1. Lecture 5—chemical shift1 Quantization and depth effects, XPS and Auger I.XPS: The Chemical Shift II.Mean free path, overlayer attenuation, etc. III.Auger spectroscopy, final state effects

2. The XPS Chemical Shift: Shifts in Core level Binding Energies with Chemical State 2 ΔE Chemical Shift In part fromC. Smart, et al., Univ. Hong Kong and UWO

3. The binding energy is defined as: E b = hv –E k –Φ Where hv= photon energy E k = kinetic energy of the photoelectron Φ = work function of the spectrometer Specifically, the CHEMICAL SHIFT is ΔE b That is the change in E b relative to some chemical standard 3Binding energies and particle size

4. 4 Chemical Shift in Au compounds vs. bulk elemental gold PHI handbook

5. EFEF E vacuum EBEB hv E kin e- Φ spectrometer Because the electron emitted from the solid has to impact on the analyzer/dectector to be counted, the relationship E kin and E B has to include the work function term of the detector (typically, 4-5 eV): E kin = hv-E B – Φ spectrometer We only need the work function term for the spectrometer, not the sample, because (for a conducting sample) the two Fermi levels are coupled. Obviously, electrically insulating samples present problems (Charging) E vacuum E kin 5

6. EFEF E vacuum EBEB hv E kin e- Φ spectrometer E vacuum E kin Changes in E B result from : 1.Changes in oxidation state of the atom (initial state effect) 2.Changes in response of the system to the core hole final state: 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 6 mainly sometimes

7. 7

8. 8 Primarily an initial state effect

9. 9 ΔE b = kΔq i + ΔV ij V ij often similar in different atoms of same material, so Δv ij is typically negligible

10. Binding energies and particle size10 ΔE b = kΔq i + ΔV ij Initial state term, often similar for diff. atoms in same molecule In principle, can be obtained from ground state Mulliken Charge Density calculations Valence charge is removed or added to an atom by interaction with surrounding atoms.

11. Binding energies and particle size11  Chemical shift is dominated by changes in ground state valence charge density:  Changes in valence charge density dominated by nearest-neighbor interactions  Qualitative interpretation on basis of differences in ground state electronegativities

12. Binding energies and particle size12 C O e- EN = 2.5EN = 3.5 CTi EN = 1.5 e- C C O withdraws valence charge from C: C(1s) shifts to higher BE relative to elemental C (diamond) at 285.0 eV Elemental C: binding energy = 285.0 eV Ti donates charge to C, binding energy shifts to smaller values relative to 285 eV

13. 13 Thus, a higher oxidation state (usually) yields a higher binding energy!

14. Binding energies and particle size14 Electron withdrawing groups shift core levels to higher binding energy

15. Binding energies and particle size15 Binding energy shifts can be used to follow the course of surface reactions for complex materials: e.g., atomic O /(Pt)NiSi (e.g., Manandhar, et al., Appl. Surf. Sci. 254(2008) 7486 = Ni = Si NiSi (Schematic, not real structure) Bulk Vacuum Atomic O

16. Binding energies and particle size16 Pauling Electronegativities, Ground State Si = 1.8 O = 3.5 Ni = 1.8 Ni-O or Si-O formation  shift of Ni or Si to higher BE Question: Ni-Si  Ni-Ni. Which way should BE move (think).

17. 17 Si SiO 2 Exposure to atomic O XPS binding energy shifts for Pt-doped NiSi as a function of exposure to atomic O at room temp. (Manadhar, et al., Appl. Surf. Sci. 254 (2008) 7486 SiO 2 peak appears (shift to higher BE) Ni (2p) shifts to lower BE. Why?

