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

Two-dimensional van der Waals heterostructures (vdWhs) have aroused tremendous attention due to valley polarization and the interlayer coupling 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 this work, heterobilayered and heteromultilayered SnS 2 /MoS 2 vdWhs were prepared for the study of EPC and valley polarization by a two-step chemical vapor deposition method. The SnS 2 layer has a good epitaxial relationship with the underneath MoS 2 monolayer. Raman red shifts of 0.3 and 1.0 cm –1 were found for the E 2g 1 mode of MoS 2 in the heterobilayer and heterotrilayer, respectively, concurrently with an isotropic-to-anisotropic transition of the E 2g 1 vibration intensity for the latter. By a helicity-resolved photoluminescence study, layer-dependent valley depolarization was revealed with the helicity of the A exciton of MoS 2 decreasing from 57 to 9% for the heterobilayer and close to zero for the heterotrilayer. Strong EPC was confirmed by low-temperature resonant Raman scattering coexisting with helicity-resolved PL, and the layer-dependent evolution demonstrates the intensive relation of valley depolarization with charge transfer and localized EPC with non-zone-center phonons. Based on the layer-dependent band structure calculated by density functional theory, intervalley charge transfer and phonon-assisted intervalley scattering were proposed for layer-dependent valley depolarization. The strong EPC effect also results in layer-dependent Raman shifts.