 So in the last class we have discussed about the high-resolution electron microscopy principles and I said that we will discuss about the image simulation today. As we know that high-resolution electron microscopy gives you images of the atoms or the columns of atoms to the resolution level of amstrung or sub-amstrung even it is possible to obtain resolution level of 0.8, 0.7 amstrung in the microscopes available today. The problem is that interpretation of such high-resolution images are really a difficult or daunting task because image contrast can vary drastically depending on the focus as we know. As I have shown you that defocus is the most important factor in obtaining the high-resolution images in the times electron microscope. So therefore during the subsequent process after we receive the images from the microscope we need to simulate it using the prescribed models available for the crystal structure of the material. If it is a crystalline, if it is a non-crystalline material then we must have a prescribed model available for the structure of the non-crystalline material like amorphous or any other structure. So during as you know during imaging process the electrons undergoes three distinct interactions if we have to list down in a very short form and each of these interactions can be simulated in a computer. So first thing or first interaction electron undergoes is called the basically because of this dynamical scattering of the electrons in this sample or the specimen and this basically this can be simulated the dynamical scattering is basically because electrons falls on a sample and then can get scattered in different directions and this is basically it is a dynamical process. So one needs to apply the dynamical diffraction theory to obtain the exit wave functions of the electrons. Now this can be done using a technique called multi-slice method in this method actually the specimen is sliced into many slices actually specimen is basically taken into many many different you know small slices and then this slices are basically taken at a normal to the electron beam or instant beam and then this after we obtain the slices we can simulate the electron interaction dynamical interaction electron with the material by using different methods there are methods like reciprocatory reciprocal space formalism or fast Fourier transformations or real space approach or block web approach depending on how precisely you want to determine the interactions. Now I do not have time to deal into talking to each of these techniques very precisely because of the time constraint but nowadays any conventional textbook on the transmission electron microscopy will provide you all the knowledge base available for these techniques. The most important technique which you day to day apply in the high-resolution microscopy is called fast Fourier transformations in this technique we basically use the actual high-resolution image which showing the columns of the atoms as a real space image and then Fourier transform the the this real space information and obtain the diffraction pattern then compare the diffraction patterns with the the ones which is can be derived from the structural model available provided we know this structure of the material very precisely and then by comparing it we can get many distinct information regarding the the interaction of the electrons with the material and input to such a multi-slice method obviously has to contain different parameters of the specimen or the object like unit cell the position of the atoms and inside the unit cell the what is called thermal factor because of the derivation the atoms as the temperature increases and the orientation of the specimen or when some of the crystals are the the object as well as the thickness so this itself is a basically not a easy simulation as you understand there are so many parameters which goes into the simulations and if you do not have distinct information or distinct values of these parameters like thickness of the specimen the object orientations values of the of the develop factors very precisely for the material which is staring we do not get very you know good simulated picture. Now results of this calculation actually gives us the wave function as we know wave function is these important result of this wave function are the exit plane of the sample is obtained by using the multi-slice method and in the second step once you obtain this wave functions the formation of the images in the electron microscopic the time electron microscope can be simulated using different equations which I have discussed this equation is actually based on the transfer functions. So transfer functions actually are doing known apary for a particular set of microscopes once you know the transfer functions and related equation how this the the points in the specimen is getting transferred to the image plane then we can basically obtain the instrumental factors in the image. So therefore in this simulation second step simulation one is to provide all the information regarding the instrument or the microscope which you are using for obtaining highly spectromyte cook images and this includes obviously the spherical abrasion constant the defocus as well as even if it is possible to give the information regarding the currents of the different lenses subsequent to the objective lens. Finally in the third step electrons intensity of the electron beams which are coming at the image plane because of this interaction can be calculated by squaring the wave function and this can be displayed like a monotone or halftone image basically halftone image on a high-reson screen on a computer. So these three steps are normally done nowadays in all kinds of computers available where the commercial software actually allows you to run this