Density of the unoccupied electronic states of the ultrathin films of the aziridinylphenylpyrrol substituted fullerene

Abstract

The ultrathin films of aziridinylphenylpyrrol-C60 (APP-C60) and of the unsubstituted C60 thermally deposited in UHV on an oxidized silicon substrate (SiO2)n-Si were studied experimentally using the very low energy electron diffraction (VLEED) method and the total current spectroscopy (TCS) measurement scheme. The density of the unoccupied electronic states (DOUS) located 2-20eV above the Fermi level (EF) of the films under study was determined using the TCS results and using the results of the density functional theory (DFT) calculations of the vacant electronic orbitals of the APP-C60 and C60 molecules. The DOUS peak structure obtained on the basis of the calculation results corresponds well to the structure of the major DOUS bands obtained from the TCS experiment. The comparison of the DOUS spectra of the APP-C60 and C60 films under study showed that the introduction of the APP substituent to the C60 molecule was accompanied by the pronounced changes of the π* DOUS bands in the energy range from 2eV to 5eV above EF and of the DOUS band composed from both π* and σ* - type orbitals in the energy range from 5eV to 9eV above EF. The formation of the low-lying σ* DOUS band in the APP-C60 film in the energy range from 8eV to 13eV above EF can be explained by the superposition of the relevant DOUS maxima from the C60 film and from the APP fragment.