Synthesis of high-quality large Cr2TiC2Tx MXene monolayers, their mechanical properties, p- type electrical transport, and positive photoresponse

Cr2TiC2Tx is an ordered double-transition-metal MXene with peculiar magnetic properties. Previous studies produced sub-1-mu m sheets of Cr2TiC2Tx, which prevented complete characterization of its intrinsic properties at a single-flake level. We report the synthesis of high-quality Cr2TiC2Tx monolayers with lateral sizes exceeding 15 mu m for single-flake measurements. These measurements establish Cr2TiC2Tx as a unique material among the MXenes experimentally tested so far. Field-effect electrical measurements on Cr2TiC2Tx monolayers revealed an average conductivity of 180 S cm(-1) and p-type transport, while established MXenes, such as Ti3C2Tx and Nb4C3Tx, demonstrated n-type behavior. In contrast to negative photoresponse reported for Ti3C2Tx flakes, Cr2TiC2Tx devices show positive photoresponse to visible and infrared light. Nanoindentation measurements of monolayer Cr2TiC2Tx membranes yielded an effective Young's modulus of 220 +/- 22 GPa. Density functional theory calculations provide insights into the p-type character of Cr2TiC2Tx and predict its potentially tunable p-/n-type behavior depending on the concentrations of Cr vacancies, oxygens substituting carbon atoms, and surface terminations.