 Well in the last class I have discussed about the efficacies of using advanced characterization techniques and I also discussed that why different techniques to be used in combination. In the subsequent lectures we are going to discuss about different microstructure related advanced characterization techniques. Microstructure is the basic theme of material science that is why most of the research studies involve only not only seeing the microstructure using different kind of machines but also to analyze them. So in this lecture we are mostly going to concentrate on electron microscope especially the transmission electron microscope before I just begin I must tell you that why the transmission electron microscope is very important and how this technique has been developed over the time period. As you know electrons are discovered in the beginning of the last century or may be end of the 19th century and then people started realizing that electrons can be used for many purposes but as far as microstructure is concerned if you want to discuss about microstructure we should know about microscope. So first let me discuss about the microscope what is a microscope we know that our normal eye can see a feature as small as 0.1 to 0.2 millimeters. So I can see any feature which is in this range 0.1 to 0.2 millimeters that means if there are two points on the space our normal eye can resolve them very precisely in the distance between these two points range from 0.1 to 0.2 millimeters. Say any machine which can resolve two points closer than these numbers from 0.1 to 0.2 millimeters can be defined as a microscope. So therefore this study of developing those kind of machine is began long back in fact in 17th century by Leuwen Hook first time in the year of 1968 Leuwen Hook discovered the optical microscope in which the normal light was used to image many important algae or even small features which are present for the small insects. So that is the beginning of the microscope the using of instrument which can resolve objects finer than our eye can do but it went on for the next subsequent two centuries or three centuries the optical microscope was refined and used for many purposes but in the end of 19th century and the beginning of the 20th century people started realizing that electrons can also be used for electron microscopes or can be used to image things which can resolve objects much finer than what an optical microscope do. So this has led to a lot of discoveries in the arena of electron microscope which I will discuss first then finally I go to how the electrons interact with the material the most major discovery which has propelled the discovery of electron microscopes by de Brocley de Brocley in the year 1925 de Brocley in France he first time told electrons can be used as a wave and he said that electron wavelength can be defined by this formula where h a lambda is the wavelength of the electron h is the constant and p is the momentum of the electron. So therefore we can write down h equal to this equal to h by m into v where in the mass of the electron and with the velocity of the electron it all started with the discovery by de Brocley in 1925 when he stated that electrons can be considered to be a wave with wavelength given by this formula lambda equal to h by p where h is the Planck's constant and p is the h is the Planck's constant and p is the momentum of the electron. So therefore we can write this is equal to h by m b where the m is the mass of the electron and with the velocity. So by knowing that electrons can be used as a wave or can be here like a wave one can use this particular feature of electron to make use in microscope within just next 7 years that is in 1932 two scientists E Uska and M Noll first time demonstrated that electrons can be used as a tool for microscope and first electron microscope first rather transmission electron microscope was demonstrated and this is a major discovery and for this discovery in fact arms Ruska own Nobel Prize in 1986 after almost 50 years. So this set the tone or that electrons can be used as an imaging tool people know it because of the fact that electrons can have very small wave length. So let me just state how this history has developed first then get back into the efficacy of electrons in the microscope first electron commercial electron microscope came up in the year of 1936 just after 4 years of first demo of electron microscope so this was by a company name metropolitan Vickers UK but this microscope was now successful. So therefore the first successful commercial microscope came up in the years of 1939 just on the year of second wall over by Simmons Hulski but this has basically made electron microscope available even before the second wall over but then after the first second wall over they down those days it was very difficult to prepare samples for electron my T M because sample needs to be very thin to observe under electron microscope especially in T under T M. So the tool for making those thin foils came up in later on by another German scientist in 1949 just after the second wall over known as hidden rich he discovered the technique to prepare very thin foil so that it can be observed under electron microscope T M but subsequently many other companies not only Simmons and Hulski but many other companies like Geol Hitachi even Philips started manufacturing time is electron microscope and later on from 1950s to onwards T M was extensively used to image many