Au 5d Research Articles

Ab initio determination of the valence electron distribution in the average structure of the incommensurately modulated calaverite AuTe2.

The valence-electron density distribution of the average structure of incommensurately modulated calaverite, AuTe2, has been computed using density-functional theory. High-density regions, centered around the Au and Te atoms, are not spheric, but present charge concentrations along the Au-Te and Te-Te bonds. The electronic band structure and its corresponding density of states reveal the presence of three electronic band groups, constituted mainly by Te 5s, Au 5d and hybrids of Te 6p + Au 6s + Au 5d orbitals. The electrons belonging to the last block are responsible for the chemical bonds.

Elastic electron backscattering from overlayer/substrate systems

AbstractElastic electron backscattering probability for Au overlayers deposited on Ni were determined experimentally in the energy range 200–1000 eV. These measurements were accompanied by characterization of the overlayer structure and the mean overlayer thickness. The elastic backscattering probabilities also were derived from extensive Monte Carlo simulations of the electron transport in overlayer/substrate systems. Two overlayer structures were considered in these calculations: a uniform Au layer, and isolated Au islands located on the uncovered substrate. Analysis of the shape of Au 4d spectra with QUASES‐Tougaard software indicated that the latter structure is more probable. It has been found that the experimental elastic backscattering probabilities compare better with the theoretical probabilities calculated for the island structure. The possibility of characterization of the overlayer thickness and structure from the elastic backscattering probabilities has been discussed. Copyright © 2001 John Wiley & Sons, Ltd.

Reactivity of Au with ultrathin Si layers: A photoemission study

We present a photoemission study on reactivity of the Au–Si system. We studied gold films evaporated atop ultrathin silicon layers previously deposited on GaAs. Following analysis of both the Si 2p core level and the Au 5d valence band spectra related response, we show that the reaction mechanism between Au and Si is affected by structural imperfections of the silicon layer. This is in sharp opposition to some current models of the reactivity mechanism.

Experimental and theoretical study of the electronic structure of AuAl 2

Synchrotron-radiation-excited angle-resolved photoemission spectra for the (100) and (111) faces of AuAl 2 are presented, along with derived experimental dispersion relations. These bands are compared with those obtained from the theoretical band-structure calculation. The density-of-state curves, band hybridization, and charge transfer for AuAl 2 are also calculated. The energy variation of the Au 5d bands for AuAl 2 shows atomic-like behavior in the photon-energy range of 80–200 eV.

Photoelectron spectroscopy study of surface alloying in the Au/Ni (s) 5(0 0 1) × (1 1 1) system

The growth of vapor-deposited Au on the stepped [ 5(0 0 1)×(1 1 1) ] nickel surface at room temperature was studied by X-ray photoelectron and Auger electron spectroscopy (XPS/XAES) and ultraviolet photoelectron spectroscopy (UPS) for Au coverages ranging from about 0.2 up to 13 Ml. The Au 4f 7/2 binding energy is initially lower than that of metallic gold and reaches its bulk value above 4 Ml. Helium II UPS exhibits an initial Au 5d doublet splitting of 1.5 eV, which increases up to the bulk value of 2.7 eV after 13 Ml of gold deposition, as well as an abnormally strong attenuation of the Ni 3d state up to a Au coverage of about 0.5 Ml. The results suggest charge transfer from Ni to Au and AuNi surface alloying in the submonolayer regime.

Valence band studies of the RTX (R=Pr, Nd; T=Cu, Ag, Au; X=Ge, Sn) compounds

The valence band in the RTX (R=Pr, Nd; T=Cu, Ag, Au; X=Ge, Sn) compounds was studied by the X-ray photoemission spectroscopy. The XPS data were compared with the ones calculated using the spin-polarized tight-binding linear muffin-tin orbital (TB LMTO) method. The obtained results show that the valence bands are determined mainly by the Cu 3d, Ag 4d and Au 5d bands. The calculation of the total energy for two models of the crystal structure: an ordered of the LiGaGe-type and a disordered one of the CaIn 2-type indicates that in the RTSn (T=Cu, Ag ,Au) compounds the LiGaGe-type structure is stable. Calculated value of the Pr magnetic moment is smaller than that of the free Pr 3+ ion value, whereas the Nd magnetic moment is equal to the free Nd 3+ ion value.

