photoemission fundamentals of data acquisition and analysis j. a. kelber, june 12 2007 texts: phi...
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Photoemission Fundamentals of Data Acquisition and Analysis
J. A. Kelber, June 12 2007
Texts: PHI handbook, Briggs and Seah
Outline:
I. Photoemission process
II. How an xray source works
III. How electrons enter the analyzer
IV. What do we mean by Pass Energy?
V. Atomic Sensitivity Factors
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Some slides adopted from…
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ACRONYMS
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Photon E = hv
Ionization of atom
Emission of photoelectron
KE = hv-BE, where BE= binding energy of electron in that atom
e-
Photoemission process
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Since the kinetic energy of an electron is directly related to the binding energy in the solid:
KE = hv –EB - Φanalyzer
We can use core level photoemission for:
(1)Quantitative analysis of surface/near surface compositions
(2)Bonding environment of a given atom (small changes in KE, the “chemical shift”
(3)Electronic structure of the valence band
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3 step model of photoemission:
Originally due to Spicer(e.g., Lindau and Spicer, J. El. Spect. and Rel. Phen. 3 (1974) 409)
1.Step 1: Excitation of photoelectron (cross sections, rel. intensities)
2.Step. 2. Response of the system to the core hole (final state effects, like screening of the core hole, shakeup)
3.Step. 3. Transport of the photoelectron to the surface and into the vacuum . (Inelastic mean free path considerations).
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Caution: Note that rigorously, the energy of a photoelectron transition is the difference in energy between the initial (ground state of the system with n electrons, and the final state, with n-1 electrons around the atom (ion) and an electron in the vacuum (n-1 + 1):
Etransition = Efinal(n-1 + 1) - Einitial (n)
Therefore, the energy of the transition therefore reflects screening of the core hole in the final state. This is generally not a factor in most uses of XPS, but can be important in, e.g., determining the size of metal nanoparticles. (see publication for Pt/SrTiO3)
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Filaments (at ground)
e-
e-
e-
e-
Al
Mg+15 KeV
Electrons emitted from one of two filaments (depending on source selected)
Electrons at 15 KeV strike Al or Mg anode, causing emission of characteristic x-rays; Kα, Kβ, etc. + background
User selects one or other anode for use
hv=1483.6eV
hv = 1253.6 eV
X-ray Source
Emits characteristic lines, but also other lines that can broaden spectra
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Photoemission Process
Some electrons will reach the analyzer without undergoing inelastic interactions with solid: KE = hv-BE-Φanalyzer. These electrons (Auger or photemission) will occur as elastic, or characteristic peaks in the electron emission spectrum
Others will interact with the solid and lose energy (and chemical information). This contributes to the secondary electron background
KE
N(E)
Background
Elastic PeakNote: Background intensity “step” increase occurs at KE< KEpeak Why?
Why does background increase towards lower KE?
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Outer Hemisphere (VO)
Inner Hemisphere VI
e- E = KE
Retarding/focussing lens
Retards Electrons to Epass
KE-Vretard = Epass (Vretard varied, Epass constant)
e- E = Epass
Detector
Pass Energy = C(V0-VI)
Only electrons with E = Epass+/- δE get thru the analyzer
δE increases with Epass
Note: Intensity Increases with Pass energy, resolution decreases!
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Sweeping the retarding voltage allows one to sweep out the electron distribution curve (photoemission spectrum)
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Detecting Photoelectrons: The Channeltron
Horned-shape device
Lined with low workfunction phosphor
Electron in many electrons out (cascade)
Gain ~ 107
e-
107 e- Vbias
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EFermi
Valence Band
3p
3s
2p
2s
1s
EB
hv
Evacuum
Work function, sample surface(Φsurface)
e-
KE ~ hv –EB - Φanalyzer
Photoemission from a core leve
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Auger: KE of (KL1L2) transition = EK-EL1-EL2 –U(final state)Is independent of excitation source energy
However, when plotting BE (along with XPS data), the peak position depends on hv.
More on Auger later on
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hv
Because the Fermi levels of the sample and spectrometer are Because the Fermi levels of the sample and spectrometer are aligned, we only need to know the spectrometer work function, aligned, we only need to know the spectrometer work function, specspec, to calculate BE(1s). , to calculate BE(1s).
EE1s1s
SampleSample SpectrometerSpectrometer
ee--
Free Electron EnergyFree Electron Energy
Fermi Level, EFermi Level, Eff
Vacuum Level, EVacuum Level, Evv
sample
KE(1s) KE(1s)
spec
BE(1s)
Sample/Spectrometer Energy Level Diagram- Conducting Sample
19
Why Φanalyzer?
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Binding Energy:
The binding energy is calculated:
BE = hv-KE-φ
where φ = detector work function (normally 3-5 eV)
φ is typically used as “fudge factor’ to align a calibration peak with accepted literature values prior to the start of the experiment
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Why do we use constant pass energy?
1. Resolution Constant, with kinetic Energy
2. Easier to quantitatively compare peaks at different energies
Why do we retard electrons?
1. If we did not retard electrons:
ΔE = 0.1 eV would require resolution of 1 part in 104, very difficult
With retardation, ΔE = 0.1 eV requires resolution of 1 part in 100 (much easier!)
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Conclusion:
Practical experience shows initial state effects dominate in XPS (with exceptions):
ΔE(Binding) = kΔqi + Vi ground state characteristics.
Thus, careful analysis of the XPS spectrum typically yields info regarding chemical bonding in the ground state.
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Exception: Nanoparticles
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Exception: Nanoparticles
Exception: Nanoparticles reflect final state screening
Binding energy decreases as Pt particle size increases
Pt(111)71.2 eV
Oxidized Pt
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Shift in BE reflects enhanced final state screening with increased particle size.
ΔEB = ΔE(in.state) – ΔR + other effects (e.g., band bending)
where ΔR = changes in the relaxation response of the system to the final state core hole (see M.K. Bahl, et al., Phys. Rev. B 21 (1980) 1344
Limited charge, small screening
Larger screening response
d
ΔR ~ d See Vamala, et al, and references therein
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Pass Energy and Analyzer Resolution
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Quantitation:
1.Cross sections, transmission functions, and intensities
2.Attenuation
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Includes instrumental transmission function, lens factors, etc.
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Transmission Functions (T)
T = T(KE) probability of an electron of KE going thru the analyzer to the detector
Typically, T~KE-1/2 , but this can be analyzer dependent.
Atomic sensitivity factors typically “adjusted by some manufactures—e.g., PHI has adjusted spot size (lens ) to change with KE. For other manufacturers, can use Scofield cross-sections
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Alloy AxBy
To a first approximation: We have the concentration of A (NA) is given byIA = NA FA where F = atomic sensitivity factor
Thus: NA/NB = (IA FB)/IBFA
More accurately, this should be modified by the mean free path λA:
NA/NB = IAFBλB/IBFAλA
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Summary:
XPS typically done with laboratory-based Al or Mg anode sources
Quantitative surface region analysis possible
Hemispherical Analyzer, Retarding mode is the preferred laboratory tool
Still to come:
Chemical Shift
Mean free path and attenuation,
Auger and final state effects