First Principles Study of Intrinsic and Extrinsic Point Defects in Monolayer WSe2

We present a detailed first principles density functional theory study of intrinsic and extrinsic point defects in monolayer (ML) WSe2. Among the intrinsic point defects, Se vacancies (Sevac) have the lowest formation energy (disregarding Se adatoms that can be removed with annealing). The defects with the next smallest formation energies (at least 1 eV larger) are SeW (Se substituting W atoms in an antisite defect), Wvac (W vacancies) and 2Sevac (Se divacancies). All these intrinsic defects have gap states that are not spin-polarized. The presence of a graphite substrate does not change the formation energies of these defects significantly. For the extrinsic point defects, we focus on O, O2, H, H2 and C interacting with perfect WSe2 and its intrinsic point defects. The preferred binding site in perfect WSe2 is the interstitial site for atomic O, H and C. These interstitial defects have no gap states. The gap states of the intrinsic defects are modified by interaction with O, O2, H, H2 and C. In particular, the gap states of Sevac and 2Sevac are completely removed by interaction with O and O2. This is consistent with the significantly larger stability of O-related defects compared to H- and C-related defects. The preferred binding site for O is Sevac, while that for H is SeW. H bonded to SeW results in spin-polarized gap states, which may be useful in defect engineering for spintronics applications. The charge transition levels and ionization energies of these defects are also computed. H in the interstitial site is an effective donor, while all the other defects are deep donors or acceptors in isolated WSe2 ML.