XPS study of bonding in ligated Au clusters

Ligated metal cluster compounds containing a core of metal atoms with well defined structure surrounded by a variety of organic and inorganic ligands are closely related to the bare metal clusters that are only now becoming available in uniform cluster size. The evolution of band structure and the development of metallic properties as a function of cluster size are of considerable interest. We report here a comparison of these two types of systems based on a study by X-ray photoelectron spectroscopy. The valence band spectra of ligated Au clusters compounds are similar in many respects to those of bare clusters, indicating significant participation of the d electrons in bonding. The core electron binding energy shifts of the central Au atom in Au₁₁(PPh₃)₇Cl₃ corresponds to the loss of approximation one 6 s electron. The total charge transferred to the halogens is accounted for by the shifts of the central and three halogenbonded Au atoms. No indication of metallic behaviour is found in the core of the ligated clusters.     

Nb 3d and O 1s core levels and chemical bonding in niobates

A set of available experimental data on binding energies of Nb 3d_5/2 and O 1s core levels in niobates has been observed with using energy difference (O 1sNb 3d_5/2) as a robust parameter for compound characterization. An empirical relationship between (O 1sNb 3d_5/2) values measured with XPS for Nb⁵⁺-niobates and mean chemical bond length L(NbO) has been discussed. A range of (O 1sNb 3d_5/2) values possible in Nb⁵⁺-niobates has been defined. An energy gap ∼1.4–1.8 eV is found between (O 1sNb 3d_5/2) values reasonable for Nb⁵⁺ and Nb⁴⁺ states in niobates.     

The role of XPS in the study and understanding of rubber-to-metal bonding

XPS has been used to study the mechanism of the adhesion of rubber to well-characterized metal surfaces. By means of a special sample preparation technique the rubber-to-brass interface could be analyzed by X-ray photoelectron spectroscopy following vulcanization of rubber against a CuZn alloy surface. Combination of these XPS results with quantitative data on the adhesion levels of brass has led to the development of a new adhesion model. Reaction of brass with rubber results in the formation of both Cu_xS and ZnS. The amount of Cu_xS formed must be carefully controlled. On the one side, it improves the adhesion as a result of a catalytic effect on the rubber vulcanization. On the other hand, excessive Cu_xS formation leads to embrittlement of the interfacial Cu_xS/ZnS film and a loss of adhesion.     

Auger and direct electron spectra in X-ray photoelectron studies of zinc,zinc oxide,gallium and gallium oxide

Auger and direct electron spectra from Zn, ZnO, Ga and Ga₂O₃ have been studied with X-ray photoelectron spectroscopy (ESCA). The chemical shift between zinc electron binding energies in Zn and ZnO is very small, whereas the zinc Auger electron signals are separated by 4.3 eV. In gallium, the oxide and metal signals are separated by 1.9 eV, but the Auger electron energy shift is three times as large. Thus the Auger signals are more sensitive to the chemical environment than the direct electron signals, which is the same relation as earlier observed for copper and copper oxides.     

Auger and XPS study of band structure of zinc phosphide

The valence band region XPS spectrum of Zn₃P₂ has been compared to loss-deconvoluted LVV Auger lineshapes and the band structure calculation of Lin-Chung [Phys. Status Solidi (b) 47 (1971) 33]. Fair agreement has been found from these comparisons.     

Melting of copper and nickel at high pressure: the role of d electrons

Melting curves of Cu and Ni were measured in the laser-heated diamond cell to 97 GPa (3800 K) and 60 GPa (2970 K), respectively. The temperatures of Cu are in good agreement with recent theoretical calculations. The Cu melting slope (dT/dP) is about 2.5 times steeper than for Ni. The present results confirm the key role d-shell electrons play in determining the temperature dependence of high pressure melting curves in transition metals that have filled or partially filled d electron bands.     

Interaction between the magnetic moments of the 3d and the 4f electrons in manganite, probed by Ga substitution

The substitution of Ga for Mn in manganite Nd_0.6Dy_0.1Sr_0.3MnO₃ with a ferromagnetic (FM) ground state has been performed to study the influence of the Mn-sublattice magnetic ordering on the magnetic rare-earth sublattice. It is found that the substitution of Mn³⁺ with Ga³⁺ ions results in a sharp decrease of T_C, reflecting the reduction of the double-exchange interactions strength J_Mn-Mn. At the same time, a depinning effect of the rare-earth magnetic moment has been observed. This behavior unambiguously proves that the exchange interaction between Mn and rare-earth ions J_Mn-R strongly influences the rare-earth magnetic ordering at temperatures below T_C and stabilizes the rare-earth magnetic ground state.     

First-principles study of electronic structure, chemical bonding, and optical properties of cubic SrHfO3

Electronic structure and optical properties of SrHfO₃ are calculated using the full potential linearized augmented plane wave plus local orbitals method. The calculated equilibrium lattice is in reasonable agreement with the experimental data. From the density of states (DOS) as well as charge density studies, we find that the bonding between Sr and HfO₃ is mainly ionic and that HfO₃ entities bond covalently. The complex dielectric functions are calculated, which are in good agreement with the available experimental results. The effect of the spin-orbit coupling on the optical properties is also investigated and found to be quite small.     

