Hello everyone.
Welcome to this material characterization
course.
In the last class, we started looking at the
transmission electron microscopy.
I just gave a brief introduction about the,
the technique as well as for the instruments
and we realize that the transmission electron
microscopy technique involves a lot of diffraction
principles and it is good that we have the
enough background about the diffraction in
the X-ray diffraction section itself.
So, I hope that you will be able to appreciate
this electron diffraction in a similar way
without any difficulty.
So, let me continue the discussion on the
instrumentation details about the TEM.
So, we just started looking at the electron
source and the lenses and then basic operations
and the types of electron source and so on.
So, in that we are we are also looking at
some of the details about the other parts
and in continuation that we will now look
at some more details.
So, we were just seen that the details of
the electromagnetic lens and how it is functioning
and all that and today we will discuss about
the lenses, apertures and their relation to
the resolution.
So, look at this schematic.
What is shown here is, it is a there is a
specimen there is a electron beam coming through
and then you have the maximum aperture collection
angle β and then you have the limiting diagram
and then you have the lens and then image
is converging.
So, we are talking about this limiting diaphragm
and the apertures.
So, some of the apertures and the diagrams
appear like this.
This is an actual photograph from this textbook
and you see that typically they are all metallic
discs which where you have the perforation
in the middle, depending upon the size or
the diameter requirement.
So, typically the diameter can be as small
as 10 to 30 micrometer and 25 to 50 micrometer
thick.
So, this is the typical dimension of this
kind of apertures and diaphragms.
And the discs are made up of typically platinum
and molybdenum.
So, one of the objective apertures is shown
here.
You can see that a small perforation.
It varies from model to model and we will
look at the actual size of this strip.
There is an objective aperture.
When we look at the in an electron microscope
operations, I will just show you what is this
for different perforations will do to the
image and image contrast and so on.
So, this is the information about the lenses.
So, what are they control?
What are the aperture and diaphragm functions?
To control the collection angles, what is
collection handle this is a β, so they control
that in objective lenses, it controls the
resolution of the image formed by the lens.
The depth of field and depth of focus also
controlled by this.
The image contrast, the collection angle of
the electron energy loss spectrometer, there
are also it is being used, the angular resolution
of the diffraction pattern.
So, the apertures and the diaphragms will
have all these factors will be involved or
it will influence the, the functions of all
these parameters.
And also you have the various pumps and you
have the diffusion pump and ion pumps and
vacuum pump and so on.
I will give the actual details when we when
I show the laboratory demonstration and what
they do is.
So, just to give a general comment, the advent
of high-quality digital recording which will
remove the need for the film in the camera
will do more to improve the quality of the
vacuums in the TEMs than any advances, advances
in pumping technology.
So, today we be completely record all the
information in a digital recording system
and some of our microscopes still use the
plate films.
And we will look at the details when we go
to the laboratory exercise.
And coming to the holders, this is how the
TEM holder will look like.
You can see the details here.
So, you this is a barrel containing specimen
controls.
This is o-ring seal and the, the section which
is shown in the shaded box is under the vacuum
and this is not under the vacuum.
So, you can see that the specimen will somewhere
sit here and then there is a bearing which
will sense the I mean the positions of this
specimens inside the column.
So, this is the principal parts of a side
entry holder that is held in a goniometer
stage.
So, the specimen is held in a goniometer stage
in this orientation wherever we make a side
entry holder.
And you can have different types of holders.
Whether you can have a rotation folder where
the sample is will rotate in this fashion
and, and you have the heating holder where
you can heat the sample locally to look at
the phase transformations and thermal response
of a material can be studied thoroughly.
And similarly a cooling holder, you can have
a quite a bit of a heat treatment can be done.
And then you have a double tilt holder.
That means, it will tilt in this axis as well
as this axis.
So, that is called a double tilt holder.
And typically most of the basic version of
this equipment will have this single tilt
holder which will rotate in this fashion in
this axis, this is an axis in this rotate
way in this fashion.
