Investigation of graphene using low energy positron annihilation induced Doppler broadening spectroscopy

We report the first measurements on 6-8 layers graphene using a low energy positron beam employing a high efficiency rare gas moderator. Doppler broadening of the annihilation gamma was measured from graphene over layers at positron kinetic energies as low as 2 eV. The efficient trapping of positrons on the surface state of graphene at these low energies has been utilised to get Doppler broadened gamma spectra predominantly from graphene over layer on Cu. The ratio of the annihilation gammas measured at low positron energies (a few eV) to that measured at 20 keV show features corresponding to carbon. W-parameter calculated from gamma spectra at different positron energies point towards trapping of positrons in defected graphene as well as at the interface of graphene over layer and Cu. This is the first ever measurements of graphene film of 2-3 nm thickness on a substrate with a depth resolved Doppler broadening spectroscopy at positron kinetic energies below 10 eV.