Anomalous dispersion effects,thermal vibrations and bonding charges in gallium arsenide

Accurate X-ray diffraction intensity data have been obtained using a spherical single crystal of GaAs at ambient temperature, 298 °K. Least squares refinement of this data yielded the Debye-Waller factors,B _Ga=0.68±0.05 Ų andB _As=0.42±0.03 Ų. The residual index for the discrepancy between measured and calculated structure factors in the present work is quite small, beingR=0.0068. Anharmonic thermal vibration was sought for and found to be negligible. The inclusion of dispersion corrections in the structure factor expressions gives rise to Bijvoet inequality between inverse reflections with the indices all odd. The present work reveals excellent agreement, both in sign and magnitude, of the Bijvoet inequalities for a large number of such reflections. Covalent charge transfer has been deduced from the experimental measurements on a few quasi-forbidden Bragg reflections.     

PLA-PMMA blends: A study by XPS and ToF-SIMS

This paper reports which are the possibilities of quantification by time of flight secondary ion mass spectrometry (ToF-SIMS) for some polymer blends. In order to assess the composition of the mixtures, we studied first different poly(l-lactide)/polymethylmethacrylate (PLA/PMMA) blends by X-ray photoelectron spectroscopy (XPS), this technique being quantitative. By XPS fitting of the C 1s level, we found a very good agreement of the measured concentrations with the initial compositions. Concerning ToF-SIMS data treatment, we used principal component analysis (PCA) on negative spectra allowing to discriminate one polymer from the other one. By partial least square regression (PLS), we found also a good agreement between the ToF-SIMS predicted and initial compositions. This shows that ToF-SIMS, in a similar way to XPS, can lead to quantitative results. In addition, the observed agreement between XPS (60-100 Å depth analyzed) and ToF-SIMS (10 Å depth analyzed) measurements show that there is no segregation of one of the two polymers onto the surface.     

Investigation of d-orbital participation in bonding by positron angular correlation

Whether or not ligand perturbation is necessary for d-orbital contraction and consequent participation in bonding has been controversial till now. A study of angular correlation of positron annihilation radiation on organic sulphides and sulphones gives a direct evidence for the conclusion that ligand perturbation is necessary.     

How itinerant are 3d electrons in transition metals? A study of magnetic circular dichroism in 2p photoemission

We show how the spectral structure of the magnetic circular dichroism (MCD) in photoemission (PE) can be analysed using a sum rule for the sublevel orbital moment. For itinerant 3d metals, such as Co, Fe and Ni, the MCD in the 2p PE does not agree with a single-particle model. On the other hand, a final-state impurity model calculation gives an excellent agreement with the experimental results for the Ni 2p PE of nickel metal. This indicates that inter-configurational mixing must be taken into account to understand the detailed MCD structure in metallic systems. Vice versa, MCD-PE can help to explain the complicated behavior of electrons in correlated materials. Received: 23 May 2001 / Accepted: 4 July 2001 / Published online: 5 October 2001     

EM, XPS and LEED study of deposition of Ag on hydrogenated Si substrate prepared by wet chemical treatments

The deposition of Ag on a hydrogenated Si(111) substrate, prepared by wet chemical treatment, was carried out at room temperature (RT), 250°C and above 350°C. The samples were examined in situ by LEED, XPS and then by SEM and TEM. Our results show that flat Ag crystallite domains are growing on the H/Si(111) surface at all three substrate temperatures. The h/w ratio of the islands increases but the density decreases with increasing substrate temperature which is attributed to the differences between hydrogenated and clean Si(111) surface. The so-called A-type and B-type domains resulting from analyses of TOF ICISS are attributed to twinning in the deposited layer.     

Photoionization of electrons and holes at oxygen donors in gallium phosphide

The spectral dependence of the photoionization cross-section for the oxygen donor in GaP is studied in the temperature range between 90 and 410°K by steady state photocurrent measurements using two different radiation sources. From a fit of Lucovsky's model to the experimental results, accurate information is obtained about the energy position of the oxygen donor at all temperatures. It is found that the oxygen level is pinned to the valence band for temperatures above 295 K and that at these temperatures the Franck-Condon shift is very small. This may indicate that there is little interaction between the impurity level and the surrounding lattice.     

XPS and XAES study of the interaction of palladium and copper overlayers with fullerene films

X-ray photoelectron and Auger spectroscopies were used to examine surface bonding and overlayer growth during palladium and copper deposition onto films of fullerene, C₆₀ The results were consistent with metal cluster formation on C₆₀. The observed positive core electron binding energy shifts in small metallic clusters supported on C₆₀ were shown to originate in metal-fullerene interaction accompanied by charge transfer. Palladium-fullerene intermixing at temperatures as low as ss 50°C was observed for small Pd coverages.     

Complex nature of the copper acceptor in gallium arsenide

The infrared excitation spectrum of the hole bound to the 0.15 eV Cu-acceptor in GaAs has been measured. A novel fine structure has been resolved which is explained by zero-field splitting. It shows that this acceptor has a symmetry lower than T_d of the hoset lattice and cannot be described by a simple substitutional Cu^- -ion on a Ga site, as heretofore assumed.