 In this module we are looking at different characterization techniques and so far we have looked at the diffraction technique, thermal analysis technique, spectroscopic techniques and in the previous lecture we have also looked at the tunneling microscopic technique and atomic force microscopy. But the most important and inevitable characterization tool for all branch of science is microscopy especially with reference to scanning electron and transmission electron microscopy. This particular microscopic tool is used by a range of researchers, biologists rely on SEM as much as the physis and chemistry researchers do. The main reason is this is one of the most commonly available instrument and also it is fairly simple for anyone to access and the requirements for studying the materials using SEM and TEM is comparatively much much more simpler than the sample preparation requirements needed for STM and AFM techniques. Therefore this is one microscopic tool which is very very popular and I am going to just run through some of the examples or principles behind using SEM microscopy. Then drive home some point as to how carefully we need to look at this microscopy or what are the informations that we can get out of it and I will also show some of the recent results which can give idea about the importance of this SEM and TEM tool in materials characterization. As you see here in the first slide we can use it for a range of materials and the picture that you see in the middle of this cartoon are the pollen that is present in air and they are mostly micron range and one can get an idea about the pollen that is present in air and how much they contribute even to pollution. So this gives an idea about the range of particles suspended in air which are of micron size and also you would see some of the other images mingled with this those which are appearing as yellow particles or golden particles are indeed gold particles the color is actually camouflaged just to drive home the point. So we can study a range of material one which are synthesized in a laboratory scale or in nano scale and those which are already existing in nature. So a range of materials can be studied using SEM and TEM microscopic techniques. Now what is the fundamental issue that is involved in studying this SEM, TEM characterization tools is the impact or the interaction of electron beam with matter how they interact and what is the secondary effect or what really comes out that determines whether it is a TEM microscopy or SEM microscopy therefore incident electron beam actually the way it interacts determines the nature of study that we are going to have and when we look at the incident electron beam there is a minimum threshold that is needed to observe something significant and to quantify the result electron beam when they interact with material usually there are some secondary effects which happen and that can be used to convert into useful information as you see here first of all there is a back scattering that occurs whenever electron beam interacts with a nuclei or with a material and that can be a primary electron the back scattered electron is a primary electron or it could be elastically scattered primary electron. So we tack this process as back scattered process so it does not really do any damage it is reflected suppose small nuclei is there the back scattering is of a different nature compared to a larger or a heavier nuclei therefore the way the back scattering occurs also tells with what sort of nuclei it is interacting with and the other thing is a secondary electron image this is another process which can give us other information than the back scattered stuff and apart from that there can also be x-rays produced because when a particular electron is knocked out from a inner core then the outer shell electrons can come in and therefore x-ray can be produced. So three distinct processes happen when electron beam interacts one is back scattered electrons the other one is secondary electrons and the third one is the x-ray that is produced. So all this can be usefully converted into a image form and we can try to derive some information about the material. Now the electron interactions have a depth and that depth also determines what is that we are looking at secondary electron the production range is of this dimension therefore since the penetration is not too high and it is very much on the peripheral the secondary electron production is usually used for spatial resolution and therefore predominantly any image that you see in SEM you correlate that to a secondary electron image but if you look at the back scattered electrons the production area is much more deeper and broader compared to secondary electron therefore this is used selectively because it the impact strength or the production area is quite big therefore you can use this for a better contrast of the material that is already present. So back scattered image is not a primary source information in SEM rather it is a back up information that you would like to use other than the secondary electron image. So this is quantitatively the way we can picture about scanning sorry secondary electron image and the back scattered image and then there is more impact which causes x-rays to be produced and therefore the x-ray mapping can be used for a analytical tool. So you can not only look at the image but you can look at the quantitative information about the element that is present because of the x-ray production area is much more larger. So three important things happen in scanning electron microscope specifically so when you look at a image from a scanning electron microscope you need to understand that there are at least three major informations that you can get one is the secondary electron image back scattered image and the x-ray that is coming out of it. The electron and the sample interaction also has its magnitude which depends on the atomic number therefore if you are looking at low atomic weight atoms usually the response or the phase contrast is very very less compared to heavier