 So, we shall consider one more example wherein we traverse across lens scales and see some interesting effects in terms of the properties of a material. The case which has been considered here is an ion sample which has not been magnetized. In other words, an ion sample which does not behave like a permanent magnet macroscopically. So, what are the lens scales we are going to traverse in this problem? It is going to be the atomic level and to be more precise even the subatomic level which is of the order of angstroms and below. We will travel to something known as a domain level which could be a few micrometer to something below that as well and finally, we look at the level of the entire sample which could be of the order of centimeters. Now, a typical ion sample is actually polycrystalline and this actually in fact complicates the issues which we are considering with respect to the magnetism of a sample. Now, let us consider a magnetic material and ion is the example chosen in the current example and we are up below the curie temperature. Below the curie temperature, we know that the material behaves like a ferromagnet and when you heat the material above the curie temperature, it becomes a paramagnet. Therefore, we are talking about the ferromagnetic condition of the ion sample. In this condition the atomic magnetic moments try to align within a region called the single domain, but this tendency for alignment is actually fighting constantly against the thermal disordering effects which is always present. Now, what is the origin of this magnetic moment in an ion sample? If you take a single atom of ion, there are three possible origins of the what you might call the net magnetic moment. One is due to the electron spin, one is due to the orbital motion of the electrons around the nucleus and here we are considering what we may call a classical view point to magnetism. And finally, there is also a nuclear spin which can give rise to magnetism, but typically nuclear spin is ignored in these systems because the nucleus is much more massive than the electron. And therefore, as the mass comes in the denominator in the calculation of the magnetic moment, you typically ignore the magnetic moment arising from the nuclear spin. Typically, therefore, there are two contributions which are prominent when you are talking about magnetic contributions which are the electron spin and orbital motion of electrons. But this is for a single ion atom and now when we try to make an ion crystal, then it might so happen that the orbital motion could be quenched. In other words, the contribution which is coming from the orbital to the magnetization, net magnetization of the sample could be ignored or could be less. And therefore, the prominent contribution could come from the electron spin. So, therefore, every atom makes a contribution to the crystal in terms of its magnetic moment. And this magnetic moment is constantly fighting against the thermal disordering. In other words, magnetic moments in neighboring ion atoms want to align themselves and that is why this material is termed as a ferromagnetic material. But there is a misalignment or the tendency for the alignment to be lost because of temperature. But typically, what we observe is that there is a net magnetic moment within a domain and but though there is less than the number of actual participating atoms in terms of the number. Now, what happens when we consider the region beyond a domain? That means, when I cross to a domain to a next domain, then there is something known as a domain wall. And if you look at a domain wall which is shown in the picture below, the magnetization vectors or the spin vectors actually go from one orientation to another orientation. And therefore, the region of the domain wall is a region which is in some sense not ordered either to the domain which is on the left which is can be called domain 1 or in that orientation which is to the domain 2. The kind of wall which is been shown here is called a block wall and in thin films, there are other walls which are possible which are called Neal walls also. Now, therefore, if you look at the size of the domain wall itself, it is typically of the order of a few hundred atomic diameters and it is a nanostructure in itself. Though, we have not gone to the level of defining all the nanostructures possible nanostructures. Now, this tells me that though there is an inherent tendency for ion to magnetize, but the magnetic alignment is not perfect due to thermal disordering. And furthermore, this ordering within a domain is broken when we traverse from one domain to the other and the two domains are connected by a domain wall wherein actually the magnetization vectors rotate from one orientation to another. And in this as in the case of the block wall shown, it actually rotates out of the plane to create this two differently ordered domains. Now, what happens when you look at the sample level? At the sample level actually, there is no net magnetization at all and this happens because there are these multiple this sample is actually split into multiple domains and the sort of an what you might call a schematic diagram is shown here in the picture on the right hand side. There is one domain which is pointing which has net magnetization in the upward direction, one in the downward direction, other in the upward direction and there are these closure domains which actually close the magnetic loops. The purpose of doing so is that the overall magnetostatic energy of the sample is reduced when the sample is actually split into domains. And therefore, this domain structure leads to an overall no lack of magnetic movement at the sample level. So, what is that we are gaining by going across these various length scales? There is an inherent tendency for atomic magnetic movements coming from three sources two of which being the electron spin and orbital motion of electrons which are important. But when iron forms a crystal, the orbital motion or the orbital contribution typically gets quenched a lot and therefore, the main contribution comes from the electron spin alone. That means, there is a loss of magnetic moment right at that scale. The next scale is a scale of the domain formation and therefore, within a magnetic domain these magnetization vectors of all