In this lecture, we learn about electron energy
loss spectroscopy. So, the overall thing
what we can see is electrons energy loss and
spectroscopy.
So, in this particular case we incident a
particular electron been of known energies
of
known kinetic energy and we let it interact
with the material. Depending on how much
energy is being lost from that we obtain a
particular spectrum to comment on the maybe
of the chemical nature or the electronic stake
state of the directive state particular
material.
So, in these particular characterising techniques
EELS we call it electron energy loss
spectroscopy. We allow a particular known
set of electron energies known kinetic energy
of electron we are literally attracted to
the material. We see how what is the overall
loss
energy loss is occurring because of certain
vibration modes or some elastic scattering
to
attend a particular spectrum.
Since the overall term is energy loss it means
that the electrons are interacting in
elastically, so there are certain losses.
So, it is not an elastic interaction, but
more or less
like a inelastic interaction with the material
result, certain scattering. In this particular
case, we allow a known narrow range of kinetic
energies, so in this particular case we
take particular known spectrum. So, we use
a we can utilise such a cylindrical mirror
analyser, so we can allow only a certain range
or narrow range of kinetic energies to pass
through the channel.
Then, once we have that known narrow range
of kinetic range of electrons, we can let
it
interact, we can bombard those electrons on
a samples office. Then, we can measure the
energy loss of a beam via this electron spectra
metre, so once we attain this is a incident
energy of the electron. Later on, after it
has interacted with the material we can attain
a
overall electron spectrum from the interaction
of electron with the sample.
Then, we can analyse that which all processes
have resulted this particular energy loss,
so the known energy of electrons bombard on
a sample then we measure the output
energy. So, the difference in the energy or
the loss of energy is now being analysed from
the processes, which are responsible for that
particular loss. So, then those particular
process can let us they can let us know what
all interactions are recurring and what is
the
characteristics of a particular atom or the
dilative nature of the material which are
causing this particular losses.
From those information we can comment on many
of the structures, it can be
compositional it can be dialectic it can be
ionisation. So, we can comment on those
particular aspects of a particular material
by using electron energy loss spectroscopy.
So, in this particular case we are measuring
the kinetic energy after they have interacted
with a material and it is definitely based
on the inelastic scattering because we are
worried about the loss, the electron energy
loss which is occurring after the interaction.
That interaction of measuring via supplying
and on the narrow range of kinetic energy
of
electron and that energy range is between
1 to 10 electron volt. Then, again we are
analysing those kinetic energy and that will
provide us the energy transfer which is
happening because of certain surface vibrational
modes.
So, depending on that we can attain different
kind of scattering with the material, so in
this particular case what we can that is we
have a sample we apply certain sort of
energies to interact with the sample. Again,
these interactions can be specular or off
specular and then once we get this particular
spectrum particular scattering in certain
cases. So, we have incident energy and this
scattered electron, so what we can get finally,
is scattered electron is equal to e i minus
certain losses, which have occurred h mu.
So, we have the overall energy of the electron
which is coming after interacting with the
sample, so we have sample here with this the
incident so incident energy minus certain
energy which is now been absorbed by the material.
Those are nothing but the energy
loses so that is what we are interested in
this particular term how much energy is being
loss. So, we can define them that how much
energy is now loss because of the inelastic
scattering and that energy is responsible
for the surface vibrational modes and that
can
give us much more information about the overall
loss spectrum.
So, in this particular case, we have energy
loss spectrum which is running between 0 to
5
to 200 electron volts and typically the resolution
is approximately 20 to 30 inverse of
centimetre. In certain special cases, the
resolution can be as better as 8 centimetre
inverse, so that is what we can get from the
EEL spectrum.
Again, the EEL spectrum we see a typical spectrum
which appears more like this the
initial a we have a 0 loss p. Then, it is
followed by a certain valley and we have the
first
p and then we have high loss region so in
this case we can see energy loss along this
side, this is the intensity of the frequency.
So, first what we see is nothing but the 0
loss
p, so if we can come to the 0 loss p, so this
is a 0 loss p and in this case we are achieving
the interaction of electron which are elastically
scattered. So, there is no loss there is no
loss of energy s in this case the overall
electron energies same as that of incident
electron.
So, in this case this is called as a 0 loss
peak because electrons are coming back coming
back after their elastic scattering after
their elastic interaction with the material
can they
result the 0 loss p. So, this is the of this
particular 0 loss p will tell us about the
resolution
of this particular ell spectrum and then later
on we have low loss region which is
consisting, comprising of this particular
region. This case we have energy loss less
than
100 electron volts and this is basically dominate
by the plasma on oscillations. So, once
we have some plasma on plasma on oscillations
or those of interactions which are
occurring because of the polarisation of electrons
in a particular in a particular area.
