Impact of interlayer distance on the electronic properties of WS₂/WSe₂ semiconducting heterobilayer

In this work, we employed density functional theory (DFT) within the generalized gradient approximation framework to investigate the electronic properties of a semiconducting WS₂/WSe₂heterobilayer with an AA' stacking configuration. The heterostructure consists of strained monolayers WS₂ and WSe₂. Fully relativistic DFT calculations reveal that spin-orbit coupling (SOC) leads to a robust K-K direct band gap and a moderate valence band offset, resulting in hole localization in the WSe₂ layer, whereas electrons remain localized in the WS₂ layer and are unaffected by SOC. These properties can be tuned by varying the interlayer distance. Specifically, gradually increasing the interlayer spacing enhances the electronic properties, leading to an increased K-K band gap and a stabilized valence band offset, thereby reinforcing the type-II band alignment. As a result, the heterostructure can support spatially indirect excitons, which are characteristic of type-II band-aligned systems. Conversely, reducing the interlayer distance shows that beyond a critical value, where the structure exhibits nearly degenerate direct and indirect band gaps, the gap becomes indirect (Γ-K) and the heterostructure adopts a type-II band alignment with a vanishing valence band offset. In this regime, holes are delocalized across the heterobilayer, while electrons remain strongly localized in the WS₂ layer due to the stable conduction band offset. Consequently, despite the type-II alignment, the negligible valence band offset and electron localization suggest the possible formation of quasi-direct excitons within the WS₂ layer.