18. PtSi  Pt 1+y Si NiSi  Ni 1+x Si Si transport and oxidation Pt 1+y Si Ni 1+x Si (B) Si transport kinetically inhibited, metal oxidation Si  SiO 2 Pt silicate formation (A) Preferential Si oxidation, Si flux creates metal-rich substrates O + O 2

19. 19 How do we estimate q, Δq? This is usually done with Mulliken atomic charge densities, originally obtained by LCAO methods: Ψ MO = c a Φ a + c b Φ b Φ a(b) atomic orbital on atom a (b)  Ψ  2 = c a c a * Φ a Φ a * + [cross terms] + c b c b * Φ b Φ b * Atomic charge on atom a Atomic charge on atom b Overlap charge

20. C 2 -B-H C-B-H B-B-H RC-BRC-B Different Boron Environments in orthocarborane derived films (B 10 C 2 H X and B 10 C 2 H X :Y) R c =Ring carbon

21. Figure 3 B 2 -B CB-B C 2 -B C 2 -B-H C-B-H B-B-H

22. Binding energies and particle size22 Chemical Shifts: Final Note Calculating ground state atomic charge populations with DFT: Minimal basis sets give best results (LCAO-MO) Such basis sets are not best for lowest energy/geometric optimization

23. Binding energies and particle size23 Attenuation: Clean surface of a film or single crystal hv e- I = I 0 d film or single crystal with overlayer of thickness d I = I 0 exp(-d/λ) hv Issues: 1.Average coverage 2.Calculating λ 3.Relative vs. Absolute intensities

24. Binding energies and particle size24 Monolayer Surface coverage = Θ 1 d = d 1 Bilayer Surface coverage = Θ 2 d = d 2 Bare surface Coverage = 1-( Θ 1 +Θ 2 ) We can only measure a total intensity from a macroscopic area of the surface: I = [1-( Θ 1 +Θ 2 )] I 0 + Θ 1 I 0 exp[-d 1 /λ] + Θ 2 1 I 0 exp[-d 2 /λ] = I 0 exp[-d ave /λ]  we can only determine average coverage with XPS!

25. Binding energies and particle size25 Consider 2 cases: 1.d ave < 1 ML (0< Θ<1) 2.d ave > 1 ML ( Θ> 1) We need to look at the RATIO of I substrate (I B ) and I overlayer (I A ) Why? Absolute intensity of I B can be impacted by: 1.Small changes in sample position 2.Changes in x-ray flux I B /I A will remain constant

26. Binding energies and particle size26 Calculation of the overlayer coverage First, we need to calculate the IMFP of the electrons of the substrate through the overlayer and the IMFP of the electrons in the overlayer. The formula to calculate the IMFP is (NIST): IMFP=E/E p 2 ([βln(γE)-(C/E)+(D/E2])

