Electronic structure of pristine and potassium-doped $\mathrm{Y}@{\mathrm{C}}_{82}$ metallofullerene

Ultraviolet photoelectron spectra (UPS) of $\mathrm{Y}@{\mathrm{C}}_{82}$ have been measured with a synchrotron light source. UPS spectra reveal the photoelectron onset at $0.3\mathrm{eV}$ below the Fermi level, indicating the semiconducting nature of the metallofullerene, and contain low-binding-energy features associated with charge transfer from the encapsulated atom to molecular orbitals. As for empty fullerenes, when the incident photon energy is varied, the relative intensities of the valence-band features of $\mathrm{Y}@{\mathrm{C}}_{82}$ oscillate, showing that the endohedral atom does not significantly disturb the photoelectron interference effects of the fullerene cage. For ${\mathrm{K}}_{2.3}\mathrm{Y}@{\mathrm{C}}_{82}$, which forms a fcc-like structure, core and valence states of $\mathrm{Y}@{\mathrm{C}}_{82}$ broaden and shift to higher binding energies but there is no evidence of density of states at the Fermi level. Finally, although core and valence states of $\mathrm{Y}@{\mathrm{C}}_{82}$ change considerably upon an intercalation of potassium, the encapsulated atom is well shielded from the outside chemical environment and its valency remains close to $3+$.

Figure 1

(Color online) Valence-band spectra of (a) $\mathrm{Y}@{\mathrm{C}}_{82}$ and (b) ${\mathrm{C}}_{84}$ acquired at different photon excitation energies. There are six distinct features within the leading $8\mathrm{eV}$ range of the $\mathrm{Y}@{\mathrm{C}}_{82}$ spectra (labeled A–F) and three features in the ${\mathrm{C}}_{84}$ spectra (labeled A–C).

Figure 2

(Color online) (a) Valence-band spectra of $\mathrm{Y}@{\mathrm{C}}_{82}$ and ${\mathrm{K}}_{2.3}\mathrm{Y}@{\mathrm{C}}_8$, with the analogous spectrum of the empty fullerene ${\mathrm{C}}_{84}$ shown for comparison. The spectra were recorded using a photon energy $h\nu =65\mathrm{eV}$. Broken lines indicate the peak positions of $\mathrm{Y}@{\mathrm{C}}_{82}$ and the Fermi level. (b) $\mathrm{C}1s$ core-level spectra, taken using $\mathrm{Mg}K\alpha$ x rays. Inset shows an enlarged view of the shake-up structures on the high-binding-energy side. The reduction in the intensity and broadening of the shake-up feature upon the potassium intercalation of $\mathrm{Y}@{\mathrm{C}}_{82}$ are apparent.

Figure 3

(Color online) Core-level photoemission spectra of the $\mathrm{Y}3d$ from $\mathrm{Y}@{\mathrm{C}}_{82}$ and ${\mathrm{K}}_{2.3}\mathrm{Y}@{\mathrm{C}}_{82}$, acquired using $\mathrm{Mg}K\alpha$ x rays. Both the spectra were normalized to the integrated intensities after the substraction of a linear background. No binding-energy shift associated with $K$ doping is observed.