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

[Display omitted] 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-NbSe 2 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 Nb 2 O 5 precursor. Scanning transmission electron microscopy and Raman measurements reveal the NbSe 2 exhibits a pure 3R stacking mode with relatively weak interlayer van der Waals interactions. Importantly, 3R-NbSe 2 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-NbSe 2. We designed a NbSe 2 /WS 2 /NbSe 2 photodetector utilizing the metallicity of 3R-NbSe 2 , 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. [ABSTRACT FROM AUTHOR]