Topologically induced confinement of collective modes in multilayer graphene nanocones measured by momentum-resolved STEM-VEELS

In the expanding field of plasmonics, accurate control of the degree of plasmon localization is of crucial importance for tailoring optical properties at the nanoscale. In this paper, the degree of plasmon localization is directly probed by recording the momentum transfer dependence (i.e. the dispersion) of plasmon resonance energies using electron energy loss spectroscopy in the aberration-corrected scanning transmission electron microscope. Limited by the uncertainty principle, resolution in momentum space can easily be tuned by the beam convergence, and it is shown that localization is clearly identifiable, even at low-momentum resolution. In this proof-of-principle study, this technique was applied to multilayer graphene cones containing a varying number of topological defects at their apex. It is shown that a high degree of confinement of the π and π+σ volume plasmons is reached for five pentagonal defects at the cone apex. This effect was attributed to the presence of the topological defects themselves. Furthermore, slight negative refraction was observed for the five-pentagon cone, predominantly affecting the collective excitation of the π electrons.