Density Of Unoccupied States Research Articles

Charge-transfer at silver/phthalocyanines interfaces

Abstract Valence band photoemission spectroscopy (VB-PES) and inverse photoemission spectroscopy (IPES) were employed to determine the occupied and unoccupied density of states upon silver deposition onto layers of two phthalocyanines (H2Pc and CuPc). The two different Pc molecules give rise to very distinct behaviour already during the initial stage of silver deposition. While in the CuPc case no shift occurs in the energy levels, the H2Pc highest occupied molecular orbital (HOMO) and lowest unoccupied molecular orbital (LUMO) are shifting simultaneously by 0.3 eV, i.e., the HOMO shifts away from the Fermi level while LUMO shifts towards the Fermi level. As the silver quantity increases the HOMO levels of both Pcs are shifting towards the Fermi level. When the Fermi level is resolved in the VB spectra, the characteristic features of H2Pc and CuPc are smeared out to some extent. Shifts in HOMO and LUMO energy positions as well as changes in line shapes are discussed in terms of charge-transfer and chemical reactions at the interfaces.

Experimental studies on the electronic structure of pyrite FeS2 films prepared by thermally sulfurizing iron films

Pyrite FeS2 films have been prepared by thermally sulfurizing iron films deposited by magnetron sputtering.The electronic structures were studies by X-ray absorption near edge structure and X-ray photoemission spectrum. The results show that an S 3p valence band with relatively higher intensity compared to the calculation exists in 2—10eV range and a high density below the Fermi level of Fe 3d states were detected.A second gap of 2.8eV in the unoccupied density of states was found above the conduction band which was 2.4eV by experimentally calculation.The difference between t2g and eg which were formed in an octahedral crystal field was computed to be 2.1eV.

Simulated electron energy loss spectra from a C70 crystal

Electron energy loss spectra are simulated for a C70 crystalline structure. It is found that the simulated spectrum is similar to the unoccupied density of states of a C70 molecule, indicating that the crystalline structure has only a small effect on the spectrum. Unlike the case of a single molecule, however, the main contribution to the second peak in the spectrum cannot be ascribed as being due to the equatorial atoms.

Bonding of thin Pd films on(100)SrTiO3substrates:Ab initiodensity functional theory investigations

The electronic structure of the PdSrTiO{sub 3} interface, which serves as a model system for more complex metal/ceramic interfaces, is studied in terms of local site- and momentum-projected densities of states. Band structures were calculated using density functional theory in the local density approximation. By comparing the local densities of unoccupied states to experimentally determined electron energy-loss near-edge structures, a TiO{sub 2} termination of the SrTiO{sub 3} substrate in contact to the Pd is verified. Furthermore, the local bonding characteristics across the PdSrTiO{sub 3} interface are analyzed by local densities of occupied states overlapping in energy, i.e., by covalent bonding interactions. It is found that the bonding between the Pd and the substrate are dominated by {sigma}-type bonds across the interface.

Some effects of electron channeling on electron energy loss spectroscopy

As an electron beam (of order 100 keV) travels through a crystalline solid it can be channeled down a zone axis of the crystal to form a channeling peak centered on the atomic columns. The channeling peak can be similar in size to the outer atomic orbitals. Electron energy loss spectroscopy (EELS) measures the losses that the electron experiences as it passes through the solid yielding information about the unoccupied density of states in the solid. The interaction matrix element for this process typically produces dipole selection rules for small angle scattering. In this paper, a theoretical calculation of the EELS cross section in the presence of strong channeling is performed for the silicon L23 edge. The presence of channeling is found to alter both the intensity and selection rules for this EELS signal as a function of depth in the solid. At some depths in the specimen small but significant non-dipole transition components can be produced, which may influence measurements of the density of states in solids.

Insights into oxygen-cation bonding in fresnoite-type structures from O K- and Ti L23-electron energy-loss spectra and ab initio calculations of the electronic structure

O K- and Ti L23-core-loss spectra of fresnoite Ba2TiSi2O8 (BTS) and Sr2TiSi2O8 (STS), which is isotypic to BTS, have been measured by electron energy-loss spectroscopy (EELS). The energy-loss near-edge structures (ELNES) of the O K edge have been identified on the basis of theoretical simulations and interpretations of the X-ray absorption near-edge structures (XANES), which have been modelled in the framework of self-consistent full multiple-scattering (FMS) theory using FEFF8. Herewith, the K-absorption spectra of oxygen μ(E) and the local partial electron density of states (DOS) of all atoms have been calculated. For BTS, the observed spectral features in the O K-edge spectra are interpreted in terms of mixing between the central O p and neighbouring Ba 5d and 4f, Si 3p and 3d, and Ti 3d orbitals. The observed differences in the O K-edge spectra for STS and BTS can mainly be attributed to three properties: (1) The lack of high local partial Sr unoccupied DOS with 4f symmetry near the Fermi level compared to the high Ba 4f unoccupied DOS results in differences of overlapping O 2p – cation orbitals. (2) The differences in the ionic radii of Sr and Ba result in a larger unit cell for BTS and, thus, in larger oxygen-cation bonding distances. (3) In comparison to STS, the strength of the incommensurate 2-D structural modulation is significantly weaker in BTS, i.e. distortions of coordination polyhedra occur to a much lesser extent. All these effects alter the oxygen-cation hybridization and, hence, result in a variation of the O 1s → p transition and consequently of the O K-edge spectral shape. The observed peak broadening in Ti L23 ELNES of STS compared to BTS is correlated with strong displacive modulations hosted in STS.

