Surface 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.

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.

Conformation and orientation effects in the X-ray photoelectron spectra of organic polymers

Conformation and orientation effects in the XPS spectra of organic polymers are small but can be observed with modern high performance equipment. This paper discusses some of the experimental factors that should be considered when attempting to detect such effects, and describes several recent studies which illustrate the subtle phenomena that can now be observed. Conformation effects, revealed by melting semi-crystalline polymer samples in the analysis chamber of an XPS spectrometer, are reported for the valence band and C 1s spectra of nylon 12 and poly(ethylene adipate), and for the valence band spectrum of poly(ethylene succinate). For nylon 12 the changes in the C 2s region of the valence band spectrum are interpreted in terms of disordering of the planar zig-zag conformation of the (CH 2) 11 segments of the polymer chain. Pendant group surface orientation effects, detected by angle resolved XPS (with emission angles as low as 5° relative to the sample surface), are reported for poly(2-chloroethyl methacrylate) (PCEMA), poly(lauryl methacrylate) (PLMA) and poly(2-hydroxyethyl methacrylate) (PHEMA). For PCEMA and PLMA the uppermost surfaces are enriched with –CH 2CH 2Cl and –(CH 2) 11CH 3 pendant groups, respectively, whereas for PHEMA the data suggest relatively few –CH 2CH 2OH pendant groups at the surface. The C 1s and Cl 2p spectra of PCEMA reveal surface core level binding energy shifts of about +0.2 eV, and the Cl L 23M 23M 23 spectrum a surface Auger kinetic energy shift of about −0.6. The C 1s spectra of PLMA and PHEMA also reveal surface core level shifts and for PLMA this is interpreted in terms of a disordered and open arrangement of the –(CH 2) 11CH 3 pendant groups at the polymer surface.

Thermally induced core-electron binding-energy shifts in transition metals: An experimental investigation of Ta(100)

High-resolution photoemission spectra from the 4 f 7/2 levels of Ta~100! have been obtained between 77 K and room temperature. The data show an increase in both the surface and bulk core-level binding energies ~BE’s ! as the temperature is raised: between 77 and 293 K the bulk and surface BE’s increase by 31 63 and 1362 meV, respectively. A model calculation of the bulk binding-energy increase, which is based upon the lattice expansion of the solid, is in good agreement with the experimental results and indicates that the shifts arise from both initial- and final-state effects that are of comparable magnitude. The model is further used to estimate thermally induced shifts for the whole 5 d transition-metal series. @S0163-1829~96!02348-X#

Surface core-level shifts for simple metals.

We have performed an ab initio study of the surface core-level binding energy shift (SCLS) for 11 of the simple metals by means of a Green's-function technique within the tight-binding linear-muffin-tin-orbitals method. Initial- and final-state effects are included within the concept of complete screening, whereby a SCLS becomes equivalent to the surface segregation energy of a core-ionized atom, a quantity we obtain by separate bulk and surface impurity calculations. The results are in good agreement with experiment in most of those cases where the data originates from single-crystal measurements. We discuss the surface shifts of the electrostatic potentials and the band centers in order to trace the microscopic origin of the SCLS in the simple metals and find that the anomalous subsurface core-level shifts in beryllium are caused by charge dipoles, which persist several layers into the bulk. We furthermore conclude that the unexpected negative sign of the SCLS in beryllium is predominantly an initial-state effect and is caused by the high electron density in this metal.

Interaction of Carbon Monoxide with Bimetallic Overlayers

Interaction of carbon monoxide with a few chosen bimetallic overlayers has been investigated along with the core-level binding energies of the deposited metals by employing X-rays as well as UV photoelectron spectroscopies. Core-level binding energies of the deposited metals around monolayer coverages (0 similar to 1) are significantly different than those at high coverages or of the pure metals. Bimetallic overlayers such as Ni/Au and Cu/Pt showing large negative shifts in the surface core-level binding energy of the deposited metal interact strongly with carbon monoxide. In the case of Ni/Au (0(Ni) similar to 0.85), CO dissociates around 280 K. In contrast to this behavior, the interaction of CO with Pd/Mo or W, showing large positive shifts in the surface core-level binding energy, is very weak, and the CO desorption temperature is much lower than that from the clean Pd metal surface. The CO desorption temperature generally increases as the surface core-level shift of the deposited metal becomes more negative; the separation between the (5 sigma + 1 pi) and 4 sigma levels of CO also increases in this direction. These results suggest that the variation in the strength of interaction of CO with bimetallic overlayers is a chemical manifestation of the shift in the surface core-level binding energies of the deposited metals at monolayer coverages.

