Ultrafast exciton and trion dynamics in highly n -doped Mo S2 monolayers: Many-body effects

Understanding many-body interactions of excitons and charge carriers in monolayer semiconductors is crucial for tuning their unique optical properties and optimizing their performance in optoelectronic devices. However, the sensitivity of these atomically thin semiconductors to doping, defects, and strain-arising from synthesis, substrate, and environmental conditions-hinders consistent observation of many-body effects. In this work, we employed linear and ultrafast transient optical absorption spectroscopy to investigate the influence of background doping on exciton many-body interactions in MoS2 monolayers. Using reversible molecular physisorption gating, we achieved a high background doping density of 4.9×1013cm-2 in an argon environment, which is significantly higher than those attainable with conventional electrical gating. Our results reveal a photoinduced A-exciton resonance redshift, attributed to band-gap renormalization at a low background-doping density of 4.3×1012cm-2 in an air environment, transitioning to a blueshift at a high background-doping density of 4.9×1013cm-2 due to dominant Pauli blocking effects and vertical excitation shifts. We further observed transient energy splitting between free-exciton and -trion states up to 57 meV due to exciton-electron interactions. The ultrafast spectroscopy further revealed exciton and trion dynamics, including fast energy splitting of exciton and trion resonances within 1 picosecond (ps) followed by a rapid decay having a lifetime of ∼5.4 ps. Our results demonstrate the critical role of background-doping conditions in tuning many-body interactions and quasiparticle dynamics in 2D semiconductors, providing valuable insights for future device design and material engineering.