X-ray Photoelectron Intensities Research Articles

Correction to “Evaluating the Internal Structure of Core–Shell Nanoparticles Using X-ray Photoelectron Intensities and Simulated Spectra”

Correction to “Evaluating the Internal Structure of Core–Shell Nanoparticles Using X-ray Photoelectron Intensities and Simulated Spectra”

Evaluating the Internal Structure of Core-Shell Nanoparticles Using X-ray Photoelectron Intensities and Simulated Spectra.

The functionality of a new version of the National Institute of Standards and Technology database Simulation of Electron Spectra for Surface Analysis (SESSA) has been extended by implementing a new geometry engine. The engine enables users to simulate Auger-electron spectra and X-ray photoelectron spectra for different predefined morphologies (planar, islands, spheres, multi-layer core-shell particles). We compared shell thicknesses of core-shell nanoparticles derived from core-shell XPS peak intensities using Shard's method, which allows one to estimate shell thicknesses of core-shell nanoparticles, and a series of SESSA simulations for a wide range of nanoparticle dimensions. We obtained very good agreement of the shell thicknesses for cases where elastic scattering within the shell can be neglected, a result that is in accordance with the underlying assumptions of the Shard model. If elastic-scattering effects are important, there can be thickness uncertainties of up to 25 %. Experimental spectra of functionalized gold nanoparticles obtained by Techane et al. were analyzed with SESSA 2.0 both with respect to the relevant peak intensities as well as the spectral shape. Good agreement between experiment and theory was found for both cases. These results show that the single-sphere model for core-shell nanoparticles is valid when just using peak intensities, but more detailed modeling is needed to describe the inelastic background.

Surface Termination Conversion during SrTiO3 Thin Film Growth Revealed by X-ray Photoelectron Spectroscopy

Emerging electrical and magnetic properties of oxide interfaces are often dominated by the termination and stoichiometry of substrates and thin films, which depend critically on the growth conditions. Currently, these quantities have to be measured separately with different sophisticated techniques. This report will demonstrate that the analysis of angle dependent X-ray photoelectron intensity ratios provides a unique tool to determine both termination and stoichiometry simultaneously in a straightforward experiment. Fitting the experimental angle dependence with a simple analytical model directly yields both values. The model is calibrated through the determination of the termination of SrTiO3 single crystals after systematic pulsed laser deposition of sub-monolayer thin films of SrO. We then use the model to demonstrate that during homoepitaxial SrTiO3 growth, excess Sr cations are consumed in a self-organized surface termination conversion before cation defects are incorporated into the film. We show that this termination conversion results in insulating properties of interfaces between polar perovskites and SrTiO3 thin films. These insights about oxide thin film growth can be utilized for interface engineering of oxide heterostructures. In particular, they suggest a recipe for obtaining two-dimensional electron gases at thin film interfaces: SrTiO3 should be deposited slightly Ti-rich to conserve the TiO2-termination.

Quantitative AES IX and quantitative XPS II: Auger and x‐ray photoelectron intensities and sensitivity factors from spectral digital databases reanalysed using a REELS database

AbstractAn extension has been made of previous analyses of peak area intensities for elemental spectra in digital Auger and x‐ray photoelectron databases. The intensities, instead of being analysed after removal of a background derived from the Tougaard Universal cross‐section, are now analysed after removal of the extrinsic characteristic loss background calculated by deconvolving the relevant angle‐averaged reflected electron energy‐loss spectroscopy (REELS) spectrum. The angle‐averaged REELS data for each element are calculated from a digital REELS database using a recently defined scaling of the measured characteristic losses from the elastic peak. The new background removal procedure leads to an improvement in the correlation between experiment and theory for intensities in both AES and XPS. Analysis of these correlations shows that a systematic divergence remains for each element, which is the same for XPS as for AES. This divergence is attributed to an inadequacy either of the angle‐averaged REELS method or of the material‐to‐material dependence of the TPP‐2M equation used in the calculation of the inelastic mean free paths. Correction for this is possible in a new matrix‐less quantification formulation using average matrix sensitivity factors. This leads to correlations between experiment and theory with scatter factors of ×/÷1.08 and ×/÷1.11 for AES and XPS, respectively, for a wide range of elements and peaks. These scatter factors are much better than for previous correlations and underpin the choice of formulae to calculate the relevant theoretical intensity predictions. These calculations lead directly to values of average matrix sensitivity factors, appropriate for use with spectrometers giving true spectra or with those spectrometers that may be calibrated to provide such spectra. © Crown Copyright 2001. Published by John Wiley & Sons, Ltd.

