X-ray photoelectron study of some silicon-oxygen compounds

More than fifty specimens of silicon-oxygen compounds have been studied by X-ray photoelectron spectroscopy. Due to the insulating properties of these specimens the measured binding energies of the various atomic levels were corrected using the C (1s) line due to hydrocarbon contamination as a reference. A statistical treatment of the experimental results has been used to verify the validity of the reference and to discuss the differences in binding energy of the Si (2p) line in various samples. Three different steps of oxidation have been found for silicon; the chemical shifts with respect to unoxidised silicon are 3.2, 4.1 and 5.0 eV for surface oxidised silicon, silicates and quartz, respectively. The influence of atoms other than oxygen in the neighbourhood of silicon, such as fluorine or alkaline cations is discussed.     

Correlation of inner shell binding energies with shifts of charge transfer bands

Inner shell binding energies were determined for the elements in the crystalline compounds A^IIIX^VO₄ where A  B, Al, Ga, Fe and X  P, As with fourfold coordination of A, and in the corresponding dihydrates of Al having a sixfold coordination of Al. The O 1s binding energies for the dihydrates could be resolved into two bands arising from XO₄ -groups and from the water molecules respectively. The O 1s binding energies from the XO₄ groups decrease by about 0.6 eV in going from the anhydrous compounds to their dihydrates whereas the Al 2p binding energies remain constant within the limits of error. These results correlate surprisingly well with shifts of the first charge transfer band of trivalent iron in these compounds. The binding energy differences between phosphates and arsenates are also in good agreement with concurrent shifts of this charge transfer band. The consequences of these results for the concept of optical electronegativities are discussed.     

Molecular spectroscopy by means of ESCA: V. Boron compounds

The B 1s binding energies of 23 compounds, including boron hydrides and carboranes have been measured in the vapor state. Correlations with Pauling charges and with shifts, calculated from the potential model using CNDO charges, are presented. The B 1s binding energies are subsequently discussed in terms of the chemical reactivity of the compounds. The iodomethanes have also been studied, and an electronegativity value for iodine suitable for ESCA correlations is given.     

Heat of mixing in HfCxN1−x compounds from XPS core level binding energy shifts

The XPS core level binding energies for the Hf4f_{$\text{7}\text{2}$}, Cls and Nls level in several nearly stoichiometric HfC_xN_1?x compounds are reported. Using the thermochemical model to calculate core level binding energy shifts in metals the heat of mixing as function of the HfC/HfN ratio is calculated from the position of the Cls binding energies. In addition it is shown that the Hf4f_{$\text{7}\text{2}$} and Nls binding energies can be used to obtain further thermochemical data for these compounds.     

Excitons in silicon nanocrystallites

Recombination and binding energies of excitons in nanocrystalline silicon quantum dots are calculated within the effective mass approximation including the effects of the induced electrostatic polarization. The calculated exciton recombination energies compare well with other calculations and with the results from photoluminescence measurements in porous silicon. The calculated exciton binding energies are far larger than the bulk exciton binding energy and show substantial dependence on the matrix that surrounds the nanocrystallites. A model is proposed that explains the main orange-red, blue and infrared luminescence peaks of porous silicon within a simple unified framework.     

Core-level binding-energy shifts at surfaces and in solids

This review presents an overview of the theory and of various successful approaches to the interpretation of core-level binding-energy shifts observed in photoelectron spectroscopy. The review concentrates specifically on shifts since most of the chemical and physical insights provided by core levels are derived not from the core-level binding energies themselves but from shifts they exhibit. The theoretical background is presented at a level readily accessible to the general reader. Particular attention is paid to relative merits of the two basically different conceptual frameworks for interpreting core-level binding-energy shifts, the initial-state-final-state approach and the equivalent-core approach.     

Determination of chemical shifts of core electron binding energies for some zinc compounds and the applicability of electron spectroscopy to environmental samples

Core electron binding energies were determined for zinc borate, zinc carbonate, zinc chloride, zinc fluoride, zinc iodide, zinc phosphate, zinc sulfate and zinc titanate. These binding energies were measured relative to that of the C 1s line (284.3 eV) and the Au 4f_7/2 line (83.9 eV). Also the shifts in electron energy were determined by measuring the electron spectra when two of the above compounds were mixed together. The former method was found to be unsatisfactory for environmental samples while the method of mixture was found to be satisfactory. The chemical shifts were compared to the electronegativities involved showing a linear relationship. The electronegativity of titanate was determined to be 2.7 ± 0.1.     

