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

The\nunique anisotropic electron–photon and electron–phonon\ninteractions of black phosphorus (BP) set it apart from other isotropic\n2D materials. These anisotropic properties can be adjusted by varying\nthe stacking thickness and sequence as well as by applying external\npressure and strain. In contrast to multilayer or bulk BP, the effects\nof pressure on bilayer BP are still not fully elucidated. This study\npresents experimental evidence that explores the high-pressure response\n(0–20 GPa) of bilayer BP within the extreme low Raman shift\nregion (5–150 cm<sup>–1</sup>), aiming at verifying\nstructural phase transitions and stacking sequence variations. Notably,\nphase transitions were observed at ∼7 GPa for the orthorhombic\nto rhombohedral transition and at ∼14–16 GPa for a further\ntransition to a simple cubic structure. The effects of pressure on\nbilayer BP with three distinct stacking sequences (AA, AB, and AC)\nwere analyzed using angular-resolved polarized Raman spectroscopy\n(ARPR). The recovery of all characteristic Raman modes after the application\nof pressure substantiates the reversibility of the structural phase\ntransitions in bilayer BP across all three stacking sequences. However,\nsignificant alterations in the stacking sequences were observed before\nand after the application of hydrostatic pressure. Specifically, AA\nand AC stacking sequences showed a tendency to relax into an optimized\ninterlayer AB stacking structure upon the release of pressure from\n20 to 0 GPa. This observed change in the stacking sequences of bilayer\nBP under pressure is detected for the first time in this work. The\npressure-induced modifications in the stacking sequences, and consequently\nin the anisotropic properties of bilayer BP, highlight its potential\nas a promising material for pressure- (or strain-) sensitive optoelectronic\nnanodevices. Additionally, this study offers novel insights into the\nanalysis of anisotropic interlayer interactions in other van der Waals-stacked\n2D materials.