Two-Dimensional MXenes Mo2Ti2C3Tz and Mo2TiC2Tz: Microscopic Conductivity and Dynamics of Photoexcited Carriers

Abstract

MXenes are a recently discovered family of two-dimensional transition metal carbides, nitrides, and carbonitrides with electronic properties that can be tuned by their chemistry and structure. Herein THz spectroscopy was used to investigate the microscopic conductivity and photoexcited charge carrier dynamics in two Mo-based MXenes: Mo2Ti2C3Tz and Mo2TiC2Tz. We find that both have high intrinsic carrier densities (∼1020 cm-3 in Mo2Ti2C3Tz and ∼1019 cm-3 in Mo2TiC2Tz) and mobilities and exhibit high conductivities within individual nanosheets. Optical excitations result in a transient conductivity increase in both compositions, in stark contrast with the most studied member of the MXene family, Ti3C2Tz, where photoexcitation suppresses the conductivity for nanoseconds. Deintercalation of water, and other species, from between the nanosheets by mild vacuum annealing at 200 °C further improves the long-range, internanosheet transport of the photoexcited carriers and increases their lifetime. High, and long-lived, photoinduced conductivity that can be engineered by substituting Mo for Ti renders these Mo-based MXenes attractive for a variety of optoelectronic, sensing, and photoelectrochemical applications.