Electron spectroscopy

1
Electron spectroscopy

• IMFP of electron
• Photoelectron spectroscopy
• 3. Microscopies PEEM, LEEM, Imaging XPS
• Auger electron spectroscopy
• 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 82 5. Web lecture
notes Roger Nix, http//www.chem.qmw.ac.uk/su
rfaces/scc/scat5.htm Jeffrey J. Weimer,
http//www.chem.qmw.ac.uk/surfaces/teach
Simon Garett, http//www.cem.msu.edu/cem924sg/Lec
tureNotes.html
2
Acronyms

• Electron Spectroscopies
• Auger Electron Spectroscopy (AES)
• X-ray Photoelectron Spectroscopy (ESCA)
• Ultraviolet Photoelectron Spectroscopy (UPS)
• Electron Energy Loss Spectroscopy (EELS)
• High Resolution EELS
• Electron Microscopies
• Scanning Auger Microscopy (SAM)
• Photoemission Electron Microscopy (PEEM)
• Low Energy Electron Microscopy (LEEM)
• Scanning X-ray Photoelectron Microscopy
• (SXPEM)
• Secondary Electron Microscopy with
• Polarization Analysis (SEMPA)

3
Why are electron spectroscopies surface sensitive
?
100
IMFP(nm)
10
1
0

• 10 100 1000 10000
• Energy (eV)

The inelastic mean free path (IMFP) of electrons
is less than 1 nm for electron energies with
101000 eV.
4
Photoelectron Spectroscopy
X-ray Photoelectron Spectroscopy (XPS)
hv2002000 eV or Electron Spectroscopy for
Chemical Analysis (ESCA) Ultraviolet
Photoelectron Spectroscopy (UPS) hv 1050 eV
e photoelectron
KE
KE kinetic energy BE binding energy F work
function
hn(E,p,q)
Ev
e(E,q,s)
f
Ef
hn
BE
KE hv BE f for soild KE hv BE(or
IP) for gas
5
PES spectra
KE
EF
Valence electrons
Core electrons
hn
Secondary electrons
Evac
EF
Valence band
Core level
6
Typical XPS spectra
KE
KE

• steplike background due to inelastic electron
  energy loss
• Electrons from deep bulk(depthgtIMFP) loose their
  KE energies (higher BE)

7
Photoemission peak intensity
I(E, hv) Nv(E) Nc(E) s(E,hv) UPS limit
Nv(E)s(E,hv) XPS
limit, where Nv(E) densities of initial
states(i) Nc(E) densities of final
states(f) s(E,hv) photoionization
cross section s ltfAPigt2

The XPS spectra represent the total
density-of-states modulated by the cross-section
for photoemission
8
Transition dipole moment
Hamiltonian under electric field H
(1/2m)(p-eA/c)2 V(r) Ho p2/2m V(r) H
-eA.p/mc Mif ltfHigt(e/mc)ltfApigt
(e/mc)ltf
Aoexp(ikx).pigt let unit vector eAo/Ao the
direction of polarization
(e/mc)ltf e.pigt Vector potential
A(x,t) Aoexp(ikx-vt) electromagnetic
wave exp(ikx) 1 ikx - (kx)21 dipole
approximation kx2p/5000?x1? 10-3 for the
visible light of l500nm k photon wave
vector Electron momentum operator p -i?
ltf?igt (i/?) ltfpigt
(mv/?)ltfrigt (mv/e?)ltferigt
(mv/e?) ltfmigt m electric
dipole moment (1/?v)(ltf?Vigt
9
Koopmans Theorem frozen orbital approximation
A(N) hv A(N-1) e
e
hv
e(KE)
initial state final state
Ei(N) hv Ef(N-1) KE BE hv KE Ef(N-1)
- Ei(N) -eiHF - erelax ecorrel
BE of core level - eiHF( ith orbital energy)
10
Binding energies
11
Chemical Shift of Binding Energy
Valence shell Electron charge q
r
Core electron e
The core electron feels an alteration in the
chemical environment when a change in the charge
of the valence shell occurs.
A change in q, dq, gives a potential change dE
e dq/r

