Propagation of low-energy electrons and the density of unoccupied states in ultrathin TCNQ layers on the oxidized silicon surface

The formation of unoccupied electronic states and the boundary potential barrier during thermal deposition of tetracyanoquinodimethane (TCNQ) films to 7 nm in thickness on a (SiO2)$n$-Si surface has been studied. The electronic characteristics of the surface under study are measured by the total current spectroscopy (TCS) using a testing electron beam with energies from 5 to 20 eV above the Fermi level. The formation of a boundary potential barrier in the (SiO2)$n$-Si/TCNQ structure is accompanied by an increase in the work function of the surface from 4.2 $\pm$ 0.1 to 4.7 $\pm$ 0.1 eV. Using the results of the TCS experiments, the DOUS dependences of the TCNQ films have been built. To analyze the experimental DOUS dependences, the orbital energies of the TCNQ molecules are calculated by the density functional theory (DFT) at the B3LYP/6-31G(d) level with subsequent correction and the inclusion of the polarization energy of the condensed medium. In the energy range indicated above, DOUS of the TCNQ films has four main maxima. The DOUS maximum at energy 7.0 eV above $E_{\mathrm{F}}$ is predominantly formed by $\pi^*$ orbitals. Three DOUS maxima in the energy range from 8.0 to 20 eV above $E_{\mathrm{F}}$ are formed by approximately the same amount of the $\pi^*$ and $\sigma^*$ orbitals.