Core-level Binding Energies Research Articles

Surface core-level binding energy shifts for MgO(100).

Theoretical and experimental results for the surface core-level binding energy, BE, shifts, SCLS, for MgO(100) are presented and the anomalous O(1s) SCLS is interpreted in terms of the surface electronic structure. While the Mg(2p) surface BE shifts to a higher value than bulk by ≈1 eV as expected from the different surface and bulk Madelung potentials, the O(1s) SCLS is almost 0 rather than ≈-1 eV, expected from the Madelung potentials. The distortion of the surface atoms from the spherical symmetry of the bulk Mg and O atoms is examined by a novel theoretical procedure. The anomalous O SCLS is shown to arise from the increase of the effective size of surface O anions.

Lattice charge models and core level shifts in disordered alloys

Differences in core level binding energies between atoms belonging to the same chemical species can be related to differences in their intra- and extra-atomic charge distributions, and differences in how their core holes are screened. With this in mind, we consider the charge-excess functional model (CEFM) for net atomic charges in alloys (Bruno et al 2003 Phys. Rev. Lett. 91 166401). We begin by deriving the CEFM energy function in order to elucidate the approximations which underpin this model. We thereafter consider the particular case of the CEFM in which the strengths of the ‘local interactions’ within all atoms are the same. We show that for binary alloys the ground state charges of this model can be expressed in terms of charge transfer between all pairs of unlike atoms analogously to the linear charge model (Magri et al 1990 Phys. Rev. B 42 11388). Hence, the model considered is a generalization of the linear charge model for alloys containing more than two chemical species. We then determine the model’s unknown ‘geometric factors’ over a wide range of parameter space. These quantities are linked to the nature of charge screening in the model, and we illustrate that the screening becomes increasingly universal as the strength of the local interactions is increased. We then use the model to derive analytical expressions for various physical quantities, including the Madelung energy and the disorder broadening in the core level binding energies. These expressions are applied to ternary random alloys, for which it is shown that the Madelung energy and magnitude of disorder broadening are maximized at the composition at which the two species with the largest ‘electronegativity difference’ are equal, while the remaining species have a vanishing concentration. This result is somewhat counterintuitive with regards to the disorder broadening since it does not correspond to the composition with the highest entropy. Finally, the model is applied to CuPd and CuZn random alloys. The model is used to deduce the effective radii associated with valence electron charge transfer for Cu, Pd and Zn in these systems for use in the ‘ESCA potential model’ of x-ray photoelectron spectroscopy. The effective radii are found to be R1/3, where R1 is the nearest neighbor distance, with only small variations between chemical elements and between different systems. The model provides a framework for rationalizing the disorder broadenings in these systems: they can be understood in terms of an interplay between the broadening in the Madelung potentials and the broadening in the intra-atomic electrostatic potentials.

Hard x-ray photoelectron spectroscopy and electrical characterization study of the surface potential in metal/Al2O3/GaAs(100) metal-oxide-semiconductor structures

Hard x-ray photoelectron spectroscopy (HAXPES) has been used to study metal-oxide-semiconductor (MOS) structures fabricated with both high (Ni) and low (Al) work-function metals on 8-nm thick Al${}_{2}$O${}_{3}$ dielectric layers, deposited on sulfur passivated $n$- and $p$-doped GaAs substrates. A binding energy difference of 0.6 eV was measured between the GaAs core levels of the $n$- and $p$-doped substrates in the absence of gate metals, indicating different Fermi level positions in the band gap. Subsequent photoemission measurements made on the MOS structures with the different work-function metals displayed very limited change in the GaAs core level binding energies, indicating that the movement of the Fermi level at the Al${}_{2}$O${}_{3}$/GaAs interface is restricted. Using a combination of HAXPES measurements and theoretical calculations, the Fermi level positions in the band gap have been determined to be in the range of 0.4--0.75 eV and 0.8--1.11 eV above the valence band maximum for $p$- and $n$-type GaAs, respectively. Analysis of capacitance voltage (C-V) measurements on identically prepared samples yield very similar Fermi level positions at zero applied gate bias. The C-V analysis also indicates a higher interface defect density ($D$${}_{it}$) in the upper half of the GaAs bandgap.

