X-ray photoelectron spectroscopy - Theory...
Transcript of X-ray photoelectron spectroscopy - Theory...
X-ray photoelectron spectroscopy
Some Synchrotron Radiation Based Methods
Jesper Andersen
Division of Synchrotron Radiation Research, Lund University
X-ray photoelectron spectroscopy
h! typically
<1500 eV
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X-ray photoelectron spectroscopy
h! typically
<1500 eV
X-ray photoelectron spectroscopy
h! typically
<1500 eV
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X-ray photoelectron spectroscopy
Binding Energy = h! -
Ekin
Note direction of x-axis
X-ray photoelectron spectroscopy
Binding Energy = h! -
Ekin
Note direction of x-axis
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Basics:A) Elemental analysis – what is in the sample (or rather on the sample) Different elements have different binding energies of the inner (core) levels.
B) Often, also the chemical state of the elements can be determined, eg. Al-metal can be distinguished from Al-oxide, O2 from O The exact binding energy of a core level depends on the chemical state.
Chemical shifts. ESCA (Electron Spectroscopy for Chemical Analysis)
X-ray photoelectron spectroscopy
Basics:A) Elemental analysis – what is in the sample (or rather on the sample) Different elements have different binding energies of the inner (core) levels.
B) Often, also the chemical state of the elements can be determined, eg. Al-metal can be distinguished from Al-oxide, O2 from O The exact binding energy of a core level depends on the chemical state.
Chemical shifts. ESCA (Electron Spectroscopy for Chemical Analysis)
X-ray photoelectron spectroscopy
! ! Advanced:
C) The surface geometry can be determined. Using diffraction effects and/or the chemical shifts of the binding energies
D) Often, also the mesoscopic scale can be addressed
E) Chemically sensitive microscopy is possible. (Note: Chemically, not just element specific)
Combine the above with either focusing of the incoming photons or magnifying electron optics.
Some 10 nanometers to micrometers are typical values for the spatial resolution.
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Elemental analysis: Photoionization cross-sectionsTo first approximation the intensity (area) of peak A from element Z is proportional to the amount of Z times the cross-section for photoionization of level A.
Crossections depend on Z and on what level we are looking at.
Elemental analysis: Photoionization cross-sectionsTo first approximation the intensity (area) of peak A from element Z is proportional to the amount of Z times the cross-section for photoionization of level A.
Crossections depend on Z and on what level we are looking at.
And on the photon energy.
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Elemental analysis: Photoionization cross-sectionsTo first approximation the intensity (area) of peak A from element Z is proportional to the amount of Z times the cross-section for photoionization of level A.
Crossections depend on Z and on what level we are looking at.
And on the photon energy.Mo valence (4d) overlaps
Yb 4f levels
Note:• Photons in – electrons out. Energy analyze electrons
• Element specific, concentrations, cross-sections depend on element, level, and photon energy
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Surface core level shifts
Rh 3d 5/2 level gives two peaks.
Surface and bulk
Surface core level shifts
Attenuation length: "
I(x) = I0 e-x/"
Rh 3d 5/2 level gives two peaks.
Surface and bulk
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Surface core level shifts
Attenuation length: "
I(x) = I0 e-x/"
Using synchrotron radiation, we can vary h!
and thus the electron kinetic energy
Rh 3d 5/2 level gives two peaks.
Surface and bulk
Surface core level shifts
Attenuation length: "
I(x) = I0 e-x/"
XPS is surface sensitive
Using synchrotron radiation, we can vary h!
and thus the electron kinetic energy
Rh 3d 5/2 level gives two peaks.
Surface and bulk
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Changing the probe depth at fixed photon energy
Electron escape depth = µ cos #µ: attenuation lengthrather than inelastic mean free pathµ includes losses from elastic scattering
µ is around 0.9 x IMFP
Note:• Photons in – electrons out
• Element specific, concentrations, cross-sections depend on element, level, and photon energy
• Surface sensitive, varies with photon energy and emission angle
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Surface core level shifts, why ?
