Pressure-tunable stacking order in few-layer WS₂ revealed by second harmonic generation

In 2D material, interlayer interaction can be tuning by internal pressure. We experimentally demonstrate a controllable interlayer translation freedom in hBN-encapsulated few-layer WS₂ under applying hydrostatic pressure, which is characterized by Raman spectroscopy and the polarization-resolved second harmonic generation (SHG) spectroscopy. The E_2g¹ and A_1g modes of all samples increased with pressure and there is no new peak observed in the Raman spectrum. The SHG patterns in monolayer WS₂ have insignificant distortion and the SHG signal in bilayer WS₂ is negligible. Meanwhile, SHG patterns in trilayer WS₂ show six asymmetric lobes. Such observations suggest that the threefold rotational symmetry is broken under pressure, which origins from a relative sliding between adjacent layers in the zigzag direction. The tunability of translation structural symmetry by using a hydrostatic pressure presented in this work provides a platform to explore novel physical properties in new class of 2D materials.