different steps and provide you actually the image which is can be compared with the actual image. Now in practice this is actually routinely done in the even while doing the microscopy one can run these things one can use these functions which are available in the software in the computer and compare and get information otherwise if the structure looks to be very complicated then one needs to go back to the structure modify it and then obtain images which can be then compared but you know these simulations obviously are dependent on many input parameters as we have seen the most important input parameters are the specimen and the microscope itself. Now as per the microscope is concerned one can actually give this input parameters very precisely the values of the CS or the specular version constant or even the de-focus values can be provided but as per the specimen is concerned it is very difficult to provide the actual values of the input parameters like you know orientation of the sample the thickness of the sample where the image is taken and many what is called other parameters. So that is why many people actually what they do they initially obtained this information or measure the thickness of the sample very precisely using converging electron diffraction technique or some other technique available and also determine the orientation of the sample very precisely and use this as input precise values of these are used as input to the simulation so that one can obtain as called good images for reasons you know these are sometimes available for reasons these are sometimes not available to do the simulations. If you are lucky enough you are having a good microscopic team then you can guess this information very easily from your experiment only experiment or from your curly experiments during the whole set of time electron microscopy and then you can run this simulations at the same time for the machine as I said it is possible to provide the precise information of specular button constant or de-focus values but many times we do not rely on that the de-focus values which are provided but rather what we do is that we take a series of de-focus images from the highly sector microscope and then compare it with the simulated images this is so therefore in a nutshell I could basically show you a simple you know simulated picture in the slide where this is the specimen you see there electron from the gun falls on the specimen after going through different no lenses and then interacts interaction basically dynamical in nature and then is basically passes through the other lenses in the microscope like intermediate political lenses and finally we obtain image in a CCD camera nowadays and to make life simpler we always get a de-focus series of images where de-focus is very of the digital is very very was called systematically and images are obtained as black and white dots as you have seen in some of the pictures and this images that are fed into the computer one this is my friend the computer computer has the software in build and these softwares are basically runs based on these three principles which I have just now told you and then reconstruct the original image by comparing with the model have a level and finally it gives you a potential map so that is actually the cycle in which we simulate the images and obtained finally a most comparable image but you understand from this whole process because there are a lot of input variables which can be changed into few variables and also the complexity of the whole technique tells you that it is not an easy one to basically get information out of high listening images that is why nowadays people provide this high listening images in the different papers or the books just for the sake of increasing the quality of the images many times we find that exact information from this images are lacking in the both in the papers and sometimes in the books also because of this problem of simulating these images and getting the quantitative information so obviously qualitatively one can explain the images using such software but obtaining quantitative information requires another kind of simulations which are much more complex and probably outs of the scope of this particular course so I will not go into a lot of this but just to give you an idea that how complex is this you know simulations I am just telling you this aspect that be careful while comparing the actual images with the simulated ones sometime this may be misleading so therefore you need to first learn how to obtain good image in the electron microscope like good ones in such as Titan and then compare this images with the best possible software available in this world to extract quantity information about the microgaps so with this I close the highly sector microscopy portion of this course and I move on to the next portion of this course that is called stem scanning transmission electron microscopy as you know scanning times electron microscopy has become an integral part of the microscope nowadays all the modern-day microscopes has a scanning electron microscopy facility scanning term is like the microscope facility this is has originated from work by Crowley crew and many others and those who have tried very hard to use scanning term sector microscopy the reasons scanning electron microscopy is important because of these aspects we can get information for by field angle dark field images high angle dark field high-ended angle dark field images which can give us jet contrast we can also let us call we integrate this stem with ills energy filter energy electron energy loss spectroscopy which can be used to obtain energy filter images one can also integrate this with commercial electron beam or nano electron beam diffractions and in one can actually use the stem to get