Observation of different charge transport regimes and large magnetoresistance in graphene oxide layers

We report a systematic study on charge transport properties of thermally reduced graphene oxide (rGO) layers, from room temperature to 2 K and in presence of magnetic fields up to 7 T. The most conductive rGO sheets follow different transport regimes: at room temperature they show an Arrhenius-like behavior. At lower temperature they exhibits a thermally activated behavior with resistance R following a R = R0exp(T0/T)^p law with p = 1/3, consistently with 2D Mott Variable Range Hopping (VRH) transport mechanism. Below a given temperature Tc, we observe a crossover from VHR to another regime, probably due to a shortening of the characteristic lengths of the disordered 2D system. The temperature Tc depends on the reduction grade of the rGO. Magnetoresistance ?R/R of our rGO films shows as well a crossover between positive and negative and below liquid He temperature ?R/R reaches values larger than ~-60%, surprisingly high for a - nominally - non magnetic material.