So, those particular interactions they are
very rapid and those can be captured easily
via
ell spectrum whereas, 0 loss p comprises mostly
the interactions. So, those basically tend
to ate up the material and those energy losses
are very fine less than 0.2, 0.3 electron
volts. So, those can be detected by the EEL
spectrum, but this plasma on oscillations
they can they appear in the low less region
and from that we can detect much more of the
dialectic nature of composition of a particular
material. So, the first p the 0 loss p
provides like the resolution of the EEL spectrum
the low loss region provides one of the
composition or the dialectic nature of a particular
material.
In the high loss region we have greater, than
we can call it greater than 100 electron volts
or greater than 50 electron volts. We can
get a spectrum which tells much about the
bonding or the ionisation structure of a particular
material, so in this case we can see
initial ionisations which is rare because
energy is low. We can see those spectrum very
easily in the high loss region around this
side, so we see high loss region we see a
low
loss region and we see a 0 loss p in the EEL
spectrum. Generally, the resolution
spectrum resolution is generally approximately
one electron volt this particular spectrum.
We cannot really measure which energy losses
when they are lesser than 1 electron volt
such as losses they comprise between 0.2,
0.3 electron volts, so that can be related
detected by the EEL spectrum.
There were a variety of manners in which a
energy can be loss and those all have to be
in
elastic interaction because some emery is
bring absorbed by the material. So, we can
have excitations we can have inter interval
transitions we can also have plasma on
oscillation excitation we can also have initialisations
and at the same time we can also
have some negative ion resonance. So, excitation
means that we are allowing some
lattice vibrational and those tend to heat
up the material and the energy losses are
very
low.
In this particular cases, we can also have
inter and intra vent transitions, so in that
particular case we can have an electron jumping
from one shell to another shell or higher
shell to another shell, then even between
two different items. So, that is nothing but
the
inter and intra vent transitions and within
a particular shell there can be some transition
between the two p 1 2 p 2 3. So, that part
can also happen with an inter intra vent
transition we can also have some plasma on
excitations that means the localisation or
the
polarisation of the electrons in a particular
entity.
Then, that can also lead to the absorption
of energy we can also have some initialisation
we can allow electron to get ionise an atom
to get ionise by release of an electron at
the
same time we can also see certain negative.
In that particular case we can have certain
different transitions which can allow electron
to stay much for much longer time in a
certain molecular orbital.
So, electron will sit state trap for certain
time, so it might appear that energy is being
observed for a certain duration of time. So,
these are certain energy loses which can
occur and those can be an incorporated in
the EEL spectrum, but the on these are the
losses which are basically contributed into
energy loss spectrum. It does not mean that
all
the losses can be easily visualise such as
losses they can be really resolved in the
EEL
spectrum.
Again, there can be variety of scattering
which can occur in this case in the dipole
scattering, we have a scattering to a long
range hundreds of Angstorm via columbic
strong columbic field. So, if electron is
coming interacting with the surface and it
goes of
via certain loose lose in the momentum in
the vertical, in the horizontal direction.
So, in
this particular case the perpendicular momentum
is now conserved, so we can see that
the perpendicular motion is now conserved.
So, we have the similar perpendicular plane,
but the horizontal part is basically being
lost, so horizontal momentum is basically
being lost, so that part we see how a here
or so
much energy is being observed by the electron
in the horizontal direction. So, in this
particular case in the dipole scattering we
can achieve scattering in case of a because
of
columbic field. In this particular case, we
have the retention of the k, momentum in the
perpendicular direction, but there is loss
of momentum in the horizontal direction, it
is
happening because of the columbic field of
the surface.
It can all attain certain out of plane scattering
can also occur and in this particular case
we have kinematic scattering. That is only
for a shorter range in couple of samples,
but
in this particular case when an electron is
basically interacting with the material, it
is
losing both perpendicular as well as the horizontal
momentum. So, in this particular case
we whatever the distances we had momentums,
we had in the horizontal and vertical
direction those are basically being lost in
both the cases. So, we have some extra loss
in
this part as well as some extra loss in the
vertical direction, so we have loss in vertical
direction as well as some loss in the horizontal
direction.
That occurs because of certain strong nuclear
motions or vibrations and that results some
potentials on the surface and because of that
electron gets scattered over wide range of
angles. So, angles are much wider, but they
are at short ranges few, so they go in and
out
of the plane of incidence and we are not able
to retain the electron momentum in both the
direction, which are both parallel in the
perpendicular direction.
