Electronic and optical properties of layered InSe structures from Density Functional Theory and Quantum Monte Carlo methods

We performed Density Functional Theory (DFT) and Quantum Monte Carlo (QMC) calculations of layered InSe to investigate its optical and electronic properties. The post-transition metal chalcogenide InSe has attracted significant attention because of its potential applications in energy conversion, spintronic device, and chemical sensing. While bulk InSe has been known to be a semiconductor with ~ 1.2 eV band gap, electronic and optical properties of the mono- or multi-layer forms are not well known experimentally. We have used ab-initio DFT method to predict their electronic properties; however, we found that the DFT results vary strongly with the choice of exchange-correlation functional. Our DFT results cannot conclusively determine if InSe polytypes possess a direct or indirect gap, and the band gap is entirely closed in bulk β-InSe when using plain DFT, while it is experimentally known as a direct gap semiconductor. Using fixed-node diffusion Monte Carlo (DMC), we seek to achieve improved electronic and optical properties and investigate the interplay between interlayer interactions and electron correlations on InSe polytypes. In addition, our QMC calculations of the monolayer and bilayer provide further insights into the electronic and optical properties of InSe structures.