Metal-insulator crossover in monolayer MoS.

We report on transport measurements in monolayer MoSdevices, close to the bottom of the conduction band edge. These devices were annealedbefore electrical measurements. This allows us to obtain good ohmic contacts at low temperatures, and to measure precisely the conductivity and mobility via four-probe measurements. The measured effective mobility up to= 180 cmVsis among the largest obtained in CVD-grown MoSmonolayer devices. These measurements show that electronic transport is of the insulating type for≤ 1.4/and≤ 1.7 × 10cm, and a crossover to a metallic regime is observed above those values. In the insulating regime, thermally activated transport dominates at high temperature (> 120 K). At lower temperatures, conductivity is driven by Efros-Schklovkii variable range hopping in all measured devices, with a universal and constant hopping prefactor, that is a clear indication that hopping is not phonon-mediated. At higher carrier density, and high temperature, the conductivity is well modeled by the Boltzmann equation for a non-interacting Fermi gas, taking into account both phonon and impurity scatterings. Finally, even if this apparent metal-insulator transition can be explained by phonon-related phenomena at high temperature, the possibility of a genuine 2D MIT cannot be ruled out, as we can observe a clear power-law diverging localization length close to the transition, and a one-parameter scaling can be realized.