Quantitative Broadband Optical Spectroscopy of Freestanding Ti₃C₂T_x MXene Films: Unveiling Intrinsic Bulk Charge...

Micrometer-thick freestanding MXene films offer a rare opportunity to probe intrinsic bulk optical and electronic properties without the influence of a substrate. Yet, quantitative optical constants over a broadband spectral range remain elusive due to the mechanically flexible nature of such films. Here, we report the first comprehensive far-infrared-to-ultraviolet (150-36,000 cm^-1) optical spectroscopic study of freestanding Ti₃C₂T_x MXene films with thicknesses of 1 and 4 μm. By a combination of attaching the freestanding films on flat and smooth brass plates and using an in situ metal-evaporation method, we obtained highly reliable reflectance spectra. Kramers-Kronig analysis, coupled with a two-Drude-Lorentz model, reveals multiple free-carrier populations at the Fermi level, yielding carrier densities of 7.31 × 10²¹ and 7.50 × 10²¹ cm^-3 and mobilities of 8.15 and 4.98 cm² V^-1 s^-1 for the 1- and 4-μm-thick freestanding films, respectively. The spectra further identify a 1.61 eV feature arising from interband-plasmon coupling and a 3.84 eV interband transition, with small thickness dependence, highlighting the bulk-dominated plasmonic response in micrometer-scale films. This work establishes a quantitative, noncontact framework for mapping intrinsic charge dynamics in MXenes, unlocking pathways for their integration into broadband plasmonic, optoelectronic, infrared, and THz devices.