Techniques for Surface Chemical CompositionX-ray Photoelectron Spectroscopy (XPS/ESCA)Auger Electron SpectroscopySecondary Ion Mass Spectrometry (static and dynamic modes)Photoelectron Spectroscopies (PES)XPS: X-ray photoelectron spectroscopyUPS: Ultraviolet photoelectron spectroscopyConsider the following excitation sources; Cr, Al, Y, He II and He IXPS X-ray Photoelectron SpectroscopyESCA Electron Spectroscopy for Chemical AnalysisUPS Ultraviolet Photoelectron SpectroscopyPES Photoemission SpectroscopyHeinrich HertzAlbert EinsteinThe XPS emission processIt is based upon a single photon in/electron out process:(1) The photon is absorbed byan atom and leading toionization(2) A core(inner shell) electron is emitted from an atomPhysical basis of photoemissionAbsorption very fast - ~10-16sSchematic Representation of the Photoelectron productionApprox.Ekin= hv - EbEbis the binding energy of the ejected core electronsIN SOLIDS: Above Equation is modified by inclusion of Work Function.Ekin= hv - Eb- - work function1) Measure kinetic energy (KE) of photoelectrons ejected from sample2) Calculate photoelectron binding energy (BE) in electronvolts, eVBE = h KE f + dh = excitation x-ray energyf= electron spectrometer work functiond= net surface chargePrinciple of XPS techniqueSet-up of XPS equipmenteAnalyserUHVDetectorhvX-ray gunBEIhvBecause the Fermi levels of the sample and spectrometer are aligned, we only need to know the spectrometer work function, spec, to calculate BE(1s).E1sSample Spectrometere-Free Electron EnergyFermi Level, EfVacuum Level, EvsampleKE(1s)KE(1s)specBE(1s)Sample/Spectrometer Energy Level Diagram- Conducting SamplehvA relative build-up ofelectrons at the spectrometer raises the Fermi level of the spectrometer relative to the sample.A potential Echwill develop.E1sSample Spectrometere-Free Electron EnergyBE(1s)Fermi Level, EfVacuum Level, EvKE(1s)specEchSample/Spectrometer Energy Level Diagram- Insulating SampleIn order to measure the proper BE of electrons using XPS, we must make sure the sample and detector are at the same potential (typically ground) have an accurate value for the energy of the source have an accurate value for the work function of the detectorIn summaryEach element has unique set of core levels, BEs can be used tofingerprint element BE follows energy of levels: BE(1s) > BE(2s) > BE(2p) > BE(3s) BE of orbital increases with Z: BE(Na 1s) < BE(Mg 1s) < BE(Al 1s) BE of orbital not affected by isotopes: BE(7Li 1s) = BE(6Li 1s)

A A+Energy conservation in the photoionization processConservation of energy requires:E(A) + H = E(A+) + E(e-)

KE = h - (E(A+) - E(A))Kinetic Energy BE: Binding EnergyAbsorption very fast - ~10-16sKoopman's TheoremThe BE of an electron is simply difference between initial state (atom with n electrons) and final state (atom with n-1 electrons (ion) and free photoelectron)BE = Efinal(n -1)- Einitial(n)If no relaxation followed photoemission, BE = - orbital energy which can becalculated from Hartree-FockOne-electron model of the photoionization process electron rearrangement to shield core hole - the frozen orbital approximation is not accurate. electron correlation (small) relativistic effects (small)Measured BE's and calculated orbital energies different by 10-30 eV because of:Really, both initial state effects and final state effects affect measured BEComparison of experimental XPS C 1s binding energies with those calculated via Koopmans theorem for C in a range of molecules.Three step modelPhotoemission process often envisaged as three steps(i) Absorption and ionization (initial state effects)(ii) Response of atom and creation of photoelectron (final state effects)(iii) Transport of electron to surface and escape (extrinsic losses)Definition: Initial state effects are those factors thatinfluence the charge state of the atom before the photon strikes it.Survey Spectrum of Silicon WaferPrimary lines(1) Photoelectron linesThe most intense photoelectron peaks are relatively symmetrical and are typically the narrowest peaks observed in the spectra(2) Auger linesThese are group of peaks in rather complex patterns. 