Methods and Tehniques in
Surface Science
Prof. Dumitru LUCA
“Alexandru Ion Cuza” University, Iasi, Romania
Historic
1887 – Discovery of the photoelectric effect (PE): Heinrich Hertz
1895 – Discovery of X-rays: Wilhelm Conrad Roentgen
1901 – Awarded (the FIRST) Nobel prize.
H. Hertz
1905 – Explanation of the PE by Albert Einstein.
1921 – Awarded Nobel prize.
Karl Manne Georg Siegbahn, (1886 – 1978) Univ. of Upsala,
Sweden. Nobel prize in 1924 for his results in X-ray spectroscopy.
W.C. Roentgen
A. Einstein
Kai M. Siegbahn (SON!) (1918 - ). Nobel prize in 1981 for his
discoveries in high-resolution electron spectroscopy.
1950’s – 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 coworkers) and the development of the UPS (Spicer and
co-workers)
1960: occurrence of commercial XPS instruments.
Hamamatsu, November 2007
K.M.G. Siegbahn
Kai M. Siegbahn
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
Auger Electron Spectroscopy
X-ray Electron Spectroscopy
XPS
AES
X-rays or electrons
X-rays
Ultraviolet Electron
Spectroscopy
UPS
UV photons
Vac
Vac

V
3s
EL2,3
2p6
EL1
2s2
EK
1s2
KE = EK-EL1-EL23-
KE = h-EL1-
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KE = h-EV-
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 = h – Eb – Er -  - δE ≈ h – 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 Kα- 1253,6 eV
Al Ka = 1486.6 eV
Cu Ka = 8047 eV
Fwhm= 0.75 eV
Fwhm= 0.95 eV
Fwhm= 2.6 eV
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KE→ BE
Mg K
330 eV
910 eV
920 eV
690 eV
720 eV
581 eV
Transformarea (KE) in EB
(BE = h  KE)
920 eV
534 eV
561 eV
343 eV
333 eV
54, 88 eV
(4s, 4p)
673 eV
0-8 (4-12) eV
(4d, 5s)
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Intensity N(E)
Ecin = hν - EB
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.
Li-metal
1s2
1s2
1s2
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.
2s
density
Li: 1s2 2s1
O: 1s2 2s2 2p4
Li2O
2p6
2s
1s2
2s
1s2
2s2
Li
O
By losing a valence electron, the BE of the
electron in a core-level of the atom becomes
larger.
1
s2
Li
Li2O
Li-metal
Binding Energy
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EF
0
Chemical shift
2.1 eV
4.3 eV

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
Hamamatsu, November 2007
The PHI VersaProbe 5000 XPS machine
View of the XPS machine in our lab with the following options:
(a) XPS with chemical imaging;
(b) AES;
(c) Depth profiling;
(d) UPS (future development)
Hamamatsu, November 2007
Quantitative analysis using XPS:
Relative elemental composition
Ii=Fx i(EK) ni i(Ek) K cos θ
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 K and Mg K)
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,
θ – 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 =
Area1Sample
ASF 1Sample
N
Area X Sample
1 ASF X Sample
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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.
Hamamatsu, November 2007
Further references
1. D. Briggs, M. P. Seah, Practical surface analysis, vol. I, Willey and Sons, 1990.
2. J. M. Walls, R. Smith, Surface Science Techniques, Pergamon, 1994.
3. H. Lüth, Surfaces and interfaces of solid materials, Springer, 1993.
4. J. W. Niemantsverdriet, Spectroscopy in Catalysis – An Introduction, Wiley-VCH, 1995.
5. http://www.chem.qmul.ac.uk/surfaces/scc/scat5_3.htm
6. C.D. Wagner, W.M. Riggs, L.E. Davis, J.F. Moulter, G.E. Muilenberg, Handbook of X-ray Photoelectron Spectroscopy,
Perkin-Elmer Corp. (1978).
7. C.D. Wagner, Practical Surface Analysis, Vol. 1, 2ª, J. Wiley and Sons, 1990.
8. W.N. Delgass, G.L. Haller, R. Kellerman, J.H. Lunsford, Spectroscopy in heterogeneous catalysis, Cap. 8: X-ray
Photoelectron Spectroscopy, Academic Press (1979).
9. H.D. Hagstrum, J.E. Rowe, J.C. Tracy, Electron spectroscopy of solid surfaces, in Experimental methods in catalytic
research, Vol. 3, R.B Anderson y P.T. Dawson (Ed.), Academic Press (1976).
10. C.D. Wagner, L.E. Davis, M.V. Zeller, J.A. Taylor, R.M. Raymond, L.H. Gale, Surf. Interf. Anal. 3 (1981) 21. (for atomic
sensitivity factors).
11. Moulder, John F., William F. Stickle, Peter E. Sobol, and Kenneth D. Bomben, Handbook of X-ray Photoelectron
Spectroscopy, ed. Jill Chastain and Roger C. King Jr. 1995: Physical Electronics, Inc., USA. 11
12. http://seallabs.com/howes1.html
13. http://srdata.nist.gov/xps/elm_in_comp_res.asp?elm1=C
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