holographic images so I will not be will discuss all of them in this in this course but I will try to give an idea how stem works and how stem can be used to obtain different sets of information as I said the concept of stamina scanning transmission electron microscopy is not a new one it has been employed by crew a long back who introduced this whole concept into the electron microscopy community and he actually used first time showed that first time scanning term is electron microscopy you need to use most notably the field emission guns so in a scanning terms electron microscopy images what we basically do is like this we have a basically a fake source or effigy source what we can say which is a very high brightness and a very small spread this effigy source can be focused by using the objective lens on the specimen obviously the effigy source when it is focused on the specimen by using these this kind of objective lenses the beam will be de magnified electron beam and basically with this beam is called convergent beam and once the convergent beam falls on the sample or the specimen it diffracts and gives you diffraction disc and this diffraction disc can be either transmission transmitted electron beam or diffracted beams now one can actually use different detectors below the specimen to obtain the images using this that is called either transmitted beam or the diffracted beam so that is the basically idea so it is shown very nicely here in the second picture where you can see this is source or electron beam which is then it is it is can be scanned over the sample by using a deflector which is there in normal scanning electron microscope and this image is falling this beam is falling on the objective lens which focuses or being de magnified the electron beam to a very small size beam called convergent electron beam on the specimen and then specimen diffracts and we can get different kinds of information which are listed there. So this is basically the idea now as you understand that you know as the convergent beam falls on the specimen some part of this convergent beam will be diffracted and some part will be transmitted so if one collect this diffracted beam intensity using a detector one can form dark field images or if one collect this transmitted beam using a detector one can actually displays as a bright field images. So and then once this beam scans over the sample and this whole imaging can goes on as a speed of the raster let us just like a scanning electron microscope and scanning electron microscope the raster scans the electron beam and signals like backscatter electron or the secondary electrons or whatever are the signals generated these signals are then used to display the image on the computer scale. Same thing can be done here also as the beam scan on the sample the diffracted of the transmitted beam can be either of them can be taken by the detector and we can can be then plotted on the computer to obtain an image. So the first thing you understand the intensities of the diffracted beams or the transmitted beam which is passing to the sample is a totally dependent on the initial intensity of the convergent beam which is falling on the specimen. So that is why in this kind of same configuration one uses something known as FIG or what is called as field emission guns reason is very simple the field emission guns intensities of the electron beams are very high almost 3 to 4 order magnitude higher than the normal lab 6 with the filaments or the tungsten herping filaments. So these beams the which are high energy or high intensity beams rather can be demagnified to about nowadays 1 nanometer or even less in many cases one can go down to 0.5 nanometers also demagnified and to get a very very fine probe of very good intensity of falling on a sample. So that the diffracted beam which is coming or the beam which are coming out on the exit surface of the specimen can have sufficient intensity to get you know informations regarding the sample. So that is why FIG sources are normally used as I have to give an idea the if I take a FIG field emission gun source 1 nanometer beam has a current of 0.5 nano amps. So that is why with a FIG bright and dark field stammages can be easily recorded within few seconds or even at a TV scan rate that is what you want we want the beam to scan on the sample and then image to be displayed at the same scan rate. So to do that you need the TV scan rate and to get a TV scan rate you need to have a sufficient intensity of the beam. So as I have discussed with you this slide this looks like a very simple but there are lot of attachments involved which I have removed for the sake of simplicity of the images. So essentially the components in a scanning transmit electron microscopy is more or less same as a conventional transmission electron microscopic instrument and not much difference is there. So except that there is a scanning coil which can make the beam scan on the sample like in raster mode just like a scanning electron microscope but you know there are practically there are lot of advantages using this scanning transmit electron microscopy. The real advantage is that the dark field images can be obtained with a very high collection efficiency in stem as compared to the normal dark field images in transmission and transmission electron microscope. This is mainly because that the all these scattered electrons outside the incident beam spot can be collected in a stem mode and once we collect all the scattered electrons or diffracted beams the intensity of the diffracted beams is sufficiently high so that it can be it can give us a very good collection efficiency. That is one of the biggest advantage and that is why almost half of the imaging detectors in the stem mode are dark field like annular dark field high angle