Lecture 11 Surface Characterization of Biomaterials in Vacuum

Transcription

1 1 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: Secondary electrons ejected by x-ray bombardment from the sample near surface ( nm) with characteristic energies Analysis of the photoelectron energies yields a quantitative measure of the surface composition

2 Electron energy analyzer (variable retardation voltage) 2 X-ray source (Ε = hν) Lens e θ P Torr Detector e e E kin E vac E F Photoelectron binding energy is characteristic of the element and bonding environment E B L III L II L I Chemical analysis! K Binding energy = incident x-ray energy photoelectron kinetic energy E B = hν - E kin

3 3 Quantitative Elemental Analysis Low-resolution spectrum Intensity O 1s C 1s N 1s Binding energy (ev) Area under peak I i number of electrons ejected (& atoms present) Only electrons in the near surface region escape without losing energy by inelastic collision Sensitivity: depends on element. Elements present in concentrations >0.1 atom% are generally detectable (H & He undetected) Quantification of atomic fraction C i (of elements detected) C i = I i / S i I j / S j j sum over detected elements S i is the sensitivity factor: - f(instrument & atomic parameters) - can be calculated

4 4 Intensity C 1s High-resolution spectrum PMMA Binding energy (ev) Ratio of peak areas gives a ratio of photoelectrons ejected from atoms in a particular bonding configuration (S i = constant) 5 carbons in total Ex. PMMA H CH 3 H CH 3 C C 3 C C (a) Lowest E B C 1s H C=O H C E B ev O CH 3 1 O CH 3 (b) Intermediate E B C 1s E B ev 1 C=O O (c) Highest E B C 1s E B ev Why does core electron E B vary with valence shell configuration?

5 5 Electronegative oxygen robs valence electrons from carbon (electron density higher toward O atoms) Carbon core electrons held tighter to the + nucleus (less screening of + charge) Slight shift to higher C binding energy 1s 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

6 6 Depth-Resolved ESCA/XPS The probability of a photoelectron escaping the sample without undergoing inelastic collision is inversely related to its depth t within the sample: t Pt () ~ exp λ e where λ e (typically ~ 5-30 Å) is the electron inelastic mean-free path, which depends on the electron kinetic energy and the material. (Physically, λ e = avg. distance traveled between inelastic collisions.) For t = 3 λ e P(t) = % of signal from t 3 λ e e θ =90 By varying the take-off angle (θ), the sampling depth can be decreased, increasing surface sensitivity e θ e θ t = 3λ e sin θ

7 7 C i 5 90 θ (degrees) Variation of composition with angle may indicate: - Preferential orientation at surface - Surface segregation - Adsorbed species (e.g., hydrocarbons) - etc. Quantifying composition as a function of depth The area under the jth peak of element i is the integral of attenuated contributions from all sample depths z: z I ij = C inst T ( E kin )L ij σ ij n (z)exp i dz λ e sinθ C inst = instrument constant T(E kin ) = analyzer transmission function Lij = angular asymmetry factor for orbital j of element i σij is the photoionization cross-section n i (z) is the atomic concen. of i at a depth z (atoms/vol)

8 8 For a semi-infinite sample of homogeneous composition: z I ij = I ij,o n i λe sinθ exp λ sinθ e 0 = I ij, o n i λ e sinθ = S i n i = I ij, where I ij o = C inst T ( E kin )L ij σ ij, Relative concentrations of elements (or atoms with a particular bond configuration) are obtained from ratios of I ij (peak area): L ij depends on electronic shell (ex. 1s or 2p); obtained from tables; cancels if taking a peak ratio from same orbitals, ex. I C1s / I O1s C inst and T(E kin ) are known for most instruments; cancel if taking a peak ratio with E kin constant, ex. I C1s (C C O) / I C1s (C CH 3 ) σ ij obtained from tables; cancels if taking a peak ratio from same atom in different bonding config., ex. I C1s (C C O ) / I C1s (C CH 3 ) λ e values can be measured or estimated from empirically-derived expressions e kin For polymers: λ (nm) = ρ (49E E kin ) For elements: λ (nm) = a e 538E (E kin kin a ) For inorganic compounds (ex. oxides): λ (nm) = a 2170E (E kin a ) e kin