samples which is either metallic or ceramic in nature so major groups which started working on this research was one in Cambridge University of Cambridge and later on in US in US to America so finally the theories of electron microscope was came up by few scientists in Cambridge especially Professor Hersch and his co-workers they develop the theory of T M in University of Cambridge and then onwards lot of work has been done on electron microscopes by different groups which I will not discuss so that is the basically the brief the history of how Tasmus electron microscopes came up in the scientific community and later on used extensively to probe different kinds of phenomenon in material science and engineering so let me just now get back into the comparison between optical microscope and electron microscope why there is a need to use electron microscope when optical microscope was already existing what was the basic reason for that then we can understand many other features we know that any microscope the most important feature is resolution resolution means ability to distinguish objects which are separated by small distance like the one I discussed few minutes back if I have two points separated by small distance whether the machine can resolve these two points successfully or not and this was dealt extensively by Rayleigh and he discovered or he gave a formula to correlate the resolution with wavelength and other parameters so this formula is given by ? equal to 0.61 ? is equal to 0.61 ? divided by ? ? where ? is the wavelength of radiation ? is refractive index of the medium and ? is the half angle of the lens of the collecting power of the lens so therefore if in a in extreme case if we consider ? ? to be equal to 1 then the resolution become 0.61 ? that is about 60% of the wavelength and if you consider the normal light which is used in the electron microscope if the normal light has different wavelengths if you consider the green light which is wavelength of about 600 nm 550 nm rather not 600 nm so the resolution power of the light microscope is approximately about 300 nm so therefore we can write down for optical microscope we can have resolution power of approximately 300 nm so we cannot resolve any object whose size is more less than 300 nm that is a basic need to develop electron microscope because we know that electrons has a wavelength much lower than the normal light so if we consider the wavelength of electron we know that wavelength of electron depends on the accelerating voltage and normally the wavelength of electron is given by a formula which can be written like this ? equal to so wavelength of electron normally can be given by H divided by 2m0E V2 to the power half or rather we can write down this is equal to ? 2m0E constant and V2 to the power half so therefore sorry H divided by 2m0E to the power half V to the power half so that means the wavelength of electron is a strong function of the voltage by which it is accelerated so if we calculate that wavelength of electron at 200 kilovolts which is used in normal in transmission electron microscope so ? will be approximately 0.0251 nm so immediately you can see that if the ? is an electron microscope is given by this then ? will be less than 1 picometer according to this formula given by this so therefore one can clearly see that it is possible to reach a resolution theoretically from 300 nanometers in optical microscopes to less than a picometer in an electron microscope but normally we do not achieve this resolution because of the problem in the lens so if we consider the optical versus electron microscope the tannous electron microscope there is a huge benefit in terms of the resolution or resolving power of the microscope by knowing this one can in fact go on and develop different kinds of microscopes to get understanding of the phenomenon happens in the materials science engineering let me just show you few of these features so this particular slide shows you how the microscope has developed from 1668 by Levenwick to the 2006 by FEI a company which has manufactured the highest best possible resolution in a tannous electron microscope obviously Ruska and Norl's contribution is extremely huge as far as the first microscope is concerned from 1932 to 2006 in this about 80 years approximately microscope microscope seen a drastic development so using the Titan microscope which is now sold across the whole world by the FEI one can reach a solution of order of less than 1 Armstrong as I mentioned that although the theoretically possible resolution in this tannous electron microscope is one picometer but we can never achieve because the problem of lenses so by correcting different lens it is possible to have a resolution of approximately 0.6 Armstrong which is the best possible solution till the achieve till today now obviously one needs to know that in a normal tannous electron microscopes what are the different things are there as compared to a optical microscope let me just go back to the optical microscopes a optical microscopes consisting of basically three things one is illumination system that is the light in optical microscope you can either have a limited system coming from down in a biological microscope or an illumination system coming from top in a metallurgical microscopes so these provide a source of the light and then we have to have a sample obviously which is placed on a sample holder at the top of which we have basically imaging system which