 In this lecture, we will learn about transmission electron microscopy as a term suggests that we are utilizing transmission mode of electrons to find out the features or see inside the material. So, in this particular case, we want the material to be transparent to electrons, so that electrons can really get transmitted through the material and provides some information about the material itself. And the way the electrons interact with the material is like if we have a specimen around here, we can get some electrons which are inelastically scattered and we can get them either as secondary electrons, we can also get them as ojo electrons, some things can also get elastically scattered back after their interaction with the material and those we can get them as a baxter electron, once we apply a some incident electron beam. So, once we are applying electron beam, I can get some signal which are basically coming back either as secondary electrons or as ojo electrons or also as baxter electrons, they can also interact with the material to produce something called x-rays. So, we can see that these are all these are all everything is happening, when the electrons can be detected or those signals can be detected back above the sample regime, they are not allowing the electrons to pass through the material, but if we have a material transparent enough, if we can attain a specimen which is thin enough and through which we can attain transparency for the electrons to pass through the material, what we can see is, we can get the incident electron beam, we can let it pass through, the energy should be high enough, so that it comes out as a direct beam or it can also get inelastically scattered. So, we can also detect the inelastically scattered electrons, but we do not really need them, why because once we are supplying certain kind of energy to the to the to the material and if the electron is undergoing some inelastic losses, so we do not know what is the input, though we know now what the output is, we will not know what the input is, the input energy of the electrons. So, we will not know their wavelength, we will not know their overall functionality of how they are interacting with the material, but we are finally getting some signal. In some cases, we can also tap those particular signals and we can it might also help in the analysis of the overall material, the overall the structure of the material or to track some sort of a crystallographic directions, but again we can get much more information from a material, if we are if we are if the electrons are getting elastically scattered. So, if I let my incident electron beam to interact with the material and I let it elastically interact with the material and I basically detect what is what are the elastically scattered electrons. So, I now I know what is my input energy, I know how they will interact with the material to give me a final elastic interaction after their interaction with the material elastically. So, I know what is my output, I know what is my input, I can get much more information from the material after this elastic interaction has occurred with the material. So, this electron interactions they can be very complicated in nature, starting from secondary electrons, back side electrons, oj electrons, they can also start inducing some producing the production of some x rays and that is actually everything is going back to the above the sample, but if I there all the signals are not really getting transmitted through the material, but if I let the electron beam be strong enough. So, that I can get some signals which are letting which are basically being transmitted through the material, I can get them as either direct beam, elastically scattered electron or also as elastically scattered electron and mostly I utilize this as my overall feature of analyzing a particular material, though I can also find some information from the elastically scattered electrons as well. So, this basically comes under the transmission electron electron microscopy, first of all what is the difference between my T M and my X R D, T M is a transmission electron microscopy, my X R D is the X ray diffraction. So, generally we see that for a micron size particle, I always get some diffraction peak. So, in this particular case I have my 2 theta, in this particular case I have my intensity. So, for a nano crystalline for a micro crystalline material which has grain size greater than 1 micron, I will see some peak which is associated with a particular 2 theta value. So, I am getting certain intensity of the from the crystal after it has diffracted. So, I can get some I am getting some information, but as soon as the material start becoming much more nano crystalline or some micron size, then what happens that my X R D is not able to detect whether the whether I have some crystallinity into the material, it starts showing broadening of the peak. Because as soon as my grains are becoming finer and finer, I start getting broadening of the peak. So, the device error crystallite size can be also calculated from the broadening of the peak itself. So, but here itself I am not I do not know whether the broadening is either because I have a amorphous material or the crystals have become nano crystalline in nature. So, that complexity can be easily analyzed by a T e m and in T e m or the transcription electron microscopy we can image and analyze all these nano crystals. So, what I can get is from electron diffraction is I can get a I can prove a very small area that can be even a nano crystal and I can obtain a electron diffraction pattern. Because now my beam is much more refined I am letting the electrons interact with the material instead of an X ray. Then X ray will interact with the cloud of a electron or an atom as a whole, but my electron is so my electron is so sensitive that once I am sending an electron to interact with the material it gets affected even by it gets affected even by a single electron or the positive charge which is there in the nucleus. So, I can see that I can get a very strong signal if I let my