Spectromicroscopy beamline at ELETTRA: Performances achieved at the end of commissioning

In the course of 1998, the Spectromicroscopy beamline at ELETTRA completed commissioning and succeeded in performing its first test experiments. The beamline is designed to perform photoemission experiments with high spatial resolution, which is obtained by focusing the radiation in a small spot on the sample by means of a multilayer-coated Schwarzschild Objective. Three objectives are currently available; these operate at photon energies of 74, 95, and 110 eV. A review is presented of the performances achieved together with an outlook on the future upgrades of the microscope. The smallest achievable spot size is currently 0.5 μm. At present, the limit to the spatial resolution is due to aberrations caused by figure errors of the objective. Typical counting rates in photoemission spectra, for example, on the Au 5d peak, are of the order of 104–105 counts per second with an energy resolution of the order of 100–200 meV. Among the first experiments in which p- and n-type GaAs layers of 0.25 μm thickness were imaged.

A study of gold ultrathin film growth on yttria-stabilized ZrO 2(100)

The initial stages, up to five equivalent monolayers, of Au growth on yttria-stabilized ZrO 2 (100) at 680 K were studied by X-ray photoelectron and X-ray induced Auger electron spectroscopy (XPS/XAES). The XPS results indicate that Au grows as three-dimensional clusters on the substrate. The coverage-dependent average coordination number for Au atoms is estimated from the observed Au 5d 5/2–5d 3/2 splitting. Analysis of the XPS and XAES results indicate that Au does not interact substantially with the substrate. The initial state contribution to the binding energy shift, as obtained by an Auger parameter analysis, and its relation with Au cluster size is discussed.

First-principles calculations of the magneto-optical Kerr effect in L1 0-type ordered alloys TM–X [formula omitted

The magneto-optical polar Kerr effect of L1 0(CuAu)-type ordered alloys TM–X ( TM= Mn, Fe, Co ; X= Pt, Au ) was calculated from the electronic structures given by the first-principles LMTO-ASA method. The calculated results agreed reasonably well with the experimental results. The important optical transitions were identified by detailed analysis of the electronic structures and transition matrix elements. As an example, it was found that the peak structure at 4 eV in the Kerr spectra of L1 0-FeAu mainly originated from the d ↓±2→f ↓±3 transitions on Au, in which the strength of the d ↓−2→f ↓−3 transition was much larger than the d ↓+2→f ↓+3 transition due to the large spin-orbit splitting in Au(5d ↓±2)-band. In this transition, the final state Au(5f ↓±3) is a small tail state hybridized with the unoccupied Fe(3d ↓) state. Thus, the transition can simply be described as the Au(5d ↓)→Fe(3d ↓) transition as predicted by Sato et al. [J. Magn. Soc. Japan 20 (S1) (1996) 35]. Another type of transition p ↓±1→d ↓±2 on Pt was found to be very strong and plays an important role in the Kerr spectra of TM–Pt, since there are many final states (Pt(5d ↓)) in the transition due to the spin polarization in Pt(5d)-bands.