27. Binding energies and particle size27 ElementNvNv ρ ( g cm -3 ) M E (ener gy) E g (Band Gap EpEp βγUCD(E p ) 2 ln(γE)(C/E) (D/E 2 ) [βln(γE)- (C/E)+(D/E 2 ] E p 2 ([βln(γE)- (C/E)+(D/E 2 ]) IMFP=E/E p 2 ([βln(γE) -(C/E)+(D/E2]) Sulfur62.07321520 17.94 2308 0.026 8203 0.1327 5418 0.3881 4372 1.616 789 45.326 6107 321.9 264 3.0046 2437 0.010 637 0.001 962 0.07190997 823.149720236.56595408 Co98.958.97651 33.58 5444 0.013 9544 0.0640 2335 1.3599 9767 0.732 402 25.112 0484 1127. 982 3.8913 6834 0.000 957 4.29E -05 0.05338727 660.2198904512.7034439 O in MgO th-- C62.2512.01722.60 30.53 4293 0.005 7446 0.1273 3333 1.1241 1749 0.947 053 30.018 3562 932.3 43 4.5219 0886 0.001 311 5.75E -05 0.02472347 923.0507637431.34820209 C in C th--C42.2512.012650 24.93 1146 0.012 6928 0.1273 3333 0.7494 1166 1.288 035 37.812 2375 621.5 62 3.5187 8287 0.004 861 0.000 5380.0403412725.0746016410.56846301 Co Thr--C92.2558.97650 16.88 68 0.030 7302 0.1273 3333 0.3438 1964 1.657 124 46.248 5516 285.1 64 4.5789 2887 0.002 166 7.9E- 050.1386242739.530652219.3520713 C in C th--Co42.2512.0112010 24.93 1146 0.012 6928 0.1273 3333 0.7494 1166 1.288 035 37.812 2375 621.5 62 5.0299 6286 0.001 072 2.62E -05 0.06279824 339.0330037430.76883368 O Thru C62.2512.015000 30.53 4293 0.005 7446 0.1273 3333 1.1241 1749 0.947 053 30.018 3562 932.3 43 4.1536 6114 0.001 894 0.000 12 0.02208713 520.5927869324.2803464 Ni Thru C152.2512.01632.20 48.27 8956 - 0.005 6184 0.1273 3333 2.8102 9373 - 0.587 367 - 5.0541 0956 2330. 858 4.3882 5884 - 0.000 93 -1.3E- 05 - 0.02373862 2-55.33134684-11.42571139 Mg thru C22.2512.01 1435. 50 17.62 8982 0.028 3767 0.1273 3333 0.3747 0583 1.629 018 45.606 1187 310.7 81 5.2083 2154 0.001 135 2.21E -05 0.14668249 445.5861344231.48983827 Fe thru C62.2512.01775.10 30.53 4293 0.005 7446 0.1273 3333 1.1241 1749 0.947 053 30.018 3562 932.3 43 4.5920 4509 0.001 2225E-05 0.02520763 323.5021609832.979946 Mg thru MgO83.5840 1435. 50 24.36 9634 0.017 1225 0.1009 4664 0.7160 3453 1.318 409 38.506 4818 593.8 79 4.9761 0525 0.000 918 1.87E -05 0.08430378 250.0662491328.67201009

28. Terms used in the excel sheet (example Carbon through MgO) ColumnTerm used 1Valence electrons of the element (O) 2Density of the over layer (Carbon) 3Mass of the over layer 4Kinetic Energy of the element(O) After you insert all the four columns, the IMFP is calculated on its own.

29. Binding energies and particle size29 overlayersubstrateoverlayersubstrate dA(Ini*IcS)B(Ic*IniS)CSi 076600 82642.42 17374.8512454.299766082642.42 27100.3174835.711766082642.42 36836.0037146.401766082642.42 46581.5289388.467766082642.42 56336.52611563.95766082642.42 66100.64413674.83766082642.42 75873.54315723.01766082642.42 85654.89717710.37766082642.42 95444.38919638.71766082642.42 105241.71821509.78766082642.42 115046.59123325.29766082642.42 124858.72925086.88766082642.42 134677.85926796.15766082642.42 144503.72228454.67766082642.42 154336.06830063.92766082642.42 164174.65531625.39766082642.42 174019.25133140.48766082642.42 183869.63134610.58766082642.42 193725.58136037.02766082642.42 203586.89437421.1766082642.42 213453.36938764.08766082642.42 223324.81540067.17766082642.42 233201.04741331.56766082642.42 243081.88542558.4766082642.42 252967.1643748.81766082642.42 262856.70644903.87766082642.42 272750.36346024.62766082642.42 282647.97847112.09766082642.42 292549.40648167.26766082642.42 302454.50249191.1766082642.42 312363.13250184.53766082642.42 322275.16251148.46766082642.42 =D6*EXP(-A6/26.36) =E6*(1-EXP(-A6/33.17)) =Area under the curve1915/0.25 =Area under the curve 54544/0.66

30. Binding energies and particle size30

31. Take-off angle variations in XPS: Definition θ Take off angle (θ) is the angle between the surface normal and the axis of the analyzer. (Some people use 90-θ) Surface normal θ = 0  normal emission θ=89   grazing emission

32. Take-off angle variations in XPS: Intensity vs. θ Intensity of a photoemission peak goes as I ~ I cosθ Therefore, intensities of adsorbates and other species are NOT enhanced at grazing emission (large θ)!

