Plasmon dispersion on epitaxial graphene studied using high-resolution electron energy-loss spectroscopy

We have investigated the momentum-space-dependent behavior of plasmons on epitaxial graphene (EG) using high-resolution electron energy-loss spectroscopy. There are significant differences in the pi plasmon behavior for single, bilayer, and 3-4 layer graphene which originate from differences in the in-plane and out-of-plane modes, as well as the different band structures between single-layer and few-layer graphene. The pi and sigma + pi surface plasmon modes in single-layer EG are recorded at 5.1 and 14.5 eV at small momentum transfer (q); these are redshifted from the values in multilayer EG. In single-layer graphene, a linear dispersion of the plasmon mode is observed, in contrast to the parabolic dispersion in multilayer EG. The overall linear pi plasmon dispersion between 4.8-6.7 eV is attributed to the mixing of electronic transitions caused by local field effects, which includes the linear dispersion features resulting from transitions within the "Dirac cone." We also observe that the intensity of the Fuches-Kliewer phonon of SiC and loss continuum of EG varies with the thickness of epitaxial graphene.