Surface core level shifts, why ?
d-band narrowing at the surface.
Assume binding energy equals the eigenvalue of the level.
Initial state model
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Surface core level shifts, why ?
d-band narrowing at the surface.
Assume binding energy equals the eigenvalue of the level.
Initial state model
BINDING ENERGY (EB) :
The energy-cost to remove the core-electron
EB = Etotal(after) - Etotal(before)
EB = h! - Ekin
Surface core level shifts, why ?
d-band narrowing at the surface.
Assume binding energy equals the eigenvalue of the level.
Initial state model
BINDING ENERGY (EB) :
The energy-cost to remove the core-electron
EB = Etotal(after) - Etotal(before)
EB = h! - Ekin
Eb =
Eb
s =
SCLS = Eb – Eb
s =
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BINDING ENERGY (EB) :
The energy-cost to remove the core-electron
EB = Etotal(after) - Etotal(before)
EB = h! - Ekin
Eb =
Eb
s =
SCLS =
Need to calculate total energy of a system
with an “impurity” placed in various sites.
In metallic systems the “impurity” is completely screened. i.e. the valence (and remaining core) electrons have relaxed to their ground state.
DFT-based supercell calculations can calculate the total energy di!erences needed.
In all-electron methods, a core electron can be explicitly removed. In pseudo-potential methods, a potential must be generated for the core-ionized state.
Note also that the “impurity” in many ways equals a Z+1 impurity, i.e. the next element in the periodic table.
Surface core level shifts and general chemical shifts, why ?
Note:• Photons in – electrons out
• Element specific, concentrations, cross-sections depend on element, level, and photon energy
• Surface sensitive, varies with photon energy and emission angle
• The binding energy shifts can be calculated accurately by DFT, total energy di"erences
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Using the substrate core level shifts: CO/Rh(111)
Theory results
Total energies: almost degenerate for CO
in top and 3fold sites (which they should be!!)
Rh 3d shifts:
Clean: –500 meV
CO ind. (top): +450 meV (no buckling) +240 meV (+0.2Å buckling)
CO ind. (3-fold): –220 meV
Conclusion
CO in on-top sites on a buckled surface
Using the substrate core level shifts: CO/Rh(111)
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Stepped surfaces:
Rh(553), seeing the step atoms and following what happens when oxygen is adsorbed
Note:• Photons in – electrons out
• Element specific, concentrations, cross-sections depend on element, level, and photon energy
• Surface sensitive, varies with photon energy and emission angle
• The binding energy shifts can be calculated accurately by DFT, total energy di!erences
• The shifts are local, i.e. mainly nearest neighbors
• The shifts can be large even though the initial state total energies are close
• Gives qualitative information on adsorption sites etc. Important input to e.g. DFT
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Using the adsorbate levels as fingerprints
• For adsorbed CO the C1s binding
energy provides a good fingerprint of
the adsorption site. Nearest neighbors.
• Ex. CO on Rh(111), pure CO, and co-
adsorbed with O and K. Electro-
positive or –negative does not matter.
• Large shifts even when ground-state
total energies are almost degenerate
• General rule: The C 1s binding energy
decreases as the coordination to the
substrate increases
Using the adsorbate levels as fingerprints
• For adsorbed CO the C1s binding
energy provides a good fingerprint of
the adsorption site. Nearest neighbors.
• Ex. CO on Rh(111), pure CO, and co-
adsorbed with O and K. Electro-
positive or –negative does not matter.
• Large shifts even when ground-state
total energies are almost degenerate
• General rule: The C 1s binding energy
decreases as the coordination to the
substrate increases
• Another rule
Watch out for diffraction !!!