9 9 where: a = monolayer thickness (nm) MW 1/ 3 a = 10 7 ρ N Av MW = molar mass (g/mol) ρ = density (g/cm 3 ) E kin = electron kinetic energy (ev) Ex: λ e for C 1s using a Mg K α x-ray source: E B = hν - E kin For Mg K α x-rays: hν = 1254 ev For C 1s : E B = 284 ev E kin = 970 ev λ (nm) = ρ ( 49E E kin ) Assume ρ = 1.1 g/cm 3 e kin λ e = 3.1 nm

10 10 For non-uniform samples, signal intensity must be deconvoluted to obtain a quantitative analysis of concentration vs. depth. Case Example: a sample comprising two layers (layer 2 semi-infinite): 1 d 2 I ij = C ins T ( z kin )L ij σ ij n i (z)exp dz λ e sinθ (1) I ij = I ij,o n i,1 λe sinθ exp z λ e sinθ d (2) I ij, o n i,2 λe sinθ exp z λ e sinθ 0 d I ij = I (1) ij, on i,1 λ e,1 sinθ 1 exp d ij,o i,2 e,2 λ λ e,1 sinθ + I (2) n λ sinθ exp d e,1 sinθ (1) d (2) d or I = I ij ij, 1 exp λ e,1 sinθ + I exp ij, λ e,1 sinθ Why λ e,1? Electrons originating in semi-infinite layer 2 are attenuated by overlayer 1 () i where I ij, = measured peak area from a uniform, semi-infinite sample of material i.

11 11 Methods to solve for d Scenario 1: n i,2 =0 (ex., C 1s peak of a polymer adsorbed on an oxide): d 1 (1) I ij = I ij, 1 exp λ sinθ e,1 2 d (1) measure a bulk sample of the upper layer material I ij, I ij d ln 1 (1) I = ij, λ e,1 sinθ obtain slope of ln 1 Iij (1) vs. cscθ d/λ e,1 I ij, for a fixed θ: I ij d = λ e,1 sinθ ln 1 (1) I ij, substitute a calculated or measured λ e,1 to obtain d

12 12 Scenario 2: n i,1 =0 (ex., M 2p peak from underlying metal oxide (MO x ): (2) d 1 I ij = I ij, exp λ e,1 sinθ 2 d measure I ij for same peak at different take-off angles (θ 1, θ 2 ) (2) d Iij, o n i,2 λ e,2 sinθ 1 exp I ij,θ λ e,1 sinθ 1 1 = I ij,θ d 2 (2) Iij, o n i,2 λ e,2 sinθ 2 exp λ e,1 sinθ 2 I ij,θ d 1 = sinθ 1 exp (cscθ cscθ 2 ) I ij,θ sinθ 2 λ 1 2 e,1 1 I ij,θ sinθ 2 1 d = λ (cscθ cscθ ) ln I ij,θ sinθ e, substitute a calculated or measured λ e,1 to obtain d

13 13 Scenario 3: element present in distinguishable bonding configurations in layers 1 & 2 (ex., O 1s peak from -C-O-C- and MO x ): (1) I ij = I ij, 1 exp d (2) d λ e,1 sinθ + I ij, exp λ sinθ e,1 (2) d I n i,2 λ e,2 sinθ exp (2) ij, o 1 d I ij λ e,1 sinθ = (1) I ij d (1) 2 I n i,1 λ e,1 sinθ 1 exp λ e,1 sinθ ij, o (1) (2) measure element peak areas I ij and I ij (1) (2) for same element and orbital: I ij, o = I ij, o n i,2 C i,2 = for same element and orbital: ni,1 C i,1 d C (2) i,2 λ e,2 exp I ij λ e,1 sinθ = (1) I ij d C i,1 λ e,1 1 exp λ e,1 sinθ solve numerically for d, substituting calculated values of λ e,2 & λ e,1

14 14 if d << λ e,1 sinθ : ax exp( ax) 1 ax + ( 2 ) 2... d C (2) i,2 λ e,2 I 1 λ sinθ ij e,1 = (1) I ij d C i,1 λ e,1 λ sinθ e,1 (2) 1 ij (1) ij d = λ e,1 sinθ I C i,1λ e,1 +1 I C i,2 λ e,2

