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

Abstract

Cr₂TiC₂T_x is an ordered double-transition-metal MXene with peculiar magnetic properties. Previous studies produced sub-1-μm sheets of Cr₂TiC₂T_x, which prevented complete characterization of its intrinsic properties at a single-flake level. We report the synthesis of high-quality Cr₂TiC₂T_x monolayers with lateral sizes exceeding 15 μm for single-flake measurements. These measurements establish Cr₂TiC₂T_x as a unique material among the MXenes experimentally tested so far. Field-effect electrical measurements on Cr₂TiC₂T_x monolayers revealed an average conductivity of 180 S cm⁻¹ and p-type transport, while established MXenes, such as Ti₃C₂T_x and Nb₄C₃T_x, demonstrated n-type behavior. In contrast to negative photoresponse reported for Ti₃C₂T_x flakes, Cr₂TiC₂T_x devices show positive photoresponse to visible and infrared light. Nanoindentation measurements of monolayer Cr₂TiC₂T_x membranes yielded an effective Young’s modulus of 220 ± 22 GPa. Density functional theory calculations provide insights into the p-type character of Cr₂TiC₂T_x and predict its potentially tunable p-/n-type behavior depending on the concentrations of Cr vacancies, oxygens substituting carbon atoms, and surface terminations.