Ultrafast Carrier and Lattice Cooling in Ti₂CT_<i>x</i> MXene Thin Films

Abstract

Metallic MXenes are promising two-dimensional materials for energy storage, (opto)electronics, and photonics due to their high electrical conductivity and strong light-matter interaction. Energy dissipation in MXenes is fundamental for photovoltaic and photothermal applications. Here we apply ultrafast laser spectroscopy across a broad time range (femto- to microseconds) to study the cooling dynamics of electrons and lattice in emerging Ti₂CT_<i>x</i> thin films compared to widely studied Ti₃C₂T_<i>x</i> thin films. The carrier cooling time in Ti₂CT_<i>x</i> is persistently ∼2.6 ps without a hot-phonon bottleneck. After hot carrier cooling is completed, the transient absorption spectra of Ti₂CT_<i>x</i> MXene can be described well by the thermochromic effect. Heat dissipation in MXene thin films occurs over hundreds of nanoseconds with thermal diffusivities ∼0.06 mm² s^-1 for Ti₂CT_<i>x</i> and ∼0.02 mm² s^-1 for Ti₃C₂T_<i>x</i>, likely due to inefficient interflake heat transfer. Our results unravel the energy dissipation dynamics in Ti₂CT_<i>x</i> films, showcasing potential applications in energy conversion.