 So, we have come now to the second module of our course and in the first module we have seen how chemical approaches are fundamental to making solids and inorganic structures. So, we have seen the power of chemical methods, how they can be cost effective and we can make molecular solids into inorganic solids and what are the fundamental approaches from chemistry point of view to make such three dimensional inorganic solids. In the second module I am going to take you through a totally different approach which is predominantly a physical approach. In physical approach I am going to tell you how using physical vapor deposition techniques which is not traditionally the chemistry roots, but these are involving high vacuum or medium vacuum level and using vacuum how you can make solids. There are different techniques by which one can make inorganic solids of different varieties. It is not confined only to metallic materials, you can make materials as oxides, you can make materials as nitrites, you can make materials of alloys, all these are possible using physical vapor deposition roots and in today's talk I am going to emphasize on the same theme of building solids from mono layers to multi layers. In other words, we are going to start from basic atomic level growth to building up three dimensional growth. So, making an atomic level deposition or just putting one atomic layer over the other is the fundamental way to build inorganic solids or three dimensional lattices. So, this is the state of thought approach as far as making materials and in this we are actually going to stumble on another important feature that we all keep hearing now and then which is called nano technology or nano science. Nano science is to study the fundamental properties in nano scale. When you confine any material in nano scale, the basic properties such as structural electronic magnetic or optical properties they vary because you have the confinement in nano scale. This is quite a departure from understanding a bulk material and its related properties. So, if we want to make materials starting from the scratch that is from an atomic plane to three dimension, now we need to understand that as we grow the way we grow an atomic plane will also determine the magnetic property and in other words the magnetic property will affect the electronic property. So, fundamental to the growth and morphology is the way we build these atomic planes. So, I am going to take you in the next 50 minutes. I am going to tell you how we can make the solids starting from the basic aspects or the minimum thickness that is called a mono layer. Mono layer is nothing but an atomic plane usually of the order of 0.2 nanometers. So, if you have 0.2 nanometer thick atomic plane can we put one atomic plane over the other to dry home the point. So, in today's lecture I am going to talk to you about how to understand this nano scale growth and how we can correlate that to some property for example, with reference to magnetic property. But I have to tell you that the interest in going down to such nanometer thickness actually comes from the Nobel prize winning work of Peter Grunberg and Albert Fert who won the Nobel prize in physics in year 2007 and they came out with very interesting results which involves nano scale thicknesses. Now, this is the work of Peter Grunberg which shows that if you deposit a material which is a trial layer where the bottom layer is actually 12 nanometer thickness in other words 120 angstrom thick ion film and then you put in between a 10 angstrom or 1 nanometer thick chromium and then if you again put on the top layer 12 nanometer thick ion. Now you can look at the basic property of this material in terms of its magneto resistance. Now you can see here for a layer like ion copper chromium ion. Now for having just 1 nanometer thick separation you can clearly see that the resistance varies with field in this fashion whereas if there is no separation of ion by such a thin film like chromium then you can see the magneto resistance is only of this order. So this is not a spectacular change in resistance as a function of magnetic field whereas the moment I put 1 nanometer thick chromium immediately the response is something like this. So what is the feature that is responsible for such a change in resistance is what we are going to see. Now before we go into those aspects which I will discuss in detail in module 5 when I talk about magneto resistive properties I want to draw your attention to this fact that 1 nanometer thin chromium film can bring about a pronounced change. Now the same thing was carried out by Albert Fert and he also made this sort of by layers with different repeats for example ion chromium like that you can have 30 repeats or ion chromium ion chromium like that as another 30 repeats or you can go for 60 repeats of this by layers. But you should notice that the resistance varies in a different way as a function of magnetic field depending on the thickness of the chromium layer. The chromium layer actually here is 1.8 nanometer in this case it is 1.2 nanometer in this case it is just 0.9 nanometer. So you are actually talking about just of 3 or 4 or 5 or 6 atomic plane separations between thick layers of ion and with respect to the chromium thickness you can see the magnitude of this magneto resistance or change in resistance is varying by orders. So the thickness of the spacer layer