2. NANOTCHNOLOGY
• Introduction
• History and origin of nanotechnology
• Fundamental definitions i.e., nanoscale, nanosciece, nanotechnology, and
nano material
• Properties differ nano material from bulk materials
• Types of nano materials
• Properties of nano materials i.e., physical, chemical, mechanical,
electrical, magnetic etc..
• Synthesis of nanotechnology- top down and bottom up
• Characterization of nano materials by x-ray diffraction, SEM and TEM
• Applications of nanotechnology
3. What is nanotechnology?
The study of objects and phenomena at a very small scale, roughly 1 to 100
nanometers (nm)
10 hydrogen atoms lined up measure about 1 nm
A grain of sand is 1 million nm, or 1 millimeter, wide
What’s interesting about the nanoscale?
Nanosized particles exhibit different properties
than larger particles of the same substance
The word ‘nano’ comes from the Greek word which means ‘dwarf’.
A nanometre (nm) is 0.000 000 001metre
(or 10-9 m). That’s one millionth of a millimetre.
Nanoparticles are very small, less than 100
nm across, but just how small is that?
4. 4
Producing materials and devices that take advantage of physical, chemical and
biological principles whose causes are found in the nanometre scale.
A nanometre (nm) is one billionth of a metre.
For comparison purposes, the width of an average hair is 100,000 nm.
Human blood cells are 2,000 to 5,000 nm long, a strand of DNA has a
diameter of 2.5 nm, and a line of ten hydrogen atoms is one nm.
Nanotechnology is distinguished by its interdisciplinary nature.
For one thing, investigations at the nanolevel are occurring in a
variety of academic fields. These areas includes, Physics,
Chemistry, Biology etc……….
NANO SCALE-HOW SMALL IS IT ?
5. How small is Nano - small?
Units in nanometers (µm)
6. NANO SCIENCE, TECHNOLOGY AND MATERIALS
NANOSCIENCE:
The study of phenomena of manipulation of materials at atomic, molecular and
micro molecular scales where the properties differ significantly from those at a
large scale.
NANOTECHNOLOGY:
A branch of engineering that deals with the design, characterization, production
and applications of structures, devices and systems by controlling shape and size at
the nano metre scale.
NANOMATERIALS:
Those materials which have structured components with size less than 100nm at
least in one dimension.
7. 7
Quantum well, Quantum wire and Quantum dots
The grain sizes of the conventional materials vary from few microns to few
millimeters and contain several billions of atoms.
When the size or dimension of a material is continuously reduced from a large or
macroscopic size, such a meter or centimeter, to a very small size, the properties
remain the same at first, then small changes begin to occur, until finally when the
size drops below100 nm, dramatic changes in properties can occur.
If one dimension is reduced to the nanorange while the other dimensions remain
large, them we obtain a structure known as quantum well. Ex: layers such as Thin
film and surface coating
If two dimensions are so reduced and one remains large, the resulting structure is
referred to as a quantum wire . Ex: Nanotubes
The extreme case of this process of size reduction in which all three dimensions
reach the low nanometer range is called a quantum dot. Ex: precipitates, colloids
8. 8
Bulk Well Wire Dot
Bulk Well Wire Dot
Quantum well, Quantum wire and Quantum dots
Progressive generation of rectangular and curvilinear nano structures
The word quantum is associated with the above three types of nanostructures because
the changes in properties arise from the quantum mechanical nature of physics in the
domain of the ultra small.
The above fig. represents the processes of diminishing the size for the case of
rectilinear geometry and the corresponding reductions in curvilinear geometry.
9.
10. Why properties of Nano Materials are different ?
The properties of Nano Materials are very much different from those at a larger scale.
Two principal factors cause the properties of Nano Materials to differ significantly from other
materials.
1.Increase in surface to volume ratio.
2.Quantum confinement effect.
These factors can charge or enhance properties such as reactivity, strength and electrical
characteristics.
In addition to these two fallowing become more important at nano scale
Gravitational forces become negligible and electromagnetic forces begin to dominate.
