5) Surface photoelectron spectroscopy. For MChem, Spring, Dr. Qiao Chen (room 3R506) University of Sussex.
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1 For MChem, Spring, ) Surface photoelectron spectroscopy Dr. Qiao Chen (room 3R506) University of Sussex
2 Today s topics 1. Element analysis with XPS Binding energy, chemical shift, quantitative measurement, depth profile. 2. Auger electron spectroscopy 3. Introduce UPS Mechanism, work function, ARUPS
3 Challenges in surface science Q: what do we want to know about a surface? What s on the surface? a. What s on the surface? Surface composition, elements, functional groups b. What s the surface structure? Periodicity and orientation of the adsorbate. c. What s the surface energy? Bonding, interaction, charging, work function and electronic states. d. What s the physical and chemical properties? Catalyst, electrodes, corrosion resist, bio-compatible, non-sticking, low friction,..the limit is your imagination. Determine Surface composition, elements, bonding, charging work function
4 Surface Analysis The Study of the Outer-Most Layers of Materials (<100 ). Electron Spectroscopies XPS: X-ray Photoelectron Spectroscopy AES: Auger Electron Spectroscopy UPS: UV photoelectron spectrocopy EELS: Electron Energy Loss Spectroscopy Optical Spectroscopies RAIRS: Reflection absorption inferred spectroscopy SERS: Surface enhanced Raman Spectroscopy Ion Spectroscopies SIMS: Secondary Ion Mass Spectrometry SNMS: Sputtered Neutral Mass Spectrometry ISS: Ion Scattering Spectroscopy
5 Photoelectron spectroscopy E kin =hv-e B - E kin : electron kinatic energy hv: photon energy E B : binding energy : workfunction
6 Incident X-rayX The Photoelectric Process Ejected Photoelectron 2p Conduction Band Valence Band Free Electron Level Fermi Level L2,L3 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 hv-be- Following this process, the atom will release energy by the emission of an Auger Electron. 2s L1 1s K A photo induced ionisation process. Comparison with electrochemistry?
7 Introduction to X-ray Photoelectron Spectroscopy (XPS)
8 Introduction to X-ray Photoelectron Spectroscopy (XPS) What is XPS?- General Theory How can we identify elements and compounds? Instrumentation for XPS
9 What is XPS? X-ray Photoelectron Spectroscopy (XPS), also known as Electron Spectroscopy for Chemical Analysis (ESCA) Photoemission as an analytical tool Kai Siegbahn, Nobel Prize 1981
10 X-ray Photoelectron Spectroscopy Small Area Detection X-ray penetration depth ~1 m. Electrons can be excited in this entire volume. X-ray Beam Electrons are extracted only from a narrow solid angle. 1 mm 2 10 nm X-ray excitation area ~1x1 cm 2. Electrons are emitted from this entire area Difficult to focus x-ray High intensity required Use electron analyser to reduce the sample area.
11 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.
12 XPS Energy Scale- Kinetic energy KE = hv - BE - spec Where: BE= = Electron Binding Energy KE= = Electron Kinetic Energy spec = Spectrometer Work Function Photoelectron line energies: Dependent on photon energy. Auger electron line energies: Not Dependent on photon energy. If XPS spectra were presented on a kinetic energy scale, one would need to know the X-ray X source energy used to collect the data in order to compare the chemical states in the sample with data collected using another source.
13 XPS Energy Scale- Binding energy BE = hv - KE - spec Where: BE= = Electron Binding Energy KE= = Electron Kinetic Energy 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.
