4. XPS.pptx

Karakterisasi:
Chemical State
Chemical Composition
X-ray Photoelectron Spectroscopy
XPS, ESCA

MEF 3100 Spring 2007
XPS and AES are surface sensitive
techniques

Photo emission process is often envisaged as
three steps
•Absorption and ionisation - (initial state effect)
•Response from atom and creation of
photoelectron -(final state effect)
•Transport of electron to surface and
escape - (extrinsic loss)

XPS – ESCA

X-ray Photoelectron Spectroscopy (XPS)
• works by irradiating a sample material with mono
energetic soft X-rays causing electrons to be ejected.
• Identification of the elements in the sample can be made
directly from the kinetic energies of these ejected
photoelectrons.
• The relative concentrations of elements can be determined
from the photoelectron intensities.

XPS - ESCA
• Surface sensitive analysis technique based on
photoelectric effect. Depth of analysis ~4-40 nm.
• All elements except Hydrogen.
• Wide range of materials: Polymers, Ceramics,
metals …. (vacuum compatible)
• Applications: corrosion, catalysis, thin films,
surface coatings, segregation …
• Gives information on chemical composition and
chemical state

History
• 1887 Heinrich Hertz /1888 Wilhelm Hallwaches
illuminating metal surfaces resulted in electronic emission
• 1900 Max Planck -black body radiation
• 1905 Einstein - light is quantized
E=h
• 1950 Kai Siegbahn 1981 : Nobel Prize
studied photoejection of electrons with x-rays
• 1950: Instrumentation 1960: Chemical Applications
Photoelectric effect
Photoelectric effect
Planck's law of black body radiation
electron spectroscopy

Absorbing X-rays – emitting Photoelectrons

XPS Peaks

Basic equations:
The relationship governing the interaction
of a photon with a core level is:
KE= h - BE – 
• KE = kinetic energy of ejected photoelectron
• h = characteristic energy of X-ray photon
• BE = binding energy of the atomic orbital from which
the electron originates.
•  = spectrometer work function

XPS example: Gold

Electronic structure and spectroscopy

Binding energies?

Spin-Orbit Coupling
965 955 945 935 925
19.8
BindingEnergy(eV)
Cu2p
2p1/2
2p3/2
PeakArea 1 : 2
Orbital=p
ls=1/2,3/2
l=1
s=+/-1/2

XPS spectra of Palladium
BE= h - KE – 

WHAT IS THIS?
Survey scan : Most elements have major photoelectron peaks below
1100 eV, so a range from 1100 - 0 eV is usually sufficient.
?
Al anode Mg anode

Energy resolution ~1 eV

Chemical shift – binding energies in
C 1s peak

Chemical shift
Functional
Group
Binding Energy
(eV)
hydrocarbon C-H, C-C 285.0
amine C-N 286.0
alcohol, ether C-O-H, C-O-C 286.5
Cl bound to C C-Cl 286.5
F bound to C C-F 287.8
carbonyl C=O 288.0

Chemical shift

Inelastic Background

Plasmon loss

Instrumentation
Surface analysis by XPS requires irradiating
Surface analysis by XPS requires irradiating
a
a solid in an Ultra
solid in an Ultra-
-high Vacuum (UHV)
high Vacuum (UHV)
chamber with monoenergetic soft X
chamber with monoenergetic soft X-
-rays
rays
and analysing the energies of the emitted
and analysing the energies of the emitted
electrons.
electrons.

XPS instrument
Kratos Axis Ultra – new in 2007

Instrument
1. Chambers
2. Vacuum and Pumps
3. Sources
4. Monochromator
5. Analysers
Specimen handling
Planning experiments

Dual anode X-ray source

Alternative X-ray sources

Monochromator
N =2dsin
For Al K  8.3 Å
_
1010 planes in quartz:
d= 4.25 Å

Monochromated X-rays

Flooded or focused X-ray beam

CHA with standard input lens

Pass Energy and Resolution

Pass Energy

Ion Etching

Survey scan : Most elements have major photoelectron peaks below
1100 eV, so a range from 1100 - 0 eV is usually sufficient.
Al anode Mg anode
?

Detail Scans
For purposes of chemical state identification, for quantitative
analysis of minor components and peak deconvolution or other
mathematical manipulation of data.
•Scan should be wide enough to encompass the background on
both sides of the region of interest - yet with small enough step
size - within a reasonable time.
•Radiation-sensitive peaks should be run first.
•Sufficient Signal / noise .
•Pass Energy ?  E same for all scans !

