Layer-dependent exciton dynamics in InSe/WS₂ heterostructures

Understanding interlayer charge transfer is crucial for elucidating interface interactions in heterostructures. As the layer number can significantly influence the interface coupling and band alignment, the charge transfer behaviors can be largely regulated. Here, we constructed two-dimensional (2D) heterostructures consisting of monolayer WS₂ and few-layer InSe to investigate the impact of InSe thickness on exciton dynamics. We performed photoluminescence (PL) spectroscopy and lifetime measurements on pristine few-layer InSe and the heterostructures with different InSe thicknesses. For pristine InSe layers, we found a non-monotonic layer dependence on PL lifetime, which can be attributed to the interplay between the indirect-to-direct bandgap transition and surface recombination effects. For heterostructures, we demonstrated that the type I band alignment of the heterostructure facilitates electron and hole transfer from monolayer WS₂ to InSe. As the InSe layer number increases, the reduction in conduction band minimum (CBM) enhances the driving force for charge transfer, thereby improving the transfer efficiency. Furthermore, we fabricated and characterized a WS₂/InSe optoelectronic device. By analyzing bias voltage dependent PL spectra, we further demonstrated that the trions in WS₂ within the heterostructure are positively charged (X⁺), and their emission intensity can be efficiently modulated by applying different biases. This study not only reveals the layer-dependent characteristics of band alignment and interlayer charge transfer in heterostructures but also provides valuable insights for the applications of 2D semiconductors in optoelectronic devices.