Observation of interlayer phonon modes in van der Waals heterostructures

We have investigated the vibrational properties of van der Waals heterostructures of monolayer transition metal dichalcogenides (TMDs), specifically $\mathrm{Mo}{\mathrm{S}}_{2}/\mathrm{WS}{\mathrm{e}}_{2}$ and $\mathrm{MoS}{\mathrm{e}}_{2}/\mathrm{Mo}{\mathrm{S}}_{2}$ heterobilayers and twisted $\mathrm{Mo}{\mathrm{S}}_{2}$ bilayers, by means of ultralow-frequency Raman spectroscopy. We discovered Raman features (at $30\char21{}40\phantom{\rule{0.16em}{0ex}}{\mathrm{cm}}^{\ensuremath{-}1}$) that arise from the layer-breathing mode (LBM) vibration 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 an atomically clean interface. Its frequency also evolves systematically with the relative orientation between the two layers. Our research demonstrates that the LBM can serve as a sensitive probe to the interface environment and interlayer interactions in van der Waals materials.