Understanding and Controlling Photothermal Responses in MXenes.

MXenes have the potential for efficient light-to-heat conversion in photothermal applications. To effectively utilize MXenes in such applications, it is important to understand the underlying nonequilibrium processes, including electron-phonon and phonon-phonon couplings. Here, we use transient electron and X-ray diffraction to investigate the heating and cooling of photoexcited MXenes at femtosecond to nanosecond time scales. Our results show extremely strong electron-phonon coupling in TiC-based MXenes, resulting in lattice heating within a few hundred femtoseconds. We also systematically study heat dissipation in MXenes with varying film thicknesses, chemical surface terminations, flake sizes, and annealing conditions. We find that the thermal boundary conductance (TBC) governs the thermal relaxation in films thinner than the optical penetration depth. We achieve a 2-fold enhancement of the TBC, reaching 20 MW m K, by controlling the flake size or chemical surface termination, which is promising for engineering heat dissipation in photothermal and thermoelectric applications of the MXenes.