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Partial vs total coverages
Making use of the site-sensitive fingerprint, ex. CO on Rh(111)
Partial coverages vs T and P
Partial vs total coverages
Making use of the site-sensitive fingerprint, ex. CO on Rh(111)
Partial coverages vs T and P
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Pt(332)
Pt(332)
Pt(332) - CO
Rh(553) - CO - H
Pt(332) - O
Fingerprints also on a more mesoscopic length scale
Note:• Photons in – electrons out
• Element specific, concentrations, cross-sections depend on element, level, and photon energy
• Surface sensitive, varies with photon energy and emission angle
• The binding energy shifts can be calculated accurately by DFT, total energy di!erences
• The shifts are local, i.e. mainly nearest neighbors
• The shifts can be large even though the initial state total energies are close
• Gives qualitative information on adsorption sites etc. Important input to e.g. DFT
• Adsorbate levels are often good fingerprints, sites, step-adsorption, dissociation
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PdO(101) on top
of Pd(100)
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Note:• Photons in – electrons out
• Element specific, concentrations, cross-sections depend on element, level, and photon energy
• Surface sensitive, varies with photon energy and emission angle
• The binding energy shifts can be calculated accurately by DFT, total energy di!erences
• The shifts are local, i.e. mainly nearest neighbors
• The shifts can be large even though the initial state total energies are close
• Gives qualitative information on adsorption sites etc. Important input to e.g. DFT
• Adsorbate levels are often good fingerprints, sites, step-adsorption, dissociation
• Core level photoemission and DFT (and STM) is an excellent combination
Monitoring an ongoing reduction of a surface oxide
CO on the Rh(111)-(9x9) surface oxide
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In-situ reduction of (2x1) and (9x9) by CO: 1x10-8 mbar and 100 C
In-situ reduction of (2x1) and (9x9) by CO: 1x10-8 mbar and 100 C
2x1
9x9
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In-situ reduction of (2x1) and (9x9) by CO: 1x10-8 mbar and 100 C
• MECHANISM for 9x9 reduction by CO: Reduction nucleates (at defects?), reduced areas grow and chemisorbed oxygen
appears on these areas and reacts with CO.
Surface oxide functions as oxygen source
2x1
9x9
In-situ reduction of (2x1) and (9x9) by CO: 1x10-8 mbar and 100 C
• MECHANISM for 9x9 reduction by CO: Reduction nucleates (at defects?), reduced areas grow and chemisorbed oxygen
appears on these areas and reacts with CO.
Surface oxide functions as oxygen source
2x1
9x9
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Note:• Photons in – electrons out
• Element specific, concentrations, cross-sections depend on element, level, and photon energy
• Surface sensitive, varies with photon energy and emission angle
• The binding energy shifts can be calculated accurately by DFT, total energy di!erences
• The shifts are local, i.e. mainly nearest neighbors
• The shifts can be large even though the initial state total energies are close
• Gives qualitative information on adsorption sites etc. Important input to e.g. DFT
• Adsorbate levels are often good fingerprints, sites, step-adsorption, dissociation
• Core level photoemission and DFT (and STM) is an excellent combination
• Time development can be monitored in-situ (but watch out for sample damage)
Vibrational splittings in Core Level Photoemission from adsorbed molecules
Not all shifts are chemical
J. N. Andersen, et al., Chem. Phys. Lett., 269, 371, 1997.
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Vibrational splittings in Core Level Photoemission from adsorbed molecules
Not all shifts are chemical
J. N. Andersen, et al., Chem. Phys. Lett., 269, 371, 1997.
Franck CondonDiatomic molecule
Ground state
Co
re i
on
ized
st
ate(
s)
Vibrational splittings in Core Level Photoemission from adsorbed molecules
Not all shifts are chemical
J. N. Andersen, et al., Chem. Phys. Lett., 269, 371, 1997.
For CHx radicals:
The number x of H atoms bonding to a particular C atom is proportional to the strength of the first vibrational component relative to the adiabatic peak.