is nothing but an objective lens and from the objective lens the image forms and we can see this on an eyepiece so that is basically a viewing or a recording system either you can view using a normal or you can put a camera this is the basic construction of an optical microscope we will see that time is electron microscope looks exactly similar except that there are a lot of complicated lens system inside and time is electron microscope and now source is obviously an electron beam not an optical light so if I describe this in terms of a simple ray diagram this one look like this so in the slide so you have basically a source basically a source here source is can be electron we will discuss how a different sources can develop on the electron microscope also then from the source the electron means are basically focused by a first condenser lens to a very small focused beam and that focus beam can be again defocused initially by second condenser lean also can be focused later on in the on the sample normally in electron microscope we put lot of apertures to select a particular electron beam so condenser apertures serves the one of such aperture fire which can control the beam size sample is kept or immersed rather within the objective lens I already told in the few minutes back how the optical lens is kept in optical microscope in electron microscope sample is just placed or immersed within the objective lens and then of these beams which falls on the electron which are falling on the sample they some of the beams are diffracted some of the beams are transmitting to the sample those beams can be used to form images by using objective lens or they can be used to form diffraction patterns by the objective lens diffraction pattern normally forms of the back focal plane of the objective lens so this fraction patterns can be either a selected area diffraction pattern or can be micro diffraction pattern or can be even convertible electron diffraction pattern depending on what kind of techniques you are using and this images are the diffraction pattern can be then magnified using intermediate lenses or the projector lenses this is in a nutshell a structure of a conventional TEM so therefore it is not as complicated as we think or as we observe or look at it in one of the lectures I am going to show you the actual terms electron microscope and how is operates but for the sake of understanding today you can clearly see that if we have a source electron means so we can use it to form an image on a plate this is mainly because the electrons cannot be seen by eye so therefore when the electron falls on a phosphorous screen then only we can see the image or the diffraction pattern so that is kept at the bottom and that is why the person sits and looks at the screen when the microscope is running as a sample in the column so whole thing has to be under vacuum because electron cannot travel so the whole thing is under vacuum if electron cannot travel in air so that is why we need to have very high vacuum system to have a very good microscope and the samples obviously are to be thin enough so that electrons can pass through it can diffract it so as you can clearly see as the electron falls on a sample there are a lot of interaction happening and so now next thing which I will discuss is that how this interaction can be used up so let me just go back and tell you how this interaction can be used in the real for different purpose of the time select on microscopes if I consider this to be my sample and let us assume that electron beam falling on a sample this is my incident electron beam we are going to see how this electron is going to interact with the sample because this interaction will give us an idea how they can be used for different kinds of analysis obviously for some of the electrons will get absorbed inside this material they can be called as absorb electrons some of the electrons will pass through they are known as a transmitted electrons some of the electrons which will pass through they are actually passing these electrons which are transmitted passing through without undergoing any kind of deviation form is original path so that means these electrons are not undergoing any scattering but some of the electrons which are passing through may undergo scattering or diffraction so these electrons can be of two types one which undergoes elastics scattering or you can call that elastically scattered electrons and other electrons can be elastically scattered electrons so elastically scattering electrons are once which undergoes the normal electron diffraction and form the electron diffraction pattern elastics scattering electrons can lead to different kinds of things obviously one who has seen the electron microscopes we have seen the Kikuchi lines or Kikuchi bands they actually form because of the elastics scattering electrons or we can use it for spectroscopic analysis like in energy loss spectroscopy which we will discuss later on now if let us look at what happens to electrons which are reflected back or scattered in the direction of the another direction that is opposite to the direction of the instant beam some of the electrons will come from the very small thickness of the tops of the sample they are called secondary electrons some of the electrons these electrons actually generated because incident beam electron is having very high energy so once this electrons fall on the sample they can eject some of the electrons from the outer