N. B. Gerasimova

Structure of vacant electronic states of an oxidized germanium surface upon deposition of perylene tetracarboxylic dianhydride films

This paper presents the results of the investigation of the interface potential barrier and vacant electronic states in the energy range of 5 to 20 eV above the Fermi level (EF) in the deposition of perylene tetracarboxylic dianhydride (PTCDA) films on the oxidized germanium surface ((GeO2)Ge). The concentration of oxide on the (GeO2)Ge surface was determined by X-ray photoelectron spectroscopy. In the experiments, we used the recording of the reflection of a test low-energy electron beam from the surface, implemented in the mode of total current spectroscopy. The theoretical analysis involves the calculation of the energy and spatial distribution of the orbitals of PTCDA molecules by the density functional theory (DFT) using B3LYP functional with the basis 6-31G(d), followed by the scaling of the calculated values of the orbital energy according to the procedure well-proven in the studies of small organic conjugated molecules. The pattern of changes in the fine structure of the total current spectra with increasing thickness of the PTCDA coating on the (GeO2)Ge surface to 6 nm was studied. At energies below 9 eV above EF, there is a maximum of the density of unoccupied electron states in the PTCDA film, formed mainly by π* molecular orbitals. The higher density maxima of unoccupied states are of σ* nature. The formation of the interface potential barrier in the deposition of PTCDA at the (GeO2)Ge surface is accompanied by an increase in the work function of the surface, Evac–EF, from 4.6 ± 0.1 to 4.9 ± 0.1 eV. This occurs when the PTCDA coating thickness increases to 3 nm, and upon further deposition of PTCDA, the work function of the surface does not change, which corresponds to the model of formation of a limited polarization layer in the deposited organic film.

Formation of the conduction band electronic structure during deposition of ultrathin dicarboximide-substituted perylene films on the oxidized silicon surface

The results of the investigation of the conduction band electronic structure and the interfacial potential barrier during deposition of ultrathin dicarboximide-substituted perylene films (PTCBI-C8) on the oxidized silicon surface have been presented. The measurements have been performed using the very low energy electron diffraction (VLEED) technique implemented in the total current spectroscopy (TCS) mode with a variation in the incident electron energy from 0 to 25 eV. Changes in the intensities of the maxima from the deposited PTCBI-C8 film and from the substrate with an increase in the organic coating thickness to 7 nm have been analyzed using TCS measurements. A comparison of the structure of the maxima of PTCBI-C8 and perylene-tetracarboxylic-dianhydride (PTCDA) films has made it possible to distinguish the energy range (8–13 eV above EF) in which distinct differences in the structures of maxima for PTCDA and PTCBI-C8 films are observed. This energy range corresponds to low-lying σ*-states of the conduction band of the films studied. The formation of the interfacial region of the PTCBI-C8 film and (SiO2)n-Si substrate is accompanied by an increase in the surface work function by 0.6 eV, which corresponds to the electron density charge transfer from the (SiO2)n-Si substrate to the PTCBI-C8 film.

Electrical conductivity of mixed structures based on conjugated organic materials and metals oxides upon adsorption of volatile organic compounds

This paper reports on the results of investigations of the electrophysical characteristics of films based on conjugated organic molecules of copper phthalocyanine (CuPc) and the perylene derivative 3,4,9,10-perylenetetracarboxylic acid dianhydride mixed with TiO2 microparticles and SnO2 nanoparticles. The electrical resistivity of the mixed films under investigation lies in the range from 5 × 108 to 5 × 109 GQ cm. The influence of adsorption on the surface of the films on the electrical conductivity is studied using vapors of water and volatile organic compounds, such as ethanol, ammonia, toluene, and acetone, at pressures in the range from 10−1 to 10 Pa. The adsorption of acetone and toluene vapors does not lead to considerable changes in the electrical conductivity of the films, whereas the exposure to vapors of water, ethanol, and ammonia results in a reversible multiple increase in the electrical conductivity of the studied films. In the case of mixed CuPc/TiO2 films, the electrical conductivity observed upon adsorption of ammonia vapors on the film surface exceeds the initial conductivity by a factor of approximately 2500. The mechanisms of the observed changes in the electrical conductivity are discussed with allowance made for the increase in the electron density of the films due to the electron exchange with adsorbed molecules of reducing gases.

Electronic structure of the conduction band of the interface region of ultrathin films of substituted perylenedicarboximides and the germanium oxide surface

The results of the investigation of the electronic structure of the conduction band and the interfacial potential barrier during the formation of interfaces of dioctyl-substituted perylenedicarboximide (PTCDI-C8) and diphenyl-substituted perylenedicarboximide (PTCDI-Ph) ultrathin films with the oxidized germanium surface have been presented. The experimental results have been obtained using the very low energy electron diffraction (VLEED) technique in the total current spectroscopy (TCS) mode at energies in the range from 5 to 20 eV above the Fermi level EF. The positions of the maxima of the fine structure of total current spectra (FSTCS) of the PTCDI-C8 and PTCDI-Ph films differ significantly in the energy range from 9 to 20 eV above the Fermi level EF, which can be associated with the difference between the substituents of the chosen molecules, dioctyl- and diphenyl-, respectively. At the same time, the positions of the lowenergy maxima in the FSTCS spectra at an energy 6–7 eV above the Fermi level EF for the PTCDI-C8 and PTCDI-Ph films almost coincide with each other. It has been suggested that these maxima are attributed to the electronic states of the perylene core of the molecules under investigation. The process of the formation of interfacial potential barriers of the PTCDI-C8 and PTCDI-Ph films with the oxidized germanium surface has been analyzed. It has been found that the work functions of the surface, Evac–EF, differ little from 4.6 ± 0.1 eV over the entire range of organic coating thicknesses from 0 to 6 nm.

