  We investigate the effects of transition metals (TM) on the electronic doping
and scattering in graphene using molecular beam epitaxy combined with in situ
transport measurements. The room temperature deposition of TM onto graphene
produces clusters that dope n-type for all TM investigated (Ti, Fe, Pt). We
also find that the scattering by TM clusters exhibits different behavior
compared to 1/r Coulomb scattering. At high coverage, Pt films are able to
produce doping that is either n-type or weakly p-type, which provides
experimental evidence for a strong interfacial dipole favoring n-type doping as
predicted theoretically.