• the oxidation state of the atom
• the chemical environment
• - electronegtivity of neighboring atoms
• - of neighboring atoms

12
Chemical shift
13
Binding energies in solids
BE BE(atom) e2q/rv UM eq charge
transferred to the ion rv radius of
valencesehll UM Madelung energy (ae2q/ d)
rv
rv
d
Chemical shift in compound A and B
DBE(A,B) e2(qA-qB)/rv UM(A)- UM(B) Ist term
the difference in the Coulomb interaction
between the core and valence
electrons 2nd term interaction of the atom to be
photoionized with the rest of
the crystal
14
Core level spectra of silicon oxides
F.J. Himpsel et al., Phys. Rev. B 38, 6084
(1988).
J.W. Kim et al (2001)
15
Example of Chemical Shift

• The chemical shift 4.6 eV
• Metals an asymmetric line shape
  (Doniach-Sunjic)
• Insulating oxides more symmetric peak

16
Spin-orbit Coupling
J LS
J L-S
Pd (3d)10 hv ? (3d)9 e L 2 ,
S ½, J LS,, L-S 5/2, 3/2 2D
5/2 g J 2x5/21 6 2D 3/2 g J
2x3/21 4

• p,d and f orbitals splitted into two peaks in XPS
  spectra
• BE (JL-S) gtBE(JLS)
• Splitting? as Z ?, n ?

17
Photoemission features

• XPS peaks Main peak and extra satellite peaks
• Source of satellite peaks
• Intrinsic part created in the photoemission
  process
• Extrinsic part interaction of photoelecton with
  the other
• electrons in the solid
• Initial state effects
• Final state effects
• Sample charging effects
• Nonmonochromic photon source

Ref Electron spectroscopy I-IV edited by C.R.
Brundle and A.D. baker Photoelectron
spectroscopy by S. Hufner
18
Extra peaks final state effect

• Initial state effect Koopmans theorem
• Final state effect 110 eV
• the created core hole after photoionzation
  affects the energy
• distribution of the emitted electrons in
  different ways.
• Relaxation effects
• Multiplet splliting
• Multielctron excitations
• shake up and shake off satellites
• electron-hole excitation (continuous satellite)
  asymmetric line shape
• Plasmon loss peaks
• Vibrational effects

19
Relaxation effect
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
BE BE(Koopman)- erelax erelax erelax(intra)
erelax(extra)
Intra-atomic relaxation - redistribution of the
electron of the excited atom - free atom
case Extra-atomic relaxation - redistribution of
electrons from neighboring atoms - molecules,
solids cases
-BE of the solid phase element is lower 510eV
than BE of the corresponding gas phase element
20
Multiplet Splitting

• due to the various possible non-degenerate total
  electronic states that
• can occur in the final states

Fe 3(3s23d5) hv Fe 4(3s13d5) e
3d
3s
Initial state Final state
Terms 6S 7S 5S
21
Shake up and shake off
Sudden approximation
-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

• 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

Excited state
Ground state
hv
A
22
Shakeup and shake off (continued)
23
Satellite peaks
Initial state 3d9L Final state Main peak
c-1d10L-1 Satellite peak c-1d9L
24
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
25
Energy loss features

• interaction between photoelectron and other
  electrons
• in the surface region
• -interband electronic transition
• -plasmon energy loss

e
hn
KE
plasmon a quantum of a plasma oscillation
vp ?4pne2/m
Bulk plasmon (hvp) 15.8 eV
surface plasmon (?2hvp)
26
Sample charging effect

• insulating sample will be charged positively due
  to the loss
• of the electrons in solids by photoionization

e
e
KE
hn
KE

Positively charged sample
Neutral sample

• shift to lower KE, thus 15 eV higher BE

• How to prevent charging ?
• use flood electron gun to compensate charging
• use thin film sample on the metallic substrate

27
Differential charging
K. Shabtai, I.Rubinstein, S. R. Cohen, and H.
Cohen J. Am. Chem. Soc., 122 (20), 4959 -4962,
2000.
Flood Gun off
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
Flood Gun on
28
Instrumentation


