Efficient Excitonic Photoluminescence in Direct and Indirect Band Gap Monolayer MoS2. 
We discuss the photoluminescence (PL) of semiconducting transition metal dichalcogenides on the basis of experiments and a microscopic theory.  The latter connects ab initio calculations of the single-particle states and Coulomb matrix elements with a many-body description of optical emission spectra.  For monolayer MoS2, we study the PL efficiency at the excitonic A and B transitions in terms of carrier populations in the band structure and provide a quantitative comparison to an (In)GaAs quantum well-structure.  Suppression and enhancement of PL under biaxial strain is quantified in terms of changes in the local extrema of the conduction and valence bands.  The large exciton binding energy in MoS2 enables two distinctly different excitation methods: above-band gap excitation and quasi-resonant excitation of excitonic resonances below the single-particle band gap.  The latter case creates a nonequilibrium distribution of carriers predominantly in the K-valleys, which leads to strong emission from the A-exciton transition and a visible B-peak even if the band gap is indirect.  For above-band gap excitation, we predict a strongly reduced emission intensity at comparable carrier densities and the absence of B-exciton emission.  The results agree well with PL measurements performed on monolayer MoS2 at excitation wavelengths of 405 nm (above) and 532 nm (below the band gap).