File Name: urface analy iby auger and x ray photoelectronpectro copy .zip
Since the interaction between solid materials and their surrounding media, whether gaseous or liquid, occurs at the surface, analytical techniques capable of providing information from the interaction region are fundamental in understanding the processes that are occurring. X-ray Photoelectron Spectroscopy XPS is one such technique, and is capable of analysing both conducting and insulating materials. During analysis, the surface is irradiated by soft X-rays and the energy of the emitted photoelectrons measured.
The energy of these electrons is determined by the atomic number of the emitting element, and is sensitive to changes in the number of electrons in the valence band, so that surface chemical state information is obtained. XPS is now a mature technique, the first commercial instruments having become available as long ago as , and judged by the number of scientific publications, it is not only the most popular of the surface analytical techniques, but also the fastest growing.
This is due to the fact that it provides relatively easily quantified surface chemical state information. This trend is likely to continue, as instruments capable of operation without expert guidance move from the laboratory to the production line, where they can assist in quality control.
Furthermore, developments in data processing have for the first time enabled quantitative surface chemical state images to be produced. In spectroscopic mode, improvements in instrumentation resulting in increased energy resolution and sensitivity have led to analysis from smaller areas. The latter is the preferred mode of operation, since it allows faster acquisition and minimises sample damage by exposure to X-rays. In spectroscopic mode, wide scan spectra are acquired at low energy resolution for quantitative analysis, and photoelectron peak areas compared with relative sensitivity factors.
For chemical state determination, high energy resolution spectra are acquired through the photoelectron region of interest, and curve-fitting techniques are often employed to resolve overlapping photoelectron peaks. Recently such instruments have been used to acquire multi-spectral data sets: that is a data set containing a spectrum at each pixel, so that by applying accepted spectroscopic data processing techniques, fully quantitative chemical state microscopy has become a reality.
In instruments employing parallel acquisition, this is achieved by acquiring a series of images incremented in energy. A schematic diagram of one such instrument4 is shown in Figure 1. Photoelectrons from the sample, which sits above a magnetic immersion lens, are projected onto the entrance aperture of a spherical mirror energy analyser, and the energy-filtered spatially orientated electrons are amplified and counted by a position sensitive detector. Since the technique is surface sensitive, the instrument is contained under ultra high vacuum.
Acquisition of multi-spectral data sets has necessitated characterisation of the instrument in imaging mode5 to establish whether peak positions, line shapes and energy calibration are consistent across the field of view, and also necessitated the development of procedures6 ,7 to measure the transmission function of the instrument at every pixel for use in quantification.
Figure 1. DLD: delay line detector. Principal component analysis PCA is widely used for this purpose. It decomposes the data into an orthogonal set of vectors ordered by variance, so that the chemical information may be separated from the noise.
The NIPALS procedure decomposes the data sequentially into abstract factors, and since there are many fewer components in such an XPS data set than there are images, terminating the procedure when sufficient factors have been calculated leads to a significant reduction in computational requirements.
In practice, a few more factors are calculated so that the number required to reconstruct the data may be determined. The data is usually filtered prior to decomposition, since the noise in pulse counting instruments is Poissonian, which is proportional to the square root of the signal intensity; PCA requires that the noise is evenly distributed. In imaging mode, as in spectroscopic mode, data may be acquired for quantification of the elements present at the surface, and for chemical state determination.
In Figure 2, an example of chemical state analysis of the degradation of a microchannel plate is shown; examples of elemental quantification are provided elsewhere. XPS analysis was carried out by acquiring and images, incremented by 0. All data processing was undertaken using CasaXPS. Figure 2 b shows the second image abstract factor and a false colour image following classification of the pixels by intensity.
Classification in this way orders the data by chemistry, which may be due to changes in peak intensities, changes in peak position caused by chemical shifts, or changes in the inelastic background caused by in-depth inhomogeneity. This allows curve fit models to be produced, which are a close approximation to the spectra to which they will be applied. Chemical state assignments are then made based on the chemical shift from the C—C, C—H peak. The results of the curve fitting were then applied to all the spectra within their own classification only, to produce chemical state images.
The validity of the curve fitting may be checked by inspecting a figure of merit composed of the root mean square deviation between the curve fit envelope and the data at each pixel in the image, normalised to the sum of the curve fit intensities. In this way, increases in intensity across the field of view indicate a poorer fit, and the spectra concerned may be examined, and the curve fitting corrected.
The results are shown in Figure 3, where peaks O2 and C4 [see Figure 2 c ], assigned as oxygen and carbon, respectively, in carbonate species, and Na are shown to correspond to the entire circular region and not just the central corrosion product.
This association would not have been possible to make conclusively using selected area analysis. It is known that enrichment of sodium occurs at the surface of microchannel plates during operation, and so it was concluded that degradation was due to sodium reacting with an aerosol droplet to form a hydroxide, which then absorbed CO 2 from the ambient. The larger corrosion feature developed at a later date. Figure 2. Imaging XPS analysis of the degradation of a microchannel plate.