Random molecular motion becomes more important
11. Increase in a Surface Area to Volume ratio
Nano Materials have a relatively larger Surface area when compared to the
same volume or mass of the material produced in a larger form.
Let us consider a Sphere of radius “r”.
Its Surface Area =4πr2.
Its volume= 4/3πr3
Surface Area to Volume Ratio= 3/r.
Thus when the radius of the Sphere decreases , its Surface to Volume ratio
increases.
12.
13. Quantum confinement
• The quantum confinement effect is observed
when the size of the particle is too small to be
comparable to the deBrogli wavelength of
the electron.
• To understand this effect we break the words
like quantum and confinement
• The word confinement means to confine the
motion of randomly moving electron to restrict
its motion in specific energy levels(
discreteness) and quantum reflects the atomic
realm of particles.
• So as the size of a particle decrease till we
reach a nano scale the decrease in confining
dimension makes the energy levels discrete
and this increases or widens up the band gap
and ultimately the band gap energy also
increases.
• This effect is known as quantum confinement
Energy
555 nm
650 nm
14. Quantum confinement
• This is very similar to the famous particle-in-a-box scenario and can be understood
by examining the Heisenberg Uncertainty Principle.
• The Uncertainty Principle states that the more precisely one knows the position of a
particle, the more uncertainty in its momentum (and vice versa).
• Therefore, the more spatially confined and localized a particle becomes, the broader
the range of its momentum/energy.
• This is manifested as an increase in the average energy of electrons in the
conduction band = increased energy level spacing = larger bandgap
• The bandgap of a spherical quantum dot is increased from its bulk value by a factor
of 1/R2, where R is the particle radius.*
• The reduction in the size of atoms in a material results in the confinement of
normally delocalized energy states.
• Electron-hole pairs become spatially confined when the diameter of a particle
approaches the de Broglie wavelength of electrons in the conduction band.
• As a result the energy difference between energy bands is increased with decreasing
particle size.
15.
16.
17. Random Molecular Motion is Significant
• Random motion at the macro scale
– Small compared the size of the substance
– We can barely detect motion of dust particles on the surface
of water
• Random motion at the the nanoscale
– Large when compared to the size of the substance
– The molecules that make up the dust particle are moving
wildly
“At the macroscale, random motion is much smaller than the
size of the substance. At the nanoscale, this motion is large
when compared to the size of the substance and therefore
has much more of an influence on the substance”
18.
19.
20. Properties of Nanomaterial
Following properties change when size of the materia is reduced to nanoscale
Physical properties:
- Inter atomic spacing decrease
- lattice parameters decrease
- Surface to volume ratio increases
Chemical properties:
- As surface to volume increases,
the reactivity of the material increase
as a result material becomes more
catalytic
Thermal properties:
- Melting point decreases as
particle size decreases
21. Properties of Nanomaterial
Optical properties:
- Due to quantum confinement optical properties change with size
- Gold sphere appears
100nm - Orange
50nm - Green
20nm - Red
Mechanical Properties:
- Much stronger and stiffer than their bulk forms.
- Both stronger and ductile.
Electrical properties:
- Due to quantum confinement energy band gap increases with size as a result
i) Nano ceramic and magneto nano compositions conductivity increases
ii) In metal conductivity decreases
iii) Some insulators become conductors in their nano forms.
e.g. SiO2
22. Properties of Nanomaterial
Magnetic properties:
- The strength of magnetic is measured in terms of coercivity
and saturation magnetization.
- These values increase with decrease in grain size
- Nano particals of even non magnetic solids are found to be
magnet
- At small size, clusters become spontaneously magnetic.
23. Synthesis of nonmaterial
Two main approaches : i) Bottom up and ii) top down
Bottom up approach:
Materials and devices are built from molecular
components which assemble chemically using
principle of molecular recognition.
Build up nano material from bottom
i.e.., atom by atom , molecule by molecule
cluster by cluster.
Simply Building what you want by assembling it
from building blocks ( atoms or molecules).
