1Surface spectroscopic techniques XPS http://www.lasurface.com http://www.chem.qmul.ac.uk/surfaces/ http://www.chem.qmul.ac.uk/surfaces/scc 2 Surfacespectroscopic techniques E l e c t r o n s p e c t r o s c o p i e s I o n s p e c t r o s c o p i e s 3 Surfacespectroscopic techniques Depthresolution Spatial resolution 4 Beamin Beamout Surface Principle of electron and ion-beamspectroscopy Analyzed by spectrometer Beamin Beamout XPS Soft X-rays (200-2000eV) Core-level Photoelectrons UPS VacuumUVradiation(10-45eV) ValencePhotoelectrons AES Electrons Auger electrons SIMS Ions Secondary ions 5 X-ray photoelectron spectroscopy (XPS) usingsoft x-rays (withaphotonenergyof 200-2000eV) to examinecore-levels 6 Outline •Introduction(XPSbasicprinciples) •Quantification. •Widescandata(lowresolution) •Narrowscandata(highresolution) •Chemical stateanalysis. •Sputterdepthprofiles. •Imaging •Applications in material sciences 7 General Introduction Photoemissionwas first detected by Hertz in1887,andexplained By Einsteinin1905. Photoemission process Photonenergy E =hν Einstein explained that experiment and showthat light behaves like particles photoelectrons Light Photons hν 8 9 • Photo-ionization the surface by monoenergeticsoftX-rays(wavelength0.1—1nm) • Photoelectrons are ejected fromthe surface A + hν → A + + e - Qualitativeanalysis • Identificationoftheelementsinthesamplecanbemadedirectly fromthekineticenergies(KE) oftheseejectedphotoelectrons. Quantitativeanalysis • Therelativeconcentrationsofelementscanbedetermined fromthephotoelectronintensities. Basic principles a photon energy of 200-2000 eV 10 (KE) A + hν → A + + e - Photo-ionization (Photoelectronformation) Conservationof energy E(A) + hν = E(A + ) + E(e - ) E(e - ) = hν -[ E(A + ) -E(A) ] KE = hν - BE BE = hν - KE Binding energy(BE) allows identification of elements 11 Linewidth Energy (hν) Radiation Anode 0.85eV 1486.6eV K(alpha) Al 0.7eV 1253.6eV K(alpha) Mg Widely usedX-ray sources BE = hν - KE known detected 12 Palladium(Metal) 1s 2 2s 2 2p 6 3s 2 3p 6 4s 2 3d 10 4p 6 5s 2 4d 8 13 wide cell/41 x 10 4 0 2 4 6 8 10 12 14 C P S 1400 1200 1000 800 600 400 200 0 Binding Energy (eV) O1s C 1s OKLL C KLL O O HO HOH 2 C HO OH O O HOH 2 C HO OH O O HOH 2 C HO OH Cellulose 14 Thedeeper-ejectedelectrons -recapturedor trappedinthesolid -loss energy duringthetravel to thesurface, resultingin spectrumbackground Surfacesensitivity of XPS to Detector Onlyelectronescapedfromatoms at first fewlayers (top1-10nm) show specific photoelectronpeak. (No energy loss) 15 Qualitative •Detection ofall elements from 3 Li to 103 Lr (Except HandHe) “Electronspectroscopy for chemical analysis”or ESCA •Detection limits that range form0.1to1.0atom% •Chemical stateanalysis (Include Bonding and oxidation state) Analysis capabilities • Relative elemental composition of the surface (depth 1-10nm) Semi-Quantitative •None destructive (some damage to X-ray beamsensitive materials) •Conducting and insulating materials. •Spatial resolution for surface mapping from>10 mm •Depth profiling capabilities (1 µm) 16 EstimatedAnalysis Time § Requires 2-4hours vacuumto pump down before analysis § Typical operating pressure is 10 -9 to 10 -11 Torr. § Could require overnight to pump down § Qualitativecan be performed in 5to 10minutes § Quantitative analysisrequires 1hour to several hours depending on the information desired 17 Identificationof elements Electrons fromall orbitalsof an atom with BE <X-ray energycan be excited Spin-Orbit Splitting Characteristic photoelectron for elements 18 Note:The far e-fromnucleus, the less BE 19 Typical XPS spectra XPS Spectroscopic mode–Wide scan (Survey scan) CleanAg (High pass energy Ex. 160eV) Doublets are present in the non-s levels as a result of spin-orbital coupling. 