Methods%20and%20Tehniques%20in%20Surface%20Science

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Methods and Tehniques in Surface Science
Prof. Dumitru LUCA Alexandru Ion Cuza
University, Iasi, Romania

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Historic
1887 Discovery of the photoelectric effect
(PE) Heinrich Hertz
1895 Discovery of X-rays Wilhelm Conrad
Roentgen 1901 Awarded (the FIRST) Nobel prize.
1905 Explanation of the PE by Albert Einstein.
1921 Awarded Nobel prize.
H. Hertz
Karl Manne Georg Siegbahn, (1886 1978) Univ.
of Upsala, Sweden. Nobel prize in 1924 for his
results in X-ray spectroscopy. Kai M. Siegbahn
(SON!) (1918 - ). Nobel prize in 1981 for his
discoveries in high-resolution electron
spectroscopy.
W.C. Roentgen
1950s huge progress in instrumentation -
increasing the resolution of the photoelectron
energy analysers, - design of X-ray sources.
- application for surface analysis (named
ESCA) by K. Siegbahn and co-workers -
also now - the discovery of the PE in gases
(Turner and co- workers) and the development of
the UPS (Spicer and co-workers)
K.M.G. Siegbahn
A. Einstein
1960 occurrence of commercial XPS instruments.
Kai M. Siegbahn
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What information can be derived from the XPS
spectra?

• The most frequently used experimental technique
  in Surface Science
• to extract information on
• The relative chemical composition in the surface
  region,
• The chemical status of elements,
• The dispersion of some phases within other
  phases,
• The depth profile of chemical composition,
• Valence band level structure.

The universal curve the dependence of the
inelastic mean free path ?inel of photoelectrons,
on their energy.
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Electron spectroscopies
Ultraviolet Electron Spectroscopy UPS
X-ray Electron Spectroscopy XPS
Auger Electron Spectroscopy AES
X-rays
X-rays or electrons
UV photons
Vac V EL2,3 EL1 EK
Vac 3s 2p6 2s2 1s2
?
KE EK-EL1-EL23-?
KE h?-EV-?
KE h?-EL1-?
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XPS vs. UPS
1. In XPS, holes are excited by X rays within
the core level of the atom to derive the binding
energy of the electrons in the lower lewels.
After a core-level electron absorbs
(integrally!) the energy of the X-ray photon,
it leaves the atom, thus becoming a
photo-electron KE hn
Eb Er - ? - dE hn Eb ?, 2. In
UPS, similar phenomena occur, except for UV
potons are used here. The
photo-electrons originate now in the valence band
of the element(s). Mg Ka- 1253,6 eV Fwhm
0.75 eV Al Ka 1486.6 eV Fwhm 0.95 eV Cu Ka
8047 eV Fwhm 2.6 eV
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KE? BE
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XPS Spectrum
Ecin h? - EB
Intensity N(E)
0 eV
Energie de legatura, EB (eV)
EF
Nivele adanci
Banda de valenta
Banda de conductie
http//www.nottingham.ac.uk/ppzpjm/sect6_1.htm
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Spin-orbital splitting
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BE-Z
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Chemical shift wat is it and why does it occur?
The charge transfer between neighboring atoms
results in the alterations of the binding
energies of the atom. A core level electron
feels the nucleus more strongly than a valence
electron (due to the differences in dimensions of
the two types of orbitals). Thus, the
electrostatic potential created by valence
electrons as experienced by a core level electron
is q/rv. By losing a valence electron, the
BE of the electron in a core-level of the atom
becomes larger.
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Chemical shift

• The BE values are affected not only by the energy
  levels, specific for any element.
• The BE values depend (to a lesser extent) on the
  chemical state of that element, since the
  core-level electrons are affected by the chemical
  state of the atom.
• Usually, chemical shift values
  are ranging between 1 and 5 eV.

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Instrumentation
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The PHI VersaProbe 5000 XPS machine

• View of the XPS machine in our lab with the
  following options
• XPS with chemical imaging
• AES
• Depth profiling
• UPS (future development)

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Quantitative analysis using XPS Relative
elemental composition
IiFx ?i(EK) ni ?i(Ek) K cos ?
where Ii intensity of the p - peak,
corresponding to the element i, ?I ionisation
cross-section (Scofield factor) of the element i
(values calculated and tabulated for all the
elements,including Al Ka and Mg Ka) ni
average concentration of the element i in the
surface region, ?I mean free path for the
inelastic collision of a photoelectron of the
element I, K all the remaining factors
involved in the detection of the
photoelectrons, ? take-off angle of
photoelectrons. Typical accuracy ? 10
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Background subtraction
step background
linear background
Shirley background D.A. Shirley, Phys. Rev. B5,
4709, 1972
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An example the atomic percentage in a catalyst
calculated from peak areas
VPO Catalyst
Element Peak area (arb. units) ASF Percentage ()
Carbon 1853 0.319 22.1
Oxygen 14240 0.75 62.0
Vanadium 3840 2.0 6.3
Phosphor 1494 0.64 9.6
Atomic Percent
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Conclusion

• The main features of the XPS
• Chemical identification all the elements
  except for H and He,
• Probing depth 1 6 nm,
• Detection limit 0.1,
• Determine the atomic environment and the
  oxidation state,
• Determine the depth-profile of the elemental
  concentration,
• Information about surface electrical
  properties from surface electrical charging,
• Lateral resolution tens of micrometers
• Energy resolution in BE 10 meV.