Diffusion Quantum Monte Carlo calculations of layered In₂Se₃ polytypes

In₂Se₃ is a semiconductor that can be found in a variety of crystal structures, most of them forming two dimensional layers stacked via van der Waals interactions. The different layered crystal structures, composed of quintuple Se-In-Se-In-Se layers, show very different electrical and optical properties that can be leveraged in a variety of device applications, including solar cells, photodetectors and phase-change memory devices. Yet, the phase ordering and the transition between them remains unclear. Density functional theory calculations show large variations in total energy differences between the different phases of In₂Se₃, dependent of the functional and the van der Waals correction used. Here we use diffusion Monte Carlo (DMC) calculations to accurately determine the total energy and charge density differences between the three most stable layered structures of In₂Se₃, namely, α, α’, and β, which differ in bonding within the quintuple layer and stacking of quintuple layers. Results for single quintuple layer will also be discussed.