20 Secondary ElectronEmission(Auger Electron) l LowEnergy “hole”produced l HighEnergy electronfills hole andenergy is emitted l Secondary Electroncanabsorbexcess energy andenter vacuum(Called“Auger Electron”) Photoemission process Auger effect 21 XPS Spectroscopic mode–Region scan (High resolution) (Lowpass energy Ex. 20eVor 40eV) Ag 3d region spectrum Area of 3d 3/ 2 = 3 Area of 3d 5/ 2 5 Spin-Orbit Splitting 22 XPSoftheAg3dregionof Agbasedcatalystonγ-Al 2 O 3 23 F 1s F KLL Auger C 1s F 2s F 1s C C F F F F n XPS Spectroscopic mode–Widescan(Survey scan) PTFE 24 100 F I F I % atomic ] A [ A A × 1 1 1 1 ] 1 ¸ , _ ¸ ¸ ∑ , _ ¸ ¸ · C C F F F F n PTFE Semi-quantification 25 XPS Spectroscopic mode–Regionscan(Highresolution) PTFE C C F F F F n C 1s F 1s No doublet peaks from1s photoelectrons C-F C-F 291.5 eV 689 eV 26 XPSofPoly(ethyleneterephthalate) Three different C atoms 27 Chemical State Identification •Since BE of a photoelectron is sensitive to the chemical surroundings of the atom → there is a chemical shift in BE. •XPS provides a tool to identify individual chemical states of an element of interest. 28 Relationship between oxidation state and BE + e Less d more attraction more BE d Li < Be < B < C < N < O <F BE (eV) : 60 120 180 285 400 530 689 More positive charge more attraction more BE Pt < Pt 2+ < Pt 4+ BE at 4f 7/2 (eV) : 71-72 72-73 76 In case of same atom In case of different atom 29 For example,the C 1s BE is observed to increase as the number of Oatoms bonded to C increases. As Ois highENatom, theattachedC is chargedpositively (C-C) < (C-O) < (O-C=O) < (O-(C=O)-O) Potential inorganic substances 285 286-287 288-289 291-291.5 BE C 1s (eV) In case of same atom More positive charge more attraction more BE (C-C) <(C-N) <(C-O) <(C-F) 285 285-6 286-7 291-3 BE C 1s (eV) 30 Fitting peaks C1: C2: C3 = 3: 1: 1 Gaussian Curve 288eV 286eV 285eV 31 OxidationStates of Titanium(Ti 3d) Ti 0 Ti 4+ Note: two spin orbit components exhibit the same chemical shift (~4.6eV) ~4.6 eV 32 Use Reference data to identify chemical state information •http://www.lasurface.com •www.NIST.gov •XPS handbook •Areas under Gaussian curve proportional to ratio of chemical species present HighResolutionXPS andFittingPeak 33 Examples of BE (C) inpolymers 34 Examples of BE (C) inpolymers 35 XPS compositional depthprofiling 1. Non-destructive depth profiling method 2. Depth profiling by erosion with inert gas ions (Destructive method) Angle-resolved XPS (ARXPS) Sputtering the surface within the spectrometer and use normal mode of XPS 36 I d = I 0 exp(-d/ λsinθ) (nm) Beer-Lambert relationship d where I d = intensity of emitted photoelectron as a function of depth (d) I 0 = intensity of emitted photoelectron froman infinity thick substrate λ =inelastic mean free pathof electron (2-5nm) θ =electron take-off angle relative to the surface Depth andsensitivity Normal mode θ =90 o I d = I 0 exp(-d/ λ) more d less I 1λ 37 Depth andsensitivity I d = I 0 exp(-d/ λ) P = I d / I 0 = exp(-d/ λ) If θ =90 o P =probability of emitted electron reaches the surface and being analyzed Surface-sensitive λ =2-5nm 38 d = [ln(I/I o )] λsinθ Angle-resolvedXPS d ∝ sinθ I d = I 0 exp(-d/ λsinθ) θ e θ e e 3λ 3λ 3λ Analysis Depth(d) =3λsinθ 39 3λ 0.8λ 2.1λ 0.26λ If λ=5 nm 15nm 10.5nm 4nm 1.3 nm Analysis depth (>95%signal) Sampling depth as a function of electron take-off angle θ 40 ARXPS of Thioltreated GaAs(100) R-SH R - S H 41 AR-XPS of aSiliconWafer withaNativeOxide Si 2p 3/2 (metal) BE ~99.0eV Si 2p 3/2 (SiO 2 ) BE ~103.0eV Si SiO 2 TOA More surface sensitive, more SiO 2 is detected. Si 0 SiO 2 42 Depthprofilingby sputtering(Destructivemethod) 43 Depthprofilingby sputtering(Destructivemethod) XPSdepthprofile ofSiO 2 onSi Widescan–quantitativeanalysis If sputter rate is known, thickness of the surface layers can be calculated. 