Why Ti3C2Tx MXenes Are Conductive but Not Plasmonic in the Optical Domain

MXenes have shown great potential in electronic and optoelectronic applications. However, optical properties of these highly conductive two-dimentional materials are not fully understood. The near-infrared (IR) optical peak (∼1.5 eV) in Ti3C2Tx with mixed terminations (T: O, OH, F, Cl) has sparked debates, attributing the peak to a localized surface plasmon resonance (LSPR) or an inter-band transition (IBT). Here, density functional theory calculations conclusively assign the peak to an IBT that exists only in Ti3C2O2. Both experiments and calculations corroborate that this peak is absent in Ti3C2Cl2. Moreover, calculations predict SPR in the mid-IR (∼0.5 eV, outside optical domain) for Ti3C2O2, but not for Ti3C2Cl2. Our results reconcile conflicting interpretations of the debate, allowing for optimized use of Ti3C2Tx MXenes, by leveraging their IBT optical signature, which is size-independent and distinct from the size-dependent plasmonic effect.