Interplay of Ultrafast Electron–Phonon and Electron–Electron Scattering in Ti<sub>3</sub>C<sub>2</sub>T<sub><i>x</i></sub> MXenes: Ab Initio Quantum Dynamics
Shiying Shen, Haoran Lu, Shriya Gumber, Oleg V. Prezhdo, Run Long
Abstract
Nonthermal electrons are vital in solar energy and optoelectronics, yet their relaxation pathways are not fully understood. Ab initio quantum dynamics reveal that in Ti 3 C 2 O 2 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 Ti 3 C 2 OS 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.