So, we had dipole scattering and that we were
able to retain the momentum in the
perpendicular direction, but because of impact
scattering those occurring because of the
nuclear motions or vibrations. In the particular
case, we achieve very wide range of
angles, but for a shorter distances and in
the particular case we destroy both the
perpendicular and the perpendicular in the
parallel motion or the momentum of the
electron.
Again, we can also has so negative and in
that particular case we can reduce some
impurities and those basically induce some
molecular orbits also that can come because
of the edge of it. So, we have incidental
electron, it interacts with the sample, but
since
then there are some, which are present on
the surface of a particular sample, they
generate some extra molecular orbitals.
As a result of that, the electron can stay
trapped out there, electron can stay transiently
trapped on those particular empty high line
molecular orbitals and because of that some
energy is being basically utilised out there.
That vibrational feature intensity strongly
depends on the incident energy or which causes
the resonance, so that is also happening,
we have dipole scattering. We had impact scattering
and we can also have negative
resonance scattering and that particular case
were able to have the electron being
transiently trapped in certain molecular orbitals
for certain duration of time.
Though these are some lost features most of
the interaction is occurring elastically,
so we
get the strongest peak is coming out because
of the elastic interaction and that basically
takes care of the 0 loss peak. So, basically
our overall spectrum of the loss spectrum
or
the overall loss spectrum is generally very
weak because major of the phenomena, they
are occurring elastically. So, that is a reason
our spectrum which is coming out as a loss
spectrum it is generally very weak. So, eventually
our detection system has to be strong
enough to be able to detect that weak spectrum
and comment on the overall nature or the
structure of the composition of the sample,
so that is what the overall field of EEL
spectroscopy is.
Depending on what kind of an information we
have, so we can see that we can have
collective oscillations, they can come out
either from plasmons or they can come out
of
phonons, so plasmons are nothing but the collective
oscillations of free electrons. So, in
this particular case we have polarisation
of electrons and electrons are generally very
free
entities. So, they tend to we dispersed like
in a metal matrix, so we have it is basically
predominated metals when we have high density
of electrons which are floating without
any bound to a particular atom.
So, they are flowing freely on a metal surface
and they cause basically the common most
common inelastic interactions, but the problem
with them is they die out very quickly in
10 to the power minus 15 seconds. They are
also localise to less than in a metre and
they
basically are scattered to less than 0.1and
there energy is approximately 5 to 25 electron
volt. So, once they have energy much greater
than 1 electron volt, these particular signals
can be easily captured by the EEL spectrum.
So, we can see that the plasmon loss is they
rise because of the localise polarisation
of the electrons because they arise from the
collective oscillations of electrons and a
very pre dominate in the metals.
So, because they have very high electron density
and because their energy loss are in the
order of 5 to 25 electron volt, they can be
easily captured by the EEL spectrum. On the
other hand, we can also have some phonon oscillations
those are the collective as
oscillations of atoms or they also arise because
of the lattice vibrations and because of
that tend to generate heat. They tend to ate
up the specimen t and they can also be
generated by certain other inelastic processes
such as energy or the x ray energy and
those can also cause the lattice vibrations
or the oscillations of the atoms. The problem
with them is they generally tend to have very
low energy loss which is of the order of
less than 0.1 electron volt.
Since that it is less than the resolution
limit of EELS which is approximately one electron
volt we are not able to detect all these losses
in the EEL spectrum and the one more thing
is that the phonon cancan get scattered to
a very large angles to the order of 1 to 15.
They
can generally give a very high order defuse
background in the case of phonon, so the
collector oscillations they are limited to
plasmons and phonons and plasmons. They are
collective oscillations of electrons, whereas
phonons tend to with the collective
oscillations of atoms and depending on what
kind of energy losses they have.
We can get a certain spectrum which can basically
grab the particular picture or the
density of electrons can be easily captured
by the plasmon peak or the low loss energy
low loss regime of the EEL spectrum.
So, overall e can see that that our phonon
excitations they are limited to 0.2 electron
volt
and they give out us a very diffuse background
and they basically come out in the 0 loss
peak. Since the overall energy loss is very
low, they can’t get resolved and because
of
that, it can create certain problem in the
resolution of the particular EEL spectrum,
at the
same time they tend to heat up the specimen
t. So, anyway those particular things appear
in the 0 loss peak and they tend to heat up
the specimen t, then later on we can also
have
some intravent transitions and that can provide
us a signature.
So, in the low loss region we can get some
signals, which are basically between the 5
to
25 electron volt and those transitions can
be easily captured, which can provide us a
structure of a particular material and plasmon
excitations. They can be limited either as
a
surface plasmons or as a bulk plasmons and
the energy again comes in the low loss
region and we can in the surface plasmons.