metals. So if you have heavier metals the interaction will be very distinct and therefore the mapping is much more easier for example you have iron and nickel in a material then it is very difficult to get a image contrast between iron and nickel because they are very close in atomic number whereas if you have iron and rhodium or platinum or palladium because of the difference in the mass or because it is heavier one is heavier than the other then the phase contrast will be easy. So when you have low atomic weight the incident beam propagates this much even at low voltage and for high atomic number the penetration or the interaction is only minimal when you have a low voltage situation therefore if you need to have more information or more contrast for higher atomic number then you have to use high voltage and the impact area is much more for low atomic number elements. So high voltage means large penetration and therefore the sample damage will also be considerable larger current means more damage and more carbon deposition but you get better x-ray signal. You do not really resort to large currents always because of the carbon deposition but then if you are looking for x-ray mapping then we need to go for higher currents. So x-ray analysis and backscattered analysis is usually kept as a backup characterization methods but predominantly you look only for scanning secondary electron images. Now this is the SCM anatomy of a microscope where you have a complicated lens arrangement which really brings in to focus the electron beam that you are shining on the sample and as you see here you have the electron gun which is taken through a series of optics and electron beam is made to fall on the specimen here and what happens when the electron beam interacts with the sample is what is the optics that is in this region and as you see you have detectors which will sense the secondary electron and this is almost kept perpendicular to the sample surface therefore you can collect enough of the secondary electrons and secondary electrons are then sent to another photo multiplier tube where you can transfer that into a image and we also have backscattering which is collected closer to this region we will see that in the next slide. So what you see here is more of the collection area whether it is a backscattered electron or a secondary electron that is actually mapped and in this sample specimen chamber you also have another important addition which is called a fluorescent screen in case if you are using a TM microscope so the design is more or less the same between a CM microscope and a TM microscope but the sample position and how you image will differ otherwise the principle by and large remains the same this cartoon tells us the process that we are referring to one is the secondary electron image which is almost kept perpendicular to the sample orientation therefore you get enough of the secondary electrons falling into the detector here is the x-ray detector that is kept mainly for x-ray analysis but here again the x-ray detector is kept in a liquid nitrogen chamber so this is a extra attachment that usually comes with the main a CM microscope and what you see here is the backscattered electron detector which will preferentially collect the backscattered primary electrons so this is a primary process this is a secondary process that happens so both are kept very close to the sample environment and we can translate that into useful information just to highlight what this secondary electron image means the secondary electrons are the main source of information in terms of viewing images and the range of secondary electrons usually depends on the energy and for secondary electron image we usually result to very high voltage we cannot work at low voltage because the number of secondary electrons that you collect has to be very large for a very good topography therefore you usually work at a very high voltage the accelerated secondary electrons are sufficiently energetic to cause the scintillator to emit flashes and that is exactly the collector and this phenomena is actually known as cathodoluminescence and this cathodoluminescence is now picked up by a photomultiplier tube because that would harvest all these signals in a very sensitive way and this is actually now converted into a two dimensional intensity distribution which finally comes to a digital image so the secondary electron image is actually transferred in the form of a cathodoluminescence by a photomultiplier tube and then it is transferred to analog video display and then there is analog to digital conversion then you get a digital display so this is quite a involved process but it all happens so therefore if you need more of the secondary electrons to come then you need to make sure that the sample preparation has to be proper and therefore the surface of the sample is preferably a flat surface if it is a three dimensional one then the range of or the orientation of the secondary electrons will be on different directions therefore you get more useful and sharper images or contrast when you deal with a flat surface that is why sample preparations have to be rather very intricate for secondary electron images now compared to secondary electron image which I said the backscattered process is another complementary information backscattered electrons consist of high energy electrons these are the ones which are reflected or backscattered out of the specimen and this is mostly the primary electron which is backscattered and this is scattered out of the specimen interaction volume by elastic scattering interactions with the specimen atoms since heavy elements are backscattered more strongly than lighter elements thus they appear brighter in image and therefore backscattered image is used as a contrasting technique so when you have a secondary electron image the same area can be twisted into or verified as a backscattered image and because the level with which the backscattering occurs is different you can get a contrast so if you are not sure about the chemical homogeneity of a