The use of reactive electropositive metal contacts is proposed to lower contact resistance in MoS2 devices, based on calculations of the sulfur vacancy in MoS2 by the screened exchange (sX) hybrid functional. sX gives band gaps of 1.88 eV and 1.34 eV for monolayer and bulk MoS2. The S vacancy has a formation energy of 2.35 eV in S-rich conditions, while the Mo vacancy has a large formation energy of 8.02 eV in Mo-rich conditions. The S vacancy introduces defect levels 0/−1 at 1.23 eV and −1/−2 at 1.28 eV in the upper gap. Its formation energy becomes small or negative for EF near the conduction band edge, leading to EF pinning near the conduction band for reactive metal contacts and lower contact resistances.

The transition metal dichalcogenides (TMDs) are two-dimensional layered solids with van der Waals bonding between layers. We calculate their Schottky barrier heights (SBHs) using supercell models and density functional theory. It is found that the SBHs without defects are quite strongly pinned, with a pinning factor S of about S=0.3, a similar value for both top and edge contact geometries. This arises because there is direct bonding between the contact metal atoms and the TMD chalcogen atoms, for both top and edge contact geometries, despite the weak interlayer bonding in the isolated materials. The Schottky barriers largely follow the metal induced gap state (MIGS) model, like those of three-dimensional semiconductors, despite the bonding in the TMDs being largely constrained within the layers. The pinning energies are found to be lower in the gap for edge contact geometries than for top contact geometries, which might be used to obtain p-type contacts on MoS 2 .

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The effect of surrounding solvents on the photoluminescence (PL) of MoS2 monolayers on Si/SiO2 substrates is studied. A redshift (up to -60 meV) is observed for MoS2 monolayers with nonhalogenated solvent surroundings. A blueshift (up to 60 meV) and intensity increase (2-50 times) are observed for monolayers with halogenated solvent surroundings.