.
So, I will show in the lab all this holder
show how they look like in I mean these are
all photographs taken from this textbook,
but you can I can also show you in our laboratory
how this holders are being used or typically
you can if you look at the usage rotation
folders they are being used to analyze some
of the orientation effects and then and then
and crystallography symmetry operation which,
which can be very effectively studied through
this holder.
And then as I said for all the phase transformation
studies, it can be used.
Double tilt holder is used especially if you
are interested in defect analysis or when
you want to obtain a very specific contrast
mechanisms are if you are interested in identifying
a very specific location and these kinds of
holders are very useful.
Especially, if you want to do a typical analytical
work, you need a double tilt holder.
For a normal conventional imaging you can
do with the single tilt holder and so on.
So, this is a brief introduction about the
holders.
And it is not that only a small sample size
can be kept and then you can also have this
kind of a big samples more than 3mm specimen.
Just for an information.
And then you can also simultaneously load
the samples in a multi sample holders in a
in a inside the vacuum you can put it in a
this kind of a multi sample holder also you
can put it in the two slot holders and so
on.
(Refer Slide time: 09:30)
So, these are all some of the varieties of
holders which is available with each of the
microscopes.
Now, how to see electrons?
See, you have to remember that, what we are
seeing in an electron microscope is it is
a beam of electrons.
Our eyes cannot see electrons.
We have to resort to the phenomenon of cathodoluminescence
in order to provide an interface between electrons
and our eyes.
So, the cathodoluminescence process converts
the energy of the electrons that is cathode
rays to produce light luminescence.
As a result, any electron display screen emits
light in proportion to the intensity of electrons
falling on it.
The fluorescent screen is coated with a long
delay phosphor.
So, what we have to appreciate here is, be
only past the electron beam and which comes
out of the sample after the electron beam
after comes out of the sample must have interacted
with the specimen accordingly the intensity
profile will change, as I just mentioned in
the yesterday's one animation I showed.
An electron beam passes through a sample the
intensity will vary according to the specimen
interaction and then that effect can be visualized
only when you put it on the fluorescent screen
which will again emitter right depending upon
the, the amplitude of the light which, which
it receives that contrast also will produce.
We will look at the much more detail about
this how the image formation and everything
in much more detail when we discuss the TEM
imaging section.
So, this is only to give you an idea, how
the electrons are electron beams are made
into an image.
And we should know what kV should you use.
So, we have a variety of microscope with different
voltages of operating voltages.
So, you always operate at the maximum available
kV unless there is a definite reason to use
a lower kV.
And most obvious reason is the beam damage.
If you recall, I just showed in the beginning
of the TEM lecture, I showed some of the images
where the specimen get affected by the radiation
damage and I also cautioned you that we should
not just misinterpret those kind of a damages
as a characteristic of material.
So, we have to be very careful with the that
is why the appropriate kV need to be chosen.
if it is a metal or if it is a ceramic if
it is a polymer or biological samples, you
need to choose an appropriate voltage to examine
the samples.
We will talk about it much more detail when
we when we go to the sample preparation section.
So, you can always operate a 300 kV machine
at 100 kV.
The threshold for a beam damage for most metals
is less than 400 kV.
For lighteror beam sensitive material such
as some ceramics and polymers, lower voltage
maybe better.
So, this comes by inexperienced it varies
with samples to sample.
So, one has to really judge this with the
previous experience and so on.
So, why do we always prefer a highest kV?
So, there are some of the reasons.
Because the higher the voltage operating voltage
or accelerating voltage, that gun brightness
is better.
We get the better gun brightness.
And as we all know that from De-Broglie relation
what we have seen yesterday, the wavelength
is wavelength in shortest many when you kV
is higher, the resolution is potentially better,
the cross section for inelastic scattering
smaller, and the heating effect is smaller.
You see, we have to remember that when the
heating of the sample also will affect your,
you know characterization purpose, because
that will that should not induce a new effect
into your material or you should not transform
ever material from the initial stage to some
other stage.