iron atoms are aligned and therefore, there is a net magnetization at the level of the domain. But if you look at the sample level again the magnetization is completely lost or there is no net magnetization at the sample level because the sample wants to reduce the magnetostatic energy. Therefore, we see that there are properties which fluctuate as you go from various to from the very small length scale starting from the angstrom length scale to length scale of the material. And when you are doing so, we also encounter structures and properties which are at the nano scale like the case of the domain wall we consider in the current sample. So, the important lesson we learn from considering these examples is that we cannot reside at a single length scale to understand the properties of a material. We will consider one more important example here wherein we go from an iron crystal or an iron sample to the entire component which is the gear wheel. So, not only are we now talking about material length scale, but we are also talking about the length scale of an entire component. And we will see how these various length scale talk to each other and how a certain property is not only arising, but also can be engineered for a particular application as in the case of a gear wheel may be. So, what does a gear wheel require? A gear wheel requires good surface hardness, it requires good abrasion resistance, it also needs good toughness. That means it needs to be shock resistance, it needs to absorb energy when certain energy is imposed on it in the form of an impact or a contact. Therefore, if my gear wheel has to have all these properties then it has to be engineered in certain way. The way the engineering is done is that the gear wheel is machined of course, you can see that there is got a certain geometry out here. And this gear wheel is made in a certain geometry and what is done is that the surface of the gear wheel is actually preferentially imposed with more carbon than the interior. So, we have a sample of a gear wheel on which we impose a carburizing medium and the carburizing medium. In fact, let us carbon diffuse into the sample. So, this is for instance the distance inside the surface and this is the percentage of carbon and with increasing time which is shown by the arrow, you see that more and more of carbon is actually diffusing into the sample. Therefore, we are preferentially increasing the carbon content of the surface. The reason for this is that when I start with the base composition for instance, I am with about 0.12 percent carbon or 0.2 percent carbon that material has good fracture toughness or it has got good toughness. So, what is this toughness property? It implies that the material will fail only after it absorbs a lot of energy. Now, often this toughness is also can be thought of as a product of the ductility and into the strength. It is the area under the curve if not tested enter impact in an attention it can be thought of the area under the stress end diagram. Now, why does the material which is never going to deform so much because if this component actually deforms by even a few microns, then it is meshing with actually start changing with respect to the remaining gear components in the assembly and therefore, any change of the order of millimeter is unacceptable as far as the gear assembly goes. Therefore, the tolerance being so high why do I require this kind of a ductility in a material because we know that the material is actually never going to deform that much. The reason lies in a length scale which is much smaller and this can be understood as follows. Suppose, I am talking at a crack tip and we already seen a crack tip in a material and during manufacturing it is unavoidable that there are some micro cracks present in a material and therefore, this crack tip we have seen already has very high stress concentration. So, this is my sigma x for instance and with distance such a sharp crack is actually extremely deleterious to the material and can lead to the failure of the material. But, if the material has suction ductility and it has got sufficient toughness what would happen is that the crack tip would actually blunt instead of being a sharp crack tip it would end up being a blunt crack tip and this would imply that the crack tip stresses do not become very large and in fact, the crack tip stresses reaches a maximum at a certain distance from the crack tip. So, a brittle or a sharp crack a sharp crack is typically found in a brittle material the crack tip stresses tend to become very large and in fact, in a mathematical form they become singular. But, in the case of a ductile material the crack tip is blunted and therefore, the crack tip stresses are reduced and in fact, the maximum crack tip stresses may not occur at the right at the crack tip, but at certain distance on the crack. This kind of a blunting of crack tip requires and this plastic deformation is what is required for this plastic deformation that we require the ductility in the material. Therefore, to understand the need for this ductility we actually have to go down to the crack tip level and to understand the plastic deformation which is occurring at the crack tip. Therefore, to summarize a gear wheel needs to have good surface hardness it needs good abrasion resistance and also needs good toughness. The reason for toughness we have seen now and the way the good surface hardness is achieved as I pointed out is by actually imposing a higher amount of carbon on the surface. The total treatment actually consists of not only putting this carbon, but also heat and this carbon is typically put at a high temperature. Then later on the sample is quenched from the high temperature, so that the a phase known as martensitic phase actually forms which gives it the requisite surface hardness. Now, if you want to start from the bonding level of the iron. So, we know that iron has a propensity for metallic bonding which ensures good ductility, good thermal conductivity etcetera which happens to be good in the case of a component like this. Because, then the it does not overheat and it actually the heat tends to get dissipated away, but it is soft as compared to say a covalently bonded material like diamond. Now, this softness is directly related to the metallic bond