Charge transfer in graphene flakes covered with metals probed by Raman spectroscopy

Abstract

In order to build graphene-based devices, it is necessary to know how graphene is going to behave when different metals are deposited on it. Mainly, the difference in work function between graphene and the metal would cause a transfer of electrons which is expected to be from the material of lower work function to the higher one. The doping implies a change on the electron-phonon interaction and the most effective method to detect this change is by means of Raman spectroscopy. Sometimes, the change observed is opposite to what would be expected in terms of work function differences, which means that other phenomena, mainly stress, must explain this anomalous change. In this study, 3nm of diverse metals were evaporated by electron beam evaporation over graphene flakes, previously grown by mechanical exfoliation on 285nm of SiO2 substrate. The Raman spectra of all the metals except that of cobalt show a similar behaviour. On the cobaltgraphene spectrum, two new peaks appear at ~1130cm-1 and ~1520cm-1 which do not correspond to uniaxial strain but maybe to more intrinsic cobaltgraphene bonding properties, definitely different from the ones of the other metals studied.