Core Level Binding Energy Shifts Research Articles

Effective work function of NiSi/HfO 2 gate stacks measured with X-ray photoelectron spectroscopy

A method to measure an effective work function, WF eff, of a metal electrode in contact with particular dielectric in a metal–oxide–semiconductor stack is proposed by in situ monitoring core level binding energy shifts of dielectric constituents upon deposition of an ultrathin metallic layer on top. The proposed method is utilized to measure relative changes in WF eff between Ni and NiSi electrodes in contact with HfO 2, as well as to investigate the role of Sb and Ti δ-layers at NiSi/HfO 2 interface.

Simulation of core-level binding energy shifts in germanium-doped lead telluride crystals

To simulate the changes in core-level binding energies in germanium-doped lead telluride, cluster calculations of the changes in the electrostatic potential at the corresponding centers have been performed. Different locations of the Ge atom in the crystal bulk have been considered: near vacancies, near another dopant site, and near the surface. For calculating the potential in the clusters that model the bulk and the surface of the lead telluride crystal (c-PbTe), the electron density obtained in the framework of the Hartree-Fock and hybrid density functional theory (DFT) methods has been used.

X-ray photoelectron spectroscopy study of dielectric constant for Si compounds

The authors measure the difference of core-level binding energy shifts for Si 1s and Si 2p, ΔE1s-ΔE2p, for various Si compounds using high-resolution high-energy synchrotron radiation. They find that the ΔE1s-ΔE2p values are in very good correlation with the dielectric constant values of the Si compounds. Using this relation, they deduce the local dielectric constant for each of the Si intermediate oxidation states formed at the SiO2∕Si interface. The results are in good agreement with values predicted by a first-principles calculation.

Core‐level shifts in complex metallic systems from first principle

AbstractWe show that core‐level binding energy shifts (CLS) can be reliably calculated within density functional theory. The scheme includes both the initial (electron energy eigenvalue) as well as final state (relaxation due to core‐hole screening) effects in the same framework. The results include CLS as a function of composition in substitutional random bulk and surface alloys. Sensitivity of the CLS to the local chemical environment in the bulk and at the surface is demonstrated. A possibility to use the CLS for structural determination is discussed. Finally, an extension of the model is made for Auger kinetic energy shift calculations. (© 2006 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

Barium and oxygen interaction on the Ni(110) surface at low coverages studied by softx-ray photoemission spectroscopy: Ba negative binding energy shifts and their correlationwith Auger electron spectroscopy shifts

In this work, barium and oxygen interaction on the Ni(110) surface is investigated, bymeans of soft x-ray photoemission spectroscopy (SXPS), mainly at submonolayer andmonolayer coverages. The results show that oxygen interacts with barium and the substrateas well forming BaO and NiO respectively. The formation of both oxides is consistent withprevious Auger electron spectroscopy (AES) results. The oxidation of barium on thesurface induces negative binding energy shifts of the Ba low core atomic levels 4d, 5s and5p. Both initial and final state effects are shown to be necessary in order to explainthese peculiar energy shifts. This interpretation is based on correlating the corelevel binding energy shifts with previously recorded AES transition line shifts,also taking into account the changes of the work function of the surface. Theanalysis shows that the extra-atomic relaxation energy decreases on going frommetallic Ba to BaO. This implies that the reduction of the free electron screeningmore than counteracts the increase in relaxation energy due to the polarizableO2− anions.

How the solid state matrix affects the chemical shift of core-level binding energies: A novel method to take the induction effect into account

The interpretation of chemical shifts in high resolution photoemission core level spectra of mixed oxides as they occur in high-k dielectrics requires that charge transfer beyond the first coordination shell be taken into account. We present a scheme which allows us to include these so-called induction effects. The method is a judicious extension of Sanderson’s original approach towards network structures. At each point of departure from Sanderson, our method is tested successfully against the rigorous Sanderson formalism. For three examples of relevance in semiconductor technology, our new method yields results in excellent agreement with experiment.

