Synthesis of High-Quality Monolayers of Cr₂TiC₂T_x MXene, Their Mechanical Properties, p-type Electrical Transport and Positive Photoresponse

In the rapidly growing family of MXenes, Cr₂TiC₂T_x is one of the most intriguing materials as an ordered double-transition-metal MXene with peculiar magnetic properties. In this work, we developed a synthetic procedure for high-quality Cr₂TiC₂T_x and produced monolayer sheets with lateral sizes exceeding 15 µm for single-flake measurements. The results of such measurements on Cr₂TiC₂T_x further establish it as a unique material among the MXenes experimentally tested so far. Field-effect electrical measurements on monolayer Cr₂TiC₂T_x flakes reveal an average conductivity of 66.18 S cm^-1 and p-type transport properties, while established MXene materials, such as Ti₃C₂T_x and Nb₄C₃T_x, demonstrated n-type behavior in similar studies. The experimental data are corroborated by DFT studies, showing that Cr vacancies are the major contributing factor to the p-type behavior of Cr₂TiC₂T_x. When illuminated with visible or infrared light, the Cr₂TiC₂T_x devices exhibit positive photoresponse in contrast to the negative photoresponse that was recently reported for Ti₃C₂T_x monolayers. Nanoindentation measurements of monolayer Cr₂TiC₂T_x membranes yield a respectable value of effective Young’s modulus of 220±22 GPa. This work provides access to large high-quality Cr₂TiC₂T_x flakes that can find numerous applications due to their attractive mechanical characteristics and transport properties that are complementary to other established MXene materials.