Signatures of Band-like Optical and Charge Transport in Ti3C2Tx MXene Flakes

MXenes are promising materials for various electronic and optoelectronic applications. However, the understanding of MXene optical properties and charge transport mechanisms remains rather incomplete. Here, we demonstrate that the photon absorption in the near-IR region (≈1.5 eV) in titanium carbide Ti3C2Tx MXenes is due to the inter-band transition of the oxygen terminations, rather than a localized surface plasmon resonance. This peak shifts to higher energy as the film thickness is increased and the trapped water is removed. We provide a unifying picture of charge transport in monoflake and multiflake MXenes. We show that in both monoflake and out-of-plane direction of multiflake Ti3C2Tx MXenes, charge transport is predominantly band-like. In multiflake MXenes, however, in-plane charge transport is governed by thermally activated interflake hopping, which is facilitated by the presence of trapped water. We also demonstrate that surface plasmon propagation on MXene surfaces can strongly depend on the termination state and is hindered by strong loss in experimentally prepared mixed terminated MXenes. Our results reconcile previous debates and shed light on both optical properties and charge transport mechanisms in Ti3C2Tx MXenes, opening avenues for further exploration in electronic and optoelectronic applications.