Doping of graphene exfoliated on SrTiO3.

We present atomic force microscopy and scanning Kelvin probe data obtained under ultra-high vacuum conditions from graphene exfoliated on crystalline SrTiO3 substrates. The contact potential difference shows a monotonic increase with the number of graphene layers until after five layers of saturation is reached. By identifying the saturation value with the work function of graphite we determine the work function of single and bilayer graphene to be Phi(SLG) = 4.409 +/- 0.039 eV and Phi(BLG) = 4.516 +/- 0.035 eV, respectively. In agreement with the higher work function of single-layer graphene with respect to free-standing graphene, our measurements indicate an accumulation of charge carriers corresponding to a doping of the exfoliated graphene layer with electrons.