1. Photoelectron Spectroscopy (XPS, UPS)
광전자 분광법
Photoelectron Spectroscopy (XPS, UPS)
김 정 원
미래융합기술본부 소재게놈측정센터 (

2. Introduction - 광전자분광법(PES)란 무엇인가 ? PES로 무엇을 할 수 있는가 ? PES의 간략한 역사
Principles of XPS
Chemical shifts

3. Photoelectron Emission
energy, polarization, angle
Low energy electrons
Vacuum UV
UPS
energy, angle, spin
x-ray
valence
XPS
core
High energy electrons
XPS: X-ray Photoelectron Spectroscopy (hv = 50~2000 eV)
UPS: Ultraviolet Photoelectron Spectroscopy (hv = 6~80 eV)

4. Schematic Diagram of PES Process
Electronic structure
Solid
UPS
Atomic composition
Chemical structure
XPS
Energy conservation
EB: element & chemistry specific
in case of core levels

5. Typical XPS spectra of Ag
3d5/2
Al Kα
Mg Kα
MNV
MNV
150
3d3/2
kCounts [a.u.]
100
3p3/2
3p3/1 3s 50
4d (valence) 4s 4p
1200
1000
800
600
400
200
Binding Energy [eV]
EB: constant irrespective of excitation source
AES transition: varies with excitation energy (constant kinetic energy)

6. 약어들 Abbreviations XPS: X-ray Photoelectron Spectroscopy
ESCA(XPS): Electron Spectroscopy for Chemical Analysis
UPS: Ultraviolet Photoelectron Spectroscopy
PES: Photoelectron (Photoemission) Spectroscopy
KE: Kinetic energy
BE (EB): Binding energy
ARXPS, UPS, PES: Angle-Resolved XPS, UPS, PES
XPD: X-ray Photoelectron Diffraction
PED: Photoelectron Diffraction
IP(E)S: Inverse Photoemission Spectroscopy
EELS: Electron Energy Loss Spectroscopy
Electron Spectroscopy: AES, EELS, PES

7. PES로 무엇을 할 수 있는가 ?
Non-destructive Elemental Identification except H and He
(다른 방법? X-ray fluorescence, SIMS, AES)
- Quantification (원소농도분석): ~ %정확도
- Chemical State Identification (e.g. Si, SiO2)
- Surface/Adsorbate Structure
- Electronic Structure
valence band level positions
band structure mapping
work function
many body effect, etc.
- Microscopy with Chemical Sensitivity

8. Available? Where? XPS (ESCA):
서울? 대부분의 종합대 및 공대, 일부 실험실 (not open), KIST, 생기연
지방? 대부분의 국립 종합대, 일부 종합 사립대
기초과학지원연구원 (대전, 부산), 화학연
대부분의 대기업 분석실 (not open), 포항방사광
UPS (Ultraviolet Photoelectron Spectroscopy):
기초과학지원연구원 (대전, 부산), KAIST, KIST
많은 개별 실험실 (not open)
vs. Evans Analytical Group (USA), Toray Research Center (Japan)
Pros: cheap and relatively fast
Cons: instrument-dependent, weak analysis

9. A Brief History of PES - 1887, H. Hertz: 광전효과(photoelectric effect) 발견
- 1899, J. J. Thompson: 전자 발견
- 1900, M. Planck: Quantum theory (quantized energy)
- 1905, A. Einstein: Quantum theory로 광전효과 설명
- 1958, W. E. Spicer: UPS spectra와 DOS 관련주장
- 1967, K. Siegbahn: XPS (ESCA) 확립
- 1969, HP - Commercial XPS
- 1970s: Synchrotron Radiation

10. History: Discovery of Electrons
Photoelectric effect
Metal plate in a vacuum, irradiated by ultraviolet light, emits charged particles (Hertz 1887), which were subsequently shown to be electrons by J.J. Thomson (1899).
Classical expectations
Light, frequency ν
Vacuum chamber
Electric field E of light exerts force F=-eE on electrons. As intensity of light increases, force increases, so KE of ejected electrons should increase.
Collecting plate
Metal plate
Electrons should be emitted whatever the frequency ν of the light, so long as E is sufficiently large I
For very low intensities, expect a time lag between light exposure and emission, while electrons absorb enough energy to escape from material
Ammeter
Potentiostat
Lecture note: Fisher (Univ. College London )

11. History: Photons and Electrons
Photoelectric effect
Einstein
Actual results:
Einstein’s interpretation (1905): light is emitted and absorbed in packets (quanta) of energy
Maximum KE of ejected electrons is independent of intensity, but dependent on ν
For ν<ν0 (i.e. for frequencies below a cut-off frequency) no electrons are emitted
Millikan
An electron absorbs a single quantum in order to leave the material
There is no time lag. However, rate of ejection of electrons depends on light intensity.
The maximum KE of an emitted electron is then predicted to be:
Verified in detail through subsequent experiments by Millikan
Work function: minimum energy needed for electron to escape from metal (depends on material, but usually 2-5eV)
Planck constant: universal constant of nature