Catalytic conversion of syngas to olefins over Mn–Fe catalysts

Abstract A series of MnO 2 –Fe 2 O 3 with various contents of Mn (0–40% atomic ratio) was synthesized by the co-precipitation method. XRD patterns indicate that the sharp and intense peaks are related to the excellent crystal quality of α-Fe 2 O 3 nanostructures. HR-TEM images showed that α-Fe 2 O 3 and MnO 2 , were partly in close contact and the lattice fringes exhibit the typical distances, i.e., α-Fe 2 O 3 (104) (2.7 A) and MnO 2 (310) (3.1 A). X-ray photoelectron spectroscopy (XPS) results of the Fe 2p core-level binding energy spectrum of the α-Fe 2 O 3 and Mn 2p, indicated the presence of Fe 3+ and Mn 4+ . The direct synthesis of olefins from syngas was carried out over the MnO 2 –Fe 2 O 3 catalysts under pressurized fixed-bed continuous flow conditions. The results showed that Mn–Fe catalyst had high catalytic activity and high olefins selectivity without the addition of any promoters at low pressure. It was found that the catalyst containing 20 at% Mn–Fe was an optimal catalyst for the conversion of synthesis gas to hydrocarbons especially light olefins. The maximum CO conversion rate was 15%, and the yield of olefins (totally about 23.77%) with a predominance of CH 4 (12.24%), and C 2 H 6 (2.80%) components. Mn–Fe catalyst can be regarded as a potential candidate for catalytic conversion of biomass-derived syngas to olefins.

XPS characterization of single crystalline SrLaGa3O7:Nd

Core-level XPS studies of single crystalline SrLaGa3O7 (SLGO) doped with Nd have been performed for the first time. The detailed analysis of the main XPS core lines of SLGO:Nd included the comparative literature data for the selected oxide compounds containing La and/or Sr, Nd or Ga cations. In particular, the binding energies (BEs) of the Ln 3d5/2 core levels (Ln=La, Nd) in SLGO:Nd were found consistent with the relevant ones for the La- and Nd-based oxides considered, thus indicating that they represent the ground final states of La3+ and Nd3+ ions (respectively) in the crystal lattice after photoexcitation. Analogous consistency has been found for the XPS-derived BEs of the Sr 3d5/2 and Ga 2p3/2 core levels related to the Sr2+ and Ga3+ cationic states in SLGO and the corresponding oxide compounds. Generally, the binding energies of the deep core levels of cations in SLGO:Nd and the other oxides considered are mainly determined by their common oxygen ligand, irrespectively of the crystalline structure.

Interfacial electronic band alignment of Ta2O5/hydrogen-terminated diamond heterojunction determined by X-ray photoelectron spectroscopy

Ta2O5 films have been deposited on hydrogen-terminated diamond (H-diamond) by a radio-frequency sputter-deposition technique at room temperature. Electronic band structure of Ta2O5/H-diamond heterojunction has been investigated by X-ray photoelectron spectroscopy. Based on the binding energies of core-levels and valence band maximum values, valence band offset has been found to be 1.5±0.2eV for the Ta2O5/H-diamond heterointerface. It shows a type-II band configuration with conduction band offset of 2.4±0.2eV. The large ΔEV value makes the Ta2O5/H-diamond heterojunction probably suitable for the application of high power and high frequency field effect transistors.

Design of an ultrahigh vacuum transfer mechanism to interconnect an oxide molecular beam epitaxy growth chamber and an x-ray photoemission spectroscopy analysis system

We designed a mechanism and the accompanying sample holders to transfer between a VEECO 930 oxide molecular beam epitaxy (MBE) and a PHI Versa Probe X-ray photoemission spectroscopy (XPS) chamber within a multiple station growth, processing, and analysis system through ultrahigh vacuum (UHV). The mechanism consists of four parts: (1) a platen compatible with the MBE growth stage, (2) a platen compatible with the XPS analysis stage, (3) a sample coupon that is transferred between the two platens, and (4) the accompanying UHV transfer line. The mechanism offers a robust design that enables transfer back and forth between the growth chamber and the analysis chamber, and yet is flexible enough to allow transfer between standard sample holders for thin film growth and masked sample holders for making electrical contacts and Schottky junctions, all without breaking vacuum. We used this mechanism to transfer a barium strontium titanate thin film into the XPS analysis chamber and performed XPS measurements before and after exposing the sample to the air. After air exposure, a thin overlayer of carbon was found to form and a significant shift (~1 eV) in the core level binding energies was observed.

