Electronic properties of ultrathin films based on pyrrolofullerene molecules on the surface of oxidized silicon

Results of the investigation into the interface formation during the deposition of the films based on aziridinylphenylpyrrolofullerene (APP-C$_{60}$) up to 8 nm thick on the surface of the oxidized silicon substrate are presented. The procedure of detecting reflection of testing low-energy electron beam from the surface implemented in the total current spectroscopy mode with a change in the incident electron energy from 0 to 25 eV is used. The structure of maxima in the total current spectra induced by the APP-C$_{60}$ deposited film is established, and the character of interrelation of these maxima with $\pi^*$- and $\sigma^*$ energy bands in the studied materials is determined. It is revealed due to analyzing the variation in intensities of the total current spectra of the deposited APP-C$_{60}$ film and the (SiO$_2$)$n$-Si substrate that the APP-C$_{60}$ film is formed at the early deposition stage with the coating thickness thinner than one monolayer without the formation of the intermediate modified organic layer. As the APP-C$_{60}$/(SiO$_2$)$n$-Si interface is formed, the work function of the surface increases by 0.7 eV, which corresponds to the transfer of the electron density from substrate (SiO$_2$)$n$-Si toward the film APP-C$_{60}$. The optical absorption spectra of the APP-C$_{60}$ films are measured and compared with the spectra of films of unsubstituted C$_{60}$.