Examples: Sol - Gel
Chemical Vapor Deposition (CVD)
physical Vapor Deposition (PVD)
24. Synthesis of nonmaterial
Top down approach:
Nano objects are constructed from larger
entities without atomic level control .
Slicing or successive cutting of bulk material
in to nano sized particles.
Simply, start with the buk material “ cut away
material” to make the what you want.
Ex: Ball Milling (Mechanical crushing)
25. Material Processing by Sol-Gel Method
Introduction
The sol-gel process is very long known since the late 1800s. The versatility of
the technique has been rediscovered in the early 1970s when glasses where
produced without high temperature melting processes.
This made possible the organic modification of silicon compounds (ORMOSIL),
which cannot withstand high temperatures.
Sol-gel is a chemical solution process used to make ceramic and glass materials
in the form of thin films, fibers , or powders .
A sol is a colloidal (the dispersed phase is so small that gravitational forces do
not exist; only Van der Waals forces and surface charges are present) or
molecular suspension of solid particles of ions in a solvent.
A gel is a semi-rigid mass that forms when the solvent from the sol begins to
evaporate and the particles or ions left behind begin to join together in a
continuous network
26. Typical precursors are metal alkoxides and metal chlorides, which undergo
hydrolysis
(Hydrolysis is a chemical reaction or process in which a chemical compound
is broken down by reaction with water) and polycondensation reactions.
(A chemical reaction in which two or more molecules combine upon the
separation of water or some other simple substance) to form a colloid, a
system composed of solid particles (size ranging from 1 nm to 1 μm)
dispersed in a solvent.
The sol evolves then towards the formation of an inorganic network
containing a liquid phase (gel).
Formation of a metal oxide involves connecting the metal centers with oxo
(M-O-M) or hydroxo (M-OH-M) bridges, therefore generating metal-oxo or
metal-hydroxo polymers in solution.
The drying process serves to remove the liquid phase from the gel thus
forming a porous material, then a thermal treatment (firing) may be
performed in order to favor further polycondensation and enhance
mechanical properties.
Sol - Gel method
27. The precursor sol can be either deposited on a substrate to form a film (e.g.
by dip-coating or spin-coating), cast into a suitable container with the desired
shape
(e.g. to obtain a monolithic ceramics, glasses, fibers, membranes, aerogels),
or used to synthesize powders (e.g. microspheres, nanospheres).
In essence, the sol-gel process usually consists of 4 steps:
(1) The desired colloidal particles once dispersed in a liquid to form a sol.
(2) The deposition of sol solution produces the coatings on the substrates by
spraying, dipping or spinning.
(3) The particles in sol are polymerized through the removal of the stabilizing
components and produce a gel in a state of a continuous network.
(4) The final heat treatments pyrolyze the remaining organic or inorganic
components and form an amorphous or crystalline coating.
Sol - gel method
28. The sol-gel approach is interesting in that it is a cheap and low-
temperature technique that allows for the fine control on the
product’s chemical composition,
as even small quantities of dopants, such as organic dyes and
rare earth metals, can be introduced in the sol and end up in the
final product finely dispersed.
An overview of the sol-gel process is presented in a simple
graphic work below.
Sol - Gel method
30. Can produce thin bond-coating to provide excellent adhesion between the
metallic substrate and the top coat.
Can produce thick coating to provide corrosion protection performance.
Can easily shape materials into complex geometries in a gel state.
Can produce high purity products because the organo-metallic precursor
of the desired ceramic oxides can be mixed, dissolved in a specified
solvent and hydrolyzed into a sol, and subsequently a gel, the composition
can be highly controllable.
Can have low temperature sintering capability, usually 200-600°C.
Can provide a simple, economic and effective method to produce high
quality coatings.
Advantages of Sol-Gel Technique:
31. It can be used in ceramics manufacturing processes, as an investment
casting material, or as a means of producing very thin films of metal
oxides for various purposes.
