Structural and optical properties of bulk and monolayer GeSe : A Quantum Monte Carlo Study

We have performed quantum Monte Carlo simulations of the monochalcogenide GeSe to study its structural and optical properties. 2D GeSe has received a great deal of attention due to its wide range of applications in industrial devices, such as photodetector, gas sensor, and anode material. Density functional theory (DFT) studies show that the monolayer has smaller lattice parameters than the bulk system, with small band gap energy (1~2 eV). However, DFT cannot conclusive determine if the monolayer has a direct or indirect band gap because of very small differences (~ 0.02 eV) between direct and indirect gaps. Moreover, the geometry for GeSe is not clear within DFT scheme because the DFT lattice parameters and atomic coordinates vary strongly with the particular exchange-correlation functional used. Using fixed-node diffusion Monte Carlo (DMC), we reproduce accurate lattice parameters and band gaps compared to the experimental values for bulk GeSe. For the monolayer, we find that the DMC optimal lattice parameters for the monolayer are close to the bulk ones, and DFT significantly underestimates its lattice parameters. Finally, we compute accurate DMC band gap energies at the optimal geometry for the monolayer.