29
Photon source

• X-ray source
• Mg/Al anode source
• - beam sieze 1cm
• flux 10101012
• Rotating anode source

• Ulraviolet light source
• He discharge source
• He(1P1) ? He(1S0) hv(21.2ev)
• flux 10101012
• Line width a few meV

30
Synchrotron radiation
hn 2.2x103E3(GeV)/R(m)

• Tunable wide photon energies (IR to hard X-ray)
• High intensity 10121015 photons/sec
• high brightness
• Polarization
• Pulsed beam

31
Pohang Light Source (PLS)
http//pal.postech.ac.kr/english.html
Beam energy 22.5 GeV Beam pulse
Length 1 ns Buch length 1720
ps Beam current 1 A
32
Photon energy
33
Photon energy
34
Electron source
J.J. Weimer, MTS273, UAH
35
Energy analyzer

• Hemispherical analyzer
• Cylnidrical mirror analyzer
• Cylindrical defelction analyzer
• Paraboloidal analyzer
• Display type analyzer
• Time of flight analyzer

From J. J. Weimer, UAH

• Hemispherical anlayzer
• E e(V2-V1)/(R2/R1- R1/R2)
• DE/E (w1w2)/Ro (da)2
• 10-3 10-4
• w1,w2 width of input and output slits
• R1,R2 radius of inner and outer spheres
• da spread of entrance angle a
• angle resolved a 1020o
• multichannel detector to enhance
• counts
• -High energy and angle resolution

36
Energy analyzer (continued)

• Cylindrical morror analyzer
• E 1.3099eVln(R2/R1)
• DE/E 0.18w/R1 1.375(da)3
• 10-2 10-3
• R1,R2 readius of inner and outer
• cylinder
• w annular slit width
• da Spread of entrance angle a
• High transmission
• Sensitive to sample position

w
37
Detector

• Electron multiplier
• Channeltron
• single channle detection
• Multichannel plate
• - multichannel detection
• - 2D-imaging

Typical gain 103106 bias voltage 1-5kV
1e 1.6 x10-19 C 1.6 x10-19x106 1.6x10-13
0.16 pA
38
Quantitative analysis

• Intensity peak area after back ground
  subtraction
• of atoms in the detected
  volume
• The intensity of a core level of an element A
• IA sAD(EA)LA(gA)JoNA T(EA) lM(EA) cosq
• sA photoionization cross section
• D(EA) Detection efficiency of the electron
  spectrometer
• LA(gA) The angular asymmetry of the emitted
  w.r.t. the angle between
• the direction of incidence and of detection
• LA(gA) 1 (1/2)bA((3/2)sin2g-1)
• Jo The flux of photon
• T(EA) The transmission of the analyzer
• NA The density of atoms A
• lM Inelastic mean free path of electron

Concentration of A CA CA IA/SA IA measured
intensity SA sensitivity factor of A

• Accuracy
• use SAlt15
• use standardslt 5
• Precision lt2

39
Background subtraction

• straight line
• Shirelys method
• Tougard and Sigmund
• Phys. Rev. B 25, 4452 (1982)
• Caution
• -change in peak position, width, height

Primary excitation spectrum F(E) J(E)
-li?K(E-E)J(E)dE liK(E-E)
B(E-E)/C(E-E)22 B2866 eV, C 1643 eV for
Cu, Ag, Au J(E) measured flux of emitted
electrons at E li 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
40
Applications

• Determination of energy levels
• Chemical bonding
• Oxidation states
• Density-of-states of valence bands
• Energy bands
• Quantum well states
• Quantitative analysis

41
Angle-Dependent Analysis
Silicon Wafer with a Native Oxide
X dcosq
x
q
d
Si 2p peak of the oxide (BE  103 eV) increases
at grazing emission angles
42
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