electron interact with the material and now I can point it to a single nano crystal and that is what will give me out its diffraction pattern. So, in X I D overall many, many grains contribute to the overall diffraction peak, but in my electron diffraction I can let a small beam of electrons to interact with the particular nano grain and I can get a diffraction pattern out of it. So, that is the advantage of my transcription electron microscopy over the X ray diffraction. So, what is so beautiful about this transcription electron microscopy is that electron interaction with the material is much more stronger. So, it is approximately 10 to power 6 to 10 to power 7 times stronger than those of X rays. So, eventually my diffracted electron will have a very high intensity as one more part we can see out here is the evolved sphere which basically provides me the diffraction pattern the radius of the evolved sphere is given by 1 by lambda. So, coming back to it the wavelength in a T m it is approximately 2 pico meters whereas, in X rays it is approximately 1 angstrom or 100 pico meters. So, in this case I have 0.02 angstroms and here I have around 1 angstrom. So, I can see that the wavelength part it is much, much higher in X rays, but in T m I have my wavelengths are very, very smaller. So, eventually my evolved sphere which is being forming which is being formed. So, my 1 by lambda is very, very less. So, I get a much bigger radius for the evolved sphere. So, that makes evolved sphere much more flatter. So, instead of touching only few points now once my evolved sphere is becoming much more flatter I can see that more number of points now start interacting with the evolved sphere and the producer diffraction spot. So, coming back to it if we can orient a particular crystal for achieving a diffraction pattern I can obtain all my diffractions within 0 to 1 degree. Whereas, in for X ray diffraction I have to rotate my crystal from up from 0 to 180 degrees to get a particular diffraction because we know that 2 d sin theta is equal to n lambda. So, as soon as I start reducing my lambda I can I have for a particular inter planar spacing. Obviously, my theta is also getting increase with increase in the lambda. So, once I reduce my lambda to very large extent my theta also will get diffracted the diffracted spots will be very, very near or they will be within a range of few 0 to 1 degree. So, that tells me that I can get all my diffraction pattern within a particular tilt of a particular tilt of particular plane which is only 0 to 1 degree along the beam that much parallel to the beam. So, if I am so my beam has to be approximately parallel because it is only 0 to 1 degree. So, my beam can be much parallel to the particular oriented crystal and still it can produce a diffraction spot it has to be within 0 to 1 degree and that also will give me a particular diffraction pattern. And one more thing about it here is that since the intensities are very, very strong because electron will interact very strongly with the particular matter it is 10 to power 6 to 10 to power 7 times stronger. So, what I have to do I have my exposure times automatically reduces to only a few seconds whereas, for taking an X R D spectrum I spend 40 minutes 45 minutes an hour or much more than that to collect the overall spectrum of how my diffraction has really occurred. And also I limited to a certain range may be say 0 to 180 degree or 20 to 90 degree. So, I have to limit my diffraction angle to theta value and then still it takes me couple of hours. Whereas, diffraction through T M through electron that is much more rapid I have to spend only 2 or 3 seconds to attain a spectrum. And so, since it is very, very rapid I can see my electron diffraction pattern I can directly view it on a fluorescent screen or I can also collect it on a particular detector. So, since my theta values are very, very narrow 0 to 1 degree I can orient my crystal along the beam direction and just by tilting it marginally I can also get a electron diffraction pattern. So, that is the beauty of it I can get a particular image I can again orient my crystal only within a couple of degrees and I can still get a diffraction pattern. And so, I can get from diffraction pattern from a very small crystals also can be obtained because my beam is very, very sharp very, very intense. So, I can focus it to a very localized location or very something called nano crystals I can focus my beam into that those nano crystals and I can still get a diffraction pattern by a particularly allowing the beam to a particular diffraction aperture. So, I can direct my beam through a diffraction aperture and I can still get a diffraction pattern from very, very fine crystals. As we can as we know that electron cloud is scattered by the positive potential which is there in the electron cloud whereas, x ray they interact with the whole of electron cloud. So, this is these are the overall differences once we go from x ray to T M that x ray will interact with the major part of the material whereas, electron beam is much more sharper much more intense. So, it will it will provides a very drastic or very very high intensity information and instead of focusing it to a very large area I can focus it to a very fine area and I can get diffraction pattern even from a very fine grain or which can be a very fine crystal or a nano crystal. I can get my diffraction pattern within few seconds I can view it on a directly on a screen as well because it is so rapid and I can also achieve all the diffraction within a tilt of 0 to 1 degree. So, those are the advantages of T M in comparison to that of x ray diffraction and moreover as we saw here that the evolved sphere is so actually so flat that many of the points they coincide with the evolved sphere the reciprocal point reciprocal lattice point they coincide with my evolved sphere to give me a diffraction pattern and as we see here that evolved sphere it comes out to be approximately 1.97 picometer once I have my energy of 300 k v electrons. So, particularly coming back to the d value which is approximately 