Alloy formation at the Ni–Al interface for nickel films deposited on Al(110) surfaces

Alloy formation at the Ni–Al interface for thin nickel films deposited on Al(110) surfaces has been studied using high-energy ion scattering/channeling (HEIS) and X-ray photoelectron spectroscopy (XPS). For nickel atoms deposited at room temperature on Al(110), a large amount of nickel–aluminum intermixing occurs at the interface. For the first two monolayers (ML) of deposited nickel, an NiAl-like compound is formed. The intermixing continues with a different rate, forming an Ni 3Al-like compound for nickel coverages from 2 to 8 ML, at which point a nickel metal film begins to grow on the surface. Nickel atoms deposited at 250°C on the Al(110) surface exhibit no surface compound formation, but diffuse up to 400 Å into the aluminum substrate. Interatomic potentials based on the embedded-atom method (EAM) are used in a Monte Carlo approach to simulate the evolution of the Ni–Al(110) interface as a function of the nickel coverage. The calculated ion-scattering yields and X-ray photoelectron intensities from nickel and aluminum atoms in these simulated interfaces are in good quantitative agreement with the experimental results. The simulations show a high-density Ni–Al alloy forming at the Al(110) surface which apparently inhibits outward diffusion of aluminum, leading to the more nickel-rich alloy and finally nickel film growth. The ion-scattering simulations show an unusually large amount of backscattering occurring below the Ni–Al(110) interface, apparently associated with defocusing of the incident ion beam.

X-Ray Fluorescence Holography of SrTiO3 Compared with X-Ray Photoelectron Holography

Strontium Kα X-ray fluorescence intensity of SrTiO3 (001) single crystal is measured as a function of azimuthal and polar angles of detection. Anisotropy is observed due to the interference effect of fluorescent X-rays in the solid. X-Ray fluorescence holography is discussed from the viewpoint of chemical analysis. X-Ray photoelectron intensity anisotropy is compared with the present measurement.

Large solid angle X-ray photoelectron intensity distributions from CO, methoxy, and formate adsorbed on Ni(110)

Adsorbed layers of CO, methoxy (CH 3O) and formate (HCOO) on a clean Ni(110) surface are investigated by angle-resolved X-ray photoelectron spectroscopy. Large solid angle stereographic projections of C 1s or O 1s core level intensity measured for these adsorbates at 85 K show strong forward scattering enhancements that unambiguously and rather directly determine the molecular orientation, and in the case of CO, the order within the layer. CO in the (2×1) p2mg structure and methoxy are tilted by 20° and 30° relative to the normal, respectively, with the projection of the tilt in the 〈001〉 azimuth. Formate is bonded through oxygen in the bidendate configuration, aligned along the 〈110〉 azimuth. The O–C–O bond angle is determined as 124°. These results are in good agreement with previous data.

Incorporation of Surface Topography in the XPS Analysis of Curved or Rough Samples Covered by Thin Multilayers

Approaches are described for the analysis of XPS signals from samples comprising thin conformal coatings on substrates with non-ideal surface topography. In particular, attention is focused on arbitrarily shaped rough or curved substrates. Relations are derived for the relative x-ray photoelectron intensities emitted from various elements in such non-ideal samples. The surface topography of arbitrarily rough samples is analysed by atomic force microscopy to provide a frequency histogram of the local slopes, which is incorporated into a global, multi-element analysis method for interpretation of the observed XPS elemental intensities. An analogous approach is used for the analysis of curved samples. Our algorithms also enable simultaneous analysis of successively coated multilayer structures. When the elemental composition of the individual layers is known, the fit procedure enables determination of the thickness values of single-layer and multilayer coatings. The range of validity of the theory is discussed. © 1997 by John Wiley & Sons, Ltd.

