Analysis of the indirect to direct bandgap transition in multilayer InSe by First Principles Calculations

Layered indium selenide (InSe) has recently emerged as a promising material, for example, as the channel material in field-effect transistors or for nonlinear optical responses upon liquid exfoliation. However, an understanding of the indirect to direct bandgap transition, as dependent on the thickness of the multilayer structure, is still lacking. Here, we investigate stacks of van der Waals-bonded InSe, assuming an ordering that corresponds to the bulk structure. By investigating the bandgap transition using first-principles methods, including G₀W₀ calculations starting from PBE+D3, we determine the structural modifications in the multilayer that lead to the observed electronic transition. Our calculations confirm the thickness dependence of the crossover between the indirect and direct bandgaps in multilayer InSe (at approximately 6 layers) and provide insight into the changes in the structure and orbital nature of the valence and conduction bands during this transition. Our work represents a foundational step in understanding the structural changes that occur during the bandgap transition that will guide the design of multi-component heterostructures containing InSe in electronic and electro-optical devices.