X-ray photoelectron spectroscopy
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X-ray Photoelectron Spectroscopy (XPS) is a quantitative spectroscopic technique that measures the elemental composition, 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 the electrons that escape from the top 1-10 nm of the material being analyzed.
XPS detects all elements (Li-Lr, Z=3 to 103), except hydrogen (H) and helium (He) with detection limits in the parts-per-thousand range for most of the elements. XPS is routinely used to analyze 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 and many others. XPS is used to measure the:
elemental composition of the surface (1-10 nm usually)
elements that contaminate a surface
chemical or electronic state of each element in the surface
uniformity of elemental composition across the top the surface (aka, line profiling or mapping)
uniformity of elemental composition as a function of ion beam etching (aka, depth profiling)
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 broad 10-30 mm beam of non-monochromatic (achromatic or polychromatic) X-rays or a highly focussed 20-2000 micron beam of monochromatic 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 these types of XPS systems are few. XPS is also known as ESCA, an abbreviation for Electron Spectroscopy for Chemical Analysis.
The energy that a particular X-ray wavelength equals is a known quantity. The binding energy of the ejected electron can then be determined from:
- Ebinding = Ephoton - Ekinetic - Φ
where Φ is a parameter that depends on the work function of the spectrometer that you're using.
History of XPS (ESCA)
Albert Einstein is one of the fathers of XPS. Kai Seigbahn was awarded the Nobel Prize for his efforts to develop XPS into a useful analytical tool.
Basic Physics of XPS
A typical XPS spectrum is a plot of the number of electrons detected (Y-axis abscissa) versus the binding energy (X-axis ordinate) of the electrons detected. Each element produces a characteristic set of XPS peaks at characteristic binding energy values that directly identify each element in the surface of the material. These characteristic peaks correspond to the electronic configuration of the electrons within the atoms (for example: 1s, 2s, 2p, 3s...). The number of detected electrons in each of the characteristic peaks is directly related to the amount of element within the area irradiated. To generate atomic percentage values, each raw XPS signal must be corrected by dividing its signal intenisty (number of electrons detected) by a "relative sensitivity factor" and normalized for 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 (1) meter away from the surface irradiated with X-rays. The number of electrons detected in an XPS The quantitative atomic percentage values that directly yield empirical formula are directly related to peak is corrected for sensitivity and converted into .
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 10-12 nm of the material. All of the deeper photo-emitted electrons, which were generated as the X-rays penetrated 1-5 microns 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.
Basic Components of an XPS System
The main components of an XPS system include: a source of X-rays, an ulta-high vacuum (UHV) stainless steel chamber with UHV pumps, an electron collection lens, an electron energy analyzer, mu-metal magnetic field shielding, an electron detector system, a moderate vacuum sample introduction chamber, sample mounts, a sample stage and a set of stage manipulators.
Common Applications & Uses
XPS has been used to determine:
what elements and how much of those elements are present in the top 1-10 nm of the sample
what contamination, if any, exists in the surface or the bulk of the sample
empirical formula of a material that is free of excessive surface contamination
the chemical state identification of one or more of the elements in the sample
the binding energy (BE) of one or more electronic states
the thickness of one or more thin layers (1-8 nm) of different materials within the top 10 nm of the surface
the density of electronic states
Advanced Capabilities of Advanced Systems
uniformity of elemental composition across the top the surface (aka, line profiling or mapping)
uniformity of elemental composition as a function of depth by ion beam etching (aka, depth profiling)
uniformity of elemental composition as a function of depth by tilting the sample (aka, angle resolved XPS)
Industries that use XPS (ESCA)
Automotive Biotech Catalyst Chemical Clothing Computer Electronic circuits Environmental Geoochemical Glass Mineralogy Mining Nuclear Paper and wood Polymer and plastic Semiconductor Steel Thin-film coating
Routine Capabilities and Limitations of XPS
Detection Limit Data Quality Analysis Area Limit Sample Size Limit
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 damaged by the energy of the X-rays (1486 eV, 8.3 Angstroms) normally used in commercial XPS instruments.
Closely Related Methods
UPS (PES), ZEKE, AES
Useful Reference Books
See also
External links
- The Science of Spectroscopy - supported by NASA. Spectroscopy education wiki and films - introduction to light, its uses in NASA, space science, astronomy, medicine & health, environmental research, and consumer products.de:Photoelektronenspektroskopie