2. 2
• Topics:
– Introduction
– Auger Electron Spectroscopy
– Electron Spectroscopy for Chemical
Analysis OR X-ray Photoelectron
Spectroscopy (XPS)
– Electron Microscopy (TEM and SEM)–
Atomic Force Microscopy (AFM)
– Fourier-Transform Infrared Spectroscopy
(FTIR)
3. 3
How do we Define the Surface
The surface behaviour of materials is crucial
to our lives.
• one considers a car body shell, a biological cell,
tissue or implant, a catalyst, a solid state electronic
device or a moving component in an engine, it is
the surface which interfaces with its environment.
The surface reactivity will determine how well the
material behaves in its intended function.
4. 4
How Many Atoms in a Surface?
The top layer of surface atoms are those
that are the immediate interface with the
other phases (gas, liquid or solid) impinging
on it, this could be regarded as the surface.
5. 5
Continue…………………...
• The most inclusive way to define a surface
is to state that a surface of interface exists
in any case where there is an abrupt
change in the system properties with
distance, with many degrees of
abruptness.
6. 6
• In a very real sense therefore, the surface
could said to be the top 2–10 atomic or
molecular layers (say, 0.5–3 nm).
• beyond 100 nm it is more appropriate to
begin to describe such a layer in terms of
its bulk solid state properties.
Continue…………………...
7. 7
The surface can be considered in terms of
three regimes:
• the top surface monolayer,
• the first ten or so layers and
• the surface film, no greater than 100 nm
Continue…………………...
8. 8
Surface of a Solid
• The surface of a solid in contact with a liquid or gaseous phase
usually has very different chemical composition and physical
properties from the interior of the solid.
• Characterization of these surface properties is often important
in many fields, including heterogeneous catalysis,
semiconductor thin-film technology, corrosion and adhesion
mechanisms, activity of metal surfaces, and studies of the
behavior and functions of biological membranes.
• To understand fully the surface of a solid material, we need
techniques that not only distinguish the surface from the bulk of
the solid, but also ones that distinguish the properties of these
three regimes.
9. 9
Surface Methods
• The chemical composition of a surface of
a solid is often different from the interior of
the solid.
• One should not focus solely on this interior
bulk composition because the chemical
composition of the surface layer of a solid
is sometimes much more important.
10. 10
Surface Measurements
• Classical methods
• They involve obtaining optical and electron
microscopic images, as well as measurements of
adsorption isotherms, surface areas, surface
roughness, pore sizes and reflectivity.
• useful information about the physical nature of
surfaces but less about their chemical nature.
11. 11
Measurements (cont.)
• Spectroscopic methods
• provided information about the chemical
nature of surfaces, as well as determine
their concentration
• began in the 1950s
13. 13
The figure below illustrates the general principle
by which a spectroscopic examination of surface
is performed.
14. 14
Electron spectroscopy
• In electron spectroscopy, the spectroscopic
measurement consists of the determination of
the power of the electron beam as a function of
the energy (or frequency hv) of the electrons.
15. 15
• The most common type is based upon the
irradiation of the sample surface with
monochromatic X-radiation.
• This is called X-ray photoelectron
spectroscopy (XPS).
• This method is also known as Electron
spectroscopy for chemical analysis
(ESCA).
16. 16
• X-Ray Photoelectron Spectroscopy (XPS),
not only provided information about the
atomic composition of the sample, but also
information about the structure and
oxidation state of the compounds being
examined.
17. 17
• The second type of electron spectroscopy
is called Auger electron spectroscopy
(AES).
• Auger spectra are most commonly excited
by a beam of electrons, although X-rays
are also used.
18. 18
• The third type of electron spectroscopy is
ultraviolet photoelectron spectroscopy
(UPS).
• In this method, a monochromatic beam of
ultraviolet radiation causes the ejection of
electrons form the analyte.
• This method is not as common as the first
two methods.
19. 19
• Electron spectroscopy can be used for the
identification of all of the elements in the
periodic table except for helium and
hydrogen.
• The method also permits the determination
of the oxidation state of an element and the
type of species to which it is bonded.
• This technique also provides useful
information about the electronic structure of
molecules.
20. 20
• Secondary-ion mass spectrometry (SIMS)
is the most highly developed mass
spectrometric surface methods, with
several manufacturers offering instruments
for this technique.
• SIMS is useful from determining both
atomic and the molecular composition of
solid surfaces.
21. 21
Secondary Mass Ion Spectroscopy
(SIMS)
In SIMS, ion bombardment sputters off
surface ions (secondary ions) that are
then counted.
Spectra are compared to a database to
determine species, quantity, orientation
information.
SIMS components: sample and ion gun,
mass analyzer (filter), and
processor/computer
SIMS can damage surface, but
gives quantitative data on
composition as a function of depth
in a sample
22. 22
• In secondary-ion mass analyzers that
serve for general surface analysis and for
depth profiling, the primary ion beam
diameter ranges from 0.3 to 0.5mm.
• Double-focusing, single-focusing, time-of-
flight and quadrapole spectrometers are
used for mass determination.
23. 23
• Ion microprobe analyzers are more
sophisticated (and thus more expensive)
instruments that are based upon a focused
beam of primary ions that has a diameter
of 1 to 2 m. This beam can be moved
across a surface for about 300 m in both
x and y directions.
24. 24
Microscopic methods
• In many fields of chemistry, material science,
geology and biology, a detailed knowledge of the
physical nature of the surface of solids is of great
importance.
• The classical method of obtaining this information
was optical microscopy.
• The resolution of optical microscopy is limited by
diffraction effects to about the wavelength of light.
25. 25
Current surface information at considerably
higher resolution is obtained by following
techniques:
Scanning electron microscopy (SEM)
Transmission Electron microscopy (TEM)
Scanning tunneling microscopy (STM)
Atomic force microscopy (AFM)
26. 26
Transmission Electron Microscoy –Scanning
Electron Microscopy (TEM, SEM)
TEM in comparison to light microscopy.
Above right: sperm cells in light
microscopy, below right: sperm cells in a
TEM
27. 27
SEM
Electrons can penetrate
deeply into a sample,
giving averaged chemical
information with depth .
SEM has great depth
of focus. Left:
osteoblast cells
cultured on a
titanium mesh.
