Observation of interlayer phonon modes in van der Waals heterostructures

TL;DR

This study uses ultralow-frequency Raman spectroscopy to identify layer-breathing modes in van der Waals heterostructures, revealing their dependence on interlayer contact and orientation, and linking them to interlayer charge transfer.

Contribution

It is the first to observe and analyze layer-breathing modes in various TMD heterostructures, establishing LBM as a probe for interlayer interactions.

Findings

LBM Raman features appear at 30-40 cm-1 in TMD heterostructures.

LBM intensity correlates with suppression of photoluminescence.

LBM frequency varies systematically with layer orientation.

Abstract

We have investigated the vibrational properties of van der Waals heterostructures of monolayer transition metal dichalcogenides (TMDs), specifically MoS2/WSe2 and MoSe2/MoS2 heterobilayers as well as twisted MoS2 bilayers, by means of ultralow-frequency Raman spectroscopy. We discovered Raman features (at 30 ~ 40 cm-1) that arise from the layer-breathing mode (LBM) vibrations between the two incommensurate TMD monolayers in these structures. The LBM Raman intensity correlates strongly with the suppression of photoluminescence that arises from interlayer charge transfer. The LBM is generated only in bilayer areas with direct layer-layer contact and atomically clean interface. Its frequency also evolves systematically with the relative orientation between of the two layers. Our research demonstrates that LBM can serve as a sensitive probe to the interface environment and interlayer interactions in van der Waals materials.