Optical excitation energies of Co@S defect of WS₂ computed with the spin-flip Bethe-Salpeter equation approach

Defected transition metal dichalcogenides are exciting materials as potential single photon emitters, quantum light sources, and room-temperature solid-state qubits. Accurate calculations of the exciton interactions in these materials is imperative to understanding these applications. Monolayer cobalt-at-sulfur-site substituted WS₂ (Co@S:WS₂) has an open-shell electronic structure due to the spin of the cobalt atom, as found from our DFT calculations of magnetism and Jahn-Teller distortions in this system. While quasi-particle energies for open-shell systems have been calculated previously–and are known to have multiplet solutions–there is at present no similar approach available for the Bethe-Salpeter equation in open-shell systems. Instead, we investigate the vertical excitation energies through the Spin-Flip Bethe-Salpeter equation approach [arXiv:2207.04549], which allows for the simultaneous calculation of ground- and excited-state energies for multiconfigurational open-shell systems. Previously, this approach has been used to calculate excitation energies for defected bulk semiconductors, and we now leverage methodological development for the calculation of optical excitations in 2D systems to calculate excitation energies for Co@S:WS₂.