Valence photoelectron diffraction and direct-transition effects.

We have measured the temperature dependence of valence and core x-ray photoelectron intensities for W(110). At 295 K, valence spectra are very sensitive to emission direction due to direct-transition effects, and Ienergy-integratedR valence and core photoelectron diffraction curves are similar in form, but with significant differences in fine structure. At 803 K, valence spectra are by contrast very insensitive to direction due to increased zone averaging, but valence and core diffraction curves retain differences due to the atomic-orbital character of the initial and final states involved.

Correlation of the Cd-to-Te ratio on CdTe surfaces with the surface structure.

We report here that reconstruction on (100), (111)A, and (111)B CdTe surfaces is either c(2\ifmmode\times\else\texttimes\fi{}2), (2\ifmmode\times\else\texttimes\fi{}2), and (1\ifmmode\times\else\texttimes\fi{}1) or (2\ifmmode\times\else\texttimes\fi{}1), (1\ifmmode\times\else\texttimes\fi{}1), and (1\ifmmode\times\else\texttimes\fi{}1) when they are Cd or Te stabilized, respectively. There is a mixed region between Cd and Te stabilization in which the reflected high-energy electron-diffraction (RHEED) patterns contain characteristics of both Cd- and Te-stabilized surfaces. We have also found that the Cd-to-Te ratio of the x-ray photoelectron intensities of their 3${\mathit{d}}_{3/2}$ core levels is about 20% larger for a Cd-stabilized (111)A, (111)B, or (100) CdTe surface than for a Te-stabilized one. According to a simple model calculation, which was normalized by means of the photoelectron intensity ratio of a Cd-stabilized (111)A and a Te-stabilized (111)B CdTe surface, the experimental data for CdTe surfaces can be explained by a linear dependence of the photoelectron-intensity ratio on the fraction of Cd in the uppermost monatomic layer. This surface composition can be correlated with the surface structure, i.e., the corresponding RHEED patterns. This correlation can in turn be employed to determine Te and Cd evaporation rates. The Te reevaporation rate is increasingly slower for the Te-stabilized (111)A, (111)B, and (100) surfaces, while the opposite is true for Cd from Cd-stabilized (111)A and (111)B surfaces. In addition, Te is much more easily evaporated from all the investigated surfaces than is Cd, if the substrate is kept at normal molecular-beam-epitaxy growth temperatures ranging from 200 \ifmmode^\circ\else\textdegree\fi{}C to 300 \ifmmode^\circ\else\textdegree\fi{}C.

Implication of X-ray photoelectron intensities for a layer model of GaAs surface

Intensities of X-ray photoelectron transitions from the As 3 d level are theoretically analysed taking into account many-electron interactions. The latter interactions profoundly affect the relative intensities of the transitions as well as binding energies. Implications for the surface structure of GaAs are then discussed and in particular we show that As atoms having the same chemical environment are spread over distances into the surface comparable to the interatomic spacing. This supports a recently proposed layer model for the untreated surface of GaAs.

Chemisorption geometry ofc(2×2)oxygen on Cu (001) from angle-resolved core-level x-ray photoemission