particular area it is always better to focus there and switch from a secondary electron mode to a backscattered mode and if there is no difference between the secondary electron image and the backscattered then you say the composition is the same but if you see two different contrast then you say that the chemical homogeneity differs in that particular region so this is therefore a very crucial mapping between chemical homogeneity in samples by choosing a secondary electron image or a backscattered image and the way the secondary electron detection and backscattered detection goes is totally different and you usually have the preferred optical alignment so that the filters are used properly in order to only sense the secondary electron and the backscattered election and this is the way the backscattered images are viewed and this is the way the lenses are arranged just to collect the secondary electron images. So the optics is very crucial in the SEM to differentiate between these two process this is one of the very old technique very old instrument that has been used as you see here the digital image in those days at least 10 years back was lacking and therefore all the conversions were usually taken in terms of photograph we never had a analog to digital conversion therefore when you have those images usually those are taken as a photograph and then we develop those images but nowadays we have everything digitalized therefore we have much sharper images can be recorded and this is one of the old instrument that you would see in any institutions this is a new generation SEM instrument where you see a picture of how the sample chamber looks like in fact we can actually mount or we can even bring down the electron gun much closer to the sample we can try to position the right sample everything is now arranged with a camera that is fitted right inside the sample chamber so lot of improvements have come with the new generation instrument typically this is a field emission SEM and I will come to this later and say why field emission SEM is much more preferred these days than ordinary SEM because of the advances in electronics now we have more refined picture of this SEM microscope just to touch on the minimum protocol that is needed in terms of sample preparation I have just put some cartoons here to just take you through the course that you need to do in order to get a SEM image first of all take any sample that you want on a stub mount it here on the stub and then this is actually gold coated any conducting sample you do not need to gold coat but if you are going to use a non conducting sample usually it needs a gold coating because any material that is interacting with the sample has to have a conducting area otherwise there will be charging of the sample you cannot see the image you cannot see the conversion of this into a useful image pattern therefore a very thin layer of the order of 1 to 2 micron thick gold coating is usually preferred on any non conducting surface for metallic samples we usually do not do that in any form it can conduct so once you have done the gold coating now this is ready to be mounted here in a sample stage and once the instrument is ready we can actually get useful pictures so this is a simple protocol for recording SEM images now I will just take you through a course of pictures because it is very interesting to see a variety of samples through SEM and not just for analysis or for understanding but it is also very enlightening to see some of the stuff which is hidden to our naked eye comes to prominence when you put it in a microscope so I will just take you through some examples of SEM pictures and tell you what are the ranges range of material that you can scan and what informations that you can look for so to start with I will show some examples of secondary electronic images what you see here is in black and white but you can actually do a color mapping also to show the platelets present in our human body and these are the platelets that one can see which is present in our serum and on the right side what you see is platinum coated snowflakes snowflakes have very preferred geometry as you can see here the symmetry of this flakes which is actually coated with the platinum then you can actually retain that or you can stabilize that otherwise the interaction with the electron beam the snowflakes will melt therefore these are platinum coated snowflakes which are frozen and they show very clear symmetry patterns here is another cartoon which gives us idea about the range of gold particles that have been studied what you see here on the left side this cartoon is a bundle of gold wires that has been prepared using chemical route and what you see here is another view graph of a secondary electron image of gold particles prepared by another solution route we see here in this cartoon is a range of gold particles with different size and with different size the symmetry of this gold particles also differ and these are the gold particles that you can see from different extracts you can see tripodal decahedral or irregular shaped ones and what you see here is triangular shaped gold particles and with each of this size and shape the plasma on effect also changes and this is the cartoon which tells that with the size how the plasma emission of gold particle changes as you would know gold is not photo luminescent but they show surface plasma on effect and typically gold particles of 1 to 2 nanometer they do not show the actual bulk color which is gold color they are actually purplish and therefore if you look at the size range and the plasma effect with each size the plasma energy surface plasma energy also changes so the gold sphere has a surface plasma energy confined in this dimension then you can also have hollow spears of gold or triangular gold particles you have rod shaped ones and tripodal shape gold particles or nano cubes of gold particle each has a very preferred orientation of showing light intensity through plasma on effect so there are several reports which talks about a controlled way to