Controlled growth of 3R phase niobium diselenide and its properties.

Inversion symmetry broken 3R phase transition metal dichalcogenides (TMDs) show fascinating prospects in spintronics, valleytronics, and nonlinear optics. However, the controlled synthesis of 3R phase TMDs is still a great challenge. In this work, two-dimensional 3R-NbSe2 single crystals up to 0.2 mm were synthesized for the first time through chemical vapor deposition method by designing a space-confined system. The crystal size and morphology can be controlled by the location of the stacked substrates and the amount of the Nb2O5 precursor. Scanning transmission electron microscopy and Raman measurements reveal the NbSe2 exhibits a pure 3R stacking mode with relatively weak interlayer van der Waals interactions. Importantly, 3R-NbSe2 shows obvious second harmonic generation signal which intensity intensified as thickness increases. Density functional theory calculations and optical absorption demonstrate the coexistence of metallic and semiconducting optical properties of 3R-NbSe2. We designed a NbSe2/WS2/NbSe2 photodetector utilizing the metallicity of 3R-NbSe2, which shows good performance especially an ultrafast response (6-7 µs, 0.5 ms - 7.9 s for Au electrodes in literature). The proposed strategy and findings are of great significance for the growth of many other 3R-TMDs and applications of nonlinear optical and ultrafast devices.

Highly Sensitive Self-Powered Humidity Sensor Based on a TaS2/Cu2S Heterostructure Driven by a Triboelectric Nanogenerator.

Self-powered humidity sensors with high response and good stability have attracted extensive interest in environmental monitoring, medical and health care, and sentiment detection. Because of its high specific surface area and good conductivity, two-dimensional material has wide application in the field of humidity sensing. In this work, we proposed a novel self-powered high-performance TaS2/Cu2S heterostructure-based humidity sensor driven by a triboelectric nanogenerator (TENG) made with the same structure. The TaS2/Cu2S heterostructure was prepared via the chemical vapor deposition method, and then, electrolytic and ultrasound treatments were introduced to further increase the surface area. The fabricated humidity sensor showed ultrahigh sensitivity (S = 3.08 × 104), fast response (2 s), low hysteresis (3.5%), and great stability. First-principles calculation results demonstrated the existence of an electron transport channel with a low energy barrier (-0.156 eV) from the Cu2S to TaS2 layer in the heterostructure, which improves the surface charge transfer of the material. The TaS2/Cu2S heterojunction-based TENG can generate an output voltage of 30 V and an output current of 2.9 µA. Furthermore, the proposed self-powered humidity sensor verified the potential ability of detecting human respiratory frequency, skin humidity, and environmental humidity. This work provides a new and feasible path for research in the field of humidity sensors and promotes the application development of self-powered electronic devices.