Observation of A$_g^1$ Raman mode splitting in few layers black phosphorus encapsulated with hexagonal boron nitride

TL;DR

This study reveals how hexagonal boron nitride encapsulation affects the Raman spectral features of few-layer black phosphorus, notably reducing phonon scattering and enabling the detection of a new inter-layer vibrational mode.

Contribution

It is the first experimental observation of the A$_g^1$ mode splitting in black phosphorus due to h-BN encapsulation, highlighting inter-layer interactions.

Findings

Encapsulation reduces Raman line width, indicating lower phonon scattering.

A new Raman peak ~4cm$^{-1}$ above A$_g^1$ mode is observed in trilayer and thicker flakes.

The new mode is linked to inter-layer interactions and surface atom vibration hardening.

Abstract

We investigate the impact of the encapsulation with hexagonal boron nitride (h-BN) on the Raman spectrum of few layer black phosphorus. The encapsulation results in a significant reduction of the line width of the Raman modes of black phosphorus, due to a reduced phonon scattering rate. We observe a so far elusive peak in the Raman spectra $\sim$4cm$^{-1}$ above the A$_{\text{g}}^1$ mode in trilayer and thicker flakes, which had not been observed experimentally. The newly observed mode originates from the strong black phosphorus inter-layer interaction, which induces a hardening of the surface atoms vibration with respect to the corresponding modes of the inner layers. The observation of this mode suggests a significant impact of h-BN encapsulation on the properties of black phosphorus and can serve as an indicator of the quality of its surface.