Kanit Hantanasirisakul1 2 Mohamed Alhabeb1 2 Alexey Lipatov3 Kathleen Maleski1 2 Babak Anasori1 2 Alexander Sinitskii3 Steven May2 Yury Gogotsi1 2

1, A.J. Drexel Nanomaterials Institute, Drexel University, Philadelphia, Pennsylvania, United States
2, Department of Materials Science and Engineering, Drexel University, Philadelphia, Pennsylvania, United States
3, University of Nebraska-Lincoln, Lincoln, Nebraska, United States

MXenes are a rapidly growing class of 2D transition metal carbides, carbonitrides, and nitrides. Their high electronic conductivities make them promising in many applications including transparent conductive coatings, electromagnetic interference shielding, and energy storage. To date, more than 20 MXenes have been synthesized mostly by selective etching using fluorine-containing etchants, and exfoliated into 2D flakes by employing organic- or inorganic-based intercalants. Using different etchants and intercalants results in variation in surface functionality, flake size, number of defects, and interlayer spacing, all of which may affect the optoelectronic properties of MXenes. In this study, we report the effects of synthesis methods and post-synthesis heat treatment on the optoelectronic properties of titanium carbonitride MXene (Ti3CN). Transport properties of free-standing MXene ‘paper’, transparent thin films, and monolayer flakes were studied by temperature-dependent resistivity measurements from 10-300 K. The results show that the films prepared using a large organic intercalant are about three orders of magnitude less conductive than films prepared with a smaller inorganic-based intercalant. Furthermore, upon heat treatment, the conductivity can be greatly improved by removing the intercalated molecules. Our results show how the electronic properties of Ti3CN MXene can be tuned by optimization of synthesis method and post-synthesis heat treatment. A similar approach can be applied to other MXenes to control their optoelectronic properties.