We have basically the transfers waves which
can provide, they are basically half the energy
of the bulk plasmons and the bulk
plasmon.
We generally have interaction with the interaction
as the longitudinal way and this
particular spectrum can be easily attain in
the low loss region and later on once we have
much higher loss. So, that is basically been
created by the initialisation and that generally
is greater than 100 electron volt and from
that we can always attain a elemental
information because we know what are the transitions
or the ionisations which are
responsible for causing this much loss of
the energy.
So, overall we can see that we have a zone
distributed in three regimes, we have 0 loss
peak 0 loss peak, basically it defines the
overall energy resolution, higher the basically
energy loss, the poor is a resolution because
the broader. The particular peak is the poor
the resolution will be and higher the k v,
the poor will be the resolution and again
this
thing is arising. This thing is arising because
of the format scattering which is to the
order of a few and as we saw that the overall
broadening is because of the phonons
which basically give us give out a diffused
pattern.
Again, this corresponds to more like the 0
spot of the affected pattern and again the
blurred peak which are to the order of 25,
they really enter the spectrometer. So, we
have
this much particular resolution which is now
been the containing factor and this also
includes energy loss of 0.3 electron volts
which comes out from the phonon interaction.
So, though it is include into phonon losses,
it is not able to resolve those phonon losses,
so we can see that 0 loss peak. It is responsible
for the overall defending, the overall
resolution or the energy resolution of the
spectrometer. Later, we have plasmon peak
and
then we have a high energy loss at peak, so
again this is again the kind of a value before
the plasmon peak and the information, what
you can get from the plasmon peak is
basically the overall dielectric nature of
a material. So, we can get the overall dielectric
nature of particular material and also we
can find the composition, whereas the high
loss
regime can provide us much more information
on the overall ionisation structure or the
bonding.
Those ionisation are occurring in this particular
high loss regime and then again we have
energy loss which is near the edge structure
this is the edge and we have energy loss near
the edge structure. This regime is the extended
energy loss structure, so we have either
fine structure or nearest structure and again
is it is in the high loss region and from
this
we can get much more of the bonding effects
how is how is the overall bonding which is
happening.
We can also attain overall the affection of
effect which can occur from the atoms
surrounding the ionised atom, so we can get
much more information as either energy loss
structure or as energy loss near the structure.
So, we have a regime which we which is
nothing but the 0 loss p and then we have
slow loss p which is limited to 5200 electron
volt and then we have high energy loss regime
which is basically that. Either, it can be
fine structure or the near edge structure
and from that we can get much more bonding
of
the ionisation effects.
So, again there are different parts of the
spectrum, so again coming to the initial part,
we
already saw that this is nothing but the 0
loss p. So, we have incident beam energy it
interacting with the material and it is getting
elastically interacted and that energy is
being captured back as a 0 loss peak because
the incident electron they have not lost any
energy while interacting with the specimen
t. So, the result of the 0 loss peak and later
on
they have plasmon peak in this case we are
observing plasmon losses those are because
of the polarization of the electrons and because
of that, it is losing certain energy via
certain oscillations.
We can we have one regime of value before
the plasmon p and then again the peak the
plasmon p. So, in this case we tend to see
this particular p because of the free electron
density and that can tell us more about the
dialectic constraint of the particular specimen
t as well as the composition of specimen t.
So, that part we can attain from the plasmon
peak of the low loss regime that we can comment
because that distribution of electrons.
The polarisation of electrons will tell about
the dilated nature of the material how easily
the electrons can flow and how easily they
can reduce the energy loss. So, that part
can
be easily obtained and again we can also form
that we can also attain the overall
composition of the specimen t from the free
electron density.
Second, from the high energy loss regime,
we have near edge structure and the fine
structure which is the accidental region and
it can arise as more than the critical energy
for the ionisation is imparted to type core
electron. So, from that we can attain what
is
the overall ionisation which is happening
out there and this is the excess energy which
is
approximately few electron volts. We can also
achieve attain some plural elastic
scattering and that can give us the bonding
structure between the two items and if the
excess energy is greater and that can be correlated
to the single scattering event.
That can provide us a local atomic arrangement,
so we can achieve the bonding between
the atoms or what is happening in terms of
the arrangement of the local atoms. So, we
can get overall structure either the bonding
of ionisation item how the ionised item is
coordinate the other items from the overall
arrangement. That can also provide us the
density of straits of solid as well as the
radius distribution function for all these
entities,
so that part we can attain from the extended
regimes of the which is near edge and the
extra extended fine structure. So, that part
that information we can get from the high
energy loss spectrum either from the energy
loss near edge structure which is ELNES or
from the extended energy loss EXELFS. So,
we can attain that much information from
out here, so later on we can also see that
the overall information what we can get from
here is if we get an intensity.