Right: schematic of an
SEM
29. 29
Scanning Probe Microscopes
• Scanning probe microscopes (SPMs) are
capable of resolving details or surfaces
down to the atomic level. Unlike optical
and electron microscopes, scanning probe
microscopes reveal details not only on the
lateral x and y axis of a sample but also
the z axis.
30. 30
Scanning Probe Microscopy (SPM)
Atomic Force Microscopy (AFM)
AFM surface topography of poly (D,L-
lactic acid)-poly(ethylene glycol)-
monomethyl ether diblock copolymer
Right: AFM instrumentation. Stylus is
placed on sample surface. Laser
tracks movement of stylus, and
cantilever deflection is monitored.
Stage is moved up and down to
maintain contact between tip and
sample.
Above right: the greater the tip radius,
the lower the spatial resolution.
31. 31
Information Required
To understand the properties and reactivity of a
surface, the following information
is required:
• physical topography,
• chemical composition,
• chemical structure,
• atomic structure, the electronic state
• a detailed description of bonding of molecules at
the surface.
32. 32
Surface analysis techniques and the information
they can provide
Radiation IN
Radiation
DETECTED
Photon
Electron
Photon
Photon
Electron
Electron
Ion
Ion
Neutron
Neutron
Surface
Information
SEM
Physical
topography
STM
Chemical
composition
ESCA – XPS AES SIMS
ISS
Chemical
structure
ESCA – XPS EXAFS
IR & SFG
EELS SIMS INS
Atomic
structure
EXAFS LEED
RHEED
ISS
Adsorbate
bonding
EXAFS
IR
EELS SIMS INS
33. 33
• ESCA/XPS – Electron analysis for chemical analysis/X-ray
photoelectron spectroscopy. X-ray photons of precisely defined
energy bombard the surface, electrons are emitted from the
orbitals of the component atoms, electron kinetic energies are
measured and their electron binding energies can be
determined enabling the component atoms to be determined.
• AES – Auger electron spectroscopy. Basically very similar to the
above except that a keV electron beam may be used to
bombard the surface.
• SIMS – Secondary ion mass spectrometry. There are two forms,
i.e. dynamic and molecular SIMS. In both a beam of high energy
(keV) primary ions bombard the surface while secondary atomic
and cluster ions are emitted and analysed with a mass
spectrometer.
• ISS – Ion scattering spectrometry. An ion beam bombards the
surface and is scattered from the atoms in the surface. The
scattering angles and energies are measured and used to
compute the composition and surface structure of the sample
target.
34. 34
• IR – Infrared (spectroscopy). Various variants on the classical
methods – irradiate with infrared photons which excite vibrational
frequencies in the surface layers; photon energy losses are detected
to generate spectra.
• EELS – Electron energy loss spectroscopy. Low energy (few eV)
electrons bombard the surface and excite vibrations – the resultant
energy loss is detected and related to the vibrations excited.
• INS – Inelastic neutron scattering. Bombard a surface with neutrons
– energy loss occurs due to the excitation of vibrations. It is most
efficient in bonds containing hydrogen.
• SFG – Sum frequency generation. Two photons irradiate and
interact with an interface (solid/gas or solid liquid) such that a single
photon merges resulting in electronic or vibrational information about
the interface region.
• LEED – Low energy electron diffraction. A beam of low energy (tens
of eV) electrons bombard a surface; the electrons are diffracted by
the surface structure enabling the structure to be deduced.
35. 35
• RHEED – Reflection high energy electron diffraction. A high
energy beam (keV) of electrons is directed at a surface at glancing
incidence. The angles of electron scattering can be related to the
surface atomic structure.
• EXAFS – Extended X-ray absorption fine structure. The fine
structure of the absorption spectrum resulting from X-ray
irradiation of the sample is analysed to obtain information on local
chemical and electronic structure.
• STM – Scanning tunnelling microscopy. A sharp tip is scanned
over a conducting surface at a very small distance above the
surface. The electron current flowing between the surface and the
tip is monitored; physical and electron density maps of the surface
can be generated with high spatial resolution.
• AFM – Atomic force microscopy (not included in table). Similar to
STM but applicable to non-conducting surfaces. The forces
developed between the surface and the tip are monitored. A
topographical map of the surface is generated.
39. What are Electron Microscopes?
Electron Microscopes are scientific instruments that use a
beam of highly energetic electrons to examine objects on a
very fine scale. This examination can yield the following
information:
Topography
The surface features of an object or "how it looks", its
texture; direct relation between these features and
materials properties (hardness, reflectivity...etc.)
Morphology
The shape and size of the particles making up the object;
direct relation between these structures and materials
properties (ductility, strength, reactivity...etc.)
40. Composition
The elements and compounds that the object is composed of and the
relative amounts of them; direct relationship between composition
and materials properties (melting point, reactivity, hardness...etc.)
Crystallographic Information
How the atoms are arranged in the object; direct relation between
these arrangements and materials properties (conductivity, electrical
properties, strength...etc.)
Electron microscopes were developed due to the limitations of Light
Microscopes which are limited by the physics of light.
In the early 1930's this theoretical limit had been reached and there
was a scientific desire to see the fine details of the interior structures
of organic cells (nucleus, mitochondria...etc.).
This required 10,000x plus magnification which was not possible
using current optical microscopes.
41. How do Electron Microscopes Work?
Electron Microscopes(EMs) function exactly as their optical
counterparts except that they use a focused beam of electrons
instead of light to "image" the specimen and gain information as to
its structure and composition.
The basic steps involved in all EMs:
A stream of electrons is formed (by the Electron Source) and
accelerated toward the specimen using a positive electrical potential
This stream is confined and focused using metal apertures and
magnetic lenses into a thin, focused, monochromatic beam.
This beam is focused onto the sample using a magnetic lens
Interactions occur inside the irradiated sample, affecting the
electron beam
These interactions and effects are detected and transformed into an
image
The above steps are carried out in all EMs regardless of type.
43. Types of Electron microscopes
TEM: transmission electron microscope
SEM: scanning electron microscope
44. Introduction of TEM
The transmission electron microscope
(TEM) was the first type of Electron
Microscope to be developed and is
patterned exactly on the light
transmission microscope except that a
focused beam of electrons is used instead
of light to "see through" the specimen.
It was developed by Max Knoll and Ernst
Ruska in Germany in 1931.
45. • In the TEM, the focused, monochromatic electron
beam interacts with and is transmitted through the
sample, focused into an image and projected onto a
phosphor coated screen which emits visible light.
