The unoccupied electronic states of the ultrathin diphenylphthalide films on the surface of the highly oriented pyrolytic graphite

The results of diagnostics of the atomic composition of a diphenylphthalide (DPP) film thermally precipitated in vacuum by the of X-ray photoelectric spectroscopy (XPS) method are presented. The results of examination of the unoccupied electronic states of the ultrathin DPP films with the thickness up to 10 nm on the surface of the highly oriented pyrolytic graphite (HOPG) by the total current spectroscopy (TCS) method in the energy range from 5 to 20 eV above $E_{\mathrm{F}}$ are presented. In this range, the main maxima in the total current spectra are identified. The analysis of the TCS results with consideration of the theoretical calculation results has shown that the low-energy maxima observed at the energies from 6 to 7.5 eV are induced predominately by $\pi$* electron orbitals of DPP films. The values of the energy $E_{\mathrm{vac}}$ in relation to $E_{\mathrm{F}}$, i.e., of the electron work function in the DPP films at the film thickness of 5–10 nm, are found experimentally at a level of 4.3 $\pm$ 0.1 eV. The negative charge transfer from an organic film to the substrate corresponds to the formation of the HOPG/DPP boundary potential barrier during the thermal deposition of the DPP film.