annular dark field these are actually routinely used to obtain all kinds of different information which we are going to discuss one by one. Not only that that is the biggest improvement advantage not only that another important advantage is that in the stem in a conventional electron microscopes the conventional TM normally two dimensional detector such as photographic plates or a CCD camera is used to record the intensities at all image points which are in parallel like as the beams goes parallel and falls on the screen the images are recorded but in stem image information is produced in a serial form like a time dependent voltage or current variation. For many years in fact this gives stem this unique possibility of online image processing to manipulate the image contrast for special uses or purposes and now we have the CCD cameras available of high quality high solutions and so therefore this gives us the serial lead out or online image processing processes and sit in conventional TM also however stem for the stem there are many other possibilities exist thus in stem detectors can be or the stem actually several detector can be used simultaneously to produce images from the different signals coming out from the sample. So there are a variety of these stem detectors available and which I can show you here or I have listed down there variety of stem detectors available to obtain different signals other than this wide field and dark field. So as this beam falls on a specimen it is one part of the beam is getting diffracted that is what is the big this diffracted disk is shown here other part is called zero beam which is not undergoing supposed to be undergoing not undergoing diffraction as far as scatter beam. So we can put a wide field detector there and we can put a dark field detector there and obtain conventional wide field dark field images just like that as the collection efficiency high product fields image quality is better even many cases resolution is also better because the beam which is basically coming out after diffractions they contain the information which are having very high resolution not only that one can actually use a integrate as I said the stem with the electron energy loss spectrometer which can allow not only allow you micro analysis of the specimen a very small area to detect elemental presence or the state of different elements present in the sample of specimen in the small area but also it allows you to from the images with electron that have lost particular amount of energy we have to understand that yields basically operates on the energy loss of the electrons. So as the electrons comes out of this the sample exit phase they carry loss information because they have under their electrons have undergone different kinds of amount of energy loss and this is nothing but because of the analysis scattering of the electrons in the specimen and so therefore we can strap this information by trapping those electrons which have undergone loss of energy because some analysis scattering and this loss can be characterized depending on the type of element present in the sample. So one can actually use a certain kind of technique in which a specific energy levels can be used to obtain image and this is all called energy filter imaging and so therefore at the what is called as the images which corresponding to electrons that have lost a particular amount of energy can be obtained and to give you an idea this is the illustration which I have shown the very first lecture for the nano crystalline copper pulse electric deposit using thiorea. So it is deemed a thought that thiorea act as a gain refiner by pinning the gain boundaries of the copper but there was no work or no experimental evidence available showing that thiorea molecule is exactly sitting at the periphery of the copper grains during electro deposition of the copper. So to show that one can actually use what is known as energy filter imaging using ills as I said that ills can be used to filter out the two image or to obtain images for the electrons which have lost a particular amount of energy. This is the left side of the picture is basically showing you the normal biofilm image and the rights of the picture is taken using sulfur energy edge which is that means the electrons which have lost energy corresponding to sulfur energy edge in ills and that can be used to obtain the byte field or image and there you can see this bright lines at the gain boundaries many places this bright line signifies the presence of sulfur and as I told you even the very first lecture the thiorea is basically a molecule which contains sulfur are the nitrogen hydrogen and carbon and sulfur is the distinct part of the molecule. So the presence of sulfur means presence of the thiorea. So by doing this kind of exercise one can actually obtain images to the resolution of the electron the term is electron microscope where we can show that where a particular element is present. So therefore this are now this does routinely in fine catalyst which is a very important field of research nowadays one can actually use this energy filter images to clearly signify where a particular element is present or not. So other than that the in the stem mode this is one of the ills is one of the most important attachment to the scanning electron microscope which many microscope has the microscope which I showed you in our campus do not have this ills but I have shown you where the ills can be attached at the bottom of the microscope just below the camera ills can be attached and the signal can be processed using a computer. So images can be formed in the stem mode by using low energy secondary electrons or algae electrons or characteristic x-rays rather. So serial nature of this image signals provides basically possibility of the