and UV Photoelectron Spectroscopy (XPS/UPS)

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1 Introduction into X-ray and UV Photoelectron Spectroscopy (XPS/UPS)

2 Introduction to X-ray and UV Photoelectron Spectroscopy (XPS/UPS) What is XPS? How can we identify elements and compounds? What is UPS? What is a work function(φ)? Examples of investigations using Φ/UPS/XPS/HIKE

3 What is XPS? X-ray Photoelectron Spectroscopy (XPS), also known as Electron Spectroscopy for Chemical Analysis (ESCA) is a widely used technique to investigate the chemical composition of surfaces. X-ray Photoelectron spectroscopy, based on the photoelectric effect, 1,2 was developed in the mid-1960 s by Kai Siegbahn and his research group at the University of Uppsala, Sweden H. Hertz, Ann. Physik 31,983 (1887). 2. A. Einstein, Ann. Physik 17,132 (1905) Nobel Prize in Physics. 3. K. Siegbahn, et. al., Nova Acta Regiae Soc.Sci., Ser. IV, Vol. 20 (1967) Nobel Prize in Physics.

4 2,0x10 6 intensity [arb.u] E kin [ev]

5 The Photoelectric Process Incident X-ray Ejected Photoelectron 2p 2s Conduction Band Valence Band Free Electron Level Fermi Level L2,L3 L1 XPS spectral lines are identified by the shell from which the electron was ejected (1s, 2s, 2p, etc.). The ejected photoelectron has kinetic energy: KE=hv-BE-Φ Following this process, the atom will release energy by the emission of an Auger Electron. 1s K

6 Auger Relation of Core Hole Emitted Auger Electron 2p 2s Conduction Band Valence Band Free Electron Level Fermi Level L2,L3 L1 L electron falls to fill core level vacancy (step 1). KLL Auger electron emitted to conserve energy released in step 1. The kinetic energy of the emitted Auger electron is: KE=E(K)-E(L2) E(L2)-E(L3). E(L3). 1s K

7 Relative Probabilities of Relaxation of a K Shell Core Hole Probab bility Auger Electron Emission X-ray Photon Emission Note: The light elements have a low cross section for X-ray emission Atomic Number B Ne P Ca Mn Zn Br Zr Elemental Symbol

8 XPS Energy Scale The XPS instrument measures the kinetic energy of all collected electrons. The electron signal includes contributions from both photoelectron and Auger electron lines.

9 1.5x10 5 Cu(111) xx0287_1 2,0x10 6 intensity [arb.u.] 1.0x x10 4 ty [arb.u] E kin [ev] intensit 3.0x x10 5 Cu(111) 2 nd order!! xx0291_1 intensity [arb.u.] 2.0x x x10 5 E kin [ev] 5.0x E kin [ev]

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

11 XPS Energy Scale - Kinetic energy KE = hv - BE - Φ spec Where: BE= Electron Binding Energy KE= Electron Kinetic Energy Φ spec = Spectrometer Work Function Photoelectron line energies: Dependent on photon energy. Auger electron line energies: Not Dependent on photon energy.

12 XPS Energy Scale- Binding energy BE = hv - KE - Φ spec Where: BE= Electron Binding Energy KE= Electron Kinetic Energy Φ spec = Spectrometer Work Function Photoelectron line energies: Not Dependent on photon energy. Auger electron line energies: Dependent on photon energy. The binding energy scale was derived to make uniform comparisons of chemical states straight forward.

13 Fermi Level Referencing Free electrons (those giving rise to conductivity) find an equal potential which is constant throughout the material. Fermi-Dirac Statistics: f(e) = 1 exp[(e-e f )/kt] + 1 f(e) T=0 K kt<<e f 1. At T=0 K: f(e)=1 for E<E f f(e)=0 for E>E f 0 E f 2. At kt<<e f (at room temperature kt=0.025 ev) f(e)=0.5 for E=E f

14 Elemental Shifts

15 Where do Binding Energy Shifts Come From? -or How Can We Identify Elements and Compounds? Pure Element Electron Electron-electron electron repulsion Electron-nucleus nucleus attraction Fermi Level Binding Energy Look for changes here by observing electron binding energies Electron- Nucleus Separation Nucleus

16 Elemental Shifts Binding Energy (ev) Element 2p3/2 3p Fe Co Ni Cu Zn Electron-nucleus attraction helps us identify the elements

17 The Sudden Approximation Assumes the remaining orbitals (often called the passive orbitals) are the same in the final state as they were in the initial state (also called the frozen-orbital orbital approximation). Under this assumption, the XPS experiment measures the negative Hartree-Fock orbital energy: Koopman s Binding Energy E B,K - ε B,K Actual binding energy will represent the readjustment of the N-1 charges to minimize energy (relaxation): E B = E N - 1 f - E N i

