STEM plasmon spectroscopy of free standing graphene

Plasmon spectroscopy of the thinnest possible membrane, a single layer of carbon atoms, graphene, has been carried out in conjunction with ab initio calculations of the low loss function. We observe π- and π + σ-surface plasmon modes in free-standing single sheets at 4.7 eV and 14.8 eV which are substantially red-shifted from their values in graphite. These modes are in very good agreement with the theoretical spectra which find the π- and π + π in-plane modes of graphene at 4.5 eV and 14.5 eV. We also find that there is little loss caused by out-of-plane modes for energies less than about 10 eV.