Sol-gel derived materials have diverse applications in optics, electronics,
energy, space, (bio)sensors, medicine (e.g. controlled drug release) and
separation (e.g. chromatography) technology. One of the more important
applications of sol-gel processing is to carry out zeolite synthesis.
Other elements (metals, metal oxides) can be easily incorporated into the
final product and the silicalite sol formed by this method is very stable.
Other products fabricated with this process include various ceramic
membranes for microfiltration, ultrafiltration, nanofiltration,
pervaporation and reverse osmosis.
Applications of Sol -Gel method
32. Introduction:
Chemical vapour deposition or CVD is a generic name for a group of
processes that involve depositing a solid material from a gaseous
phase.
Micro fabrication processes widely use CVD to deposit materials in
various forms, including: monocrystalline, polycrystalline, amorphous,
and epitaxial.
These materials include: silicon, carbon fiber, carbon nanofibers,
filaments, carbon nanotubes, SiO2, silicon-germanium, tungsten,
silicon carbide, silicon nitride, silicon oxynitride and titanium nitride.
CVD process is also used to produce synthetic diamonds.
Chemical Vapour Deposition
33. Working Concept
• Chemical vapor deposition (CVD) results from the chemical reaction of gaseous
precursor(s) at a heated substrate to yield a fully dense deposit.
• Thermodynamics and kinetics drive both precursor generation and
decomposition.
• Control of thermodynamics and kinetics through
temperature, pressure, and concentrations yields
the desired deposit.
• A simplified concept diagram is shown as Fig
• Metal deposition
metal halide (g) → metal(s) + byproduct (g)
• Ceramic deposition
metal halide (g) + oxygen/carbon/nitrogen/boron source (g) →
ceramic(s) + byproduct (g)
g- gas; s-solid
Chemical Vapour Deposition
CVD REACTION
34. A typical CVD system consists of the following parts:
sources of and feed lines for gases;
mass flow controllers for metering the gases into the
system;
a reaction chamber or reactor;
a system for heating up the wafer on which the film is to be
deposited; and
temperature sensors.
Chemical Vapour Deposition
35. CVD Process Advantages Disadvantages Applications
APCVD
Simple,
Fast Deposition,
Low Temperature
Poor Step Coverage,
Contamination
Low-temperature Oxides
LPCVD
Excellent Purity,
Excellent Uniformity,
Good Step Coverage,
Large Wafer Capacity
High Temperature,
Slow Deposition
High-temperature Oxides,
Silicon Nitride, Poly-
Si, W, WSi2
PECVD
Low Temperature,
Good Step Coverage
Chemical and Particle
Contamination
Low-temperature
Insulators over
Metals, Nitride
Passivation
Types of chemical vapor deposition
A number of forms of CVD are in wide use
AP – Atmosphere pressure, LP –liquid pressure, PE- Plasma Enhanced
36. • Can be used for a wide range of metals and ceramics
• Can be used for coatings or freestanding structures
• Fabricates net or near-net complex shapes
• Is self-cleaning—extremely high purity deposits (>99.995% purity)
• Conforms homogeneously to contours of substrate surface
• Has near-theoretical as-deposited density
• Has controllable thickness and morphology
• Forms alloys
• Infiltrates fiber preforms and foam structures
• Coats internal passages with high length-to-diameter ratios
• Can simultaneously coat multiple components
• Coats powders
Advantages of CVD
37. • CVD processes are used on a surprisingly wide range of industrial components,
from aircraft and land gas turbine blades, timing chain pins for the automotive
industry, radiant grills for gas cookers and items of chemical plant, to resist
various attacks by carbon, oxygen and sulphur.
• Some important applications are listed below.
• Surface modification to prevent or promote adhesion
•
• Photoresist adhesion for semiconductor wafers Silane/substrate adhesion for
microarrays (DNA, gene, protein, antibody, tissue)
• MEMS coating to reduce stiction
• BioMEMS and biosensor coating to reduce "drift" in device performance
• Promote biocompatibility between natural and synthetic materials Copper
capping Anti-corrosive coating
Applications of CVD
38. Physical Vapour Deposition(PVD)
Introduction
1. Physical vapour deposition (PVD) is fundamentally a vaporisation
coating technique, involving transfer of material on an atomic level. It is an
alternative process to electroplating
2. The process is similar to chemical vapour deposition (CVD) except that
the raw materials/precursors, i.e. the material that is going to be
deposited starts out in solid form, whereas in CVD, the precursors are
introduced to the reaction chamber in the gaseous state.