cells of the sample and because of this this electrons is ejected from the outer cell of the samples will have low energy they will just come out from the surface and can be called as a secondary electrons some of the electrons which are falling directly from the sample may go and hit the nucleus of the atoms and they can be then back scattered so they are called as a back scattered electrons okay not only that some of the incident electron may even cause ejection of the electrons from the outer cells or the inner cells not sorry outer cells so some of the incident electrons can actually eject electrons from the inner cells of the atoms in the sample and once this ejection happens some of the electrons which is outer cell can jump in into the inner cell which has become vacant now and because of this transition from the outer cell to inner cell the energy can be released in terms of x-ray so therefore we can have x-rays coming out because of the interaction with the incident beam and the sample in many cases it has been found that other electrons can also come out from the top surface of the samples some of the samples like semiconductors they can even produce photo luminescence so once we know that electrons during interaction with the sample can generate so many different types of signals we can use each of the signal for different kinds of purposes the signals which are passing through or other transmitting and getting either getting diffracted or not getting diffracted they can be used basically for the transmission electron microscopes okay x-rays generated by this kind of interaction with the sample can also be used for the time select from microscope which we will discuss when we discuss about the EDS or Electron energies spectroscopy the electrons which are described as secondary electron or basket electron they are used in SEM for imaging other electrons are used in other spectroscopy for getting composition and the event to know the state of the electrons x-rays as I said can be used for another dispersive spectroscopy or EDS analysis both in SEM and in the TEM and as I said photo luminescence can also be used for different kinds of semiconductor material analyzing the photo luminescence characteristics so by knowing that electron can interact to the sample and need to different kinds of signals I can we can now device the techniques in the electron microscopes especially in transmission and scanning electron microscopes to utilize each of the signals and provide the information from the sample which you are probing so next I am going to tell you how this can be used so this slide what has been shown is the basic features not all the features which I have shown in the board basic features so if you see the incident electron beam falls on the sample and then we have a direct beam the beam or the transmitted beam electron which I have written there which has not undergone any kind of scattering so that can be used to me to form what is known as bright field image in TEM on the other hand elastic is catering electrons which is shown even on the blackboard can also be used to form diffraction patterns English is catering electrons are normally used in energy loss spectroscopy electron energy loss spectroscopy and for the electrons which like backscatter or a second electron as I said they can be used in the scanning electron microscope X-rays can be both used in the TEM as well as HEM for energy disperses spectroscopic analysis to get composition analysis in the sample so that is in a nutshell what is basically used what I have not talked about is that is backscatter electron can also be used to obtain crystallographic information in HEM whether there is a new technique which has been developed up late from 1990s known as electrons backscatter diffraction or EBSD that is used extensively now to get diffraction information and as well as texture studies in the material so therefore we shall discuss in our subsequent lectures the imaging in time electron microscope, bright field and also the dark field images as well as diffraction pattern and other contrast mechanisms we shall discuss about the EDS spectroscopy analysis in one of the lectures we shall discuss about the EBSD in subsequent lectures also we shall discuss about something related to high lucent microscopy so I will also like to tell you one more thing about the electron microscopes which is known as depth of field before we go into the details of the other features depth of field is basically measure of how the object is in focus at the same time like if a very object which is very you know ups and downs are there are a lot of undivided surface of the object whether can you bring the different portion of the sample objects in focus or not as you know the optical microscope as a very low depth of field so therefore any sample which is very rough cannot be bought in focus in optical microscope very easily on the other hand electron microscopes as a very large depth of field so therefore they can be bought in focus in electron microscope very easily that is why those of you already seen the electron microscope or TEM especially you have seen that many cases we record the images in a plate and the plates are kept at a distance much lower than the view skin so the depth of field is not high so high we cannot even record this images under same focusing conditions because depth of field is very high so therefore we can use both the TEM and ACM to get good understanding