which leads to low pulse stress. Low pulse stress being the inherent lattice friction for the motion of a dislocation. This ductility further helps in improving the microstructure level properties like tolerance to cracks which we have just now seen and this leads to high fracture toughness. Sharp cracks as we saw lead to high stress amplification and should be avoided in a real component and therefore, we by making a material making this sort of material like a ductile material like iron with a low pulse stress as compared to a material like diamond which has high pulse stress we can actually achieve this toughness. The ease of deformation and good tolerance to cracks implies good ductility in a material and this available ductility in can be actually used and actually forming the component or making the component from the metallic components. Now, if we reside at this length scale of the component we already see that there are certain important design parameters which will come into play, but also we are also going to the microstructural level wherein we are seeing that the surface is actually martensitic. We form martensite on the surface of the gear wheel and below this we have a mixed microstructure which actually gives us the good toughness. Now, though iron has a low pulse stress as compared to some of the ceramic materials, but it has a much higher pulse stress as compared to material if it were an FCC form of iron which is found at high temperatures because this pulse stress is a strong function of the burgers vector and also of the kind of bonding which is present in the material. So, we have two sides of the pulse stress one coming due to the bonding characteristics and other coming from the crystal structure. The crystal structure of course determines the burgers vector and the burgers sitter sits in the exponential when it comes to the pulse stress. Now, we said that we are starting with the base material which consists of an alloy of iron and carbon. The carbon actually sits in the interstitial position that means, now I have to focus at the level of the individual crystal level or the individual unit cell level and typically in the octahedral void in BCC iron and gives size to solid solution hardening. In other words a material which is a pure iron with vis-a-vis a material with carbon dissolved in it, we notice that the carbon dissolved material is actually harder. And the slowly cool mixture consists of alpha which is BCCI solid solution and Fe3C which is a hard phase and orthorhombic hard phase. And therefore, this microstructure of a combination of alpha and Fe3C is a harder microstructure which is called a politic microstructure. But the important hardness does not alone come from the solid solution level or the microstructure consisting of Fe3C and alpha phase, but actually it is coming from the formation of martensite on the surface which is hard, but unfortunately is brittle. But by now actually engineering the composition and engineering the heat treatment, I make sure that the martensite actually forms on the surface. The interior has a more slow which experience actually a slower cooling rate, it would have a politic kind of a microstructure or a alpha and Fe3C combination of the phases and this would give it the good requisite toughness at the inside of the material. Therefore, this kind of an example of a gear wheel is and what you may call a classic example of a functionally graded material, wherein each part of the component has a different kind of a property which suits the need of that region of the component. However, we should not forget the usual good design practices which we need to follow when you are actually designing the component that means no sharp coordination be present in the component. Because, these sharp coordinates at the macroscopic level act like cracks at the microscopic level and we act will lead to stress amplification which would be zones wherein failure can initiate. So, to summarize the consideration of the gear wheel, we see that when we want to understand the properties and also want to understand how a component has to be designed, we have to traverse from the atomic lens scale to the crystal lens scale to the lens scale of the entire component. At the level of component we need to follow certain sound engineering practices including the avoidance of sharp corners providing a smooth surface finish. So, that there is no abrasion taking place at the surface etcetera. At the level of the at the other end of the spectrum, we note that carbon which is a present in the starting material of iron is actually giving rise to solid solution hardening and further in a cold slowly cooled sample. This carbon can form an Fe 3 C phase along with the alpha phase and this kind of a microstructure gives it further hardening up and above that present in a pure iron sample or even the solid solution sample. Now, but we go further and we actually engineer the component by actually putting in more carbon on the surface by what is the process known as case carburizing. And in case carburizing the surface the carbon slowly diffuses into the sample and the surface content of carbon increases and therefore, when you quench the sample from high temperatures wherein the carburizing treatment is favorable to be conducted because the at high temperatures the solubility of as we notice that the iron is in gamma form which can actually dissolve more carbon than the alpha form. Therefore, we dissolve more carbon into the iron sample and then quench it to produce a martensitic phase and the martensitic phase is very hard and this gives it the good what you might call the good abrasion resistance and good surface scratch resistance. Therefore, we see that if you want to consider a property or design a component then we have to traverse length scale from the where the interstitial atom of carbon sits to the scale of the entire component. And we can actually not only engineer the component, but actually engineer the microstructure by doing a suitable heat treatment. So, this is another nice example wherein we are traversing multiple length scales to understand the properties. Now, it is time for us to actually jump into the world of nano materials and let us start with a few basic definitions because often in the world of nano