Scanning tunneling microscopy/spectroscopy studies of two isomers of Ce@C₈₂ on Si(111)-(7 X 7) surfaces

Scanning tunneling microscopy images for two isomers of Ce@C₈₂ were observed on Si(111)-(7X7) at 295 K. The Ce@C₈₂ molecules in the first layer were bound to the Si surfaces, and the motions were frozen even at 295 K. The multilayer of the Ce@C₈₂ isomer I (Ce@C₈₂-I) produced a close-packed structure in the surface layer by annealing the Si substrate at 473 K. The distance between the nearest-neighboring molecules was 1.15(4) nm whose value was consistent with that, 1.12 nm, estimated from x-ray diffraction of the Ce@C₈₂-I crystals. This implies that the close-packed structure is dominated by van der Waals forces, as in crystals of Ce@C₈₂-I. The internal structure of Ce@C₈₂-I was observed in the first layer due to a freeze of molecular motion caused by strong interactions between the molecule and the Si adatoms in the surface. Scanning tunneling spectroscopy revealed that the energy gaps for Ce@C₈₂-I and -II in the first layer opened to gap energies, E_g of 0.7 and 1.0 eV, respectively. This fact suggests that these molecules are semiconductors with smaller value of E_g than those for C₆₀ and C₇₀.