Xps

X-ray Photoelectron Spectroscopy (XPS)

Surface Analysis The Study of the Outer-Most Layers of Materials (<100  ). Electron Spectroscopies XPS: X-ray Photoelectron Spectroscopy AES: Auger Electron Spectroscopy EELS: Electron Energy Loss Spectroscopy Ion Spectroscopies SIMS: Secondary Ion Mass Spectrometry SNMS: Sputtered Neutral Mass Spectrometry ISS: Ion Scattering Spectroscopy

Introduction to X-ray Photoelectron Spectroscopy (XPS)

Introduction to X-ray Photoelectron Spectroscopy (XPS) What is XPS?- General Theory How can we identify elements and compounds? Instrumentation for XPS Examples of materials analysis with XPS

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.

What is XPS? 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.

X-ray Photoelectron Spectroscopy Small Area Detection X-ray Beam X-ray penetration depth ~1  m. Electrons can be excited in this entire volume. X-ray excitation area ~1x1 cm 2 . Electrons are emitted from this entire area Electrons are extracted only from a narrow solid angle. 1 mm 2 10 nm

The Photoelectric Process 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-  Following this process, the atom will release energy by the emission of an Auger Electron. Conduction Band Valence Band L2,L3 L1 K Fermi Level Free Electron Level Incident X-ray Ejected Photoelectron 1s 2s 2p

Auger Relation of Core Hole 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(L3). Conduction Band Valence Band L2,L3 L1 K Fermi Level Free Electron Level Emitted Auger Electron 1s 2s 2p

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.

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 source energy used to collect the data in order to compare the chemical states in the sample with data collected using another source.

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.

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 1.0 f(E) 0 0.5 E f 1. At T=0 K: f(E)=1 for E<E f f(E)=0 for E>E f 2. At kT<<E f (at room temperature kT=0.025 eV) f(E)=0.5 for E=E f T=0 K kT<<E f

Sample/Spectrometer Energy Level Diagram- Conducting Sample 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). E 1s Sample Spectrometer e - Free Electron Energy Fermi Level, E f Vacuum Level, E v  sample KE(1s) KE(1s)  spec BE(1s)

Sample/Spectrometer Energy Level Diagram- Insulating Sample hv A relative build-up of electrons at the spectrometer raises the Fermi level of the spectrometer relative to the sample. A potential E ch will develop. E 1s Sample Spectrometer e - Free Electron Energy BE(1s) Fermi Level, E f Vacuum Level, E v KE(1s)  spec E ch

Binding Energy Referencing BE = hv - KE -  spec - E ch Where: BE = Electron Binding Energy KE = Electron Kinetic Energy  spec = Spectrometer Work Function E ch = Surface Charge Energy E ch can be determined by electrically calibrating the instrument to a spectral feature. C1s at 285.0 eV Au4f 7/2 at 84.0 eV

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

Binding Energy Determination The photoelectron’s binding energy will be based on the element’s final-state configuration. Conduction Band Valence Band Fermi Level Free Electon Level Conduction Band Valence Band 1s 2s 2p Initial State Final State

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

Chemical Shifts- Electronegativity Effects

Electronic Effects Spin-Orbit Coupling

Electronic Effects Spin-OrbitCoupling

Electronic Effects- Spin-Orbit Coupling Ti Metal Ti Oxide

Electron Scattering Effects Energy Loss Peaks 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). e ph + e solid e* ph + e** solid

Electron Scattering Effects Plasmon Loss Peak

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

Quantitative Analysis by XPS N = I/  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.

Relative Sensitivities of the Elements 1s 2p 3d 4d 4f

Instrumentation for X-ray Photoelectron Spectroscopy

Instrumentation for XPS Surface analysis by XPS requires irradiating a solid in an Ultra-high Vacuum (UHV) chamber with monoenergetic soft X-rays and analyzing the energies of the emitted electrons.

Why UHV for Surface Analysis? Remove adsorbed gases from the sample. Eliminate adsorption of contaminants on the sample. Prevent arcing and high voltage breakdown. Increase the mean free path for electrons, ions and photons. Degree of Vacuum 10 10 10 10 10 2 -1 -4 -8 -11 Low Vacuum Medium Vacuum High Vacuum Ultra-High Vacuum Pressure Torr

X-ray Photoelectron Spectrometer

X-ray Photoelectron Spectrometer X-ray Source Electron Optics Hemispherical Energy Analyzer Position Sensitive Detector (PSD) Magnetic Shield Outer Sphere Inner Sphere Sample Computer System Analyzer Control Multi-Channel Plate Electron Multiplier Resistive Anode Encoder Lenses for Energy Adjustment (Retardation) Lenses for Analysis Area Definition Position Computer Position Address Converter

X-ray Generation Conduction Band Valence Band 1s 2s 2p Conduction Band Valence Band L2,L3 L1 K Fermi Level Free Electron Level 1s 2s 2p Secondary electron Incident electron X-ray Photon

Relative Probabilities of Relaxation of a K Shell Core Hole Note: The light elements have a low cross section for X-ray emission. Auger Electron Emission X-ray Photon Emission

Schematic of Dual Anode X-ray Source Anode Fence Anode 1 Anode 2 Filament 1 Filament 2 Fence Cooling Water Cooling Water Water Outlet Water Inlet Anode Assembly Filament 1 Anode 1 Fence Filament 2 Anode 2

Schematic of X-ray Monochromator Sample X-ray Anode Energy Analyzer Quartz Crystal Disperser Rowland Circle e -

Applications of X-ray Photoelectron Spectroscopy (XPS)

XPS Analysis of Pigment from Mummy Artwork 150 145 140 135 130 Binding Energy (eV) PbO 2 Pb 3 O 4 500 400 300 200 100 0 Binding Energy (eV) O Pb Pb Pb N Ca C Na Cl XPS analysis showed that the pigment used on the mummy wrapping was Pb 3 O 4 rather than Fe 2 O 3 Egyptian Mummy 2nd Century AD World Heritage Museum University of Illinois

Analysis of Carbon Fiber- Polymer Composite Material by XPS Woven carbon fiber composite XPS analysis identifies the functional groups present on composite surface. Chemical nature of fiber-polymer interface will influence its properties. -C-C- -C-O -C=O

Analysis of Materials for Solar Energy Collection by XPS Depth Profiling- The amorphous-SiC/SnO 2 Interface The profile indicates a reduction of the SnO 2 occurred at the interface during deposition. Such a reduction would effect the collector’s efficiency. Photo-voltaic Collector Conductive Oxide- SnO 2 p-type a-SiC a-Si Solar Energy SnO 2 Sn Depth 500 496 492 488 484 480 Binding Energy, eV Data courtesy A. Nurrudin and J. Abelson, University of Illinois

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  

Angle-resolved XPS Analysis of Self-Assembling Monolayers Angle Resolved XPS Can Determine Over-layer Thickness Over-layer Coverage Data courtesy L. Ge, R. Haasch and A. Gewirth, University of Illinois

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