and UV Photoelectron Spectroscopy (XPS/UPS)

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

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

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. 3 1. H. Hertz, Ann. Physik 31,983 (1887). 2. A. Einstein, Ann. Physik 17,132 (1905). 1921 Nobel Prize in Physics. 3. K. Siegbahn, et. al., Nova Acta Regiae Soc.Sci., Ser. IV, Vol. 20 (1967). 1981 Nobel Prize in Physics.

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

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

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

Relative Probabilities of Relaxation of a K Shell Core Hole Probab bility 1.0 0.8 0.6 0.4 0.2 0 Auger Electron Emission X-ray Photon Emission Note: The light elements have a low cross section for X-ray emission. 5 10 15 20 25 30 35 40 Atomic Number B Ne P Ca Mn Zn Br Zr Elemental Symbol

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.

1.5x10 5 Cu(111) xx0287_1 2,0x10 6 intensity [arb.u.] 1.0x10 5 5.0x10 4 ty [arb.u] 0.0 700 800 900 1000 1100 1200 E kin [ev] intensit 3.0x10 5 2.5x10 5 Cu(111) 2 nd order!! xx0291_1 intensity [arb.u.] 2.0x10 5 1.5x10 5 1.0x10 5 E kin [ev] 5.0x10 4 0.0 E kin [ev]

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).

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.

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.

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) 1.0 0.5 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

Elemental Shifts

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

Elemental Shifts Binding Energy (ev) Element 2p3/2 3p Fe 707 53 654 Co 778 60 718 Ni 853 67 786 Cu 933 75 858 Zn 1022 89 933 Electron-nucleus attraction helps us identify the elements

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

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

Chemical Shifts- Electronegativity Effects Functional Binding Energy Group (ev) hydrocarbon C-H, C-C 285.0 amine C-N 286.0 alcohol, ether C-O-H, C-O-C 286.5 Cl bound to C C-Cl 286.5 F bound to C C-F 287.8 carbonyl C=O 288.0

C1s of ca. ML PFN on intensity (arb. units) 290 288 286 284 282 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 290 288 286 284 282 280 Intensity [arb.u.] 1,0 0,8 0,6 0,4 0,2 C1s, TTNI C1s, OTNI 290 288 286 284 282 280 binding energy (ev) 280 282 284 286 288 290 292 294 296 298 Binding Energy [ev]

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=3/2,5/2 290 288 284 280 276 Binding Energy (ev) Peak Area 2 : 3 378 374 370 366 362 Binding Energy (ev) Cu 2p 2p 3/2 2p 1/2 19.8 Peak Area 1 : 2 Orbital=p l=1 s=+/-1/2 ls=1/2,3/2 3.65 Peak Area 3 : 4 4f 5/2 Orbital=f l=3 s=+/-1/2 ls=5/2,7/2 4f 7/2 965 955 945 935 925 Binding Energy (ev) 91 87 83 79 Binding Energy (ev)

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)

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

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.

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).

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

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

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

Relative Sensitivities of the Elements 12 10 3d Relative Sensitivity 8 6 4 2p 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

3.0x10 5 2.5x10 5 HATCN on Cu(111) xx0291_1 xx0306_1 2.0x10 5 intens sity [a.u.] 1.5x10 5 1.0x10 5 N1s C1s 5.0x10 4 0.0 0 700 E kin [ev]

XPS of Copper-Nickel alloy 120 100 80 Cu 2p Peak Area Mct-eV/sec Rel. Sens. Atomic Conc % Ni 2.65 4.044 49 Cu 3.65 5.321 51 N(E)/E Thousands 60 40 Ni 2p Cu LMM Cu Ni LMM Cu LMM Ni LMM Ni LMM LMM 20 Ni 3p Cu 3p 0-1100 -900-700 -500-300 -100 Binding Energy (ev)

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].

from www.lasurface.com 2,0x10 1,5x10 6 6 1,0x10 6 5,0x10 5 100 200 300 400 Ekin [ev] 500 600

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) 90 85 80 75 E B [ev] 95 90 85 80 E B [ev] Au 2 O 3 : 7/2 at 85.7eV binding energy, Au (metallic) at 84eV, chemical shift = 1.7eV

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 1480 1460 1440 1420 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 920 940 960eV Binding Energy / ev Above approximately 5 kev excitation energy the substrate becomes visible! 2500eV 2000 1500 1000 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.units] survey @ 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 / 100 160 120 80 40 3d5/2 2-3d5/2 0 3d3/2 2-3d5/2 6+ measurement 900 ev background 3d3/2 0 3d3/2 6+ plasmon peak 0 0 200 400 600 800 1000 Ekin[eV] used 0 565 570 575 580 585 590 595 E b (ev) Intensity [arb.units] survey @ 900eV photon energy 4.0x10 3.5x10 Cs3 3d sample 3, used DESY 3.0x10 4 2.5x10 4 2.0x10 4 1.5x10 4 1.0x10 4 5.0x10 3 F1s Te 3d Augers C1s Cs and Te signals 0.0 0 200 400 600 800 1000 Ekin[eV] Teflon Contaminated exposed to bad vacuum counts 5000 4500 4000 3500 3000 2500 2000 1500 1000 #90.1 (fresh) #92.1 (used) photo emissive band valence bands core like Cs 5p 500 0 0 2 4 6 8 10 12 14 16 18 20 E b (ev)

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

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.] 2.4x10 5 2.2x10 5 2.0x10 5 1.8x10 5 1.6x10 5 1.4x10 5 1.2x10 5 1.0x10 5 8.0x10 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 4.0x10 4 2.0x10 4 0.0 20 25 30 35 40 E kin [ev]

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, 2529-2548 (2003)

Intensity [arb.units] 1,0 0,8 0,6 0,4 0,2 Cu(111) Ag(111) 14 12 10 8 6 4 2 0 E binding [ev] intensity (arb. units) θ e = 45 (1) (2) 3 2 1 0 binding energy (ev) MT(Å) 0 2 4 8 16 50 110 intensity (arb. units) MT(Å) PEN 0 2 4 8 16 50 110 9 10 11 kinetic energy (ev) energy level diagrams of PEN/ E va c E F φ Au =5.50 va c,pen =0.95 φ PEN =4.55 0.60 0.90 (1) (2)

2,0x10 1,5x10 6 6 1,0x10 6 5,0x10 5 100 200 300 400 Ekin [ev] 500 600

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

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