XPS Peak Shape Research Articles

Determination of electronic properties of nanostructures using reflection electron energy loss spectroscopy: Nano-metalized polymer as case study

In this work, Au was deposited with nominal effective thickness of 0.8nm on polystyrene (PS) at room temperature. According to previous study, using XPS peak shape analysis [S. Hajati, V. Zaporojtchenko, F. Faupel, S. Tougaard, Surf. Sci. 601 (2007) 3261–3267], Au nanoparticles (Au-NPs) of sizes 5.5nm were formed corresponding to such effective thickness (0.8nm). Then the sample was annealed to 200°C, which is far above the glass transition of PS. At this temperature, the Au-NPs were diffused within the depth 0.5nm–6.5nm as found using nondestructive XPS peak shape analysis. Electrons with primary energy 500eV were used because the electronic properties will then be probed in utmost surface (∼1 IMFP range of depths that is 1.8nm for PS). By using QUEELS software, theoretical and experimental electron inelastic cross section, energy loss function, electron inelastic mean free path and surface excitation parameters were obtained for the sample. The information obtained here, does not rely on any previously known information on the sample. This means that the method, applied here, is suitable for the determination of the electronic properties of new and unknown composite nanostructures.

Three-Dimensional X-Ray Photoelectron Tomography on the Nanoscale: Limits of Data Processing by Principal Component Analysis

In a previous article, we studied the influence of spectral noise on a new method for three-dimensional X-ray photoelectron spectroscopy (3D XPS) imaging, which is based on analysis of the XPS peak shape [Hajati, S., Tougaard, S., Walton, J. & Fairley, N. (2008). Surf Sci 602, 3064-3070]. Here, we study in more detail the influence of noise reduction by principal component analysis (PCA) on 3D XPS images of carbon contamination of a patterned oxidized silicon sample and on 3D XPS images of Ag covered by a nanoscale patterned octadiene layer. PCA is very efficient for noise reduction, and using only the three most significant PCA factors to reconstruct the spectra restores essentially all physical information in both the intensity and shape of the XPS spectra. The corresponding signal-to-noise improvement was estimated to be equivalent to a reduction by a factor of 200 in the required data acquisition time. A small additional amount of information is obtained by using up to five PCA factors, but due to the increased noise level, this information can only be extracted if the intensity of the start and end points for each spectrum are obtained as averages over several energy points.

Non-Destructive Depth Profiling by XPS Peak Shape Analysis

It is well known that the usual procedure for quantification by electron spectroscopy that is based on measured peak intensities is highly unreliable. An improved method is to take into account that the peak shape in a wide energy range, on the low kinetic energy side of the peak varies considerably with the surface morphology on the nano-meter depth scale. This observation has in recent years been applied in the formulation of a by now well known method for quantification that is based on quantitative analysis of measured peak shapes. The technique is sensitive on the 1 - 10 nm depth scale and it is non-destructive. The method suggested for the extraction of quantitative information from the large variation of the inelastic background with atom depth distribution will be reviewed briefly here. An example of practical application of the technique is also shown.

Surface structure of Sn-doped In2O3 (111) thin films by STM

High-quality Sn-doped In2O3 (ITO) films were grown epitaxially on yttria stabilized zirconia (111) with oxygen-plasma assisted molecular beam epitaxy (MBE). The 12 nm thick films, containing 2–6% Sn, are fully oxidized. Angle-resolved x-ray photoelectron spectroscopy (ARXPS) confirms that the Sn dopant substitutes In atoms in the bixbyite lattice. From XPS peak shape analysis and spectroscopic ellipsometry measurements it is estimated that, in a film with 6 at.% Sn, ∼1/3 of the Sn atoms are electrically active. Reflection high energy electron diffraction (RHEED) shows a flat surface morphology and scanning tunneling microscopy (STM) shows terraces several hundred nanometers in width. The terraces consist of 10 nm wide orientational domains, which are attributed to the initial nucleation of the film. Low energy electron diffraction (LEED) and STM results show a bulk-terminated (1 × 1) surface, which is supported by first-principles density functional theory (DFT) calculations. Atomically resolved STM images are consistent with Tersoff–Hamann calculations that show that surface In atoms are imaged bright or dark, depending on the configuration of their O neighbors. The coordination of surface atoms on the In2O3(111)–1×1 surface is analyzed in terms of their possible role in surface chemical reactions.