which we call it this is a spacer layer or we can even call this a non-magnetic layers and if this non-magnetic layers which are alternated to ion layers if they are made very very thin then there is a pronounced activity in this one. Smallest the thickness I can make and in one sense that is the birth for nanotechnology. Since then many groups have worked on making thin films not only in such alloy structures but also in oxides nitrates everywhere people have made thicknesses which are of a very low dimension and to see when you confined to low dimensions whether you can bring about a pronounced activity in the properties that you are studying. So this is a classic example for how a rush to nanotechnology has originated. Now the next slide I want to show professor Peter Groenberg's noble prize winning moment because he was the one who was actually responsible to say that if I put 1 nanometer thin chromium between 2 ion layers then you can see a magneto resistance of a pronounced nature and therefore I just want to recognize his contribution which is actually provided a pathway for many people to study different materials in nano dimensions. Now we are all working most of us who are listening to this module must be working on nano materials at least in bulk form some of you may be working in nano thin films some of you may be working on nano structuring but there is a mass confusion as to what is nano structuring and what is nano materials. Fundamentally there is a difference between nano materials and nano structures. Nano structures are intentionally built or you can start from a bulk and you can go down to a two dimension or a one dimensional stripes where you can confine the material in nano form or you can make nano materials which are of bulk in nature and predominantly these nano materials are three dimensional whereas nano structures can be either one dimensional two dimensional or it can even be three dimensional. Therefore there is a fundamental difference between nano structuring and nano materials predominantly chemical roots takes care of nano materials while nano structuring is taken care by physical approaches. Another thing that we need to consider the properties of ultra thin ferromagnetic films of a few monolayers thick differ from the magnetic properties of the same material in bulk form. So when you confine any material in nano size there is a fundamental difference in the property. Another issue that we need to bear in mind is the growth mode or the way we grow this nano structures by and large depend determine the interfacial effects. Suppose you are making a bilayer of two different metals we need to understand that the interface between two materials is has to be very very sensitive or very very sharp otherwise there will be mix up of the properties. Therefore interfacial effects are dominant and as a result the growth mode how you grow this nano structures are important and second if you want to make atomic layer by layer growth then we need to have a control over it because you cannot make a island like a shape and then call that as a nanometer nano structure because it has to be extremely flat in order to give such sharp interfaces. Now how do we achieve this nano structures one of the most predominantly refined and the most expensive way of realizing nano structures is to use molecular beam epitaxy to build nano structures. Molecular beam epitaxy is usually referred to MBE growth and therefore this is one of the state of art method by which anyone can make nano structures. Now I want to start with some basic definitions which are very important for understanding this one is the word epitaxy what is epitaxy molecular beam epitaxy to grow films. So what this epitaxy means it refers to the method of depositing a monocrystalline film on a monocrystalline substrate we can even call that as a single crystalline film on a single crystalline substrate that deposited film is denoted as epitaxial film or epitaxial layer if it is growing on the same geometry and on the same orientation as that of the substrate. And the term epitaxy comes from the greek word epi meaning above and taxes meaning in ordered manner. So if you can order some atomic planes over on the top of a substrate then you can call such a growth layer as epitaxial growth it can be translated as to arrange upon. So if you have a c axis oriented substrate then if it is a epitaxial film it has to grow in c axis. So this is of fundamental importance the next definition is homo epitaxy there is a term called homo epitaxy there is a term called heteropytaxy. Now homo and heteropytaxy is very important because there are fundamental differences or ease with which you can make such solids. Now in homo epitaxy it is a kind of epitaxy performed with only one material in homo epitaxy a crystalline film is grown on a substrate or film of the same material. For example, if I want to grow copper atoms on copper 100 substrate now this is called as homo epitaxy and as you would guess building copper atoms on copper is going to be very easy because both of them share the same crystal symmetry both of them share the same surface energy. Therefore, to grow a purest form of atomic layer on the same material which is single crystalline is a fairly easy method and therefore, this is called as a homo epitaxy whereas if you think of heteropytaxy it is a kind of epitaxy performed with the materials that are