Electron spectroscopy 1. IMFP of electron 2. Photoelectron spectroscopy 3. Microscopies: PEEM, LEEM, Imaging XPS 4. Auger electron spectroscopy 5. Extended X-ray Absorption Fine Stru cture s: n spectroscopy, theory, techniques, and applications I-IV, dited by C.R. Brundle (Academic, NY, 1977) l and J. Kuppers, Low energy electrons and surface chemistry (VCH, 1985) druff and T.A. Delchar, Modern techniques of surface science ge, 1986) surface analysis, I,II, edited by D. Briggs and M.P.Seah (Salle & Sauerlander) ers, Photoelectron spectroscopy, in Springer series in sol. state phys. v ure notes: Nix, http://www.chem.qmw.ac.uk/surfaces/scc/scat5.htm y J. Weimer, http://www.chem.qmw.ac.uk/surfaces/#teach ett, http://www.cem.msu.edu/~cem924sg/LectureNotes.html
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Electron spectroscopy 1.IMFP of electron 2.Photoelectron spectroscopy 3. Microscopies: PEEM, LEEM, Imaging XPS 4.Auger electron spectroscopy 5.Extended.
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Electron spectroscopy1. IMFP of electron2. Photoelectron spectroscopy 3. Microscopies: PEEM, LEEM, Imaging XPS4. Auger electron spectroscopy5. Extended X-ray Absorption Fine Structure
References:1. Electron spectroscopy, theory, techniques, and applications I-IV, edited by C.R. Brundle (Academic, NY, 1977)2. G. Ertl and J. Kuppers, Low energy electrons and surface chemistry (VCH, 1985)3. D.P. Woodruff and T.A. Delchar, Modern techniques of surface science (Cambridge, 1986)3. Practical surface analysis, I,II, edited by D. Briggs and M.P.Seah (Salle & Sauerlander)4. S. Hufers, Photoelectron spectroscopy, in Springer series in sol. state phys. v 825. Web lecture notes: Roger Nix, http://www.chem.qmw.ac.uk/surfaces/scc/scat5.htm Jeffrey J. Weimer, http://www.chem.qmw.ac.uk/surfaces/#teach Simon Garett, http://www.cem.msu.edu/~cem924sg/LectureNotes.html
The inelastic mean free path (IMFP) of electrons isless than 1 nm for electron energies with 10~1000 eV.
Why are electron spectroscopies surface sensitive ?
Photoelectron SpectroscopyX-ray Photoelectron Spectroscopy (XPS) :hv=200~2000 eVor Electron Spectroscopy for Chemical Analysis (ESCA) Ultraviolet Photoelectron Spectroscopy (UPS): hv =10~50 eV
Ev
Ef
KE
BE
e photoelectron
h
KE = hv – BE – for soildKE = hv – BE(or IP) for gas
KE: kinetic energyBE: binding energy: work function
hE,p e(E,)
PES spectra
h
KE
EF
Evac
EF
Valence band
Core level
Secondary electrons
Core electrons
Valence electrons
Typical XPS spectra
-steplike background due to inelastic electron energy loss-Electrons from deep bulk(depth>IMFP) loose their KE energies (higher BE)
KEKE
Photoemission peak intensity
I(E, hv) ~ Nv(E) Nc(E) (E,hv): UPS limit
~ Nv(E)(E,hv): XPS limit,
where Nv(E): densities of initial states(i) Nc(E): densities of final states(f) (E,hv): photoionization cross section ~ |<f|AP|i>|2
The XPS spectra represent the total density-of-statesmodulated by the cross-section for photoemission
Hamiltonian under electric fieldH = (1/2m)(p-eA/c)2 + V(r)Ho= p2/2m + V(r)H’= -eA.p/mc
Mif = <f|H’|i>=(e/mc)<f|Ap|i> = (e/mc)<f| Aoexp(ikx).p|i>let unit vector =Ao/|Ao|: the direction of polarization = (e/mc)<f| p|i>Vector potential A(x,t) = Aoexp(ikx-t): electromagnetic waveexp(ikx) = 1 +ikx - (kx)2~1: dipole approximationkx=2/5000Ǻx1Ǻ ~10-3 for the visible light of 500nmk: photon wave vectorElectron momentum operator p = -i
The core electron feels an alteration in the chemical environment when a change in the charge of the valence shell occurs.
r
A change in q, q, gives a potential change E = e q/r
• the oxidation state of the atom • the chemical environment - electronegtivity of neighboring atoms - # of neighboring atoms
Chemical shift
Binding energies in solids
BE = BE(atom) + e2q/rv + UM
eq: charge transferred to the ionrv: radius of valencesehllUM: Madelung energy (e2q/ d)
rv rvd
Chemical shift in compound A and B
BE(A,B) = e2(qA-qB)/rv + UM(A)- UM(B)
Ist term: the difference in the Coulomb interaction between the core and valence electrons2nd term: interaction of the atom to be photoionized with the rest of the crystal
Core level spectra of silicon oxides
J.W. Kim et al (2001) F.J. Himpsel et al., Phys. Rev. B 38, 6084 (1988).