Quantification of Au deposited on Ni: XPS peak shape analysis compared to RBS

We investigated the degree of quantitative agreement between XPS peak shape analysis and RBS with respect to the average thickness of Au layers deposited on Ni. Gold layers in the range of 5–100 Å were prepared on five substrate samples. By peak shape analysis of the Au 4d XPS peaks, the structure and amount of Au was determined. Likewise, by measuring with RBS on 3–5 regions of each sample the Au thickness in these regions was determined. Comparing the results, we determined the deviation between XPS and the mean value of the RBS thickness. For three of the samples the deviations were 6–7%. For the remaining two samples the agreement was worse. However, as shown by the scatter in the RBS results for each sample, the disagreement was due to a non-uniform Au layer and not to the techniques. It is found that the absolute accuracy of the XPS peak shape analysis technique is ∽7% for depths of ⩽7–8 IMFP. © 1999 John Wiley & Sons, Ltd.

Characterization of Silica-Supported Pd−Au Clusters by X-ray Absorption Spectroscopy

Silica-supported Pd−Au clusters were characterized by Pd K and Au L3 extended X-ray absorption fine structure (EXAFS) spectroscopy and Au L2,3 X-ray absorption near-edge structure (XANES) spectroscopy. Pd−Au/SiO2 catalysts with Pd/Au atomic ratios of approximately 1:1 were prepared by coadsorption of [Pd(NH3)4][NO3]2 and [Au(en)2]Cl3 (en = ethylenediamine) onto silica gel from pH 7 and pH 10 slurries. EXAFS spectroscopy of the Pd−Au/SiO2 (1.3:1, pH 7) catalyst after in situ reduction at 300 °C indicates the formation of bimetallic clusters exhibiting Pd surface enrichment. The EXAFS results can be approximated using a cluster “decoration” model in which a Au-rich core is covered by a partial monolayer of Pd. In contrast, EXAFS spectroscopy of the Pd−Au/SiO2 (1:1, pH 10) catalyst after in situ reduction at 350 °C indicates the formation of homogeneous Pd−Au alloy clusters. The homonuclear bond distances in these clusters agree closely with the Vegard's Law prediction for a 1:1 Pd−Au alloy; however, the heteronuclear bond distances are ∼0.02 Å shorter. Au L2,3 XANES spectroscopy of the supported Pd−Au clusters evidences a vanishingly small density of unoccupied Au 5d states. Excellent correspondence is observed between the XANES spectra of supported Pd−Au clusters and those of Pd−Au alloy films having similar compositions.

PHOTOEMISSION STUDY OF THE ELECTRONIC STRUCTURES OF AuAl 2 AND AuIn 2

Synchroton-radiation-excited angle-integrated photoemission spectra of AuAl 2 and AuIn 2 are presented. The spacings of the three Au 5d bands of AuIn 2 were determined and were interpreted to yield the crystal-field and the spin–orbit parameters of 0.95±0.05 and 0.52±0.05 eV, respectively. The binding energies at the Γ points for AuAl 2 and AuIn 2 agree with those obtained from the band structure calculation and the angle-resolved photoemission spectroscopic study to within 8%. The Au 5d charges deplete more for AuAl 2 than for AuIn 2.

Electronic structure of Au–Ta alloys:: An X-ray spectroscopy study

Au–Ta alloys with the compositions of AuTa, AuTa 2 and AuTa 3, prepared by quenching from the melt, has been studied with X-ray diffraction, photoelectron spectroscopy (XPS) and X-ray absorption near-edge structure (XANES) measurements. It was found that while AuTa 3 is a single-phase solid solution, AuTa 2 and AuTa have mixed phases and that the Au and Ta 4f levels of the alloys shift to higher binding energy, relative to the pure metal; this is accompanied by a narrowing of the Au 5d component of the alloy d-band, which moves away from the Fermi level. This observation is interpreted in terms of a charge compensation model in which Au loses d charge but is overcompensated by s–p charge gain, resulting in a small net charge flow from Ta to Au. The observed Ta 4f binding energy shift is as predicted by electronegativity and indicates charge depletion at the Ta site. The notion of d charge depletion at both Au and Ta sites upon alloying is confirmed independently by XANES measurements which showed that the L 2,3 edge whiteline intensity for both Au and Ta increases as they become more dilute in the host, indicating an increase in d hole count. The experimental results compare favorably with a recent linear-augmented Slater-type-orbital (LASTO) calculations. The implications of these results are discussed.

