Layer-Dependent Valley Depolarization and Raman Phonon Softening of SnS₂/MoS₂ Vertical van der Waals Heterostructures

Two-dimensional van der Waals heterostructures (vdWhs)have arousedtremendous attention due to valley polarization and the interlayercoupling effect, providing a promising platform for valleytronics.However, the correlation between electron-phonon coupling (EPC)and valley depolarization is still less studied for vdWhs. In thiswork, heterobilayered and heteromultilayered SnS2/MoS2 vdWhs were prepared for the study of EPC and valley polarizationby a two-step chemical vapor deposition method. The SnS2 layer has a good epitaxial relationship with the underneath MoS2 monolayer. Raman red shifts of 0.3 and 1.0 cm(-1) were found for the E-2g (1) mode of MoS2 in the heterobilayer and heterotrilayer, respectively, concurrentlywith an isotropic-to-anisotropic transition of the E-2g (1) vibration intensity for the latter. By a helicity-resolvedphotoluminescence study, layer-dependent valley depolarization wasrevealed with the helicity of the A exciton of MoS2 decreasingfrom 57 to 9% for the heterobilayer and close to zero for the heterotrilayer.Strong EPC was confirmed by low-temperature resonant Raman scatteringcoexisting with helicity-resolved PL, and the layer-dependent evolutiondemonstrates the intensive relation of valley depolarization withcharge transfer and localized EPC with non-zone-center phonons. Basedon the layer-dependent band structure calculated by density functionaltheory, intervalley charge transfer and phonon-assisted intervalleyscattering were proposed for layer-dependent valley depolarization.The strong EPC effect also results in layer-dependent Raman shifts.