33. Take-off angle variations in XPS: Sampling Depth (d) normal emission (θ = 0) d ~ λ (inelastic mean free path) λ λ λcosθ θ increased take-off angle: d~ λ cosθ (reduced sampling depth)

34. d~ λ cosθ: Effective sampling depth (d) decreases as θ increases Relative intensities of surface species enhanced relative to those of subsurface: Si SiO 2 λ Si SiO 2 λcosθ SiO 2 Si

35. In Dragon and other systems: Si SiO 2 Ta sample holders Arrangement of sample holder may cause increased signal from Ta or other extraneous materials. These should be monitored. However, enhancement of SiO 2 relative to Si will remain the same.

36. Binding energies and particle size36

37. Binding energies and particle size37 Multiplet Splitting: 1.Valence electrons give rise to different spin states (crystal field, etc.  Cu 2p 3/2 vs. ½ states 2.Formation of a core hole shell yields an unpaired electron left in the shell 3.Coupling between the core electron spin and valence spins gives rise to final states with different total angular momentum.

38. Binding energies and particle size38 2p 1/2 2p 3/2 Multiplet splitting in Cu

39. Binding energies and particle size39 Auger Spectroscopy: Final State Effects hv or e- XPS initial State XPS Final State Auger Initial State Auger Final State

40. Binding energies and particle size40 Kinetic Energy of Auger Electron: This transition is denoted as (KLL) K (1s) L 1 (2s) L 2,3 (2p) e- K (1s) L 1 (2s) L 2,3 (2p) Initial state Final State KE Auger = E K - E L1 – E L2,3 - U eff ~ E K – E L -E L - U eff Note: Auger transitions are broad, and small changes in BE (E L1 vs. E L2,3 ) sometimes don’t matter that much (sloppy notation) What is U eff ? e-detector

41. Binding energies and particle size41 K (1s) L 1 (2s) L 2,3 (2p) U eff is the coulombic interaction of the final state holes, as screened by the final state response of the system: e.g., Jennison, Kelber and Rye “Auger Final States in Covalent Systems”, Phys. Rev. B. 25 (1982) 1384

42. Binding energies and particle size42 For a typical metal, the final state holes are often delocalized (completely screened), and U eff ~ 0 eV. However, for adsorbed molecules, or nanoparticles, the holes are constrained in proximity to each other. U eff can be large, as large as 10 eV or more. Nanoparticle, U eff ~ 1/R R Heat in UHV Agglomeration, should see shift in Auger peak as U eff decreases

43. Binding energies and particle size43 KE(LVV) = E L –E V – E V – U eff as particle size increases, U eff decreases Note shift in Cu(LVV) Auger as nanoparticles on surface agglomerate J. Tong, et al. Appl. Surf. Sci. 187 (2002) 253 Cu/Si:O:C:H

44. Binding energies and particle size44 Similar effects in Auger KE are seen for agglomeration during Cu deposition at room temp. (Tong et al.) Note corresponding change in Cu(2p 3/2 ) binding energy. Cu(LVV) shift with increasing Cu coverage

45. Binding energies and particle size45 Auger in derivative vs. integral mode When doing XPS, x-ray excited Auger spectra are acquired along with photoemission lines

46. Binding energies and particle size46 Auger spectra, though broad, can give information on the chemical state (esp. if the XPS BE shift is small as in Cu(0) vs. Cu(I) Above spectra are presented in the N(E) vs. E mode—or “integral mode”

47. Binding energies and particle size47  However, in some cases Auger spectroscopy is used simply to monitor surface cleanliness, elemental composition, etc. This often involves using electron stimulated Auger (no photoemission lines).  Auger spectra are typically broad, and on a rising background. Presenting spectra in the differential mode (dN(E)/dE) eliminates the background.  Peak-to-peak height (rather than peak area) is proportional to total signal intensity, and the background issue is eliminated. Except in certain cases, however, (e.g., C(KVV)) most chemical bonding info is lost.

48. Binding energies and particle size48 Auger (derivative mode) of graphene growth on Co 3 O 4 (111)/Co(0001) (Zhou, et al., JPCM 24 (2012) 072201 Homework: explain the data on the right.

49. Binding energies and particle size49 N(E) KE Peak-to-peak height