Working Concept
PVD processes are carried out under vacuum conditions. The process
involved four steps:
1.Evaporation
2.Transportation
3.Reaction
4.Deposition
39. Evaporation
During this stage, a target, consisting of the material to be deposited is
bombarded by a high energy source such as a beam of electrons or ions. This
dislodges atoms from the surface of the target, ‘vaporising’ them.
Transport
This process simply consists of the movement of ‘vaporised’ atoms from the
target to the substrate to be coated and will generally be a straight line
affair.
Reaction
In some cases coatings will consist of metal oxides, nitrides, carbides and
other such materials.
In these cases, the target will consist of the metal.
The atoms of metal will then react with the appropriate gas during the
transport stage.
For the above examples, the reactive gases may be oxygen, nitrogen and
methane.
In instances where the coating consists of the target material alone, this step
would not be part of the process.
Physical Vapour Deposition(PVD)
40. Deposition
This is the process of coating build up on the substrate surface.
Depending on the actual process, some reactions between
target materials and the reactive gases may also take place at
the substrate surface simultaneously with the deposition
process.
Fig. shows a schematic diagram of the principles behind one
common PVD method.
The component that is to be coated is placed in a vacuum
chamber. The coating material is evaporated by intense heat
from, for example, a tungsten filament.
An alternative method is to evaporate the coating material by
a complex ion bombardment technique.
The coating is then formed by atoms of the coating material
being deposited onto the surface of the component being
treated.
Physical Vapour Deposition(PVD)
The vacuum evaporation
PVD process
41. Variants of PVD include, in order of increasing novelty:
Evaporative Deposition: In which the material to be deposited is heated to a high vapor
pressure by electrically resistive heating in "high" vacuum.
Electron Beam Physical Vapor Deposition: In which the material to be deposited is
heated to a high vapor pressure by electron bombardment in "high" vacuum.
Sputter Deposition: In which a glow plasma discharge (usually localized around the
"target" by a magnet) bombards the material sputtering some away as a vapor.
Cathodic Arc Deposition: In which a high power arc directed at the target material blasts
away some into a vapor.
Pulsed Laser Deposition: In which a high power laser ablates material from the target into
a vapor.
Different types of PVDs
42. Advantages
• Materials can be deposited with improved properties compared to the substrate
material
• Almost any type of inorganic material can be used as well as some kinds of organic
materials
• The process is more environmentally friendly than processes such as electroplating
Disadvantages
• It is a line of sight technique meaning that it is extremely difficult to coat undercuts and
similar surface features
• High capital cost
• Some processes operate at high vacuums and temperatures requiring skilled operators
• Processes requiring large amounts of heat require appropriate cooling systems
• The rate of coating deposition is usually quite slow
Physical Vapour Deposition(PVD)
43. Characterization of Nano particles
Various techniques are used to characterize nanao particls.
- X-ray diffraction (XRD)
- Scanning Electron Micro scope (SEM)
- Transmission Electron Microscope (TEM)
- Scanning Probe Microscope
- Scanning Tunneling Microscope
- Atomic Force Microscope
- Fourier Transform infra-red (FTIR)
- UV-VIS (UV)
- Thermal Analysis
45. Electron Microscopy Techniques
Introduction
Electron Microscopes are scientific instruments that use a beam of highly
energetic electrons to examine objects on a very fine scale.
The main advantage of Electron Microscopy is the unusual short wavelength of the
electron beams, substituted for light energy.
The wavelengths of about 0.005 nm increases the resolving power of the
instrument to fractions
Uses:
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.)
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.)
46. Working Concept
SEM allows surfaces of objects
to be seen in their natural state
without staining.