The brighter areas of the image represent areas
where more electrons have passed through the
sample. The darker areas represent areas where
fewer electrons have passed through as a result of
higher specimen density. A TEM can magnify up to
about 500,000x
Basic Principle of TEM
48. The first scanning electron microscope
(SEM) debuted in 1938 ( Von Ardenne)
with the first commercial instruments
around 1965. Its late development was
due to the electronics involved in
"scanning" the beam of electrons
across the sample.
Introduction of SEM
49. • In the SEM, a set of scan coils moves the electron beam
across the specimen in a 2 dimensional grid fashion.
When the electron beam scans across the specimens,
different interactions take place. These interactions are
decoded with various detectors situated in the chamber
above the specimen. Some electrons from the surface
material are knocked out of their orbitals by the electron
beam, and are called SECONDARY ELECTRONS.
These electrons are detected by the secondary electron
detector. Different interactions give images based on
topography, elemental composition or density of the
sample. A SEM can magnify up to about 100,000x.
Basic Principle of SEM
51. SEM Images
Budding yeast cell, original
magnification 32 000X
E. coli bacteria, original
magnification 30 000X
52. Advantages of Using SEM over OM
The SEM has a large depth of field, which allows
a large amount of the sample to be in focus at one
time and produces an image that is a good
representation of the three-dimensional sample.
The combination of higher magnification, larger
depth of field, greater resolution, compositional
and crystallographic information makes the SEM
one of the most heavily used instruments in
academic/national lab research areas and industry.
53.
54. Differences between SEM and TEM
TEM SEM
Electron beam passes through thin
sample.
Electron beam scans over surface of
sample.
Specially prepared thin samples or
particulate material are supported on
TEM grids.
Sample can be any thickness and is
mounted on an aluminum stub.
Specimen stage halfway down column. Specimen stage in the chamber at
the bottom of the column.
Image shown on fluorescent screen. Image shown on TV monitor.
Image is a two dimensional projection of
the sample.
Image is of the surface of the sample.
55. Resolution of microscopes
Microscope Resolution Magnification
Optical ± 200 nm ± 1000X
TEM ± 0.2 nm ± 500 000X
SEM ± 2 nm ± 200 000X
The RESOLUTION or RESOLVING POWER of a microscope is the
instrument's ability to separate two objects that are close together.
56. Atomic Force Microscopy
Introduction
In all SPM techniques a tip interacts with the sample
surface through a physical phenomenon. Measuring a
“local” physical quantity related with the interaction,
allows constructing an image of the studied surface. All
the data are transferred to a PC, where, with the use of
the appropriate software, an image of the surface is
created.
The scanning tunneling microscope (STM) is the
ancestor of all scanning probe microscopes. It was
invented in 1982 by Gerd Binning and Heinrich Rohrer at
IBM Zurich. Five years later they were awarded the
Nobel Prize in Physics for their invention.
The atomic force microscope (AFM) was also invented
by Binning et al. in 1986.
57. Introduction continue
the STM measures the tunneling current
(conducting surface), the AFM measures the
forces acting between a fine tip and a sample. The
tip is attached to the free end of a cantilever and is
brought very close to a surface. Attractive or
repulsive forces resulting from interactions
between the tip and the surface will cause a
positive or negative bending of the cantilever.
The bending is detected by means of a laser beam,
which is reflected from the back side of the
cantilever. Following figure shows the basic
concept of STM and AFM.
58.
59. Basic Principle of AFM
AFM provides a 3D profile of the surface
on a nanoscale, by measuring forces
between a sharp probe (<10 nm) and
surface at very short distance (0.2-10 nm
probe-sample separation). The probe is
supported on a flexible cantilever. The
AFM tip “gently” touches the surface and
records the small force between the probe
and the surface.
60. Figure shows Spring depiction of cantilever b) SEM image of
triangular SPM cantilever with probe (tip).
62. Introduction
Electron Microscopes are scientific instruments that use a beam of
highly energetic electrons to examine objects on a very fine scale.
Topography (surface features of an object)
Morphology (shape and size of the particles making up the object)
Composition (elements and compounds that the object is composed of
and the relative amounts of them)
Crystallographic information (how the atoms are arranged in the
object).
63. Electron microscope is a valuable tool has
developed scientific theory and it contributed
greatly to biology, medicine and material
sciences.
This wide spread use of electron microscopes
is based on the fact that they permit the
observation and characterization of materials
on a nanometer (nm) to micrometer (μm) scale.
64. SEM
• Penetrate into the sample within a small depth, so that it is
suitable for surface topology, for every kind of samples (metals,
ceramics, glass, dust, hair, teeth, bones, minerals, wood, paper,
plastics, polymers, etc)
• The SEM permits the observation of materials in macro and
submicron ranges.
• The instrument is capable of generating three-dimensional images
for analysis of topographic features .
• chemical composition of the sample’s surface When used in
conjunction with EDS i.e. an elemental analysis of the material or
contaminants that may be present.
65. SEM
SEM was used to examine the contamination at a higher magnification to
determine
Surface contamination, contamination was lying on the surface of the ceramic
Embedded in the surface (contamination from the pressing operation) or within
the surface (contamination in the glass powder)
dark specks of sintered glass ceramic Contamination
inside the holder ceramic
66. EDX
EDX use to quantify the particle of contamination and compared to the
composition of the holder ceramic
EDX spectra of particle (red) and the normal ceramic (black outline)
67. Forensic Applications of Scanning
Electron Microscopy
• One of the most well know
applications of SEM in forensics
is the automated detection and
classification of gun shot residue
(GSR).
• Automated SEM has also been
used for the classification of
minerals in soil and the detection
of very small pieces of bone in
fire debris
68. Find out the cause of death
The fiber-end has a flat top with a lip
and it can be clearly seen that there is a
tool mark in the end surface.
post explosion residue from flash powder
Fibers investigate fiber fracture and damage
Fiber-end of a parachute cord that probably has been cut with a knife
Post explosion residues
Improvised explosives may be based on
pyrotechnic mixtures and these can
suspect to prove they had been in the
vicinity of the scene of crime
69. Ballistics
• The examination of microtraces of
foreign material embedded in or
adhered to bullets provides
critical information in the
trajectory reconstruction of spent
bullets.
Surface of a bullet that hit MDF after hitting Greenboard
(gypsum). MDF (black) is found on top of gypsum (grey).
70. Biology
Sometimes SEM can be used to
detect small bloodstains in order
to reconstruct the trajectory of
bullets.