18 Chemical Shifts - Electronegativity Effects Carbon-Fluorine Bond Valence Level C 2p Core Level C 1s Fluorine Electro- negativity Electron-nucleus nucleus attraction (Loss of Electronic Screening) C 1s Binding Energy Shift to higher binding energy

19 Chemical Shifts- Electronegativity Effects Functional Binding Energy Group (ev) hydrocarbon C-H, C-C amine C-N alcohol, ether C-O-H, C-O-C Cl bound to C C-Cl F bound to C C-F carbonyl C=O 288.0

20 C1s of ca. ML PFN on intensity (arb. units) binding energy (ev) F F F F F F F F F F F F F F intensity (arb. units) C1s of ca. ML PFP on 1.5 ev multilayer Intensity [arb.u.] 1,0 0,8 0,6 0,4 0,2 C1s, TTNI C1s, OTNI binding energy (ev) Binding Energy [ev]

21 Electronic Effects - Spin-Orbit Coupling C 1s Orbital=s l=0 s=+/-1/2 ls=1/2 Ag 3d 3d 3/2 3d 5/2 6.0 Orbital=d l=2 s=+/-1/2 ls=3/2,5/ Binding Energy (ev) Peak Area 2 : Binding Energy (ev) Cu 2p 2p 3/2 2p 1/ Peak Area 1 : 2 Orbital=p l=1 s=+/-1/2 ls=1/2,3/ Peak Area 3 : 4 4f 5/2 Orbital=f l=3 s=+/-1/2 ls=5/2,7/2 4f 7/ Binding Energy (ev) Binding Energy (ev)

Shake-up: Relaxation energy used to excite electrons in valence levels to bound states (monopole excitation).

22 Final State Effects - Shake-up/ Shake-off Results from energy made available in the relaxation of the final state configuration (due to a loss of the screening effect of the core level electron which underwent photoemission). Shake-up: Relaxation energy used to excite electrons in valence levels to bound states (monopole excitation). Shake-off: Relaxation energy used to excite electrons in valence levels to unbound states (monopole ionization). L(2p) -> Cu(3d)

23 Final State Effects- Shake-up/ Shake-off Ni Metal Ni Oxide

24 Final State Effects- Multiplet Splitting Following photoelectron emission, the remaining unpaired electron may couple with other unpaired electrons in the atom, resulting in an ion with several possible final state configurations with as many different energies. This produces a line which is split asymmetrically into several components.

25 Electron Scattering Effects Energy Loss Peaks e ph + e solid e* ph + e** solid Photoelectrons travelling through the solid can interact with other electrons in the material. These interactions can result in the photoelectron exciting an electronic transition, thus losing some of its energy (inelastic scattering).

26 Electron Scattering Effects Plasmon Loss Peak Al 2s a a a a Metal A=15.3 ev

27 Electron Scattering Effects Plasmon Loss Peak Insulating Material O 1s x4 21 ev

28 Quantitative Analysis by XPS For a Homogeneous sample: I = Nσ DJLλAT where: N = atoms/cm 3 σ = photoelectric cross- section, cm 2 D = detector efficiency J = X-ray flux, photon/cm 2 - sec L = orbital symmetry factor λ = inelastic electron mean-free path, cm A = analysis area, cm 2 T = analyzer transmission efficiency

29 Relative Sensitivities of the Elements d Relative Sensitivity p 4f 4d 2 1s 0 Li B N F Na Al P Cl K Sc V M Co Cu G As Br Rb Y Nb TcRh Ag In Sb I Cs La Pr P Eu TbHo T Lu Ta Re Ir Au Tl Bi Be C O Ne M Si S Ar Ca Ti Cr Fe Ni Zn G Se Kr Sr Zr M Ru Pd Cd SnTe XeBa Ce Nd S G Dy Er Yb Hf W Os Pt Hg Pb Elemental Symbol

30 3.0x x10 5 HATCN on Cu(111) xx0291_1 xx0306_1 2.0x10 5 intens sity [a.u.] 1.5x x10 5 N1s C1s 5.0x E kin [ev]

31 XPS of Copper-Nickel alloy Cu 2p Peak Area Mct-eV/sec Rel. Sens. Atomic Conc % Ni Cu N(E)/E Thousands Ni 2p Cu LMM Cu Ni LMM Cu LMM Ni LMM Ni LMM LMM 20 Ni 3p Cu 3p Binding Energy (ev)

32 Escape depth mean free path Gezeigt ist die Abhängigkeit der freien Weglänge von Elektronen in Festkörpern in Abhängigkeit von deren kinetischer Energie über dem Ferminiveau. Der exakte Streumechanismus ist materialabhängig, aber insgesamt folgt der Zusammenhang einer universellen Kurve, die hier als Band gezeigt ist. Diese hat ein Minimum bei ca. 50 ev. Zu beachten ist die logarithmische Auftragung an beiden Achsen [87], [88].

