Synchrotron Radiation Excited Photoelectron Research Articles

Defect-Induced Water Bilayer Growth on Anatase TiO2(101).

Preparing an anatase TiO2(101) surface with a high density of oxygen vacancies and associated reduced Ti species in the near-surface region results in drastic changes in the water adsorption chemistry compared to adsorption on a highly stoichiometric surface. Using synchrotron radiation excited photoelectron spectroscopy, we observe a change in the water growth mode, from layer-by-layer growth on the highly stoichiometric surface to bilayer growth on the reduced surface. Furthermore, we have been able to observe Ti3+ enrichment at the surface upon water adsorption. The Ti3+ enrichment occurs concomitant with effective water dissociation into hydroxyls with a very high thermal stability. The water bilayer on the reduced surface is thermally more stable than that on the stoichiometric surface, and it is more efficient in promoting further water dissociation upon heating. The results thus show how the presence of subsurface defects can alter the wetting mechanism of an oxide surface.

Experimental and theoretical study of 3 p photoionization and subsequent Auger decay in atomic chromium

Abstract 3p photoionization and subsequent low kinetic energy Coster–Kronig and super Coster–Kronig Auger decay have been studied in atomic chromium. The binding energies, line widths, and relative intensities for the transitions seen in the synchrotron radiation excited 3p photoelectron spectrum are determined. The high resolution M 2 , 3 M 4 , 5 M 4 , 5 and M 2 , 3 M 4 , 5 N 1 Auger electron spectra following the electron impact excited 3p ionization are presented and the kinetic energies, relative intensities, and identifications are given for the main lines. The experimental findings are compared with the theoretical predictions obtained from Hartree–Fock and multiconfiguration Dirac–Fock approaches.

Breakdown of ionic character of molecular alkali bromides in inner-valence photoionization.

The inner-valence region of alkali bromide XBr (X=Li, Na, K, Rb) vapours has been studied experimentally by means of synchrotron radiation excited photoelectron spectroscopy. Experimental spectra were analyzed by comparing them with available theoretical results and previous experiments. Ionic character of alkali bromides is seen to change in the inner-valence region with increasing atomic number of the alkali atom. A mechanism involving mixing between Br 4s and Rb 4p orbitals has been suggested to account for the fine structure observed in inner-valence ionization region of RbBr.

In Situ XPS Studies of Electrochemically Positively Polarized Molybdenum Carbide Derived Carbon Double Layer Capacitor Electrode

Electrochemical processes at positively polarized Mo2C derived carbon (C(Mo2C)) electrical double layer capacitor (EDLC) electrode in 1-Ethyl-3-Methylimidazolium tetrafluoroborate ionic liquid at different cell potentials have been studied using in situ synchrotron radiation excited X-ray photoelectron spectroscopy (in-situ XPS) method. Symmetric two-electrode cells were studied using in situ XPS method within very wide cell potential region up to 4.0 V. At cell potentials higher than 2.6 V the cross-over of the limit of the ideal polarizability of the capacitor electrodes has been observed and different faradaic processes have been verified by the cyclic voltammetry and in situ XPS methods. Thus, the supercapacitor cell potential vs. in-situ XPS signal relationship have been analysed identifying the possible electrochemical reactions taking place at the positively polarized working electrode | ionic liquid interface at the extreme EDLC cell potentials applied.

3p photoionization and subsequent Auger decay of atomic gallium

The 3p photoionization and subsequent Coster–Kronig type of Auger decay of initially neutral atomic gallium were studied both experimentally and theoretically. The binding energies and relative intensities of the 3p photoelectron spectrum together with a satellite structure are given. The M2, 3MM and M2, 3MN Auger electron spectrum leading to doubly ionized final states is presented. The synchrotron radiation excited photoelectron and Auger electron spectra were interpreted using multiconfiguration Dirac–Fock calculations providing spectral identification for the main observed features.