Bloodstain with some tiny
fragments of bone near damage
in a doorpost saying that the
victim was hit first
Another area in which SEM is
used is the identification of
animal hairs. SEM is very useful
to visualize the characteristic
scale patterns on hairs.
Scale pattern on hair of a civet
bloodstain with bone fragments
71. Atomic Force Microscope
• Form of microscopy in which a sharp tip is scanned over the surface
of a sample, while sensing the interaction force between the tip and
the sample.
• Atomic force microscopy is currently applied to various environments
(air, liquid, vacuum)
• Materials of types such as metal semiconductors, soft biological
samples, conductive and non-conductive materials.
• With this technique size measurements or even manipulations of
nano-objects may be performed
72. Some possible applications of AFM are:
- Substrate roughness analysis.
- Step formation in thin film epitaxial deposition.
- Pin-holes formation or other defects in oxides growth.
- Grain size analysis.
- Phase mode is very sensitive to variations in material properties,
including surface stiffness, elasticity and adhesion.
- Obtaining information of what is happening under indentation at
very small loads.
73. Biological Applications of AFM
One of the advantages of AFM is that it can image the non-conducting
surfaces.
Immediately extended to the biological systems, such as analyzing the
crystals of amino acids and organic monolayers.
Biosciences
DNA and RNA analysis; Protein nucleic acid complexes; Chromosomes,
Cellular membranes; Proteins and peptides; molecular crystals; Polymers
and biomaterials; Ligand-receptor binding.
Cell Biology
Unique capabilities of AFM's to study the dynamic behavior of living and
fixed cells such as red and white blood cells, bacteria, platelets, cardiac
myocytes, living renal epithelial cells.
74. Microbiology
The AFM has been used to viewing and analyzing the ultra structure of microbial cell
surface studies.
Function-related conformational changes in single proteins, Surface ultra structure of
living cells, Cell surface dynamics, and Morphology of biofilms.
The physical properties and biomolecular interactions such as Stiffness of cell walls,
Local surface charge and h y d r o p h o b i c i t y, E l a s t i c i t y a n d
conformational properties of single molecules,
AFM image of Saccharomyces cerevisiae
Yeast cell immobilized in a porous membrane.
Surface of Phanerochaete chrysosporium Fungal
Spores.
75. Nucleic acid Research
• One area of significant progress is the imaging
of nucleic acids. The ability to generate
nanometer-resolved images of unmodified
nucleic acids has broad biological applications.
Chromosome mapping, transcription, translation
and small molecule-DNA interactions such as
intercalating mutagens.
76. AFM to Forensic Science
AFM is an emerging powerful tool in forensic science
At the present moment in forensic investigation AFM is used:
• To analyse documents
visualising ink deposis and differentiating them according to their
origin
• To analyse fibres
Examination of the morphological changes in textile fibres exposed
to different environmental stresses
• To distinguish between the chemical domains on the fingerprints.
79. AFM in Polymer Materials
• AFM is extremely useful for studying the local surface molecular
composition and mechanical properties of a broad range of polymer
materials, including block copolymers, bulk polymers, thin-film polymers,
polymer composites, and polymer blends.
AFM topographic images of polymer isotactic polypropylene
with a scan size of 1.5 μm x 1.5 μm
Styrene with a scan size of 1μm x 1μm
80. TEM
A Transmission Electron Microscope is ideal for a number of different
fields such as life sciences, nanotechnology, medical, biological and
material research, forensic analysis, gemology and metallurgy as well as
industry and education.
TEMs provide topographical, morphological, compositional and crystalline
information.
This information is useful in the study of crystals and metals, but also has
industrial applications.
TEMs can be used in semiconductor analysis and production and the
manufacturing of computer and silicon chips.
The TEM allows imaging of the internal structure of a wide range of
samples from biological, medical, and materials sciences, up to
magnifications of 600,000x. Samples must be extremely thin < 60nm thick.
81. • Material study
- Composite and nano-structured materials study
- Defects from the manufacturing process in semi-conductors
- Ceramic systems
- Plastic Deformations
- The study of layers and structures
- Phase transformations
- Nanometric systems
- Ordered alloy structures
- The study of cellular structures
84. Application of transmission electron
microscopy to the clinical study
Clinical study of viral and bacterial infections
Preparation of a herpesvirus from a skin
lesion negatively stained with PTA. The
virion is surrounded by a limiting lipid bi-
layer.
Thin section of a cultured cell containing
replicating adenoviruses. Note crystalline
arrays of virus assembling within the cell
nucleus. Adenoviruses are about 80 nm in
diameter.
86. Microscopy
Microscopy is the technical field of
using microscopes to view samples and
objects that cannot be seen with the
unaided eye (objects that are not within
the resolution range of the normal eye).
87. There are three well-known branches of
microscopy, optical, electron, and scanning
probe microscopy.
• Optical and electron microscopy involves the
diffraction, reflection, or refraction of radiation
incident upon the subject of study, and the
subsequent collection of this scattered radiation in
order to build up an image.
• Scanning probe microscopy involves the
interaction of a scanning probe with the surface or
object of interest.
88. Electron Microscopy
• Developed in the 1930s that use electron beams instead of
light.
• Because of the much lower wavelength of the electron
beam than of light, resolution is far higher.
Most commonly used:
• Scanning electron microscopy (SEM)
• Transmission electron microscopy (TEM)
90. • It is a type of electron microscope capable of
producing high-resolution images of a
sample surface.
• Due to the manner in which the image is
created, SEM images have a characteristic
3D appearance and are useful for judging
the surface structure of the sample.
• It is not high enough to image individual
atoms, as is possible in the TEM … as it is 1-
20 nm
Scanning Electron Microscopy (SEM)
93. Components
• Vacuum system:
• To increase the mean free path of the electron-gas
interaction, a standard TEM is evacuated to low
pressures, typically on the order of 10−4 Pa.
Specimen stage:
The specimen holders are adapted to hold a standard size of grid
upon which the sample is placed or a standard size of self-
supporting specimen.
Standard TEM grid sizes is a 3.05 mm diameter ring, with a
thickness and mesh size ranging from a few to 100 μm.
The sample is placed onto the inner meshed area having diameter
of approximately 2.5 mm.
Usual grid materials are copper, molybdenum, gold or platinum.
This grid is placed into the sample holder which is paired with the
specimen stage.
