  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.
