|
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 micrometre 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. The foundations of this equation were developed by Rutherford in 1914.
History of XPS (ESCA)
In 1887, Henrich Hertz discovered the photoelectric effect. Twenty years later, in 1907, P.D. Innes experimented with a Rontgen tube, Helmholz coils, a magnetic field hemisphere and photographic plates to record broad bands of emitted electrons as a function of velocity, in effect reocrding 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 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 micrometres 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: 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.
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
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)
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
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.
Related methods
UPS (PES), ZEKE, AES
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.
|
