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Rutgers University BME 583 - Electron Spectroscopy for Chemical Analysis (ESCA)

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PowerPoint PresentationOutline of LectureMotivation1. Introduction2. Principles of ESCASlide 6Slide 7Instrumentation: How are measurements made?3. Analysis CapabilitiesChemical State IdentificationExplanation of chemical shiftsSlide 13Slide 14Slide 15Slide 16Slide 17Slide 18Applications to biomaterials and biointerfacesSlide 20Discussion of Journal PaperExperimental ProceduresThe resultsSlide 24Professor Theodore Madey Case Study Discussant: Prabhas MogheElectron Spectroscopy for Chemical Analysis (ESCA)125: 583Biointerfacial CharacterizationOutline of Lecture•Introduction•Principles of ESCA–The photoelectron effect–Instrumentation -- how measurements are made•Analysis Capabilities–Elemental analysis–Chemical state analysis (core level shifts)–More complex effects•Surface Sensitivity•Applications•Comparisons with other techniques•Discussion of Journal Paper (Biointerfacial Case Study)Motivation•Why is surface analysis important for Biomaterials and Biological Systems?•Interactions between solid surfaces and biological systems--- Biocompatibility--- Biomolecular separations--- Cell culture--- Marine Fouling--- Biosensors1. Introduction-- ESCA provides unique information about chemical compositionAnd chemical state of a surface-- useful for biomaterials-- advantages-- surface sensitive (top few monolayers)-- wide range of solids-- relatively non-destructive-- disadvantages-- expensive, slow, poor spatial resolution, requires high vacuum2. Principles of ESCA•ESCA (also known as X-ray photoelectron spectroscopy, XPS) is based on the photoelectron effect. A high energy X-ray photon can ionize an atom, producing an ejected free electron with kinetic energy KE:• =photon energy (e.g., for BE=energy necessary to remove a specific electron from an atom. BE orbital energyKE =hυ −BEAl Kα, hυ =1486.6eV)hυ≈Light can take on many forms. Radio waves, microwaves, infrared, visible, ultraviolet, X-ray and gamma radiation are all different forms of light.The energy of the photon tells what kind of light it is. Radio waves are composed of low energy photons. Optical photons--the only photons perceived by the human eye--are a million times more energetic than the typical radio photon. The energies of X-ray photons range from hundreds to thousands of times higher than that of optical photons.Very low temperatures (hundreds of degrees below zero Celsius) produce low energy radio and microwave photons, whereas cool bodies like ours (about 30 degrees Celsius) produce infrared radiation. Very high temperatures (millions of degrees Celsius) produce X-rays.Basics of Light, EM Spectrum, and X-rayshν• Different orbitals giveDifferent peaks in spectrum• Peak intensities depend onPhotoionization cross section(largest for C 1s)• Extra peak:Auger emission•All energies expressed in electron volts (eV);•1 eV=1.6x10-19 J•In ESCA, you know & you measure KE; this determines BE.•Photoelectron process: Consider an ensemble of C atoms. Each C atom has 6 electrons in 1s, 2s, 2 p orbitals: C 1s2 2s2 2 p2Instrumentation: How are measurements made?•Essential components:•Sample: usually 1 cm2•X-ray source: Al: 1486.6 eV; Mg 1256.6 eV•Electron Energy Analyzer: 100 mm radius concentric hemispherical analyzer; vary voltages to vary pass energy.•Detector: electron multiplier (channeltron)•Electronics, Computer•Note: All in ultrahigh vacuum (<10-8 Torr) (<10-11 atm)•State-of-the-art small spot ESCA: 10 m spot size.3. Analysis Capabilities•Elemental Analysis: atoms have valence and core electrons: Core-level Binding energies provide unique signature of elements.•Quantitative analysis: measure intensities, use standards or tables of sensitivity factor3d3/2,5/2Ag: Z=47Be careful: elements with similar BEs C1s & Ru3d; Ar2p & Rb 3pChemical State IdentificationCore level chemical shifts:For the same atom in two different chemical states:ΔBE = BE(2) − BE(1) = KE(1) − KE(2)C1s – 4 peaks!Explanation of chemical shiftsIf a charge q is added to (or removed from) thevalence shell due to chemical bond formation, theelectrostatic potential felt by the electron insidethe atom is changed.r• When atom loses valence charge (Si0 --> Si4+ ) BE increases.• When atom gains valence charge (O --> O--) BE decreases. (Si2p BE increases)E ~ q/r ~  BE - (BE)o• Chemical shift of C1sAlso: • final state effects• more complex effects -spin-orbit splitting -shake-up, shake-off -Auger electron emissionImportant factor is surface sensitivity; short mean free path  for Inelastic electron scattering.• 95% of signal comes from top layer (t=3)-e.g., 50 eV electrons, ~5Å, t < 15Å1200 eV electrons, ~20Å , t< 60ÅEnhance surface sensitivity by grazing take-off.5. Applications-- Surface contamination-- Failure analysis-- Effects of surface treatments-- Coating, films-- Tribological effects-- Depth Profiling (Ar+ sputtering)F1sC1s• ESCA studies of polyimidePyromellitic dianhydride -- oxydianiline PMDA - ODAC KLL AugerApplications to biomaterials and biointerfaces• Biological interfaces have a limited number of elements(C, H, O, N, S, P, Si)• Extracting useful surface information is challenging.• ESCA can be used to (a) detect the presence of adsorbed proteins. (b) estimate the amount of protein present (c) resolution of one protein from another is difficult since many proteins share chemical features. When spectra are taken as a function of take-off angle, Useful information can be obtained, for example, for the uniformity of an overlayer; fraction covered; protein film thickness; and orientation of protein in the film.The table below is used to determine which surface analysis techniques would be most appropriate to solve problems in specific application areas.AES XPS TOF-SIMSProbe beam Analysis beamElectronsElectronsPhotonsElectronsIonsIonsSampling Depth 5-50 Å 5-50 Å 1-10 ÅDetection Limits 1 x 10-31 x 10-41 x 10-6Information Elemental, SEM Elemental, Chemical Elemental, Chemical, MolecularSpatial Resolution ~100 Ao~10 m ~1000 AoRestriction Inorganics(e-beam damage of organics a major problem)Few Quantification StandardsRequiredDiscussion of Journal Paper•Biomaterials 27 (2006) 691-701; Fabrication, characterization, and biological assessment of multilayered DNA-coatings for biomaterial purposes•van den Beucken JJ, Vos MR, Thune PC, Hayakawa T,

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