Effects of Chemical Treatments and Ultrathin Al2O3 Deposition on InAlN Surface Investigated by X-ray Photoelectron Spectroscopy

The effects of chemical treatments and Al2O3 deposition on the InAlN surface were investigated by X-ray photoelectron spectroscopy (XPS). Independent of the extent of native oxide removal, the In 4d core-level binding energy was the same for untreated, HCl-treated, and HF-treated InAlN. This indicated a strong pinning tendency of the Fermi level at the InAlN bare surface. However, a 300 meV decrease in the In 4d binding energy was observed after atomic layer deposition (ALD) of Al2O3, which indicated an increase in the negative surface potential at the InAlN surface. The reduction of positive charge at the InAlN surface is discussed.

Temperature dependence study of Pd–Cu supported bimetallic films by photoelectron spectroscopy and cyclic voltammetry

Vapor deposition method is used to prepare layered Pd–Cu bimetallic films, with different thickness of Pd over a Cu film supported on glassy carbon. X-ray photoelectron spectroscopy (XPS) measurements of core level binding energy (BE) and valence band region are used to investigate the contribution of charge transfer resulting from the bonding of these two dissimilar metals in a layered structure. As this layered bimetallic film is annealed at increasing temperature from 298 to 650K, atomic inter-diffusion occurs to form an alloy. We differentiate this interfacial charge transfer effect between these two dissimilar metals from that of surface alloying resulting from the annealing effect. Cyclic voltammetry (CV) tests combined with XPS confirm that (1) the surface is free of Cu atoms for Pd films with thicknesses between 2.0 and 7.0±0.3nm at room temperature and (2) Pd–Cu inter-diffusion propagates to the surface and the formation of surface alloys take place at temperature greater than 350K. For these Pd thicknesses, the difference in BE shifts of Cu 2p and Pd 3d peaks between layered and alloyed structures are +0.16eV and −0.17eV, respectively. This difference in BE shifts allows for a clear distinction between these two bimetallic structures and the tailoring of the optimum configuration that enhances catalytic activity toward formic acid. Additionally, angle resolved XPS data as a function of temperature and XPS depth profiling at room temperature provide insight about the inter-diffusion length. Furthermore, CV measurements in H2SO4 and formic acid confirm that for a Pd–Cu bimetallic surface with a 3.8±0.3nm Pd film over Cu shows significantly improved activity toward formic acid oxidation compared to bulk Pd at 298K and after being annealed at temperature of 350K.

Charateristics of Low Temperature High Quality Silicon Oxide by Plasma Enhanced Atomic Layer Deposition with In Situ Plasma Densification Process

We have researched low temperature silicon oxide (SiO2) with improved electrical properties and excellent film step-coverage. In-situ O2 plasma densification (DENSIFICATION) effect on SiO2 that has been deposited by PE-ALD at temperature (<400°C) was investigated. The wet etch rate was controllable as a function of DENSIFICAION time due to repairing of the SiO2 network defects by the oxygen radicals. Angle Resolved X-ray Photoelectron Spectroscopy (AR-XPS) analysis was carried out to observe the core-level binding energies shifts (chemical shifts) in the different SiO2 films. The characteristics of wet etch rate of high quality low temperature SiO2 demonstrated lower than high temperature LP-CVD SiO2 values. Compared to LP-CVD SiO2, PE-ALD SiO2 with DENSIFICATION showed excellent I-V characteristics with lower leakage current and similar to the thermal SiO2 carrier transport plot.

Photoemission and density functional theory study of Ge(100): Clean surface and Yb-induced (2×4) reconstruction

Clean and metal-adsorbed (100) surfaces of group-IV semiconductors, such as Si and Ge, often exhibit electronically and structurally similar reconstructions. However, the fundamental bulk properties of group-IV materials can have an impact on particular features of such systems, which are related, e.g., to final-state relaxation in photoemission and thus determine their spectral line shape. Here we have studied Yb/Ge(100)(2×4) reconstruction as well as clean Ge(100) surface by high-resolution photoelectron spectroscopy and ab initio calculations. An atomic geometry of both surfaces is thoroughly investigated. A detailed analysis of Ge 3d core-level photoemission, atomic origins of surface-shifted components, and final-state screening effects is presented. In particular, it is demonstrated that the core-hole screening plays an essential role in Ge 3d measurements, and that its amount in the complete screening model correlates well with the core-level binding energy of respective Ge atoms in the initial state. The results are discussed in the proper context of related reconstructions on Si(100).