2 to 3 angstrom for a for particular crystals if I put this value in my Bragg's equation 2 d psi theta is equal to n lambda. So, my lambda is known now my theta is also d is also known because d is approximately 2 to 3 angstroms and I can see that the theta value comes out to be 0.28 degrees only. So, generally as a rule the scattering angles in the electron diffractions are very very small they vary between 0 and 1 degree. So, I can see that my evolved sphere becomes very much flatter because my lambda value is very very very very less and compared to that of lambda value of a of an x r d b. So, in my x r d I use a wavelength of approximately 1 to 2 angstroms or so which actually narrows down to less than it becomes around 2 picometer for 300 k v electrons and that basically brings down the scattering angle to around 0.28 degrees. So, eventually my scattering angles are very very small in transmission electron microscopy. So, there are certain rules I have my incident electron beam it interacts with the lattice planes and it gives me a diffracted beam at an angle of 2 theta and since my theta values are very very very low the reflecting planes are almost parallel to the direct beam. So, that is what we can see that my beam which is being getting diffracted it is tilted only at very fine angles of 0 to 1 degree. So, they are almost parallel to the direct beam and secondly the incident electron beam the beam which is basically falling on to the particular to interact with the particular lattice plane becomes the zone axis of the reflecting set of lattice plane because it is approximately perpendicular to the plane which is basically it is interacting. So, the normal to it will be perpendicular to that and my electron beam is again perpendicular to the normal of the particular plane. So, my incident beam incident electron beam becomes the zone axis. So, this becomes a zone axis in terms of defining all the other diffraction planes. So, that is the those certain rules which are being followed out here that my reflecting plane is parallel to the direct beam and secondly my incident electron beam becomes the zone axis of the reflecting set of lattice planes. So, because there will be so many lattice planes. So, those become those my this incident beam becomes the zone axis for all such planes. This is the overall construct of a particular TEM transmission electron microscope that initially I have a source for the electron source of electron source here I generate my all the electrons. Then I have set of magnetic electromagnetic lenses the certain condenser lenses generally 3 to 4 condenser lenses are utilized. Then I have my condenser aperture along this particular part. So, it is required for the alignment part and then I have my objective aperture objective condenser lenses out here. Then I have my objective aperture out here and this is what decides the overall resolution of my TEM. Then again I have some objective lenses then again I have selected area aperture and then again I have some diffraction lenses I have intermediate lenses I have projector lenses and I keep my sample actually in between which I will come to in the next slide. But this is the overall construct of a TEM that I have my electron source a set of electromagnetic lenses first the condenser lenses and then my objective lenses and then the projector lenses to finally, get a image. So, I can see out here if I keep my specimen at this particular location I have the I have I have it already passed with the condenser lens. So, I can see that my specimen is at particular location and then I have my objective lens which basically gathers information from gathers information from a particular specimen. So, electron beam comes interacts with the specimen and that information is being collected by the objective lens. These are not really some tangible or something like optical they are not like optical lenses they are more of electromagnetic lenses. So, there is nothing hard as such as we see in the optical microscopy that we have really glass or lenses which are guiding the light, but in this particular case we have electromagnetic lenses which are directing the electrons. So, once I collect the information so I can see I have my objective lenses which is gathering the information from the specimen after the electron beam is interacted with the specimen. Then I can see I have an intermediate which is called intermediate diffraction pattern which what I called back focal plane here I form my intermediate diffraction pattern or later on I can also form a intermediate image plane. So, this is a image plane and this is my back focal plane. So, if I keep my aperture at the back focal plane finally, what I get is an image and if I if I keep my aperture at the image plane finally, what I get is the diffraction pattern. So, I can see that I am forming my intermediate image at the certain location. So, I can have aperture one thing is called objective aperture or something called back focal plane objective aperture. So, here I am forming my diffraction pattern and I if I keep my aperture at this particular location I can get bright field or dark field image or at the second location where I have something called SIED aperture here I am forming my intermediate image and if I keep my aperture out here what I get is a diffraction pattern. So, these two are more complementary kind of features which I can really tap and I can I can form an intermediate image as well. So, I can form my intermediate image or intermediate diffraction pattern to finally, get a image or a diffraction pattern. My viewing screen can be the fluorescent screen on which electrons can interact and they can come and fall or it can also be a CCD camera. So, I can see that I have a particular specimen I let the electron beam pass through condenser lenses when it interacts with the particular material it passes through and then I have a set of a set of objective lens which gathers the information and lets it through a certain aperture. It can be at the back focal plane or it can also be at the image plane and depending on