Ultralow-frequency collective compression mode and strong interlayer coupling in multilayer black phosphorus

TL;DR

This study uncovers an ultralow-frequency collective compression mode in multilayer black phosphorus, revealing strong interlayer coupling and enabling precise layer thickness measurement, which advances understanding and potential device applications of 2D materials.

Contribution

It reports the discovery of an ultralow-frequency collective compression mode in multilayer black phosphorus, demonstrating strong interlayer coupling and a new method for layer thickness determination.

Findings

Discovery of an ultralow-frequency collective compression mode in BP

Strong interlayer coupling evidenced by phonon analysis and calculations

Layer-dependent Raman shifts enable accurate thickness measurement

Abstract

The recent renaissance of black phosphorus (BP) as a two-dimensional 2D layered material has generated tremendous interest in its tunable electronic band gap and highly anisotropic transport properties that offer new opportunities for device applications. Many of these outstanding properties are attributed to its unique structural characters that still need elucidation. Here we show Raman measurements that reveal an ultralow-frequency collective compression mode (CCM), which is unprecedented among similar 2D layered materials. This novel CCM indicates an unusually strong interlayer coupling in BP, which is quantitatively supported by a phonon frequency analysis and first-principles calculations. Moreover, the CCM and another branch of low-frequency Raman modes shift sensitively with changing number of layers, allowing an accurate determination of the thickness up to tens of atomic layers, which is considerably higher than those previously achieved by using high-frequency Raman modes. These results offer fundamental insights and practical tools for exploring multilayer BP in new device applications.