Resonant Raman x-ray scattering at the S 2p edge of iron pyrite

The x-ray absorption and resonant inelastic x-ray spectra of iron pyrite(FeS2) have been measured at the S 2p edge and compared with published electronicstructure calculations. A minimum in the x-ray absorption intensity interpreted asindicating a gap in the unoccupied density of states is found from about 4 to6 eV above the bottom of the conduction band, in agreement with some recentcalculations. Resonant Raman scattering conditions were set up at the onset of S2p3/2 absorptionand a constant energy loss peak at 1.9 eV was observed. This is assigned to transitions from occupiedtg tounoccupied eg states, both of which are predominantly of Fe 3d character but hybridized with the Svalence states. This demonstrates that Fe dd excitations can be probed via S 2p resonantspectroscopy, as has been done recently at the O 1s edge for oxide materials.

Spin splitting of s and p states in single atoms and magnetic coupling in dimers on a surface.

Electronic states of magnetic atoms (Mn, Fe, and Co) and artificially assembled dimers (Mn2, Fe2, and Co2) on a NiAl(110) surface were probed by scanning tunneling spectroscopy at 17 K. Resonance peaks characteristic of each adsorbed species were observed in the unoccupied density of states. Comparison of the measured spectra with calculations by density functional theory revealed spin splitting in the unoccupied states with s and p characters for the single magnetic adatoms and addimers. The magnitude of the resonance splitting for the adatoms increased with the calculated values of magnetic moments. The resonance structures for the addimers exhibited signatures of their internal magnetic coupling.

A study of amorphous Ti–Ni alloys by X-ray photoelectron spectroscopy

Amorphous thin films of Ti–Ni alloys with 48, 50, and 51.5% atomic nickel concentration have been studied by X-ray photoelectron spectroscopy (XPS). The films were prepared by rf magnetron sputtering technique. The 2p core levels and the X-ray excited Auger regions of titanium and nickel were investigated. The aluminum anode was employed as the source of X-ray excitation. The experimental shifts were found to be positive for both titanium and nickel, indicating that the criterion of electronegativity cannot be used to determine the direction of charge transfer in these films. The Auger parameters (APs) were determined from the binding energies (BEs) of the 2p 3/2 core levels and the most prominent peak in the Auger regions. The Auger parameter data show that the metallic nature to the films comes mainly from titanium. The shift in the Auger parameter is found to be positive for titanium and negative for nickel, suggesting a charge transfer from nickel to titanium. The chemical shifts have been calculated by subtracting the relaxation shifts from the experimental core level shifts. These shifts were observed to be negative for titanium and positive for nickel, establishing the direction of charge transfer from nickel to titanium. This direction of charge transfer was also confirmed by calculating the charges on the ionized atoms using a simple electrostatic model. These values are observed to be positive for titanium and negative for nickel, in complete agreement with the earlier conclusions. The data on the Auger parameter, the chemical shifts, and the charge on the ions show similar trend. The Shirely and Tougaard backgrounds under the core level peaks were also estimated. The difference between these backgrounds when normalized with respect to the elemental values provides information on the density of states at the Fermi level. A decrease in the density of states at the Fermi level was observed both for titanium and nickel in the films. This is in agreement with the observed increase in the unoccupied density of states at the Fermi level studied in the earlier investigations.

Extension of the Constant Photocurrent Method to Determine Densities of Occupied and Unoccupied Localised States

ABSTRACTThis paper examines the use of Constant Photocurrent (CPM) measurements on thin film semiconductors, employing steady (DC) and modulated (AC) sub-gap illumination, to determine the density of localised states (DOS) in the bandgap. AC and DC measurements often result in different apparent absorption spectra. It is demonstrated that it is possible not only to extract information from the respective 'absorption' spectra, on the DOS below the Fermi level - i.e. occupied states, but also on the density of unoccupied states above the Fermi level. The ability to discriminate between these two groups of states by using DC and AC modulated sub-gap light arises from the frequency dependence of the different excitation pathways by which free electrons can be produced. AC modulated excitation will reveal absorption associated with transitions from occupied states into the conduction band, while DC excitation will include transitions from the valence band into unoccupied defect states, followed by slow thermal emission to the conduction band. We examine the temperature dependence of the CPM spectra and present a simple analysis of the DC and AC absorption spectra which allows the two regions of the DOS, above and below the Fermi level, to be determined.