Oxidation of Titanium
Oxidation of Ti:
Titanium has a big shift. Ti-metal
(Tio) to TiO2
(Ti4+).
Ti 2p-scan of Ti-metal and TiO2
Binding energies:
metal:Ti 2p3/2
=451.4 Ti 2p1/2
=457.57
oxide:Ti 2p3/2
=458.8 Ti 2p1/2
=464.34
1.Not the same shift for the two spin doublets.
2. Metals have more asymmetric peaks than insulators/oxides.
3. Weak peak at 450.7eV - “ghost” -
(X-ray with higher energy = K)

Density of state DOS
DOS Ti-metal
Degree of asymmetry proportional to DOS at EF

Depth information from XPS

Depth information from Ag-anode
• Greater analysis depth
The Al 2p spectra shown (Figure 3)
were acquired with
• (a) Al and
• (b) Ag sources. Spectrum
• (b) is a clear illustration of the greater
analysis depth of the Ag source as the
element : oxide ratio is greater than
that acquired with the Al source. An
oxide thickness of 1.8nm indicated
here was confirmed by independent
angle resolved measurements.

Binding Energy Referencing
BE
BE =
= hv
hv -
- KE
KE -
- 
spec
spec
--E
E
ch
ch
Where:
Where: BE
BE=
= Electron Binding Energy
Electron Binding Energy
KE
KE=
= Electron Kinetic Energy
Electron Kinetic Energy

spec
spec
=
= Spectrometer Work Function
Spectrometer Work Function
E
Ech
ch
=
= Surface Charge Energy
Surface Charge Energy
E
Ech
ch can be determined by calibrating the instrument
can be determined by calibrating the instrument
to a spectral feature.
to a spectral feature.
C1s at 285.0 eV
C1s at 285.0 eV
Au4f
Au4f7/2
7/2 at 84.0 eV
at 84.0 eV

Binding Energy Referencing
When analysing insulating samples more care is required
because of sample charging and the uncertainty in the
location of the Fermi Level within the band gap.
The term Binding energy is often used without specifying
the reference level.
Take care when evaluating spectra !

Charge neutralization for insulating samples
• Low-energy electrons from a cold cathode flood gun alleviates positive charging
• Low-energy source of positive ions alleviates the surrounding negative charge

Auger parameter
The Auger parameter is defined as:
A.P. = K.E(Auger) - K.E.(photoelectron) + Photon Excitation Energy
= K.E.(Auger) + B.E.(photoemission)
Where K.E.(Auger) and B.E.(photoemission) are normally
measured for the most intense photoemission and
sharpest Auger lines available.
Its advantage as a probe of charges in screening energy is that (unlike photoemission or Auger line measurements
taken separately) it is independent of energy referencing problems whilst still being measurable to a high
precision. Hence small shifts in its value can be measured and interpreted.

• http://www.uksaf.org/data/table.html

Specimen preparation and
handling

Analysis of Multi-Layer Paint Cross
Section

Analysis of Materials for Solar Energy
Collection by XPS Depth Profiling-
The amorphous-SiC/SnO2
Interface
The profile indicates a reduction of the SnO
The profile indicates a reduction of the SnO2
2
occurred at the interface during deposition.
occurred at the interface during deposition.
Such a reduction would effect the collector’s
Such a reduction would effect the collector’s
efficiency.
efficiency.
Photo
Photo-
-voltaic Collector
voltaic Collector
Conductive Oxide
Conductive Oxide-
- SnO
SnO2
2
p
p--type a
type a-
-SiC
SiC
a
a--Si
Si
Solar Energy
Solar Energy
SnO
SnO2
2
Sn
Sn
Depth
500 496 492 488 484 480
Binding Energy, eV
Data courtesy A. Nurrudin and J. Abelson, University of Illinois

Probing Glass Coatings
Windows are coated with complex
Multi-layer thin films to meet demands:
1) Energy conservation
2) Appearance
3) Durability

XPS Analysis of Pigment from Mummy
Artwork
150 145 140 135 130
Binding Energy (
Binding Energy (eV
eV)
)
PbO2
Pb3
O4
500 400 300 200 100 0
Binding Energy (
Binding Energy (eV
eV)
)
O
Pb Pb
Pb
N
Ca
C
Na
Cl
XPS analysis showed
XPS analysis showed
that the pigment used
that the pigment used
on the mummy
on the mummy
w
wrapping was Pb
rapping was Pb3
3
O
O4
4
rather than Fe
rather than Fe2
2
O
O3
3
Egyptian Mummy
Egyptian Mummy
2nd Century AD
2nd Century AD
World Heritage Museum
World Heritage Museum
University of Illinois
University of Illinois

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4. XPS.pptx

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