Quasiparticle band gaps, excitonic effects, and anisotropic optical properties of the monolayer distorted 1T diamond-chain structures ReS2 and ReSe2

We report many-body perturbation theory calculations of excited-state properties of distorted 1T diamond-chain monolayer rhenium disulfide (ReS2) and diselenide (ReSe2). Electronic self-energy substantially enhances their quasiparticle band gaps and, surprisingly, converts monolayer ReSe2 to a direct-gap semiconductor, which was, however, regarded to be an indirect one by density-functional-theory calculations. Their optical absorption spectra are dictated by strongly bound excitons. Unlike hexagonal structures, the lowest-energy bright exciton of distorted 1TReS2 exhibits a perfect figure-eight shape polarization dependence but those of ReSe2 only exhibit a partial polarization dependence, which results from two nearly degenerated bright excitons whose polarization preferences are not aligned. Our first-principles calculations are in excellent agreement with experiments and pave the way for optoelectronic applications.