12. 광과 시료원자와의 상호작용 표면탈출: XPS 이차전자 증폭 발생: SEM 탄성 충돌: XRD, LEED, RHEED
비탄성 충돌: EELS
X-선, UV
(EDX/WDX)
XES

13. Theory of Photoemission (binding energy)
frozen orbital approximation (Koopman’s theorem)
+ e (KE)
initial state
A(N)
final state
A+(N-1)
Considering energy conservation
relativistic energy
kth orbital energy by Hatree-Fock calculation
first approximation
correlation energy
Relaxation energy

14. Element-specific Core Levels

15. All core levels undergo same chemical shift
Chemical Shifts (Initial state effect?)
valence shell
(charge qi)
All core levels undergo same chemical shift
(approximation)
Surroundings
(charge qj)
core shell
electrons
rij
similar to Madelung potential
Charge potential model in ionic bonds
EB ↑ as q ↑ (note sign)
(Fig. 3.3)
for neutral atom
Coulomb interaction between core electron and nucleus screened by valence electron charge
Dependent on chemical environment such as
oxidation state
electronegativity of neighboring atoms
number of surrounding atoms
J.C. Vickerman and I.S. Gilmore, Surface Analysis, The principal Techniques, 2nd ed. (Wiley, 2008)

16. Chemical Shifts of S 1s and S 2p
Formal oxidation state is good indication of EB
It is only valid in ionic bonds
If there is covalent/ionic bond character mixing,
Charge density on an atom is the best criterion

17. A Example of Chemical Shifts
Not Always Possible
Tabulated Values Vary Widely

18. Final state consideration
hot electron
electron
ejection e- Ei Δt
hole
hole Ef
valence shell
final state
Instant excitation
Relaxation
(dynamic screening,
solvation…)
initial state
Ei(N) = Ef(N-1) + KE (e-)
BE = KE (e-) = Ef(N-1) – Ei(N) ≈ orbital energy
But, final-state relaxation is much dependent on its atomic environment
Relaxation energy –> binding energy (time-dependent)
Relaxation time –> peak width

19. Spin-orbit splitting Unpaired electron after ionization J=L+S 3d5/2
Ag 3d
Unpaired electron
after ionization
J=L-S
3d3/2
Ag 3d: (3d) → (3d)9 + e-
L=2, S=1/2, J=L+S,,, L−S = 5/2, 3/2
2D5/2, gJ = 2×(5/2)+1 = 6
2D3/2, gJ = 2×(3/2)+1 = 4
Branching ratio = 6:4 = 3:2
p, d, f core levels split into two doublet peaks
BE (J=L−S) > BE (J=L+S) considering final state energy
Splitting ↑ as Z ↑
What about p and f levels?

20. Final State Effects Initial state effect: Koopman’s theorem
Final state effect: : 1~10 eV
-the created core hole after photoionization affects the energy
distribution of the emitted electrons in different ways.
Relaxation effect
Multiplet splitting
Multielectron excitations
-shake up and shake off satellites
-electron-hole excitation (continuous satellite): asymmetric line
shape
Plasmon loss peaks
Vibrational effects
Ref. Electron spectroscopy, theory, techniques, and applications Vol.2 (Academic Press. 1978)
Chap. 1, C.S. Fadley, Basic concepts of x-ray photoelectron spectroscopy

21. Relaxation effect Atomic relaxation (gas phase)
by Franck-Condon principle
Extra-atomic relaxation
by surroundings (sold phase)
As Ef ↓ , BE ↓
Instantaneous electronic transition (A)
but relaxation afterward (F)

22. Typical shake-up in C 1s

23. Instrumentation - Vacuum System - Photon Source
Electron Energy Analyzer
Resolution
Binding Energy Referencing
Charging compenstation

24. Universal Curve
Inelastic electron mean free path (IMFP) ~ Intensity attenuation length
IMFP

25. Other consideration (Escape depth)
Comparison of AES and EDX analysis volume
Hard X-ray PES (HAXPES)
HAXPES
SX-PES
Conventional VUV
Probing depth (3λ) up to > 10 nm
Accessing bulk or buried interface
Depth profile of thin film
Lecture note: R. Claessen (Univ. Wϋrzburg)

