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

• 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)

Acronyms

100

10

1

01 10 100 1000 10000 Energy (eV)

IMFP

(nm

)

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

Transition dipole moment

<f||i> = (i/ℏ) <f|p|i>

= (m/ℏ)<f|r|i>

= (m/eℏ)<f|er|i> = (m/eℏ) <f||i>

electric dipole moment

= (/ℏ)(<f|V|i>

Koopman’s Theorem: frozen orbital approximation

A(N) + hv A+(N-1) + e

hv + e(KE)

initial state final state

Ei(N) + hv = Ef(N-1) + KEBE = hv – KE = Ef(N-1) - Ei(N) = -i

HF - relax + correl

BE of core level ≈ -iHF( ith orbital energy)

e

Binding energies

Chemical Shift of Binding Energy

Valence shell Electron charge q

Core electron e

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).

Example of Chemical Shift

• 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

Instrumentation

Photon source

X-ray sourceMg/Al anode source- beam sieze: 1cm- flux: 1010~1012

Rotating anode source

Ulraviolet light sourceHe discharge sourceHe(1P1) He(1S0) + hv(21.2ev)- flux: 1010~1012

- Line width ~ a few meV

Synchrotron radiation

-Tunable: wide photon energies (IR to hard X-ray)-High intensity: 1012~1015 photons/sec-high brightness-Polarization-Pulsed beam

h= 2.2x103E3(GeV)/R(m)

Pohang Light Source (PLS)

Beam energy 2~2.5 GeVBeam pulse Length ~1 nsBuch length 17~20 psBeam current ~1 A

http://pal.postech.ac.kr/english.html

Photon energy

Photon energy

Electron source

J.J. Weimer, MTS273, UAH

Energy analyzer

Hemispherical anlayzer-E = e(V2-V1)/(R2/R1- R1/R2)E/E = (w1+w2)/Ro + (

~ 10-3 ~10-4

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

Present: Submicron resolution

Low Energy Electron Microscopies

Step decoration of Si(111)

LEEM: E Bauer, Rep. Prog. Phys. 57, 895 (1994)

•Strong elastic backward scattering of slow electrons (10~100eV)•Several monolayer sensitivity•Resolution: ~ 20 nm

Photoelectron Emission Microscopy

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).

hv =10-1000 eV= 0.1-10 nm

Auger Electron Spectroscopy

Auger electron

KE = EK – EL1 – EL23

L2,3

L1

KKinetic Energy of Auger Electrons for KLL Transition

Element Specific

ehv or e

e

1s

2s

2p3/2,1/2

Auger Electron Energies

Auger Spectra From R. Nix lecture note

Applications

• Chemical identification: 1% monolayer

• Quantitative analysis

• Auger depth profiling

• Scanning Auger Microscopy (SAM)

- Spatially-resolved compositional

information

SEM and SAM

Focused electron gun

Energy Analyser

Detector

SEM topograph of Au-SiC codeposits

SAM image of Ag particles (d=1nm)

Secondary electrons

Auger electrons

SiC grain size= 0.04 m

Extended X-ray Absorption Fine Structure (EXAFS)X-ray Absorption Near Edge Structure (XANES or NEXAFS)

http://feff.phys.washington.edu/~ravel/

Rehr and Albers, Rev. Mod. Phys. 72, 621 (2000)

Informations

EXAFSBond lengthRMS displacements about bond lengthsCoordinationPartial pair distributionThree body effects

XANESFermi energyprojected DOS for absorbercharge transfer from theorytotal DOS from theory

http://feff.phys.washington.edu/~ravel/

Instrumentation

quantity to be measured - transmitted light - secondary electrons - fluorescence light - Auger electrons

Theory of EXAFS

Oscillating part of x-ray absorption coefficient (E) (E) = (E)(1+ (E))(E): atomic-like absorption coefficient(E): EXAFS modulation due to interference effect(E) = ((E) - (E))/(E) = ( - )/(- )

I

hv

hv k

(hv) (k)

r

(R)

FT

I = Ioe- d

I Io

Factors affecting to (k)

(k) = NjF(k)exp(-22k2) exp(-2rj/)[sin (2krj+ )]/krj

2

How to determine distance?- use standard to determine for a given atom pair - use this to determine unknown r

Nj: # of neighboring atomsF(k) : back scattering amplitude: Debye-Waller factor : total phase shift

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

Example: EXAFS

http://feff.phys.washington.edu/~ravel/

XANES

-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

Search light effect

E E

http://feff.phys.washington.edu/~ravel/

Example: XANES

Example: XANES

http://feff.phys.washington.edu/~ravel/

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

Self Assembled Monolayer