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 energyloss 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. Copyright c © EPLA, 2012 Introduction. – The unique electronic and structural properties of two-dimensional (2D) graphene have caused a lot of excitement recently. At low energies charge carriers in graphene are a massless 2D gas of Dirac electrons which results in unique electronic and structural properties including extremely high charge mobility, fractional quantum Hall effect among others [1,2]. Furthermore, graphene is considered to be the basic structural unit of several forms of carbon including fullerenes, single-wall carbon nanotubes (SWCNT) and graphite [3–5]. Recently a lot of interest has also been focussed on the nature of 2D plasmons in graphene [3,6–19]. 2D plasmons can be broadly described as collective excitations of charge density localized on a 2D system [20]. Plasmon oscillations are a result of Coulomb interaction between electrons and can be described based on their dispersion relation (dependence of plasmon wavelength on energy) [21]. Insights (a)E-mail: [email protected] (b)Present address: University of Vienna, Faculty of Physics Strudlhofgasse 4, A-1090 Vienna, Austria, EU. into the nature of 2D plasmons in graphene are not only important for the fundamental understanding of the electronic structure and collective behaviour in graphene, but also for potential applications in nano-plasmonics [2,22]. This paper is concerned with the dispersion of high-energy interband plasmons also known as π plasmons in freestanding graphene monolayer. Studies on plasmons in graphene have mainly focused on the low-energy plasmons (or charge carrier plasmons) which occur at 0–3 eV. The reason being the position and dispersion of these charge carrier plasmons is directly related to the electronic transport properties [2]. In addition all these studies are based on epitaxial graphene on semi-conducting or metallic substrates. This includes graphene on semiconducting SiC (0001) substrate [9,10,12,18], and on metallic Ir (111) [11], Pt (111) [13], Ni (111) and Au/Ni (111) [14] substrates. However, much less is known about the nature of π and π+σ plasmons which are observed at higher energies, above 4 eV and 14 eV, respectively. While several papers including Mak et al. [15], Eberlein et al. [16], and Yuan et al. [17]