Interplay of Ultrafast Electron-Phonon and Electron-Electron Scattering in Ti₃C₂T MXenes: Ab Initio Quantum Dynamics

Nonthermal electrons are vital in solar energy and optoelectronics, yet their relaxation pathways are not fully understood. Ab initio quantum dynamics reveal that in Ti₃C₂O₂ electron-phonon (e-ph) relaxation is faster than electron-electron (e-e) scattering due to strong coupling with the A_1g phonon at 190 cm^-1 and the presence of light C and O atoms. Nuclear quantum effects are minimal; vibrations influence e-e scattering only indirectly, and the A_1g mode' zero-point energy is much lower than thermal energy at ambient conditions. Substituting O with heavier S in Ti₃C₂OS slows e-ph relaxation and enhances e-e scattering, making it a faster process. However, both channels proceed concurrently, challenging the e-e and e-ph time scale separation often used for metals. These results underscore the need for atomistic-level understanding of nonthermal electron dynamics, especially in light-element systems such as MXenes, and provide guidance for optimizing electronic relaxation in advanced optoelectronic materials and devices.