Raman spectra of graphene exfoliated on insulating crystalline substrates

We have investigated single layer, bilayer and few-layer graphene exfoliated on SiO2 and on single crystal surfaces of SrTiO3, Al2O3 and TiO2 using Raman spectroscopy. The typical 'fingerprint' 2D peak turns out to be indicative of the number of graphene layers independent of the substrate material. The morphological quality of the graphene is as good as on SiO2 substrates for all the materials. We find evidence for substrate-induced changes due to doping. With most substrates, hole doping is observed, but with SrTiO3 we have identified a dielectric substrate with which electron accumulation in graphene can be achieved.