Understanding the Origin and Implication of the Indirect-to-Direct Bandgap Transition in Multilayer InSe

Indium selenide (InSe) multilayers have attracted much interest recently due to their electronic and optical properties, partially dependent on the existence of an indirect-to-direct bandgap transition that is correlated to the multilayer thickness. In this work, we investigate stacks of van der Waals-bonded multilayer InSe, ordered similarly to the γ phase of bulk InSe. We analyze the indirect-to-direct bandgap transition using first-principles methods, identifying the structural changes in the multilayer structures that cause the electronic modifications that result in this transition. We highlight differences between InSe and transition-metal dichalcogenides. Our calculations confirm the thickness dependence of the crossover between the indirect and direct bandgaps in multilayer InSe and emphasize changes in the structure and orbital nature of the valence and conduction bands. We compare the optical spectra predictions performed at a high level of theory to our thorough experimental photoluminescence characterization of our high-quality γ phase bulk InSe. These predictions are correlated to electronic transitions and elucidate the relative contributions of in-plane and out-of-plane dipoles. The insights gained from our study could contribute to the design of multicomponent heterostructures with InSe for future electronic and electro-optical devices.