Ultrabroadband Optical Properties of 2D Titanium Carbide MXene

MXenes have rapidly ascended as a prominent class of two-dimensional (2D) materials, renowned for their distinctive optical and electrical properties. Despite extensive exploration of MXenes' optical properties, existing studies predominantly focus on the near-infrared (NIR) to the ultraviolet spectral range, leaving the mid-infrared (mid-IR) range relatively uncharted. In this study, we conducted a comprehensive characterization of the intrinsic optical properties of Ti₃C₂T<i>_x</i> MXene across an ultrabroadband spectral range, spanning from mid-IR (28 meV) to vacuum ultraviolet (VUV, 6.4 eV). For this purpose, Ti₃C₂T<i>_x</i> MXene films of varying thicknesses were coated on quartz substrates, resulting in two distinct categories: thin film samples with thicknesses below 50 nm and bulk-like samples with thicknesses exceeding 500 nm. Using spectroscopic ellipsometry, we analyzed the optical properties of films of various thicknesses and extracted detailed information on their dielectric functions. Our findings reveal resonances in the mid-IR to VUV range. Employing the Lorentz-Drude model to examine these resonances has uncovered the optical resistivity of MXene films and led to the identification of multiple plasmonic modes active in the visible to NIR range, as well as broad band-to-band transition-like resonances in the mid-IR range. This ultrabroadband optical versatility of Ti₃C₂T<i>_x</i> MXene is anticipated to bring about a wide range of thermal and optical applications.