94. Electron lenses are designed to act in a manner emulating that of an optical
lens, by focusing parallel rays at some constant focal length.
The majority of electron lenses for TEM utilise electromagnetic coils to
generate a convex lens.
Electron lenses are manufactured from iron, iron-cobalt or nickel cobalt
alloys,such as permalloy. These are selected for their magnetic properties,
such as magnetic saturation, hysteresis and permeability.
95. Why do we need a lens?
Because all electron sources generally produce a diverging beam of
electrons. This beam must be "focussed" onto the specimen, to
increase the intensity and thus to making the probe "smaller".
Lenses in an TEM/STEM utilize either or combinations of Magnetic
and Electrostatic Fields to direct the beams as desired.
96. The electron gun is formed from several components: the filament, a
biasing circuit, a Wehnelt cap, and an extraction anode. By connecting the
filament to the negative component power supply, electrons can be
"pumped" from the electron gun to the anode plate, and TEM column, thus
completing the circuit. The gun is designed to create a beam of electrons
exiting from the assembly
97. TEM consists of an emission source, which may be a tungsten filament,
or a lanthanum hexaboride (LaB6) source. For tungsten, this will be
of the form of either a hairpin-style filament, or a small spike-
shaped filament. LaB6 sources utilize small single crystals.
98. Secondary Electron Detector
The electrons are detected by an Everhart-Thornley detector,18 which is
a type of scintillator-photomultiplier system. The secondary electrons are
first collected by attracting them towards an electrically biased grid at
about +400 V, and then further accelerated towards a phosphor or
scintillator positively biased to about +2,000 V.
99. Backscatter electron detector
Dedicated backscattered electron detectors are positioned above the
sample in a "doughnut" type arrangement, concentric with the electron
beam, maximising the solid angle of collection. BSE detectors are
usually either of scintillator or of semiconductor types.
Semiconductor detectors can be made in radial segments that can be
switched in or out to control the type of contrast produced and its
directionality.
100. Transmission Electron Microscopy (TEM)
• A beam of electrons is transmitted through a sample, then
an image is formed, magnified and directed to appear
either on a fluorescent screen or layer of photographic
film or to be detected by a sensor (e.g. charge-coupled
device, CCD camera.
• It involves a high voltage electron beam emitted by a
cathode, usually a tungsten filament and focused by
electrostatic and electromagnetic lenses.
• Electron beam that has been transmitted through a
specimen that carries information about the inner
structure of the specimen which reaches the imaging
system of the microscope. Then magnified by a series of
electromagnetic lenses until it is recorded by hitting a
fluorescent screen, photographic plate, or CCD camera.
103. (GUN: Electrons are emitted from a
tungsten filament a thin wire)
(Electrons are accelerated with
an electric field to sample)
104. Invented by Binnig, Quate, and Gerber at Stanford University in
1986.
Atomic Force Microscopy (AFM) measures the interaction
force between the tip and surface. The tip may be dragged
across the surface, or may vibrate as it moves. The interaction
force will depend on the nature of the sample, the probe tip and
the distance between them.
The AFM makes use of a sharp tip attached to a cantilever,
acting as a spring.
Unlike STM, where the tunneling current is a measure of the
interaction, the force between tip and sample is detected via its
mechanical influence on the cantilever deflection or resonance.
The AFM can be used to study insulating, semiconducting and
conducting samples.
Atomic Force Microscopy (AFM)
105. Fig. Schematic set-up of an AFM using the beam deflection method.
The piezo tube scans and approaches the sample to the tip attached to
a cantilever. The force acting on the highly flexible cantilever is
transduced into an electronic signal via a beam deflected onto a four
quadrant photo-detector.
107. Introduction
Identification of elements on surfaces of materials
Quantitative determination of elements on surfaces
Depth profiling by inert gas sputtering Phenomena such
as adsorption, desorption, and surface segregation from
the bulk
Determination of chemical states of elements
In situ analysis to determine the chemical reactivity at a
surface
Auger electron elemental map of the system
110. AES Spectrometer
The essential components
of an AES spectrometer
are
UHV environment
Electron gun
Electron energy
analyzer
Electron detector
Data recording,
processing, and output
system
111. UHV Environment
• Until 1960 the advancements in surface analysis
techniques were inhibited by two difficulties:
constructing an apparatus suitable for operation in a UHV
environment
production and measurement of UHV the glass enclosures were
replaced by standardized, stainless steel hardware.
• The UHV environment could be easily achieved by
pumping a stainless steel chamber with a suitable
combination of ion, cryo, turbo molecular, or oil diffusion
pumps.
• The surface analysis necessitates the use of a UHV
environment because in order to minimize the influence
of residual gases in surface
112. Electron Gun
• The nature of the electron gun used for AES analysis
depends on a number of factors:
– The speed of analysis (requires a high beam current)
– The desired spatial resolution (sets an upper limit on the beam
current)
– Beam-induced changes to the sample surface (sets an upper
limit to current density)
• The electron gun optical system has two critical
components: the electron source and the focusing
forming lens The commonly used electron sources are
113. Electron sources
• A tungsten cathode source consists of a wire filament,
which is bent in the form of a hairpin. The filament
operates at ~2700 K by resistive heating. The tungsten
cathodes are widely used, because they are both reliable
and inexpensive.
• A lanthanum hexaboride (LaB6) cathode provides higher
current densities because LaB6 has a lower work
function than tungsten.
• A field emission electron source consists of a very sharp
tungsten point at which the electric field can be >107
V/cm. Hence, electrons tunnel directly through the
barrier and leave the emitter. A field emission gun
provides the brightest beam
114. Electron Energy Analyzer
• The function of an electron energy analyzer is to
disperse the secondary emitted electrons from the
sample according to their energies. An analyzer may be
either magnetic or electrostatic. Because electrons are
influenced by stray magnetic fields (including the earth’s
magnetic field), it is essential to cancel these fields within
the enclosed volume of the analyzer. The stray magnetic
field cancellation is accomplished by using Mu metal
shielding. Electrostatic analyzers are used in all
commercial spectrometer.
115. Types of Electron Energy
Analyzer
• The Cylindrical Mirror Analyzer (CMA)
• Concentric Hemispherical Analyzer (CHA)
116. The Cylindrical Mirror Analyzer
(CMA)
• The CMA consists of two coaxial cylinders
with a negative potential (V ) applied to the
outer cylinder (with radius r2) and ground
potential applied to the inner cylinder (with
radius r1).
