Lecture 11 Surface Characterization

8/10/2019 Lecture 11 Surface Characterization

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Lecture 11

Surface Characterization of Biomaterials in Vacuum

The structure and chemistry of a biomaterial surface greatly dictates thedegree of biocompatibility of an implant. Surface characterization isthus a central aspect of biomaterials research.

Surface chemistrycan be investigated directly using high vacuummethods:

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 ejectedby x-ray bombardment from thesample near surface (0.5-10 nm)with characteristic energies

Analysisof the photoelectron energiesyields a quantitativemeasure of the surface composition


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23.051J/20.340JElectron energy analyzer

( = h)

(variable retardation voltage)

Lens

e

e

e

P 10-10Torr

X-ray source Detector

E

K

EF

LILII

LIII

Evac

EBenergy is characteristic

elementand

kin

Photoelectron binding

of thebonding environment

Chemical analysis!

Binding energy = incident x-ray energy photoelectron kinetic energy

EB= h- Ekin


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Quantitative Elemental Analysis

C1s

N1s

O1sIntensity

Low-resolution spectrum

500 300

Bindingenergy (eV)

Area under peakIi 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 Ci (of elements detected)

Ci =Ii / Si Siis the sensitivity factor:

Ij / Sjj -f(instrument & atomic parameters)

- can be calculated

sum over detectedelements


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High-resolution spectrumC1s

Intensity

PMMA

290 285

Bindingenergy (eV)

Ratio of peak areas gives a ratio of photoelectrons ejected from atomsin a particular bonding configuration (Si = constant)

Ex. PMMA5carbons in total H CH3

H CH3C C

3 C C (a) Lowest EBC1s

H C=O

H CEB 285.0 eV O

CH3

1 OCH3

(b) Intermediate EBC1sEB 286.8 eV

Why does core electron EBvary with valence shell

1C=O

O

(c) Highest EBC1sEB 289.0 eV

configuration?


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from carbon

Slight shift to 1s

Electronegative oxygen robs valence electrons(electron density higher toward O atoms)

Carbon core electrons held tighterto the + nucleus(less screening of + charge)

higher C binding energy

Similarly, different oxidation states of metals can be distinguished.

Ex. Fe FeO Fe3O4 Fe2O3

Fe2pbinding 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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Depth-Resolved ESCA/XPS

The probability of a photoelectron escaping the sample without

undergoing inelastic collision is inversely related to its depth t withinthe sample:

t( ) ~ expP t

e

where e(typically ~ 5-30 ) is the electron inelastic mean-free path,which depends on the electron kinetic energy and the material.

(Physically, e= avg. distance traveled between inelastic collisions.)

For t= 3e P(t) = 0.05e

=90

95% of signalfrom t 3e

By varying the take-off angle (), the sampling depth can bedecreased, increasing surface sensitivity

e e

t =3 sine


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Ci

5 90

(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 thejth peak of element i is the integral of attenuatedcontributions from all sample depthsz:

z (Iij =CinstT Ekin )Lijij n (z) exp dzi

sine

L

Cinst= instrument constantT(Ekin) = analyzer transmission function

ij= angular asymmetry factor for orbitaljof element i

ijis the photoionization cross-sectionni(z) is the atomic concen. of iat a depthz(atoms/vol)


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where:

MW 1/ 3

a= monolayer thickness (nm) a =107

NAv

MW = molar mass (g/mol)= density (g/cm3)

Ekin= electron kinetic energy (eV)

Ex: efor C1susing a Mg Kx-ray source:

EB= h- Ekin

For Mg Kx-rays: h= 1254 eVEkin= 970 eVFor C1s: EB= 284 eV

1 2 0.5 (nm) = (

49E +0.11Ekin ) Assume = 1.1 g/cm3e kin

e= 3.1 nm


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For non-uniform samples, signal intensity must be deconvolutedtoobtain a quantitative analysis of concentration vs. depth.

Case Example: a sample comprising two layers (layer 2 semi-infinite):

1 d

2

z (Iij = Cins T kin )Lijij ni (z)exp sin

dze

ij ij o i,1 e,1

ij ,o i,2 e,2 e,1 sin,

e,1 sin

(1)

d (2) d or Iij =Iij ,

1 exp

e,1 sin

+Iij ,exp

e,1 sin

(1)

z Iij = Iij,oni,1 sin exp

sin

ee

I =I (1) n sin1 exp

d

d

(2)

z I ij oni,2 sin exp

sin

, ee0 d

+I (2)n sinexp

d

Why e,1? Electrons originating insemi-infinite layer 2 are attenuatedby overlayer 1

( )where Iij

i

, = measured peak area from a uniform, semi-infinite sample

of material i.