where I choose my aperture I can get if I keep my aperture at the back focal plane I get something with some image if I keep my aperture at the image plane I get a diffraction pattern and in between I can have some intermediate lenses for basically magnifying a particular image or a particular image. So, that is what I can see in this particular construct of the ray diagram. So, essentially to construct TEM my first requirement is the electron gun because I need to generate electrons at some point. So, that I can let it pass through the specimen. So, I need a some source of electron gun. So, electrons are basically generated out here and they are after that they have to be accelerated. So, that at very high energy so that they can come and interact with the material. So, I have some sources of electrons those that can be either tungsten filament or lab 6 filament or even the field emission gun. So, I can get something called tungsten filament it can also be lab 6 filament or it can also be field emission gun and depending on that actually tungsten filament is the low cost, but lower emission source lab 6 can improve the intensity of the electron. Similarly, field emission gun can also enhance the intensity of electrons to a very drastic very high extent and the beam size also reduces to very fine. So, there are certain ways we can utilize all those sources of electrons to generate the electrons so that they can come and react with the particular specimen. So, once the electrons have been generated I need to get a parallel beam of that. So, that I can accelerate them. So, they have to be accelerated by a node and after that they are basically parallel they have been they are made parallel. So, that they we can get a parallel beam or they have to be made to fall on a as a very fine beam. So, there is some set of magnetic lenses and certain apertures which will allow me to basically condense the beam make it parallel and then basically I can focus it further for imaging part. So, I can get a parallel beam so I can make it like a more like a micro probe. So, I can get a micro probe beam of electrons again I can also allow it to get convergent. So, I can instead of getting a parallel beam I can make it converge we get a convergent beam for certain applications. So, for probing I need it like for scanning tunneling electron microscopy I want a nano probe whereas for getting a lattice fringe imaging or getting more diffraction patterns I can more diffraction intersection of more lowest zone at particular location I can also utilize my convergent beam electron diffraction. So, there are certain ways I can utilize either to achieve a parallel beam or a convergent beam. So, for that I need condenser lenses which can really direct my electron beam. So, if I can direct my electron beam if I can control the electron beam I can get information what I am really looking for. Additionally, I can also tilt my electron beam to get a dark field imaging. So, dark field transition electron microscope imaging can also be attained once I have control on the condenser lenses so that I can guide my electron beam. So, that is the importance of the condenser lenses systems electromagnetic lens systems and finally, the objective lens decides the overall resolution of the final image and objective lens is one of the most important lenses because it is generating the first intermediate image and the quality of that will be essential to get the overall resolution. So, once I am able to control the beam that is good enough, but object lens is the one which will collect the information from the specimen and since it is also creating the first intermediate image this is highly this is very critical factor in deciding the overall resolution of the final image and in between I can have diffraction or intermediate lenses either to get a diffraction mode or an imaging mode because they are being formatted different locations. So, I need to have two apertures or lenses which will guide me in terms of other switching from imaging to diffraction mode or being able to select a particular aperture and there can be as well as some projective lenses they will magnify the second intermediate image they can be either image or it can also be the diffraction pattern and so projective lenses they help in the magnification of the second intermediate image. Once I have found my image and I have basically magnified it I also need to see the image because I cannot see the electrons so I need to see electrons how they have interacted with the material. So, either I can view them on a fluorescent screen or I can also project it on a some TV camera I can also record it on either on a negative film or I can also record it on a slow scan CCD camera. So, these are required for the image observation because we cannot really see the electrons. So, we can we will have to let it interact with the fluorescent screen or see it on a TV camera or I can also get it on a negative film or I can also capture it on a slow scan CCD camera. So, it can also be on a imaging plate. So, these are certain ways through which I can observe my particular image and all these things are all the setup of team involves interaction of electrons with the matter that so the travelling of electron is highly necessary and that can happen because I am also looking for achieving a information which is elastically which is elastically scattered electron. So, I need to avoid any interaction of electron with the atmosphere. So, for that I definitely need something called a vacuum system because I want to I want the electrons to pass through the beam through the particular channel without interacting with anything else any of the atmosphere. So, if you interact with the atmosphere it is basically losing its energy. So, that will become in the elastic interaction. So, to allow to disallow any interaction of the matter of with the gas. So, the gas particle should be absent in the column. So, for that I definitely need a vacuum system and here I require a very high vacuum which is approximately 10 to power 6 to 10 to power 7 minus 6 to 10 to