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
