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

¹ University of Ulm, Central Facility of Electron Microscopy - Albert-Einstein-Allee 11, D-89081 Ulm, Germany, EU
² University of Vienna, Faculty of Physics - Strudlhofgasse 4, A-1090 Vienna, Austria, EU
³ Carl Zeiss NTS GmbH - Carl-Zeiss-Str. 56, D-73447 Oberkochen, Germany, EU

^a michael.kinyanjui@uni-ulm.de

Received: 11 January 2012
Accepted: 26 January 2012

Abstract

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 π plasmons in free-standing graphene monolayers at the momentum range of 0⩽|q|⩽0.5 Å⁻¹ and parallel to the Γ-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 π 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 π plasmon response.