CVD Synthesis of Twisted Bilayer WS₂ with Tunable Second Harmonic Generation

The introduction of rotational freedom by twist angles in twisted bilayer (TB) transition metal dichalcogenides (TMDCs) can tailor the inherent properties of the TMDCs, which provides a promising platform to investigate the exotic physical properties. However, direct synthesis of high-quality TB-TMDCs with full twist angles is significantly challenging due to the substantial energy barriers during crystal growth. Here, a modified chemical vapor deposition strategy is proposed to synthesize TB-WS₂ with a wide twist angle range from 0° to 120°. Utilizing a tilted SiO₂/Si substrate, a gas flow disturbance is generated in the furnace tube to create a heterogeneous concentration gradient of the metal precursor, which provides an extra driving force for the growth of TB-WS₂. The Raman and photoluminescence results confirm a weak interlayer coupling of the TB-WS₂. High-quality periodic Moiré patterns are observed in the scanning transmission electron microscopy images. Moreover, owing to the strong correlation between the nonlinear optical response and the twisted crystal structure, tunable second harmonic generation behaviors are realized in the TB-WS₂. This approach opens up a new avenue for the direct growth of high-crystalline-quality and pristine TB-TMDCs and their potential applications in nonlinear optical devices.