Size Effects in Electronic and Catalytic Properties of Unsupported Palladium Nanoparticles in Electrooxidation of Formic Acid

We report a combined X-ray photoelectron spectroscopy (XPS), cyclic voltammetry (CV), and chronoamperometry (CA) study of formic acid electrooxidation on unsupported palladium nanoparticle catalysts in the particle size range from 9 to 40 nm. The CV and CA measurements show that the most active catalyst is made of the smallest (9 and 11 nm) Pd nanoparticles. Besides the high reactivity, XPS data show that such nanoparticles display the highest core-level binding energy (BE) shift and the highest valence band (VB) center downshift with respect to the Fermi level. We believe therefore that we found a correlation between formic acid oxidation current and BE and VB center shifts, which, in turn, can directly be related to the electronic structure of palladium nanoparticles of different particle sizes. Clearly, such a trend using unsupported catalysts has never been reported. According to the density functional theory of heterogeneous catalysis, and mechanistic considerations, the observed shifts are caused by a weakening of the bond strength of the COOH intermediate adsorption on the catalyst surface. This, in turn, results in the increase in the formic acid oxidation rate to CO2 (and in the associated oxidation current). Overall, our measurements demonstrate the particle size effect on the electronic properties of palladium that yields different catalytic activity in the HCOOH oxidation reaction. Our work highlights the significance of the core-level binding energy and center of the d-band shifts in electrocatalysis and underlines the value of the theory that connects the center of the d-band shifts to catalytic reactivity.

Core-level binding energy shift as a probe of morphological changes in two-dimensionalAg−Coalloy

The core-level binding energy shifts can be used as a supplementary tool to microprobe techniques to describe morphological changes in the monolayer $\mathrm{Ag}\text{\ensuremath{-}}\mathrm{Co}$ films grown on the $\mathrm{Ru}(0001)$ surface. We have carefully measured the binding energy positions of the $\mathrm{Ag}\phantom{\rule{0.3em}{0ex}}3d$ and $\mathrm{Co}\phantom{\rule{0.3em}{0ex}}2p$ with respect to the Fermi level of the $\mathrm{AgCo}(1:1)$ and $\mathrm{CoAg}(1:1)$ complete layer as deposited at $350\phantom{\rule{0.3em}{0ex}}\mathrm{K}$ and after successive annealing at higher temperatures. The structural changes in the layer upon heat treatment, deduced from experimentally observed shifts, are in general agreement with the results presented in the literature.

A (4×2) reconstruction of CuInSe2 (001) studied by low-energy electron diffraction and soft x-ray photoemission spectroscopy

Clean and flat (001) surfaces of CuInSe2∕GaAs grown by molecular-beam epitaxy could be prepared by the combination of a Se capping and decapping process and subsequent Ar+ ion sputtering and annealing. The formation of a (4×2) reconstruction was observed with low-energy electron diffraction. Soft x-ray photoemission spectroscopy was performed on the prepared surfaces and revealed surface core-level binding energy shifts in the Cu2p3∕2, Se3d, and In4d levels which are associated with surface atoms. The structure model of a combined metal adatom-Se dimer structure is proposed to refer to the (4×2) reconstruction.

Initial and final state contributions to binding-energy shifts due to lattice strain: Validation of Auger parameter analyses

The validity of Auger parameter analyses to separate initial and final state contributions to core-level binding energy shifts in the growth of nanoparticles is tested; the specific concern is shifts due to lattice strain. Theoretical energies for the hole-states involved are used to avoid the approximations and assumptions normally required to justify the analyses. When the Auger transitions involve states with high lying d-holes, the Auger parameter analysis of the origin of the shifts is incorrect. When only core–hole Auger states are used, a suitable formulation will accurately reproduce the initial state origin of shifts due to lattice strain.

On the surface chemistry of silicon under reducing conditions: An SRPES investigation

Silicon electrodes were conditioned in alkaline solution at potentials negative from open circuit (ocp). The dark I–V characteristic of n-Si(1 1 1) in accumulation reveals Butler–Volmer behaviour for outer sphere charge transfer of the hydrogen evolution reaction. Deconvoluted synchrotron radiation photoelectron spectroscopy (SRPES) data are used to identify reaction intermediates of the chemical and electrochemical pathways and are compared to existing models on a molecular base. The results indicate the existence of postulated negatively charged surface species. Comparison of density functional theory (DFT) calculations and core level binding energy shifts of the Si 2p 3/2, the 2p 1/2 line and the O 1s line allows the assignment of twofold coordinated surface reaction intermediates.