So, we are getting certain intensity out here
and then in this case we have energy loss
which is e V, so we have energy, so we are
having this, which is coming out exactly at
at
0. Then, we have a very fine plasmon losses
and that is it and then we have high loss
period, so if you can we strait basically
extending it or increasing magnifying it.
So, this
is nothing but 0 peak 0 loss peak and then
we can go back and see this particular regime,
so this is nothing but the plasmon peak of
a strait magnifying, then we can we can see
that.
We are seeing certain peaks, so at certain
higher peaks we can see that there is some
information which can be available certain
information which can be available and these
regimes occur basically at around 100 electron
volts. So, this is the low loss region and
we can start seeing something at much at the
high loss regime high loss again this is high
loss. So, we can start magnifying it, so in
this particular case we are seeing 0 loss
peak
those are nothing but arising from the elastic
scattering and that can tell us more about
the resolution part of this particular EEL
spectrum.
Then, once we once we are going and seeing
the plasmon, we can tell much more about
the dialectic nature of the composition and
as we see here we can have some certain
spectrums, so in this case it can be silicon
L shell. So, we can get certain information
about the bonding that what kind of energy
irresponsible in creating this particular
ionisation it can also be arising from some
other materials say k of carbon S. This
particular part tells us two things first
of all what overall bonding is because this
is
nothing but ionisation energy. So, that is
telling directly about the bonding and then
it
can also tell us about the oscillation strait
whether how much energy is base basically
being provided here.
So, from that we can also get the oscillation
strait and we can also know what the overall
concentration because depending on the height
of this particular peak. We can know how
much carbons are really interacting to give
out this particular intensity or giving out
this
particular regime and we can also see that
there can be certain secondary secondary
basically peaks out here. So, we can further
magnify it, so we can see the extension of
this one, so if this can be 50 x, this can
be 500 x and this can again be 1000 x. so,
later on
we can all again see some more peaks which
can belong to this. So, from one spectrum
we can keep magnifying it, so the blue one
what we saw earlier is the initial spectrum.
Then, we magnify it by 50 times and then we
see certain spectrum that tells much more
information about whether there is some silicon
in it. It will tell us the bonding or the
oscillation strait we magnify it further to
say 5 to 500 x and then what we get we can
see
some presence of some carbon. That will tell
us about the overall concentration carbon
we can magnify it further in what to see is
some presence of some oxygen. So, we can
get couple of information from here the bonding
the state and the concentration even the
electronic structure of this particular entities.
Those are what we can get very easily from
the EEL spectrum, so that is what we can see
in this particular case.
Further, we can also do some EELS microanalysis
and in this particular case, we can’t
even detect a single atom of theorem even
on a carbon film. So, if we see we can get
a
particular spectrum, we can get a particular
spectrum and in the high in the in the low
loss regime we can see some presence of some
theorem if I had a particular film. So, I
can get some signals which are basically for
carbon this peak is coming from a theorem
with the cluster of theorem, but if I only
have one single atom of theorem, this is the
energy loss the intensity. I can also compare
it with a say if I had certain peaks of carbon
and then if I will allow only one theorem
atom to basically get deposit get absorbed
on
the surface to see some bump out here and
with the similar kind of a carbon peak.
So, I can even did back even when I have a
single theorem atom which is now absorbed
on the carbon surface I still will be able
to detect that particular part. So, that tells
how
sensitive that EEL spectrum that I can detect
even a single theorem atom, so I can detect
that because the theorem atom if it forms
a cluster I can still see the similar peak
at the at
the similar energy loss Regime. That might
be at either at lower energy or maybe less
than 100 electron volt, so that part I can
see from the EELS micro analysis and I can
detect even a single atom of theorem on a
carbon film. So, that is the overall sensitivity
of the EEL spectrum, so in this particular
case I have carbon, I had a carbon film.
So, I do peaks which are arising, because
of carbon and those are generally in the height
of regime, so they can tell me about the overall
excitation nature or the bonding nature of
the electron and in the ionisation strait.
So, I can get much more information of what
are
the transition which are occurring in the
carbon alone, so depending on what kind of
carbon it is I can the pie interactions or
the sigma interactions which are occurring
for the
electrons and for the theorem. It is now absorbed
on the carbon surface, so I am getting a
much bulk or the peak on the carbon film and
that is because of the cluster of theorem
atoms, but when I have single theorem atoms
sitting on the surface of carbon as I can
still detect that, so that is the overall
sensitivity of the EEL spectrum.