33 from 2,0x10 1,5x ,0x10 6 5,0x Ekin [ev]

34 surface sensitive data bulk sensitive data hν=190ev hν=630ev bulk sensitive data: the oxidic signal exceeds the metallic Au signal; counts [a.u.] c Au 2 O 3 Au met (b) surface sensitive data: metallic signal more intense than oxidic signal. (a) (c) (b) (a) E B [ev] E B [ev] Au 2 O 3 : 7/2 at 85.7eV binding energy, Au (metallic) at 84eV, chemical shift = 1.7eV

35 HIGH KINETIC ENERGY PHOTOELECTRON SPECTROSCOPY (HIKE) on Thin Film Solar Cells Intensity / a.u. 9000eV 8000eV 7000eV 6500eV 6000eV 5500eV 5000eV 4000eV 3000eV Intensity / a.u. Se2p 1500eV Binding Energy / ev Glass + Mo Se2p Ga2p Zn(O,S) - 15 nm buffer layer Cu(In,Ga)Se 2 (CIGS) Cu2p Intensity / a. u. Cu2p eV Binding Energy / ev Above approximately 5 kev excitation energy the substrate becomes visible! 2500eV Binding Energy / ev Depth profiling of the device is possible! Johansson, Platzer-Björkman, Gorgoi, et al, Rev. Sci. Instrum. 78 (2007) 1.

Cs 2 Te photocathodes for electron accelerators: new, contaminated, used, oxidised composition contamination chemical analysis Intensity [arb.

36 Cs 2 Te photocathodes for electron accelerators: new, contaminated, used, oxidised composition contamination chemical analysis Intensity [arb.units] 900eV photon energy 5x10 4 Cs3 3d sample 1, new 4x10 4 3x10 4 2x10 4 1x10 4 Te 3d O1s Mo 3d Augers Cs and Te signals new oxidised counts / d5/2 2-3d5/2 0 3d3/2 2-3d5/2 6+ measurement 900 ev background 3d3/2 0 3d3/2 6+ plasmon peak Ekin[eV] used E b (ev) Intensity [arb.units] 900eV photon energy 4.0x10 3.5x10 Cs3 3d sample 3, used DESY 3.0x x x x x x10 3 F1s Te 3d Augers C1s Cs and Te signals Ekin[eV] Teflon Contaminated exposed to bad vacuum counts #90.1 (fresh) #92.1 (used) photo emissive band valence bands core like Cs 5p E b (ev)

37 Counts C 2,0x10 6 Secondary electron cutoff (SECO) HOMO or E F E kin,seco E kin,homo Ekin,EF E kin

38 Counts 2,0x10 6 Secondary electron cutoff (SECO) valence states UPS: valence orbitals HOMO or E F interface states E kin,seco E E E kin,homo kin,ef E kin ionization energy = hν (E kin,homo E kin,seco ) work function = hν (E kin,ef E kin,seco ) intensity [arb.u.] 2.4x x x x x x x x x10 4 C60 on CH8000 Z0115_1 Z0117_1 Z0119_1 Z0120_1 Z0121_1 Z0131_1 Z0132_1 Z0133_1 Z0136_1 6.0x10 4 hole injection barrier = E kin,ef E kin,homo 4.0x x E kin [ev]

bond formation Permanent dipoles Ishii, Sugiyama, Ito, Seki, Adv.

39 Workfunction Free Electron Vacuum Level, E v hv Fermi Level, E f Φ sample E 1s Binding energy Push-back effect Charge transfer Covalent bond formation Permanent dipoles Ishii, Sugiyama, Ito, Seki, Adv. Mater. 11, 605 (1999); Kahn, Koch, Gao, J. Poly. Sci. B 41, (2003)

40 Intensity [arb.units] 1,0 0,8 0,6 0,4 0,2 Cu(111) Ag(111) E binding [ev] intensity (arb. units) θ e = 45 (1) (2) binding energy (ev) MT(Å) intensity (arb. units) MT(Å) PEN kinetic energy (ev) energy level diagrams of PEN/ E va c E F φ Au =5.50 va c,pen =0.95 φ PEN = (1) (2)

41 2,0x10 1,5x ,0x10 6 5,0x Ekin [ev]

42 k 2,0x10 6 REMINDER e y c s Strommessung e - Photoemission provides information on: e e e e e e e e e e e e e e e e e e e e e e e e e s V l z Schale: M L E - Chemical composition - Chemical state/reaction - Electronic state - Interfaces/interface reactions - Quantitative analysis - Depth profiling e e K 4 ertins / Ch. Jung / M. Mast ekt.dsf

43 Thank you!

Introduction to X-ray Photoelectron Spectroscopy (XPS) XPS which makes use of the photoelectric effect, was developed in the mid-1960

Introduction to X-ray Photoelectron Spectroscopy (XPS) X-ray Photoelectron Spectroscopy (XPS), also known as Electron Spectroscopy for Chemical Analysis (ESCA) is a widely used technique to investigate