So, the heating is very important.
So, you have to be very careful about the,
the type of kV you choose before you operate
the I mean before you put your sample inside
the TEM column to analyze it.
So, these are some of the general information
about the kV.
Then we will talk about very important aspect
of again a specimen electrode electron beam
interaction that is contamination.
Vacuum can be a source of contamination particularly
residual hydrocarbon from the pump oil which
cracked under the electron beam.
Carbonaceous material then deposits on your
thin specimen making it difficult to do a
sensible high resolution imaging or microanalysis.
So, this we have to be very careful and any
material which is being coated on your sample
and then whatever you will see as a new feature
may not be belong to your specimen at all.
So, we have to be careful about this.
So, that is that is exactly the contamination
is always avoided.
Contamination also occurs through air lock
with the specimen.
It can be minimized by heating the sample
above 100° in a heating or cooling the specimen
to a liquid nitrogen temperature.
Polymers and biological samples can easily
introduce hydrocarbon contaminants as the
out gas in the vacuum.
So, it is sensible to cool the specimen.
When you cool the specimen, it attracts water
vapor which condenses as ice on the surface.
So, here again the some of the basic idea
of contamination is given.
But then you have to take a lot more care
while preparing the sample for a TEM analysis.
These are all general guidelines within the
column.
But even before even putting the sample inside
the TEM column you need to be very extra careful.
which some of the aspects we will discuss
when you prepare when we go to the sample
preparation class.
I will just show you some of the live examples.
Now, we will just go to the some of the basic
you know instrumental details and it is instrumental
operation.
And what you are now seeing is one of the
schematics which shows the operation that
is called a parallel beam operation.
The first one that schematic shown here is
you see that you have the C1 lens C1 cross
over and C2 lens and then it is falling on
the specimen.
So, which has got some the semi aperture angle
is there α.
On the right-hand, head side you see that
the beam is made parallel.
Why parallel?
Because some of your you know diffraction
experiments, we when you do only with the
parallel beam you get will be able to focus
the diffraction spot to the sharpest as possible.
So, parallel beam operation is important in
a menu when you do a diffraction analysis.
So, for that you need to get this condition.
So, this is done by the insertion of another
objective lens, where the which is the front
focal plane of the objective lens.
You have another objective lens there, upper
objective lens.
So, which is being made being made into parallel.
And the same line, you can see another schematic
where it shows the how the convergence angle
control the parallel beam operation.
So, for example you see that effect of C2
aperture on the parallel beam.
A smaller the aperture creates more parallel
beam.
You can see that, smaller the aperture create
more parallel beam.
You can see that α you can have a different
α will give you a different kind of a converging
beam and then what you have to understand
here is the, the nomenclature for the, the
lens is C1 and C2 are pertaining to a particular
system.
You can have condenser lens 1, condenser lens
2 and then objective lens 1, something like
that.
So, do not worry about this designation of
the lenses but it is you have to just see
that what the operation of that particular
lens and what is the effect of the effect
of that particular lens our diaphragm on the
probing electromagnetic radiation.
That is all we are interested.
And then you can see that a focused C2 lines
eliminates a small area of specimen with their
nonparallel beam.
Suppose, if you have the focused lens then
you will have a focused beam here, here the
C2 lens is focused and this is a C2 diaphragm
then you are able to get the focused and nonparallel
beam.
So, just give you an idea inside the column
what kind of beam designation or what kind
of beam conditions under which they the specimens
are examined.
The convergent beam is a probe.
We use such a probe when we want to localize
the signals coming from the specimen.
As in microanalysis or a converging beam also
known as a micro or nano diffraction.
This is again a very important operations
of diffraction.
Suppose, if you are wanted to obtain a diffraction
information from the very localized region
for example a small second phase particle,
you just want a diffraction from only from
that particle, then you need this converging
beam operation.
I will show you the actual diffraction pattern
when we discuss the diffraction in TEM in
much more detail in the following section.
So, how the converging beam operation is done?
