Scanning tunneling spectroscopy of CVD graphene

We investigate the electronic properties of large crystal CVD grown graphene using scanning tunneling microscopy. Mono- and bilayer crystals were prepared by transferring graphene from copper onto exfoliated boron nitride flakes on 300 nm SiO$_2$ substrates. The boron nitride provides an ultra flat surface for the graphene. Samples were measured in ultra high vacuum by scanning tunneling spectroscopy at 5 K. In these experiments, we have investigated the effect of two types of charged impurities, either random impurities from the growth process or controlled doping of the graphene with potassium atoms. Potassium atoms are controllably deposited on the graphene at low temperature by heating a getter source.