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Methods to solve ford

Scenario 1: ni,2=0(ex., C1speak of a polymer adsorbed on an oxide):

d d 1(1)

Iij =Iij ,

1 exp

sin

e,1 2

(1)measure a bulk sample of the upper layer material Iij ,

Iij

dln 1 (1)I ij,

=

e,1 sin

obtain slope of ln 1

I (1

ij

) vs. csc d/e,1 Iij ,

for a fixed :

d = e,1 sinln 1I

(1

ij

)

Iij ,

substitute a calculated or measured e,1to obtain d


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Scenario 2: ni,1=0(ex., M2ppeak from underlying metal oxide (MOx):

d(2)

d 1Iij =Iij ,exp

e,1 sin

2

measure Iijfor same peak at different take-off angles (1, 2)

(2) d

I ni,2e,2 sin exp Iij ,ij o 1,

e,1 sin11 = d

2 (2)Iij , I ni,2e,2 sin exp ij o 2,e,1 sin2

d 1

Iij , =sin1 exp (csc csc2 )Iij ,

sin2 e,11

2

1 Iij ,

sin21d = (csc csc )

ln Iij , sin

e,1 2 12 1

substitute a calculated or measured e,1to obtain d


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Scenario 3: element present in distinguishable bonding configurations inlayers 1 & 2 (ex., O1speak from -C-O-C- and MOx):

(1)

d (2) d

Iij =Iij ,1 exp

e,1 sin

+Iij,

exp e,1 sin

(2 ) d

I ni,2e,2 sinexp 1 dIij(2) ij o,

e,1 sin=(1) d Iij (1)

2I ni,1e,1 sin

1exp

e,1 sin

,ij o

(1) (2)measure element peak areasIij and Iij

(1)

(2), ,for same element and orbital: Iij o =Iij o

ni,2 Ci,2=for same element and orbital: ni,1 Ci,1

I

d

ij

(2)C

i,2e,2 exp e,1 sin

I =

ij

d

(1)

Ci,1e,1 1 exp

e,1 sin

solve numerically for d, substituting calculated values of e,2& e,1


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if d


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Ion Etching

Depth profiling for depths > 10 nm (100 nm 1 m)

Ar+, Xe+or He+ions etch surface layer

Signal

Intensity

6(OH)2) coatingHydroxyapatite (Ca10(PO4)

P2pTi2p

XPS spectra re-recorded

on Ti implant

sputter time

Calibration of sputter rates: time depth


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2. Auger Electron Spectroscopy

Theoretical Basis:

Auger electrons createdby electron bombardment of sample areejected from near surface (1-3 nm)with characteristic energies

Analysisof the Auger electron energiesyields a quantitativemeasure of the surface composition

ejected core electron

EK EM= EN+ Ekin EvacAuger electronNEkin = EK EM ENM

ELIII

xyz (ECVVor ECCV)

LLII

I

ejection from x shell

electronic transition yx K

release of z-level Auger with Ekin

INFORMATION: Exyzis characteristic to element & bonding

AES vs. XPS

Advantages Disadvantages

- focused e-beam gives highx,y spatial resolution(5 nm vs. ~1 m)

- charging effects onnonconductive samples(unsuitable)

- larger bonding effects - degradation of organics


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3. Secondary Ion Mass Spectroscopy (SIMS)

Experimental Approach:

Energetic ions (1-15 keV)bombard sample surface

Secondary ions/charged fragmentsare ejected from surface anddetected

ion gun

filter mass filter

ionenergy

detector

sample

Ion Guns

Nobel gas: Ar+, Xe+

liquid metal ion: Ga+, Cs+(~1nm beam size x,y mapping)types

pulsed LMI (time-of-flight source)

low currents used: 10-8-10-11A/cm2

Ion beam current(A/cm2)

surface monolayerlifetime (s)

10-5 16

10-7 160010-9 1.6105

10-11 1.6107

1 Amp = 6.21018 ions/sec


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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< 103amu

Oscillating RF fielddestabilizes ions: only ionswith specified m/zcan pass

Magnetic sector: m> 104amu; m/m~ 10,000

R

Applied B-field results incircular trajectory (radius=R)of charged particles

1 2mV1/ 2

R =

B

z

acceleratingvoltage

Time-of-flight (TOF): m~ 103-104amu; m/m~ 10,000

Pulsed primary beamgenerates secondary ion

m 1/ 2

pulses detected at distance Ltime =

2zV

L

flight tubelength


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Modes of Operation

Static SIMS

low energy ions: 1-2 keV; penetration ~5-10

low ion doses: < 1013ions/cm2sec 1 cm2 1015 atoms

195% of signal from

statomic layer!

Information:

surface composition

surface bonding chemistry (sputtered fragments)

Example: SIMS of silica powderNegative spectrum

Intensity(arb. units)

O

OH

SiO2

SiO2H

SiO3 SiO3H

(SiO2)2OH

16 17 60 61 76 77 137

m/z

OH HO OHWhich structure is

suggested from SIMS? SiSiOO O O

O


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Positive spectrum

Intensity(arb. units) O+

CH3+

H2O+

OH

+

15 16 17 18

SIMS suggests presence m/zof adsorbed methanol

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


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beam pathprimary

10-100 m

10-100 m

N

Signal

Intensity

N ion-implanted Ti (for wear resistance)

sputter time

References

J.C. Vickerman, Ed., Surface AnalysisThe 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, 1998.

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 comparisonof properties of depth-profile reconstruction methods,J. Electron Spectroscopy 73(1995) 25

52.

B. Elsener and A. Rossi, XPS investigation of passive films on amorphous Fe-Cr alloys,Electrochimica Acta 37(1992) 2269-2276.

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) 3635-3653.

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Lecture 11 Surface Characterization

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