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

Abstract

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 SnS2/MoS2 vdWhs were prepared for the study of EPC and valley polarization by 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 E2g 1 mode of MoS2 in the heterobilayer and heterotrilayer, respectively, concurrently with an isotropic-to-anisotropic transition of the E2g 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 MoS2 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.

Contact Geometry-Dependent Excitonic Emission in Mixed-Dimensional van der Waals Heterostructures

“… , Figure S8 illustrates a statistical comparison of these peak shifts in our heterostructures according to the contact geometry, where c-PbI 2 /WSe 2 exhibits a relatively lower degree of shift (0.18 ± 0.06 cm –1 ) in the E 1 2g (A 1g ) mode compared to a-PbI 2 /WSe 2 (0.26 ± 0.03 cm –1 ). Although these shifts are minor, they suggest that heterointerfacial atomic configurations result in distinctive electron–phonon interaction between PbI 2 and the underlying WSe 2 , alongside variations in electron-doping efficiency. , Consequently, the local charge transfer at the heterojunction can vary based on the structural characteristics of the vdW heterostructure, which is likely contingent upon changes in the atomic contact geometry at the junction area, as will be further discussed below.…”

“…Although these shifts are minor, they suggest that heterointerfacial atomic configurations result in distinctive electron−phonon interaction between PbI 2 and the underlying WSe 2 , alongside variations in electron-doping efficiency. 40,41 Consequently, the local charge transfer at the heterojunction can vary based on the structural characteristics of the vdW heterostructure, which is likely contingent upon changes in the atomic contact geometry at the junction area, as will be further discussed below. Figure 1g demonstrates the photoluminescence (PL) spectra of a WSe 2 flake, as well as c-PbI 2 and a-PbI 2 nanowires.…”