Accurate core binding energies of ions from Dirac-Fock calculations combined with experimental atomic binding energies

Core binding energies are reported for the singly-charged alkali ions obtained by removing the outer s electron and for the singly-charged halide ions obtained by adding a P_{$\text{3}\text{2}$} electron. The levels studied are Li 1s, Na 1s, K 2p, Rb 3p, Cs 3d, F 1s, Cl 2p, Br 3p and I 3d. The binding energies were obtained by combining DiracF?ock ΔSCF binding-energy shifts between the ions and the corresponding isoelectronic rare gases with experimental rare-gas binding energies, and also by combining the calculated atomīon shifts with experimental atomic binding energies where available. It is shown that the former approach corrects accurately for the correlation energy, which is not included in single-configuration calculations.     

Chemical shifts in ESCA and NMR

A detailed examination of the relationship between chemical shifts in ESCA and NMR is presented. It is demonstrated that even in series of closely related compounds a linear correlation between ESCA and NMR shifts cannot be expected. Included is a discussion of reorganization effects on electron binding energies. The close relation between chemical shifts of ESCA and NMR and the spin—rotation constants of molecules is pointed out.     

On the binding energies of excitons in polar quantum well structures in a weak electric field

The binding energies of excitons in quantum well structures subjected to an applied uniform electric field by taking into account the exciton longitudinal optical phonon interaction is calculated. The binding energies and corresponding Stark shifts for Ⅲ-Ⅴ and Ⅱ-Ⅵ compound semiconductor quantum well structures have been numerically computed. The results for GaAs/A1GaAs and ZnCdSe/ZnSe quantum wells are given and discussed. Theoretical results show that the exciton-phonon coupling reduces both the exciton binding energies and the Stark shifts by screening the Coulomb interaction. This effect is observable experimentally and cannot be neglected.     

The prediction of core-level binding-energy shifts from CNDO molecular orbitals

A theory is described for calculating core-level binding-energy shifts with potential models that employ “intermediate-level” molecular-orbital wave functions. The relaxation-energy term in atomic core-level binding energies is considered first. The ground-state potential model (GPM) and relaxation-potential model (RPM) are developed for calculating core-level binding energy shifts in molecules from CNDO wavefunctions. It is shown that neglect of certain two- and three-center integrals in these models limits their accuracy when unlike molecules are compared. The models are modified by calculating r^?1 integrals, to be sensitive to bond directions of p orbitals. The pp′ modification, in which a subset of the neglected integrals is retained to recover invariance to coordinate transformations, is thereby necessitated. The GPM approach yields shifts in very good agreement with experiment when comparisons are restricted to similar molecules. The RPM version gives better agreement especially over wider classes of molecules. It also provides relaxation energies V_R that can be combined with ab initio orbital energies to give binding energies. Several applications of these potential models are discussed.     

Reexamine structures and relative stability of medium-sized silicon clusters: Low-lying endohedral fullerene-like clusters Si30-Si38

We report improved results of lowest-lying silicon clusters Si₃₀-Si₃₈. A large population of low-energy clusters are collected from previous searches by several research groups and the binding energies of these clusters are computed using density-functional theory (DFT) methods. Best candidates (isomers with high binding energies) are identified from the screening calculations. Additional constrained search is then performed for the best candidates using the basin-hopping method combined with DFT geometry optimization. The obtained low-lying clusters are classified according to binding energies computed using either the Perdew-Burke-Ernzerhof (PBE) functional or the Becke exchange and Lee-Yang-Parr correlation (BLYP) functional. We propose to rank low-lying clusters according to the mean PBE/BLYP binding energies in view that the PBE functional tends to give greater binding energies for more compact clusters whereas the BLYP functional tends to give greater binding energies for less compact clusters or clusters composed of small-sized magic-number clusters. Except for Si₃₀, the new search confirms again that medium-size silicon clusters Si₃₁-Si₃₈ constructed with proper fullerene cage motifs are most promising to be the lowest-energy structures.     

X-ray photoelectron spectroscopy of copper compounds

The X-ray photoelectron spectra of some forty-six copper compounds and complexes have been measured. The chemical shifts obtained from accurate determinations of the binding energies have been qualitatively explained on the basis of the Pauling electronegativity concept using the group electronegatives of Huheey for the polyatomic counter anions. The chemical shifts of the copper atoms as well as the atoms in the ligands were found to be dependent not only on the oxidation state but also on the kind and number of ligand atoms.