This which is being shown as a schematic with
the in the ray diagram here.
You can see that this left hand head shows
how the convergent beam are obtained and here
also you can see that how a small probe and
a large probe size are controlled by this
lens system.
Just give you an idea, so, you should have
the large u by v ratio which promotes the
convergent beam of the probe.
So, this is just shown with the two condenser
lenses.
One is a strong C1 cross over use a small
probe and other is a weak C1 crossover will
give a large probe.
So, how to get the microprobe or a nano probe
is it, because the controlling the C1crossover.
Now, we will just look at some of the alignments
of the aperture.
So, we have just seen what types of apertures
and then if they are not aligned properly
what is the issue.
So, these are the some of the schematic which
shows that.
So, you can see that now a distorted image
of a beam off axis and this is an optic axis.
This is a viewing screen.
And the focused image of the beam on axis
and then when it is distorted you will see
at this kind of a configuration of the beam
on the screen.
So here, if everything is focused here you
can carry the focused beam on the axis and
the defocused beam on the axis.
This is on the viewing screen.
So, these two explains how a beam supposed
to appear like ideally on the fluorescent
screen, when you do an alignment setting.
So, this is just to give you an idea, what
is that alignment is about.
So, if the C2 aperture is misaligned under-focusing
our or over-focusing the C2 lines causes the
image sweep of axis.
So, it could be under focusing our over focusing
from the C2 lens.
If the C2 lens is aligned, the image of the
beam remains circular and expands or contracts
about the optic axis as the lenses under focused
are over focused.
So, it is completely aligned but you can just
under focus and war focus it will just open
and close in a symmetric manner.
So, then we can make sure that the beam is
aligned in the column then we are ready to
go.
So, these operations we will see it in the
laboratory.
And very importantly, how do we identify whether
we are beam is having astigmatism?
So, suppose this is a shape of your beam on
the fluorescent screen.
Then you can simply say that there is some
issue.
So, you have this distorted and under focused
beam and this is a distorted over focused
beam and this is supposed to be a circularly
focused beam should appear like this.
And if it deviates from this circularity and
then it shows a oval shape like this either
it could be under focused beam or it could
be a over focused beam you have to cut it
for the astigmatism.
So, the effect of astigmatism in the illumination
system is to distort the image of the image
beam elliptically as C2 lenses under focused
are over focused.
So, this is one aspect we have to very important
aspect of alignment and this is one of the
major part of the operation.
So, we will now go to the imaging system.
And I will just play some of the schematic
ray diagram which will show what kind of imaging
operation one need to carry out in a TEM.
So, what you are seeing here is, the first
one is where diffraction beam will be formed
the second one is image will be formed.
So, I will you use you see that this is specimen
and there is an objective lens and you have
objective aperture and then SAD aperture and
then you have the other intermediate lens
and then finally it reaches the screen.
So, what you are to appreciate is, when you
want to record a diffraction pattern, what
are you supposed to do.
So, I will play this schematic again.
You just observe.
So, I have the objective lens.
This is an optic axis.
So, you have objective aperture and a SAD
aperture, Suppose, if I want record SAD, then
objective of aperture should be removed from
the optic axis.
That is what shown in the schematic.
Suppose, if you want to record image, your
SAD aperture should be out of the optic axis.
So, you can see that correspondingly how this
you know the intermediate image is being further
demagnetized and then magnified and finally
it reaches the screen.
And you can see that, how the back focal plane
again get projected and then how the image
formation occurs.
So, the two important a physical operation
here.
One is forming a diffraction pattern, one
is forming an image, a basic thing.
We will talk about actual image formation
in another better example.
This is only to show that what kind of physical
operation one need to do when you do an image,
TEm imaging.
So, this is the description of what I just
shown.
To see the diffraction pattern, you have to
adjust the imaging system lenses.
So, that a back focal plane of the objective
lens acts as the object plane for the intermediate
lens.
Then the diffraction pattern is projected
on the viewing screen.
Let us go and see.
