Electronic structure of the conduction band upon the formation of ultrathin fullerene films on the germanium oxide surface

The results of the investigation of the electronic structure of the conduction band in the energy range 5–25 eV above the Fermi level $E_{\mathrm{F}}$ and the interfacial potential barrier upon deposition of aziridinylphenylpyrrolofullerene (APP-C$_{60}$) and fullerene (C$_{60}$) films on the surface of the real germanium oxide ((GeO$_2$)Ge) have been presented. The content of the oxide on the (GeO$_2$)Ge surface has been determined using X-ray photoelectron spectroscopy. The electronic properties have been measured using the very low energy electron diffraction (VLEED) technique in the total current spectroscopy (TCS) mode. The regularities of the change in the fine structure of total current spectra (FSTCS) with an increase in the thickness of the APP-C$_{60}$ and C$_{60}$ coatings to 7 nm have been investigated. A comparison of the structures of the FSTCS maxima for the C$_{60}$ and APP-C$_{60}$ films has made it possible to reveal the energy range (6–10 eV above the Fermi level $E_{\mathrm{F}}$) in which the energy states are determined by both the $\pi^{*}$ and $\sigma^{*}$ states and the FSTCS spectra have different structures of the maxima for the APP-C$_{60}$ and unsubstituted C$_{60}$ films. The formation of the interfacial potential barrier upon deposition of APP-C$_{60}$ and C$_{60}$ on the (GeO$_2$)Ge surface is accompanied by an increase in the work function of the surface $E_{\operatorname{vac}}$–$E_{\mathrm{F}}$ by the value of 0.2–0.3 eV, which corresponds to the transfer of the electron density from the substrate to the organic films under investigation. The largest changes occur with an increase in the coating thickness to 3 nm, and with further deposition of APP-C$_{60}$ and C$_{60}$, the work function of the surface changes only slightly.