synthesize there are also reports where researchers have used bacterial effect on controlling the size and shape of gold nanoparticles and we have plenty of literature data to support these cases so all this can be studied using a second electron image picture from SEM and here is another cartoon which tells about the gold nano particles which are deposited from auric chloride and tetra-octyl ammonium bromide and these are different cartoons which talks about the density and shape control of the gold nanoparticles in a variety of or in different dimension of the substrate length as you see you can actually code this in 4.3 millimeter substrate or you can go up to 14 mm substrate so with the chemical approaches you can actually deposit in a very controlled way gold nanoparticles in a variety of substrate so you can map those things conveniently using SEM microscopy in these two view graphs you can study how zinc oxide can be prepared and these are zinc oxide bundle of nano wires as you see here and these are irregularly oriented in a random way like a bunch of nano wires you can prepare whereas in the cartoon that you see here you have oriented zinc oxide bundles and the insert shows how you can map that these are electro chemically grown zinc oxide wires with a specified orientation and this also you can laterally see through a SEM micrograph and these two micrographs talk about carbon nanotube and here is a carbon nanotube in a random orientation and here you can see carbon nanowires can be grown in a specific way and just to compliment our understanding you can also see through the TME images these are TME images of these carbon wires or fibers where you can see preferred deposition of cobalt atoms inside carbon nanotubes this is for cobalt atoms and here we can also incorporate silica in carbon nanotubes this is silica in multi wall carbon nanotubes and here the same TEM pattern can be used for studying a single wall or for a multi wall these are multi wall carbon nanotubes and these are single wall carbon nanotubes so TEM and SEM can offer a combined approach can give very useful information to study a particular system SEM can be used also for several other materials here is a example of cardiovascular stent which is coated with polymer composite and we can actually make the contrast between how a polymer coating can be made on thin specimens as you would see here the dimension of this is less than 4 millimeter these are 4 millimeter dia cardiovascular stents and this is the expanded stent once you put it in the artery you can expand this and these are uncoated stents and what information we can get out of it if we try to expose this cardiovascular stents to human platelet for a preferred time period and then try to look at the surface as to what has happened to the coated and the uncoated cardiovascular stents here is the view graph that tells clearly that the platelets are totally kept away from the surface in the coated stent whereas the platelets are adhering to the surface of a uncoated stent so this information can give us some idea whether the platelet which is sticking to the surface of the cardiovascular stent can initiate thrombogenic activities in the case of the cardiovascular stent which is coated with polymer you do not see any platelet adhesion which means the thrombogenic activity will be very very minimum in other words it is anti thrombogenic so such useful information you can get out of it here is another view graph of the same what I had shown here if you take a close look at the surface of these coated stents you see almost practically you do not see any platelet there whereas the whole surface is plagued with the platelet adhesion in a uncoated surface suppose the platelet adhered surface is there then it is possible to see the influence of another coating of the polymer composite and here is another view graph which tells how ACM can go hand in hand with your deposition process and if you take this platelet adhered surface and then you try to coat the polymer once again this whole surface now gets transformed almost as a unadhered surface so this shows the influence that you can map even a very thin coating of the polymer on the surface of the stent so ACM can be used for such intricate analysis here is another range of material which gives us idea about the influence of the particle morphology specially with addition of ruthenium this is a well known battery material which is called a lithium manganite it is a layered material suppose you prepare this using solid state method at 800 degree C the particles usually show this sort of morphology and if you further heat this sample then you can see they are coalescing together into larger particles and if you try to substitute ruthenium into this LIMN2O4 and if you look at the morphology just with little amount of ruthenium you can see the whole morphology of this particle changes what does that has to do with this because this battery materials have to be compacted loosely but in a battery and the particle morphology therefore becomes very very important and you can see from irregular or rocky type of material you can bring down the particles considerably even with little amount of ruthenium doping and you can see a very mono sized uniform grain growth of this materials when you go from 950 to 1050 degree C so you can get some understanding about the particle morphology not only with respect to temperature but also with respect to the elemental composition so it gives you a indirect clue that ruthenium is actually playing a role on the particle morphology so it is very useful even for preparing for devices to have a closer check using ACM micrographs here is another view graph of cunaline precursors if you take gallium q3 or indium q3 or aluminum q3 you can see that the morphology of this aluminum gallium and indium are more spherical in nature when they are formed as a precursor complexes whereas if you take aluminum 0.5 indium 0.2 gallium 0.3 q3 if this is the composition which we can refer it as AIG q3 complex then the morphology of this solid