quantifications basically and then the information can be correlated with the specimen compositions crystallography and as well as the morphology. And another important detector is basically added into the microscope or into the stem is known as HARDIF high angle annular detector. This was basically done or the developed by Crewe et al and they have seen that if we use annular detector, annular detector means is just like this, like this suppose this is the specimen, this is the specimen here as you can see and this is my convergent beam coming from this objective lens stem and then obviously electrons are undergoing diffractions. So if you use a detector which is annular type so what actually happens is that we can we can collect the diffraction information in a disk surrounding by this central circle and this is what is called annular detector. In fact the reasons for it which I will tell you within few minutes time. So if we can put these detectors at a very high angle, very high angle means not very high angle. In electron diffraction you have to remember the scattering happens at a small angle maybe one red micro radian or few actually micro radians or few one or two micro radians like that. The high angle means of that nature not that several couple of 20, 30, 40 radians or so no. So therefore these detectors can be placed just like this one source here at a little distance from the transmitted electron beams or the zero beam and they contain information regarding the atomic number contrast. It has been shown by QA at all the basically that intensity of the image which forms by the detectors is proportional to z to the power 3 by 2 where z is basically the atomic number of the element present in the specimen. So therefore these detectors if we collect the information detector and display will tell us chemical compositions of the phases present in the microscope. This is very clear and this is routinely done nowadays. In fact the microscope which I showed you has a hardy detector and hardy detector takes this signal which are coming out or coming from the specimen at a little larger angle than the diffracted commonly diffracted beam and then a annular detector and displays on the computer scheme. So to show you one such image here this one I have shown you in the very first lecture of this course. This is basically a picture the one and the taken in a hardy detector in a microscope where there is a fake gun. So as you see this is the lot of large number of nanoparticles present in this specimen and this nanoparticles embedded in a matrix it can be it is basically here is the aluminum matrix and this nanoparticles are late in eutectic alloys and as you can clearly see that in a high angle annular dark field TM image we can clearly see these particles having showing distinctly two phase contrast. One is late solution or distinct solution. Aluminum is a dark so as we know that aluminum having very low atomic number so therefore jet contrast because of the aluminum will be very low on the other hand lead and tin having very high atomic number so jet contrast will be very high but there will be distinctly different jet contrast variation in aluminum and it from the tin and the lead which is seen from the brightness of this different portion of the particles. As I said that the bulk of the particle as you see on the right hand side picture is tin or the cap is basically lead and lead having high atomic number than the tin will be looking brighter in the hard of images. So this is again taken from our own work so one can actually use this particular technique to decipher this kind of informations and other rather to decipher different kind of phases presence. This is exactly equivalent to the backscatter electron imaging in scanning electron microscope where the backscatter electrons carry the information regarding the jet contrast and we routinely do in the scanning electron microscope to obtain the jet contrast images so that we can see different phases very clearly exactly same thing can be done in the stem but at a resolution of highly seen electron microscope. Now one of the biggest fallout of this particular hard detector is this we can use this hard detector to obtain highly seen electron micrographs okay so how to do it I will explain you within few minutes time is like this suppose you have a very thin portion of the sample specimen and you are using the stem mode so that the convergent beam is focused on to the particular column of atoms and obviously as I said you using a fake good fake guns one can actually convert the electron beams to 1 nanometer or 1.5 nanometer level. So if the small beam of very high intensity falls on a column of atoms and then this column of atoms contains suppose different atoms of different atomic numbers some of them are like aluminium which is very low some of them let us suppose contains holomium or maybe some other very heavy lead maybe high atomic numbers. So obviously the atoms which are heavy they will diffract strongly then the atoms which are having low atomic numbers so if we take or if you collect this information in a hard detector in a high elution mode we can clearly depict that which atom is what actually whether it is aluminium atom or it is a holomium atom or it is basically a lead atom one can clearly show on the microscope at the resolution of the high elution micrograph. So this is one of the another important thing which normally people do and if we collect this high elution high elution image is in a hard if mode or in this in the from the hard of detectors serially then one can build the whole structure of the crystal slow by slowly one by one well nowadays using Titan we do not need to do it because in a Titan basically one can get the distinct contrast from different atoms like one can contrast