrasonic sonication, it is found that both the piezocatalytic, photocatalytic and piezo-photocatalytic performances of ZnO/ZnS nanoarrays are much higher than those of pure ZnO. In addition, the piezo-photocatalytic degradation rate of ZnO/ZnS nanoarrays can be further increased by improving the ultrasonic power. The mechanism can be attributed to the synergetic effect of piezoelectric effect and heterojunction photocatalysis.