Characterization of Au nano-cluster formation on and diffusion in polystyrene using XPS peak shape analysis

XPS peak shape analysis is used as a novel and nondestructive method to study Au nano-cluster growth mechanism on polystyrene (PS) as a function of deposition as well as diffusion and distribution of the nano-clusters in PS as a function of subsequent annealing at temperatures in the range from room temperature to above the glass transition temperature of PS. The Au nano-cluster size and density are determined for four different amounts of Au deposition. It is shown that this nondestructive method can give all mentioned information on such a metalized polymer without the need for any other complimentary and time consuming technique such as AFM, TEM and the destructive technique XTEM. Thus this method is suitable to monitor and control the degree of intermixing of metal nano-clusters and polymers which is of high technological interest.

Study of the first nucleation steps of thin films by XPS inelastic peak shape analysis

AbstractThe initial steps in the formation of thin films have been investigated by analysis of the peak shape (both inelastic background and elastic contributions) of X‐ray photoelectron spectra. Surface coverage and averaged height of the deposited particles have been estimated for several overlayers (nanometre range) after successive deposition cycles. This study has permitted the assessment of the type of nucleation and growth mechanisms of the films. The experiments have been carried out in situ in the preparation chamber of an XPS spectrometer. To check the performance of the method, several materials (i.e. cerium oxide, vanadium oxide and cadmium sulfide) have been deposited on different substrates using a variety of preparation procedures (i.e. thermal evaporation, ion beam assisted deposition and plasma enhanced chemical vapour deposition). It is shown that the first deposited nuclei of the films are usually formed by three‐dimensional particles whose heights and degree of surface coverage depend on the chemical characteristics of the growing thin film and substrate materials, as well as the deposition procedure. It is concluded that XPS peak shape analysis can be satisfactorily used as a general method to characterize morphologically the first nanometric moieties that nucleate a thin film. Copyright © 2006 John Wiley & Sons, Ltd.

Determination of amount of substance for nanometre‐thin deposits: consistency between XPS, RBS and XRF quantification

AbstractQuantification on the nanometre scale is a key task in quality control and for the development of new materials in nanotechnology. In this paper we have studied the consistency in the determination of the amount of substance found by XPS peak shape analysis, Rutherford backscattering spectroscopy (RBS) and x‐ray fluorescence spectrometry (XRF). To this end, ZnO was deposited by plasma‐enhanced chemical vapour deposition on the three substrates. Four different sets of samples were produced, with the amount of ZnO deposited in the range 1–10 nm. From XPS analysis it is found that ZnO grows in the form of islands on all three substrates. For each system, the analysis was done independently with two XPS peaks from the overlayer with widely different kinetic energy and one XPS peak from the substrate. The growth mechanism found from analysis of each of the three peaks was consistent and the total amount of determined ZnO material was identical to within 15%. The root‐mean‐square deviation from the XPS quantification of the relative AOS was 20% for XRF and 16% for RBS. Because the absolute amount of substance determined from analysis of the three XPS peaks for each sample was consistent, it is concluded that the energy dependence of the applied inelastic mean free paths (taken here from the empirical TPP‐2M formula) is correct. It was found that the absolute amounts of substance determined by RBS and XRF are consistently factors of 2.1 and 1.5 lower than the that determined by XPS peak shape analysis. It is suggested that the main reason for this large discrepancy is inaccuracy in the applied ‘effective’ inelastic electron mean free path. Copyright © 2003 John Wiley & Sons, Ltd.

