lecture11

lecture11 - 3.051J/20.340J Lecture 11 Surface...

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1 3.051 J / 20 .340 J Lecture 11 Surface Characterization of Biomaterials in Vacuum The structure and chemistry of a biomaterial surface greatly dictates the degree of biocompatibility of an implant. Surface characterization is thus a central aspect of biomaterials research. Surface chemistry can be investigated directly using high vacuum methods: Electron spectroscopy for Chemical Analysis (ESCA)/X-ray Photoelectron Spectroscopy (XPS) Auger Electron Spectroscopy (AES) Secondary Ion Mass Spectroscopy (SIMS) 1. XPS/ESCA Theoretical Basis: ¾ Secondary electrons ejected by x-ray bombardment from the sample near surface (0.5-10 nm) with characteristic energies ¾ Analysis of the photoelectron energies yields a quantitative measure of the surface composition
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2 3.051 J / 20 .340 J Electron energy analyzer θ ( Ε = h ν ) (variable retardation voltage) Lens e e e P 10 -10 Torr X-ray source Detector E K E F L I L II L III E vac E B energy is characteristic element and kin Photoelectron binding of the bonding environment Chemical analysis! Binding energy = incident x-ray energy photoelectron kinetic energy E B = h ν - E kin
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3 3.051 J / 20 .340 J Quantitative Elemental Analysis C 1s N 1s O 1s Intensity Low-resolution spectrum 500 300 Binding energy (eV) ¾ Area under peak I i number of electrons ejected (& atoms present) ¾ Only electrons in the near surface region escape without losing energy by inelastic collision ¾ Sensitivity: depends on element. Elements present in concentrations >0.1 atom% are generally detectable (H & He undetected) ¾ Quantification of atomic fraction C i (of elements detected) C i = I i / S i S i is the sensitivity factor : I j / S j j - f (instrument & atomic parameters) - can be calculated sum over detected elements
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4 3.051 J / 20 .340 J High-resolution spectrum C 1s Intensity PMMA 290 285 Binding energy (eV) ¾ Ratio of peak areas gives a ratio of photoelectrons ejected from atoms in a particular bonding configuration ( S i = constant) Ex. PMMA 5 carbons in total H C H 3 H C H 3 C C 3 C C (a) Lowest E B C 1s H C =O H C E B 285.0 eV O C H 3 1 O C H 3 (b) Intermediate E B C 1s E B 286.8 eV Why does core electron E B vary with valence shell 1 C =O O (c) Highest E B C 1s E B 289.0 eV configuration?
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5 3.051 J / 20 .340 J from carbon Slight shift to 1s Electronegative oxygen “robs” valence electrons (electron density higher toward O atoms) Carbon core electrons held “tighter” to the + nucleus (less screening of + charge) higher C binding energy Similarly, different oxidation states of metals can be distinguished. Ex. Fe FeO Fe 3 O 4 Fe 2 O 3 Fe 2p binding energy XPS signal comes from first ~10 nm of sample surface. What if the sample has a concentration gradient within this depth? Surface-segregating species Adsorbed species 10nm Multivalent oxide layer
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6 3.051 J / 20 .340 J Depth-Resolved ESCA/XPS ¾ The probability of a photoelectron escaping the sample without undergoing inelastic collision is inversely related to its depth t within the sample: t () ~ exp Pt λ e where λ e (typically ~ 5-30 Å) is the
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lecture11 - 3.051J/20.340J Lecture 11 Surface...

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