Bulk and surface core level binding energies in the Pt 80Fe 20(111) alloy

Core level photoemission spectroscopy of the Pt 4f 7 2 level is used to investigate the surface and bulk Pt environments in the ordered catalytic alloy Pt 80Fe 20. The results obtained support a LEED model of Pt segregation to the surface. This implies the presence of two Pt surface sites.

Investigation of the SMSI phenomenon with TiO 2/Ru/SiO 2 model-dispersed catalysts

The response of ternary SiO 2-supported Ru/TiO 2 catalysts to a series of characteristic treatments has been examined in detail by chemisorption measurements, XPS, SIMS, and HREM observations, appropriate comparisons being made with binary Ru/SiO 2 and Ru/TiO 2 reference samples. The SMSI state is shown to be air-sensitive and the active highly dispersed phases on the ternary TiO 2/ Ru/SiO 2 catalysts exhibit SMSI behavior even after low-temperature reduction. SIMS and XPS data indicate that the SMSI condition does not involve intermixing of the Ru and TiO 2 phases. Rather it is associated with highly dispersed metal entities whose XP spectra exhibit significant surface core-level binding energy shifts. Further aspects of the SMSI state are also discussed.

Surface science studies of the electronic and chemical properties of bimetallic systems

Recent studies dealing with the electronic, chemical, and catalytic properties of well-defined bimetallic surfaces (prepared by vapor-depositing one metal onto a crystal face of a second metal) are discussed. The result show that a metal atom supported on a dissimilar metal can be electronically perturbed and this perturbation can dramatically alter the chemical and catalytic properties of both constituents of the bimetallic system. In many cases, the metal adatoms exhibit properties toward the chemisorption of H{sub 2}, O{sub 2}, CO, and CO{sub 2} and reactivities toward small hydrocarbons that are significantly different from those seen for the pure metal. For supported monolayers of Ni, Cu, and Pd a correlation is observed between shifts in surface core-level binding energies and changes in the desorption temperature of CO from the metal adlayers. The shifts in core-level binding energies and CO desorption temperatures are a consequence of (1) electronic interactions between the metal overlayer and metal substrate and (2) variations that occur in the admetal-admetal interactions when the admetal adopts the lattice parameters of the substrate. Examples are provided which demonstrate the relevance of single-crystal studies for modeling the behavior of high surface area supported bimetallic catalysts. The coupling of an apparatus formore » the measurement of reaction kinetics at elevated pressures with an ultrahigh-vacuum system for surface analysis allows detailed studies of structure sensitivity, the effects of surface composition on catalytic activity, and, in certain cases, identification of reaction intermediates by postreaction analysis. The roles of ensemble' and ligand' effects in mixed-metal catalysts are discussed in the light of data obtained by using well-defined bimetallic surfaces.« less

Electronic interactions in bimetallic systems: Core-level binding energy shifts

The electronic properties of Cu, Ni, and Pd films supported on single-crystal metal surfaces have been examined by means of x-ray photoelectron spectroscopy (XPS). The shifts in core-level binding energies indicate that adatoms in a monolayer of Ni, Cu, or Pd are electronically perturbed with respect to surface atoms of Ni(100), Cu(100), or Pd(100). The present results show a correlation between the shifts in XPS surface core-level binding energies and variations in the desorption temperature of CO from metal adlayers. The shifts in XPS binding energies and CO desorption temperatures can be explained in terms of (1) variations that occur in the admetal–admetal interactions when the admetal adopts the lattice parameters of the substrate and (2) effects of the admetal–substrate interaction. Chemisorption of CO induces a large decrease in the electron density of the metal adlayers. This is a consequence of (1) charge transfer from the metal adlayer to the substrate (induced by a repulsive interaction between the admetal σ charge and electrons in the 5σ orbital of CO) and (2) transfer of electrons from the admetal into the 2π orbitals of CO (π backbonding).

Interaction of ultrathin films of copper with rhodium(100) and ruthenium(0001): an XPS study

The interaction of ultrathin films of Cu with Rh(100) and Ru(0001) has been examined by using X-ray photoelectron spectroscopy (XPS). The effects of surface annealing temperature, adsorbate coverage (film thickness), and CO chemisorption were investigated. The XPS data show that the atoms in a monolayer of Cu supported on Rh(100) or Ru(0001) are electronically perturbed with respect to the surface atoms of Cu(100). The magnitude of the electronic pertubations is larger for Cu/Rh(100). Measurements of the Cu(2p{sub 3/2}) XPS peak position of Cu/Rh(100) and Cu/Ru(0001) as a function of film thickness show that the Cu-Rh and Cu-Ru interactions affect the electronic properties of two or three layers of Cu atoms. The present results show a correlation between the shifts in the XPS surface core-level binding energies and the variations in the desorption temperatures of CO from Cu adlayers. The shifts in XPS binding energies and CO desorption temperatures can be explained in terms of (1) variations that occur in the Cu-Cu interaction when Cu adopts the lattice parameters of Rh(100) or Ru(0001) in a pseudomorphic adlayer and (2) modifications in the electronic properties of the Cu adatoms, caused by the Cu-Rh and Cu-Ru interactions. Chemisorption of CO induces a large more » decrease in the electron density of the Cu adlayers. « less

Surface core-level spectroscopy of Cu(100) and Al(100).