XPS study of the adsorption of NH3 on nickel oxide on Ni(111)

The aim of the present work was to investigate the initial stages of NH3 interaction with nickel oxide layers formed on well-defined Ni(111) surfaces. The selected technique was x-ray photoelectron spectroscopy. Ammonia adsorption was performed at 10−7 mbar and room temperature in a preparation chamber attached to the analysis chamber. After each adsorption, the N 1s core level was analysed and its intensity was plotted as a function of exposure (at the same pressure) to obtain the adsorption kinetnetics. The XPS peak shapes and the chemical shifts were related to the nature of the adsorption products. In our experimental conditions, we observed that the more reactive initial surface was the one covered by the O-adsorbed phase corresponding to the precursor state of the NiO formation. The NiO alone does not exhibit significant reactivity, whereas a mixture of NiO and the O-adsorbed phase shows a similar behaviour to that of the pure O-adsorbed phase. From the first adsorption until the last one, two nitrogen adspecies were detected: one with an N 1s binding energy (BE) of 397.8 eV and the other at 399.7 eV. The data indicate that the surface reactivity is structure-sensitive for the NiO layer and the mechanisms of adsorption seem to involve NH3 dissociation into NHx adspecies, along with OH formation. Copyright © 2000 John Wiley & Sons, Ltd.

ESCA Studies of Yttrium Orthoaluminum Perovskites

Room temperature X-ray photoelectron spectroscopy (XPS or ESCA) has been employed to investigate pure single crystals of yttrium aluminum perovskite (YAlO3 (YAP)) and YAlO3 doped with several rare earth elements, Y1-xLnxAlO3 (Ln = Pr, Er, Ho). XPS peak shapes of the main elements forming the YAP structure, in structures that have been doped with ions of varying oxidation states, are rationalized as a function of structural environment and the oxidation state of doping ions. The significant changes in chemical shifts and full widths at half-maximum of the signals are caused by doping and the presence of color centers. The most distinctive change in chemical shifts is observed for the O(1s) peaks. Annealing in H2/N2 atmosphere (1200 °C) changes the color of the sample, removing color centers due to relocation of oxygen ions in the structure. Although the total amount of oxygen in the samples does not change, the relative ratio of oxygen ions occupying two independent positions in the structure changes on an...

ESCA Studies of Yttrium Aluminum Garnets

X-ray photoelectron spectroscopy (XPS/ESCA) has been employed to investigate pure single crystals and powdered samples of yttrium aluminum garnet, Y3Al5O12 (YAG), and YAG crystals doped with several rare earth elements (Ln = Pr, Er, Ho, Tm) and a transition metal (Cr). Core level XPS peak shapes of the main elements forming the garnet structure can be rationalized due to different structural environments of particular ions modified by doping. The change of dopant oxidation state also results in variation of XPS peaks and helps to identify the sites in which it takes place. Single-crystal and powder samples give different XPS spectra. Possible sources of these differences are discussed. Similarities and differences between simple and mixed oxides are shown. The structure of YAG suggests the presence of only one independent oxygen ion; however the O(1s) spectra of all YAG systems exhibit two readily discerned peaks. An explanation for this dichotomy is discussed, involving the possible polarization of the oxygen valence electron density between the aluminum and yttrium. Alternative explanations are also considered.

Quantification of Au deposited on Ni: XPS peak shape analysis compared to RBS

We investigated the degree of quantitative agreement between XPS peak shape analysis and RBS with respect to the average thickness of Au layers deposited on Ni. Gold layers in the range of 5–100 Å were prepared on five substrate samples. By peak shape analysis of the Au 4d XPS peaks, the structure and amount of Au was determined. Likewise, by measuring with RBS on 3–5 regions of each sample the Au thickness in these regions was determined. Comparing the results, we determined the deviation between XPS and the mean value of the RBS thickness. For three of the samples the deviations were 6–7%. For the remaining two samples the agreement was worse. However, as shown by the scatter in the RBS results for each sample, the disagreement was due to a non-uniform Au layer and not to the techniques. It is found that the absolute accuracy of the XPS peak shape analysis technique is ∽7% for depths of ⩽7–8 IMFP. © 1999 John Wiley & Sons, Ltd.

