Unoccupied electronic states and potential barrier in films of substituted diphenylphthalides on the surface of highly ordered pyrolytic graphite

The results of a study of the unoccupied electronic states of ultrathin films of bis-carboxyphenyl-phthalide (DCA-DPP) and bis-methylphenyl-phthalide (DM-DPP) up to 8 nm thick are presented. The studies were carried out by total current spectroscopy (TCS) technique in the energy range from 5 eV to 20 eV above $E_{\mathrm F}$ during thermal vacuum deposition of these organic films on the surface of highly oriented pyrolytic graphite (HOPG). The energy $E_{\operatorname{vac}}$ relative to $E_{\mathrm F}$, that is, the electronic work function of the DM-DPP films, at a film thickness of 5–8 nm was 4.3 $\pm$ 0.1 eV. The electronic work function of the DCA-DPP films was 3.7 $\pm$ 0.1 eV. The structure of the maxima of the unoccupied electronic states of DCA-DPP films and DM-DPP films in the studied energy range is determined. The properties determined of DCA-DPP and DM-DPP films are compared with the properties of films of unsubstituted diphenylphthalide (DPP). According to our analysis, –CH3 substitution of the DPP molecule practically did not affect the height of the potential barrier between the film and the HOPG surface, and –COOH substitution of the DPP molecule led to an increase in the height of the potential barrier between the film and the HOPG substrate surface by 0.5–0.6 eV. Substitution of DPP molecules with –COOH functional groups which represents formation of DCA-DPP molecules led to a shift of two peaks of the experimental total current spectra located at energies in the range from 5 eV to 8 eV above $E_{\mathrm F}$, by about 1 eV towards lower electron energies.