Ｌ１０型規則合金の磁気光学Ｋｅｒｒ効果の第一原理計算

The magneto-optical polar Kerr effect of L10-type ordered alloys TM-X (TM = Mn, Fe, Co, X = Pt, Au) was calculated from the electronic structures given by the first-principles LMTO-ASA method. The calculated results agreed reasonably well with the experimental results. The important optical transitions were clarified by detailed analysis of the electronic structures. For example, it was found that the peak structure at about 4 eV in the Kerr spectra of L10-FeAu mainly originated from the d↓→f↓ transition on Au. In this transition, the final state Au (f↓) is a small tail state hybridized with the unoccupied Fe (3d↓) state. Thus, the transition can simply be expressed as the Au (5d↓)→Fe (3d↓) transition.

Quantitative XPS: Influence of Elastic Electron Scattering in Quantification by Peak Shape Analysis

A new method for non-destructive quantitative analysis of surface nanostructures was developed in recent years. It relies on analysis of the XPS peak shape and usually the effects of elastic electron scattering are neglected in practical applications of the method. In the present paper we study the influence of elastic electron scattering for interpretation of the analysed experimental data. Spectra of the Au 4d peak [with inelastic mean free path (IMFP)=16 A] from a thin layer of Au atoms situated at varying depths in a Ni solid were analysed. When elastic scattering is neglected, the accuracy of the quantification is ∽10% for depths smaller than 1.5 IMFP but for a depth of 2.5 IMFP the determined quantitative amount of gold is 25% too low. The effect of elastic scattering on the peak intensity was <10% for marker depths up to 1 IMFP but it grows to ∽40% at 2.5 IMFP. After correction for elastic scattering effects, the accuracy of the analysis is improved and the root-mean-square scatter in the determined amount of Au around the mean value is reduced from 15.4% when elastic scattering effects are neglected to 11% when they are included in the analysis. For the present system, it is then concluded that the effect of elastic scattering is quite small for marker depths smaller than ∽1 IMFP but for larger depths the effect is substantial. © 1997 John Wiley & Sons, Ltd.

Time-resolved valence band photoelectron spectroscopy of liquid AuSn

The solid - liquid - solid-phase transitions of a alloy have been investigated by means of the recently developed method of time-resolved photoelectron spectroscopy, using dye laser pulses to melt the sample surface on a microsecond scale. Starting at a temperature of the liquid sample just above the melting point (740 K), the laser pulse heating allowed us to obtain, for the first time, valence band spectra of an alloy at temperatures of up to 1620 K, distinctly above the vapour pressure limit of steady-state photoelectron spectroscopy. The photoemission results of the liquid at different temperatures are compared with data from the corresponding amorphous alloy, prepared by vapour condensation at liquid-nitrogen temperature. By increasing the temperature of the disordered phase, the Au 5d band is found to shift continuously toward a lower binding energy, showing a linear behaviour over the whole temperature range. Our measurements confirm earlier results obtained with conventional photoemission within a restricted temperature range. The results presented here clearly show the potential of the new technique to investigate the electronic structure of alloys at conditions under which standard photoemission experiments cannot be performed.

Comparison of Self-Assembled Monolayers ofn-Alkanethiols and Phenylthioureas on the Surface of Gold

Long-chain alkanethiols [R(CH 2 ) n SH] and substituted phenylthioureas (RPhNHCSNH 2 ) adsorb from solutions on the surface of gold. Analysis by XPS indicates that the binding energies of the S 2p and Au 4d 5 peaks in alkanethiol monolayers are 162.3 and 335.4 eV, which are 1.4 and 0.4 eV higher than those in phenyl-thiourea monolayers, respectively ; the widths (FWHM) of the S 2p XPS peaks in alkanethiols are less than those in phenylthioureas. The alkyl chain in alkanethiol monolayers on gold is canted 30° from the normal to the surface, while the longitudinal dimension of the phenylthiourea molecule is perpendicular to the surface. It is suggested that the covalent bond between sulphur and gold dominates the conversion of alkanethiols to gold alkane-thiolates, whereas the σ-π delocalized coordinate bond between sulphur and gold controls the adsorption of phenylthioureas on the surface of gold because of the existence of the conjugated π 4 6 bond in thioureas.

