Atomic sensitivity factor

The resulting data were then treated by use of standard XPS methods. An asymmetric background correction using the Shirley function (described in detail by Castle and Salvi 16)), was used for all spectra, and peaks were fit by using a mixed Gaussian-Lorenzian distribution. Quantitation was performed with the equation (A/Sj) + EA/S, where A, is the peak area and 5/ is the atomic sensitivity factor for the element i being determined 17). Atomic sensitivity factors are empirical constants determined on standards of the elements 18). This yields an atom percentage (atom %) for each element at the surface of the material. 

Superficial (surface) compositions are also listed in Table I as determined by XPS with atomic sensitivity factors characteristic of the electron energy analyzer used for these studies. The superficial compositions agree reasonably well with the bulk values for all samples with the exceptions of SAPO-5 and the two Si-VPI-5 samples which show a significant enrichment of Si at the surface. Note that samples which were examined with and without deposited gold (the reference for the binding energy scale) showed little difference in measured compositions with XPS. 

The intensity of an XPS peak (Ia) is a strong function of (i) the incoming photon flux, (ii) the concentration of the given element, (hi) its photoionization cross-section (which is excitation-energy dependent), (iv) the mean free path of the emitted photoelectron, and (v) further instrumental parameters (such as photoelectron collection and detection efficiency). By defining atomic sensitivity factors (S, as an overall factor summing up the effects of iii-v), the atom fraction of any element in a sample can be calculated as ... 

For XPS characterisation, we used an Omicron UHV system with hemispherical analyser [27]. Spectra were obtained with Mg K excitation at room temperature and were corrected for differences in contact potential. Elemental ratios were calculated fi om the XPS intensities, modified with the specific atomic sensitivity factors (ASFs) when necessary [28]. 

This use of atomic sensitivity factors will give semiquantitative results, within 10-20% of the value, for most homogeneous materials. While the absolute sensitivity of XPS is high, the actual sample volume analyzed is small, so the amount of an element that can be detected is in the % range with sensitivity of about 0.5 atom%. 

As already suggested, quantification in electron spectroscopy has been the active research area of a number of the best practitioners in the field. Efforts in ESCA were originally championed by Wagner [46,47], who has researched empirically derived, atomic, sensitivity factor scales [47]. Further development has been provided by Wagner et al. [48], Nefedov et al. [49], Seah (primarily for Auger spectroscopy) [50] and Powell [51] (primarily for ESCA). 

Quantification is simple if the atoms are homogeneously distributed with depth since the intensity of each XPS peak is then directly related to the abundance of that particular element at the specimen surface. The peak intensity will usually be reported as a peak area and this will be normalized using atomic sensitivity factors (the intensity of the photoelectron transition of interest, I, is related to the concentration of that element within the XPS analysis volume, and the sensitivity factor, S, in the following way F= concentrationxS). Such atomic sensitivity factors are a function of the basic physical parameters, such as the relative photoelectron cross-sections of the different elements, electron attenuation lengths, and instrumental parameters, such as analyzer transmission functions, of the XPS experiment. The ratio of normalized peak area to the sum of normalized peak areas for the major peaks of all elements detected in the spectrum provides an analysis as an atomic fraction (or when multiplied by 100, atomic %). 

Most analyses use empirical calibration constants, ASF called atomic sensitivity factors) derived from standards... 

For a given transition, the last six terms are constant, and we can write the atomic sensitivity factors as... 

The denominator term is defined as the atomic sensitivity factor, S, which differs for various elemental photoelectron transitions. Equation (15) can then be further extended to a generalized expression for determining the mole fraction of any constituent element from the sum of the peak intensities from all elements in the sample being analyzed ... 

Figure 1 presents the survey spectrum of an original powder sample. The spectram demonstrates photoelectron C Is and O Is peaks and related to them C KW and O KW Auger peaks. No either peak has been found. Elemental composition determined by XPS using atomic sensitivity factors (ASF) is close to It should be noted... 

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