  By employing x-ray photoelectron spectroscopy (XPS), we have been able to
establish the occurrence of charge-transfer doping in few-layer graphene
covered with electron acceptor (TCNE) and donor (TTF) molecules. We have
performed quantitative estimates of the extent of charge transfer in these
complexes and elucidated the origin of unusual shifts of their Raman G bands
and explained the differences in the dependence of conductivity on n- and
p-doping. The study unravels the cause of the apparent difference between the
charge-transfer doping and electrochemical doping.