Unoccupied states evolution with oxidation of ultrathin Mg, Zn and Cd layers on SrTiO3(100) surfaces

An unoccupied density of states (DOS) evolution under oxidation of ultrathin Mg, Zn and Cd layers on SrTiO3(100)-(2×2) substrates are investigated. The metal deposition and subsequent oxidation were studied in situ under UHV conditions by means of LEED, AES, TCS and work function measurements. An appearance of metal- and oxygen-derived bands of the unoccupied DOS was traced in the TC spectra. Metal oxidation was followed by the hybridization of metal and oxygen orbitals and by the appearance of the mainly O 2p oxygen-derived bands, which are characterized by a set of DOS maxima, located at the following energies (above the Fermi level): 8.0, 11.5, 14.3, 15.0 and 19.0eV for MgO; 6.0, 8.1, 16.8 and 20.5eV for ZnO and 7.5, 10.0 and 12.7eV for CdO. The results obtained are discussed in comparison with previously published experimental and theoretical data on unoccupied DOS in metal oxides.

Auger electron appearance potential spectroscopy study of CrNx films

AbstractThin films of chromium nitride deposited with varying concentrations of nitrogen on stainless steel substrates have been studied using Auger electron appearance potential spectroscopy (AEAPS). This technique provides information on the density of states in the conduction band. The chromium L2,3 levels have been investigated. The normalized AEAPS chromium spectra in the films have been compared with the elemental chromium spectrum. The signal strength at the Fermi level (EF) is observed to be larger in the films. This represents an increase in the density of states at EF for the films as compared with that of elemental chromium. However, in the films, the density of states is found to decrease with increasing nitrogen concentration. The binding energy of the chromium L3 level for the films exhibits a positive chemical shift with respect to elemental chromium. These spectral data indicate a direction of charge transfer from chromium to nitrogen. In the films, the charge transfer is observed to decrease with increasing nitrogen content. We also present a scheme to estimate the Coulomb correlation energy using the chromium L3 core‐level binding energy data obtained from AEAPS and x‐ray photoelectron spectroscopy (XPS). A decrease in the correlation energy with increasing nitrogen content is observed for the films. We also have estimated the total density of unoccupied states above EF from the experimental data. This total density is observed to be more in the films than in elemental chromium. In the films, the total density of states decreases with nitrogen concentration. The AEAPS spectral results have been correlated with our earlier XPS results on these films. Copyright © 2001 John Wiley & Sons, Ltd.

Orientation dependence of ionization edges in EELS

Anisotropy in the density of unoccupied states can be detected in the fine structure of ionization edges in angle-resolved EELS. It is shown that in a crystal an interference term occurs in the inelastic signal, and how it relates to electron channeling and site selection. The combination of orientation and site selection induces subtle variations in the ELNES. It is shown how this technique can be used to analyze local anisotropy related to the point group of the target atom. A second example shows how to extract non-dipole transitions at small scattering angles.

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

Unoccupied densities of states (DOS) for the ordered and atomicallyclean TiO2(110) and SrTiO3(100) surfaces were studied using the total currentspectroscopy (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 and10.5 eV; Sr 5p/O 2p - 15 eV) maxima of unoccupied DOS. Main oxygen-derived maximaare located at 11.2 and 15.0 eV for TiO2 and at 11.8, 18.0 and 19.6 eV forSrTiO3. The results are discussed in comparison with the previously reporteddata on DOS of these materials and they provide additional detailedinformation on the energy structure of unoccupied DOS on TiO2(110) andSrTiO3(100) surfaces in the 4-25 eV range above the Fermi level.

Resonant elastic x-ray scattering of graphite and diamond at the carbonKthreshold

Resonant elastic x-ray scattering (REXS) was observed at the carbon K threshold of graphite powder, and a small amount of excitonic x-ray scattering was observed in diamond. A comparison of the energy spectra of REXS in graphite with the carbon K x-ray absorption spectra, and with the calculated energy spectra of the unoccupied density of states, suggests that REXS is caused by the transition of the ``participator'' electrons via $p{\ensuremath{\pi}}^{*}$ states in ${\mathrm{sp}}^{2}$-carbon atoms. The probabilities of transition to ``participator'' REXS or to ``spectator'' resonant inelastic x-ray scattering in the resonant radiative-decay process of graphite powder, were determined from REXS spectroscopy to be approximately 47% and 53%, respectively.

