Chemical-doping-driven crossover from graphene to "ordinary metal" in epitaxial graphene grown on SiC

Atmospheric chemical doping can be used to modify the electronic properties of graphene. Here we report that the chemical atmospheric doping (derived from air, oxygen and water vapor) of low-carrierdensity monolayer epitaxial graphene on SiC can be readily tuned by a simple low-temperature (T <= 450 K), in situ vacuum gentle heating method. Interestingly, such an approach allows, for the first time, the observation of a crossover from graphene (mu(t)/mu(q) approximate to 2) to an "ordinary metal" (mu(t)/mu(q) approximate to 1) with decreasing carrier density, where mu(t) and mu(q) are transport mobility and quantum mobility, respectively. In the low carrier density limit, our results are consistent with the theoretical prediction that mu(t) is inversely proportional to charged impurity density. Our data also suggest that atmospheric chemical doping can be used to vary intervalley scattering in graphene which plays a crucial role in backward scattering events.