118. Electron Detector
• Having passed through the analyzer, the
secondary electrons of a particular energy
are spatially separated from electrons of
different energies.
• Various detectors are used to detect these
electrons.
119. Electron multiplier
• An electron multiplier consists of a series of
electrodes called dynodes. Each is connected
along a resistor string . The dynode potentials
differ in equal steps along the chain. When a
particle (electron, ion, high energy neutral, or high
energy photon) strikes the first dynode, it produces
secondary electrons. The secondary electrons are
then accelerated to the next dynode. A cascade of
secondary electrons ensues. The cascade is
collected at the anode. The resulting current is
then electronically amplified and measured
123. Ion guns
• An AES system is commonly equipped
with an argon ion beam. The Ar+ ion beam
is used to sputter the sample surface.
• The ion gun is employed for:
• Cleaning the sample surface, and
• Depth profiling
125. Data Recording, Processing, and
Output System
• At present, the data in commercial
instruments are acquired digitally and can
be presented in either analog or digital
mode. The majority of AES instruments
are controlled by computer. Major
functions of the computer control system
are to acquire and store data efficiently.
127. 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
XPS is a technique used to investigate elemental composition of surfaces.
X-ray Photoelectron Spectroscopy (XPS), also known as Electron
Spectroscopy for Chemical Analysis (ESCA)
XPS was developed in the mid-1960’’s by Siegbahn in Sweden.
128. • XPS is also known as ESCA
(Electron Spectroscopy for
Chemical Analysis).
• The technique is widely used
because it is very simple to
use and the data is easily
analyzed.
• XPS works by irradiating
atoms of a surface of any
solid material with X-Ray
photons, causing the
ejection of electrons.
129. The XPS is controlled by
using a computer
system.
The computer system will
control the X-Ray type
and prepare the
instrument for analysis.
130. • The instrument uses
different pump
systems to reach the
goal of an Ultra High
Vacuum (UHV)
environment.
• The Ultra High
Vacuum environment
will prevent
contamination of the
surface and aid an
accurate analysis of
132. X-Ray source
Ion source
Axial Electron Gun
Detector
CMA
sample
SIMS
Analyzer
Sample introduction
Chamber
Sample
Holder
Ion Pump
Roughing Pump Slits
133.
134.
135.
136. • The sample will be introduced
through a chamber that is in
contact with the outside
environment
• It will be closed and pumped to
low vacuum.
• After the first chamber is at low
vacuum the sample will be
introduced into the second
chamber in which a UHV
environment exists. First Chamber
Second Chamber UHV
137. • Contamination of surface
XPS is a surface sensitive technique.
Contaminates will produce an XPS signal and lead to incorrect analysis of
the surface of composition.
• The pressure of the vacuum system is < 10-9 Torr
• Removing contamination
To remove the contamination the sample surface is bombarded with argon
ions (Ar+ = 3KeV).
Heat and oxygen can be used to remove hydrocarbons
• The XPS technique could cause damage to the
surface, but it is negligible.
138. X-Rays
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.
139. Electrons from filament are accelerated to
~30 kV, then directed towards the anode
which is coated with the target element of
interest (Mg or Al here).
The power dissipated by the electron
beam is ~300-400 Watts, which requires
cooling of the anode targets with flowing
water.
This is a very serious design problem,
since the anodes are at 30 kV, and normal
water is conductive.
In some sources, both targets are Mg or Al
to allow both sources to operate
simultaneously, with double the output
power.
140. • 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
X-Rays
141. The Atom and the X-Ray
Core electrons
Valence electrons
X-Ray
Free electron
proton
neutron
electron
electron vacancy
The core electrons
respond very well to
the X-Ray energy
142. Atoms layers
e- top layer
e- lower layer
with collisions
e- lower layer
but no collisions
X-Rays
Outer surface
Inner surface
143. • The X-Rays will penetrate to the core e- of the atoms
in the sample.
• Some e-s are going to be released without any
problem giving the Kinetic Energies (KE) characteristic
of their elements.
• Other e-s will come from inner layers and collide with
other e-s of upper layers
– These e- will be lower in lower energy.
– They will contribute to the noise signal of the spectrum.
144. X-Ray
Electron without collision
Electron with collision
The noise signal comes
from the electrons that
collide with other electrons
of different layers. The
collisions cause a decrease
in energy of the electron
and it no longer will
contribute to the
characteristic energy of the
element.
145. For best spectral purity, a
monochromator selects only
the central intense X-ray
emission, but this reduces flux
by at least an order of
magnitude
146. Kinetic energy and hence binding energy is measured
using a hemispherical analyzer
Electrons are injected at S along a tangent of the spherical sections.
Electrons that match the pass energy are refocused back to the exit slit
F, where they are detected using a channeltron or electron multiplier.
147. Detectors:
How can we detect the electrons once they
have passed through the analyzer?
148. Each dynode at progressively more +ve potential to
accelerate electrons giving more and more electrons
149. The front of the channeltron ideally should be at a high positive potential to attract the
electrons.
The rear of the device must be at an even more positive potential to accelerate the
electron cascade down to the detector.
Since most detection systems presume a source signal near ground potential, some level
decoupling must be used.
151. Context
• X-RAY INTERACTIONS WITH MATTER
• Introduction
• ATOMIC STRUCTURE
• XRF – A PHYSICAL DESCRIPTION
• XPS Principles
• Photoelectric Effect
• Photoelectron vs Other Spectroscopies
• Summary
152. X-RAY INTERACTIONS WITH MATTER
When X-rays encounter matter, they can be:
Absorbed or transmitted through the sample
(Medical X-Rays – used to see inside materials)
Diffracted or scattered from an ordered crystal
(X-Ray Diffraction – used to study crystal structure)
Cause the generation of X-rays of different “colours”
(X-Ray Fluorescence – used to determine elemental composition)
153. • X-ray Photoelectron Spectroscopy (XPS), also
known as Electron Spectroscopy for Chemical
Analysis (ESCA) is a widely used technique to
investigate the chemical composition of
surfaces.
