Disentanglement of the unoccupied electronic structure in metallic and semiconducting C-60 peapods

We provide insight into the C1s x-ray absorption response of C-60 encaged in metallicity-sorted single-walled carbon nanotubes. The partial density of states observed for different constituent species unveils their distinct electronic properties. We identify the relative positions and filling of the one-dimensional van Hove singularities in the hosting nanotubes and the molecular orbitals of the encaged fullerenes. We directly address the possible charge transfer and especially the predicted existence of a shallow half-filled electronic t(1u) band in metallic peapods. In the experiment, we find the t(1u) of C-60 unoccupied in the semiconducting nanotubes. In metallic nanotubes, it is observed 100-200 meV above the Fermi level E-F.