Variable range hopping and thermally activated transport in molybdenum-based MXenes

Abstract

The magnetotransport of freestanding, vacuum filtered, thin films of $\mathrm{M}{\mathrm{o}}_{2}\mathrm{C}{\mathrm{T}}_{z}$, $\mathrm{M}{\mathrm{o}}_{1.33}\mathrm{C}{\mathrm{T}}_{z}$, $\mathrm{M}{\mathrm{o}}_{2}\mathrm{Ti}{\mathrm{C}}_{2}{\mathrm{T}}_{z}$, and $\mathrm{M}{\mathrm{o}}_{2}\mathrm{T}{\mathrm{i}}_{2}{\mathrm{C}}_{3}{\mathrm{T}}_{z}$ was measured in the 10--300-K temperature (T) range. Some of the films were annealed before measuring their transport properties. Analysis of the results suggest that---with the exception of the heavily defective $\mathrm{M}{\mathrm{o}}_{1.33}\mathrm{C}{\mathrm{T}}_{z}$ composition---in the 10- to 200-K temperature regime, variable range hopping between individual MXene sheets is the operative conduction mechanism. For $\mathrm{M}{\mathrm{o}}_{1.33}\mathrm{C}{\mathrm{T}}_{z}$ it is more likely that variable range hopping within individual flakes is rate limiting. At higher temperatures, a thermally activated process emerges in all cases. It follows that improved fabrication processes should lead to considerable improvements in the electrical transport of Mo-based MXenes.