solution changes entirely from the respective morphology patterns so this also gives us some idea about whether the solid solution is indeed forming or not so ACM can prove very useful to understand how the influence of the substitution affects the morphology of the samples we will look at the backscattered image and see what information we can get out of backscattered image processing and we will see that in the next few slides. Let us take the case of magnesium which is doped in zirconia system and then it becomes very easy to understand how backscattered image can give us information for example if you take zirconia system and then keep on increasing the substitution with the magnesium the idea is to reinforce more strength because zirconia is a tough and ceramic it has very high mechanical strength so if you put little bit more of magnesium it is reported that you can increase the strength of the zirconia particles as a result we can actually go through a range of substitution to increase the solubility of magnesium into zirconia and as you see here from the x-ray pattern the magnesium does not seem to be coming out as a MGO impurity because MGO is a cubic system and zirconia is also a cubic system and fortunately you see here only the cubic zirconia pattern that we can infer from the XRD so magnesium oxide is actually absent in other words magnesium is getting substituted so with this information one can go ahead trying to sinter this and make it into a very strong solid but what we should understand is when we heat it to high temperature these solids can disintegrate or the phases can come out so the only way that you can characterize that is using SEM so I will give you some information about how backscattered image can give us a clue about the limiting composition as well as where to limit the sintering temperature this is a view graph that shows fine grained grain growth for zirconia and these are not polished surface as you see here there seems to be hole but this is not hole this is the surface defect because it is not a polished surface but once you polish you would actually see almost this sort of small pittings are not there which means even at 1100 degree C these nano particles of magnesium doped zirconia can be sintered almost to 99 percent theoretical density but if you keep on increasing the annealing temperature say from 900 to 1000 to 1400 if you keep going the more you go the more you sinter it into a 100 percent theoretical density compacts therefore you usually prefer a very high temperature so having seen that there is a very clear grain growth and you also do not see much of a phase contrast in this grain boundary because this can be detrimental for toughening mechanism meaning if there is any phase segregation usually the tendency for that secondary phase is to be present in this grain boundary so this particular image shows that there is no phase segregation now let us go to the case of 1400 sintered sample if you look at a 1400 sintered sample suddenly you start seeing this sort of grains protruding out of a uniformly grained situation which you can see from the previous picture in this picture you do not see any such grains coming these are all uniform but when you go to 1400 you see several such things coming out of so when you take a closer look at that and now instead of secondary image you can see here this is written Se Se means secondary electron image from there if you go to backscattered image that is what you see here it is written ASB this is backscattered so both are same only thing I have switched over from secondary electron to a backscattered image now immediately you can see that there is a clear phase contrast between the macro grains and the ones which are segregating out so what does this mean that the chemical composition in this area and the chemical composition in this is entirely different and if you carefully look at this this black regions are turning out to be MGO and this region is relating to zirconia so what we infer from this backscattered images at 1100 there is no phase segregation whereas when you go to very high temperature even though x-ray remains clean it does not mean that there will be no phase segregation so room temperature x-ray pattern is not a conclusive proof to say that there is no phase segregation so when you sinter it usually the several things that happens inside the sample comes out and backscattered image can prove to be very very useful to understand that here is another image where you see the secondary electron image does not seem to show any phase contrast whereas backscattered image shows that along the grains there is a compositional difference compared to the macroscopic image so backscattered image therefore provides a very important clue to the chemical inhomogeneity or homogeneity apart from that we can also look at the x-ray analysis that is possible through SEM and edax is a very useful technique which is always attached to the SEM microscope and edax is nothing but energy dispersive analysis of x-rays of x-rays so whatever x-ray that is coming out can be used to quantitatively measure the amount of that particular element that is present which is also present in the same area and this is the way the x-rays come out and when the secondary electron is removed then you have the outer electrons coming to the knocked out electrons therefore you can actually get different sort of radiation one is k alpha radiation k beta or l alpha and so on if you are looking for very higher elements usually you get m alpha or l alpha radiation but for smaller atomic numbers you usually look for k alpha or k beta radiation if you are looking for lanthanites usually go for l or m so in a typical energy dispersive analysis you can find out that this sort of peaks do come you can also get contrast for oxygen but usually in edax we do not rely on anything less than oxygen atomic number below 13 we do not really take that very seriously although they can still give contrast and specially from beryllium boron downwards we do not even do that therefore we do