Monitoring the doping of graphene on SiO2/Si substrates during the thermal annealing process

Various doping levels of graphene on SiO2/Si substrates are reported in the literature. We show by in situ Raman spectroscopy that the heating of chemical vapor deposited graphene on SiO2/Si during the transfer process is the main factor causing this unintended doping of graphene samples. Large areas of graphene were analyzed using Raman spectroscopy, before and after the thermal treatment, to demonstrate that the effects of heating are spread throughout the graphene layer. The perturbations caused by the exposure of supported graphene during the first heating cycle (in vacuum) are irreversible, even though the samples were later in contact with the atmosphere. These results clarify deviations found in the Raman data obtained for transferred chemical vapor deposited graphene by different authors.