• The chemical shift: ~4.6 eV• Metals: an asymmetric line shape (Doniach-Sunjic)• Insulating oxides: more symmetric peak
Spin-orbit Coupling
Pd: (3d)10 + hv (3d)9 + e L =2 , S = ½, JL+S,…, L-S = 2D 5/2 g J = 2x{5/2}+1 = 6 2D 3/2 g J = 2x{3/2}+1 = 4
JL+S
JL-S
-p,d and f orbitals splitted into two peaks in XPS spectra-BE (J=L-S) >BE(J=L+S)-Splitting as Z , n
Photoemission features
Ref: Electron spectroscopy I-IV edited by C.R. Brundle and A.D. baker Photoelectron spectroscopy by S. Hufner
-XPS peaks: Main peak and extra satellite peaksSource of satellite peaks-Intrinsic part : created in the photoemission process-Extrinsic part : interaction of photoelecton with the other electrons in the solidInitial state effectsFinal state effectsSample charging effectsNonmonochromic photon source
Extra peaks: final state effectInitial state effect: Koopman’s theoremFinal state effect: : 1~10 eV
-the created core hole after photoionzation affects the energy distribution of the emitted electrons in different ways.Relaxation effectsMultiplet splliting Multielctron excitations-shake up and shake off satellites-electron-hole excitation (continuous satellite): asymmetric line shapePlasmon loss peaksVibrational effects
Relaxation effect
-BE of the solid phase element is lower 5~10eV than BE of the corresponding gas phase element
BE = BE(Koopman)- relax
relax = relax(intra) + relax(extra)
Intra-atomic relaxation - redistribution of the electron of the excited atom - free atom caseExtra-atomic relaxation- redistribution of electrons from neighboring atoms- molecules, solids cases
Localized core hole created by photoionization is delocalized by The movement of electrons from the photoexcited atom or neighboring atoms, causing the photolectrons with lower BE than the adiabatic BE
Multiplet Splitting
Fe 3+(3s23d5) + hv Fe 4+(3s13d5) + e
Initial state Final state
3d
3s
Terms: 6S 7S 5S
-due to the various possible non-degenerate total electronic states that can occur in the final states
Shake up and shake off
•Multi-electronic transitions after creation of Ne 1s core hole - Excitation of electron to higher bound state: shake up - Excitation to continuum state: shake off
-In experiment, one cannot observe adiabatic energy since atom does not have time to relax to ground ionic state-photolectron is ejected while the atom is in the excited state
Ground state Excited state
Sudden approximation
Ahv
Shakeup and shake off (continued)
Satellite peaks
Initial state 3d9LFinal state: Main peak: c-1d10L-1
Satellite peak: c-1d9L
Multielectron excitations in metals: Doniach-Sunjic line shape-asymmetric line shape due to the electron-hole excitation near Fermi level-proprotinal to the density-of-state at Fermi level
Energy loss features-interaction between photoelectron and other electrons in the surface region-interband electronic transition-plasmon energy loss
surface plasmon(2ħp)
Bulk plasmon (ħp) = 15.8 eV
KEhe
p= 4ne2/m
plasmon: a quantum of a plasma oscillation
Sample charging effect-insulating sample will be charged positively due to the lossof the electrons in solids by photoionization
Neutral samplePositively chargedsample
+ + + + + + + + + +KE KE
-shift to lower KE, thus 1~5 eV higher BE
h
How to prevent charging ?-use flood electron gun to compensate charging-use thin film sample on the metallic substrate