Exciton and core-level electron confinement effects in transparent ZnO thin films

The excitonic light emission of ZnO films have been investigated by means of photoluminescence measurements in ultraviolet-visible region. Exciton confinement effects have been observed in thin ZnO coatings with thickness below 20 nm. This is enhanced by a rise of the intensity and a blue shift of the photoluminescence peak after extraction of the adsorbed species upon annealing in air. It is found experimentally that the free exciton energy (determined by the photoluminescence peak) is inversely proportional to the square of the thickness while core-level binding energy is inversely proportional to the thickness. These findings correlate very well with the theory of kinetic and potential confinements.

Quantitative analysis of complex amino acids and RGD peptides by X‐ray photoelectron spectroscopy (XPS)

The C 1 s, N 1 s, and O 1 s core level binding energies (BEs) of the functional groups in amino acids (glycine, aspartic acid, glutamic acid, arginine, and histidine) with varied side‐chains and cell‐binding RGD‐based peptides have been determined and characterized by X‐ray photoelectron spectroscopy with a monochromatic Al Kα source. The zwitterionic nature of the amino acids in the solid state is unequivocally evident from the N 1 s signals of the protonated amine groups and the C 1 s signature of carboxylate groups. Significant adventitious carbon contamination is evident for all samples but can be quantitatively accounted for. No intrinsic differences in the XP spectra are evident between two polymorphs (α and γ) of glycine, indicating that the crystallographic differences have a minor influence on the core level BEs for this system. The two nitrogen centers in the imidazole group of histidine exhibit an N 1 s BE shift that is in line with previously reported data for theophylline and aqueous imidazole solutions, while the nitrogen and carbon chemical shifts reflect the unusual guanidinium chemical environment in arginine. It is shown that the complex envelopes of C 1 s and O 1 s photoemission spectra for short‐chain peptides can be analyzed quantitatively by reference to the less complex XP spectra of the constituent amino acids, provided the peptides are of high enough purity. The distinctive N 1 s photoemission from the amide linkages provides an indicator of peptide formation even in the presence of common impurities, and variations in the relative intensities of N 1 s were found to be diagnostic for each of the three peptides investigated (RGD, RGDS, and RGDSC). Copyright © 2013 John Wiley &amp; Sons, Ltd.

Band offsets in complex-oxide thin films and heterostructures of SrTiO3/LaNiO3 and SrTiO3/GdTiO3 by soft and hard X-ray photoelectron spectroscopy

The experimental determination of valence band offsets (VBOs) at interfaces in complex-oxide heterostructures using conventional soft x-ray photoelectron spectroscopy (SXPS, hν ≤ 1500 eV) and reference core-level binding energies can present challenges because of surface charging when photoelectrons are emitted and insufficient probing depth to clearly resolve the interfaces. In this paper, we compare VBOs measured with SXPS and its multi-keV hard x-ray analogue (HXPS, hν &amp;gt; 2000 eV). We demonstrate that the use of HXPS allows one to minimize charging effects and to probe more deeply buried interfaces in heterostructures such as SrTiO3/LaNiO3 and SrTiO3/GdTiO3. The VBO values obtained by HXPS for these interfaces are furthermore found to be close to those determined by first-principles calculations.

Chemisorption of uracil on gold surfaces via density functional theory

We have performed a periodic density functional theory study, including a correction for the dispersive forces (DFT-D), of the adsorption of the pyrimidine nucleobase uracil on the Au(100) and Au(111) surfaces. We illustrate the influence of the inclusion of dispersive interactions in the calculation on the geometries and energetics of adsorption. A systematic analysis has been undertaken on both surfaces, where we have explored two possible deprotonation of the two nitrogen positions of the uracil and compared the core-level binding energies of the chemisorbed phase with previous experimental findings and calculations.

