Resonance Raman Process and Exciton Engineering in MoS₂-WS₂ Vertical Heterostructure

Two-dimensional (2D) materials have emerged as a fascinating platform to explore novel optical and electrical properties at the nanoscale. Designing suitable heterostructures has the potential to offer enhanced and tunable physical properties and to broaden their applications in the field of nanoelectronics and optoelectronics, which are not achievable using individual monolayers. In this study, we present the optical characterization of a salt-assisted chemical vapor deposition-grown MoS2-WS2 vertical heterostructure, where a continuous monolayer of MoS2 is transferred over WS2 layers with different stacking configurations using a PMMA-assisted transfer method. The heterostructures are characterized by micro-Raman and photoluminescence spectroscopy under various conditions, revealing a few interesting results. The resonance Raman effect with different laser excitations leads to a noticeable increase in intensity for both the characteristic and low-frequency vibrational modes in WS2 compared to MoS2. Additionally, as the layer number of WS2 increases, two novel Raman modes have emerged from the subject heterostructure, which show crossed polarization dependency. Low-temperature photoluminescence spectroscopy reveals the evolution of interlayer exciton peaks as a consequence of type-II band alignment in the MoS2-WS2 heterostructures. Furthermore, our first-principles density functional theory (DFT) calculations shed light on the observed optical properties of the vertical heterostructure. These findings highlight the potential for tailoring the structural and vibrational properties of 2D heterostructures by engineering a van der Waals heterostructure.