So, again in certain cases, we have diamond
graphite and fullering and again they only
have carbon in it nothing else. So, when considering
three systems diamond graphite and
fullering and they all consist because basically
carbon, but they have absorption peak of
around 284 electron volt in the EELS in the
EEL spectrum which correspond to the
existence of carbon atom. So, we see two 84,
we have certain regime carbon which is
present, so we do see that the carbon is present
out there and from the fine structure of
the absorption peak the difference in the
bonding state in the local electronic state
can be
detected.
So, you can find the bonding and the local
electronics strait from the fine structure
of the
EEL spectrum and again if you go further we
can see for the graphite we have this
particular bonding which is 1 s as or the
pi bonding 1 s to pi bonding. We have certain
energy absorption, so we are able to see this
particular peak in the graphite and the
similar p we can also see in the c 60, but
that peak is totally absent in the diamond
structure. So, we can see that because the
diamond we s p 3, so we don’t have a
transition which is which can really see 1
s to going to the empty pi bond. So, we can
see
that carbon electron from 1 s it is not able
to bond to EEL a pi bonding orbital in case
of
diamond.
So, that can be very nicely being that is
very nicely being observed out here it is
sharp
peak at the absorption h which corresponds
to the excitation of the carbon electron.
The
1 s electron to the empty orbital that is
present only in the graphite as well as in
the c 60
because they both have s p 2 type. So, they
still have 1orbital, which is empty, so there
can be a transition from the s shell or the
1 s shell to the p bond and that particular
energy regime is following in the regime out
here. From this, we can say that diamond
we have and there is no empty pie electron
basically which can come out here, so there
is
no 1 s electron which can jump to the empty
pi electron.
That is the reason we have this particular
energy level vacant out there, so that is
a kind
of information. We can get that from a difference
between the graphite c 60 and a
diamond where everything is now composed of
carbon only and that thing is now being
captured from the EEL spectrum.
So, in the EEL spectrum we can see that if
you have if you have large number of
particular electrons at say 284 electron volt,
what we saw? So, we have overall spectrum
and then from 280, we are seeing very large
number of spectrum, this thing is that 284
electron volt. So, from this we directly know
that that we have energy loss happening
much more at the 284 and it means that there
entity some ionisation which is happening
at this particular level only because some
particular atom is present. That is leading
to
this particular energy loss, so this happens
to be electron energy loss, which equals to
the
ionisation of carbon from k shell.
So, if we k shell electron present in the
carbon atom and that equals to the energy
of that
particular electron equals to 284 eV. So,
directly means that if you have loss which
is
happening at 2854 electron volt, so we have
energy loss out here in eV and we have
intensity out here. So, once we have once
we see a high intensity exactly 284 electron
volt we can directly correlate it to the ionisation
of carbon from the k shell. So, that
directly tells us that we have some atom present
out here and that leading to the
ionisation of this particular atom and incident
beam is somehow interactive of this
particular atom and it is ionising the carbon.
So, that is the first information what we
can get from here that is equal to the energy
which is required to remove and electron from
the k shell of the carbon. So, this tells
we
have presence of carbon and the intensity
of this also tells us what is approximate
composition or the approximate content of
this particular carbon because the more the
carbon the higher the intensity of this peak
at 184 electron volt. So, we can also say
that
there is a significant amount of carbon which
is present the material and from that from
the interaction that we saw if we have certain
bonding or a bonding present, so from s
shell to the p of from the pi shell if you
can see, we see that particular p.
It means that type of a bonding can be present,
if not then it means it is not there, so we
can also find what is the type of atom its
overall, its overall amount and number of
atoms
of each type. So, in this case we have certain,
which is happening, so we can know
whether which have s p 2 or s p 3, which is
present in a particular case and also we can
find the scattering angles. So, depending
on that, we can also find the overall scattering
angles and that can provide us much more of
dispersion relations for a particular atom.
So, from this overall information from the
EEL spectrum first of all we can identify
where the particular peak is getting where
we are getting the exact p and that will tell
us
how much energy loss is occurring.
From that, we can correlate it to the ionisation
of a particular entity say carbon in this
case they will tell us the amount of carbon
type of bonding which is happening the
overall scattering angle from the dispersion
for a for a particular entity. So, all this
information we can get easily from a EEL spectrum
and how do we get that is we
initially we have electron source. First of
all, you have providing a narrow range of
kinetic energy, so we have narrow range of
kinetic energy and that thing is being
achieved from a cylindrical mirror analyser
or we have certain two cylinders out there
and we apply certain bias to it.