Binding of oxygen on vacuum fractured pyrite surfaces: Reactivity of iron and sulfur surface sites

Synchrotron radiation excited photoelectron spectroscopy (SXPS) has been used to study the interaction of oxygen with vacuum fractured pyrite surfaces. Especially valence band spectra obtained with 30eV photon energy were analyzed to provide a mechanism of the incipient steps of pyrite oxidation. These spectra are far more sensitive to the oxidation than sulfur or iron core level spectra. It is shown that oxygen is adsorbed on Fe(II) surface sites restoring the octahedral coordination of the Fe(II) sites. This process leads to the removal of two surface states in the valence band which are located at the low and high binding energy sides of the outer valence band, respectively. The existence of these surface states which have been proposed by calculations is experimentally proven. Furthermore, it is shown, that the sulfur sites are more reactive than expected. Sulfite like species are already formed after the lowest oxygen exposure of 10L. This oxidation occurs at sulfur sites neighboring the Fe(II) surface sites. Oxidation of the S2− surface sites which were considered as the most reactive species in former studies is second. No iron(III) oxides are formed during oxygen exposure, supporting the assumption that water plays an important role in the oxidation mechanism of pyrite surfaces.

In Situ XPS Studies of Electrochemically Negatively Polarized Molybdenum Carbide Derived Carbon Double Layer Capacitor Electrode

Electrochemical processes at negatively polarized electrical double layer capacitor (EDLC) electrode at different cell potentials have been studied using in situ synchrotron radiation excited X-ray photoelectron spectroscopy (XPS). 1-Ethyl-3-methylimidazolium tetrafluoroborate (EMImBF4) room-temperature ionic liquid (RTIL), as an electrolyte, and Mo2C derived carbon (C(Mo2C)) based micromesoporous electrodes, as an EDLC electrodes, were studied within very wide cell potential region (up to 4.0 V). To store more charge in a RTIL based EDLC, higher cell potentials have been applied leading to the cross-over of the limit of the ideal polarization of the capacitor electrodes and to the initiation of different faradaic processes. Therefore, parallel to XPS measurements, the cyclic voltammetry was used to obtain electrochemical data, correlated with previously calculated electrochemical (including electrochemical impedance spectroscopic) characteristics for C(Mo2C) | RTIL interface. In this paper we have, according to our knowledge, first polarized the RTIL based EDLC electrodes in real in situ vacuum conditions, measured and analyzed the supercapacitor two-electrode cell potential vs. XPS spectra relationship and discussed in the light of XPS data possible electrochemical reactions taking place at the negatively charged working electrode | RTIL interface at the different cell potential applied.

Photoelectron spectroscopy of unsupported bismuth clusters: Size related effects of metallic properties

Evolution of metallic properties of free and initially neutral nanoscale Bi clusters has been studied using synchrotron radiation excited photoelectron spectroscopy. The 4f and 5d core as well as the valence levels have been probed. The cross-level analysis indicates metallic properties in Bi clusters in the observed size range from 0.5 nm to 1.4 nm. The behavior of the core-level and valence binding energies as a function of cluster size has been observed to be smooth and relatively consistent. Valence responses for the largest clusters have their shape and width similar to those of the polycrystalline solid Bi.

Layer-resolved photoelectron diffraction: Electron attenuation anisotropy in GaAs

Photoelectron diffraction was used to get polar plots of the low-energy photoemission intensities from AlAs monolayer buried 2, 3 and 4 GaAs monolayers below the GaAs(001)-c(4×4) reconstructed surface. Unexpected differences in polar plots of the layer-resolved, synchrotron radiation excited photoelectron emission from even and odd AlAs monolayers were observed experimentally and obtained theoretically. This 26eV kinetic energy photoelectron emission is accompanied by electron attenuation anisotropy and a non-exponential intensity decay in the near-surface region. The importance of the top surface layer and its reconstruction is demonstrated by theoretical simulations. Enhanced contributions of photoelectrons from individual subsurface layers in specific directions offer a way to investigate the desired subsurface atomic layers.

Atom–solid binding energy shifts for K 2p and Rb 3d sublevels

Abstract Binding energy shifts between free and solid state atoms for K 2p and Rb 3d photolines have been determined by measuring the vapor and solid state spectra simultaneously in similar experimental conditions applying synchrotron radiation excited photoelectron spectroscopy. This method has the important benefit that the work function is not needed to correct for different reference energy levels, therefore much more accurate values for binding energy shifts are obtained.