different from each other in heteropytaxy at crystalline film grows on a crystalline substrate or film of a different material. This technology is often used to grow crystalline films of materials for which single crystals cannot otherwise be obtained and to fabricate integrated crystalline layers of different materials. For example, gallium nitride you can grow it on sapphire sapphire is nothing but iron doped alumina or you can try to dope aluminum gallium indium phosphide on gallium arsenide substrates. So, this is an example of heteropytaxy this is an example of homo epitaxy. So, growing heteropytaxy is much more challenging than growing homo epitaxy. So, I want to draw all these fundamental definitions about growing nanostructures but I should also tell you because molecular beam epitaxy is a more refined technique it is impossible to have this facility all over the world and almost in every laboratory. But now we have another sophisticated machine which is coming which is called as focused iron beam and this is more affordable because the way focus iron beam looks like it looks like a ordinary ACM. But it has special facility not only to look at surfaces you can also try to do variety of things. This is a ACM image of a polymeric film which is deposited by another technique but you can actually look at the ACM picture of this and you can also write IITK as a nanostructure because the thickness of this letter is just 20 nanometers you can try to write such nanostructures on a film. Same way you can also write nano deposition nano machining which is possible to write using this focus iron beam. The advantage of focus iron beam is you can actually have a top down approach to make nanostructures meaning you start from bulk and you can reduce it to nanostructure or you can even do some limited deposition to grow some nanostructures as you are trying to do this sort of machining. So you can actually try to either scale it down to nanostructure or you can build upon both are possible using this FIB and therefore, this is the most sophisticated instrument as of now which can nearly substitute for molecular beam epitaxy. The next slide I want to tell you what is this molecular beam epitaxy and why this is very important to make nano materials. Molecular epitaxy for example, a machine can be as big as this we should not get discreet looking at the machine, but typically if you want to have all the characterization facilities attached to a single instrument then the machine will be of the order of this size. And this is a picture view of William or Wiley environmental molecular sciences laboratory in USA and this is not to scare you, but to tell you that molecular beam epitaxy is actually the most sophisticated instrument and the most costliest deposition technique in the world as of now. Now I want to also say what this molecular beam epitaxy is it takes place in high vacuum high vacuum of the order of 10 power minus 8 Pascal or we it can even go up to 10 power minus 11 Torr and therefore, it is called as high vacuum or ultra high vacuum. The most important aspect of MBE is the slow deposition rate, the atomic layers grow one over the top in a very slow fashion typically less than 100 nanometer per hour. In other words the 10000 angstrom thick layer can be deposited in one hour time which allows the films to grow epitaxially. The slow deposition rates requires proportionally better vacuum to achieve the same impurity levels as the other deposition techniques. In solid source MBE ultra pure elements like gallium, arsenic or even iron, copper, cobalt any metallic substrates that you want to choose you can have it in sort of effusion cells and until they begin to slowly sub limit you can keep heating this substrates now they will sort of sub limit the caches elements then condense on the wafer or the single crystal or the substrate where they may react with each other in the example of gallium and arsenic single crystalline gallium arsenide is formed if two things are parallely evaporated same way you can also make metallic films like iron chromium which I showed you can have iron and then you can have chromium whenever you want iron you can open the shutter whenever you want chromium you can open the shutter by this way you can keep alternating such non structures. In the term beam what is this term molecular beam means that the evaporated atoms do not interact with each other or vacuum chamber gases until there is the wafer due to the long mean free parts of the atom. So, it is like a steady stream of atoms which are going without any interaction therefore, they all carry the same kinetic energy therefore, the growth is going to be exactly homogenous at the interface there would not be a Gaussian distribution of the kinetic energy of the arriving atoms which might moderate or manipulate the growth mode. So, all the atoms reach with same kinetic energy to the surface as a result you have a very beautiful growth atomic growth that is ensured that is why it is called molecular beam epitaxy and this is expensive that way in today's talk I am going to confine and show some examples how small this atomic layers can be and how we can make such layers. As case studies of MBE grown films I will talk about how to grow monolayers or ultra thin films of iron on copper and iron on copper 111 see the if you choose copper 100 as a substrate or copper 111 as a substrate I will show