Observation of Low-Frequency Vibrational Modes in Anisotropic 2D Violet Phosphorus

Violet phosphorus (VP), the most stable layered allotrope of phosphorus, has emerged as a promising 2D optoelectronic material due to its large direct bandgap and highly anisotropic lightmatter interactions. While bulk properties of VP have been extensively studied, its layered characteristics remain largely unexplored. In this work, low-frequency (LF) (below 50 (Formula presented.)) interlayer shear (S) and layer-breathing (LB) modes of few-layer VP flakes are observed and characterized for the first time using linearly polarized Raman spectroscopy. Polarization-dependent measurements and group theory analysis show that S and LB modes exhibit (Formula presented.) - and (Formula presented.) -symmetry, respectively. The frequency evolution of S and LB modes as a function of the layer number enables the interlayer coupling strength estimation using a linear chain model. Notably, the in-plane anisotropy of VP gives rise to two distinct branches of the S mode. Beyond LF interlayer vibrational modes, substrate-induced (Formula presented.) -symmetry phonon modes appear in the ultralow-frequency range (below 20 (Formula presented.)) from two- to eight-layer VP and are described by a simple harmonic oscillator model. This work provides fundamental insights into the anisotropic phonon dynamics and interlayer interactions in VP, establishing a foundation for its exploration as a next-generation 2D optoelectronic semiconductor.