With a direct band gap of 1.88eV and a reasonable carrier mobility of ~100 cm2/(Vs), 2D MoS2 is a promising candidate for next generation semiconductor devices. Despite those excellent features, intrinsic defects reduce carrier mobility and quench photoluminescence. Sulfur vacancies are the dominant defect. Therefore, passivation of the Sulfur vacancy is highly useful. Here, we investigate possible passivation schemes which have been proposed by experimentalists. Unlike in HfO2, substitutional doping to complete a closed shell  does not passivate because the Mo-S bond is not ionic, so there is no atomic relaxation around the vacancy due to its charge state. An alternative is to use molecules which might be chemically adsorbed at the vacancy. The dehydrogenated cysteine acid cation  and neutral O2  might combine with the vacancy and move the Fermi level to the middle of the gap. However, both of them give rise to localized states just above the valence band maximum (VBM). Finally we consider adsorption of organic super acid TFSI, which is strong protonating agent, and is known to greatly improve the PL efficiency . We consider the effect of hydrogen. The vacancy has one doubly degenerate state of e symmetry at the VBM and a pair of doubly degenerate states of e symmetry in the upper gap, whose energy becomes split by occupancy. To avoid producing half-filled states in the gap, we need a spin-paired system with trigonal symmetry. Unlike in a-Si, the bonding in MoS2 is multi-centered. Thus one Mo dangling bond does not bond to one H atom. We place three hydrogens with four electrons symmetrically around vacancy center, which is calculated to be an energetically favorable configuration. Each hydrogen bonds symmetrically to a Mo dangling bond, forming bonds and anti-bonds with the a1 state and double degenerate e states which originally lie in the upper band gap . The hydrogen forms a filled deep bonding state and an empty strongly anti-bonding state with a1 symmetry. The anti-bonding state eH* is pushed up to conduction band. The Mo dxy, dyz, dxz and dx2-y2 states form bonding states eH with hydrogen, which are occupied by four electrons. These lie below VBM. Therefore the combination of three hydrogens and 4 electrons passivation produces a MoS2 with a clear band gap, and the Fermi level in mid-gap. The mechanism will improve the performance of MoS2.
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