E. F. Lazneva

A total current spectroscopy study of metal oxide surfaces: I. Unoccupied electronic states of ZnO and MgO

Unoccupied density of states (DOS) for the polar (0001) Zn, (000) O and non-polar (100) ZnO surfaces and for a MgO(001)-c(2 × 2) thin film were studied using total current spectroscopy (TCS). TC spectra of the ZnO surfaces reveal the Zn 4s-O 2p band (6.2 eV) and two DOS maxima in the Zn 4p-O 2p band which exhibited different energy positions on the Zn-terminated (8.1 eV) and O-terminated (11.0 eV) surfaces. In the TC spectra of the MgO thin film, the energy location of Mg-derived (8.0, 14.8 and 21.0 eV) and O-derived (11.6, 14.2 and 18.5 eV) maxima of DOS were determined. Previously reported data on DOS of these materials is discussed, and it is shown that additional detailed information on the energy structure of unoccupied DOS in ZnO and MgO in the 5-25 eV range above EF has been found.

Interface formation between oligo(phenylele–vinylene) films and highly ordered pyrolytic graphite and Ge(1 1 1) surfaces

Abstract Thin films of trioligo(phenylene–vinylene) end terminated by dibutylthiole (tOPV) were thermally deposited in UHV on highly ordered pyrolytic graphite (HOPG) and on Ge (1 1 1) substrates. The surface potential and the structure of unoccupied electron states (DOUS) located 1–25 eV above vacuum level were monitored during the film deposition, using an incident beam of low-energy electrons according to the total current electron spectroscopy (TCS) method. The electronic work function of the tOPV films changed during the film deposition until it reached a stable value of 4.3±0.1 eV at the film thickness of 8–10 nm. Deposition of the tOPV under 3 nm led to formation of intermediate DOUS structures that were different for the cases of the two substrates used in the study. The intermediate structures got replaced by a DOUS structure of the tOPV films which was the same for the two substrates, along with the increase of the deposit thickness to 8–10 nm. A significant electron transfer from the tOPV film to Ge (1 1 1) surface was observed and the charge transfer layer in the film extended up to 10 nm. No significant charge transfer was observed at the interfaces between the tOPV film and the HOPG substrate. A substantial reconfiguration of the electronic structure of the tOPV films due to the interaction with the Ge(1 1 1) and also the HOPG surfaces was deduced and the charge transfer at the tOPV–Ge(1 1 1) interface was found to be consistent with the extended interface dipole model.

Low-energy electron mean free path in thin films of copper phthalocyanine

The formation of thin organic films of copper phthalocyanine (CuPc) deposited onto the surface of gold-coated quartz crystal resonator was studied in situ under ultrahigh vacuum conditions by means of total electron-beam-induced current spectroscopy in combination with deposit thickness determination by piezocrystal microbalance technique. Variations in the fine structure of the total current spectra of CuPc layers of var-ious thicknesses in the 0–8 nm interval have been analyzed and the electron mean free path in thin CuPc films was determined as a function of the electron energy. For electron energies of 5.0, 7.2, 14.4, and 18.0 eV above the Fermi level, the mean free path is 6.4, 3.9, 2.6, and 2.3 nm.

A total current spectroscopy study of metal oxide surfaces: II. Unoccupied electronic states on TiO2(110) and SrTiO3(100) surfaces

Unoccupied densities of states (DOS) for the ordered and atomically clean TiO2(110) and SrTiO3(100) surfaces were studied using the total current spectroscopy (TCS) technique. Obtained TC spectra reveal the Ti-derived (Ti 3d(eg) - 5.2 eV, Ti 4s - 23.5 eV) and Sr-derived (Sr 3d - triplet: 7.3, 9.0 and 10.5 eV; Sr 5p/O 2p - 15 eV) maxima of unoccupied DOS. Main oxygen-derived maxima are located at 11.2 and 15.0 eV for TiO2 and at 11.8, 18.0 and 19.6 eV for SrTiO3. The results are discussed in comparison with the previously reported data on DOS of these materials and they provide additional detailed information on the energy structure of unoccupied DOS on TiO2(110) and SrTiO3(100) surfaces in the 4-25 eV range above the Fermi level.

Electronic Properties of the Interface between Hexadecafluoro Copper Phthalocyanine and Unsubstituted Copper Phthalocyanine Films

The formation of an interface during the deposition of unsubstituted copper phthalocyanine (CuPc) films on the surface of hexadecafluoro copper phthalocyanine (F16-CuPc) films is studied. An incident low-energy electron beam with energies from 0 to 25 eV is used to test the surface under study according to the very-low-energy electron-diffraction technique (VLEED) in the mode of total current spectroscopy. For F16-CuPc films, the structure of the maxima in the total current spectra and its main differences from the structure of the maxima for the CuPc film are determined in the energy range from 5 to 15 eV above the Fermi level. The differences in the structure of vacant electron orbitals for CuPc and F16-CuPc are also revealed using density functional theory calculations. As a result of an analysis of variations in the intensities of the total current spectra of the CuPc and F16-CuPc films, it is assumed that an intermediate layer up to 1 nm thick appears during the formation of an interface between these films, which is characterized by a spread of the features in the total current spectrum. The height, width, and change in the work function are determined for the studied F16-CuPc/ÑuPc interface barrier. A decrease in the level of vacuum by 0.7 eV occurs in the boundary region, which corresponds to electron density transfer from the CuPc film toward the F16-CuPc substrate.

