Survey spectrum: high sensitivity mode- identifies elements at surfaceXPS-elemental informationMultiplex spectrum: high resolution mode- identifies peak position and peak shape (chemical state)XPS Chemical state information97 98 99 100 101 102Binding Energy (eV)Intensity (A.U.)Si 2pSi 2p3/2Si 2p1/2Binding Energy (eV)96 98 100 102 104Intensity (A. U.)Si 2pHigh resolution spectrumLow resolution spectrumPeak fittingWhy is peak fitting needed? In many cases the information provided by XPS is contained in a spectrum that consists of a number of overlapping chemically shifted peaks, often of different peak shapes and intensities. Peak fitting have to be used to resolve the overlapping peak into individual components.A number of software are available for peak fitting. You can just click computer key and it will do fitting automatically.http://www.phy.cuhk.edu.hk/~surface/XPSPEAK/http://www.uwo.ca/ssw/archives/oct98/0019.htmlhttp://www.uwo.ca/ssw/archives/oct98/0020.htmlhttp://www.uwo.ca/ssw/archives/oct98/0021.htmlSome important thing done manually(1) Background removal method(2) FWHM(3) Symmetric or asymmetric(4) Guass or Lorentz(5) Tail height, Tail Mix, Tail exponential in asymmetric modePeaks widths or energy resolutionThe full width at half-maximum (FWHM) EE = (En2+ Ep2+ Ea2)1/2En:the intrinsic width of the initial level and the lifetime of the final stateEp: the line-width of the incident radiation (x-ray)Ea: the resolving power of the electron-energy analyserIn most cases, the second factor is the major contribution to the overall line width XPS lineEn(eV)Ag 3d5/20.38Ag 3d3/20.37Au 4f0.45Si 2p0.33SourceEp(eV)Mg k 0.69Al k 0.85mono 600.20mono 1500.22mono 1508000.25mono 25010000.30Ep(pass energy (eV)): 51020 50 100 Ea(eV) :0.09 0.18 0.360.90 1.80Inelastic BackgroundXPS spectra show characteristic "stepped" background (intensity of background to high BE of photoemission peak is always greater than low BE) due to inelastic processes (extrinsic losses) from deep in bulkBackground removalThree types of background in XPS studies (1) Linear background This is a simple subtraction method, achieved by removal of a linear background drawn as a straight line between the first and last set ofpoints in a spectrum.(2)Shirley backgroundThis is an approximation method for determining the background under an XPS peak. It requires the lower kinetic energy point to have a higher count value than the upper kinetic energy point.(3) Tougaard backgroundThis is the most accurate removal of background. However, complex calculation is need for this approaches. It is not supported for XPS quantification.776 777 778 779 780 781 782200300400500600700800Bindi ng Energy (eV)Intensity (a.u.)Co 2pB i n d i n g E n e r g y3 6 6 3 6 7 3 6 8 3 6 9 3 7 0 3 7 1IntensityBlue-GaussianRead- Ag 3d5Green-LorentzianShirley backgroundGuass or Lorentz mixture Charge compensation and referencing for insulatorsIn XPS, photoelectrons are emitted from a sample surface. If the sample is an insulator, electrons ejected from the surface cannot be replaced from the bulk, and consequently a charge will build up on the sample surface.Mo oxide on silica real catalyst is powder sampleThree general problems of surface charging:(1) General instability leading to spectral noise making analysis impossible.(2) A shift of the spectrum on the energy scale, leading to difficulties in interpreting chemical states.(3) Differential charging in different regions of the surface may cause a given peak to broaden or to split into more than one peak. How to solve charging problem?(1) Low energy electron flood gun(2) Special sample preparation and mounting Deposited surface layers Cover up any insulating material outside the analysis spot using masks or meshes. Press the powder to soft metal foil such as In and Au. (3) Put x-ray sources as close as possible to sample if using unmonochromated x-ray source.Charge referencing(1) Reference via adventitious carbon-based contaminationC 1s at a binding energy of 285.00.2 eVThis is the most convenient and commonly applied method(2) Implanted noble gas ions such as Ar ionsAr 2p3/2at a binding energy of 242.30.2 eV(3) MixturesThis method can be used for powder sample. Powdered graphite was the most commonly used reference material. (4) Internal standards (polymer system) This is the most reliable method of spectral referencing. However, a knowledge of the molecular formula of the material is required.