x ray photoelectron spectroscopy
TRANSCRIPT
X-ray photoelectron
spectroscopy
Electron Spectroscopy for Chemical
Analysis (ESCA)
Kai M. Siegbahn
Obtained Nobel Prize
For his work on XPS
Introduction:X-ray photoelectron spectroscopy (XPS) is a surface-
sensitive spectroscopic technique widely used to
investigate the chemical composition of surfaces.
XPS technique is based on Einstein’s idea about the
photoelectric effect, developed around 1905
The concept of photons was used to describe the
ejection of electrons from a surface when photons were
impinged upon it
L2,L3
L1
K
Incident X-rayEjected Photoelectron
1s
2s
2p
The Photoelectric Process
XPS spectral lines are
identified by the shell
from which electrons
were emitted (1s, 2s, 2p
etc.)
The kinetic energy of the
ejected photoelectron
KE=hv-BE-F
L2,L3
L1
K
Emitted Auger Electron
1s
2s
2p
Auger Relation of Core Hole
L electron falls to fill
core level vacancy
(step1).
KLL Auger electron
emitted to conserve
energy released in
step1.
X-Rays
Irradiate the sample surface, hitting the core electrons (e-) of the atoms.
The X-Rays penetrate the sample to a depth on the order of a micrometer.
Useful e- signal is obtained only from a depth of around 10 to 100 Å on the surface.
The X-Ray source produces photons with certain energies:
MgK photon with an energy of 1253.6 eV
AlK photon with an energy of 1486.6 eV
Normally, the sample will be radiated with photons of a single energy (MgK or AlK). This is known as a monoenergetic X-Ray beam.
Why the Core Electrons?
An electron near the Fermi level is far from the nucleus,
moving in different directions all over the place, and will
not carry information about any single atom.
Fermi level is the highest energy level occupied by an electron in
a neutral solid at absolute 0 temperature.
The core e-s are local close to the nucleus and have
binding energies characteristic of their particular element.
Core e-
Valence e-
Atom
How Does XPS Technology Work?
A monoenergetic x-ray beam emits photoelectrons from the from the
surface of the sample.
The x-ray photons The penetration about a micrometer of the
sample
The XPS spectrum contains information only about the top 10 - 100
Ǻ of the sample.
Ultrahigh vacuum environment to eliminate excessive surface
contamination.
Cylindrical Mirror Analyzer (CMA) measures the KE of emitted e-s.
The spectrum plotted by the computer from the analyzer signal.
The binding energies can be determined from the peak positions
and the elements present in the sample identified.
XP SPECTROMETERS
COMPONENTS OF XPS:
A source of X-rays
An ultra high vacuum (UHV)
An electron energy analyzer
magnetic field shielding
An electron detector system
A set of stage manipulators
XPS Instrument
X-Ray Source
Ion Source
CMA
Sample introduction
Chamber
WHY WE USE UHV?
Remove adsorbed gases from the sample.
Eliminate adsorption of contaminants on the sample.
Prevent arcing and high voltage breakdown.
Increase the mean free path for electrons, ions and photons.
Dual Anode X-ray Source
Cylindrical Mirror Analyzer (CMA)
Slit
Detector
Electron Pathway through the CMA
0 V
+V
0 V 0 V
0 V
+V
+V
+V
X-RaysSource
SampleHolder
KE versus BE
E E E
KE can be plotted depending on BE
Each peak represents the amount of e-s at a certain energy that is characteristic of some element.
1000 eV 0 eV
BE increase from right to left
KE increase from left to rightBinding energy
# o
f ele
ctr
on
s
(eV)
Interpreting XPS Spectrum:
Background
The X-Ray will hit the e-s in the bulk (inner e- layers) of the sample
e- will collide with other e-
from top layers, decreasing its energy to contribute to the noise, at lower kinetic energy than the peak .
The background noise increases with BE because the SUM of all noise is taken from the beginning of the analysis.
Binding energy
# o
f ele
ctr
on
sN1
N2
N3
N4
Ntot= N1 + N2 + N3 + N4
N = noise
XPS peak identification
Electronic Effect:
Auger lines
Chemical shifts
X-ray satellites
X-ray “Ghost”
Energy loss lines
Example of Chemical Shift
Example of Chemical Shift
XPS Study of Changes in the Chemical Composition of
Langasite Crystal Thin Surface Layers during Vacuum
Annealing
The aim of this work was to study the chemical composition
of LGS La3Ga5SiO14 langasite crystal wafer surface after
thermal vacuum annealing at 650°C and at elevated
temperatures (1000°C). To analyze the surface and
subsurface layer chemical composition we used X-ray
photoelectron spectroscopy (XPS).
The 1050°C, 30 min, and 1000 °C, 5 h annealing
experiments showed that in both cases the crystals lost color,
and the chemical composition of the wafer surface changed:
the gallium concentration decreased abruptly
Figure 1 shows the photoelectron spectra of the crystal
surfaces before and after annealing from which the gallium
line intensity can be seen to decrease by an order of
magnitude.
Advantages and Disadvantages
Advantages
Non-destructive technique.
Surface Sensitive (10-100
Å).
Quantitative
measurements are
obtained.
Provides information about
chemical bonding.
Elemental mapping.
Limitations
Very expensive
technique.
High vacuum is required.
Slow processing (1/2 to
8 hours/sample).
Large area analysis is
required.
H and He can not be
identified.
Data collection is slow 5
to 10 min.
XPS is used to measure:
Elemental composition of
the surface (top 1–12 nm
usually).
Chemical or electronic
state of each element in
the surface.
Uniformity of
composition across the
top surface (line
profiling).
Applications in the
industry:
Failure analysis
Polymer surface
Corrosion
Adhesion
Semiconductors
Thin film coatings
Uses
References: Siegbahn, K.; Edvarson, K. I. Al (1956). "β-Ray spectroscopy in the
precision range of 1 : 1e6". Nuclear Physics
Turner, D. W.; Jobory, M. I. Al (1962). "Determination of Ionization
Potentials by Photoelectron Energy Measurement".
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