Direct experimental determination of atom–molecule–solid binding energy shifts for Sb and Bi

Binding energy shifts Bi–Bi2–Bi (solid) and Sb–Sb2–Sb4–Sb (solid) have been measured for the first time directly for Bi4f and Sb4d core levels by applying synchrotron radiation excited photoelectron spectroscopy. Atomic, molecular and solid state spectra are obtained by varying the temperature of evaporated metal vapours by means of a special multichamber oven and letting the vapour condensate on a cool solid tip. Vapour and solid state spectra are created simultaneously in similar experimental conditions, making direct and accurate shift determinations possible. The effects of the multiplet fine structure of the open shell atoms are pointed out.

Size-varied photoelectron spectroscopy of metal clusters using the Exchange Metal Cluster Source

The recently developed Exchange Metal Cluster Source (EXMEC) is described. The source is designed for electron-spectroscopic studies of neutral metal clusters. Preliminary results on the rubidium and tin clusters using synchrotron radiation excited photoelectron spectroscopy show the capability for cluster production in the size regime of few tens to few hundred of atoms per cluster. A wide range of metal evaporation temperatures is covered applying resistive and induction heating methods.

Initial stages of ZrO2 chemical vapor deposition on Si(1 0 0)-(2 × 1) from zirconium tetra-tert-butoxide

The initial stages of chemical vapor deposition of ZrO2 from zirconium tetra-tert-butoxide (ZTB) on Si(100)-(2×1) have been studied by scanning tunnelling microscopy (STM) and synchrotron radiation excited photoelectron spectroscopy (PES). The STM images and core level (PES) spectra indicate that the predominant surface modifications induced by ZTB are due to silicon carbonization and formation of zirconium dioxide. The carbonization reaction leads to formation of subsurface carbon and two types of reconstructions are discussed: dimer vacancies and dimer vacancies in conjunction with a rotated surface Si-dimer. Indications for the formation of small amounts of zirconium silicide are also found. No evidence for silicon oxidation can be observed with PES, in contrast to the interface properties previously found after larger exposures to ZTB.

XPS and AFM investigation of hafnium dioxide thin films prepared by atomic layer deposition on silicon

The nucleation and growth of technologically important hafnia (HfO 2) ultrathin films on Si-substrates and formation of interface layers were investigated using ex situ synchrotron radiation excited photoelectron spectroscopy (XPS) and atomic force microscopy (AFM) techniques. Atomic layer deposition based on HfCl 4–H 2O and HfI 4–O 2 precursor systems was applied to prepare the films. XPS studies showed that the deposition temperature had substantial effect on the first stages of the film growth as well as formation of the interface layers. Considerable delay of hafnia film growth and Hf-silicate (HfSi x O y ) interface layer formation were observed at high temperature (600 °C) whereas at lower temperature (300 °C), the film started to grow during the first growth cycles and the silicate interface layer was absent. AFM investigations affirmed the XPS results, and showed also that the surface topography had to be known for proper interpretation of XPS data.

Multielectron effects in 4p photoionization of atomic Cs

The effect of electron correlation has been studied in the case of 4p photoionization of atomic cesium. High resolution synchrotron radiation excited photoelectron spectrum of vapour phase Cs has been measured and compared to the theoretical predictions. The multiconfiguration Dirack–Fock method has been applied in the calculations.

Metal–support interactions in systems palladium deposited on oxidized tungsten surfaces

The morphology of the palladium (Pd) overlayers on oxidized tungsten (W) tips has been studied by Field Emission Microscopy (FEM). The effect of thermal treatment on the interaction of Pd with the support and chemisorption of CO on variously treated Pd-containing samples has been investigated. The results are discussed in relation to complementary macroscopic experiments by synchrotron radiation excited photoelectron spectroscopy (SRPES) and thermally programmed desorption (TPD) of carbon monoxide (CO) on a polycrystalline W foil. A distinct influence of support pre-oxidation on the Pd layer growth has been demonstrated. Two types of oxidized supports have been used: tungsten with oxygen pre-adsorbed at room temperature (RT) and then heated to 700 K (WO x /W (RT) system) and tungsten oxidized at 1300 K (WO x /W (1300 K) system) in situ. The surface of WO x /W (1300 K) sample is fully oxidized in contrast to WO x /W (RT), where the presence of un-oxidized patches has been demonstrated by SRPES measurements. A Pd layer grows on the WO x /W (RT) surface mostly on the densely populated planes (1 1 0) and (2 1 1) of the W tip. Heating of this system up to 700 K results in disaggregation of the original Pd layer. Pd clusters on the tungsten tip oxidized at 1300 K are localized on the atomically rough (1 1 1) plane. The observed differences in CO adsorption on the aforementioned types of investigated samples can be attributed to differences in the chemical nature of their surfaces.

