Direct Determination of Band-Gap Renormalization in the Photoexcited Monolayer MoS2

A key feature of monolayer semiconductors, such as transition-metal dichalcogenides, is the poorly screened Coulomb potential, which leads to a large exciton binding energy (E-b) and strong renormalization of the quasiparticle band gap (E-g) by carriers. The latter has been difficult to determine due to a cancellation in changes of E-b and E-g, resulting in little change in optical transition energy at different carrier densities. Here, we quantify band-gap renormalization in macroscopic single crystal MoS2 monolayers on SiO2 using time and angle-resolved photoemission spectroscopy. At an excitation density above the Mott threshold, E-g decreases by as much as 360 meV. We compare the carrier density-dependent E-g with previous theoretical calculations and show the necessity of knowing both doping and excitation densities in quantifying the band gap.