Electronic structure and optical properties of concentric-shell fullerenes from electron-energy-loss spectroscopy in transmission

We present momentum-dependent measurements of the loss function of bulk samples of concentric-shell fullerenes using electron-energy-loss spectroscopy in transmission. It is shown that the π and π+σ plasmons of these so-called carbon onions exhibit a significant momentum dependence, with a dispersion coefficient of about two-thirds of that of the corresponding plasmons in graphite. The optical properties derived from a Kramers-Kronig analysis are reminiscent of those found for polycrystalline disordered graphite. Furthermore, we find no changes of the electronic properties by alternating the average diameter of these carbon nanoparticles between 4 and 8 nm. Consequently, concentric-shell fullerenes can be regarded as spherical graphitic nanoparticles, whose electronic structure and optical properties, down to a particle size of at least 4 nm, are strongly governed by the band structure of graphite.