Water-soluble copper phthalocyanine for optimization of gas-sensor characteristics of tin dioxide upon adsorption of ammonia

This paper presents the results of the investigation into the electrical conductivity of thin films based on tin dioxide (SnO2) nanoparticles, a film consisting of copper phthalocyanine-3,4′,4″,4‴-tetrasulfonic acid tetrasodium salt (CuPc–4SO3Na) molecules, and a composite film based on a mixture of equal parts by volume of the two materials upon adsorption of ammonia. The adsorption experiments have been carried out in a vacuum at room temperature with an increase in the ammonia pressure to 3 × 103 Pa from the residual gas base pressure of 5 × 10–1 Pa. It has been found that, in the case of a single-component film based on SnO2 nanoparticles, an increase in the electrical conductivity reaches 100% and is not completely reversible at room temperature after evacuation of the gas. For the single-component CuPc–4SO3Na film and the composite CuPc–4SO3Na/SnO2 film, the electrical conductivities increase by a factor of 400 and 150, respectively. Upon evacuation of ammonia to the base pressure, the electrical conductivity decreases to the initial values for 1 s. The mechanism of the influence of adsorbed ammonia molecules on the electrical conductivity of the composite material under investigation has been discussed using the model of the formation of the composite sample, according to which the organic component is located in voids between the SnO2 nanoparticles.

Erratum to: “Electronic properties of ultrathin films based on pyrrolofullerene molecules on the surface of oxidized silicon”

Results of the investigation into the interface formation during the deposition of the films based on aziridinylphenylpyrrolofullerene (APP-C60) up to 8 nm thick on the surface of the oxidized silicon substrate are presented. The procedure of detecting reflection of testing low-energy electron beam from the surface implemented in the total current spectroscopy mode with a change in the incident electron energy from 0 to 25 eV is used. The structure of maxima in the total current spectra induced by the APP-C60 deposited film is established, and the character of interrelation of these maxima with π* and σ* energy bands in the studied materials is determined. It is revealed due to analyzing the variation in intensities of the total current spectra of the deposited APP-C60 film and the (SiO2)n-Si substrate that the APP-C60 film is formed at the early deposition stage with the coating thickness thinner than one monolayer without the formation of the intermediate modified organic layer. As the APP-C60/(SiO2)n-Si interface is formed, the work function of the surface increases by 0.7 eV, which corresponds to the transfer of the electron density from substrate (SiO2)n-Si toward the film APP-C60. The optical absorption spectra of the APP-C60 films are measured and compared with the spectra of films of unsubstituted C60.

Gas-sensor properties of composite semiconductor films of substituted perylene and tin dioxide nanoparticles

Electrical conductivity of film samples of a composite constituted by a perylene derivative (3,4,9,10-perylenetetracarboxylic-dianhydride) and SnO2 nanoparticles was studied in adsorption of vapors of ammonia, toluene, aqueous hydrogen peroxide, ethanol, and water. A model of formation of the composite sample under study and a mechanism by which adsorbed molecules affect its electrical conductivity are suggested.

Electron energy loss spectra of NTCDA organic films

The electron energy loss (EEL) spectra of thin NTCDA films on the ZnO(0001) surface were measured. In the low-energy range (<60 eV), the EEL spectra (displaying the characteristic loss peaks at Eloss=4.0, 5.6, 12.5, and 14.5 eV) reflect the structure of transitions between the valence and conduction bands. As the primary electron energy increases, the dominating energy losses are due to the excitation of plasma oscillations at ħω1=6.5 eV (π plasmon) and ħω2=25 eV (π−σ plasmon).

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 (EF) 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 EF, there are two main maxima of the density of unoccupied electronic states predominantly formed by the π*-orbitals of the molecules. The higher-lying maxima have essentially a σ*-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 π*-maxima, the relative shift has been observed at an energy of approximately 1 eV. In the region of σ*-electronic states, there is a small transformation of the structure of the maxima.

Electronic structure of the conduction band upon the formation of ultrathin fullerene films on the germanium oxide surface

The results of the investigation of the electronic structure of the conduction band in the energy range 5–25 eV above the Fermi level EF and the interfacial potential barrier upon deposition of aziridinylphenylpyrrolofullerene (APP-C60) and fullerene (C60) films on the surface of the real germanium oxide ((GeO2)Ge) have been presented. The content of the oxide on the (GeO2)Ge surface has been determined using X-ray photoelectron spectroscopy. The electronic properties have been measured using the very low energy electron diffraction (VLEED) technique in the total current spectroscopy (TCS) mode. The regularities of the change in the fine structure of total current spectra (FSTCS) with an increase in the thickness of the APP-C60 and C60 coatings to 7 nm have been investigated. A comparison of the structures of the FSTCS maxima for the C60 and APP-C60 films has made it possible to reveal the energy range (6–10 eV above the Fermi level EF) in which the energy states are determined by both the π* and σ* states and the FSTCS spectra have different structures of the maxima for the APP-C60 and unsubstituted C60 films. The formation of the interfacial potential barrier upon deposition of APP-C60 and C60 on the (GeO2)Ge surface is accompanied by an increase in the work function of the surface Evac–EF by the value of 0.2–0.3 eV, which corresponds to the transfer of the electron density from the substrate to the organic films under investigation. The largest changes occur with an increase in the coating thickness to 3 nm, and with further deposition of APP-C60 and C60, the work function of the surface changes only slightly.