26. Photon Source Intensity, Focus, Monochromatic, energy selection
 dual anode, monochromatic, discharge lamp, SR

27. Characteristic X-ray Lines
Width (eV)
2.6
3.8
3.0
0.77
0.47
0.85
0.7
Line
Cu Kα
Cu Lα
Cr Lα
Zr Mζ
Y Mζ
Al Kα
Mg Kα
Energy (eV)
8048.0
929.7
572.8
151.4
132.3
1486.6
1253.6
1.0
Si Kα
1739.5
Satellites of
Mg Kα
Relative Intensity
Line
Separation (eV)
100
α12
0.0
8.0 α3
8.4
0.55 α5
17.5
4.1 α4
10.2
0.45 α6 20
0.5 β
48.5

28. Al K Monochromatic X-ray Source의 구조
PHI Quantera

29. C 1s: Effect of monochromatization
10 ~ 40 times intensity reduction
Focusing at the sample (up to 10 ㎛ recently)
- Intense beam
- Imaging

30. Resonance Lines of Rare Gas Discharge
UV sources : each monochromator available
Intensity (%) 15 2
100
Resonance Line
Kr I
Ne I
He II
He I
Ar I
Energy (eV) 50
9.5695
Xe I
8.4363

31. Synchrotron Radiation
- High Intensity and resolution, energy tunability, focused Beam, polarization, pulsed beam
- Big Facility means Big Money ! – Shared facility
Pohang Acceleration Lab.

32. Concentric Hemispherical Analyzer : CHA
Hemispherical Energy Analyzer
Concentric Hemispherical Analyzer : CHA

33. Microchannel Plate (MCP)
Electron Detection
Channeltron (CEM)
Microchannel Plate (MCP)

34. Position-sensitive Energy Analyzer
Channeltron array
MCP + CCD (or DLD)

35. Energy Resolution of Analyzer
Small Epass  Large Eretard
Large Epass  Small Eretard
High resolution
Low throughput (low count rates)
Low resolution
High throughput (high count rates)

36. Total Energy Resolution
ΔETotal =
√(ΔEanal.2 + ΔEphoton2 + ΔEthermal broad2 + ΔEinhomogen )
ΔEanal.= Epass(d/2Ro + α2/4) for HEA type
d= slit width
Ro=mean radius
α=a half acceptance angle
ΔEthermal broad ~ 3/2kT
<Practical measurement
of total resolution>

37. Differential surface charging
Peak Broadening or shift

38. Charging Compensation

39. Charging Compensation
With a Magnetic lens system (Cratos)
Nearly complete charging compensation !

40. Modern ESCA Features 가격 $1M? Monochromatic x-ray (1486 eV)
Angle-resolved analyzer
UV discharge lamp (optional)
Automatic sample transfer
Charge compensation
가격 $1M?

41. Data Processing - Smoothing - Background and X-ray Satellite 제거
- Peak Fitting and Deconvolution
Practical Programs

42. Smoothening - NOT recommended, but - 너무 오래 걸리거나, 변하는 시료, 또는 미분하기 전에
- Savitsky-Golay method
- Adjacent Averaging

43. Background Removal Background Removal Methods
Inelastic Energy Loss Mechanism
Complicated Process
Sample Dependent
Geometry Dependent
Instrument Dependent
Must be Removed for Quantification
Removal Methods
Linear, Shirley, Tougaard

44. Linear Background Removal

45. Shirley Method integrated background
Key point: 어떤 점 x에서 BG 는 x 보다 높은 K. E.를 가진
전자들의 total intensity에 비례한다

46. Shirley BG Removal in Action

47. Peak Fitting and Deconvolution
Lorentzian
Gaussian
Voigt
Lorentzian width: core hole life time: ΔE·τp≥ ħ
Gaussian width: instrumental resolution, system inhomogeneity

48. Least Square Peak Fitting

49. Peak Fitting in action C 1s urea formaldehyde/epoxy coating
J.F. Watts and J. Wolstenholme, An Introduction to Surface Analysis by XPS and AES (Wiley, 2003)
C 1s
urea formaldehyde/epoxy coating
Any principles?
Proper background subtraction should be made.
Initial guess is important.
Any peak should have physical meaning.
Fitting is just fitting (no result from nothing)
Peak splitting within an system resolution

50. Fitting Programs Non-commercial programs
CasaXPS
Unifit
NIST Database for the Simulation of Electron Spectra for Surface Analysis (SESSA)
Non-commercial programs
Fitt-win
XPSPEAK 4.1
FitXPS
Compro

51. Applications - Elemental Identification Quantitative Analysis
Chemical Shifts
Work function Measurement
Angle-resolved Techniques
Depth profile
Microscopy

52. Elemental Identification

53. 정량분석: Basic Concept 시료 중 원소 A의 농도 시료에서 측정된 원소 A의 XPS 피크 세기
원소 A의 XPS 피크 세기 (Atom Sensitivity Factor)
값은 알기 어려우나,
는 비교적 알기 쉬움

54. 정량분석: RASF 이용 다른 조건들이 동일할 때 원소들 피크의 상대적 세기
Analyzer, Source, Geometry dependent parameter
Measured Intensity
Element
RASF
6443.2
Sr 3d
1.843
6009.1
Ti 2p
2.001
1080.6
C 1s
0.296
5339.7
O 1s
0.771
Concentration
20.4
17.5
21.3
40.8

55. RASF
Relative elemental sensitivity 3d 4f 2p 4d 1s

56. 정량분석: complicated problem
Material-dependent inelastic mean free path (IMFP)
Probing different volumes in mixed (A and B) materials
Correction must be made !!

