Observation of Low-frequency Interlayer Breathing Modes in Few-layer Black Phosphorus

TL;DR

This study reports the first observation of low-frequency interlayer breathing modes in few-layer black phosphorus using Raman spectroscopy and theoretical analysis, revealing their sensitivity to layer number and potential for material characterization.

Contribution

First experimental and theoretical identification of low-frequency interlayer breathing modes in few-layer black phosphorus, highlighting their use in probing layer thickness and orientation.

Findings

LF breathing modes are sensitive to interlayer coupling.

Breathing modes depend strongly on the number of layers.

Modes exhibit harmonic behavior with temperature.

Abstract

As a new two-dimensional layered material, black phosphorus (BP) is a promising material for nanoelectronics and nano-optoelectronics. We use Raman spectroscopy and first-principles theory to report our findings related to low-frequency (LF) interlayer breathing modes (<100 cm-1) in few-layer BP for the first time. The breathing modes are assigned to Ag symmetry by the laser polarization dependence study and group theory analysis. Compared to the high-frequency (HF) Raman modes, the LF breathing modes are much more sensitive to interlayer coupling and thus their frequencies show much stronger dependence on the number of layers. Hence, they could be used as effective means to probe both the crystalline orientation and thickness for few-layer BP. Furthermore, the temperature dependence study shows that the breathing modes have a harmonic behavior, in contrast to HF Raman modes which are known to exhibit anharmonicity.