Quasicontinuous electron and hole doping of ${\mathrm{C}}_{60}$ peapods

We report on the doping dependence of so-called peapods where ${\mathrm{C}}_{60}$ is encapsulated inside single-wall carbon nanotubes (SWCNT’s) using Raman spectroscopy as probe. The charge transfer in intercalated ${\mathrm{C}}_{60}$ peapods is analyzed as a function of doping with ${\mathrm{FeCl}}_3$ $(p$ type) and potassium $(n$ type). A competition between a charge transfer to the SWCNT pods and to the ${\mathrm{C}}_{60}$ peas is observed. At low intercalation levels, the charge transfer is predominantly to the SWCNT pods. The Fermi level shifts beyond the second van Hove singularity of the semiconducting tubes and below the first singularity of the metallic tubes, respectively. The doping of the valence band for ${\mathrm{FeCl}}_3$ intercalation $(p$ doping) and of the conduction band for K intercalation $(n$ type) is symmetric. At high levels of potassium intercalation, the Fermi level shifts beyond the third singularity of the semiconducting tubes and the ${\mathrm{C}}_{60}$ molecules are charged as well finally yielding a ${\mathrm{C}}_{60}^{6-}$ state that forms a one-dimensional metallic single-bonded ${\mathrm{C}}_{60}$ polymer. For intermediate charging levels of the ${\mathrm{C}}_{60}$ molecules, no line phases are observed in contrast to the fullerides. Rather, a quasicontinuous charge transfer is observed. This provides a unique procedure to establish a continuous charge transfer to ${\mathrm{C}}_{60}$ by intercalation.