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X-ray Photoelectron Spectroscopy (XPS)

Surface analysis by XPS involves irradiation of the sample in vacuum with mono energetic soft x-rays and sorting the emitted electrons by energy. The spectrum obtained is a plot of the number of emitted electrons per energy interval versus their binding energy. Each element has a unique elemental spectrum, and the spectral peaks from a mixture are approximately the sum of the elemental peaks from the individual constituents. Since the mean free path of the electrons is very small, the electrons, which are detected, originate from only the top few atomic layers. The depth of the solid material sampled varies from the top 2 atomic layers to 15-20 layers. Quantitative data can be obtained from the peak heights or areas and identification of chemical states often can be made from the exact positions and separations of the peaks, as well as from certain spectral contours. The samples can be gaseous, liquid, or solid but a majority of electron spectrometers are designed to deal with solids. This method is the least destructive of all the electron or ion spectroscopy techniques and can be applied to organic, polymeric, inorganic, organometallic and biological materials through metals, ceramics and semiconductors. There is always a possibility that other surface techniques can be combined with XPS to satisfy the individual requirements. The sample can also be sputtered and different layers can be analyzed. If the depth of interest is beyond the capability of sputtering, then the sample can be polished down or sectioned or etched and then analyzed.

The electrons leaving the sample are detected by an electron spectrometer according to their kinetic energy. The analyser normally is operated as an 'energy window', accepting only those electrons having an energy within the range of this fixed window, referred to as the pass energy. Electrons are detected as discrete events and the number of electrons for a given detection time and energy is stored digitally or recorded using analog circuitry.

Figure : XPS AXIS ULTRA from KRATOS ANALYTICAL Inc.

Range of Elements: All except Hydrogen and Helium

Destructive: No, some beam damage to X-ray sensitive materials

Elemental Analysis: Yes, semiquantitative without standards; quantitative with standards. Not a trace element method

Chemical State Information: Yes

Depth Probed: 5-50

Depth Profiling: Yes, over the top 50Ao ; greater depths require sputter profiling

Depth Resolution: A few to several tens of , depending on conditions

Lateral Resolution: 5mm-75m; down to 5m in special instruments.

Sample Requirements: All vacuum-compatible materials; flat samples best; size accepted depends on particular instrument

Main Uses: Determinations of elemental and chemical state compositions in the top 30

Instrument Cost: $600,000

Size: 10ft-12ft


                Director:     Dr. Sharmila M . Mukhopadhyay

         Lab Manager:     Mr. Gregory Wilt

                 People:     Center's students

Center for Nanoscale Multifunctional Materials, Wright State University, Dayton, Ohio-45435, USA

Last updated on 07/21/11