Strain-modulated Ultrafast Hot-Electron Cooling and Many-body Effects in Flexible MoSe₂ Monolayer

Understanding how strain modulates carrier dynamics is crucial for incorporating 2D transition metal dichalcogenides into flexible optoelectronics. Moving beyond exciton-centric research, we employ femtosecond transient absorption spectroscopy to probe the strain-dependent ultrafast cooling and many-body effects of hot-electrons in tensile-strained monolayer MoSe₂. The relaxation exhibits biphasic dynamics: a strain-insensitive sub 2 ps cooling stage followed by a sub 20 ps slower phase that accelerates under strain. Pump-fluence dependence reveals strong hot-phonon effects in both stages, indicative of robust electron–LO phonon coupling. The strain-independent initial phase implies negligible strain effect on electron–optical phonon scattering, whereas the accelerated slower phase suggests enhanced optical–acoustic phonon coupling that facilitates thermal dissipation. Many-body effects drive an initial rapid redshift of the exciton resonance, succeeded by a blueshift comprising a sub 1 ps strain-invariant component and a sub 5 ps strain-accelerated one. The fast component reflects an early balance between band filling and bandgap renormalization, while the slower one stems from phonon-scattering-enhanced band filling. In contrast, exciton dynamics show strain-insensitive time constants, suggesting compensatory electron and hole dynamics, and the exciton resonance blueshifts more markedly with strain. These results offer deeper insight into strain-engineered carrier processes in two-dimensional semiconductors.