So, this is the back focal plane which of
the objective lens will act as an image for
the intermediate lens.
Then the diffraction pattern is projected
on the viewing screen.
If you want to look at an image instead, you
readjust the intermediate lens so that its
object plane is the image plane of the objective
lens.
Then the image is projected under the viewing
screen like what I have just shown here.
So, another important imaging system in a
TEM is producing a bright field or a dark
field.
We have seen that in the optical system itself.
So, you know the basic principle of obtaining
a bright field and dark field.
So, in TEM, it is it is the same thing but
the schematic clearly illustrates that how
the bright field and a dark field imaging
are realized.
So, you have this specimen.
This is an optic axis and then you have the
whatever you get on the screen is given on
the 2D projection here.
So, this is the optic axis and this is also
a zero-order zero- order beam, that is transmitted
beam.
And then you have a diffracted spot around
this.
So, when you want to do a bright field image,
the objective aperture just stays on this
transmitted beam and then you will get an
image.
But, on the other hand, if you are interested
in taking a dark field, then you can see that
your aperture is moved to from the optic axis
of transmitted beam to one of the diffracted
beam.
So, you can do this dark field imaging on
any using any one of this diffracted spots
of your interest.
So, each one will give a different information
from a different set of planes from the specimen.
So, based on that interest you can do a bright
field or dark field and so on.
So, normally this is done through a beam tilt
operation.
Beam is tilted, rather than an objective is
aperture is just brought down to this spot.
A beam tilt operation will make the beam the
diffracted beam come to the center and then
aperture is being put on that then you will
get the dark field image.
So, that is the primary information from this
slide is, you should get an idea, a transmitted
spot is being used to form a bright field
image.
Any one of the diffracted spot is used to
form a dark field image.
So, that is the information from this slide.
Now, we will look at the other important imaging
system in a TEM, called STEM (Scanning Transmission
Electron Microscopy) imaging.
So, you have the go through this schematic,
you have the electron source and then you
have the condenser lens.
Please recall, what we have discussed in a
scanning electron microscopy lecture where
we had the deflection scan coils.
So, similar setup is here which will manipulate
the electron beam according to the scanning
action.
We have very we have gone through the scanning
action in much more detailing the scanning
electron microscopy.
You just recall those aspects.
Similarly, beam will scan through the sample
on the complete surface.
The only difference between the scanning electron
microscopy and this is, here that your specimen
is transparent.
But then the scanning action is the same.
So, you get the signals.
Again, you have the detector and then which
will collect the signal and so on.
Rest all the operations are same, except that
the specimen is transparent here.
So, that is one point.
The right hand side schematic shows that how
exactly the you know the ray diagram looks
like.
So, you have the pivot point of a scanning
system.
front focal plane of an objective lens here
and then you have an upper pole piece of an
objective lens and then you have the convergence
scanning beam which scans on the specimen
like this and then you have the lower pole
piece of objective lens, where you can see
the direct beaming a stationary diffraction
pattern and a diffracted beam in a stationary
diffraction pattern.
So, you can see that this is falling on a
back focal plane of an objective lens.
So, you have the scanning the converging probe
for a stem image formation using two pairs
of scanned coils between the C2 lenses and
the upper of the two-pole piece.
So, this is the exact location of the scan
coils between C2 lens and upper objective
pole piece.
The probe remains a parallel to the optic
axis as it scans.
So, this is a simple way of putting this transmission
as well as scanning system together.
And you have to just you know you can appreciate
that the operations are similar to the scanning
electron microscopy with regard to scanning
action is concerned, but the difference is
this specimen is here at transparent electron
transparent specimen where are in a SEM, you
have only the surface which is being, I mean
you gives out all these signals after the
scanning.
So, that is one point you have to remember.
So here, a principle of forming a scanning
image showing how the same scan coils in the
microscope controls the beam scanned on the
specimen and the beam scan on the CRT.
Thus, lenses are required to form the image.
So, it is similar to scanning electron microscopy,
as I told you before.
