Metal-insulator crossover in monolayer MoS 2 .

We report on transport measurements in monolayer MoS 2 devices, close to the bottom of the
conduction band edge. These devices were annealed in situ before 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 μ eff = 180 cm 2 /Vs is among the largest obtained in CVD-grown MoS 2 monolayer
devices. These measurements show that electronic transport is of the insulating type for σ≤ 1.4e 2 /h
and n ≤ 1.7×10 12 cm -2 , and a crossover to a metallic regime is observed above those values. In
the insulating regime, thermally activated transport dominates at high temperature (T > 100 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.&#xD.