Reprinted with permission from J. Interface Anal. Published: Online 13 th November in Wiley Interscience. Figure 3. Chemical state images produced by applying the curve fit models of Figure 2 c to the spectra within their own classification. Published: Online 13th November in Wiley Interscience. Classification of pixels in both atomic concentration images and image abstract factors may also be used to aid visualisation of wide scan spectra without recourse to multivariate curve resolution and its attendant uncertainties.
In future, as quantitative imaging becomes more widespread, improvements in lens and analyser design will lead to increased spatial resolution and sensitivity. Improvements in multivariate analytical techniques may eliminate the need to scale the data, and parallelisation could be used to speed computationally intensive procedures such as curve fitting.
Attempts have already been made to provide in-depth analysis within images,11 —13 and given the demand for nano-structural analysis, may be expected to continue.
Skip to main content. You are here Home. Quantitative surface chemical microscopy by X-ray photoelectron spectroscopy. Full-Text PDF:. Introduction Since the interaction between solid materials and their surrounding media, whether gaseous or liquid, occurs at the surface, analytical techniques capable of providing information from the interaction region are fundamental in understanding the processes that are occurring.
Instrumentation In spectroscopic mode, wide scan spectra are acquired at low energy resolution for quantitative analysis, and photoelectron peak areas compared with relative sensitivity factors. Analysis In imaging mode, as in spectroscopic mode, data may be acquired for quantification of the elements present at the surface, and for chemical state determination. References For a detailed explanation, see D.
Briggs and J. Walton and N. Fairley, Surf. Kratos Analytical, Manchester, UK. Fairley, J. Electron Spectrosc. For more information, see E. Malinowski, Factor Analysis in Chemistry , 3 rd Edn.
Smith, D. Briggs and N. Hajati, S. Coultas, C. Blomfield and S. Tougaard, Surf. Rate this Article:. Log in or register to post comments.
X-ray photoelectron spectroscopy XPS is a surface-sensitive quantitative spectroscopic technique based on the photoelectric effect that can identify the elements that exist within a material elemental composition or are covering its surface, as well as their chemical state , and the overall electronic structure and density of the electronic states in the material. XPS is a powerful measurement technique because it not only shows what elements are present, but also what other elements they are bonded to. The technique can be used in line profiling of the elemental composition across the surface, or in depth profiling when paired with ion-beam etching. It is often applied to study chemical processes in the materials in their as-received state or after cleavage, scraping, exposure to heat, reactive gasses or solutions, ultraviolet light, or during ion implantation. XPS belongs to the family of photoemission spectroscopies in which electron population spectra are obtained by irradiating a material with a beam of X-rays. Material properties are inferred from the measurement of the kinetic energy and the number of the ejected electrons. When laboratory X-ray sources are used, XPS easily detects all elements except hydrogen and helium.
This article provides a detailed account of the principles, instrumentation,and applications of x-ray photoelectron spectroscopy XPS , a technique used for elemental and compositional analysis of surfaces and thin films. It reviews the nomenclature of energy states and sensitivity of electrons at the surface that are capable of producing peaks in XPS. Additionally, it presents information on the instrumentation and the preparation and mounting of samples for XPS analysis. The article explains qualitative analysis, namely, measuring of shifts in the binding energy of core electrons, multiplet splitting, and the Auger parameter; and quantitative analysis such as depth analysis carried out using XPS. It also discusses the applications of XPS with examples.
Unrivalled large area spectroscopic performance allows photoelectron spectra to be acquired. Fast, high spatial resolution XPS imaging reveals the lateral distribution of surface chemistry and aids further characterisation with selected area analysis. Unattended sample holder transfer and exchange during analysis is achieved through coordination of the Flexi-lock sample magazine and sample analysis chamber autostage. Efficient collection of photoelectrons combined with high transmission electron optics ensures unrivalled sensitivity and resolution at large analysis areas. As well as conventional scanned acquisition, spectra may be acquired in fast, unscanned snap-shot mode in less than a second making use of the channel Delay-Line Detector DLD.
Since the interaction between solid materials and their surrounding media, whether gaseous or liquid, occurs at the surface, analytical techniques capable of providing information from the interaction region are fundamental in understanding the processes that are occurring. X-ray Photoelectron Spectroscopy XPS is one such technique, and is capable of analysing both conducting and insulating materials. During analysis, the surface is irradiated by soft X-rays and the energy of the emitted photoelectrons measured. The energy of these electrons is determined by the atomic number of the emitting element, and is sensitive to changes in the number of electrons in the valence band, so that surface chemical state information is obtained. XPS is now a mature technique, the first commercial instruments having become available as long ago as , and judged by the number of scientific publications, it is not only the most popular of the surface analytical techniques, but also the fastest growing.
and students associated with The Surface Analysis Laboratory have pro- vided a stimulating This book is largely concerned with X-ray photoelectron spectroscopy. (XPS) and most modern instruments and, when operating in this mode, a spectro- copy. Although the principles are equally applicable to Auger electron.
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