in the next few slides how the whole pattern of iron growing on copper will be modified. I will also tell you if we take pulse laser deposition and thermal deposition as two different approaches in the same MBE then you can try to improve or you can modify the growth mode in a very very characteristic way. So, in the first example I will show you how iron grows on two different subsets and I will show you in the next example how iron grows on based on two different methods and lastly I will also give you a classic example if you have control over this monolayer growth then you can even make adventure into making new artificial alloys which are not present in mother nature. So, three examples I will give before I conclude this talk in a typical molecular beam epitaxy chamber you have several such gadgets attached what you see here is a instrument attached to the deposition chamber of your MBE which can be used for magnetic measurements. You can also use instruments which are for OJ and you can also have facilities for STM. So, you can study the structure, you can study the elemental purity, you can study magnetic property all without exposing the atomic layers or nano structures to atmosphere which means you are actually doing all the studies in the same condition where it is deposited which we call it as inside two characterization because we are not breaking the vacuum you are keeping the film in the same high vacuum condition, but you are ensuring that the material is not contaminated by exposing it to air or to even low vacuum therefore, whatever property you see in OJ or magnetic or STM will be the virgin property of the material before it is exposed to atmosphere that is the state of art or the most big emphasis of MBE growth that you do not break vacuum that you conduct all the studies in high vacuum. Now, to understand this machine in a simpler form this is the machine with whose deposition chamber is the biggest in size and it is attached to a facility called reed because if you want to know whether you are making a atomic layer growth you need to have some idea whether you are in the right track therefore, you use a expensive characterization tool called reed which is reflection high energy electron diffraction and you can actually ablate the material and make this thin films using XMR layer as I told you or you can actually use thermal evaporation. So, either way in a MBE system you can actually try to make films using different evaporation source and as I told you you have this magnet which can be used for measuring the magnetic signal then you have the reed chamber to study the structural property and to know the morphology of your nanostructures you use scanning tunneling microscopy. STM stands for scanning tunneling microscopy so all the characterization tools are combined to the basic deposition chamber. Now, before I start showing some of the examples of how this nanostructures look and what are the properties I will stress on the different growth mode which is fundamental to growth of nanostructures. What you see here is a cross section views of three primary modes of thin film growth including former Weber island formation which is A and Frank Wender Mervie layer by layer growth which is called as B and the Strancki Krusnow layer plus island growth which is categorized as C. What you see here is the development of this nanostructure as a function of coverage this theta is nothing but coverage and it is expressed in one monolayer here it is expressed in between one to two monolayers here it is greater than three monolayers monolayer the definition is one atomic plane it is one atomic plane so it is one atomic plane. So, if it is less than one atomic plane that means few atoms you are going to throw on the substrate then this is the way it will proceed if it is island growth you will have you will see the pattern of island that is forming but if it is going to spread uniformly throughout which we call it as a two dimension then it will go as a flat terrace it will go as a flat terrace. Now, if it is going to be island plus layer growth first it might proceed just as a flat growth and then as you increase the coverage it can actually top up with some islands and it can grow in this fashion. Now, perfectly one would desire to have very sharp interfaces in structures as I discussed like iron chromium iron where you have the chromium in one nanometer thickness if you want such a crucial separation then you would actually go for a ideal growth of chromium in this fashion where you would have a very sharp interface in other words this iron at iron layer will not coupled with this iron layer there will be a perfect separation between these two layers otherwise you will not see such sharp variation in the magnetic field in the presence of the magnetic field. So, what one desires is actually a two dimensional growth this we need to register in our mind because if you are thinking of a nano structure then always you should ensure a two dimensional growth. Now, each mode is shown with several different amounts of surface coverage now as you proceed further you will see this can translate even into a two dimension or this can translate into something like this therefore, anything can happen as you keep growing the structure. But if you are going to play with the nano structures what is more desired or the ideal growth is two dimensional growth now this is a