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<i>_x</i> 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<i>_x</i> and produced monolayer sheets with lateral sizes exceeding 15 µm for single-flake measurements. The results of such measurements on Cr₂TiC₂T<i>_x</i> further establish it as a unique material among the MXenes experimentally tested so far. Field-effect electrical measurements on monolayer Cr₂TiC₂T<i>_x</i> flakes reveal an average conductivity of 66.18 S cm^-1 and <i>p</i>-type transport properties, while established MXene materials, such as Ti₃C₂T<i>_x</i> and Nb₄C₃T<i>_x</i>, demonstrated <i>n</i>-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<i>_x</i>. When illuminated with visible or infrared light, the Cr₂TiC₂T<i>_x</i> devices exhibit <i>positive</i> photoresponse in contrast to the <i>negative </i>photoresponse that was recently reported for Ti₃C₂T<i>_x</i> monolayers. Nanoindentation measurements of monolayer Cr₂TiC₂T<i>_x</i> membranes yield a respectable value of effective Young’s modulus of 220±22 GPa. This work provides access to large high-quality Cr₂TiC₂T<i>_x</i> flakes that can find numerous applications due to their attractive mechanical characteristics and transport properties that are complementary to other established MXene materials.