The x-ray photoelectron intensity of the $1s$ core level of adsorbed oxygen in the form of a $c(2\ifmmode\times\else\texttimes\fi{}2)$ overlayer on Cu (001) exhibits strong angular dependence due to final-state diffraction at low electron take-off angles. The azimuthal anisotropies of the intensity, measured as $\frac{({I}_{max}\ensuremath{-}{I}_{min})}{{I}_{max}}$, are 24%, 23%, 16%, 13%, and 18% for scans at fixed electron take-off angles of 7\ifmmode^\circ\else\textdegree\fi{}, 10\ifmmode^\circ\else\textdegree\fi{}, 13\ifmmode^\circ\else\textdegree\fi{}, 15\ifmmode^\circ\else\textdegree\fi{}, and 17\ifmmode^\circ\else\textdegree\fi{} with respect to the surface, respectively. A strong sensitivity of the azimuthal pattern to the electron take-off angle is also noted. Similar azimuthal anisotropies measured on the substrate Cu $3p$ and $2{p}_{\frac{3}{2}}$ core levels are considerably stronger, with $\frac{({I}_{max}\ensuremath{-}{I}_{min})}{{I}_{max}}\ensuremath{\lesssim}73%$ at electron take-off angles from 7\ifmmode^\circ\else\textdegree\fi{} to 45\ifmmode^\circ\else\textdegree\fi{}. Single-scattering cluster (SSC) calculations have been performed for both substrate and adsorbate core emission. The calculated azimuthal patterns for the substrate core emission show very good agreement with experiment at low electron take-off angles and reasonable agreement at high electron take-off angles, thereby verifying the applicability of the SSC model to the adsorbate core emission as well. Within several choices of the oxygen overlayer geometry tentatively proposed previously, the SSC results for the adsorbate core emission show by far the best agreement with experiment for oxygen atoms in fourfold-hollow sites, coplanar with the first Cu layer. The validity of various approximations made in the SSC model is also discussed, together with the estimated accuracy of the method.

Relation among x-ray photo-electron intensities, cross-sections, and kinetic energies in insulating solids

Relation among x-ray photo-electron intensities, cross-sections, and kinetic energies in insulating solids

Quantitative X-ray photoelectron diffraction studies of single-crystal silicates

The interpretation of angle-resolved X-ray photoelectron intensity measurements on single crystals which contain an element in more than one structurally distinct location, each of which is individually characteristic of a different element, to yield directly quantitative information concerning the extent of isomorphous substitution in such crystals is described. These principles are applied to quantify the distribution of aluminum between tetrahedrally and octahedrally coordinated sites in four 2:1 layered silicates. An accuracy of ⩽ 5% is demonstrated by comparison with a less direct surface analytical route to the same information.

Relation among x-ray photoelectron intensities, cross sections and kinetic energies in insulating solids

The fundamental relation among X-ray (MgK α) photoelectron intensities, theoretical orbital ionization cross sections, and photoelectron kinetic energies has been investigated through precisely measured relative intensities of orbital electrons from sublimed-condensed samples of KF, NaF, CaF 2 and KG, and other ionic solids. Although the results confirm within approximately 15% the relation and establish a linear dependence of the intensities on analyzer transmission, they suggest that either a slight deviation from the relation exists or a cancellation of errors occurs. Empirically it is found that the relative intensities are proportional to the square of the ratio of kinetic energies, and with this relation, the intensities agree with the theoretical cross sections within a few per cent. This relation suggests that the dependence on the angular parameter is inconsequential and that the mean free path is proportional to the kinetic energy.

Azimuthal Anisotropy in Deep Core-Level X-Ray Photoemission from an Adsorbed Atom: Oxygen on Copper(001)

For oxygen adsorbed on a Cu(001) surface, the O $1s$ x-ray photoelectron intensity is found to exhibit a complex azimuthal anisotropy with an overall amplitude of \ensuremath{\sim} 26% for a grazing 10\ifmmode^\circ\else\textdegree\fi{} angle of emission. No significant anisotropy is observed at 45\ifmmode^\circ\else\textdegree\fi{}. Several reasons for expecting a direct relationship between such final-state scattering effects and atomic geometry are discussed, and the data are found to be suggestive of a surface-chemisorbed $c(2\ifmmode\times\else\texttimes\fi{}2)$ oxygen overlayer in fourfold coordination.

On the influence of surface-roughness on X-ray photoelectron intensities

Abstract Surface roughness decreases the fluorescent intensities in XRFA (X-ray fluorescence analysis). We expect the same influence for XPS (X-ray photoelectron spectrometry). Therefore, the cube model, which was applied successfully to XRFA, is used for a quantitative description in the case of XPS as well. In spite of the fact that the agreement between theory and experiment for XPS is not as good as it was for XRFA, we learn that sample preparation should be a careful polishing procedure, which is sufficient to get results not influenced by surface roughness.