classic example of a clean surface of copper one one one which is a single crystal and this crystal if it is very carefully prepared this copper one one one crystal will actually have your step width which is approximately of twenty to forty nanometer this may be different. But each one is a step and each one has a step edge like this this is the way a crystal grows to relieve its energy. So, these are all step edges and these are all step terraces now when you try to put some atoms these atoms can either come and fall here or the atoms can occupy the step edges both can be possible. Now, the way you adopt the technique you can try to modify it I will come to that in a few minutes time. But what you should understand is that these are all highly pure single crystals this is the atomic resolution of the copper one one one terraces which clearly shows that each one is a copper atom and it is highly ordered now when I am going to put iron actually each of this iron atom has to sit on each of this copper and that is what we call it as epitaxy. If they are not going to form like this then it is not a epitaxial layer. So, you are by molecular beam epitaxy you are actually making a dissimilar element to actually sit on each of this copper atoms therefore, you call this as epitaxial films they are growing. One more thing that we should understand if you try to grow iron in bulk is BCC alpha BCC iron. But what you would see if you take a FCC lattice in molecular beam epitaxy condition it is possible to force a BCC element to grow in a FCC lattice until the stress strain issues are not coming into focus. So, it is possible to enforce a two dimensional growth with the different crystal structure for even a material in bulk which is known to have a different crystal symmetry and that is the power of molecular beam epitaxy. Now, this is a classic example of what really happens to iron grown on copper one one one using a thermal deposition technique this is different from pulse laser deposition technique I will come to those comparisons later, but just have this in mind if you thermally grow this film iron on copper one one one initially we are talking now about 0.3 monolayers that is less than one atomic plane. So, if you try to give 0.3 monolayer coverage you can see the stripes are growing in this fashion growing in this fashion and atoms are not occupying the step terraces of the steps, but they are occupying the step edges therefore, this growth is called as one dimensional growth because they are growing in one dimension or we can call them as one dimensional wires because the length of this wire if all the islands here they are all connected nicely then they can be called as one dimensional wires typically of the order of 800 angstrom or so you can make long wires or this is also called as one dimensional stripes. So, you can actually make such one dimensional magnetic wires and one dimensional stripes by decorating this preferentially along this step edges, but note that in the next few slides I will show to you the same deposition technique of iron, but if it is grown on copper one zero zero instead of one one one the growth mode will be completely different it would not grow on the step edges that is unique of the substrate as well as the deposition mode. Now, if you proceed further to nearly to 2.8 monolayer which is nearly to one atomic layer growth you can see that it has grown and some of the islands are now growing on the top of the first island and also some of the islands are sort of crisscrossing now it is growing along the stripes of the next step edge. Now, if you go further this is for 1.4 monolayer you can clearly see that there is lot of crisscrossing talking between the islands of adjacent layers and you call this as a two dimensional progression. So, from one dimensional if you keep on depositing this layers you transform to two dimensional growth now if you proceed further this is for 1.8 monolayer now if you go to 2.3 monolayer it is nearly complete. So, it is a flat growth of a two dimensional film if you are achieve and you can achieve such a growth around 1.3. So, the dimensionality changes with the same growth in for critical thicknesses first you start with one dimension then you go to two dimension, but if you go from there to nine monolays you see this is not looking flat at all this is showing lot of bumpy surface in other words this is nothing, but three dimensional growth. So, within one system as you proceed from 0.3 monolayers if you go to just nine atomic planes you see there is a change in the dimensionality of the growth mode from one dimension to two dimension and two three dimension. Now, when such a transformation occurs linearly you will also understand that there is a problem that is associated with other transformations when dimensionality changes there is also a change in the lattice or from FCC to a BCC and from from the magnetic point of view from a in plane to a out of plane geometry. So, in the next few slides I will show you what how this dimensionality changes the magnetic property as well as the structural property. So, I will try to show a closer image of the first few monolayer growth say 0.3 monolayers and 0.89 monolayers you can see the growth mode is slowly transforming to a continuous stripe here, but here in this case you can see there is some discontinuity between the atomic islands. Now, if you go further you can