Density of unoccupied electronic states of vapor-deposited films of dioctyl-substituted and diphenyl-substituted perylenedicarboximides

The results of the investigation of the density of unoccupied electronic states (DOUS) in the energy range from 5 to 20 eV above the Fermi level $(E_{F})$ in dioctyl-substituted perylenedicarboximide (PTCDI-C8) and diphenyl-substituted perylenedicarboximide (PTCDI-Ph) ultrathin films have been presented. The experimental results have been obtained from measurements of the secondary low-energy electron current with the use of the total current spectroscopy (TCS) technique. A theoretical analysis has been performed, including the density functional theory calculation of the energies and spatial distribution of the orbitals of the molecules under investigation and the subsequent scaling of the calculated orbital energies according to the procedure well-proven previously in studies of small conjugated organic molecules. It has been found that, for each of the two types of the studied films, at energies below 8 eV above the Fermi level $E_{F}$, there are two main maxima of the density of unoccupied electronic states predominantly formed by the $\pi^*$-orbitals of the molecules. The higher-lying maxima have essentially a $\sigma^*$-character. The influence of dioctyl- and diphenyl-substituent groups on the density of unoccupied electronic states has been analyzed in comparison with the results obtained for the studied types of films. In the case of the $\pi^*$-maxima, the relative shift has been observed at an energy of approximately 1 eV. In the region of $\sigma^*$-electronic states, there is a small transformation of the structure of the maxima.