57. TPP-2M equation for IMFP
By Tanimura, Powell, Penn
Surf. Interface Anal. 21, 165 (1994)
(Å)
density (g/cm3)
Band gap (eV)
atomic or molecular weight
free electron plasmon energy (eV)
number of valence electrons

58. Valence band spectra final state XPS UPS EF initial state core levels
Zn 3d
XPS
UPS EF
initial state
core levels

59. Photoionization intensity (cross section)
Lecture note: S. Kim (KAIST)

60. Atomic Cross-section
UPS
XPS

61. UPS의 상대적인 특징 1. Peak is broad
Why? Due to electronic band structure or orbital hybridization
2. Feels more surface-sensitive than with XPS
Why? Low electron attenuation length & high O atomic photoioinization cross-section at low photon energy
3. Very difficult to quantify
Why? This is not atomic character but bonding character. It means that you need more clear and physical idea of your specimen in simplified way
4. Resolution is better
Why? Gas discharge source provides a sharp fluorescent light

62. Example of XPS analysis-1
Pigment from Mummy Artwork

63. Example of XPS analysis-2
Valence electron state of Pt in PtPc 4+ 2+

64. Metal-semiconductor Contact
Energy Level Diagram

65. Organic-Metal contact in OLED
Device performance depends on the balanced carrier injection and transport
To maximize device efficiency : Enhance charge injection

66. Energy levels and UPS spectra
interface dipole UV
(X-ray)
ionization potential
work function
HOMO level
S. Braun et al., Adv. Mat.(2009)

67. Angle-Resolved PES
Better Surface Sensitivity
SiO2
SiO2 Si Si

68. XPS에 의한 SiO2 박막 두께 측정 L = Attenuation length of Si 2pq
cos  L e
Sample
Analyzer
x-ray
(hν) Si
L = Attenuation length of Si 2p
correct measurement of SiO2 thickness from R0 and L

69. XPS에 의한 SiO2 박막 두께 측정 결과
SiO2
K. J. Kim, Thin Solid Films 500, 356 (2006)
K. J. Kim, Surf. Interface Anal. 39, 512 (2007)
a-Si
0 offset and the same linearity in the sub-nm region

70. Solid in periodic potential

71. Angle-Resolved UPS
Band Mapping using Valence Spectra

72. Band Structure of Graphite

73. ESCA sputter depth profile
단점: ① sample damage
② very slow

74. Sputtering conditions

75. Photoemission Electron Microscopy
SPEM
PEEM

76. Photoelectron Spectromicroscopy
Contaminated Coronary Stent
Secondary Electron Image
XPS Spectrum of Contaminated Area
100 µm F F O O C Cl Si Si F
1000
800
600
400
200
Binding Energy (eV)

77. Contaminated Coronary Stent
Secondary Electron Image
Carbon Spectrum of Contaminated Area
100 µm
CF3
CF2O
C-C
CFO
O=C-O
C-O CF
294
290
286
282
Binding Energy (eV)

78. Contaminated Coronary Stent
Fluorocarbon Map
Hydrocarbon Map
100 µm
100 µm
500 x 500um

79. Summary XPS Characteristics UPS Characteristics Surface sensitive
Accurate quantification
Detection of all elements above He
Provides chemical state information
Can be applied to both inorganic and organic materials
Can be applied to both conductors and non-conductors
UPS Characteristics
More surface sensitive
No quantification
Information on chemical bonds and electronic band structure

80. References
1. Electron spectroscopy, theory, techniques, and applications I-IV, edited by C.R. Brundle (Academic, NY, 1977)
2. Photoemission in Solids I&II, edited by M. Cardona and L. Ley (Springer-Verlag 1978)
3. G. Ertl and J. Kuppers, Low energy electrons and surface chemistry (VCH, 1985)
4. D.P. Woodruff and T.A. Delchar, Modern techniques of surface science (Cambridge, 1986)
5. Practical surface analysis, Vol.I, edited by D. Briggs and M.P.Seah (Salle & Sauerlander)
6. S. Hufner, Photoelectron Spectroscopy (Springer 1996)
7. J.F. Watts and J. Wolstenholme, An Introduction to Surface Analysis by XPS and AES (Wiley 2003)
8. J.C. Vickerman and I.S. Gilmore, Surface Analysis, The principal Techniques, 2nd ed. (Wiley, 2008)
9. Website