Band-like Optical Signatures of Ti ₃ C ₂ T <i> _x </i> MXenes

MXenes have shown great potential in electronic and optoelectronic applications. However, the optical properties of these highly conductive two-dimensional materials are not fully understood. The broad near-infrared (IR) optical extinction (∼1.5 eV) in Ti 3 C 2 T x with mixed terminations (T x: ═O, −OH, −F, −Cl) has been widely attributed to a localized surface plasmon resonance (LSPR). However, previous simulations suggest this peak may be due to an interband transition (IBT). Here, we show that the real component of the dielectric constant of Ti 3 C 2 T x at this peak is positive (ε 1 > 0), as measured by spectroscopic ellipsometry (SE), ruling out the possibility of LSPR. Moreover, this band nearly vanishes for experimentally synthesized chlorine-terminated Ti 3 C 2 Cl 2 . Density functional theory (DFT) calculations confirm an IBT origin for this band, specifically due to the oxygen terminations (Ti 3 C 2 O 2 ). Metallic behavior is only experimentally observed below 1 eV (ε 1 < 0), and DFT calculations predict surface plasmon polaritons (SPPs) in the mid-IR (∼0.5 eV, outside the optical domain) and only for Ti 3 C 2 O 2, but not for Ti 3 C 2 Cl 2 or other terminations. Additionally, we demonstrate that making thicker Ti 3 C 2 T x MXene films and/or removing the intercalated water can induce a blue shift in this IBT due to the influence of water in facilitating the out-of-plane conductivity. The IBT assignment is critical because its light-matter interaction is fundamentally different from that of an LSPR, providing a new foundation for designing innovative MXene-based optoelectronic materials, which can be tailored through their termination states, while an LSPR would be insensitive to such synthetic variations.