power minus 7 tau. So, that part that much that much vacuum I require 10 to power 6 to 10 to power 7 minus 7 tau. Therefore, I require for the creating the vacuum. So, this is very high vacuum which is being utilized out here. So, for this I require very high vacuum which is approximately 10 to power minus 6 to 10 to power minus star. So, that electrons can continue without any interaction with the nearby gases. So, I can get an information which is truly from the interaction with the specimen and I can achieve my vacuum by utilizing a pre rotary pump which is some sort of a pre vacuum pump and later on I can go for a diffusion pump or a ion gator pump to create such high vacuum. So, that is what is highly required for a TEM imaging. So, here we see that we require a projective lenses for magnifying the second intermediate image and then for visualizing the image I need to have some sort of a TV camera or a CCD camera or I can also record it on a imaging plate or I can also have a look it on a negative film. And since everything is happening with the electrons, the electrons need not get interacted with the nearby gases. So, I need to allow a vacuum to be there. So, I get information only which is truly from the material. So, vacuum to the order of 10 to power minus 6 to 10 to power minus 7 star is required and which I can attain either by utilizing a pre vacuum rotary pump or followed it backing it up with diffusion pump or a ion gator pump to become the main source for creating the vacuum. So, coming back to the picturesque view of the electron gun, I have my filament this can be the tungsten filament or a lab 6 filament. So, this is this becomes my source of electrons, I apply certain bias to it, I apply certain bias to it and I generate electrons. Once electrons are being generated my vionette cylinder it has a negative potential. So, once the electrons are being generated it is now basically focusing the electrons to some particular spot. So, that is what is being given then by the negative potential out here that I am generating my electrons, electrons are negatively charged particles and they are they now get focused or they get repel by the negative bias. And from here they are now accelerated because as I note I apply certain potential positive potential. So, now I help the electrons to accelerate to finally, get a electron beam. So, in this particular case I can utilize tungsten or lab 6 filament or even field emission gun as my electron source, I apply certain negative bias to basically construct my electron beam. Later on I can create some positive potential so that electrons can get attracted and they can get accelerated to finally, I can get my electron beam. And the sample preparation it is very highly critical because I need to make my sample truly transparent to electrons. And this is how overall construct of a T M looks like I have my electron gun to supply me electrons. Then I have set of condenser apertures and condenser lenses which basically focus the beam or control the beam I can get either parallel or a convergent beam. Then here is my specimen port where I keep my specimen then I have certain objective aperture which collect the light from the specimen, I have objective lenses I can have some diffraction lenses or intermediate lenses. And again some projected lenses to finally, be able to observe it on a fluorescent screen or a image recording system. So, that is the overall construct of my transmission electron microscope, but again here the very much requirement is of the sample to be transparent to the electrons. So, generally we have some sort of holders which can either provide me a tilt in case when I require it it can be up down tilt, it can be side tilt or it can also be a rotation which I can get from the holder itself. So, this is what it is out here that I keep my sample on a particular copper grid and it has some locking ring on which I keep my sample which is approximately 3 millimeter in diameter. So, I can keep my power I can make as very thin film which is approximately 3 millimeter in diameter, but which is a which the center part of that is transparent to electron. I can also have some powders which are very fine enough and those are basically transparent to electrons, but to prepare a sample it is very critical to attain transparent to electrons. What I can do? Since my samples will be mostly bulk in nature. So, even to select a particular piece of sample from a bulk is very very challenging because first of all my sample has to be representative of the overall bulk structure what I have really intend to look at. In say for an example, I am excluding a particular rod. So, the structure at the surface will undergo much more shear. So, the grain refinement might occur much more at the surface whereas the grains may remain unaltered at the core. So, if I am taking my sample only from the surface and I say it to be representative of what is happening in the core of that particular rod I am totally wrong. So, I need to carefully select my sample as such and then be able to relate it to the particular surface or particular area where I have taken my sample from. So, I might require sample either from the surface as well as from the core to be able to say what is happening at the surface and what is happening inside the core of that particular rod. So, once I want my sample to be representative of the particular bulk material. So, I will take so many samples like from the surface as well as from the core because what I am seeing in T M is only a very fine or very small regime of the overall bulk. So, I need to very carefully select the region or the sample which is represented truly representative of my overall bulk material. So, in order to make my material very fine or transparent to electron initially my samples may not be transparent to electron. So, what I will do I will first of all I will section my material into very fine slices which might be approximately a millimeter or less than that may be couple of microns as fine as I can cut them through a saw. Then I will start thinning my sample from