XPS core-level shifts and XANES studies of Cu-Pt and Co-Pt alloys

The K and L 2,3 absorption edges and core-level binding-energy shifts for pure Co, Cu and Pt and Co-Pt and Cu-Pt alloys are measured to investigate changes in electronic structures of these alloys. Enhancements and depletion of the white lines in the alloys as compared to pure elements were correlated with changes in the unoccupied density of states upon alloying. We find that the Pt atoms lost d 3/2 electrons and gained d 5/2 electrons when alloyed with Cu and Co. However, the total 5d occupancy changes of the Pt were very little in Cu-Pt alloys, and this is consistent with the small shifts of Pt 4f core-level binding energy. From the quantitative analysis of Cu and Co K-edge absorption spectra, it is found that Cu and Co have significant valence occupation changes due to dehybridization of p-d states. From these results, we suggest that the core-level shifts are affected mainly by the d-valence electron changes in these alloys.

Structural versus Magnetic Properties at the Surface of Fe Films during Oxygen-Assisted Homoepitaxial Growth

The correlation between structural and magnetic properties at the surface of Fe films deposited on $\mathrm{Fe}(001)\ensuremath{-}p(1\ifmmode\times\else\texttimes\fi{}1)\mathrm{O}$ has been investigated in real time during growth by He reflectivity ( ${R}_{\mathrm{He}}$), spin polarized metastable deexcitation spectroscopy (SPMDS), and spin resolved adsorbed curve spectroscopy (ACS). Surface oxygen acts as a surfactant for Fe growth as demonstrated by the well-resolved oscillations of ${R}_{\mathrm{He}}$. The asymmetries measured both by SPMDS and ACS, sensitive to the spin polarization of occupied and unoccupied density of states, respectively, oscillate in phase with ${R}_{\mathrm{He}}$. This demonstrates a strict correlation between morphology and the intensity of magnetization at surface.

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.

First-principles calculations of electron-energy-loss near-edge structure and near-edge x-ray-absorption fine structure of BN polytypes using model clusters

Effects of a core hole in the unoccupied density of states are systematically studied using model clusters of $h\ensuremath{-}\mathrm{BN},$ $c\ensuremath{-}\mathrm{BN},$ and $w\ensuremath{-}\mathrm{BN},$ in order to reproduce and interpret experimental electron-energy-loss near-edge structure /near-edge x-ray absorption fine structure at both B- and N-K edges. Wave functions are found to localize significantly near the core hole, thereby changing their energies as well as spatial distribution. They are very different from the Bloch states assumed in a band-structure calculation on the basis of a structural unit cell. When the presence of the core hole is ignored, in other words at the ground state, small clusters such as $h\ensuremath{-}({\mathrm{B}}_{7}{\mathrm{N}}_{12})$ exhibit better agreement with the experiment as compared with large clusters such as $h\ensuremath{-}({\mathrm{B}}_{91}{\mathrm{N}}_{108})$ because wave functions are made to localize in the small clusters. Features appeared in the experimental spectra are interpreted in terms of chemical bondings among atomic orbitals using overlap population diagrams. Absolute transition energies by Slater's transition state method agree with experimental values within an error of 1%.

Core hole effect in NEXAFS spectroscopy of polycyclic aromatic hydrocarbons: Benzene, chrysene, perylene, and coronene

Carbon K-edge NEXAFS spectra of chrysene, perylene, and coronene were measured using synchrotron radiation, and the results were analyzed by ab initio molecular orbital (MO) calculations. The spectra do not agree well with the calculated density of unoccupied states (DOUS), indicating significant core-hole effect leading to deviation of the NEXAFS spectra from the DOUS. On the other hand, the observed spectra were well simulated by theoretical calculations taking this effect into account by the improved virtual orbital method. This allowed a detailed analysis of the core-hole effect, which affects both transition energy and intensity. During the course of this analysis, the core-hole effect in benzene was also analyzed. It was found that the magnitude of the core-hole effect is strongly dependent on the combination of the excited site and final vacant orbital. This dependence could be semiquantitatively explained in terms of the combination of the excitation site and the orbital patterns of the final state. The severe deviation of the NEXAFS spectra from the DOUS is ascribed mainly to such variation of the core-hole effect, with additional effect by the site-dependent core ionization energy. In some cases, even the transitions to the same vacant orbital contribute to different spectral features, or an apparently single spectral feature consists of transitions to different vacant orbitals. Thus direct information about DOUS of a molecule with such inequivalent sites is not obtainable from the NEXAFS spectrum, although smaller deviation is suggested for other systems with single site, such as the C atoms in C60 and graphite, and the N atoms in Zn tetraphenyl porphyrin.