Quantification of Au deposited on Ni: XPS peak shape analysis compared to RBS

Quantification of Au deposited on Ni: XPS peak shape analysis compared to RBS

Nanostructure of Ge deposited on Si(001): a study by XPS peak shape analysis and AFM

The morphology of Ge overlayers with different thicknesses deposited on Si(001) at 560°C, has been investigated with XPS peak shape analysis and AFM. From the peak shape analysis of the Ge 2p and the Si KLL lines we find island growth for all evaporations and no wetting layer. In the analysis, the structural parameters of the islands (coverage and height) are determined after each evaporation. The AFM images confirm island formation, and the magnitude of the structural parameters are in qualitative agreement with the XPS results. Additionally the amount of evaporated Ge is measured with RBS and compared with the amount determined from the structural parameters of the islands. There is a systematic discrepancy which might be due to inaccuracy in the inelastic mean free path or to forward focusing effects. Our study demonstrates that combining the complementary techniques, XPS peak shape analysis and AFM, gives a much more detailed picture of the surface nanostructure than with any of the techniques alone.

Accuracy of the non‐destructive surface nanostructure quantification technique based on analysis of the XPS or AES peak shape

The accuracy of XPS and AES quantification by peak shape analysis was established from a detailed analysis of a range of model spectra and three sets of experiments. It was found that information on the concentration–depth profile in the surface region up to depths of ∽5λi (where λi is the inelastic electron mean free path) is primarily contained in the spectral energy region up to ∽100 eV below the peak energy and is essentially completely contained by the energy region up to ∽200 eV below the peak. Analysis of a larger energy range than 100 eV does not add much to the information on the details of the structure in the outermost 5λi but gives the possibility to determine additional structural parameters that describe the composition at larger depths. The structural parameters that describe the chemical composition of the outermost 5–10λi of the solid were divided into primary and secondary parameters: the primary parameters are the three most important parameters needed to describe the main characteristics of the distribution of atoms; the secondary parameters are parameters other than the three primary parameters that describe the finer details of the depth distribution of atoms in the outermost 5–10λi of the surface region. The uncertainty in the determined three primary parameters is typically 5–10%. The uncertainty in the determined secondary parameters is typically ≳35%. Different models of depth profiles can be distinguished when they differ significantly over a width of more than ∽1/3λi at any depth ≲5λi. The uncertainty in the total determined amounts of atoms within the surface region is ∽5–10% as long as the depths are within the primary probing depth of the method (i.e. <5λi). The absolute quantification of a set of samples where the in-depth distribution varies considerably gives a root-mean-square scatter of 15%. This is reduced to ∽10% when elastic scattering effects are modelled by a simple analytical expression. © 1998 John Wiley & Sons, Ltd.

Accuracy of the non-destructive surface nanostructure quantification technique based on analysis of the XPS or AES peak shape

The accuracy of XPS and AES quantification by peak shape analysis was established from a detailed analysis of a range of model spectra and three sets of experiments. It was found that information on the concentration–depth profile in the surface region up to depths of ∽5λi (where λi is the inelastic electron mean free path) is primarily contained in the spectral energy region up to ∽100 eV below the peak energy and is essentially completely contained by the energy region up to ∽200 eV below the peak. Analysis of a larger energy range than 100 eV does not add much to the information on the details of the structure in the outermost 5λi but gives the possibility to determine additional structural parameters that describe the composition at larger depths. The structural parameters that describe the chemical composition of the outermost 5–10λi of the solid were divided into primary and secondary parameters: the primary parameters are the three most important parameters needed to describe the main characteristics of the distribution of atoms; the secondary parameters are parameters other than the three primary parameters that describe the finer details of the depth distribution of atoms in the outermost 5–10λi of the surface region. The uncertainty in the determined three primary parameters is typically 5–10%. The uncertainty in the determined secondary parameters is typically ≳35%. Different models of depth profiles can be distinguished when they differ significantly over a width of more than ∽1/3λi at any depth ≲5λi. The uncertainty in the total determined amounts of atoms within the surface region is ∽5–10% as long as the depths are within the primary probing depth of the method (i.e. <5λi). The absolute quantification of a set of samples where the in-depth distribution varies considerably gives a root-mean-square scatter of 15%. This is reduced to ∽10% when elastic scattering effects are modelled by a simple analytical expression. © 1998 John Wiley & Sons, Ltd.

