Geometry, electronic properties, and optical characteristics of monolayer GeSe monochalcogenides from parameter-free Quantum Monte Carlo simulation*

The GeSe monochalcogenide has received a great deal of attention due to its unique thermoelectric and electronic properties that can be exploited in wide range of industrial devices. While bulk GeSe has been known as a semiconductor with a 1.2 ~ 1.5 eV band gap, the optical characteristics of the GeSe monolayer form are not well known experimentally. Theoretical studies based on density functional theory (DFT) have been predicting slightly larger band gap energy for monolayer than bulk; however, DFT cannot conclusively determine detailed optical characteristics because the DFT-optimized geometry and computed band gap energy vary strongly with the choice of exchange-correlation functional. Using fixed-node diffusion Monte Carlo (DMC), we fully optimize atomic coordinates and lattice parameters for the monolayer by minimizing DMC energy based on the DFT energy Hessian. In the optimized geometry, we find band structure and band gap energies that differ from the DFT results. Based on our DMC band gap energies, we conclude that GeSe optical properties are sensitive to strain and can therefore be manipulated, and furthermore that DFT underestimates gap energies for the monolayer.