Density of vacant electronic states of semiconductor films of molecules of naphthalene and diphenylphthalide modified by electroactive functional groups

A comparative study has been performed for the structures of the maxima of the density of unoccupied electronic states (DOUS) in ultrathin films of the naphthalene-1,4,5,8-tetracarboxyl acid dianhydride (NTCDA) and naphthalene-1,8-dicarboxyl acid anhydride (NDCA) and two types of films based on phthalides: 3,3-diphenylphthalide (DPP) and 3,3-diphenylphthalide-4',4'-dicarboxyl acid (DPP–DCA). The structures of unoccupied electronic states of the 8–10-nm-thick films have been measured by total current spectroscopy (TCS) in the energy range from 5 to 20 eV above the Fermi level. The experimental results are analyzed by constructing the model total current spectra and the DOUS dependences using the results of calculation of the orbital energy of the molecules under study by the density functional theory (DFT) at the B3LYP/6-31G(d) level. The difference in the DOUS spectra of the NTCDA and NDCA films is the shift of the main DOUS maxima of the NTCDA film to lower energies approximately by 1 eV at energies lower than 12.5 eV; at higher energies, the DOUS maxima are shifted by 1.5–2 eV. The energy positions of the maxima of the total current spectra of the DPP–DCA and DPP films are changed only slightly as various substrates are used: highly ordered pyrolitic graphite (HOPG) and a CdS deposited layer-by-layer. The relative intensities of the maxima are different as various substrates are used. The characteristic shifts of the maxima of the total current spectra of the DPP–DCA films are approximately 1 eV at energies lower 12.5 eV above the Fermi level and 1.5–2 eV at higher energies, as compared to the positions of the corresponding maxima of the DPP films.