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X-ray photoelectron spectroscopy
X-ray Photoelectron Spectroscopy (XPS) is a quantitative spectroscopic technique that measures the elemental composition, empirical formula, chemical state and electronic state of the elements that exist within a material. XPS spectra are obtained by irradiating a material with a beam of X-rays while simultaneously measuring the kinetic energy (KE) and number of electrons that escape from the top 1 to 10 nm of the material being analyzed. XPS requires ultra-high vacuum (UHV) conditions.
XPS is a surface chemical analysis technique that can be used to analyze the surface chemistry of a material in its "as received" state, or after some treatment such as: fracturing, cutting or scraping in air or UHV to expose the bulk chemistry, ion beam etching to clean off some of the surface contamination, exposure to heat to study the changes due to heating, exposure to reactive gases or solutions, exposure to ion beam implant, exposure to ultraviolet light, for example.
XPS is used to measure:
XPS can be performed using either a commercially built XPS system, a privately built XPS system or a Synchrotron-based light source combined with a custom designed electron analyzer. Commercial XPS instruments in the year 2005 used either a highly focused 20 to 200 micrometer beam of monochromatic aluminum K-alpha X-rays or a broad 10-30 mm beam of non-monochromatic (achromatic or polychromatic) magnesium X-rays. A few, special design XPS instruments can analyze volatile liquids or gases, materials at low or high temperatures or materials at roughly 1 torr vacuum, but there are relatively few of these types of XPS systems.
Because the energy of a particular X-ray wavelength equals a known quantity, we can determine the electron binding energy (BE) of each of the emitted electrons by using an equation that is based on the work of Ernest Rutherford (1914):
where Ebinding is the energy of the electron emitted from one electron configuration within the atom, Ephoton is the energy of the X-ray photons being used, Ekinetic is the kinetic energy of the emitted electron as measured by the instrument and Φ is the work function of the spectrometer (not the material).
History of XPS
In 1887, Heinrich Rudolf Hertz discovered the photoelectric effect. Twenty years later, in 1907, P.D. Innes experimented with a Röntgen tube, Helmholtz coils, a magnetic field hemisphere (electron energy analyzer) and photographic plates to record broad bands of emitted electrons as a function of velocity, in effect recording the first XPS spectrum. Other researchers, Moseley, Rawlinson and Robinson, independently performed various experiments trying to sort out the details in the broad bands. Due to the wars, research on XPS came to a halt. After WWII, Kai Siegbahn and his group in Sweden developed several significant improvements in the equipment and in 1954 recorded the first high energy resolution XPS spectrum of cleaved sodium chloride (NaCl) revealing the potential of XPS. A few years later in 1967, Siegbahn published a comprehensive study on XPS bringing instant recognition of the utility of XPS. In cooperation with Siegbahn, Hewlett-Packard in the USA produced the first commercial monochromatic XPS instrument in 1969. Siegbahn received the Nobel Prize in 1981 to acknowledge his extensive efforts to develop XPS into a useful analytical tool.
Physics of XPS
A typical XPS spectrum is a plot of the number of electrons detected (Y-axis, ordinate) versus the binding energy of the electrons detected (X-axis, abscissa). Each element produces a characteristic set of XPS peaks at characteristic binding energy values that directly identify each element that exist in or on the surface of the material being analyzed. These characteristic peaks correspond to the electron configuration of the electrons within the atoms, e.g., 1s, 2s, 2p, 3s, etc. The number of detected electrons in each of the characteristic peaks is directly related to the amount of element within the area (volume) irradiated. To generate atomic percentage values, each raw XPS signal must be corrected by dividing its signal intensity (number of electrons detected) by a "relative sensitivity factor" (RSF) and normalized over all of the elements detected.
To count the number of electrons at each KE value, with the minimum of error, XPS must be performed under ultra-high vacuum (UHV) conditions because electron counting detectors in XPS instruments are typically one meter away from the material irradiated with X-rays.
It is important to note that XPS detects only those electrons that have actually escaped into the vacuum of the instrument. The photo-emitted electrons that have escaped into the vacuum of the instrument are those that originated from within the top 10 to 12 nm of the material. All of the deeper photo-emitted electrons, which were generated as the X-rays penetrated 1–5 micrometers of the material, are either recaptured or trapped in various excited states within the material. For most applications, it is, in effect, a non-destructive technique that measures the surface chemistry of any material.
Components of an XPS system
The main components of an XPS system include:
Monochromatic aluminum K-alpha X-rays are normally produced by diffracting and focusing a beam of non-monochromatic X-rays off of a thin disc of natural, crystalline quartz with a <1010> lattice. The resulting wavelength is 8.3386 angstroms (0.83386 nm) which corresponds to a photon energy of 1486.7 eV. The energy width of the monochromated X-rays is 0.16 eV, but the common electron energy analyzer (spectrometer) produces an ultimate energy resolution on the order of 0.25 eV which, in effect, is the ultimate energy resolution of most commercial systems. When working under everyday conditions, the typical high energy resolution (FWHM) is usually 0.4-0.6 eV.
Non-monochromatic magnesium X-rays have a wavelength of 9.89 angstroms (0.989 nm) which corresponds to a photon energy of 1253 eV. The energy width of the non-monochromated X-ray is roughly 0.70 eV, which, in effect is the ultimate energy resolution of a system using non-monochromatic X-rays. Non-monochromatic X-ray sources do not diffract out the other nearby X-ray energies and also allow the full range of high energy Bremsstrahlung X-rays (1–12 keV) to reach the surface. The typical ultimate high energy resolution (FWHM) for this source is 0.9–1.0 eV, which includes with the spectrometer-induced broadening, pass-energy settings and the peak-width of the non-monochromatic magnesium X-ray source.
Uses and capabilities
XPS is routinely used to determine:
Capabilities of advanced systems
Industries that use XPS
Adhesion, Agriculture, Automotive, Battery, Beverage, Biotech, Canning, Catalyst, Ceramic, Chemical, Computer, Cosmetic, Electronics, Environmental, Fabrics, Food, Fuel cells, Geology, Glass, Laser, Lighting, Lubrication, Magnetic memory, Mineralogy, Mining, Nuclear, Packaging, Paper and wood, Plating, Polymer and plastic, Printing, Recording, Semiconductor, Steel, Textiles, Thin-film coating, Welding
Routine limits of XPS
Analysis area limits
Sample size limits
Degradation during analysis
Materials routinely analyzed by XPS
Inorganic compounds, metal alloys, semiconductors, polymers, pure elements, catalysts, glasses, ceramics, paints, papers, inks, woods, plant parts, make-up, teeth, bones, human implants, bio-materials, viscous oils, glues, ion modified materials
Organic chemicals are not routinely analyzed by XPS because they are readily degraded by the either the energy of the X-rays or the heat from non-monochromatic X-ray sources.
Charge compensation techniques
Charge referencing insulators
|This article is licensed under the GNU Free Documentation License. It uses material from the Wikipedia article "X-ray_photoelectron_spectroscopy". A list of authors is available in Wikipedia.|