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

Abstract

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 $\ensuremath{\pi}$ 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 $\ensuremath{\pi}$ and $\ensuremath{\sigma}+\ensuremath{\pi}$ 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 $\ensuremath{\pi}$ 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.