Terahertz spectroscopy of intrinsic and photoexcited carrier dynamics in Ti ₂ CT _x MXene films

Abstract Metallic MXenes hold strong potential for applications in flexible electronics, electromagnetic interference shielding, photonics, and photothermal energy conversion. Realizing these applications requires a detailed understanding of carrier transport and its response to optical excitation. Here, we use terahertz (THz) time-domain spectroscopy and THz time-resolved spectroscopy to investigate the intrinsic conductivity and ultrafast photoresponse of Ti ₂ CT _x MXene, a less-explored member of the MXene family. Analysis of the complex THz conductivity spectra suggests that oxidation fragments Ti ₂ CT _x flakes into small, conductive regions separated by non-conductive areas. Within these conductive regions, electrons exhibit scattering times and local mobilities comparable to those in Ti ₃ C ₂ T _x . These mobilities represent intrinsic upper limits that could be realized in Ti ₂ CT _x at macroscopic scales with improved oxidative stability and processing. Photoexcitation at both 400 nm and 800 nm induces a transient enhancement in THz conductivity, in contrast to Ti ₃ C ₂ T _x , where photothermal heating suppresses mobility. Most photoexcited carriers in Ti ₂ CT _x recombine or become trapped within a few picoseconds, while a small fraction persists for hundreds of picoseconds. Photoinduced carriers exhibit longer scattering times and reduced localization compared to intrinsic carriers, suggesting that their excess kinetic energy allows them to overcome barriers from defects and oxidation. Excitation wavelength influences early-time dynamics: 800 nm excitation results in a slower rise and decay than 400 nm, consistent with inter-valley scattering between conduction-band valleys of differing mobility, or with initial excitation into low-mobility, defect-associated states followed by thermally activated de-trapping. These observations provide new insights into the complex pathways of photoexcitation and carrier evolution in Ti ₂ CT _x .