44 Regionscan–qualitativeanalysis Depthprofilingby sputtering(Destructivemethod) XPSdepthprofileofSiO 2 onSi Si =99.3eV SiO 2 =103.3eV Sputter time 45 XPS-Instrumentation http://en.wikipedia.org/wiki/Image:System2.gif 1. Vacuumsystem 2. Samplepreparation 3. X-ray source 4. Electronenergy analyzer 5. Detectionsystem 46 Sample preparation § Solid samples are preferable. (Films, powder (pressed)) § Stable in the UHV chamber § Sample can be conducting or insulating. Mountingthe sample Clips Double sided adhesive tape Single bar Special holder stubs/cups are available for powder samples. 47 X-ray sources for XPS < 5000 eV 48 X-ray sources for XPS Formation of X-ray Theenergy of X-ray (Photon) must behighenough(at least 900eV) to excitetheelectroninK-shell of all elements. Most popular anodematerials areAl andMg Bombard metal target by high energy electrons – then photon with known energyare emitted 49 Al Kα MgKα Photons with known energy 50 Monochromatic X-ray X-ray with a specific energy Monochromation 51 X-ray Sources 2.1eV 5417 K(alpha) Cr 2.0eV 4510.9 K(alpha) Ti 2.6eV 2984.3 L(alpha) Ag 1.6eV 2042.4 L(alpha) Zr 0.85eV 1486.6 K(alpha) Al 0.7eV 1253.6 K(alpha) Mg Line Width Energy (eV) Radiation Material http:/ / www.thermo.com/ eThermo/ CDA/ Products/ Product_Detail/ 0,1075,15955-158-X-1-13080,00.html 52 Electron energy analyzer 1. Double Cylindrical Mirror Analyzer (CMA) Lowresolution dE/E ~1% Outer -Negative potential Inner –Ground Only e-of a fixed energy can be passed and detected 53 2. Hemispherical Mirror Analyzer 54 l A potential difference between the cylinders or hemispheres allow only electrons with specific kinetic energies to make it to the electron detector. l Varying potentials measure different kinetic energies l Computer calculates binding energy High resolution dE/E <0.5 % 55 Kratos Ultra (UK) www.latrobe.edu.au SampleMounting 56 Applications of XPS inMaterial Science Metallurgy andCorrosion science 1. Interaction of a metal surface with its environment and the formation of passive layer(Oxide film) 2. The breakdown of the surface filmby a localized phenomenon such as pitting. 3. Elemental distributions on mineral particle surfaces 4.Depth profiles of corroded materials 57 Metallurgy Stainless steel Surface Cr richCr-oxide Bulk Cr 18%, Ni 8%, Fe72% 58 http://www.eaglabs.com/techniques/analytical_techniques/xps_esca.php W WOxide Identificationof Pittingcorrosion 59 CorrosionProblems onSteel Wires a significantly higher concentration of iron in the corroded areas coupled with lower levels of calciumand sodium which are attributed to the drawing lubricants (Ca and Na stearates). XPS images of a corroded stainless steel wire Fe Cl Ca C 60 Applications of XPS in Material Science Microelectronics andSemiconductor Materials XPS depthprofilingis used to confirmthelayer uniformity Semiconductor distributedBraggReflector Stack 61 62 • Location of ppm/ppb levels of elements in the microstructure •Characterisation of ceramic compositional change with depth •Analysis of nmthick surface layers Ceramics Applications of XPS in Material Science 63 Thin film coatings on glass Architectural glass, lenses, mirrors, and many other products are coated to provide specific optical properties. These coatings are often a stack of thin layers that can be characterized using Depth-profiling XPS to verify the composition of the layers, detect contaminants, and estimate layer thickness. Fig.1:XPS sputter depth profile of an architectural glass coating 64 Example – Vanadium Phosphous Oxide Catalyst Sample 65 Polymer –surface modification 66 67 C60sputter depthprofilingof polymer films andsurfaces RecentlyC60sputteringhas beenshownto bevery effectivefor removingsurface contaminants andsputteringthroughpolymer films whilecausingminimal chemical damage. Fig.2:AC60sputter depthprofilethroughawax layer onpolyurethaneshows theabilityto sputter throughorganic andpolymer materials without causingsignificant chemical damageto thematerials.