43
Example polymer
The number of CH2 groups Ns (2-15RO/C)/RO/C at
the BA surface RO/C O to C atomic ratio Ns
(6-15RF/C)/RF/C at the FBA surface RF/C F to C
atomic ratio
L. Li et al, Macromolecules 33, 8002(2000)
44
Example SAM/AU
M-C Bourg et al, J. Phys. Chem. B 104, 6562 (2000)
45
Example Biomolecules/SAM/Au
C-M Yam et al, Coll. Surf. B21, 317 (2001)
46
Imaging XPS
Si SiOx

• Scan Analyzer
• Parallel Direct Imaging
• X-ray microprobe/Zone plate

Present Submicron resolution
47
Low Energy Electron Microscopies
Step decoration of Si(111)

• Strong elastic backward scattering of slow
  electrons
• (10100eV)
• Several monolayer sensitivity
• Resolution 20 nm

LEEM E Bauer, Rep. Prog. Phys. 57, 895 (1994)
48
Photoelectron Emission Microscopy
Array detector
Cathode Lens
hv
e
Ground
Resolution 20100nm
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 gtgt 4.9
eV) and Pd appears BRIGHT (W 5.12 eV 4.9 eV)
49
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).
hv 10-1000 eV l 0.1-10 nm
50
Auger Electron Spectroscopy
Auger electron
e
2p3/2,1/2
L2,3 L1 K
2s
hv or e
e
1s
KE EK EL1 EL23
Kinetic Energy of Auger Electrons for KLL
Transition
Element Specific
51
Auger Electron Energies


52
Auger Spectra
From R. Nix lecture note
53
Applications

• Chemical identification 1 monolayer
• Quantitative analysis
• Auger depth profiling
• Scanning Auger Microscopy (SAM)
• - Spatially-resolved compositional
• information

54
SEM and SAM
Focused electron gun
Detector
SiC grain size 0.04 ?m
Secondary electrons
SEM topograph of Au-SiC codeposits
Energy Analyser
Auger electrons
SAM image of Ag particles (d1nm)
55
Extended X-ray Absorption Fine Structure (EXAFS)
X-ray Absorption Near Edge Structure (XANES or
NEXAFS)
Rehr and Albers, Rev. Mod. Phys. 72, 621 (2000)

http//feff.phys.washington.edu/ravel/
56
Informations
EXAFS Bond length RMS displacements about bond
lengths Coordination Partial pair
distribution Three body effects
XANES Fermi energy projected DOS for
absorber charge transfer from theory total DOS
from theory
http//feff.phys.washington.edu/ravel/
57
Instrumentation
quantity to be measured - transmitted light
- secondary electrons - fluorescence
light - Auger electrons
58
Theory of EXAFS
I Ioe- m d
I
Io
Oscillating part of x-ray absorption coefficient
m(E) m(E) mo(E)(1 c(E)) mo(E) atomic-like
absorption coefficient c(E) EXAFS modulation due
to interference effect c(E) (m(E) -
mo(E))/mo(E) (g - go)/(go- g)
go
I
g
hv
c(R)
FT
c(hv)
c(k)
hv
k
r
59
Factors affecting to c(k)
c(k) NjF(k)exp(-2s2k2)
exp(-2rj/l)sin (2krj f)/krj2
Nj of neighboring atoms F(k) back scattering
amplitude s Debye-Waller factor f total phase
shift
How to determine distance? - use standard to
determine f for a given atom pair - use this
to determine unknown r
60
Surface EXAFS Cl/Ag(111)

• Probability of creating core hole
• Single scattering effect
• Needs synchrotron radiation
• Detection methods
• Auger electron yield for low Z elements
• Fluorescence yiled for high Z elements
• Total or partial electron yield
• Ion yield
• Informations
• nearest neighbor distance
• Adsorption structure

61
Example EXAFS
http//feff.phys.washington.edu/ravel/
62
XANES

• Near edge absorption 050eV
• Multiple scattering effect dominant
• Mean free path 5A
• Intramolecular scattering process for
• chemisorbed molecules
• Molecular orientation
• Electronic structure
• Bond length
• Density of states

Search light effect
E
E
63
Example XANES
http//feff.phys.washington.edu/ravel/
64
Example XANES
http//feff.phys.washington.edu/ravel/
65
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

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Self Assembled Monolayer