Enhancement of the WS₂ A1g Raman Mode in MoS₂/WS₂ Heterostructures

When combined into van der Waals heterostructures, transition metal dichalcogenide monolayers enable the exploration of novel physics beyond their unique individual properties. However, for interesting phenomena such as interlayer charge transfer and interlayer excitons to occur, precise control of the interface and ensuring high-quality interlayer contact is crucial. Here, we investigate bilayer heterostructures fabricated by combining chemical-vapor-deposition-grown MoS₂ and exfoliated WS₂ monolayers, allowing us to form several heterostructures with various twist angles within one preparation step. In case of sufficiently good interfacial contact, evaluated by photoluminescence quenching, we observe a twist-angle-dependent enhancement of the WS₂ A1g Raman mode. In contrast, other WS₂ and MoS₂ Raman modes (in particular, the MoS₂ A1g mode) do not show a clear enhancement under the same experimental conditions. We present a systematic study of this mode-selective effect using nonresonant Raman measurements that are complemented with ab-initio calculations of Raman spectra. We find that the selective enhancement of the WS₂ A1g mode exhibits a strong dependence on interlayer distance. We show that this selectivity is related to the A1g eigenvectors in the heterolayer: the eigenvectors are predominantly localized on one of the two layers; yet, the intensity of the MoS₂ mode is attenuated because the WS₂ layer is vibrating (albeit with much lower amplitude) out of phase, while the WS₂ mode is amplified because the atoms on the MoS₂ layer are vibrating in phase. To separate this eigenmode effect from resonant Raman enhancement, our study is extended with near-resonant Raman measurements.