Metallic Conductivity of Ti3C2Tx MXene Confirmed by Temperature-Dependent Electrical Measurements

Ti3C2Tx, the most popular MXene to date, is widely regarded as a metallic material, based on numerous theoretical predictions and the results of experimental studies. Yet, despite this general consensus on the metallic nature of Ti3C2Tx, there have not been reports on its temperature-dependent resistivity (ρ) measurements that would demonstrate the expected increase of resistivity with temperature with dρ/dT > 0 in a wide temperature range. Instead, all ρ(T) data reported so far, which were mostly collected on macroscopic films of percolating Ti3C2Tx flakes, demonstrate dependences with minima, which were observed in the range from 90 to 250 K in different measurements. In this study, we fabricated electronic devices based on individual high-quality Ti3C2Tx flakes and performed their temperature-dependent resistivity measurements. The resistivity of flakes was found to increase with temperature in the 10–300 K range, and the resulting ρ(T) dependences can be accurately described by the Bloch–Grüneisen model for the temperature dependence of the resistivity of metals, confirming the metallic nature of Ti3C2Tx. We also demonstrate that an oxidation of a Ti3C2Tx monolayer transforms a monotonically increasing ρ(T) curve into a dependence with a minimum that looks similar to the previously reported results for percolating MXene films. The emerging low-temperature tail with a semiconductor-like dρ/dT < 0 behavior can be explained by the stronger electron scattering in a partially oxidized MXene due to an increased level of disorder, and the resulting ρ(T) curves can be accurately fitted using Matthiessen's rule, which incorporates the effect of all types of scatterers on the transport properties of metals. These experiments verify the metallic nature of Ti3C2Tx (dρ/dT > 0) and provide insights into the origin of the emergence of a low-temperature tail with dρ/dT < 0. We also demonstrate that multilayer Ti3C2Tx flakes retain their purely metallic dρ/dT > 0 behavior even after annealing in air, suggesting that the outer layers of multilayer flakes effectively protect the core layers from oxidation. This result suggests that certain applications may benefit from the use of multilayer flakes because of their improved environmental stability.