not quantify any hydrogen that is present in the sample hydrogen or helium or lithium we do not really consider that as a useful technique to quantify those presence of those elements so when we go for higher atomic numbers then the contrast is more and for example you can see typical ion k alpha k beta reflections do come here and we have magnesium ion k alpha so each of this binding energies are given here and that the area under the curve gives the quantification of how much of that is present but this is usually a semi qualitative image because there are also surface impurities which can distort the elemental composition one of the reason is that you do not have a very sensitive detector but thanks to the advancement in electronics today that we have a detector which is much more sharper to pick up the signals in the form of silicon drift detector it is called STD which is nowadays coming in later versions of SEM therefore energy dispersive analysis can now be treated as a more quantitative measure but usually it is used as a qualitative way to figure out whether something is there for example if I have gold coated a sample then the sample if you analyze the x-ray it will also show gold pattern so it is so sensitive even to very small or very small thicknesses of deposited elements therefore energy dispersive analysis by and large we can say it is a qualitative measure but with the sophisticated electronics today we can call this more as a quantitative analysis. Here is another view graph just to show how magnesium doped zirconia can be treated and estimated you can use you can usually quantify the data and find out how much of magnesium is present in zirconia this is a reflection for ZR and this is reflection for MG so this can prove useful FE SEM is another technique which is much more refined compared to normal where a field emission cathode is actually replaced for a electron gun and because this provides a narrower probing beams at low as well as high electron energy you can actually get a more improved spatial resolution and the charging is very less and damage to the sample is very less. So field emission microscopes are much more convenient and it is used nowadays compared to traditional SEM one of the main reason is that you get a narrower electron beam and it improves the spatial resolution and this is FE SEM that is available today's market therefore we can see some of the advantages why field emission SEM can be used it gives a clearer image spatial resolution and therefore we can go down to even 2 nanometers which otherwise was not possible using SEM conventional SEM you can map only up to say 50 to 100 nanometer thick particles but we cannot go down on the phase contrast but nowadays with the field emission SEM it is much more sharper by 3 to 6 times smaller area contamination spots can be examined using field emission therefore even if the surface is not really flat it is possible for you to go down to a 3 dimension sample and we can collect the x-rays also more easily in such samples and reduced penetration of low kinetic energy electron probes closer to immediate material surface as a result we can go in for even materials which will which will demand very low voltage for the inorganic samples usually you look for very high voltages so we can actually play around with the voltage say from 0.5 to 30 kilo volts as a result even biological samples can be easily probed with much contrast because your accelerating voltage can be brought down significantly as you see here it is impossible to operate SEM at 0.5 kilo volt but because we can play around even with the vacuum then it is possible to operate the SEM even at voltage which is less than even 1 kilo volt as a result we can go for a range of samples now conventional SEM it is not possible to study any biological samples because the moment you focus the electron beam it will immediately rupture the sample but today's the field emission SEM is giving a very different landscape for studying materials another technique which has emerged but not very popular but physicists really rely on is something called SEM STM suppose I want to do a scanning tunneling microscopy on a particular area it is impossible to take the STM tip to that particular area therefore if you have a combined technique of SEM in SEM you can first probe the macroscopic view from a SEM and once you have spotted a particular region you can take your SEM tip there so this is called SEM STM combine which is not very popular because of the sophistication it requires very ultra high vacuum atmosphere and as you can see here this is a SEM image where you can bring the SEM tip to that particular desired region and you can get a SEM image of that this for gold overlays so it proves very useful for you to bring down your focus to a particular point but just left alone to STM it is very difficult for you to take to the right locations this is the view graph that shows how a SEM image can help you to take the SEM STM tip to a preferred location and this is another image of a SSTM tip going to a preferred place and not only that there are several companies which are bringing a hybrid instrument and here is a four probe station which combines SEM STM and XPS also all in one mode so you can go for a inside to characterization using different techniques and this is the same view graph of this instrument where you have your SEM STM and then you have the surface probe spectroscopic tool. TEM microscopy is another complementary one I may not be able to run through full details on the TEM but just to tell you where we really differ if you think about SEM and STM the sample projection area and the way we harvest the image differs in case of SEM sample is actually kept here whereas in case of TEM sample is actually kept here and the image of this sample is actually taken in a fluorescent screen and when you try to take the image you remove the fluorescent screen and then you can photograph the image. So this is the main difference between a transmission and a SEM sample but another requirement for transmission