So, we have energy less than a naught it gets
basically gets interrupt out there if I have
e
greater than e naught then it gets interrupt
out here, so what we can get out is a narrow
range of energy which has limited e naught.
That particular energy of known e naught
value will now interact with the sample and
once it interacts with the sample after the
interaction how much energy is being lost
by the electron that thing is again sent back
to
the analyser. We can again do the same operation
and we can select what all energies are
coming out.
So, by putting the biased at a basis to the
particular analyser, we can somehow detect
the
overall spectrum from a detector. So, we can
get overall spectrum of what is overall
energy loss which is happening because once
you know the incident energy minus the
energy which is being observed. Later on,
that is nothing but equal to the energy loss
which is occurring for the foe a particular
electron, so that is what we are getting from
the overall spectrum that we are sending.
Second energy we have electron source we are
sending it through a mono chromator to
get a particular set of non narrow range of
kinetic energy for electron. Those are
interacting with the sample and upon interaction
with the sample it is now sent to an
analyser. So, those will separate out the
energies once we are able to separate out
the
energies we know the incident energy we know
the final energy as the subtraction of that
e i minus e s.
This will tell us how much energy loss is
now occurring and that energy loss is now
being an overall structure of a particular
material or even form for an even. We can
also
find out the overall dialectic nature or the
ionisation strait or the bonding strait for
a
particular material and that thing we are
getting from this EELS spectrum.
Again, the advantages of the EEL spectator,
it basically can be applied materials which
are normally unstable under electron beam,
because in this particular case our energies
are very low. So, because our energies are
very low we can also apply it to the materials
which are basically unstable under the electron
beam and again in this particular case we
can have multiple scattering occurring because
we have a dipole scattering. We can also
have a specular scattering we have impact
scattering dipole scattering and we can also
have and ion resonance. So, all those mechanisms
have behave very differently in this in
one case, we can retain the momentum in the
perpendicular strait in as second case.
The momentum was not at all conserved both
parallel as well as perpendicular and all
those at basically scattering mechanisms are
allowed and that will allow us to observe
the overall energy loss. So, we can observe
all the modes which are parallel and
perpendicular to the surface, so that can
reveal much more information at the same time
we can observe modes between 0 to 4000 inverse
of centimetre. Also, we can attain
spectrums which are which have a lower energy
or the lower frequency modes we can
also attain the spectrum for that and that
basically we can also observe the molecular
surface. So, that part gives us the information
much more about the what is happening at
the surface level or the low energy level
low energy losses also we can attain.
We can perform vibrational electronic loss
spectroscopy, we can also induce an prove
current induce interactions with the variable
incident energy. This case we don’t need
any background subtraction because we know
what the incident beam is and what is the
beam, which is coming out. So, we don’t
need to any background subtraction and it
is
very common technique for the surface and
bulk phonon measurements because that will
create the initial broadening of the 0 loss
peak. So, that part we can tell very nicely
what
is the overall surface and bulk phonon interaction
which is happening with the material.
In this particular case we can find the atomic
composition, so we can also attain what is
atomic composition which is out there what
is the overall chemical bonding which is
happening in the mutual h because from the
intensity of a particular intensity. If a
particular EELS peak, we can find what is
the overall how much is the overall presence
of a particular entity, which is causing this
particulate increase in the peak intensity.
So,
that can tell us about the overall composition
and it can tell us about the overall bonding
nature because depending on where the particular
peak is appearing.
So, like in the case of carbon from 1 s to
pi shell we saw that peak was operating in
284
electron volt, which is not present we can
tell much about the bonding nature of a
bonding nature of carbon itself. So, we know
whether its s p 2 or s p 3 and again it can
tell us about the electron profile. So, that
part can also be obtained from the ionisation
part, we can also get much more of surface
properties because we saw even when a
single theorem atom is present on a carbon
surface. It can be basically detected, so
this
particular instrument is highly sensitive,
so we can much more information even from
the EEL spectrum.
Again, we can find the elements specific distribution
function as we know as we have
seen earlier from the distribution to the
distribution of the secular scattering we
can find
the correlation between the radiant profile.
So, that can also provide us much more
information about how the specific instance
is now being distributed. So, that part we
can also obtain in this particular EEL spectrum,
so overall advantage is that it is basically
we can find atomic composition, we can also
find the chemical bonding, we can also find
the variants and conduction profiles, we can
get surface properties.
We can also get some elements specific properties
and it can also give us the modes
which are lower frequency modeless than four
hundred centimetre inverse and we can
perform many vibration electronic loss spectroscopy.