Intense satellite lines were found in the 2p and 2s bands of the cupric compounds; the number and splitting of the satellites were found to be sensitive to the chemical environment. A correlation was found between the satellite splitting and the binding energies and this is explained by a 3d→4s, 4p ‘shake-up’ mechanism.     

Core level binding energy shifts in Ni on Au and Au on Ni overlayers

Core level binding energy shifts of the Ni-2p and Au-4? lines have been measured for Ni on Au and Au on Ni overlayers down to mean coverages of less than 0.1 monolayers. When normalized to the maximum shift at submonolayer coverage the Ni on Au and Au on Ni shifts show the same dependence as function of the monolayer coverage. Using the thermodynamical approach for the calculation of binding energy shifts in metals, recently developed by Mårtensson and Johansson, the submonolayer shifts together with experimental results of core level binding energy shifts in dilute NiAu and AuNi alloys are used to calculate surface segregation energies in these alloys. They are compared with semiempirical determinations of these energies by Seah.     

Useful chemical information obtained by use of common counterions as internal calibrants in X-ray photo-electron spectra of inorganic complex ions

It is shown that the accuracy and precision, and hence the value for bonding-structure studies, in relative binding energy measurements can be enhanced if a common counterion is employed. The differences in chemical shifts between the peaks for the atoms under consideration and for a common counterion in twenty-nine compounds are measured. This technique reduces charging effect errors, which otherwise often occur when non-conducting samples (e.g., salts) are measured relative to a traditional external calibrant. The improvements in accuracy and precision are demonstrated by using the cesium salt of twenty-nine heteropoly and isopoly anions in more than seventy-five different runs. Oxygen 1s, tungsten 4d, and molybdenum 3d binding energies are measured relative to the cesium 3d ionization potential. In this work the cesium counterion is assumed to be chemically invariant. For the relative binding energies that are studied, no dependence on the charge of the anion is observed. A linear relation seems to exist between the oxygen Is binding energies (measured relative to Cs) and the oxygen-to-tungsten ratio in five isopoly anions. This latter finding may serve as a useful aid in studies related to the synthesis of new compounds.     

A CNDO/2 study of σ- and π- relaxation energies and core electron binding energy shifts in some simple molecules

A relaxation potential model has been used to study relaxation energies and shifts in core electron binding energies for some substituted alkenes and carbonyl compounds in terms of σ- and π-contributions.The conclusions from this study may be summarized as follows:(A) For the series R₁R₂C^*CH₂ and R₁R₂C^*O: (i) The total shifts vary in a regular manner, similar to the shifts in saturated systems; (ii) The variation in σ-relaxation energies is greater than the variation in π-relaxation energies.(B) For the series R₁R₂CCH₂ and R₁R₂CO: (i) There is little variation in the σ-relaxation energies; (ii) The π-relaxation energies show moderately large variations and the higher values are associated with unsaturated substituents which can donate π-electrons to the core-ionized atom via the conjugated π-system; (iii) The π-relaxation energies in the fluorinated systems are similar to those in the unsubstituted molecules; and (iv) The large -ve π-shift in the fluorinated systems results from a ground state build up of electron density at the site of core ionization.     

Electron-spectroscopic investigations of Ba and Ba compounds

Electron spectroscopic investigations of barium, barium compounds and barium thin film structures are reported. It is found that the barium 3d photoelectrons from BaO are at lower binding energies than for the case of pure barium metal. This effect is ascribed to relaxation processes. In some cases the compounds studied exhibited charging. In these cases plots of the Auger—Photoelectron energy separations are measured which were found to provide a using means of identifying the chemical environment of the barium atoms. Finally, measurements of the ionization loss spectra, principally the barium 4d lines, and changes in the plasmon energies for this series of barium compounds are reported.     

Correlation between core level chemical shifts obtained by ultrasoft x-ray emission and ESCA

High resolution C:K X-ray emission spectra have been recorded from six molecules in the gas phase. The spectra are interpreted by comparing the relative X-ray energies with the vertical ionization energies of the valence orbitals and comparing the intensities predicted by the C 2p populations from CNDO calculations with the measured intensities. The obtained C 1s binding energy shifts are found to be in excellent agreement with shifts measured with ESCA.     

Correlation between core level chemical shifts obtained by ultrasoft X-ray emission and ESCA

High resolution C:K X-ray emission spectra have been recorded from six molecules in the gas phase. The spectra are interpreted by comparing the relative X-ray energies with the vertical ionization energies of the valence orbitals and comparing the intensities predicted by the C 2p populations from CNDO calculations with the measured intensities. The obtained C 1s binding energy shifts are found to be in excellent agreement with shifts measured with ESCA.