from the I have shown you the example of staunchium titanate in the last lecture from staunchium titanium titanium and the oxygen differently. So you do not need actually hard detector that way so this is another advantage of the stem the stem actually has changed the whole color of the electron microscopy so much. Now as I said that the one of the what is called advantage of these high angle detector is to obtain jet contrast images but it has its own you know limitation also limitation in the sense that it has to be properly sample has to be properly oriented and in many cases we found that very signals depending on the sample thickness the sample thickness is high the hard if signals are very good the sample things are small highly signals are bad so that is why one needs to use very high intensity beam at the as incident beam on the sample that is to be focused by the objective means very properly on the sample and so that the intensities of the hard if detector or the hard in the hard detector for the thin regions of the sample can be comparable another important technique which is attached to the stem is the converging electron diffraction and the nanofim electron diffraction specifically the nanofim electron diffraction is nowadays widely used we know that many techniques allows us to form grains or the particles which is very small suppose less than 5 nanometers. So in those cases it is very difficult to obtain diffraction information using conventional diffraction like the the selectively diffraction pattern so one is to use the use the C-bait or the converging electron diffraction or specifically nanofim electron diffraction to give you an example suppose there is a particle here small which are marked there in the image size is approximately 10 nanometers. So to obtain diffraction information from such a small particle one needs to use a very fine probe and that kind of fine probe can be obtained in a stem mod so that the beam which is highly convergent size of the beam of the order of suppose few nanometers can be allowed to fall on this particle and then diffraction can be collected this is also now understood routinely in the fake guns one has to understand that the nanofim electron diffraction depends on the available intensities of the incident beam because a sample is particle is not so thin the diffracted beam intensity may not be sufficient enough to be recorded by the recording device. So that is why in many cases what is done is that the beams are whether we use the fake beam which is having very high intensity and allow to fall on the specimen and then diffracted beam is collected. The advantage of this is that one can actually obtained the information for large number of particles by this way and then get full scale diffraction information from the particles. So as you see I am not going to discuss about holography here itself is a big subject as you see that using all these kinds of detectors in the scanning transmission electron microscopy one can obtain information in compressing normal byte field, anode duct field, z contrast images, composition analysis by yields, energy filter images by yields as well as diffraction information by nanofim electron diffraction. So this actually seems to like that it gives all kinds of informations and advantages are many obviously advantages are many but there are disadvantages. The disadvantage is that the images in stem mode is obtained in a skierial manner. So therefore it has practical problems like recording the images sometime can be long and that is why in that time long recording time sometime images can shift that will be drift of the images. Not only that it is also possible that during this taking the images even the current of the beam electron beam which is falling from the stem detector stem sorry from the objective lens can degrade as a function of time. So this can actually give sticky images many times in stem those of you who have used the stem will find if your microscope is not functioning at the optimum level you will get sticky images in the computer skin and not only that if you use a very highly convergent high you know current electron beams this can damage the sample. Many times you will find if you do nanofim electron diffraction on a particular particle after taking the diffraction pattern particle is basically change. So the sample damage is very high that is why in stem mode. So one has to be very careful about the particle specimen sensitivity of the specimen towards the electron beam if the specimen is very sensitive to electron beam once you not used stem in a FEG gun rather once would use FEG in the lab 6 filament where the intensity of electron beam is normally low. So basically the uses of the stem depends on the that particular type of material the material is good and stable you can go ahead with the normal FEG microscope stem configuration and obtain all kind of information a material is beam sensitive you cannot do it. So you have to change to the normal microscope. So in a nutshell I have discussed with you the hyalosine electron microscope stem in the last 3-4 lectures in the next lecture I am going to show you two important things one is the in-situ electron microscopy briefly I will discuss and to some extent I will discuss about the EDS detector which is normally attached to the microscope nowadays in fact new concept have come where super EDS is used instead of using one EDS detector one can use several EDS detector in the electron microscope which is then in Titan. So I am going to discuss about that and then I will move on to the scanning electron microscope where different kinds of scanning electron microscopy techniques like the EBSD electron beam a back straight electron diffraction patterns or in-situ ACM can be used to obtain lots of information.