So, how the image formation occurs in a scanning
electron microscopy is demonstrated in this.
So, you have already and knowledge on this
so I will skip this.
So, typically, you have the what kind of an
image will see through a STEM image.
This is taken from this text book again.
So, you have the bright field image and this
is a dark field image.
This is called, you know, another dark field
image and this is a typical diffraction pattern.
What you have is, you suppose if you have
the specimen and this is a scanning beam and
you have the bright field I mean image can
be formed from this transmitted one and this
is a diffracted beam.
You have ADF is called annular dark fig that
means, suppose if you have this is the diffraction
pattern a ring pattern.
An annular ring will collect complete the
dark field intensity and completely block
the transmitted intensity, then you get this
kind of very interesting image that is called
annular dark field and then you have the right
field which is formed with this image in this
system.
Please remember, a scanning transmission electron
microscopy is very specifically used for a
chemical analysis, very useful in chemical
analysis and microanalysis are the elemental
mapping across the particular location of
the material feature.
And mostly the probe which is being used in
the STEM is much more smaller as compared
to normal TEM operation.
We will just show you some of the demonstrations
about this also.
So, as an introduction to this course, you
should know what are all the possible techniques,
possible in a transmission electron microscopy.
In that respect only I just brought the brought
the introductory slides like this.
Then we will get into this basic idea when
we actually operate the TEM in the lab.
So now, another important aspect of TEM operation
Is a camera length calibration.
The schematic which shows a basic geometry
geometrical relation between the distance
between the specimen and the screen and then
and what you can do with that camera geometry.
So, let us see that.
This is a specimen; this is an incident beam.
Suppose, this is the back focal plane here
and this is a diffracted beam. this is 2θ
and this distance is ‘L’ and then this
distance is ‘R’.
This is from the transmitted beam to one of
the diffracted beam.
The distance is R. Then the relationship what
we have is Rd is equals to Lλ.
So, this is a very important relation.
We will we will look at that derivation when
we look at the diffraction of the diffraction
phenomena and TEM in much more detail but
this relation is a very important relation
in identifying or indexing the diffraction
pattern.
And the λL is kept constant, always.
That is why it is called it a camera constant.
λL is a camera constant and this requires
some calibration.
Before you do any analysis, your camera length
need to be calibrated with a standard sample
and then it has to be fixed.
So, then only you can use this relation for
any of your diffraction analysis and so on.
We will do a demonstration or we will work
out some examples also
And this is the typical table 1 generate after
the calibration of the camera constant and
I leave it there.
We will use this parameter when we do a diffraction
analysis.
You will appreciate this importance of camera
constant.
I am showing this slide because, when you
when you decide to use this kind of an analysis,
like if you want to use a camera length and
then if then it has to be calibrated.
First of all you have to check, whatever it
is showing in the display in a TEM display,
whatever the camera considered it shows we
have to we should not blindly take it up.
We have to check whether it is calibrated
or not.
Then only you can take that value and use
it for the diffraction analysis later.
So, that for that information only I brought
this.
Now I slowly move on to the other important
aspect in TEM, that is a diffraction in TEM.
Before I get into this in a very exhaustive
topic in TEM, I would like to just recall
some of the principles we discussed about
diffraction in X-ray course or an X-ray diffraction
lectures.
See, the there is no difference in those principles
for whether in terms of you know diffraction
condition, we talked about a reciprocal lattice,
we talked about Ewald sphere concepts and
then all this phenomenon whatever we discussed
they are all going to be the same in this
electron microscopy as well.
And then, another important thing is though
you have to just find out what is the fundamental
difference between the electron diffraction
and an X-ray diffraction.
You yourself will no now, that only the wavelength
is the difference.
Because, with increasing the accelerating
voltage we will be able to control the Lλ.
That is what we have seen in the beginning
of the course.
so through de Broglie relation.
And in X-ray we have a much higher wavelength.
So, that is one basic fundamental difference.
Otherwise all the diffraction phenomena are
same.