of the surface features of the sample especially in ACM so far we have discussed about the different types of signals which can be generated from the sample when electron beam interacts with the sample let us now look at seriously each of this in a time is electron microscope in time is in electron microscope electron beams accelerated by about 200 to 300 kilo volts are allowed to follow the sample so therefore incident beam which is very high intense in false and thin sample and then interacts the sample as I said said that during interaction it can either have it can either create a scatter electron beam which is elastic scatter or inertially scatter or it can create it can generate a beam which is not undergone any kind of scattering so in a normal transmissive electron microscope we can use the forward scatter or the transmitted electron beam for bright field image this is done by putting a small aperture here so once we put an aperture nothing but a thin plate with a hole so we can block all the other radiations except this transmitted electron beam so therefore image formed by this can be easily called as bright field image because this image will have bright contrast or the image will be brighter contrast that is why it is called as a bright field image to give an idea how this is done let us see here this is taken from a sample which is tib 2 or titanium diboride thinned by to the electron transparency so if I see the diffraction pattern obtained from this grain which is shown here a diffraction pattern will look like this with the term is electron beam here and that means that this one is formed because of the forward scatter or the electron which is directly passed through without undergoing any kind of diffraction on the other hand the electrons which have undergone scattering or diffraction can be seen like this these are elastically scatter diffraction electrons which has led to the diffracted spots now if I put my aperture only on the transmitted beam that is what I said you I generate this image at the top left which is known as bright field image on the other hand if I consider one of this electron diffracted beams like this one here 101 and put my aperture in around this and then I form image what I get is known as dark field image or the central dark field image center dark field image this is obtained by bringing this weaker reflection on the opposite side which is same as this one to the center and put the aperture if we instead of bringing this weaker reflection of center if you bring this one the strong reflection 101 to the center and we can form an image is known as the weak beam dark field image as you can see in the bright field image there are lot of dislocations inside the sample and this dislocations can be very nicely seen in a weak beam dark field image or can be easily captured in a weak dark field image the reason is this in a center dark field image the whole grain is illuminated strongly because it is the diffracting strongly. So therefore the the defect structure cannot be seen so nicely on the other hand in a weak beam dark field image you can easily reduce the intensities because you are bringing the strong parts in the central part of the intensity will be reduced so therefore you can create a much better image so this is in a national in a conventional microscope is done routinely that means you can go to a sample go to a region of a sample which is thin enough then oriented to particular crystallographic axis and get the diffraction pattern using the diffraction pattern you can either take a bright field image or a center dark field image or a weak beam image this is a very routine thing normally people do but in advanced characterization course like this we are going to learn how we can see even much better how that we can see even atoms or column of atoms in a electron microscope or not in the with the use of the advent of the new generating electron microscope we can see very nicely the atoms a column of atom in a sample that is what I will tell you now okay let me just get into that this again is basically today describe you how a electron microscope can be judiciously used to obtain a lot of informations particularly the spectroscopic and the real space information as I said a diffraction pattern can be used to generate by field dark field weak beam with dark field a center dark field images the diffraction pattern is basically tells you a reciprocal space information as we have already learned from the normal the course on characterization techniques so the spots actually related to the reciprocal space. So therefore to obtain the actually the atomic structure the how the atoms looks like in a sample one need to get a real space information this where the atoms are sitting in a sample so that is what is called real space information that can be obtained by something known as the phase contrast so therefore the contrast which I discussed here is basically coming from diffraction and they are known as diffraction contrast so they can be changed depending on the diffraction conditions on the other end one can use something known as phase contrast which you will discuss in detail in the next class phase contrast what is that let us now look at first the sample this is zinc cobalt gallium oxide nothing but a cobalt oak zinc gallium oxide zinc cobalt and gallium to zinc oxide and if you look at the diffraction pattern which is shown here it shows a particular symmetry and zinc