1. 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

2. Some slides adopted from…

4. ACRONYMS

5. Photon E = hv Ionization of atom Emission of photoelectron KE = hv-BE, where BE= binding energy of electron in that atom e - Photoemission process

6. Since the kinetic energy of an electron is directly related to the binding energy in the solid: KE = hv –E B - Φ analyzer 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

7. 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).

8. 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): E transition = E final (n-1 + 1) - E initial (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/SrTiO 3 )

9. Filaments (at ground) 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

12. Photoemission Process Some electrons will reach the analyzer without undergoing inelastic interactions with solid: KE = hv-BE- Φ analyzer. 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 Peak Note: Background intensity “step” increase occurs at KE< KE peak Why? Why does background increase towards lower KE?

13. Outer Hemisphere (V O ) Inner Hemisphere V I e - E = KE Retarding/focussing lens Retards Electrons to E pass KE-V retard = E pass (V retard varied, E pass constant) e - E = E pass Detector Pass Energy = C(V 0 -V I ) Only electrons with E = E pass +/- δE get thru the analyzer δE increases with E pass Note: Intensity Increases with Pass energy, resolution decreases!

14. Sweeping the retarding voltage allows one to sweep out the electron distribution curve (photoemission spectrum)

16. Detecting Photoelectrons: The Channeltron Horned-shape device Lined with low workfunction phosphor Electron in  many electrons out (cascade) Gain ~ 10 7 e- 10 7 e- V bias

17. E Fermi Valence Band 3p 3s 2p 2s 1s EBEB hv E vacuum Work function, sample surface( Φ surface ) e-e- KE ~ hv –E B - Φ analyzer Photoemission from a core leve

18. Auger: KE of (KL 1 L 2 ) transition = E K -E L1 -E L2 –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

19. hv Because the Fermi levels of the sample and spectrometer are aligned, we only need to know the spectrometer work function,  spec, to calculate BE(1s). Because the Fermi levels of the sample and spectrometer are aligned, we only need to know the spectrometer work function,  spec, to calculate BE(1s). E 1s SampleSpectrometer e-e-e-e- Free Electron Energy Fermi Level, E f Vacuum Level, E v  sample KE(1s)  spec BE(1s) Sample/Spectrometer Energy Level Diagram- Conducting Sample 19 Why Φ analyzer ?

20. 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

21. 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 10 4, very difficult With retardation, ΔE = 0.1 eV requires resolution of 1 part in 100 (much easier!)

23. Conclusion: Practical experience shows initial state effects dominate in XPS (with exceptions): ΔE(Binding) = kΔq i + V i  ground state characteristics. Thus, careful analysis of the XPS spectrum typically yields info regarding chemical bonding in the ground state.

24. Exception: Nanoparticles

25. Exception: Nanoparticles reflect final state screening Binding energy decreases as Pt particle size increases Pt(111) 71.2 eV Oxidized Pt

26. Shift in BE reflects enhanced final state screening with increased particle size. ΔE B = 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

27. Pass Energy and Analyzer Resolution

32. Quantitation: 1.Cross sections, transmission functions, and intensities 2.Attenuation

33. Includes instrumental transmission function, lens factors, etc.

36. 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

37. Alloy A x B y To a first approximation: We have the concentration of A (N A ) is given by I A = N A F A where F = atomic sensitivity factor Thus: N A /N B = (I A F B )/I B F A More accurately, this should be modified by the mean free path λ A : N A /N B = I A F B λ B /I B F A λ A

38. 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