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Ethanol on Rh surfaces at 300K
Ethanol on Rh surfaces at 300K
CO The C1s spectra and the fact that we never see any atomic O indicates that the CO group of the ethanol molecule stays intact
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Ethanol on Rh surfaces at 300K
But what about the peaks below 285 eV ?
Ethanol on Rh surfaces at 300K
But what about the peaks below 285 eV ?
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Ethanol on Rh surfaces at 300K
But what about the peaks below 285 eV ?
On Rh(111) C-H vibration Intensity à CH
On Rh(553) ????
Surface Site Eads (eV) CLS (eV)
Rh(111) Hcp -6.54 0.00
Fcc -6.34 -0.16
Rh(553) Fcc(Up) -6.58 -0.13
Hcp(Terrace) -6.55 0.00
Hcp(Up) -6.53 -0.03
Hcp(Low) -6.44 0.55
Fcc(Low) -6.39 0.04
Fcc(Terrace) -6.28 -0.14
Step(Bridge) -6.16 -0.12
Surface Site Eads (eV) CLS (eV)
Rh(111) Hcp -7.18 0.03
Rh(553) Hcp(Low) -7.54 0.46
Fcc(Up) -7.00 0.33
Hcp(Up) -7.26 0.23
Hcp(Terrace) -7.15 0.05
Fcc(Terrace) -6.75 0.01
Fcc(Low) -6.79 0.06
Bridge(Step) -6.80 0.08
Eads relative to CH in gas-phase
CLS relative CH in hcp site on Rh(111)
Eads relative to C in gas-phase
CLS relative CH in hcp site on Rh(111)
CH C
From the core level shifts, the dominant peak on Rh(553) could
be CH. However, the lack of vibrational structure excludes this.
" " Must be atomic Carbon at the step
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Adsorption energies (referenced to a CH radical in gas phase) for the initial state (left), the transition state (middle), and the final state (right) for the lowest energy dissociation paths on (a): Rh(111) (full) and (b): Rh(553) (dashed). Insert shows the initial (left) and final (right) state geometry for the two surfaces.
It fits:Activation energy for dissociation is lowered by 0.4 eV on Rh(553) and dissociation becomes exothermic
Note:• Photons in – electrons out
• Element specific, concentrations, cross-sections depend on element, level, and photon energy
• Surface sensitive, varies with photon energy and emission angle
• The binding energy shifts can be calculated accurately by DFT, total energy di!erences
• The shifts are local, i.e. mainly nearest neighbors
• The shifts can be large even though the initial state total energies are close
• Gives qualitative information on adsorption sites etc. Important input to e.g. DFT
• Adsorbate levels are often good fingerprints, sites, step-adsorption, dissociation
• Core level photoemission and DFT (and STM) is an excellent combination
• Time development can be monitored in-situ (but watch out for sample damage)
• Not all shifts are chemical. Vibrational shake-up. Electronic shake-ups also exist
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Things were much easier in the old days
Siegbahn, Nordling, Fahlman, Nordberg, Hamrin, Hedman, Johansson, Bergmark, Karlsson, Lindgren, Lindberg,
ESCA - Atomic, Molecular and Solid State Structure Studied by Means of Electron Spectroscopy,
Nova Acta Regiae Societatis Scientiarum Upsaliensis, Almqvist & Wiksell, Uppsala 1967
Conclusions• Photons in – electrons out
• Element specific, concentrations, cross-sections depend on element, level, and photon energy
• Surface sensitive, varies with photon energy and emission angle
• The binding energy shifts can be calculated accurately by DFT, total energy di!erences
• The shifts are local, i.e. mainly nearest neighbors
• The shifts can be large even though the initial state total energies are close
• Gives qualitative information on adsorption sites etc. Important input to e.g. DFT
• Adsorbate levels are often good fingerprints, sites, step-adsorption, dissociation
• Core level photoemission and DFT (and STM) is an excellent combination
• Time development can be monitored in-situ (but watch out for sample damage)
• Not all shifts are chemical. Vibrational shake-up. Electronic shake-ups also exist
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