oxide as you know is hexagonal crystal structure therefore this diffraction pattern is taken along 001 of the zinc cobalt gallium oxide and the crystals are the by taking the white filmage by selecting aperture from the central beam of the the porous cutters beam we can see the crystals of these zinc cobalt gallium oxides are of the size of about approximately 80 to 60 to 100 nanometers and they are elongated so morphology shapes and everything can be seen some of the crystals are black some of the crystals are a little less black so they some of the dark contrast some of the so light contours so this dark contours light contours mainly because of the diffraction the ones which is undergoing strong diffraction will show the dark contours the ones which are not only strong diffraction they will be showing the light contrast now once I if I if I want to take if I want to take basically the phase contrast image so what do you need to do is that you can select a set of spots okay like first six and as long as the transmitted beam you can select that spots and put a big aperture like that and then you can form an image which is shown on the right hand side so as you know that the transmitted one or the beam which has passed through the sample has not been undergone any kind of diffraction it is considered so therefore it contains a certain phase of the electrons on the other hand the diffracted beams have been undergone diffraction so therefore they contain a phase information phase means the electron phase so if I select all the beams and allow to interfere them in a normal time electron microscope I get something known as interference pattern and interface pattern will show me the columns of the atoms which is shown here very clearly you can see the different columns of the atoms sitting on the surface not only that in fact one can actually take this real space atomic arrangement image and do Fourier transformation in a shop using as particular different kind of soft as which available in the market and then one can easily get back this diffraction pattern which we have been obtained in a normal electron microscopes so therefore this can be used to prove that whatever image is formed using the phase contrast mechanism is basically reflects the real space informations on the left bottom corner of this slide we have shown the edacs or let energy disperses their spectroscopic information from this one can see the peaks correspond to cobalt zinc gallium and everything this one corresponds to oxygen so by knowing this you can see here also oxygen zinc zinc gallium cobalt peaks very easily so by knowing all this information a normal white field the spectroscopic information as well as these the phase contrast image you can get lots lots of information from the term select the microscopes this is a zoom view one can really see of this high resolution micro image one can really see each column of atoms very specifically in a high resolution image this can be done routinely now in a many microscopes because microscopes capabilities has been increased as I said many microscopes have been produced obtained in the market now with a resolution less than one Armstrong so that is why this are no longer a big problem nowadays in fact even a routine user can get this kind of images this another one whereas cadmium sulphide nano crystal has been image one can even see here the hexagonal asment of the atoms very nicely cadmium sulphide is basically having hexagonal crystal structure so this is a very small crystal approximately four nanometers even you can see the atoms at the surface very clearly here at the edges of the sample is a very nicely faceted crystal the advance has happened so much that in the the microscopes called Titan which has the best possible resolution in the world can even tell you the type of atom present on the sample if I go back here I cannot see from this I cannot tell from this high resolution image whether this is a cadmium atom or this is sulphide atom it is very difficult to say the type of atom cadmium of sulphur very difficult to do a normal electron microscopy but with the advent of the Titan with the new kinds of contrast mechanism one can basically tell what kind of atoms are present on the surface by something known as hard if stem or high angle dark field image scanning tasmish electron microscopy this image is taken from germanium crystal which are oriented along one one two directions you can even see clearly germanium dumbbell structures on the surface taken from this Titan microscope this is again taken from a zinc sulphide crystals where cadmium sulphide crystals where you can see the twin structure in a summary I can say that I have discussed today you the interaction of the electron beams with the material and how this interaction can be easily used to generate different kinds of images either diffraction contest images or a fetch contest images in a Titan electron microscopes in addition at the beginning I have discussed you how the Titan electron microscope has come up from 1925 to 2006 in this about 80 90 years of time period and also I discussed resolution the depth of field along with different mechanisms of the image formation the Titan electron microscope in the next class first thing I will go is I will take you to a times electron microscope and show you the real terms electron microscopes and demonstrating you how this different techniques can be used for analyzing the samples in a microscope.