Photoinduced Bandgap Renormalization and Exciton Binding Energy Reduction in WS2

Strong Coulomb attraction in monolayer transition metal dichalcogenides gives rise to tightly bound excitons and many-body interactions that dominate their optoelectronic properties. However, this Coulomb interaction can be screened through control of the surrounding dielectric environment as well as through applied voltage, which provides a potential means of tuning the bandgap, exciton binding energy, and emission wavelength. Here, we directly show that the bandgap and exciton binding energy can be optically tuned by means of the intensity of the incident light. Using transient absorption spectroscopy, we identify a sub-picosecond decay component in the excited-state dynamics of WS2 that emerges for incident photon energies above the A-exciton resonance, which originates from a nonequilibrium population of charge carriers that form excitons as they cool. The generation of this charge-carrier population exhibits two distinct energy thresholds. The higher threshold is coincident with the onset of continuum states and therefore provides a direct optical means of determining both the bandgap and exciton binding energy. Using this technique, we observe a reduction in the exciton binding energy from 310 +/- 30 to 220 +/- 20 meV as the excitation density is increased from 3 X 10(11) to 1.2 X 10(12) photons/cm(2). This reduction is due to dynamic dipolar screening of Coulomb interactions by excitons, which is the underlying physical process that initiates bandgap renormalization and leads to the insulator metal transition in monolayer transition metal dichalcogenides.