Introduction
154. An atom consists of a nucleus (protons and neutrons) and electrons
Z is used to represent the atomic number of an element
(the number of protons and electrons)
Electrons spin in shells at specific distances from the nucleus
Electrons take on discrete (quantized) energy levels (cannot occupy
levels between shells
Inner shell electrons are bound more tightly and are harder to remove
from the atom
ATOMIC STRUCTURE
155. K shell - 2 electrons
L shell - 8 electrons
M shell - 18 electrons
N shell - 32 electrons
Shells have specific names (i.e., K, L, M) and
only hold a certain number of electrons
The shells are labelled from the
nucleus outward
ELECTRON SHELLS
X-rays typically affect only inner shell (K, L) electrons
156. Step 1: When an X-ray photon of sufficient energy strikes an atom, it dislodges an electron
from one of its inner shells (K in this case)
Step 2a: The atom fills the vacant K shell with an electron from the L shell; as the electron
drops to the lower energy state, excess energy is released as a K X-ray
Step 2b: The atom fills the vacant K shell with an electron from the M shell; as the
electron drops to the lower energy state, excess energy is released as a K X-ray
XRF – A PHYSICAL DESCRIPTION
Step 1:
Step 2a:
Step 2b:
http://www.niton.com/images/XRF-Excitation-Model.gif
157. XPS Principles
• If we consider a single atom with just one x-ray photon on the way, the total energy
is hv+Ei, where hv is the photon energy and Ei the energy of the atom in its initial
state.
• Following the absorption of the photon and the emission of the photoelectron, the
total energy is now KE+Ef, where KE is the electron kinetic energy and Ef the final
state energy of the atom (now an ion).
Because total energy is conserved
hv+Ei = KE+Ef Or
hv-KE = Ef-Ei = BE
where we call the difference between the photon energy (which we know) and the
electron energy (which we measure), the binding energy of the orbital from which
the electron was expelled. The binding energy is determined by the difference
between the total energies of the initial-state atom and the final-state ion.
• It is roughly equal to the Hartree-Fock energy of the electron orbital and so peaks in
the photoelectron spectrum can be identified with specific atoms and hence, a
surface compositional analysis performed.
158. Ionization occurs when matter interacts with light of
sufficient energy (Heinrich Hertz, 1886)
Ehn = electron kinetic energy + electron binding energy
hn
e-
e- e-
Photoelectric Effect
Photoelectron spectroscopy uses this phenomenon
to learn about the electronic structure of matter
159. Photoelectrons
• When light strikes an atom an electron may be ejected if the energy of the
light is high enough. The energy in the light is determined by its
wavelength or frequency (short wavelength = high energy and high
frequency = high energy) X-rays have high energy. When X-rays strike a
solid electrons are always ejected from the near-surface region of the solid.
160. XPS spectral lines are
identified by the shell from
which the electron was
ejected (1s, 2s, 2p, etc.).
The ejected photoelectron has
kinetic energy:
KE=hv-BE-
Following this process, the
atom will release energy by
the emission of an Auger
Electron.
Conduction Band
Valence Band
L2,L3
L1
K
Fermi
Level
Free
Electron
Level
Incident X-ray
Ejected Photoelectron
1s
2s
2p
The Photoelectric Process
161. Kai Seigbahn: Development of
X-ray Photoelectron Spectroscopy
Nobel Prize in Physics 1981
(His father, Manne Siegbahn, won the Nobel Prize in Physics in 1924
for the development of X-ray spectroscopy)
C. Nordling E. Sokolowski and K. Siegbahn, Phys. Rev. 1957, 105, 1676.
162. Measurements with XPS
If we measure the energy of the ejected photoelectrons
we can calculate its Binding Energy which is the energy
required to remove the electron from its atom. From the
binding energy we can learn some important facts about
the sample under investigation:
• The elements from which it is made
• The relative quantity of each element
• The chemical state of the elements present
• Modern XPS instruments can also produce images or
maps showing the distribution of the elements or their
chemical states over the surface. A good instrument
would have a spatial resolution of a few microns.
163. Electron Spectroscopy for Chemical Analysis
Exciting Radiation Outcoming Radiation
UV (~ 20 eV) UPS
electrons from occupied
valence states
X-Ray (~ 10 keV)
XPS
electrons from
(occupied) core states
Ultraviolet photoelectron spectroscopy (UPS) refers to the
measurement of kinetic energy spectra of photoelectrons emitted by
ultraviolet photons, to determine molecular energy levels in the
valence region.
164. Photoelectron vs Other
Spectroscopies
Others
• Photon must be in
resonance with transition
energy
• Measure absorbance or
transmittance of photons
• Scan photon energies
Photoelectron
• Photon just needs enough
energy to eject electron
• Measure kinetic energy of
ejected electrons
• Monochromatic photon
source
165. Summary
ESCA provides unique information about chemical
composition
And chemical state of a surface
useful for biomaterials
Advantages
surface sensitive (top few monolayers)
wide range of solids
relatively non-destructive
Disadvantages
expensive, slow, poor spatial resolution, requires high
vacuum
167. Applications
• X-ray photoelectron spectroscopy
(XPS) is a quantitative spectroscopic
technique that measures,
• The elemental composition,
• Empirical formula,
• Chemical state and electronic state of the
elements that exist within a material.
168. • Identifying stains and discolorations
• Characterizing cleaning processes
• Analyzing the composition of powders and
debris
• Determining contaminant sources
• Examining polymer functionality before
and after processing to identify and
quantify surface changes
169. • Measuring lube thickness on hard disks
• Obtaining depth profiles of thin film stacks
(both conducting and non-conducting) for
matrix level constituents
• Assessing the differences in oxide
thickness between samples
170. • It is a surface analysis
technique with a sampling
volume that extends from the
surface to a depth of
approximately 50-70
Angstroms
171. Where do Binding Energy Shifts
Come From?
-or How Can We Identify Elements and Compounds?
Electron-electron
repulsion
Electron-nucleus
attraction
Electron
Nucleus
Binding
Energy
Pure Element
Electron-
Nucleus
Separation
Fermi Level
Look for changes here
by observing electron
binding energies
172. 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 right
Binding energy
#
of
electrons
(eV)
173. 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
#
of
electrons
N1
N2
N3
N4
Ntot= N1 + N2 + N3 + N4
N = noise
174. XPS Spectrum
• The XPS peaks are sharp.
• In a XPS graph it is possible to see
Auger electron peaks.
• The Auger peaks are usually wider
peaks in a XPS spectrum.
175. Identification of XPS Peaks
• The plot has characteristic peaks for each
element found in the surface of the sample.
• There are tables with the KE and BE already
assigned to each element.
• After the spectrum is plotted you can look for
the designated value of the peak energy from
the graph and find the element present on the
surface.
