Metallic Conductivity in Ti₃C₂T_x Evidenced by Temperature-Dependent Resistivity Measurements.

Ti₃C₂T_x, 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 Ti₃C₂T_x, 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, mainly collected on macroscopic films of percolating Ti₃C₂T_x flakes, demonstrate dependencies with minima, which were observed in the range of 90-250 K in different measurements. In this work, we fabricated electronic devices based on individual high-quality Ti₃C₂T_x flakes and measured their temperature-dependent resistivity. The resistivity of flakes was found to increase with temperature in the entire 10-300 K range, and the resulting ρ (T) dependences can be accurately fitted by the Bloch—Grüneisen formula for the temperature dependence of resistivity of metals, confirming the metallic nature of Ti₃C₂T_x. We also demonstrate that 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. We also demonstrate that multilayer Ti₃C₂T_x 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 multilayer flakes' improved environmental stability.