ee
Differential chargingFlood Gun off
Flood Gun on
K. Shabtai, I.Rubinstein, S. R. Cohen, and H. CohenJ. Am. Chem. Soc., 122 (20), 4959 -4962, 2000.
Monolayer Self-Assembly. The surface was pretreated by UV-ozone + ethanol dip. Decanethiol (DT) was adsorbed (2 h, 4 mM solution in bicyclohexyl), the sample was rinsed with chloroform followed by octadecane trichlorosilane (OTS) adsorption (2 min, 2 mM solution in bicyclohexyl) and rinsing with chloroform
w1,w2: width of input and output slitsR1,R2: radius of inner and outer spheres : spread of entrance angle -angle resolved = 10~20o
-multichannel detector to enhance counts-High energy and angle resolution
-Hemispherical analyzer-Cylnidrical mirror analyzer-Cylindrical defelction analyzer-Paraboloidal analyzer-Display type analyzer-Time of flight analyzer
From J. J. Weimer, UAH
Cylindrical morror analyzer
-E = 1.3099eVln(R2/R1)
E/E = 0.18w/R1 + 1.375(
~ 10-2 ~10-3
R1,R2: readius of inner and outer
cylinder
w : annular slit width
: Spread of entrance angle
-High transmission
-Sensitive to sample position
Energy analyzer (continued)
w
Detector
Electron multiplierChanneltron-single channle detectionMultichannel plate - multichannel detection - 2D-imaging
Typical gain : 103~106
bias voltage: 1-5kV
1e = 1.6 x10-19 C1.6 x10-19x106 = 1.6x10-13 = 0.16 pA
Quantitative analysisIntensity = peak area after back ground subtraction ~ # of atoms in the detected volumeThe intensity of a core level of an element AIA = AD(EA)LA(A)JoNA T(EA) M(EA) cosA: photoionization cross sectionD(EA) : Detection efficiency of the electron spectrometerLA(A) : The angular asymmetry of the emitted w.r.t. the angle between the direction of incidence and of detectionLA(A) = 1 + (1/2)A((3/2)sin2-1)Jo: The flux of photonT(EA) : The transmission of the analyzerNA: The density of atoms AM : Inelastic mean free path of electron
Concentration of A CA
CA = IA/SA
IA = measured intensitySA= sensitivity factor of A
-Accuracy use SA<15% use standards< 5%-Precision <2%
Background subtraction
-straight line-Shirely’s method-Tougard and Sigmund: Phys. Rev. B 25, 4452 (1982)Caution:-change in peak position, width, height
Primary excitation spectrum
F(E) = J(E) -iK(E’-E)J(E’)dE’
iK(E’-E) = B(E’-E)/[C+(E’-E)2]2
B=2866 eV, C = 1643 eV for Cu, Ag, Au
J(E): measured flux of emitted electrons at E
i: the mean free path of electron
K(E’-E): the probability that an electron of E shall lose
energy E’-E per unit path length travelled in the solid
Applications
• Determination of energy levels
• Chemical bonding
• Oxidation states
• Density-of-states of valence bands
• Energy bands
• Quantum well states
• Quantitative analysis
Angle-Dependent Analysis
Si 2p peak of the oxide (BE ~ 103 eV) increases at grazing emission angles
d
x
X= dcos
Silicon Wafer with a Native Oxide
Example: CdSe-Nanoparticles
TOPO: n-trioctylphosphine oxide
Se 3d •Unstable to photoxidation•Chalcogenide at the surface is oxidized to sulfate or selenate and evaporate as molecular species
Example: polymer
The number of CH2 groups Ns= (2-15RO/C)/RO/C at the BA surfaceRO/C: O to C atomic ratioNs= (6-15RF/C)/RF/C at the FBA surfaceRF/C: F to C atomic ratio
L. Li et al, Macromolecules 33, 8002(2000)
Example: SAM/AU
M-C Bourg et al, J. Phys. Chem. B 104, 6562 (2000)
Example: Biomolecules/SAM/Au
C-M Yam et al, Coll. Surf. B21, 317 (2001)
Imaging XPS
Si SiOx
• Scan Analyzer• Parallel Direct Imaging • X-ray microprobe/Zone plate
A threshold excitation lamp. g-Arc lamp, operating at 4.9 eV Due to the presence of oxide at the silicon surface,Si appears DARK (W >> 4.9 eV) and Pd appears BRIGHT (W = 5.12 eV ~ 4.9 eV)
hv
Ground
e
CathodeLens
Array detector
Resolution : 20~100nm
Scanning Transmission X-ray Microscopy
X-ray microscope image, and protein and DNA map, of air-dried bull sperm. Images were taken at six x-ray absorption resonance wavelengths, and were used to derive the quantitative maps. Zhang et al., J. Struct. Biol. 116, 335 (1996).
-Near edge absorption: 0~50eV-Multiple scattering effect dominant-Mean free path ~5A-Intramolecular scattering process for chemisorbed molecules-Molecular orientation-Electronic structure-Bond length-Density of states
Future Directions for Characterization of Nanosystems
• Instrument for analysis of biomolecules, and polymers• 3-D structure determination• Nanostructure chemical determination• Functional parallel probe arrays• Standardization and metrology• New nano-manipulator• Non-SPM probes that use ions or electrons• In-situ nondestructive monitoring techniques