microscopy is the sample has to be very very thin so that the light can transmit through this one whereas in the scanning you do not look for the sample thickness. So that is the main advantage so you can see here the sample is placed here for a TEM whereas here for SEM and then you capture the TEM image in a fluorescent screen here and the TEM sample preparation is much much more cumbersome than SEM therefore we do not see TEM as popular like SEM and here is some example of how TEM can be used for example this is a sample with the nickel doped in zirconia. The transmission mode can be viewed either as a image in this form or as a electron diffraction spot so if you have electron diffraction spot the information that you get here is that of a polycrystalline nature but the spots really show whether nickel is actually coming out as impurity or as zirconia and you can see here zirconia does not show any nickel precipitates coming out but for nickel that is doped up to 50 percent and 60 percent you can clearly see that these are all the nickel oxide particles that are segregating out of zirconia. So you can get very useful information from TEM and these are cobalt doped zirconia particles you can see for up to 10 percent there is no phase aggregation of cobalt oxide whereas in the case of 20, 30 percent you can see cobalt oxide that is coming out in a polycrystalline zirconia patterns. We can also you study TEM for going down to nanometer size and these are cobalt platinum alloys usually when you look at this alloy you see a amorphous picture in the electron diffraction but if you keep on focusing the electron beam you can see as you focus the particle actually goes from amorphous to a crystalline pattern so that you can see from here. So TEM can be used to probe nanoparticles. Here is another example of how we can use TEM this is for iron platinum nanoloys prepared by chemical route as you can see here we can get the image of this iron platinum nanoparticles which are much less than even 2 nanometers. So with such contrast we can get the nanoloys mapped and we can also get several interesting features about these nanoloys if you take a careful look at the TEM picture. This is another example I have actually touched upon this in module 2 when I discussed about multi layers TEM microscopy can be used even to look at the interface between multi layers that you grow using thin films and here is a YSE that is Itria stabilized zirconia which is a substrate and on this substrate if you try to dope cerium doped lutecium oxide you can find out how epita actually this film can grow on a zirconia substrate. This is the zirconia substrate diffraction pattern and once you look at the interface where exactly the film is growing immediately you see the pattern is changing but once you go to higher thicknesses and look at it this is exactly the TEM pattern of lutecium oxide alone and this is the interface and this is the zirconia bulk substrate. So you can clearly see whether epita actual growth is following or not another thing you can see that the diffraction spot distances between these three spots is equal to the distance between these two. In fact the lattice constant of this is actually half of the lattice constant of Itria stabilized zirconia. So by mapping this we can find out whether we can epita actually grow highly oriented films so TEM can become very useful again in mapping the interface region whether you can make a epita actually grown film we can use manganese samples grown on lanthanum aluminate using pulse laser deposition and find out whether oriented films can be grown. So TEM can be used for mapping whether any atomic level defects are there which can be mapped and another useful information that we can get nowadays is using focus ion beam. Focus ion beam is almost similar to SEM even if you look at today's images of the instrument they look by and large the same and what you essentially do here is instead of using a electron gun you are using gallium ions and the gallium ions can be used in the same way to map the topography of your sample not only that the gallium ions can be used for making any impressions on your surface. So that is why it is called as focus ion beam because you are using a gallium beam instead of a electron gun to study the surface and this is a example of how focus ion beam can be used in the place of SEM not only for mapping the topography but you can also preferentially do cutting this is possible and we can use a combine of FIB and SEM to probe a particular place to prepare sample for TEM therefore it has become a very useful complementary tool other than SEM and FIB can be used not only for mapping but also for writing something or for etching both can be done and in this is a important view graph to say how on silicon we can try to do etching. So if you take xenon fluoride then it will react with silicon and it will knock out silicon fluoride and then you can actually etch those patterns. So gallium assisted etching can be made using FIB we can make writings of this order and this is the contour of your FIB which involves gallium ion and this is the FIB column suppose you are going to study the image then this is the orientation that you do and suppose you are going to cut it you tilt the sample orientation and essentially this is a new form of imaging technique that is coming but the cost of the FIB is now three times more than the traditional SEM therefore this is proving out to be a very useful technique as you see here this is not a virus image you can actually use FIB to not only map but also to construct several structures because you can do deposition you can do cutting and you can do imaging all in one and therefore we have a new generation microscopy tool that is coming which is FIB that is focus ion beam. So we have seen in this lecture at different examples where a combined technique of SEM and FIB can give you a very comprehensive analysis of the material that you are preparing so all this are useful for imaging your materials that you synthesize using variety of techniques.