Band alignment of ZnTe/GaAs heterointerface investigated by synchrotron radiation photoemission spectroscopy

ZnTe films have been grown on GaAs substrate by metalorganic vapor phase epitaxy. High-resolution synchrotron radiation photoemission spectroscopy measurements are used to determine the valence band offset of ZnTe/GaAs heterojunctions. Based on the binding energies of Zn 3d and Ga 3d core levels and valence band maximum values, the valence band offset has been determined to be 0.19 eV for ZnTe/GaAs heterojunction. The heterojunction shows type I band configuration with a conduction band offset of 0.65 eV. The accurate determination of the band alignment of ZnTe/GaAs heterojunction facilitates the design of optical and electronic devices based on ZnTe/GaAs structure.

Influence of Axial and Peripheral Ligands on the Electronic Structure of Titanium Phthalocyanines

To discover how molecular changes affect the electronic structure of dye molecules for solar cells, we have investigated four titanium phthalocyanines customized by axial and peripheral ligands (monodentate oxo versus bidentate catechol and tert-butyl versus tert-butylphenoxy, respectively). X-ray absorption spectroscopy and photoelectron spectroscopy were combined with density functional theory (DFT) and crystal-field multiplet calculations to characterize the Ti 3d and N 2p valence electrons that form the frontier orbitals. When a monodentate oxo axial ligand was replaced by a bidentate catechol ligand, the multiplet structure of the Ti 2p-to-3d transitions was found to change systematically. The most noticeable change was an additional transition into the low-lying 3dxy level, which is attributed to a reduction in local symmetry from 4-fold to 2-fold at the Ti center. An increase of the Ti 2p core-level binding energy was observed in the bidentate complex and compared to a calculated core-level stabilization. DFT predicts a change of the LUMO from the inner phthalocyanine ring to the Ti dxy orbital and a reversal of the high-lying dx 2 −y 2 and dz 2 orbitals. The N 1s edge was calculated using time-dependent density functional theory (TDDFT) and compared to experiment.

Solvation at nanoscale: Alkali-halides in water clusters

The solvation of alkali-halides in water clusters at nanoscale is studied by photoelectron spectroscopy using synchrotron radiation. The Na 2p, K 3p, Cl 2p, Br 3d, and I 4d core level binding energies have been measured for salt-containing water clusters. The results have been compared to those of alkali halide clusters and the dilute aqueous salt solutions. It is found that the alkali halides dissolve in small water clusters as ions.

Depth profiling of ultra-thin alumina layers grown on Co(0001)

Epitaxial thin oxide layers were grown by simultaneous aluminum deposition and oxidation on a Co(0001) single crystal, and the metal–oxide interface between the substrate and the grown layer was studied using photoelectron spectroscopy. The oxide layers were composed of two kinds of chemically different layers. Angle-resolved measurements were used to determine the compositions of oxide sub-layers and to reveal their respective thicknesses. The topmost oxide layers were up to 0.23 nm thick, determined by analysis of O 1s and Co 2p3/2 photoelectron spectra. The results of the analysis show that the interface layer is composed of a mixture of oxygen and cobalt atoms and its thickness is approximately 0.6 nm. The analysis of Co 2p3/2, Al 2p3/2 and O 1s core level binding energies confirmed the presence of CoO in the interface layer and Al2O3 in the topmost oxide layer.

Evaluating the Structure of Catalysts Using Core-Level Binding Energies Calculated from First Principles

X-ray photoelectron spectroscopy (XPS) is a powerful and popular surface characterization technique, and the measured shifts in the core electron binding energies are sensitive to the chemical structure and local environment of the surface species. C 1s binding energies were calculated with density functional theory (DFT) for 17 structures including eight well-characterized structures on a Co(0001) surface and nine on a Pt(111) surface, while B 1s binding energies were calculated for six well-characterized structures and compared with experimental values. DFT calculations describe the 2.8 eV variation in the C 1s binding energies on Co surfaces, the 4.2 eV variation in the C 1s binding energies on Pt surfaces, and the 5.5 eV variation in the B 1s binding energies in the test sets with average deviations of 85, 73, and 53 meV, respectively. The shift in the C 1s and the B 1s binding energies can be correlated with the calculated charges, though only within homologous series. To illustrate how binding energ...