15 15 Ion Etching Depth profiling for depths > 10 nm (100 nm 1 µm) Ar +, Xe + or He + ions etch surface layer XPS spectra re-recorded Signal Intensity P 2p Ti 2p Hydroxyapatite (Ca 10 (PO 4 ) 6 (OH) 2 ) coating on Ti implant sputter time Calibration of sputter rates: time depth

16 16 2. Auger Electron Spectroscopy Theoretical Basis: Auger electrons created by electron bombardment of sample are ejected from near surface (1-3 nm) with characteristic energies Analysis of the Auger electron energies yields a quantitative measure of the surface composition ejected core electron E K E M = E N + E kin E kin = E K E M E N Exyz (E CVV or E CCV ) ejection from x shell electronic transition y x release of z-level Auger with E kin E vac Auger electron N M L III L II LI K INFORMATION: E xyz is characteristic to element & bonding AES vs. XPS Advantages - focused e-beam gives high x,y spatial resolution (5 nm vs. ~1 µm) Disadvantages - charging effects on nonconductive samples (unsuitable) - larger bonding effects - degradation of organics

17 17 3. Secondary Ion Mass Spectroscopy (SIMS) Experimental Approach: Energetic ions (1-15 kev) bombard sample surface Secondary ions/charged fragments are ejected from surface and detected ion gun energy filter mass filter ion detector sample Ion Guns types Nobel gas: Ar +, Xe + liquid metal ion: Ga +, Cs + (~1nm beam size x,y mapping) pulsed LMI (time-of-flight source) low currents used: A/cm 2 Ion beam current (A/cm 2 ) 1 Amp = ions/sec surface monolayer lifetime (s)

18 18 Detectors sensitive to the ratio of mass/charge (m/z) resolution defined as m/ m (larger = better!) Quadrupole (RF-DC): resol. m/ m ~ 2000; detects m < 10 3 amu Oscillating RF field destabilizes ions: only ions with specified m/z can pass Magnetic sector: m > 10 4 amu; m/ m ~ 10,000 R Applied B-field results in circular trajectory (radius=r) of charged particles 1 2mV 1/ 2 R = B z accelerating voltage Time-of-flight (TOF): m ~ amu; m/ m ~ 10,000 m 1/ 2 time = 2zV L flight tube length Pulsed primary beam generates secondary ion pulses detected at distance L

19 19 Modes of Operation Static SIMS low energy ions: 1-2 kev; penetration ~5-10 Å low ion doses: < ions/cm 2 sec 1 cm atoms Information: 95% of signal from 1 st atomic layer! surface composition surface bonding chemistry (sputtered fragments) Example: SIMS of silica powder Negative spectrum Intensity (arb. units) O OH SiO 2 SiO 3 H SiO 3 SiO 2 H (SiO 2 ) 2 OH m/z OH HO OH Si Which structure is suggested from SIMS? Si O O O O O

20 20 Positive spectrum CH 3 + Intensity (arb. units) O + OH + H 2 O + SIMS suggests presence of adsorbed methanol m/z SIMS vs. XPS/AES Advantages Disadvantages - high sensitivity (ppm ppb) - not quantitative - more sensitive to top surface - applicable to any solid Dynamic SIMS 1-20 kev primary beam rastered beam sputters a crater in sample secondary ions gives depth profiling

21 primary beam path µm µm N ion-implanted Ti (for wear resistance) Signal Intensity N sputter time References J.C. Vickerman, Ed., Surface Analysis The Principle Techniques, J. Wiley & Sons: New York, NY, 1997, Chapters 3, 4 & 5. D. Briggs, Surface Analysis of Polymers by XPS and Static SIMS, Cambridge University Press: United Kingdom, C.-M. Chan, Polymer Surface Modification and Characterization, Hanson: Cincinnati, OH: 1994, Chapters 3 & 4. P.J. Cumpson, Angle-resolved XPS and AES: depth-resolution limits and a general comparison of properties of depth-profile reconstruction methods, J. Electron Spectroscopy 73 (1995) B. Elsener and A. Rossi, XPS investigation of passive films on amorphous Fe-Cr alloys, Electrochimica Acta 37 (1992) A.M. Belu, D.J. Graham and D.G. Castner, Time-of-flight secondary ion mass spectroscopy: techniques and applications for the characterization of biomaterial surfaces, Biomaterials 24, (2003)