there are a single definition could have multiple meanings. We have many terms which come into play and in the process we need to understand how various properties arise in the case of nano materials, nano structures and various other nano terms which we shall be soon introducing ourselves to. So, what are the nano terms which we need to learn? So, let us start with the most basic the word nano itself. In day to day terminology the word nano is quite often loosely used and we should be careful with the use of the word in multiple context. Simply speaking nano is just a prefix to define a factor of 10 power minus 9. In other words the term nano itself is not a measure of length mass or time and hence should be used as a prefix to standard units like we can have nano grams, we can have nano meters, we can have nano herds or any one of the standard using units like nano pascals. So, it essentially tells us that we are 9 orders of magnitude lower than the unit which is being mentioned. In the context of nano materials typically the word nano implies a size range between 100 nanometers. Though there is nothing hard and fast for this size range, but typically in usual accepted terminology we are referring to size range of 1 to 100 nanometers. Often this is as I pointed out not a hard and fast rule and often we will notice that if a material is of the order of even for instance tending towards about 300 or 400 nanometers then such a material should actually be called sub micrometer, but is often sometimes included in the regime of nano materials. So, even though when we are saying nanometer we actually should mean 10 power minus 9 meter, but sometimes we extend up to 10 power minus 7 meter in even larger in terms of sizes, but essentially we understand that the importance of the definition lies with respect to the properties. In other words if there is an property arising which is special which is nice which is interesting then the use of nano actually makes sense in our context. So, if it is just a prefix for definition of a scale then in that case though it is important to consider the word nano, but it does not really is not that exciting for us to consider what this nano actually can give us new prospects. So, what we are considering here is that the appearance of interesting properties especially which are absent in their bulk counterparts. So, often we have a certain accentuation of properties as do we go down to the nano scale, but if there are new properties new phenomena and new kind of structures arising when you go down to the nano scale that makes our study of nano materials nano structures more worthwhile and also opens of the door for application of these kind of materials into a kind of technology which we will define soon as nano technology. So, to summarize this slide nano is just a prefix to a certain standard unit, but typically in the context of nano materials and we imply that there is a certain length scale in the problem which is 1 to 100 nanometers or sometimes slightly larger than that also. So, there are two important terms which we will come across one is a word called nano material another word which is called nano structure. Now, what is the technical difference between nano structure and nano material is very important for us to note. So, let us start with the definition of a nano material when we say a material we actually can visualize a certain kind of a tangible amount of matter. In a nano material we typically do not imply that the material is itself in the nano scale what we imply is that there is a material of tangible size and there is some component which could be 1 or more components in the material which is what makes up this material is of nano scale. So, it is not essential that every component of this material is in the nano scale it is not essential that there is just one component in the nano scale, but typically 1 or more components is in the nano scale and which is what is finally, giving us that important set of properties which we are interested. These components could be for instance grains in a poly crystal for instance this could be nano tubes which have been grown in a line fashion there could be nano spheres etcetera whether this form a material and that material can be called a nano material. Often the word nano material is also used in a more liberal or a relaxed fashion where in various other kind of things like nano structures are also called nano materials, but as long as we understand the context in which the word is or the terminology is being used it is absolutely perfect for us. Examples of nano materials would be nano crystalline copper in this case the copper sample itself is not nano sized. What is nano sized is the grains from which this nano or nano poly crystalline sample is made of. So, it has to absolutely clear that in this case the sample is definitely a bulk sample it could be outer outer centimeters it could be at least millimeters, but definitely a tangible amount of sample and what is nano in this sample is the grain size and therefore, this is absolutely clear. A crystal can also be made out of silicon nano spheres and the nano spheres themselves may be amorphous actually are typically amorphous, but we are not referring to the silicon nano spheres as a nano material. We are referring to the assemblage of this which is giving rise to the entire material which we call a nano material. Now, we have to clearly distinguish this nano material from the freestanding nano structures which we will soon discover what are these freestanding nano structures what kind of geometries can they have we will soon understand when we dwell into deeper. So, when I am talking about a nano material I see that the material is made up of certain components which is of the nano scale or which has a length scale in the nanometers. Later on we will ask ourselves this question what is a bulk material we have been using the term bulk very much. So, we will ask ourselves this question what is a bulk material and what can be nano in a nano material. So, these two questions we will again once ourselves ask ourselves to clarify some of the points which are not clear from this slide. Whenever I am loosely referring to some kind of an object without actually being specific. So, sometimes the word nano entity can