A newly developed computational technique is used to predict the detailed atomic structure and optical properties of the 2D nanomaterial GeSe. Given the potential of 2D materials an interesting question is the properties of GeSe monolayer, and how to calculate them. In the bulk, this material forms a 3D layered structure with layers bonded by van der Waals interactions. Different density functional theory (DFT) results have yielded significantly different geometries and band gaps. The highly accurate many-body diffusion Monte Carlo (DMC) methods were used to optimize the GeSe structure and to calculate the charged quasiparticle and neutral excitonic gaps in the optimized structure. DMC was first verified to yield the correct experimental structure and electronic properties for bulk GeSe. The structure of monolayer GeSe was then found using a newly-developed structural optimization method within DMC. This method has a much lower computational cost compared to previous techniques, can be applied to heavier elements, and is based on using a surrogate Hessian. All the atomic positions and lattice constants are fully relaxed, marking the first full structural relaxation of a periodic nanostructure with quantum Monte Carlo techniques. We then calculate band gaps. The DMC energy surface has a shallow minimum at the optimal structure, while the electronic properties vary strongly with strain: not only does the magnitude of the bandgap change with strain, but a transition from direct to indirect gap can be induced with strain. This shows that the optical absorption properties of monolayer GeSe are highly tunable, which can be exploited in applications. This may be a general feature of this class of materials. We also determined that no DFT exchange-correlation functional tested could simultaneously yield both accurate band gaps (position and ordering) and structure, indicating the importance of many-body methods such as DMC for mono- and few-layer van der Waals materials. Calculations were performed at the Argonne and Oak Ridge Leadership Computing Facilities.
