Unoccupied molecular orbitals of C60 molecules adsorbed on Si(0 0 1)-(2×1) and Si(1 1 1)-(7×7) surfaces studied by NEXAFS

Interaction between C60 molecules and Si surfaces at room temperature has been investigated by near-edge X-ray absorption fine structure (NEXAFS). The NEXAFS spectra of 1.0 monolayer (ML) C60 films adsorbed on Si(0 0 1)-(2×1) and Si(1 1 1)-(7×7) surfaces show the same spectral profile as those of C60 solids. At 0.25 ML, the full-width at half-maximumʹs of the unoccupied molecular orbitals increase, the energy levels of the lowest unoccupied molecular orbital (LUMO) shift, and the intensities of the LUMO+1 decrease on both Si surfaces. These results indicate that C60 molecules are chemisorbed at 0.25 ML, and 75% of C60 molecules are physisorbed and 25% are chemisorbed at 1.0 ML on both Si surfaces. Further, we conclude that the character of the chemisorption is covalent, to which the LUMO+1 contributes on both Si(0 0 1)-(2×1) and Si(1 1 1)-(7×7) surfaces.