be used because then that avoids us being very specific. So, this is a general term where in specificity is being avoid and there is something nano in the whole system that is what we are referring to when you are talking about a nano entity. The next important term is nano science. Nano science is the study of fundamental principles of nano materials and these could be made up of molecules and structures with one dimension at least which is between 100 nanometer. The concept of dimension based classification will take up later, but the important thing is that the study of fundamental principles of nano materials is what comprises nano science this includes physical, chemical and biological aspects of nano entities. The study of nano materials is extended the frontiers of science and has warranted the existence of a new domain of research called nano science. In many cases we actually may not actually observe very many drastic new phenomena, but still the study of whatever properties interesting properties or interesting accentuation of properties which occur at the nano scale has warranted us to create a new domain called nano science. Research in nano science coupled with nano technology which we will define soon has fueled the growth of conventional scientific disciplines as well. So, not only is nano science is separate discipline by itself, but growth in nano science is helping us actually study the conventional kind of materials better and also has fuel research in other allied areas which are actually not directly related to nano science. Example, understanding the physics of super paramagnetism appearing on the reduction of size of ion nano crystals because of easy alignment of nano domains along the externally applied magnetic field which is otherwise ferromagnetic in nature. So, if I am studying a phenomenon like super paramagnetism as stated here or equivalently I could be talking about a phenomena like giant magneto resistance. Then I would like to know the fundamental principles like for instance I may be talking about spin diffusion length, I may be talking about spin dependence scattering etcetera. So, these fundamental aspects of study of say phenomena like super paramagnetism or a phenomena like a giant magneto resistance is what would comprise this area of nano science. So, all the and we will again worry about all the kind of properties we have been worrying about in the conventional materials like hardness, we could be talking about mechanical behavior, optical transmittivity, dielectric constant etcetera. So, it could be one of those conventional looking parameters, but now we are in the realm of nano science and some of these properties arise which are very unique to these what you may call nano materials as we saw the case of the giant magneto resistance which has no conventional bulk counterpart. Now, nano science of course, is a study of fundamental aspects, but in the broader sense nano technology is application of the principles of nano science into useful deliverables. So, we have this nano science, we have this beautiful properties arising, but we would like to put them to good use like in any technology. And this application aspect of nano science is what comprises the broad area of nano technology. And so we are not only we are using the principles of nano science to not only make useful products, but also useful devices which can actually further even the study of nano science itself. So, that is why the use of nano science can actually into nano technology can fuel the growth of nano science itself. This includes the application of nano structures, nano materials into useful devices and components. And these devices could be small devices in the nano scale itself or this could be much larger devices in the micron scale or even larger devices. Further by tailoring or manipulating the concepts of nano science, nano technology aims at improving the lifestyle of humans. When I meaning lifestyle we actually imply the standard of living of human beings that is certain tasks and certain kind of issues which could not be solved for the human race at the level of the macro level, nano science and nano technology could actually prove extremely useful. Like for one example would be to insert a micro chip into the body for control drug delivery. So, if you have a control drug delivery automatically it implies that we are not going to damage any side issues. The side effect aspects is going to be lower the amount of drug needed for the delivery is going to be much less. And if the chip has some time delay or time release mechanism then we are very sure that the drug is only delivered when needed by the tissue. Therefore, when I am putting this nano science into use it is clear that there are certain benefits which can arise from arise in the standard of living of human beings. Of course, towards the end of this chapter we will also see that all is not green with nano materials. There are obviously, serious concerns with use of nano materials can they be harmful to human beings, can they cause certain kind of diseases or allergies in human beings which are not been discovered before. So, these aspects also have to be kept in mind, but that is the area of nano ethics and which poses new challenges in terms of the growth of nano science and nano technology. But here we clearly see there are benefits and these benefits are going to drive the growth of nano science and nano technologies. So, some of these applications are those which otherwise would not be possible without the use of conventional in the case with the use of conventional technology. Like for instance I would I can use some nano structures to make ropes for construction of space elevators which are extremely strong. So, some of these things may not be possible with the help of or with the use of normal conventional materials. So, nano technology is application of nano science to useful products and devices, useful deliverables. Important thing is that some of these deliverables can actually improve the quality or standard of human living. Then nano technology itself can lead to the growth of nano science. For instance we can use nano tubes as a probe and therefore, nano science might grow. So, new phenomena may be discovered by the use of nano technology itself. Therefore, it