say less than from more than which is greater than micro maybe say approximately 100 micrometer in thickness I start thinning in down to less than 1 micrometer. So, once I have a particular disk a particular sample which is less than 1 micrometer I will punch out a small regime which is approximately 3 millimeter in diameter and it is thickness will be less than 1 micrometer. So, I will attain a disk which is approximately 1 micron and then less than 1 micrometer thin once I have this particular disk I start thinning it mechanically. So, I start thinning it down mechanically and I can keep mechanically thinning it till it has reached much lesser than approximately 200 nanometers or even as fine as I can go. So, what all I have is a thin disk which is approximately 100 to 200 nanometer in thickness. If the sample is conducting I can apply some corrosive or some media which can start eating away my this particular material. So, what I can get is I can start throwing some media it this is enlarge view I can start throwing in some media from both the sides until it starts eating away the material and creates a very fine hole in the center of the disk. So, what I eventually get is what I eventually get is more like this I get a material which is a very fine hole in the center. So, this particular point is nothing but a hole and the just the area nearby this particular hole is now transparent to electrons. So, ideally if I see it will be more like this it will be transparent to electrons. So, this is what I am really targeting at to get a material which is transparent to electrons or I can also utilize something called ion beam milling this is for conducting samples I can use something called jet the twin jet polishing I am sending two jets on each one on each side to start eating away the material and make it transparent to electrons at the center. Once I create a hole the center the part near the hole is transparent to electrons. So, seeing it from the top view I can also this particular thing will look more like this I have a disk which is a hole in the center. So, that particular hole just the area nearby that particular hole is now transparent to electrons. So, that I can create by twin jet polishing or alternately I can also supply some ion beam I will take a source of argon I can start throwing my ions on the sample and I start rotating my sample. So, I can create a similar way I can create a hole in the center and on the region near this particular hole is again transparent to electrons. So, I can look around in this particular regime to learn more about the sample itself. So, there are certain of certain ways. So, my overall disk diameter remains around 3 millimeter, but the central part is now it has a certain hole in it and the area nearby that particular hole is now transparent to electrons and I utilize that particular area to be observed or to be analyzed under transmission electron microscope. So, I can see how critical particular sample preparation is I need to make the sample big enough. So, that I can handle it at the same time some part of it or the center part of it has to be transparent to electrons. So, I can see or visualize or observe or analyze the region in that particular regime. So, that is what is the overall thing about the sample preparation in the transmission electron microscope and once my sample is ready I inserted in the particular time holder or the transmission electron microscope holder. So, I can observe it further and how the overall magnetic lens looks like is I have a magnetic lens which consists of a copper wire with some iron pole pieces. So, what I am seeing is a copper wires which are out they out these and these are nothing but the pole pieces of iron and once I supply some electric current to it it will start creating a magnetic field. So, I will get some magnetic field which are represented by this red lines and so what I can do I can get a rotationally symmetric magnetic field, but this is again inhomogeneous because I will get a weak field in the center and very strong field in the on the sides near the pole pieces. So, what I can see is if my electron is passing through it electron will get electron will basically get strongly deflected once it undergoes a very high field. So, once the electron is traversing along this near the pole piece it will get deflected very to very sharp very sharply where as an electron get passing through in the center will be deflected to a very lesser extent. So, I can see that they will undergo a kind of a crossover. So, they may not get focused at the same point. So, that that actually results the results that the electrons which are close to the center they are less strongly deflected then those passing the lens from the far from the axis. So, far from axis they get deflected very quickly where as in the center they keep going to much further extent and they get deflected to a very small extent. So, instead of getting a parallel beam. So, we can try to focus all this beam as a into a spot and this spot now becomes a it is called so called crossover because I have a particular regime instead of a spot it is more like a regime on which my electrons are basically being targeted or being focused at. So, I get instead of getting a fine very fine spot I get a kind of a spot or a region which is called a crossover and that happens because I have my electromagnetic lenses and they create a stronger field near the pole piece and the weak field is much weaker at the center part. So, electrons which are travelling at the center they get deflected to a smaller extent as compared to the near the pole pieces. So, I can get some instead of getting a very fine or a single spot I get some sort of a regime and that is called a crossover of the electron beam and since it is a magnetic field which is being applied. So, my electrons will experience a Lorentzian force and since Lorentzian force is a component of the electric field as well as the magnetic field and it also depends on the charge by velocity ratio of the electrons. So, basically this particular part is more like this that