Microscopic conductivity and ultrafast carrier dynamics in molybdenum-based MXenes: THz spectroscopy study

MXenes are a new family of two-dimensional transition metal carbides, nitride and carbonitrides with high conductivity and versatile chemical structures. Here we have used THz spectroscopy to study microscopic conductivity and photoinduced carrier dynamics in two Mo-based MXenes, Mo₂Ti₂C₃T_z and Mo₂TiC₂T_z. Both exhibit high intrinsic carrier densities (~ 10²⁰ cm^-3 in Mo₂Ti₂C₃T_z, and ~ 10¹⁹ cm^-3 in Mo₂TiC₂T_z), mobilities, and high conductivities within individual nanosheets. We also observe that optical excitation increases their conductivity, unlike Ti₃C₂T_z, in which photoexcitation suppresses conductivity for nanoseconds. Vacuum annealing improves the long-range transport of photoinduced carriers and further increases their lifetime, as it results in de-intercalation of water and other species from van der Waals gaps between the nanosheets in the films. High and long-lived photoinduced conductivity suggests Mo-based MXenes a promising candidate for optoelectronic, sensing and photoelectrochemical applications.