is a self or it is a nice positive feedback loop which can lead to a better growth of nano science. A closely related term to nano technology is the word nano manipulation and nano manufacturing. Now, we said that we want to use the nano science to make useful deliverables. This is definitely not possible if you do not have the concepts of nano manipulation and nano manufacturing. This is because now we are dealing with material which is very little in size or very small in size and therefore, there are special ways of handling this material and also special ways of manufacturing using these materials. So, in nano manufacturing, nano entities are to be manipulated. Retaining their nano structures is very important. The entire manipulation should not itself alter the material which we are trying to what you may call manipulate to evolve manufacturing to evolve manufacture nano or macro components. So, as I pointed out the component itself could be actually small or could be big, but we want to manipulate these nano structures and components of nano materials to actually make these components. And if the benefits of the nano science have to reach masses, nano manufacturing has to play a very key role because unless you can mass produce some of these using certain new technologies and new manufacturing techniques, it will the benefits of nano science will not reach the masses. Physicist and chemist if you note have been doing nano say what you call nano science or scale, science of the nano scale for a some time now, but it is only with advent of modern tools like field emission scanning electron microscopy, high resolution transmission electron microscopy, atomic force microscopy, scanning probe microscopy and certain important manufacturing tools like e beam lithography, focus to n beam that has become possible to manipulate an engineer atoms at the atomic levels at its extreme and sometime at the nano particle level to make a component or device. So, not only that I have my nano structures and nano material components, I am able to manipulate them of course, I am able to observe them first in a for instance technique using like a field emission scanning electron microscope. Then I am able to manipulate them and finally, leading to the process of nano manufacturing in which we actually produce a entire component. An example would be the field emission gun can fuse a end of a carbon nano tube to a specific dna to extract signals by applying a small current or voltage. This can act as a sensor for genetic diseases that might be in its in its incipient stage. This is possible obviously, because of manipulation of the nano scale. So, what I am doing here I have a very nice example here of an application, where in I take a carbon nano tube and fuse its end to a dna by using small amount of voltage a current I extract the signal from the dna and I actually see from the signal I will deduce if the dna has some fault. If there is any incipient genetic disease and early deduction can actually perhaps lead to even prevention or perhaps a cure if not. Therefore, nano manipulation and nano manufacturing would form an integral part of nano nanotechnology. If I am going to get the full benefit out of the nano science or the nano the concepts which I have discovered in the case of nano science. So, let us now get on to certain specific geometries and specific structures which the examples of which typically pervade literature. The first of them we start with is the concept of a nano particle as in the case of the definition of any other nano term. What I imply here is there is a particle in the size range of 1 to 10 nanometers. And when I am using the word nano particle I am actually not being very specific the kind of particle I am referring to here. The particle itself for instance could be a single crystal and you will see examples of that in coming slides. It could be a poly crystal it could even be a quasi crystal or it could be amorphous. One example of a amorphous nanoparticle is shown here as you can see here each one of these spherical entities is a silica nanoparticle. So, and the silica nanoparticles can assemble in the form of a nano material like in the case you can see here this is in the form of a mono layer or in the case other cases it could be in the form of multilayers. The size of these particles is in the 300 nanometer regime that means we have now extended our definition to beyond the 100 nanometer regime which we have citing before. But then other cases you could actually take up 100 nanometers or 20 nanometers or 30 nanometers particles and arrange them in the form of mono layers. So, when I am referring to a nanoparticle I typically think of it as an entity which exist in isolation. Even if it exist as an assembly as in the picture below at least it is clearly identifiable to be a separate particle. For instance it is not a grain in a poly crystal. So, that is what when I am saying it is a nanoparticle I mean it is a free standing entity or at least it can be easily isolated or visualized as a free standing entity. However, nanoparticles may form a part of a composite also may be part as a larger composite and in this case of course, it would be it would assume a new role and it would be called a reinforcement or certain other thing. But nevertheless when I am referring to a nanoparticle I assume that it is definitely identifiable as a separate entity. The importance of nanoparticles is not diminished even when they are embedded in a matrix and this is because I am actually expecting certain important properties to arise by putting these nanoparticles in a certain medium. If there no such benefit then I would not be attempting such a task and therefore, I would expect that there could be some reduction there could be some amplification in the properties. But in some sense the properties of this nanoparticle which are special in some sense is retained when I make a composite out of it. An example of such a role could be their role in accelerated catalytic conversion. In fact, it may often be that in the presence of a certain substrate the catalytic activity may actually increase as seen in the case of a gold nanoparticles or suppose I am dispersing this nanoparticles in a solution. For instance the color comes out from the solution