I am applying I am letting the electrons get accelerate I am applying a electric field I am applying a magnetic field. So, ideally with the magnetic field which is being controlled by a coil current it results in some sort of force which is perpendicular both to velocity part as well as the magnetic part. So, what happens that basically that basically is trying to pull the electron the same time it is trying to push the electron to certain direction. So, that creates a helical trajectory because my electron is travelling like this and I am at the same time I am trying to pull it or I am also trying to push it depending on the kind of field I apply. So, my so my magnetic lenses they are they are resulting some forces which are perpendicular to v. So, I have some forces which are perpendicular to v as well as my b. So, I can get a more like a helical trajectory. So, electrons will traverse more like a helicity once they are traversing in a magnetic field. So, I can see that the magnetic rotation is caused with respect to the object. So, that basically is very essential because depending on the kind of magnetic field I am applying it will tend to rotate my image because electrons the way they are flowing if they are very stronger field they might to they might get deflected to a larger extent. So, applying a very smaller electric magnetic field they might get deflected to a lesser extent and that is nothing but my magnification because I am applying field very strongly I am allowing to form a very bigger image. So, my image can also get rotated once I am applying a certain field. So, this is very one of the very important points in TEM that electrons are traversing a helical trajectory that depends on the magnetic field which is being applied because that magnetic field strongly creates a force which is perpendicular to both the velocity and the v part of it or the magnetic field part of it. So, that eventually forms a helical trajectory and then it creates a some sort of a rotation of the object itself. So, the features of TEM when they basically involve like this that my electrons are interacting with the material either inelastically or elastically, but I want to avoid the inelastic scattering because it is not containing an information it is similar to like leading to absorption because I have I am applying a certain electron beam I know its energy I know what are its features and if it starts getting scattered inelastically I do not know how what kind of losses it has gone through once it is interacted inelastically and after its diffraction with the material I get some I am getting some information, but I do not know how it is being generated what was the incident energy of the electron. So, I always need to avoid what is happening inelastically because it will not contain any local information, but if I am letting it inelastically interact or elastic diffraction can occur then basically I can also modulate its either amplitude or its phase with the primary beam. So, I can get both the information from either defects or the lattices from that I can get what is the kind of amplitude change or what is the kind of phase change with respect to the primary beam, primary beam and. So, I can get the information I can extract the information from the diffracted beams what is happening locally that is the advantage of my elastic scattering with the material I can avoid I. So, I tend to I need to avoid inelastically inelastically scattering part of it and again the energy which is basically what I am utilizing in the T m is approximately 100 to 400 k v to result me a wall square which is a inverse radius of 2 pico meters and these states we can go energy as high as 1.5 mega e v. So, that in that particular cases I can have sample which is even more than a micrometer in thickness. So, that part basically decides the overall energy which can pass through the material. So, conventionally or these states we generally utilize energy which is approximately 100 to 400 k v and that is enough for getting giving us a good information, but instruments as high as 1.5 mega e v are available which can penetrate down to penetrate into a material which is approximately more than 1 micrometer in thickness, but generally thickness of specimen has to be approximately 10 nanometer or may be the lesser the better and again my resolution part depends on the thickness and again if you want to get a high resolution T m imaging that basically for that particularly we need to have the specimen to be thin enough approximately nanometer in thickness. So, I can extract much more information in terms of its lattice fringes or high resolution grain boundary imaging and so on. So, overall features of T m are that I need to get I need to avoid the elastic scattering I need to get the elastic scattering and from that I can get extract the information either for the lattices or for the defects and then I can measure either the amplitude or the phase and I can tell what is happening locally in the particular material. The overall energy what I the electron beam energy what is being interacted with the material that generally is approximately 100 to 400 k v and for that we need to have a specimen thin approximately 10 nanometer and then these states some other instruments also available which can increase the energy to 1.5 m e v and for that my samples as can be as thick as 1 micro and for high resolution imaging I definitely need to have a material which is thin enough less than couple of nanometers. So, coming back to the ray diagram I can see that my specimen lies here and then I have my back focal plane then I have my intermediate image which is forming then I have some intermediate lenses and then finally, I get my image on the either as a image or as a diffraction pattern I can have a screen. So, that is what is required out here that I can selectively take a particular beam in back focal plane I can allow only a transmitted beam to pass through or I can also allow only a diffracted beam to pass through. So, if I am allowing only my transmitted beam to pass through what I get