Low-energy electron transmission for the analysis of the interface barrier formation and the density of the unoccupied electronic states in the ultra-thin layers of fluorinated copper-phthalocyanine

Abstract

Abstract:The interfacial structure made from the thermally deposited 5 -7 nm thick layers of hexadecafluoro copper phthalocyanine (F 16 -CuPc) and of the unsubstituted copper phthalocyanine (CuPc) was subjected to the studies. The surface work function and the density of the unoccupied electron states (DOUS) located 5-20 eV above the Fermi level (E F ) were investigated during the CuPc/F 16 -CuPc interface formation using the very low energy electron diffraction (VLEED) method and the total current spectroscopy (TCS) measurement scheme. The DOUS peak structure of the organic films studied obtained from the TCS results showed a good correspondence to the main π* and σ* DOUS bands obtained from the density functional theory (DFT) calculations. The interfacial barrier was characterized by the negative charge transfer from the CuPc overlayer to the F 16 -CuPc underlayer occurred within the 5 nm thick interfacial region in the CuPc overlayer which was accompanied by the decrease of the surface work function from 4.9±0.1 eV to 4.3±0.1 eV. The stabilization of the π* DOUS bands, as well as restructuring of the low lying σ* bands was observed in the in the case of the fluorinated film (F 16 -CuPc) compared to the case of the unsubstituted CuPc film.

Density of the unoccupied electronic states of the ultrathin films of the aziridinylphenylpyrrol substituted fullerene

“…Their energy position corresponds well to the position of the peaks B 1 -B 8 of the DOUS obtained from the TCS experiment (Fig.2b). The three unlabeled peaks of the calculated DOUS in Fig.2c situated in the energy range from 2 eV to 5 eV correspond well to the three low-lying DOUS peaks observed by the other authors in the C 60 vacant band, for which the * character was suggested [2,15,38]. The DOUS bands in the energy range from 5 eV to 7 eV, marked as the maxima C 1 , C 2 in Fig.2b and B 1 , B 2 in Fig.2c are analogous to the bands which were assigned as * bands according to the refs.…”

“…[2,15,[38][39][40] and using the DOUS obtained as a result of the DFT calculations (Fig.2c). The calculations at the B3LYP/6-31G(d)) level provided over 150 values of the orbital energies in the energy range from 0 eV to 25 eV, the scaled virtual orbital energies (see section 2) were used to plot the DOUS function.…”

“…In ref. [38] there was suggested that these bands might have been composed from a mixture of * and * excitations, while the higher DOUS maxima would correspond to * bands.…”