The specimen is put into the
vacuum chamber and covered
with a thin coating of gold to
increase electrical conductivity
and thus forms a less blurred
image.
The electron beam then sweeps
across the object building an
image line by line as in a TV
Camera.
As electrons strike the object,
they knock loose showers of
electrons that are captured by a
detector to form the image.
Scanning Electron Microscope (SEM)
Scanning electron microscopy, which looks at the surface of a solid object.
47. The "Virtual Source" at the top
represents the electron gun,
producing a stream of monochromatic
electrons.
The stream is condensed by the first
condenser lens (usually controlled by
the "coarse probe current knob").
This lens is used to both form the
beam and limit the amount of current
in the beam.
It works in conjunction with the
condenser aperture to eliminate the
high-angle electrons from the beam
The beam is then constricted by the
condenser aperture, eliminating some
high-angle electrons
The second condenser lens forms the
electrons into a thin, tight, coherent
beam and is usually controlled by the
"fine probe current knob"
Scanning Electron Microscope (SEM)
48. A set of coils then "scan" or "sweep" the beam in a grid fashion (like a
television), dwelling on points for a period of time determined by the scan
speed (usually in the microsecond range)
The final lens, the Objective, focuses the scanning beam onto the part of
the specimen desired.
When the beam strikes the sample (and dwells for a few microseconds)
interactions occur inside the sample and are detected with various
instruments
Before the beam moves to its next dwell point these instruments count
the number of interactions and display a pixel on a CRT whose intensity is
determined by this number (the more reactions the brighter the pixel).
This process is repeated until the grid scan is finished and then repeated,
the entire pattern can be scanned 30 times per second.
Scanning Electron Microscope (SEM)
49. Specimen Interactions and utilization:
Backscattered Electrons
Formation
Caused by an incident electron colliding with an atom in the specimen which
is nearly normal to the incident's path.
The incident electron is then scattered "backward" 180 degrees.
Utilization
The production of backscattered electrons varies directly with the specimen's
atomic number.
This differing production rates causes higher atomic number elements to
appear brighter than lower atomic number elements.
This interaction is utilized to differentiate parts of the specimen that have
different average atomic number.
Scanning Electron Microscope (SEM)
50. Secondary Electrons
Source
Caused by an incident electron passing "near" an atom in the specimen, near
enough to impart some of its energy to a lower energy electron (usually in the
K-shell).
This causes a slight energy loss and path change in the incident electron and
the ionization of the electron in the specimen atom.
This ionized electron then leaves the atom with a very small kinetic energy
(5eV) and is then termed a "secondary electron".
Each incident electron can produce several secondary electrons.
Utilization
Production of secondary electrons is very topography related.
Due to their low energy, 5eV, only secondaries that are very near the surface
(<10nm,) can exit the sample and be examined.
Any changes in topography in the sample that are larger than this sampling
depth will change the yield of secondaries due to collection efficiencies.
Collection of these electrons is aided by using a "collector" in conjunction with
the secondary electron detector.
The collector is a grid or mesh with a +100V potential applied to it which is
placed in front of the detector, attracting the negatively charged secondary
electrons to it which then pass through the grid-holes and into the detector to
be counted.
Scanning Electron Microscope (SEM)
51. UNIT IV LECTURE 5 51
Auger Electrons
Source
Caused by the de-energization of the specimen atom after a secondary
electron is produced.
Since a lower (usually K-shell) electron was emitted from the atom
during the secondary electron process an inner (lower energy) shell
now has a vacancy.
A higher energy electron from the same atom can "fall" to a lower
energy, filling the vacancy.
This creates and energy surplus in the atom which can be corrected by
emitting an outer (lower energy) electron; an Auger Electron.
Utilization
Auger Electrons have a characteristic energy, unique to each element
from which it was emitted from.
These electrons are collected and sorted according to energy to give
compositional information about the specimen
Scanning Electron Microscope (SEM)
52. X-rays Source
Caused by the de-energization of the specimen atom after a
secondary electron is produced.