4 main Auger series observed in XPS are KLL, LMM, MNN and NOO series(3) Valence linesPrimary Structure in XPS spectraLines of low intensity occur in the low binding energy region of 0-20 eV. These lines are produced by photoelectron emission from molecular orbitals and from solid state energy bandsThe initial state effects 1:Spin-Orbit SplittingXPS Pd 3d spectrumFor any electron in orbital with orbital angular momentum, coupling betweenmagnetic fields of spin (s) and angular momentum (l) occursThis splitting arises from spin-orbit coupling effects in the initial stateThe relative intensities of the two peaks reflects the degeneracies of the final states (gJ= 2J+1)S-level L= 0, S = 1/2no spin-orbit splittingP-level L= 1, S = 1/2 J = 3/2, 1/2g3/2/g1/2= 2:1D-level L = 2, S = 1/2J= 5/2, 3/2g5/2/g3/2= 3:2F-level L = 3, S = 1/2J = 7/2, 5/2g7/2/g5/2= 4:3Observations:- s orbitals are not spin-orbit split - singlet in XPS- p, d, f orbitals are spin-orbit split - doublets in XPS- BE of lower j value in doublet is higher (BE 2p1/2 > BE 2p3/2)- Magnitude of spin-orbit splitting increases with Z- Magnitude of spin-orbit splitting decreases with distance from nucleus (increased nuclear shielding)Any change in bonding of an atom that changes the BE of the electron of interest will cause a corresponding shift in the peak position. This call the chemical shift. Usually chemical shifts are thought of as initial state effect (i.e. relaxation processes are similar magnitude in all cases)Hybridizationexample - C in H2C=CH2 versus C in H3CCH3oxidation stateexample - Fe in FeO versus Fe in Fe2O3degree of polar covalent or ionic bondingexamples - *C in [C*CCl]n versus *C in [C*CF]n- Cl in NaCl versus Cl in KClThe initial state effects 2: chemical shiftTi 2p1/2and 2p3/2chemical shift for Ti and Ti 4+. Charge withdrawn Ti Ti4+ so 2p orbital relaxes to higher BEChemical shift information very powerful tool for functional group, chemical enviroment, oxidation state, but not surface structure the binding energy of the electron in the orbital of an atom in an ordered lattice versus the same situation in a disordered lattice.Longer excited states live more likely to see final state satellitesEF4scinitial state c3d94s+c-1final state main linec-13d104s3d104sfinal state satellite linec-13d94s2++c-14s2The Ni 2p satellitee-1 hiMain processaji he-1Satellite process6eVc-13d94s2c-13d104s EBEF6eVInformation from secondary features in XPS spectra-Satellite peaksInformation from secondary features in XPS spectra-Satellite peak and energy loss peak (plasmon, phonon, band to band)Oxidized metals show enhanced satellitesThe bond shake-up satellite due to to *transition Extrinsic Satellites - Occur during transport of electron to surface - discrete loss structureElectronic excitation (interband or plasmons (bulk or surface))1. Intrinsic Satellite peaksPhotoelectron is created while ion is in various electronically excited states2. Extrinsic satellite peaks (energy loss peaks)- occur during transport of electron to surface3. X-ray satellite and ghost peaksSecondary peaks in XPS SpectrumX-ray SatellitesThese are due to excitation arising from some minor x-ray components1,23456Mg displacement, eV 0 8.4 10.117.6 20.648.7relative height 1008.0 4.10.6 0.50.5 Al displacement, eV 0 9.811.8 20.1 23.469.7relative height 100 6.4 3.2 0.4 0.3 0.6X-ray Satellite Energies and IntensitiesXPS spectra of a cellulose nitrate filmX-ray Ghost peaksX-ray ghosts are due to excitations arising from impurity elements in the X-ray sourceContaminating RadiationAnode Materials MgAlO(K)728.7 961.7Cu(L)323.9 556.9Mg(K) 233.0Al(K) -233.0Displacement of X-ray Ghost Lines (eV)Peak asymmetry in metals caused by small energy electron-hole excitations near EFof metal. Degree of asymmetry proportional to DOS at EF