Core-level shifts for two- and three-dimensional bimetallicPdxCu1−xandPdxAg1−xalloys on Ru(0001)

Core-level binding energy shifts (CLSs) and surface CLS (SCLSs) are determined by experiment and theory for ultrathin Pd as well as for PdCu and PdAg surface alloys which vary in thickness from 1 t ...

The Growth of Silver on an Ordered Alumina Surface

The growth of Ag on an ordered Al2O3 surface was studied by low energy ion scattering spectroscopy (LEIS), scanning tunneling microscopy (STM), X-ray photoelectron spectroscopy (XPS), and temperature programmed desorption (TPD). Three-dimensional (3D) growth of Ag clusters was observed with STM and LEIS, with the cluster size increasing with Ag coverage. The XPS core level binding energies and the Auger parameters indicate a weak interaction between the Ag clusters and the Al2O3 support. Final state effects are determined to be the primary contribution to the Ag core level binding energy shift. Nonzero order kinetics was observed for Ag desorption in TPD with the Ag sublimation energy decreasing with decreasing cluster size.

Acid−Base Characterization of Aluminum Oxide Surfaces with XPS

The localized acid−base properties of different, aluminum oxide thin layer surfaces have been evaluated with X-ray photoelectron spectroscopy (XPS). Five types of oxide layers were studied, which were produced by oxidizing aluminum in a vacuum, with an alkaline and acidic pretreatment, and in boiling water. The photoelectron core level binding energies, as measured with XPS, are evaluated for this purpose, while taking into consideration the initial and final state effects. For the structurally comparable oxides, the shifts in the O 1s binding energies are determined by their initial state chemistry. The values of the O 1s binding energy can be directly related to the surface-averaged charge on the O anions. For the Al cations, a correlation between the photoelectron core level binding energy shift and changes in the initial state chemistry was observed, but the Al 2p binding energy shifts were found to be partially due to changes in extra-atomic relaxation. The measured Al 2p binding energies and the binding energies of the resolved OH and O components in the O 1s peak showed that the studied aluminum oxides have OH sites with the same Brönsted/Lewis acid−base properties, O sites with the same Lewis base properties, and Al sites with very similar Lewis acid properties. The pseudoboehmite oxide, obtained by boiling aluminum in water, exhibits more basic O, OH, and Al sites. This oxide deviates structurally from the other oxides studied, resulting in a lower extra-atomic relaxation and Madelung potential contribution to the binding energies.

Surface and nanosolid core-level shift: Impact of atomic coordination-number imperfection

Identifying the origin for core-level binding-energy shift induced by surface relaxation or nanosolid formation, and quantifying the crystal binding energy in a bulk solid has been a challenge. Here we show that a recent bond order-length-strength correlation mechanism allows us to unify the effects of surface relaxation and nanosolid formation on the core-level binding-energy shift into the atomic-coordination number imperfection. A new and simple method has been developed that enables us to elucidate the intra-atomic trapping energy (the core-level position of an isolated atom) and the crystal binding strength (core-level bulk shift) to the core electrons at the energy levels of Si and a number of metals by matching the predictions to the measurements.

Electronic structure of Co-Pt alloys: X-ray spectroscopy and density-functional calculations

The K and L 2 . 3 absorption edges and core-level binding-energy shifts for pure Co and Pt, and Co-Pt alloys are measured to investigate the changes in electronic structures of Co-Pt alloys. The results agree well with those of the first-principles calculations of the electronic structures by using a full-potential linearized-augmented-plane-wave method. It is found that there is an enhancement of the hybridization between the Co 3d and Pt 5d orbitals upon alloying. Consequently, as the Pt concentration increases, both Pt and Co gains d electrons, and there is a concomitant decrease in s and p electrons at both sites. This enhanced hybridization in the alloy leads to a net charge transfer from the Co to Pt site in accordance with the predictions based on the electronegativity. For the core-level shifts in Co-Pt alloys the initial-state effects are more important to the Pt 4f level, while the relaxation effects play a more important role for the Co 2p level.