Since a lower (usually K-shell) electron was emitted from the
atom during the secondary electron process an inner (lower
energy) shell now has a vacancy.
A higher energy electron can "fall" into the lower energy shell,
filling the vacancy.
As the electron "falls" it emits energy, usually X-rays to
balance the total energy of the atom so it .
X-rays or Light emitted from the atom will have a
characteristic energy which is unique to the element from
which it originated.
Scanning Electron Microscope (SEM)
53. Working Concept
TEM works much like a slide projector.
A projector shines a beam of light through (transmits) the slide, as the
light passes through it is affected by the structures and objects on the
slide.
These effects result in only certain parts of the light beam being
transmitted through certain parts of the slide.
This transmitted beam is then projected onto the viewing screen, forming
an enlarged image of the slide.
TEMs work the same way except that they shine a beam of electrons (like
the light) through the specimen (like the slide).
Whatever part is transmitted is projected onto a phosphor screen for the
user to see.
A more technical explanation of typical TEMs workings is as follows
Transmission Electron Microscope (TEM)
Transmission electron microscopy, which essentially looks through a thin slice of a
specimen.
54. The "Virtual Source" at the top
represents the electron gun, producing a
stream of monochromatic electrons.
This stream is focused to a small, thin,
coherent beam by the use of condenser
lenses 1 and 2. The first lens (usually
controlled by the "spot size knob")
largely determines the "spot size"; the
general size range of the final spot that
strikes the sample.
The second lens (usually controlled by
the "intensity or brightness knob"
actually changes the size of the spot on
the sample; changing it from a wide
dispersed spot to a pinpoint beam.
The beam is restricted by the condenser
aperture (usually user selectable),
knocking out high angle electrons (those
far from the optic axis, the dotted line
down the center)
The beam strikes the specimen and parts
of it are transmitted
Transmission Electron Microscope (TEM)
55. This transmitted portion is focused by the objective lens into an image
The image is passed down the column through the projector lenses,
being enlarged all the way.
The image strikes the phosphor image screen and light is generated,
allowing the user to see the image
Specimen Interactions and utilization
Unscattered Electrons
Source
Incident electrons which are transmitted through the thin specimen
without any interaction occurring inside the specimen.
Utilization
The transmission of unscattered electrons is inversely proportional to the
specimen thickness.
Areas of the specimen that are thicker will have fewer transmitted
unscattered electrons and so will appear darker, conversely the thinner
areas will have more transmitted and thus will appear lighter.
Transmission Electron Microscope (TEM)
56. Elasticity Scattered electrons
Source
Incident electrons that are scattered (deflected from their original path)
by atoms in the specimen in an elastic fashion (no loss of energy).
These scattered electrons are then transmitted through the remaining
portions of the specimen.
Utilization
• All electrons follow Bragg's Law and thus are scattered according to
Wavelength=2*Space between the atoms in the specimen*sin(angle of
scattering).
• All incident electrons have the same energy(thus wavelength) and enter
the specimen normal to its surface
These "similar angle" scattered electrons can be collated using magnetic
lenses to form a pattern of spots; each spot corresponding to a specific
atomic spacing (a plane).
This pattern can then yield information about the orientation, atomic
arrangements and phases present in the area being examined.
Transmission Electron Microscope (TEM)
57. UNIT IV LECTURE 5 57
Transmission Electron Microscope (TEM)
Inelastically scattered electrons can be utilized two ways
Electron Energy Loss Spectroscopy:
The inelastic loss of energy by the incident electrons is characteristic of the
elements that were interacted with.
These energies are unique to each bonding state of each element and thus
can be used to extract both compositional and bonding (i.e. oxidation state)
information on the specimen region being examined.
Kakuchi Bands: Bands of alternating light and dark lines that are formed by
inelastic scattering interactions that are related to atomic spacings in the
specimen.
These bands can be either measured (their width is inversely proportional to
atomic spacing) or "followed" like a roadmap to the "real" elasticity scattered
electron pattern.
