Scanning Tunneling Microscopy/Spectroscopy of La2@C72 Multilayer Islands on Si(100)-2*1-H Surfaces

We have investigated structural and electronic properties of La2@C72 multi layer islands grown on a hydrogen-terminated Si(100)-2×1-H surface using ultra-high-vacuum scanning tunneling microscopy/spectroscopy (STM/STS) and theoretical calculations based on first principles. The La2@C72 molecules are observed by STM as ellipsoids, consistent with the NMR results [H. Kato et al.: J. Am. Chem. Sci. 125 (2003) 7782], indicating that, out of 24 isomers only two, with D2 symmetry and with a non-IPR (isolated pentagon rule) structure are possible. The energy gap for the carbon cage of the La2@C72 multi layer islands is measured by STS as 1.4 eV, suggesting that the C72 carbon cage is stabilized by a charge transfer from the encaged La atoms to the fullerene cage. The La-atom derived LUMO states are localized near the encapsulated La atoms and observed as a midgap-like density of states by STS.