177. Carbon-Oxygen Bond
Valence Level
C 2p
Core Level
C 1s
Carbon Nucleus
Oxygen Atom
C 1s
Binding
Energy
Electron-oxygen
atom attraction
(Oxygen Electro-
negativity)
Electron-nucleus
attraction (Loss of
Electronic Screening)
Shift to higher
binding energy
Chemical Shifts-
Electronegativity Effects
184. Relative Sensitivities of the
Elements
0
2
4
6
8
10
12
Elemental Symbol
Relative
Sensitivity
Li
Be
B
C
N
O
F
Ne
Na
M
Al
Si
P
S
Cl
Ar
K
Ca
Sc
Ti
V
Cr
M
Fe
Co
Ni
Cu
Zn
G
G
As
Se
Br
Kr
Rb
Sr
Y
Zr
Nb
M
Tc
Ru
Rh
Pd
Ag
Cd
In
Sn
Sb
Te
I
Xe
Cs
Ba
La
Ce
Pr
Nd
P
S
Eu
G
Tb
Dy
Ho
Er
T
Yb
Lu
Hf
Ta
W
Re
Os
Ir
Pt
Au
Hg
Tl
Pb
Bi
1s
2p
3d
4d
4f
185. XPS Analysis of Pigment from Mummy
Artwork
150 145 140 135 130
Binding Energy (eV)
PbO2
Pb3O4
500 400 300 200 100 0
Binding Energy (eV)
O
Pb Pb
Pb
N
Ca
C
Na
Cl
XPS analysis showed
that the pigment used
on the mummy
wrapping was Pb3O4
rather than Fe2O3
Egyptian Mummy
2nd Century AD
World Heritage Museum
University of Illinois
186. Analysis of Carbon Fiber- Polymer
Composite Material by XPS
Woven carbon
fiber composite
XPS analysis identifies the functional
groups present on composite surface.
Chemical nature of fiber-polymer
interface will influence its properties.
-C-C-
-C-O
-C=O
-300 -295 -290 -285 -280
Binding energy (eV)
N(E)/E
187. Analysis of Materials for Solar Energy Collection
by XPS Depth Profiling-
The amorphous-SiC/SnO2 Interface
The profile indicates a reduction of the SnO2
occurred at the interface during deposition.
Such a reduction would effect the
collector’s efficiency.
Photo-voltaic Collector
Conductive Oxide- SnO2
p-type a-SiC
a-Si
Solar Energy
SnO2
Sn
Depth
500 496 492 488 484 480
Binding Energy, eV
Data courtesy A. Nurrudin and J. Abelson, University of Illinois
190. Strengths of XPS Analysis
• Chemical state identification on surfaces
• Identification of all elements except for H and He
• Quantitative analysis, including chemical state
differences between samples
• Applicable for a wide variety of materials,
including insulating samples (paper, plastics,
and glass)
• Depth profiling with matrix-level concentrations
• Oxide thickness measurements
191. Limitations of XPS Analysis
• Detection limits typically ~ 0.1 at%
• Smallest analytical area ~10 µm
• Limited specific organic information
• Sample compatibility with UHV
environmen
194. Definition
A common analytical technique used specifically
in the study of surfaces and, more generally, in
the area of materials science
It is a surface specific technique utilising the
emission of low energy electrons in the Auger
process and is one of the most commonly
employed surface analytical techniques for
determining the composition of the surface
layers of a sample.
195. Discovery
Discovered independently by both Lise Meitner and
Pierre Auger in the 1920s.
Though the discovery was made by Meitner and
initially reported in the journal Zeitschrift für Physik
in 1922.
Working with X rays and using a
Wilson cloud chamber.
Tracks corresponding to ejected
electrons were observed along a
beam of X rays
196. Basic Principle
Auger spectroscopy can be considered as involving
three basic steps :
(1) Atomic ionization (by removal of a core electron)
(2) Electron emission (the Auger process)
(3) Analysis of the emitted Auger electrons
197. Auger effect
Based on the analysis of energetic electrons emitted from an
excited atom after a series of internal relaxation events
The incident electron with sufficient primary energy 2 keV
to 50 keV, Ep, ionizes the core level, such as a K level.
• The vacancy thus produced is immediately filled by another
electron from L1.
• The energy (EK – EL1) released from this transition can be
transferred to another electron, as in the L2 level. This
electron is ejected from the atom as an Auger electron
198.
199. The transition energy can be coupled to a
second outer shell electron which will be
emitted from the atom if the transferred
energy is greater than the orbital binding
energy
An emitted electron will have a kinetic energy
of:
Ekin = ECore State − EB − EC'
where ECore State, EB, EC' are respectively the
core level, first outer shell, and second outer
shell electron energies, measured from the
vacuum level
202. • This excitation process is denoted as a KL1L2
Auger transition.
• It is obvious that at least two energy states and
three electrons must take part in an Auger
process. Therefore, H and He atoms cannot give
rise to Auger electrons.
• Several transitions (KL1L1, KL1L2, LM1M2, etc.)
can occur with various transition probabilities.
• The Auger electron energies are characteristic of
the target material and independent of the incident
beam energy
203. • Isolated Li atoms having a single electron in
the outermost level cannot give rise to Auger
electrons.
• However, in a solid the valence electrons are
shared and the Auger transitions of the type
KVV occur involving the valence electrons of
the solid.
• In general, the kinetic energy of Auger
electrons originating from an ABC transition
can be estimated from the empirical relation
204. • Until the early 1950s Auger transitions were
considered nuisance effects by
spectroscopists, not containing much relevant
material information, but studied so as to
explain anomalies in x-ray spectroscopy data.
• Since 1953 however, AES has become a
practical and straightforward characterization
technique for probing chemical and
compositional surface environments and has
found applications in metallurgy, gas-phase
chemistry, and throughout the microelectronics
industry
205. Advantages
• High sensitivity for chemical analysis in the 5-
to 20-Å region near the surface.
• A rapid data acquisition speed.
• Its ability to detect all elements above helium,
and its capability of high-spatial resolution.
• The high-spatial resolution is achieved
because the specimen is excited by an electron
beam that can be focused into a fine probe.
206. Disadvantages
Analyzes conducting and semiconducting
samples.
Special procedures are required for
nonconducting samples.
Only solid specimens can be analyzed.
Samples that decompose under electron
beam irradiation cannot be studied.
Quantification is not easy.
