Van der Waals TMDC Heterostructures: electronic structure dependence on number of layers

Two-dimensional (2D) transition metal dichalcogenides (TMDCs) of type MX₂ (M = Mo, W; X = S, Se) have attracted considerable attention in electronics and optoelectronics due to their extraordinary properties, which differ strongly between single layers and bulk materials. Moreover, weak van der Waals interactions between layers make it possible to exfoliate them and stack with different TMDC layers to form heterostructures, which offer remarkable electronic properties. In this work, we have studied interlayer coupling between various TMDC layers, which differ either in chalcogen or transition-metal atoms. Furthermore, we have investigated stacking on 3, 4, and 5 layers. The property we were interested in was the electronic structure, in particular, which stacking yields direct band gap materials. For instance, stacking a monolayer of WSe₂ on top of MoS₂ multi-layer (2 or more layers) can yield direct (type-II) band gap for up to 6-layers system (i.e., 1L WSe₂ and 5L MoS₂) whereas vice versa yields in-direct gap for even 5-layers system (i.e., 1L MoS₂ and 4L WSe₂). We believe that our findings could greatly contribute to the potential applications of heterostructure TMDCs in photovoltaics, energy harvesting, and solar cells.