Direct probe of linearly dispersing 2D interband plasmons in a free-standing graphene monolayer

In low-dimensional systems, a detailed understanding of plasmons and their dispersion relation is crucial for applying their optical response in the field of plasmonics. Electron energy-loss spectroscopy is a direct probe of these excitations. Here we report on electron energy-loss spectroscopy results on the dispersion of the pi plasmons in free-standing graphene monolayers at the momentum range of 0 <= vertical bar q vertical bar <= 0.5 angstrom(-1) and parallel to the Gamma-M direction of the graphene Brillouin zone. In contrast to the parabolic dispersion in graphite and in good agreement with theoretical predictions of a 2D electron gas of Dirac electrons, linear pi plasmon dispersion is observed. As with previous EELS results obtained from single-wall carbon nanotubes, this can be explained by local-field effects in the anisotropic 2D system yielding a significant contribution of the low-energy band structure on the high-energy pi plasmon response. Copyright (C) EPLA, 2012