Interface doping of conjugated organic films by means of diffusion of atomic components from the surfaces of semiconductors and of metal oxides

The paper reports the results on the interface formation of 5-10 nm thick conjugated layers of Cu-phthalocyanine (CuPc) with a number of solid surfaces: polycrystalline Au, (SiO(2))n-Si, ZnO(0 0 0 1), Si(1 0 0), Ge(1 1 1), CdS(0 0 0 1) and GaAs(1 0 0). The results were obtained using Auger electron spectroscopy (AES) and low-energy target current electron spectroscopy (TCS). The organic overlayers were thermally deposited in situ in UHV onto substrate surfaces. The island-like organic deposits were excluded from the analysis so that only uniform organic deposits were considered. In the cases of polycrystalline Au, Si(1 0 0) and Ge(1 1 1) substrates the AES peaks of the substrate material attenuated down to the zero noise level upon the increase of the CuPc film thickness of 8-10 nm. The peaks corresponding to oxygen atoms in the case of SiO(2) substrate, and to atoms from the ZnO, GaAs and CdS substrates were clearly registered in the AES spectra of the 8-10 nm thick CuPc deposits. The relative concentration of the substrate atomic components diffused into the film was different from their relative concentration at the pure substrate surface. The concentration of the substrate dopant atoms in the CuPc film was estimated as one atom per one CuPc molecule. Using the target current electron spectroscopy, it was shown that the substrate atoms admixed in the CuPc film account for the appearance of a new peak in the density of unoccupied electronic states. Formation of intermediate TCS spectra until the CuPc deposit reaches 2-3 nm was observed in the cases of GaAs(1 0 0), ZnO(0 0 0 1), Ge(1 1 1) surfaces. The intermediate spectra show a less pronounced peak structure different from the one typical for the CuPc films. It was suggested that the intermediate layer was formed by the CuPc molecules fully or partially decomposed due to the interaction with the relatively reactive semiconductor surfaces.

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.

Hypothesis for the Mechanism of Ascorbic Acid Activity in Living Cells Related to Its Electron-Accepting Properties

Electron-accepting properties, and in particular resonance dissociative electron attachment (DEA) to ascorbic acid (AA), are investigated by means of DEA spectroscopy in vacuo. The experimental features are assigned in silico and discussed in relation to expected dissociative electron transfer processes in vivo with the support of density functional theory calculations and the polarizable continuum model. It is shown that formation of the two most abundant AA metabolites in living cells, namely monodehydroascorbic acid and dehydroascorbic acid, can be stimulated by cellular electron transfer to AA under reductive conditions. Prooxidant effects caused by AA are suggested to be mediated by hydroxyl radicals formation via the DEA mechanism. The involvement of excited electronic states under UV-irradiation in plants could open additional DEA channels leading to specific AA activity forbidden under dark state conditions.

Laser-induced O2 desorption from TiO2 surfaces

Abstract Laser-induced oxygen molecule desorption from crystalline (110) and compressed-powder rutile TiO 2 surfaces is investigated by time-of-flight distributions and desorption signal dependencies on laser fluence. A double-component distribution of desorbed O 2 molecules (a fast one and a slow one) justifies validity of both photoelectronic and thermo-activation mechanisms. The defected surfaces are characterized by the more intensive oxygen escape supporting the desorption flow formation via the surface defect sites.

Resonance Electron Attachment to Tetracyanoquinodimethane

Resonance interaction of low energy (0-14 eV) electrons with gas-phase 7,7,8,8-tetracyanoquinodimethane (TCNQ) was investigated using dissociative electron attachment (DEA) spectroscopy. Spectral features associated with formation of long-lived TCNQ molecular negative ions are detected at incident electron energies of 0.3, 1.3, and 3.0 eV. A variety of negative fragments is observed around 4 eV, and slow (microseconds) dissociative decay channels are detected at about 3 eV, in competition with simple re-emission of the captured electron. The average electron detachment time from the TCNQ(-) negative ions formed at 3 eV was evaluated to be 250 μs. The experimental findings are interpreted with the support of density functional theory (DFT) calculations of the empty orbital energies, scaled with an empirical equation, and by comparison with earlier electron transmission spectroscopy (ETS) data. A possible mechanism for the unusual formation of long-lived molecular anions above zero energy (up to 3 eV) is briefly discussed. The present results on the interactions between electrons and isolated TCNQ molecules could give more insight into processes observed in TCNQ adsorbates under conditions of excess negative charge. In particular, electron-stimulated surface reactions are hypothesized, likely occurring when condensed TCNQ molecules are exposed to electron beam irradiation.