Ru-promoted alloying of Au and Cu ultrathin films: photoemission studies

For the co-adsorption of Au and Cu on Ru(001), the Ru substrate promotes the alloying of the noble metals by lowering the activation energy required for the process. High-resolution photoemission spectroscopy with synchrotron radiation and X-ray photoelectron spectroscopy have been used to investigate the core-level binding energy shifts and the valence-band behaviour in a series of Au and Cu overlayers on Ru(001): 1 ML Au, 1 ML Cu, 1 ML Au and 1 ML Cu, 3 ML Au and 1 ML Cu, as well as 1 ML Au and 3 ML Cu. A monolayer of Au in contact with Ru(001) has a Au 4 f 7 2 binding energy ∼0.23 eV higher than that of Au surface atoms on pure Au but still ∼0.1 eV smaller than that of the bulk Au. In contrast a monolayer of Cu bonded to Ru(001) exhibits a Cu 2 p 3 2 binding energy ∼0.1 eV lower than that of a Cu surface atom on pure Cu. Most of the co-deposited AuCu systems formed ultrathin alloys on top of the Ru(001) surface. The strain and electronic perturbations induced by the Ru substrate on the Au and Cu overlayers favour the formation of AuCu alloys. Binding energy shifts of the Au 5d band components and the accompanying change to the overall d-band width were found to be very sensitive to the alloying process, as has been observed in bulk AuCu alloys. The order of deposition, annealing, and the temperature of the substrate were all found to have an effect on the formation of AuCu ultrathin alloys. These ultrathin alloys exhibit Au 4 f 7 2 and Cu 2 p 3 2 binding energies that are significantly different from those of bulk AuCu alloys as a consequence of the interactions with the Ru(001) substrate.

Role of Surface Effects in the Inelastic Background of X-ray Photoelectron Spectroscopy

We have derived a new deconvolution formula to obtain the original no-loss XPS spectra, the so-called source function, by taking surface effects into account. With this formula the primary XPS spectra of Au 4d and Au 4f are carried out from the experimental data. The primary excitation spectra are compared to the results derived by Tougaard's method in which surface effects were neglected. The present result is markedly different from Tougaard's result, which consists of a tail extending ∼50 eV below the peak. The result reveals that the influence of surface effects on the background removal of the spectra is considerably significant for the energy range ∼50 eV below the peak energy. It is also found that the large tail in Tougaard's results can be essentially removed when surface effects are considered.

Electronic structure of dilute alloys

The electronic structure of dilute alloys has been studied by synchrotron-radiation-induced photoemission and bremsstrahlung isochromat spectroscopy. The use of photon energies at the Au 5d Cooper minimum and the Ni 2p - 3d absorption threshold strongly enhances the spectral distribution of the Ni 3d impurity states with respect to the Au states. It displays a Lorentzian-shaped quasiparticle peak near the Fermi energy and a final-state satellite. In accordance with the Friedel - Anderson model the former is interpreted as due to a single spin-degenerate virtual bound state. The satellite reflects strong electronic correlations in the 3d shell and displays a giant resonance at the Ni 2p edge, allowing an identification of the various multiplet terms. The effective Coulomb integral of the 3d electrons is estimated from the experimental data, showing a significant reduction from that of Ni-rich alloys. The application of Anderson's criterion for the existence of a local magnetic moment yields the result that the Ni impurities are non-magnetic, in agreement with the Pauli paramagnetism observed in dilute .