So, with that background, you will be able
to appreciate some of the concepts in TEM
also.
So, I will be moving little bit faster in
this lecture I do not spend much time because
the most of the concepts are same.
But I will spend much more time in some of
the important diffraction experiments like
converging beam electron diffraction or you
have the kikuchi pattern and so on which are
very important diffraction experiments in
transmission electron microscope.
I will spend little more time on that.
Otherwise, they are all the same.
So, if you look at the diffraction pattern
in TEM, you have to just ask few questions.
Suppose, if you ask what is it and what can
be learned from it, why do we see it what
determines the scale, what determined the
distance between the spots or the positions
of the lines.
So, something like this if you try to answer
these four or five questions, you will you
will see that surprisingly you will get lot
of information about the specimen characteristics.
So, some of the related questions is the specimen
crystalline?
If it is crystalline then what are the crystallographic
characteristics?
Is the specimen mono crystalline?
If not, what is that grain morphology?
How large are the grains?
What is the grain size distribution?
Etc.
What is the orientation of the specimen or
of individual grains with respect to electron
beam? is more than one phase present in the
specimen?
So, some of these basic questions related
to what I mean I have put some fundamental
questions in the previous slide they are all
related and.
You will be able to get all these informations
is from the diffraction.
So, you have to remember in a transmission
electron microscopy, diffraction is most important.
In fact, the whole microscopy operates with
the I mean operation lies mostly on the principle
of diffraction.
So, you have to give an importance to the
diffraction analysis when you really want
to use this or exploit this TEM.
So, I will I will talk about the importance
and much more when me I mean I actually give
a practical example as well.
First look at the typical is SAD in TEM.
That means, selected area diffraction in TEM.
So, what is shown in this photograph?
So, this is a typical selected area diffraction
taken from this textbook.
Once you have chosen the area from which you
wish to obtain a diffraction pattern, select
the required camera length, that is the magnification
of the diffraction pattern.
So, the camera length is also called you know
where whenever you turn this magnification
knob in a diffraction mode that camera will
I mean the camera length will increase or
decrease.
So, this typical diffraction pattern is taken
at two different camera lengths.
You can see that this is the orientation of
the two patterns are same, but only thing
is you can see that the distance between the
spots are bigger.
That means, this diffraction pattern is taken
it one camera length and this diffraction
pattern is taken at much a little more higher
camera length.
So, that is what it is.
So, you have to choose a camera length of
your interest.
That is what it is shown here.
And then I as I said before we have before,
we do all this and that your camera length
need to be calibrated and then you choose
appropriate camera length to do the experiments.
So, diffraction using small probes.
It is possible to obtain diffraction from
small area.
In conventional selected area diffraction,
and approximately parallel electron beam is
incident on the specimen.
The resulting diffraction pattern from as
periodic sample consists of an array of sharp
spots in the back focal plane of the objective
lens.
So, as Introduced using that converging beam
electron diffraction, I said we will be able
to do a microprobe or a nano probe.
That means you can make the probe into a micro
micron size diameter or a nano size diameter.
We will also see the meaning of what I am
saying now.
What is the meaning of I am seeing a micro
micron-sized a beam on the screen or at nano
meter size screen.
What is the physical meaning of it that?
We will see.
But then we can obtain a diffraction information
using this either a microprobe or a nano probe
from a localized region of our interest.
That is what we are discussing here.
In a conventional SAD, as I just mentioned,
when I talked about converging beam or a parallel
beam, I said that a parallel beam operation
is important to in order to get a sharp diffracted
spot.
So, that is what is shown here in a conventional
SAD, a parallel electron beam is incident
a specimen where you get a very sharp pattern.
Whereas, in a in a small probe, a converging
beam has to be used.
So, we will continue this discussion on the
diffraction and its effectiveness and also
what are the details you are going to get.
It is very powerful a tool or I would say
that the powerful technique to obtain a most
information about the material characteristics.
We will continue in the next class.
Thank you.
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