is dependent on the size of the particle. And therefore, we can see that there are these special properties which may be retained when I put this in a certain medium and this putting in a certain medium just could be for its preservation for instance could be for its isolation. In other words so that they do not coagulate or what could be certain other what you call synergistic benefits. But nevertheless the nanoparticle has its own entity and its own specific properties literally when I am using the word nanoparticle I assume a single nano entity which behaves like a complete unit. So, it is a unit by itself and it may have sub components that is an important thing which I need to note. So, there are some examples we can think of we can think of particles of amorphous alumina which are. So, it is an amorphous state we can think of crystalline gold which could be a nanoparticle and it could even be a polycrystals. So, therefore, when I am talking about a particle a nanoparticle I am not ascribing anything more than the fact that it is a particle and this particle could have its own geometry like it could be have faceted faces like I could it could have a particle which has a faceted geometry. So, this could be a particle a nanoparticle could have certain other kind of a shape it could spherical a nanoparticle could also have actually components to itself that means it could be a polycrystal with these being grains. So, each one of this is a grain a nanoparticle for instance could have read more than one component for instance a nanoparticle could have a spherical nanoparticle could have one face here and this could be a different face and this could be a different face. So, there are all these possibilities when I am talking about nanoparticles and I am not being specific about any of these aspects. In other words phase one could be crystalline and phase two could be amorphous in a polycrystal for instance each one of these grains could be randomly oriented. But what I am referring to a nanoparticle is a fact that this is a particle and it has a certain size which is in the range of one to 100 nanometers. And in the example here I have consider certain spherical particles of silica, but that is just one example and there could be many other possible examples of such nanoparticles. One specific class of a nanoparticle could be a nanocrystal. In other words here I am being very specific regarding the kind of atomic order in a nanoparticle and I am saying it is crystalline order it is not quasi crystalline it is not amorphous, but it is crystalline order. So, one example is shown here for instance these are nanocrystals of gold and these particles have about size of about 100 nanometers. And the as you can see these are freestanding gold nanoparticles. If you look at it more carefully actually you observe that these particles have certain nice well developed facets. This is because of the anisotropy in the surface energy and therefore, this particle would like to put out those surfaces will give it a low surface energy and the overall energy of the system is low. So, a nanocrystal is typically a freestanding monocrystalline nanomaterial with at least one dimension the size of one to 100 nanometer. It must be noted. So, this concept of at least one dimension we will elaborate a little later in during this lectures, but essentially we note that at least one of the dimensions has to be nanometer or one to 100 nanometer regime for us to classify it as a nanocrystal. So, if we have a polycrystalline particle and this particle as we say could be nanometer regime then strictly speaking this should not be termed as a nanocrystal. So, because then it becomes a nanopolycrystal and not a nanocrystal. Examples are gold nanocrystals diamond nanocrystals etcetera. Sometimes we also refer to nanocrystals as I pointed out to those things which are embedded in a matrix and one such example is a case of a precipitate shown here and this is produced in an aluminum 6061 alloy which has been severely deformed and heat treated at some low temperature close to 200 degree Celsius to allow for nanoprecipitation to form. So, we see a region here which is a nanoprecipitate. So, from the lattice fringes which you can see these lattice fringes lying here we can see that this region of the precipitate which I will highlight may be here is actually crystalline. However, this is now embedded in a matrix. So, this is my precipitate or a crystal and the size of this crystal is about 30 nanometer and this is embedded in a matrix. In some cases even this kind of a configuration when the nanocrystal is not a free standing nanocrystal as in the case of the gold coaster here we may still want to call this a nanocrystal because it still is an independent identifiable entity of the nanoskeleton and it is crystalline and it exists in a matrix. Other such kind of nanocrystals have been produced and typical examples are good examples of these are lead nanocrystals which are suspended in or has been distributed in aluminum matrix and one of the ways of doing so is actually taking lead and aluminum and melt spinning the mixture and this gives us thin foils in which lead has because lead is typically not soluble in aluminum. But then by doing this fast rapid solidification process we can actually get lead nanocrystals which are distributed in an aluminum matrix. So, when I use the word nanocrystal it implies that the material is crystalline typically it is a single crystal and typically it is a free standing entity. However, in some cases as we just saw a nanocrystal could actually be embedded in a matrix and still we may want to call it a nanocrystal though it is not free standing if it is embedded in a matrix. So, there are abundant examples of nanocrystalline materials like gold nanocrystals which has be which is typically used for you know catalysis and other kind of important applications that can be diamond nanocrystals which has nice properties as well and so forth. Now, we come to the next important definition the definition of a nano structure. A nano structure has to be differentiated from a nano material and has to also be differentiated from terms we just came across like nanocrystals and nanoparticles and other kind of similar kind of terminology which is may sound very close to what