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X-ray Photoelectron Spectroscopy

XX--ray Photoelectron Spectroscopyray Photoelectron SpectroscopyRoger Smart, Stewart McIntyre, Mike Roger Smart, Stewart McIntyre, Mike Bancroft, Igor Bancroft, Igor Bello Bello & Friends& FriendsDepartment of Physics and Materials ScienceDepartment of Physics and Materials ScienceCity University of Hong KongCity University of Hong KongSurface Science Western, UWOS urface Science Western, UWOP hotoelectric effectPhotoelectric effectEinstein, Nobel Prize 1921 Photoemission as an analytical toolKai Siegbahn, Nobel Prize 1981 IntroductionXPSX-ray Photoelectron SpectroscopyESCAE lectron Spectroscopy for Chemical AnalysisUPSU ltraviolet Photoelectron SpectroscopyPESP hotoemission SpectroscopyXPS, also known as ESCA, is the most widely used surface analysis technique because of its relative simplicity in use and data Analytical Methods Analytical Methods KE = h -(EB+ ) Elemental identification and

ESCA Electron Spectroscopy for Chemical Analysis UPS Ultraviolet Photoelectron Spectroscopy PES Photoemission Spectroscopy XPS, also known as ESCA, is the most widely used surface analysis technique because of its relative simplicity in use and data interpretation. 1s 2s 2p 3s VB Ef Ev Photoelectron 0 Binding Energy 0 Kinetic Energy Photon h ...

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Transcription of X-ray Photoelectron Spectroscopy

1 XX--ray Photoelectron Spectroscopyray Photoelectron SpectroscopyRoger Smart, Stewart McIntyre, Mike Roger Smart, Stewart McIntyre, Mike Bancroft, Igor Bancroft, Igor Bello Bello & Friends& FriendsDepartment of Physics and Materials ScienceDepartment of Physics and Materials ScienceCity University of Hong KongCity University of Hong KongSurface Science Western, UWOS urface Science Western, UWOP hotoelectric effectPhotoelectric effectEinstein, Nobel Prize 1921 Photoemission as an analytical toolKai Siegbahn, Nobel Prize 1981 IntroductionXPSX-ray Photoelectron SpectroscopyESCAE lectron Spectroscopy for Chemical AnalysisUPSU ltraviolet Photoelectron SpectroscopyPESP hotoemission SpectroscopyXPS, also known as ESCA, is the most widely used surface analysis technique because of its relative simplicity in use and data Analytical Methods Analytical Methods KE = h -(EB+ ) Elemental identification and chemical state of element Relative composition of the constituents in the surface region Valence band structure------XX--ray Photoelectron Spectroscopy (XPS)ray Photoelectron Spectroscopy (XPS)

2 XPS spectrum:Intensities of photoelectrons versus EBor = h - - .Fspec e-Va c u u m l e v e lFermi levelcore levelFermi levelVa c u u m l e v e l sample = h - . = h - - () = h - .Fsample Energy ReferenceInstrumentation electron energy analyzer X-ray source Ar ion gun Neutralizer Vacuum system Electronic controls Computer systemUltrahigh vacuum system< 10-9 Torr (< 10-7 Pa) Detection of electrons Avoid surface reactions/contaminationsPeak:Photoelectr ons withoutenergy lossBackground:Photoelectrons withenergy lossRelative binding energies and ionization cross-section for UFor p, d and f peaks, two peaks are separation between the two peaks are named spin orbital splitting.

3 The values of spin orbital splitting of a core level of an element in different compounds are nearly the peak area ratiosof a core level of an element in different compounds are also nearly the orbital splitting and peak area ratios assist in element NotationsL-S Coupling ( j = ls)e-s=12s=1212j = l +12j = l Spin-orbital splittingGold XPS wide scan spectrumAuger PeaksN67O45O45N5N6N67N4N6N67N5N67 VBindingEnergies1416134213241247 Qualitative analysis57748488110335353547643763 Binding energies5p3/25f1/24f7/24f5/25s4d5/24d3/2 4p3/24p1/24sPhotoelectron is independent of the X-ray photon energy.

4 However, in the scale, Auger peak positions depend on the X-ray Induced Auger ElectronsGeneral methods in assisting peak identification(1) Check peak positions and relative peak intensities of 2 or more peaks (photoemission lines and Auger lines) of an element(2) Check spin orbital splitting and area ratios for p, d, f peaksA marine sediment sample from Victoria HarborThe following elements were found: O, C, Cl, Si, F, N, S, Al, Na, Fe, K, Cu, Mn, Ca, Cr, Ni, Sn, Zn, Ti, Pb, VAl 2pAl 2sSi 2pSi 2sXPS Sampling Depth i = inelastic mean free path of an electron in a solidFor an electron of intensity Ioemitted at a depth d belowThe surface, the intensity is attenuated according to theBeer-Lambert law.