We present experimental and theoretical results for the surface core-level binding-energy shifts of Al(100), representative of an sp metal, and Cu(100), representative of a transition metal. Our analysis of these results leads to a unified interpretation for the different behavior of sp and transition metals. The d-electron contribution to smaller surface core-level binding energies is elucidated.

Constant final state measurements of surface core-level binding energy shifts: InP(110) and GaAs(110)

The surface core-level binding energy shifts have been obtained for the In4d and the P2p core-levels on the InP(110) surface. In agreement with previous studies of core-level shifts on the cleavage face of III–V semiconductors, the anion and cation shifts are of almost equal magnitude and are of opposite polarity (−0.31 and +0.30 eV respectively). The results are compared with a similar investigation of the GaAs(110) surface and discussed in terms of a recent calculation of surface core-level shifts for the (110) cleavage face of III–V semiconductors.

Constant final state measurements of surface core-level binding energy shifts: InP(110) and GaAs(110)

The surface core-level binding energy shifts have been obtained for the In 4d and the P2p core-levels on the InP(110) surface. In agreement with previous studies of core-level shifts on the cleavage face of III–V semiconductors, the anion and cation shifts are of almost equal magnitude and are of opposite polarity (−0.31 and +0.30 eV respectively). The results are compared with a similar investigation of the GaAs(110) surface and discussed in terms of a recent calculation of surface core-level shifts for the (110) cleavage face of III–V semiconductors.

Surface and bulk core level binding energy shifts in PtAu alloys

The surface core level shift of the 4 f 7 2 level in a Pt2%Au and a Pt15%Au alloy has been investigated using high resolution synchrotron radiation excited photoelectron spectroscopy. The bulk and surface components in the recorded spectra have been extracted using a curve fitting procedure. The chemical shifts and the bulk-to-surface intensity ratios are consistent with a strong enrichments of Au to the alloy surfaces. The chemical shifts on nonsegregated surfaces have been measured on these alloys and on a Pt90%Au alloy using conventional XPS.

Surface and bulk core level binding energy shifts in Pt-Au alloys

Surface and bulk core level binding energy shifts in Pt-Au alloys

Electronic properties and surface geometry of GaAs and ZnO surfaces

Some basic data and models for the low-index surfaces of GaAs and ZnO are briefly reviewed putting emphasis on LEED, Auger electron spectroscopy and photoemission. Angle-resolved UV photoelectron spectra are compared with initial-state calculations. Recent photoemission results on surface core-level binding energies are shown to measure potential changes at the surface. An oxidation study using a ring-shaped GaAs single crystal is discussed from which information about step heights on vicinal surfaces was evaluated. Finally, first results are presented which indicate that ARUPS from Xe adsorbed on ZnO at low temperature may supply structural information for the ZnO surfaces.

Electronic properties and surface geometry of GaAs and ZnO surfaces

Abstract Some basic data and models for the low-index surfaces of GaAs and ZnO are briefly reviewed putting emphasis on LEED, Auger electron spectroscopy and photoemission. Angle-resolved UV photoelectron spectra are compared with initial-state calculations. Recent photoemission results on surface core-level binding energies are shown to measure potential changes at the surface. An oxidation study using a ring-shaped GaAs single crystal is discussed from which information about step heights on vicinal surfaces was evaluated. Finally, first results are presented which indicate that ARUPS from Xe adsorbed on ZnO at low temperature may supply structural information for the ZnO surfaces.

Chemically shifted surface core-levels and surface segregation in EuAu and YbAu alloys

Chemically shifted surface core-level binding energies are observed for the rare-earth alloy component in Eu 1− x Au x and Yb 1− x Au x . Furthermore, these shifts are different from the chemical shifts of the bulk 4 f levels. The surface core-levels are used to identify surface segregation of the lanthanide metal in the present alloys.

Surface core-level binding energy shifts in alloys

A simple theory for the core-level binding energy shifts at the surfaces of binary alloys A x B 1− x is presented. Results are given for the surface core-level shifts of Ni in Ni x Cu 1− x alloys and Pd in Pd x Ag 1− x alloys. It has been shown that the surface core-level shifts may depend sensitively on surface segregation.