Pressure-induced physical changes of noble gases implanted in highly stressed amorphous carbon films

Noble gases (Ar, Kr, and Xe) were trapped in an amorphous carbon matrix in the 1--11-GPa pressure range. Extended and near-edge x-ray-absorption spectroscopies indicate clustering of noble gases induced by the host matrix internal pressure. Simultaneously, the matrix pressure promotes a shift of the noble-gas core-level binding energy of $\ensuremath{\sim}1\mathrm{eV}.$ The Auger parameter reveals that both the initial state and the host relaxation terms contribute to the binding-energy shift. Ab initio calculations performed on an ${\mathrm{Ar}}_{7}$ cluster and on Ar atoms clustered in aromatic molecules support the experimental findings.

Interactions of ultrathin Pb films with Ru(0001) and Pd(111)

The interaction of ultrathin Pb films with Ru(0001) and Pd(111) has been studied with x-ray photoelectron spectroscopy (XPS) and thermal desorption spectroscopy (TDS). XPS and TDS show that at room temperature Pb grows onRu(0001) via a Stranski-Krastanov mechanism, i.e., after completion of the first adlayer of Pb on Ru(0001), further Pb deposition leads to the formation of three-dimensional islands. Upon annealing to 500 K, the overlayer Pb undergoes significant clustering with the extent dictated by the initial coverages. XPS results show that the Pb 4f 7 / 2 core level binding energy (BE) increases slightly (+0.3 eV) as the Pb coverage increases from 0.07 ML to multilayers. When the first monolayer of Pb/Ru(0001) has been completed, the Pb 4f 7 / 2 BE has nearly reached the BE value for bulk Pb; it is only 0.1 eV lower. At ∼ 2 ML, the Pb 4f 7 / 2 BE is equivalent to that of bulk Pb. In contrast, at room temperature, Pb alloys with the Pd(111) surface, the Pd 3d 5 / 2 BE increasing ∼ 0.8 eV with coverage from 0.07- to 4.3-ML Pb. The Pb 4f 7 / 2 BEs also increase with increasing coverage reaching the bulk value of Pb at a coverage of ∼ 2 ML. The Pb 4f 7 / 2 BE at 0.07 ML is 0.55 eV smaller than that of bulk Pb. The lineshape and position of the x-ray induced Pd Auger features (M 4 , 5 VV) also display significant changes upon alloying. For Pb/Pd alloys, annealing to 500 K does not induce further changes in the Pd (M 4 , 5 VV) Auger features. The core level BE shifts are discussed in terms of possible initial and final state contributions. In particular, the initial state BE shift mechanisms of hybridization and environmental conduction band charge density are explained in the context of prior theoretical studies. It is shown that 6s→6p hybridization in Pb can be expected to be sufficiently large to contribute to the observed BE shifts.

High-resolution photoemission study of hydrogen interaction with polar and nonpolar GaAs surfaces

We have used synchrotron-based high-resolution core-level photoemission and valence-band emission measurements to characterize hydrogen chemisorption on nonpolar GaAs(110) and polar GaAs(100) and GaAs(001) surfaces. Chemisorbed atomic hydrogen forms both Ga-H and As-H bonds on all three surfaces, causing chemical shifts of core-level binding energies and changing the valence-band emission. For low hydrogen exposures arsenic desorbs from all surfaces. However, at higher exposures, the (110) surface transforms into a Ga-rich structure with traces of metallic Ga, while the (100) surface transforms into an As-rich structure. We have also observed some additional changes in the binding energy of bulk components of Ga and As core levels as a function of hydrogen exposure, which may be explained by hydrogen-induced changes in band bending.

Oxide‐supported Metal Clusters: Models for Heterogeneous Catalysts

AbstractFor Abstract see ChemInform Abstract in Full Text.