Twist Angle-Dependent Optical Responses in Controllably Grown WS2 Vertical Homojunctions

Interlayer coupling in two-dimensional (2D) materials with specific stacking modes results in angle-dependent electrical and optical behaviors. Compared to multilayered WS2 with a 0 degrees twist angle grown under normal conditions, WS2 sheets with a 60 degrees twist angle are prepared by chemical vapor deposition (CVD) with the assistance of tin (Sn) for reducing stacking energy. The layer number can be effectively regulated by the growth temperature and hydrogen flow. The 2H-like phase of the 60 degrees bilayer structure is revealed by scanning transmission electron microscopy (STEM), displaying intense photoluminescence (PL) variation and diverse second harmonic generation due to interlayer coupling. The bilayer WS2 with various twist angles can be obtained for angle-dependent Raman/PL response. Density functional theory (DFT) calculations demonstrate that heteroatom Sn can tip the balance between 0 degrees and 60 degrees bilayers, facilitating the formation of the bilayer with a 60 degrees stacking angle. The heteroatom-assisted approach provides a general strategy for the selfassembly of 2D materials. The heterostructures with tunable twist angles can be prepared, which broadens their applications in moire excitons, spintronics, and valley electronics of transition metal dichalcogenides.