We can also find some phonon
measurements, we can also do some phonon measurements
and also we have no need to
do any background corrections for in this
particular spectrum. So, these are certain
advantages of the EELS spectrum, what we can
get, we can get very fine detailed
information.
There are couple of challenges out here that
it is a poor resolution the best it can achieve
as approximately 6 to 8 centimetre. Since
we are very low energy electron which are
utilised in this particular case we can work
only in the ultra high vacuum or to the order
ten to power minus 10 to 12. So, that is overall
regime, we need to work in because the
electrons need to travel very large distances.
So, they can be a detector, so we need to
perform all these operations under ultra high
vacuum, so that is the most problem with it
and that makes it very expensive. It is very
complex because we need to generate a first
of all a monochromatic beam and then let it
pass through analyser and then again get it
detected.
So, again it is very complex and it requires
a delicate instrumentation because we also
need to detect what is the beam energy and
what is the energy we are getting after it
interactive with the sample. At the same time,
electron should be maintained well within
the ultra high vacuum so that makes it very
delicate, it requires very instrumentation
and
also we detect that some spectral information
in terms of their dispersion or the diffused
spectrum what we get?
So, that tells more about the phonon interactions
and that makes the instrumentation
much delicate and thus attending the spectrum
is little slower 15 to 60 minutes per
spectrum. Again, it can also result some surface
charging because we are dealing with
the flow of electrons on the sample surface
some plasmon oscillations. So, it can also
lead to surface charging so generally conducting
samples are much more preferred in this
particular case. So, again it also has a difficult
theory because what all ionisation are
occurring how the loses are occurring they
require exact identification of how those
how
those processes are leading to the loss and
energy.
That makes the theory little difficult to
grasp, so the overall advantage include that
it
worked in vacuum, it requires very sophisticated
instrumentation it is very complex and
its very expensive and it is a little slower
15 to 60 minutes. It is a poor resolution
in
comparison to the reflection spectroscopy,
so that makes it much more poorer resolution,
but in spite that it is still very appealing.
In comparison to the EDX, the EELS we have,
we can just basically have a correlation
between the EELS and the EDX. That means like
an EDX, we have sensitivity for higher
atomic number elements whereas, for the EELS
we have high detection efficiency for the
low atomic number elements.
So, that is one part out here and in EDX,
we can get only elemental information, but
in
this case EELS we can get elemental we can
get chemical as well as we can get the
dielectric information. So, in that part we
can get the composition, we can also get the
ionisation strait and those all information
we can get easily from the EELS. In the EDX
we have energy resolution limited to 100 electron
volts, whereas in this particular case
approximately 1 electron volt. So, in this
particular case energy resolution is much
higher
as compared to EDX and we can also get generally
tends to give less number of peak
over lapse it can also provide us very fine
structure for the ionisation edge ionisation
edge.
So, this part also we can get from the EELS
and in case of EDX, we have an inefficient
similar collection and that makes it very
time consuming that elemental mapping
generate which is very time consuming in this
particular case. In EELS, we have very
high efficiently very high or efficient signal
collection because it is highly sensitive
and
it can detect EDX a single theorem atom which
is which is sitting on a carbon film and
that makes it very, very efficient in terms
of mapping. So, it can provide efficient
elemental mapping on the particular surface
at the same times since it is very efficient,
it
becomes very fast technique its fast technique
nut.
It requires very complex theory though EDX
is little slow, it was much more simpler and
we can write a spectrum very quick, but again
this is a slow process, slow technique, but
it can have very simpler processing is required
in the EDX. So, that is the overall thing
about EELS that we can have, known electron
energy and we are letting it interact with
the material and from the detection of the
loss which is occurred upon interaction of
electron with the material and from that energy.
We can distinguish the overall spectrum
either at 0 loss low loss and the high loss
in the 0 loss, we have mostly the elastic
scattering or that tells us most about the
phonon loses which are occurring in the
material.
Then, later on we have the plasmon losses
and then plasmon losses basically are the
main thing which we can have more about the
overall composition of a information
compositional order dialectic nature of the
materials. In the high loss regime, we can
see
the nearest structure or the fine structure
which is near the extended which is called
the
extended energy loss fine structure.
So, we can get some information out from there
which tells us more about the ionisation
strait or what is the ionisation of a particular
material and also about the bonding which
is predominant in that particular location.
All these information we can get from the
high
loss regime in the EEL spectrum and as we
see it is a very sensitive technique and it
can
detect a very fine edge of it which are there
on the surface of a particular entity or a
particular surface. This can serve as a very
fine tool in terms of resolution in terms
of
resolving any element, which are there on
the sample surface, so basically with that
I will
end my lecture here.
Thank you.
