x-ray photoelectron spectroscopy · – observed: n, l, and ls-coupling for unpaired electrons...
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X-rayPhotoelectron
Spectroscopy
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ContentsInformation conveyed by XPSInstrumentation
vacuum, sample preparation, detectorIngredientsX-rays, photoelectrons, spectrum
ExtensionsXPD, ARPES, SRPES, PEEM
Conclusion
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InformationgivesElemental Chemical info
does notstructureimages / spatial resolution defects , phase
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A bit of historyTime line– Hertz 1887: photo-electricity
• UV on -ve electrode facilitate the spark between two electrodes
– Thompson1897: discovery of electron & e/m – Einstein 1905: quantum energetics
• hν= (KE )e +(KE)atom + (Eo-E+) ≅ (KE)e + Φ + (BE)e
– Siegban’s & Robison 1960s: techniqueEarly days– collect photo electric current vs. retarding potential– differentiate
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Instrumentation
Charged electrons: reactive with atoms– to the analyzer (min. p. 10^-6 bar)– on the solid => surface technique (5-20A)– surface cleanness– adsorbate layer (min. 10^-9bar /UHV)
cleaning surface:– cleaving or scratching under vacuum,– ion etching, annealing
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Detectors
Combination of retardation and deflection analyzers with electron multiplier or a multi channel plate at the end
low current:– sweeping/stability, – speed
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Ingredients
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Ingredients
Incoming radiation:x.rays
Outcoming: photoelectrons
EDC by the analyzer
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X-RaysParameters:energy,polarization , intensity, spot size, angle
UPS: He lamb (pressure to choose from 21.22-52.2415)XPS: x-ray tube , 12-15KeV electrons hit a cooled target:– Y,Mξ (132,.5) Ti,Mξ (452) Mg,Kα(1253,.7)
Al,Kα(1486.7,.8) SiKα(1740,.9) CuKα(8047.8,2.5)– secondary lines or satalites + bremsstrahlung background
Monochromate by crystal diffraction (2dsinθ=nλ)Monochromaticity vs. brightness
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Synchrotron radiation
Tunable bright energy– 20-5000ev– utilize ionization cross section
without loss of signal you can:– monchormatize– collimate / focus
Polarized controllable time pulsation
16
6
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PhotoElectrons (bound)
While bound– Fully characterized by four quantum numbers:
(principal, angular, magnetic, and spin)– negotiated by QM/Coulomb interactions with all all
parties including: the nucleus, other electrons and the immediate chemical environment
– Observed: n, l, and LS-coupling for unpaired electronsSome theories tries to predict BE for all lines
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PhotoElectrons (leaving)Some electrons leave the sample intact and form XPS lines While leaving the sample: charged,energetic interacting with:– collective oscillation of electron (plasmons ≅ bulk 10,
Surface < 10eV)– surface charge ( global effect )
– bound e-h pair (exciton)– crystal vibrations (phonons ≅.1eV)– spin waves / magnons
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PhotoElectrons (as a beam)
As a beam out of the sample– beam with direction (polar, azimuthal)– beam out of a certain sized spot – intensity distributed particles over a range of K.E
XPS is angle integrated and wide area (cm2) analysis
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Elemental Info
Well tabulated
different lines will make it sure in case of confusion
n, l, LS
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Chemical Info
Not all lines are sensitive
shifts are towards higher BE
area ratios -> relative concentration
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Shake up/off lines:weak / unfavorable (more than one e)
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Auger process
3 electrons are involved
KE of Auger is not hνdependant
broad peak
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The spectrum (E)
Peaks:– XPS lines– Auger– Others:
• palsmons, shakeup,
libraries and standard’s spectra and pure samples
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Spectrum (I)
Intensity– atomic:
• no. of electrons• photoionization cross section (hν dep.)
– Instrumental• sample area• radiation flux• KE detection efficiency • angular detection efficiency
->Relative concentration using peak areas
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Extended techniques
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X-ray Photoelectron Diffraction (XPD)
Track angular variation of a certain peak intensity. It varies only if it belongs to the second layer
Consideration: preparation, propped depth, time->surface structure + enhanced surface sensitivity
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Spin resolved XPS (SRPES)
Orientation of the spin of the photoelectron can be maintained from emission to detectionMott detector can tell the spin -> up/down
Band structure of a magnetic material affects the up/down ratioBand structure <-- XPS
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Photoelectron Emission Microscopy (PEEM)
By focusing the x-rays only a chosen spot of the sample may contribute to the spectrum --> imageFocusing can be done via:– collimating (signal↓ -> synchrotron)– x-ray optics (under development)
Advantage: Element specific,chemical status
Applications: diffusion, segregation, Shottky barrier
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Conclusion
A look back
what does the future holds?
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Information +veElemental (but no trace analysis .01-.3%)Chemical info (resolution, globality)-vestructure (-> XPD)images (-> PEEM)defects , phase
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Future
Wide range tunable, sharp, bright, well focused, controlled pulsed , and affordable x-ray source (synchrotron)
High resolution, fast/efficient detectors
automated processing
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Angle resolved XPS (ARPES)Anisotropy of electron photo emission direction Function of:– atom, molu. orientation – E,hν of radiation
•Difference•ARPES:molecule, atom of the crystal and adsorbate•XPD: structure of an atomic layer
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