Photoelectron Spectroscopic Investigations of Chemical Bonding in Organically Stabilized PbS Nanocrystals

The surface structure of organically capped PbS nanocrystals using synchrotron radiation excited core-level photoelectron spectroscopy has been studied. The nanocrystallites prepared by methods of colloidal chemistry have average diameters of 3.1, 3.9, 4.6, and 7.6 nm with narrow size distributions and are stabilized either with oleic acid only or with a combination of trioctylphosphine and oleic acid as ligands. High resolution photoelectron spectroscopy measurements allowed the surface structure to be studied and in particular how the organic ligands bind to the surface of the PbS nanocrystals to be elucidated. The results indicate that the trioctylphosphine ligands passivate only the surface S sites while oleic acid ligands appear to bind mainly to Pb sites.

Multielectron effects in the4dphotoionization of atomic indium

Observed intensity distribution in the high resolution synchrotron radiation excited 4d photoelectron spectrum of In is compared with the predictions based on the relativistic single configuration (SC) Dirac-Fock calculations. Many-electron effects beyond the SC approximation were found to cause additional fine structures in some of the photoelectron lines.

Solid-state reaction between Pr and SiO 2 studied by photoelectron spectroscopy and ab initio calculations

We report on the structural and electrical properties of Pr-based high- k dielectric films fabricated by solid-state reaction between metallic Pr and SiO 2 underlayers. A non-destructive depth profiling using synchrotron radiation excited photoelectron spectroscopy (SR–PES), X-ray photoelectron spectroscopy (XPS) and transmission electron microscopy (TEM) were employed to examine the chemical composition and microstructure. Ab initio calculations were done to gain insight into the physical processes involved. SR-PES results indicate that Pr deposition at room temperature (RT) leads to the formation of a Pr silicide and a Pr oxide, what is in good agreement with the scenario expected from ab initio calculations. As revealed by TEM and electrical measurements, oxidation of the reacted structures, followed by annealing, results in a stacked dielectric composed of a SiO 2-based buffer with an enhanced permittivity and a Pr silicate film with a high dielectric constant. The leakage current density of 10 −4 A/cm 2 was measured for stacks with capacitance equivalent thickness (CET) of 1.5 nm prepared by evaporation of the Pr layer on a 1.8 nm SiO 2 film, followed by oxidation in air ambient and annealing in N 2 atmosphere. The capacitance-voltage ( C– V) curves exhibit a large flatband voltage ( V FB) shift indicating the presence of a positive charge in the stack. Switching away from the Al contacts to Au gate electrodes introduces a significant reduction of the V FB by 1.3 eV, what is much more than the change expected from the work function difference between Al and Au (∼0.9 eV). This in turn implies that V FB is strongly affected by the gate interface electrode.

Ab initio study of valence correlation satellites in photoelectron spectra of neon

Ab initio calculations of energies and widths of the valence correlation satellites in the binding energy region between 48 and 99 eV of the photoelectron spectrum of neon are presented. The results are compared with the high-resolution, synchrotron radiation excited valence photoelectron spectra recorded by Kikas et al. [ J. Electron Spectrosc. Relat. Phenom. 77 (1996) 241] using photon energies of 96 and 169 eV and with previous synchrotron radiation and conventional radiation studies. Our calculations allow a deeper insight in the regions where a bunch of states, different in spin and angular momentum, are concentrated, and add considerable information on assignment and width of several transitions. The theoretical spectrum calculated in the region between 74 and 94 eV compares quite well with the experimental one.