Quantitative XPS: Influence of Elastic Electron Scattering in Quantification by Peak Shape Analysis

A new method for non-destructive quantitative analysis of surface nanostructures was developed in recent years. It relies on analysis of the XPS peak shape and usually the effects of elastic electron scattering are neglected in practical applications of the method. In the present paper we study the influence of elastic electron scattering for interpretation of the analysed experimental data. Spectra of the Au 4d peak [with inelastic mean free path (IMFP)=16 A] from a thin layer of Au atoms situated at varying depths in a Ni solid were analysed. When elastic scattering is neglected, the accuracy of the quantification is ∽10% for depths smaller than 1.5 IMFP but for a depth of 2.5 IMFP the determined quantitative amount of gold is 25% too low. The effect of elastic scattering on the peak intensity was <10% for marker depths up to 1 IMFP but it grows to ∽40% at 2.5 IMFP. After correction for elastic scattering effects, the accuracy of the analysis is improved and the root-mean-square scatter in the determined amount of Au around the mean value is reduced from 15.4% when elastic scattering effects are neglected to 11% when they are included in the analysis. For the present system, it is then concluded that the effect of elastic scattering is quite small for marker depths smaller than ∽1 IMFP but for larger depths the effect is substantial. © 1997 John Wiley & Sons, Ltd.

Angle resolved XPS study of inhomogeneous specimens of polycrystalline silver covered with uniform graphite overlayers

The depth dependence of the XPS peak shape parameter is determined experimentally for inhomogeneous specimens of the type f( x) = 0 for x 0 > x > 0 and f( x) = constant for x > x 0, where f( x) is the electron emitter concentration as a function of depth ( x) and x 0 is the overlayer thickness. In order to do this, specimens with sharp interfaces are prepared by depositing uniform overlayers of graphite on flat surfaces of polycrystalline silver. The peak shape parameter thus determined is found to be a function of the overlayer thickness. The observed dependence has been explained in terms of the existing first order analytical expression for this parameter. Angle resolved studies have also been done to explore the possible contributions arising out of surface excitation losses. Such studies clearly pointed out the presence of such an effect. Experimental results are discussed with a special emphasis on the possible sources of systematic errors which may contribute to the measured values of this parameter.

On the XPS peak shape analysis

Inelastic background or the peak shape parameter is defined as the ratio of the area under the primary photoelectron peak to the height of the accompanying inelastic loss background measured at 30 eV below the peak energy. This parameter has been experimentally determined for homogeneous polycrystalline samples of Au, Ag and Cu. Inhomogeneous specimens prepared by depositing graphite overlayers on Au films are also investigated. The effect of the choice of background level on this parameter is determined. A simple experimental test is proposed to determine the validity of the existing first-order analytic expressions for this parameter. Angle-resolved studies are performed to explore the possibility of contribution arising out of surface excitations.

XPS peak shapes accounting for the inelastic scattering of photoelectrons

AbstractA simple and fast method to account for inelastic scattering of photoelectrons is presented. A new peak shape class is introduced, which consists of a conventional photopeak and a tail originating from the inelastic scattering of the electrons, created originally in the photopeak. The proposed peak shapes with inelastic tail can be described using up to three additional parameters. The method has both practical and computational advantages.

Parallel core and valence photoemission studies of nominal (Bi0.9Pb0.1)2Sr2Ca2Cu3O10+x

Clean surfaces of ceramic samples of the oxide superconductor (Bi0.9Pb0.1)2Sr2Ca2Cu3O10+x have been prepared by in situ annealing under pure oxygen at atmospheric pressure inside a UHV system. Scraping of these surfaces under argon is shown to re-expose contaminated grain boundaries, whilst oxygen annealing removes this contamination. Analysis of Pb 4f XPS peak shapes suggests that the metallic character of the Bi-O planes is enhanced by Pb2+ substitution for Bi3+. Ultraviolet photoemission spectra indicate some small changes in the valence band DOS from the Bi2(Sr,Ca)3Cu2O8+x superconductors; in particular, the density of states at Ef is significantly enhanced, as expected from the considerably higher Tc of these materials.