5 So, the intensity Isof the same electronas it reaches the surface is Is= Ioe-d/ With a path length ofone 63% of allelectrons are scatteredSampling Depth Sampling Depth is defined as the depth fromwhich 95% of all photoelectrons are scatteredby the time they reach the surface ( 3 ) Most s are in the range of 1 nm for AlK radiation So the sampling depth (3 ) for XPS under these conditions is 3-10 nmdepends of the photoelectronthe specific materialnm (nanometers) 1 monolayer = nm Universal Curve for IMFPQ uantitative XPS: ISome XPS quantitative measurements are as accurateas 10%Ii= Ni i i Kwhere.

6 Ii= intensity of Photoelectron peak p for element i Ni = average atomic concentration of element i in thesurface under analysis i = Photoelectron cross-section (Scofield factor)for element i as expressed by peak p i = inelastic mean free path of a photoelectronfrom element i as expressed by peak p K = all other factors related to quantitative detection ofa signal (assumed to remain constant during exp t)WorstBestHow to measure ImeasuredAccuracy better than 15%using ASF sUse of standards measuredon same instrument or fullexpression above accuracybetter than 5%In both cases, reproducibility(precision) better than 2%Transmission FunctionTransmission function is the detection efficiency of the electron energy analyzer, which is a function of electron energies.

7 Transmission function also depends on the parameters of the electron energy analyzer, such as pass Au after Ar+sputteringQuantitative Analysis: IIScofield Cross-section Factors( i)have been calculated for each element from scattering theory, specifically forAlK and MgK radiationInelastic Mean Free Paths( i )varies with the kineticenergy of the Photoelectron . It can be estimated from a universal curve or calculated (better).For a multi-element surface layer consisting of elements i, j, = IiNi+Nj+Nk ( i i )Ii+ Ij+ Ik i i j j k k Examples of Quantitation IExamples of Quantitation IIErrors in QuantitationIi= sometimes difficult to separate intrinsic photoelectrons for the extrinsic scatteredphotoelectrons which comprise the background ( 5 - 100%) i = calculated value (unknown magnitude)

8 I= estimated error 50%Session 2 Chemical shifts in XPSI nitial and final statesKoopman s theoremEquivalent core approximationCalculations for binding energies and chemical shiftsLine widths and resolutionChemical Effects in XPSC hemical shift: change in binding energy of a core electron of an element due to a changein the chemical bonding of that view: Core binding energies are determined by: electrostatic interaction between it and the nucleus, and reduced by: the electrostatic shielding of the nuclear charge from allother electrons in the atom (including valence electrons) removal or addition of electronic charge as a result of changes in bonding will alter the shieldingWithdrawal of valence electron charge increase in BE(oxidation) Addition of valence electron charge decrease in BEChemical Shifts.

9 Oxide Compared to MetalLi-metal1s22s Density1s22sLi1s22s202s6Li2O1s22sLiEFerm iLi2 OLi-metalBinding Energy is lower due to increased screening of the nucleus by 2s conduction by 2s electronsBinding Energy is higher because Li 2s electron density is lost to oxygenPE spectrum1s21s2 Binding EnergyLi 1sPhotoemission Processcan be thought ofas 3 steps:(a) photon absorption and ionisation (initial state effects)(b) Response of atom and creation of Photoelectron (final state effects)(c) Transport of electron to surface (extrinsic effects)(one additional+ve charge)AABBBB+Koopman s TheoremThe BE of an electron is simply the difference between the initial state (atom with n electrons) and final state (atom with n-1electrons (ion) and free Photoelectron )BE = Efinal(n-1) Einitial(n)If no relaxation* followed photoemission, BE = - orbital energy, which can be calculated from Hartree Fock.

10 *this relaxation refers to electronic rearrangement following photoemission not to be confused with relaxation of surface Chemical Shift: Charged Sphere ModelFor a single atom j:E = qve2qv= no. of valence electrons rvrv= average radius of valenceelectrons Eb= qve2rvAdd change in interatomic potentialEb= qve2- Vijwhere Vij= potential of atom i on jrvrvqvEb288285C (1s)CH3 COCH3CH3C=OPeak width = eV291281 Eb qve2- VijrvTiC2C-C-CC-CF3 Examples of Chemical ShiftsFe 2p/1x 1 02681012141618202272 071 871 671 471 271 070 870 670 470 2700 Bin ding Energy (eV)Detailed Iron 2p Spectrum of High Purity IronMetallic FeFe2O3Fe 2p_ HSS2_3/33x 1 02152025303572 071 871 671 471 271 070 870 670 470 2700 Bin ding Energy (eV)


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