Structural Transition Analysis of Bilayer Black Phosphorus under High Pressure via Ultralow-Frequency Raman Spectroscopy

The unique anisotropic electron-photon and electron-phonon interactions of black phosphorus (BP) set it apart from other isotropic 2D materials. These anisotropic properties can be adjusted by varying the stacking thickness and sequence as well as by applying external pressure and strain. In contrast to multilayer or bulk BP, the effects of pressure on bilayer BP are still not fully elucidated. This study presents experimental evidence that explores the high-pressure response (0-20 GPa) of bilayer BP within the extreme low Raman shift region (5-150 cm