14 Elemental Shifts-binding energy Binding Energy (ev) Element 2p 3/2 3p Fe Co Ni Cu Zn Electron-nucleus attraction helps us identify the elements
15 Elemental Shifts
16 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
17 Chemical Shifts- Electronegativity Effects Carbon-Oxygen Bond Oxygen Atom Valence Level C 2p Core Level C 1s Electron-oxygen oxygen atom attraction (Oxygen Electro- negativity) Electron-nucleus nucleus attraction (Loss of Electronic Screening) C 1s Binding Energy Shift to higher binding energy Carbon Nucleus
18 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
19 Electronic Effects Spin-Orbit Coupling C 1s Cu 2p 2p 3/2 Orbital=s l=0 s=+/-1/2 ls=1/2 2p 1/ Peak Area 1 : 2 Orbital=p l=1 s=+/-1/2 ls=1/2,3/ Binding Energy (ev) Binding Energy (ev) 3d 5/2 Ag 3d 3d 3/2 6.0 Peak Area 2 : Binding Energy (ev) Orbital=d l=2 s=+/-1/2 ls=3/2,5/2 Au 4f 3.65 Peak Area 3 : 4 4f 5/2 Orbital=f l=3 4f 7/2 s=+/-1/2 ls=5/2,7/ Binding Energy (ev)
20 Quantitative Analysis by XPS For a Homogeneous sample: I = N DJL AT where: N = atoms/cm 3 = = photoelectric cross-section, section, cm 2 D = detector efficiency J = X-ray X flux, photon/cm 2 -sec L = orbital symmetry factor = inelastic electron mean-free path, cm A = analysis area, cm 2 T = analyzer transmission efficiency
21 Quantitative Analysis by XPS N = I/ DJL DJL AT Let denominator = elemental sensitivity factor, S N = I / S Can describe Relative Concentration of observed elements as a number fraction by: C x = N x / N i C x = I x /S x / I i /S i The values of S are based on empirical data.
22 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)
23 Angle-resolved XPS =15 = 90 More Surface Sensitive Less Surface Sensitive Information depth = dsin d = Escape depth ~ 3 = = Emission angle relative to surface = Inelastic Mean Free Path
24 Angle-resolved XPS Analysis of Self-Assembling Monolayers SiW12O40 d C(W) C(Au) Au Expt. Data Model Angle Resolved XPS Can Determine Over-layer Thickness Over-layer Coverage Electron Emission Angle, degrees Data courtesy L. Ge,, R. Haasch and A. Gewirth,, University of Illinois
25 Instrumentation for X-ray Photoelectron Spectroscopy
26 X-ray Photoelectron Spectrometer
27 X-ray Photoelectron Spectrometer Hemispherical Energy Analyzer Computer System Outer Sphere Magnetic Shield Inner Sphere Analyzer Control X-ray Source Electron Optics Sample 54.7 Lenses for Energy Adjustment (Retardation) Lenses for Analysis Area Definition Position Sensitive Detector (PSD) Multi-Channel Plate Electron Multiplier Resistive Anode Encoder Position Computer Position Address Converter
28 Schematic of Dual Anode X-ray Source Anode Assembly Water Outlet Water Inlet Anode Fence Anode 1 Anode 2 Filament 1 Filament 2 Anode 1 Fence Anode 2 Fence Cooling Water Cooling Water Filament 1 Filament 2
29 UPS (UV photoelectron spectroscopy) E b =h -E kin - Energy range: 10 ~ 45eV Measuring of valence electronic structure of: 1. Substrate band structure 2. Organic adsorbate states near HOMO HOMO=highest occupied molecular orbitals
30 Detailed measurements with UPS: 1. Substrate density state near Fermi edge. (energy spectrum) 2. Substrate band structures. (Momentum resolved energy spectrum, energy spectrum projected in reciprocal space, angle resolved UPS) 3. Surface workfunction 4. Adsorbate electronic states. (energy spectrum) 5. Adsorbate geometry reflected by the orientation of its electronic states (angle resolved UPS)
31
32 Example of energy spectrum Cu(110) UPS with He I UV source (21.2eV) Incident UV Work function Ejected Photoelectron Free Electron Level Fermi Level 4s Valence Band 3d 3p 3s 2p 2s 1s Electronic configuration: K 3d 10 4s 1 Full width of the spectrum=hv ev
33 High binding energy cut off Full spectra Near Fermi edge HeI UPS spectra of NiPc/Ag.
34 Angle resolved UV photoelectron spectroscopy (ARUPS) ARUPS geometry Polarisation Allow measuring the electron emitting angle. Or angular distribution of photoelectrons.
35 Orientation of planar molecule: tetracene -states electron For a flat-lying aromatic molecule, the emission from orbitals has maximum along the surface normal. For an upright aromatic molecule, the emission from orbitals has maximum paralle to the surface.
36 Summary 1. Element analysis with XPS Binding energy, chemical shift, quantitative measurement, depth profile. 2. Auger electron spectroscopy 3. Introduce UPS Mechanism, work function, ARUPS
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