Direct Observation of Photoinduced Bandgap and Binding Energy reduction in WS₂

In 2-D semiconductors, strong Coulombic interactions give rise to tightly bound excitons. Control of surrounding dielectrics can screen this interaction. It has long been suggested that photo injected charge and excitons can dynamically screen the Coulombic interaction giving rise to bandgap renormalization. However, direct experimental evidence has only revealed small shifts in the exciton resonance due to an assumed imbalance in the cancellation of bandgap and binding energy reduction. Here we present a novel means of optically determining the electronic band gap and exciton binding energy in monolayer WS₂. We use transient absorption spectroscopy to identify the exciton formation from initially photogenerated hot charges. The photon energy threshold for this feature occurs at the electronic bandgap, allowing for direct determination of the exciton binding energy. Using this method, we show the first direct measurement of bandgap renormalization and binding energy reduction upon high fluence photoexcitation in monolayer transition metal dichalcogenides. By changing the fluence form 3x10¹¹ to >1x10¹² cm^-2 we find that the exciton binding energy in WS₂ is reduced from 300 to 220 meV due to dynamic screening.