Ultrafast Photoexcitations in Ti₃C₂T_x —Sodium Polyphosphate Thin Films

2D titanium carbide MXene, Ti₃C₂T<i>_x</i>, where T<i>_x</i> stands for surface terminations -OH, -F, and -O, exhibits high intrinsic free carrier density, conductivity greater than 1,000 S/cm, unprecedented light-to-heat conversion efficiency and unusually slow heat dissipation after excitation with a short laser pulse [1-4]. In our recent study, we have demonstrated that rapid increase in lattice temperature arising from strong electron−phonon coupling results in long-lived (&gt; 1 ns) changes in optical properties of Ti₃C₂T<i>_x</i> films in near-infrared and THz spectral range: bleaching of the localized surface plasmon at ~ 750 nm and suppression of THz optical conductivity [3]. Monitoring recovery kinetics of those effects thus provides a view into thermal properties of MXene films. Here, we investigate how capping MXene nanosheets with sodium polyphosphate, which has been shown to improve their chemical stability in aqueous solution [5], impacts thermal relaxation following photoexcitation. We find that polyphosphates do not significantly impact thermal relaxation kinetics, suggesting that it is possible to achieve long-shelf life MXene colloidal suspensions that can be used for flexible photothermal devices.<br/><br/>Approved for public release, approval number PR2024-1259.<br/><br/>[1] A. VahidMohammadi, J. Rosen, Y. Gogotsi, Science 372, 1165 (2021).<br/>[2] G. Li et al. Nano Lett. 20, 636-643 (2020).<br/>[3] E. Colin-Ulloa, A. Fitzgerald, et. al. Materials 35, 2208659 (2023).<br/>[4] B. Guzelturk et al. Nano Letters 23, 2677-2686 (2023).<br/>[5] V. Natu et al. Angew. Chem. Int. Ed. 58, 12655 (2019).