is a nano structure. So, what is the definitive difference between a nano structure and a nano material? In the case of a nano structure we are talking about a structural or geometrical entity with a distinct shape having and this whole entity has at least one dimension in the nano scale and we already said that one nano scale implies 1 to 100 nanometer. So, when I am referring to nano structure I imply it is a geometrical entity or a structural entity and it has a distinct shape and the shape could be a spiral, the shape could be a rod, the shape could be a sphere or any specific shape and that shape should be is associated with a certain geometry. That means if I have a material like carbon and now I make a structure like this a cylindrical structure the other possibility is that I make a hollow cylinder of course, again of the nanometer dimension. So, now I have a hollow cylinder. So, this hollow cylinder is like a tube from the external shape of course, they look very similar, but this is one kind of a nano structure because its geometry is different from the geometry of this structure and therefore, these represent two different nano structures. Therefore, when I am talking about nano structure I have to worry about the geometry and the reason that I need to worry about the geometry is that even the same material in different geometries actually give rise to different properties. And therefore, nano structures have a specific geometry and that geometry has to be specified whenever you are talking about a certain kind of structure. However, from this example it should not imply that every kind of structure can be found in every kind of geometry. It is certain structures are only found in certain geometries for instance, fullerines have a certain geometry. I cannot take a fullerine and make it in for instance, a plate like geometry. It is not possible a fullerine molecule has a geometry that which is resembles a football and I cannot take a fullerine and make it in a geometry which is like a plate. Therefore, I have to remember that though I would like to differentiate two different structures, but it is not possible to produce every kind of material or every kind of nano structure in any desirable kind of a geometry. And this also implies that if there is a structure produced in a certain geometry the properties are very specific to that kind of a geometry. Examples of nano structures include carbon nano tubes. So, carbon nano tubes could be single layer carbon nano tubes or single wall nano carbon nano tubes or multi wall carbon nano tubes. And you can see in the picture below here, where in certain carbon nano tubes have been shown and these are typically multi walled carbon nano tubes. So, these are all nano tubes and these are multi walled carbon nano tubes. Fullerines are other nice example of nano structures, carbon onions which are concentric shells of carbon layers or graphitic like layers, nano fiber of zinc oxide, nano spheres are some of the few common examples of nano structures. One nice example of a nano structure is shown in the figure below, wherein I have certain pillars which have now grown in a nice crystalline array. If I now consider not the entire crystalline array, but I just worry about one single entity in the crystalline array which forms a motif for this crystal and this unit cell of this crystal would be this unit cell. So, therefore, this is now the unit cell of my crystal. Now, of course, this is not a strict crystal in the strictest sense, but in an average sense is would qualify as a crystal. So, this structure I see here is a nano structure, because now this has got a specifics geometry in other words at least this one dimension which we are talking about which is visible in this micro graph is of the order of 200 nanometers which qualifies it to be a nano structure. But the entire crystalline array then can be thought of as a nano material and this one single entity which is what is now my nano structure. In biology we have many more examples of nano structures these include the DNA double helix the DNA strand is typically about 2.2 to 2.6 nanometer in size, the bacterial cell wall is another nano structure, protein nano layers in knacker another examples in nano structures. And in the case of the protein nano layers the it does not exist in isolation actually it exist in combination with other layers which actually gives knacker the extremely or the ablon shell it is extreme toughness. Other examples of nano structures and nano spheres, nano pillar, nano cage, fullerines, nano fiber, nano flakes, nano rings, nano belts, nano helices, nano bows, nano nano sphere of course repeated again here nano tubes, quantum dots, micelles, nano cones, nano flower and you can see one example which seems like a small nano flower here with fold fold kind of a dent right. So, nano flowers, nano brushes etcetera. We will see one example in a upcoming slide which resembles some kind of a nano brush. We already seen we talked about block walls. So, block walls in ferromagnetic materials can also be thought of as nano structures because they have a specific geometry, they have a specific arrangement of spins within that wall. And this is what comprises a magnetic domain wall which can it is itself a nano structure in it is own right. Therefore, to summarize this slide we come across very many terms when we read literature regarding nano materials. And we come across terms which have been described here like the nano full flower, nano ring, nano flake etcetera. And when I am talking about this kind of a classification I am referring to a specific geometry. Whenever a specific geometry is being referred to the term I would use for the entity or the nano entity would be a nano structure. So, a nano structure is characterized by specific geometry because this specific geometry would also determine the properties which come from this kind of a geometry. Certain examples shown by a micrographs we considered like a multi walled carbon nano tube in the right hand side. We also talked about this nano pillars which are basically made by focused ion beam lithography of a thermally operated gold film. So, this is done by photo lithography focused ion beam lithography. And therefore, you see that each one of these entities is a nano structure.