Surface analysis with X Ray Photoelectron Spectroscopy Marta Cerruti Materials Engineering Wong 2M020 marta.cerruti@mcgill.ca Lecture for CHEM 634, February 3 rd, 2012
The importance of surfaces When a plate of gold shall be bonded with a plate of silver, or joined thereto, it is necessary to beware of three things, of dust, of wind and of moisture: for if any come between the gold and silver they may not be joined together Bartholomeus Anglicus, De Proprietatibus Rerum, 1323 Can you give a few examples showing why surfaces are important? How thick of a surface layer do these examples involve?
The surface as a defect
Surface sensitive techniques Probe Electrons Ions Photons Electric/ magnetic field Analyzed Electrons Ions Photons Auger spectroscopy SEM TEM XPS UPS STM AFM Secondaryion mass spectrometry EDS FTIR Raman EXAFS Surface sensitivity < 10 nm ~ 1 m Either
XPS spectroscopy Benoit, «La spectroscopie de photoélectrons X», Le Vide, 2003, 58, pp. 219 229 (can be found on lasurface.com)
2p L1 or 2s X-Rays photoelectron Kor1s E k =E b -h spe Kinetic energy Binding energy Spectrometer work function
Why is XPS surface sensitive? Inelastic mean free path Kinetic energy determines surface sensitivity! The exact depth of analysis depends on the angle at which I m collecting the electrons
XPS components Do you remember how X-rays are generated? Sometimes there is a monochromator before the sample Ultra high vacuum! Al: K =1486.6 ev; line width= 0.85 ev Mg: K =1253.6 ev; line width= 0.70 ev
Real systems Analysis chamber (ultra high vacuum) Transfer arm Load lock (high vacuum)
Hemispherical analyzer By varying the potential between the plates, we can select for different kinetic energies reaching the detector, t and scan through h the whole spectrum of energies
Detector V Avalanche effect: gains of 10 7-10 8 electrons
XPS spectrum of chromate coating Auger electron L 2,3 or 2p L 1 or 2s K or 1s 1. Multiple peaks for same element, for electrons emitted from different orbitals 2. Auger peaks visible too
XPS spectrum of Au More than one peak for same orbital!
Why do electrons from the same orbital show more than one peak? Spin/orbital interactions! Where did we see this before? Degenerate orbitals: p, d, f,.. Orbital angular momentum quantum number L Intrinsic angular momentum (spin) quantum number S What s L for p orbitals? What s L for d orbitals? What are the possible values for S? Total electronic angular momentum quantum number J J = L + S What are the possible values of J for electrons in f orbitals?
Why is the intensity of the peak corresponding the 4f 5/2 lower than 4f 7/2? Intensity is related to the multiplicity of each state (how many microstates have the same energy) Multiplicity = 2J+1 Degeneracy for 4f 5/2? Degeneracy for 4f 7/2? Intensity of 4f 7/2 peak : Intensity of 4f 7/2 peak =? What s the relative intensity of the peaks in the p doublet?
Let s rotate the XPS spectrum to understand it better Energy XPS intensity
Chemical environment Al O Valence electrons pulled by electronegative elements => core electrons are more tightly bound to the nucleus. Increasing B.E.
Chemical shift for Sulphur http://www.lasurface.com/database/liaisonxps.php http://srdata.nist.gov/xps/selenergytype.aspx
XPS for Pb coating exposed to air and heat
Peak deconvolution
Element quantification Number of atoms contributing to peak Estimated from universal curve Mean free path All other instrument-related parameters Intensity: peak area Cross section Assume they cancel out when we ratio different elements Calculated from theory Accuracy: +/- 10%
How do I change surface sensitivity? 1. Measure spectra at different tilt angles Detector Atom Electron path Analysis depth
2. Use an ion gun e- Gas in (Ar) Gas out Ionized gas (Ar + ) Measure XPS spectra after each etching cycle
XPS XPS Depth profile on a piece of Al foil O C Al
Al peak 73 ev: Al 76 ev: Al 2 O 3 Cou unts Etching time Binding Energy (ev)
Examples of XPS applications 1. Contamination analysis 2. SAMs characterization 3. Explosive detection!
1. Surface contamination
Polymer scaffolds Polymer solution (typically organic solvent) Mold Polymer scaffold PDLLA fabricated in silicone mold 1.80E+05 1.60E+05 1.40E+05 O C Counts /s 1.20E+05 1.00E+05 8.00E+04 6.00E+04 4.00E+04 2.00E+04 Si 0.00E+00 1400 1200 1000 800 600 Binding Energy (ev) 400 200 0
25 Silicon contamination of PDLLA from different molds 20 licone (%) Sil 15 10 5 0 => Protective coating necessary to prevent Si leaching from flexible molds
2. SAM characterization
2. Poly(EG) SAMs on Au and SiN Poly(ethylene glycol) Cerruti et al, Langmuir, 2008, 24 (19), 10646-10653
Molecules analyzed
PEG SH XPS spectra C 1s Bare gold SH-PEG7 SH-PEG16 SH-uPEG3 SH-uPEG6 C1 C2 C2 C1C2 C1C2 O 1s S 2p C2 @ 286.5 ev C1 @ 285 ev 533 ev 162 ev = SH; 163 ev = S-S
PEG Silanes XPS spectra C 1s O 1s C-Si! C1 C2
Quantitative comparisons Divided by n. of PEG units
Aging of PEG SH SAMs SH-PEG16 SH-uPEG3 Fresh 2 week old Fresh 2 week old C 1s C1! O 1s S=O S2p S=O groups!
