Ultrafast Carrier and Lattice Cooling in Ti<sub>2</sub>CT<sub><i>x</i></sub> MXene Thin Films

Metallic MXenes are\npromising two-dimensional materials for energy\nstorage, (opto)electronics, and photonics due to their high electrical\nconductivity and strong light–matter interaction. Energy dissipation\nin MXenes is fundamental for photovoltaic and photothermal applications.\nHere we apply ultrafast laser spectroscopy across a broad time range\n(femto- to microseconds) to study the cooling dynamics of electrons\nand lattice in emerging Ti<sub>2</sub>CT<sub><i>x</i></sub> thin films compared to widely studied Ti<sub>3</sub>C<sub>2</sub>T<sub><i>x</i></sub> thin films. The carrier cooling time\nin Ti<sub>2</sub>CT<sub><i>x</i></sub> is persistently ∼2.6\nps without a hot-phonon bottleneck. After hot carrier cooling is completed,\nthe transient absorption spectra of Ti<sub>2</sub>CT<sub><i>x</i></sub> MXene can be described well by the thermochromic effect. Heat\ndissipation in MXene thin films occurs over hundreds of nanoseconds\nwith thermal diffusivities ∼0.06 mm<sup>2</sup> s<sup>–1</sup> for Ti<sub>2</sub>CT<sub><i>x</i></sub> and ∼0.02\nmm<sup>2</sup> s<sup>–1</sup> for Ti<sub>3</sub>C<sub>2</sub>T<sub><i>x</i></sub>, likely due to inefficient interflake\nheat transfer. Our results unravel the energy dissipation dynamics\nin Ti<sub>2</sub>CT<sub><i>x</i></sub> films, showcasing\n potential applications in energy conversion.