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X-ray Photoelectron Spectroscopy

surface analysis technique

Dawin Ng

on 28 September 2012

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Transcript of X-ray Photoelectron Spectroscopy

X-ray Photoelectron Spectroscopy (XPS) XPS, also known as ESCA (Electron Spectroscopy for Chemical Analysis), is the most widely used technique to investigate chemical composition of surfaces. What is XPS ?
XPS technique is based on Einstein’s idea about the photoelectric effect, developed around 1905.

In 1981, Dr. Siegbahn was awarded the Nobel Prize in Physics for the development of the XPS technique. XPS Background How does XPS work ? Binding Energy Reference Photoemission Process Photoemission is usually assumed to occur in a 3-step process:

Absorption and ionization (initial state effects)
Response of atom and creation of photoelectron (final state effects)
Transport of electron to surface and escape (extrinsic losses)

XPS spectra consist of 3 principal components: Core Electron Core levels
Auger Series
Valence levels Auger Electron 25oC 65oC Valence levels are those occupied by electrons of low binding energy (0-20eV) which are involved in de-localized or bonding orbitals.

This figure shows that Vanadium dioxide (VO2) undergoes a transition from insulator to metallic conductor at the higher temperature. Valence levels Valence level Auger peak Pd XPS Spectrum
Core level structure is a direct reflection of the electron structure present in the atom.

For non-s levels, separated peaks (doublets) are observed and arise through spin-orbit (j-j) coupling. Core Electron Concentric hemispherical analyzer (CHA)
X-ray source : Al
Photon energy=1486.7eV
Line width=0.85eV
UHV-vacuum system
Electronic controls
Computer system
X-ray monochromator XPS Instrumentation Main Chamber Electron Energy Analyzer X-ray Monochromator Al α X-ray Source XPS Instrumentation Reduced background
Narrow peak width
No satellite & ghost peaks Chemical shift: change in binding energy of a core electron of an element due to a change in the chemical bonding of that element.

Core binding energies are determined by:
electrostatic interaction between electron and nucleus
electrostatic shielding of the nuclear charge from all other electrons in the atom (including valence electrons)

Withdrawal of valence electron (oxidation)= increase in BE
Addition of valence electron = decrease in BE EXAMPLE 1- Chemical Shift For some materials, there is a finite probability that the photoemission process leads to substantial reorganization of the valence electrons (relaxation).

This major perturbation may involve excitation of one of them to a higher unfilled level (shake-up) or an unbounded continuum state (shake-off). EXAMPLE 2- Satellite Peak The two-electron process leads to discrete structure on the high BE side of the photoelectron peak.

3d orbital to empty 4s orbital in ferric compounds
π bond to anti-π bond in C Multiplet (exchange) splitting of core level peaks can occur when the system has unpaired electrons in the valence levels. EXAMPLE 3 –Multiplet Splitting Multiplet splitting for non-s levels is more complex due to the coupling both orbital and spin angular momentum in the core ionised state.

In Fe3+ ion, the 2p level is split into four levels. √ × In the relative intensity computation, intensity is defined as the peak area rather than the peak height.

The experimental spectra are fitted by using the Shirley-type or Tougaard type background to remove most of the extrinsic loss structure.

Atomic sensitivity factor (ASF) are used to scale the measured areas so that percentage atomic concentrations can be obtained Analytical Methods Deconvolution-CasaXPS Analytical Methods Charging Compensation Non-destructive
Quantitative method for elemental composition
Chemical shifts give information about
(i) oxidation states
(ii) chemical environment
Extensive databases of chemical shift information
Spectra complicated by secondary features
(i) x-ray satellites
(ii) extrinsic losses
(iii) final state effects
Surface charging in insulators shifts BE scale
Cannot detect H, He with good sensitivity Summary THE END XPS CASE STUDIES Electron Energy Analyser
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