is something called bright field image or if I let only one of the diffracted beams to pass through diffracted beam to pass through what I get is something called dark field image. So, if I use a particular aperture back focal plane aperture through which I am letting only my transmitted beam to pass through I get something called bright field image if I let only one of the diffracted beams to pass through what I get is a dark field image and conversely I can look at a certain particular area if I have particular microstructure in the bright field imaging say I had a particular kind of a particular image which is being formed in the bright field image and if I want to see what say this can be very different faces this can be phase A this can be phase B and to confirm that I need to get a diffraction pattern because diffraction pattern is arriving from the elastic interaction of the beam with the material. So, if I want to see what is this particular phase say I want to see what this particular phase is. So, I can let my beam concentrate on this particular part and it can give me a diffraction pattern I can get a particular diffraction pattern that will be consistent to the kind of orientation. So, I can again tilt my particular sample to get a different orientation because within a tilt of few degrees I can get diffraction pattern from any different planes. So, I can get diffraction pattern depending on the orientation of this particular crystal. So, if I can align my beam according to this particular phase I can get some diffraction pattern. So, I can I can do that part as well or I can go back to it more like this that I can select one of the diffracted diffracting spots. If I take an area and as I can take as bigger area instead of a small focused regime I can have an aperture which can accommodate more number of more number of grains out there. If I choose say this much area which has more number of grains and I am getting some diffracted spots I alternately I can choose a particular diffracted spot I put my aperture here and then I can again come back and see that this diffraction spot is resulting because of which particular grain. So, which all those particular grains which are contributing to diffraction spot will start appearing brighter and this thing is called dark field image because I am letting only one of the diffracted spots to pass through and only certain areas which are contributing to this of my diffraction spot because they are oriented favorably they will start appearing bright and rest of the field will be nothing but. So, I have a dark field, but my features are bright which are resulting this particular diffraction spot and this thing is called dark field imaging. So, that part I can achieve with the transmission electron microscope I can either get an image or I can also get a information about a particular crystal through its diffraction. So, that is the overall capability of my transmission electron microscope. So, eventually what we can see I have my back focal plane this gives me diffraction pattern out here and if I keep my aperture somewhere I can get either a bright field image if I let my transmitted beam pass through or if I let my diffraction beam pass through I can get some image and that will be the dark field image or if I can put my aperture at this particular location which is nothing but a Gaussian image plane and that will eventually form a diffraction pattern. So, I can see that in transmission electron microscopy I am capturing the beam which is passing through the material. So, my particular specimen or the sample is to be transparent to electrons. So, if I am using something which is much thicker through which electrons cannot pass through I will not get any information. So, the information like secondary electrons, back centered electrons they have to be collected back like not through the material, but back on the surface of the particular material or the sample, but in transmission electron microscopy I make the sample thin enough. So, my electrons can pass through and they can interact with the material elastically and then I get the information. So, for the diffraction the planes have to be almost parallel to the incident beam and then they get diffracted within a regime of 0 to 1 degree. So, I can see that for a particular plane to be aligned I have to treat it to a very marginal extent. So, that I can align it and I can get a diffraction pattern from that particular plane and again the zone axis becomes the incident beam itself. The direction of incident beam becomes the zone axis of the planes which are diffracting my which are getting which are basically diffracting the electron beam. So, I can see that the zone axis of that the incident beam itself itself becomes the zone axis and then again the sample preparation is very critical because for electrons to pass through they need to be they need to basically interact with the material and the sample itself should be thin that the energy which is being supplied to the electron is enough for it to come out of the material. So, that is the requirement for that and again my volts will become so huge in comparison to that of which is constructed in the x ray diffraction. But now my volts will can touch more number of points in the reciprocal lattice spacing. So, more number of planes basically produce a diffraction pattern and now I can analyze the diffraction pattern to come out with what is the overall material of what how the basically what all what is the overall phase which is present out there in my particular material. At the same time I can get an image I can see the crystalline nature of the material I can also focus a nano crystalline terrain itself and I can get some information out of it or I can do a vice versa that from a particular diffraction pattern I can look which all grains are oriented favorably to my incident being. So, I can see this kind of things in the transmission electron microscope and there is much more to be explore which I will basically cover in part 2 and I end my this particular lecture here. Thanks a lot.