Electronic properties of potassium-intercalated ${\mathrm{C}}_{60}$ peapods

We present a study of the electronic structure of potassium-intercalated ${\mathrm{C}}_{60}$-filled single-wall carbon nanotubes (SWCNT’s), so-called peapods, in comparison to the corresponding reference SWCNT’s. The structural changes and the variation of the electronic properties were characterized by electron energy-loss spectroscopy in transmission. The analysis of the $\mathrm{C}1s$ core-level excitations shows that the doping level is nearly the same for peapods and the reference SWCNT’s and that a competitive charge transfer of the $\mathrm{K}4s$ electrons to both the ${\mathrm{C}}_{60}$ peas and the SWCNT pods occurs. The intercalation process causes an expansion of the intertube distance in the bundle lattices and a decrease of the intermolecular distance between the ${\mathrm{C}}_{60}$ peas in the doped peapods. Regarding the optical properties, the charge transfer to the peapods (SWCNT’s) yields the formation of a free-charge-carrier plasmon at about 1.3 eV (1.45 eV). An analysis by an effective Drude-Lorentz model shows that the lower plasmon energy in the doped peapods can be explained by a higher effective screening in these hybrid compounds.