defects in tungsten disulfide*

We use density functional theory (DFT) to explore the physical properties of an Er_W point defect in monolayer WS₂. Our calculations indicate that electrons localize at the dangling bonds associated with a tungsten vacancy (V_W) and at the Er³⁺ ion site, even in the presence of a net negative charge in the supercell. The system features a set of intra-gap defect states, some of which are reminiscent of those present in isolated Er³⁺ ions. In both instances, the level of hybridization is low, i.e., orbitals show either strong Er or W character. Through the calculation of the absorption spectrum as a function of wavelength, we identify a broad set of transitions, including one possibly consistent with the Er^{3+ 4}I_15/2→⁴I_13/2 observed in other hosts. Combined with the low native concentration of spin-active nuclei as well as the two-dimensional nature of the host, these properties reveal Er:WS₂ as a potential platform for realizing spin qubits that can be subsequently integrated with other nanoscale optoelectronic devices.