Aging of Si PEG3 SAMs Charging! Fresh 2 month old Better conductor! Supercritically dried C 1s O 1s Si 2p No energy calibration here!
Quantitative aging data ~ 30% left ~ 40% left ~ 42% left, increase in contamination Only contamination gone
SH PEG16 heating RT 120 C 160 C C 1s O 1s S 2p C2/Au after 160 C = 37% of starting
Poor quality Si PEG6/9 SAM: effect of heating RT 160 C C 1s O 1s Selective desorption of surface contamination!
3. Explosive detection
Today and the future Traditional methods for gas sensing: expensive slow professional staff needed Our final goal: reach the performances of a GC/MS in a hand held device
Schematic of a sensor In Receptor coating Transducer Out Response Important parameters Sensitivity: what s the smallest amount of target that can be detected? Selectivity: how many true vs. false positivesare detected?
The challenge ppt! Science and Technology for Army Homeland Security: Report 1 (2003)
Our targets TNT 2,4,6 Trinitrotoluene DNT 2,6 Dinitrotoluene
A very selective receptor Multiple binding sites.. Like an enzyme!
Our approach to find receptors Bacteriophages Receptors: oligopeptides that recognize specific sites on E. coli membrane
Phage display Phage (Virus) Screening Wash away non specific binders 10 9 different candidate receptors TNT Elution TNT REPEAT: Use more difficult binding conditions No Identified Selective Receptor Capable of Multivalent Binding Yes Consensus sequence? Sequence Analysis x 10 6 Bacterial amplification oligopeptide
Sensor s s coating Target Receptor As specific as possible Matrix Increase receptor density Avoid adsorption of competitors Does not interfere with receptor s activity
Matrix Poly(ethylene co glycidyl methacrylate) (PEGM) y=8% TNT receptor Trp His Trp Asn Phe Lys Pro Pro His Asp Leu Leu DNT receptor His Pro Asn Trp Ser Lys Tyr Ile Leu His Gln Arg Can we really bind them? Will the receptors keep their affinity towards DNT and TNT? Cerruti et al, Anal. Chem. 2009, 81, 4192
Oligopeptide on Au Au+receptor After rinsing CO, CN Ar NH 3 + NH 2 Binding Energy (ev) Binding Energy (ev)
Oligopeptide on PEGM PEGM PEGM+receptor After rinsing
Is the receptor still active? NO 2 has a very distinguishable signature PEGM PEGM + TNT 1600 2000 408.5 1200 800 400 0 392 396 400 404 408 412 Binding energy (ev) 1500 1000 500 0-500 392 396 400 404 408 412 Binding energy (ev)
PEGM+TNT receptor PEGM+TNT receptor + TNT NH 3 + NH 3 + 30000 403.2 25000 25000 20000 403.6 20000 15000 15000 10000 NO 2 10000 5000 409.8 5000 0 392 396 400 404 408 412 Binding energy (ev) NH + 3 50000 402.1 40000 0 392 396 400 404 408 412 Binding energy (ev) 30000 20000 NH 2 Soaked in H 2 O 399.99 NO 2 overnight 10000 408.1 0 392 396 400 404 408 412 Binding energy (ev)
TNT left after rinsing (A NO2 / A N tot ) AFTER RINSING / (A NO2 / A N tot ) BEFORE RINSING
Quantitative tests with Gas Chromatography/Mass Spectroscopy TNT or DNT droplet deposition Rinsing i in water GC/MS measurement of residual amount PEGM/Receptor coating
Droplet of TNT on PEGM with or Droplets of DNT and TNT on PEGM without receptors: with TNT receptor: 20 10 TNT and DNT left on substrates with TNT receptors (pmol) 20 10 TNT left on substrates (pmol) 0 TNT DNT TNT recepto ceptor DNT recepto ceptor PEGM 0 Highly specific interactions! One NO 2 group difference drastically changes the binding to the receptor
Test in vapor phase 60 C GC/MS measurement of adsorbed amount DNT or TNT PEGM/DNT receptor sorbed (pmo ol) Amount ad 2 1 0 DNT TNT Receptors work also in gas phase!
Real time liquid detection with Quartz Crystal Microbalance Quartz is piezoeletric: voltage applied induces oscillation When something is adsorbed on the electrode, a change in frequency of oscillation ( f) is observed. f is proportional to m (mass adsorbed) for rigid systems
DNT or TNT solution DI water Crystal holder: crystal coated with PEGM/TNT receptor uency (Hz) Ch hange in Freq 2 TNT injection 0-2 -4-6 -8 A -10 DI water injection -12 0 50 100 150 200 Time (min) ency (Hz) Cha ange in Frequ 3 DNT injection 2 1 0-1 -2 DI water injection B -3 0 20 40 60 80 100 Time (min) No frequency change observed if DNT solution flows on the crystal
Summary about XPS Surface sensitive technique Elemental composition Chemical species Applications from surface coatings to sensing!
Our system at McGill: Thermo K Alpha Analyzer Ar ion gun Detector Cameras X-Ray Source (Al K =1486.6 ev) Analysis chamber Load lock Compact, fully automated, depth profiles, x-y maps Easy to use! 3 inches Sample holder: as many as 36 small samples at a time, or large samples, up to ~1 thick