see from the lead that is L E E D this is low energy electron diffraction pattern we can get clue about what is the structural transformation that is happening we have already seen there is a transformation from one dimensional to three dimensional growth. Now, if you carefully monitor the lead pattern this is the situation of your copper 1 1 1 surface it shows a electron diffraction pattern like this. So, this is typical for a FCC lattice this is called P 1 cross 1 pattern which is typical for a FCC. Now, if you progress along the line say iron on copper if you keep depositing 1.8 monolayer you typically see the same in other words you can say this is still in FCC because the electron pattern is still the same. Now, if you go to 2.3 you see this dots are smearing out and it is more smeared here and when you come here to 9 monolayer you see a 3 cross 1 pattern in this case this is P 1 cross 1 pattern in this case it is 3 cross 1 pattern. Therefore, when you go from a clean substrate to 9 monolayer there is a transformation from FCC to BCC. So, along with the dimensionality change there is also a change in the structure structural transformation is there which is FCC to BCC. Now, if you go further we can correlate that to magnetic property, but this is a IV lead spectrum of the 0 0 spot for iron on copper 1 1 1 which gives you idea about the new peaks that are coming here which is typically due to the BCC transformation. So, as you go from a pure FCC lattice of copper you know you do not see this signature of BCC there, but as the system is transforming somewhere around 3.6 you can see this new peaks coming. So, if you have a IV lead spectrum recorded that will also clearly show you where the transition onset is and therefore, you can clearly map the structural transition. This is the magnetic transition we can try to quantify magnetic structure, magnetic structural transition which shows how as a function of layer coverage you can see the magnetic axis rotating. Now, this is nothing but in plane magnetization and this is called perpendicular magnetization. For a kind of information for most of the applications it is the perpendicular magnetization which is preferred over in plane magnetization nevertheless for understanding the structure and the related transitions let us see how it originates. For example, if you take iron on copper which is grown sorry copper 1 1 1 then for 1.1 mono layer you can see a small hysteresis there which means it is ferromagnetic. Now, if you go for further higher thicknesses you can see this magnetic hysteresis more pronounced and it is more pronounced this way and as you go through 3 mono layers you can see slowly the nature of this magnetic signal is changing and as it goes to 5 mono layer or so it is nearly paramagnetic. In other words it is not magnetic along this direction or it is showing a hard axis of magnetization this is called as easy axis of magnetization. So, in in plane you see there is a change from easy axis of magnetization to hard axis of magnetization whereas, if you go to the perpendicular geometry in the perpendicular geometry up to 2 mono layer here in is in plane it is magnetic whereas, in out of plane that is perpendicular it is not showing any magnetic signature. But you can see as it proceeds through this layer thickness it is slowly transforming from a non magnetic or to a magnetic situation. In other words the there is a rotation in the magnetic axis wherever it is easy it is hard here wherever it is hard here it becomes the easy axis of magnetization. As you would see now in the last slide I tried to show you one dimensional to three dimensional transformation occurs along this progression where FCC to BCC also occurs. Now, I also told you from the magnetic signature that there is a in plane to out of plane transition that is a happening and all are going hand in hand. If you see the transition and limits exactly where the FCC is transforming to BCC you also see the change from in plane to out of plane. So, what we understand from here that growth mode is important number one second thing when there is when you affect the growth mode it affects the crystal symmetry and it also affects the magnetic signature therefore, this is very important as we understand the nanostructures. So, let me go back to the place where I left we can also be very careful to understand this magnetic signatures in such low nanostructures because it can even be paramagnetic. The stresses loop has to be verified and the best way to verify is time dependent magnetization you try to magnetize a compound you put the field on here that means it will get magnetize and you put the field of you should you see here for lower thicknesses immediately it falls down. So, we cannot be very sure if they are ferromagnetic whereas, if you grow thicker films you can see that it takes a long time to decay this area is building up. So, that means it is authentically ferromagnetic up to say 2 monolayers whereas, at 0.3 monolayers we need to be more cautious whether to call it ferromagnetic or paramagnetic. So, time dependent magnetization can tell us that fact now if you actually plot the saturation magnetization as a function of coverage layer you can see here as you go from 1 to 2.2 monolayer the magnetization actually linearly varies which has to be the case, but once you are FCC to BCC transformation is onset and 1 dimensional to 3 dimensional stripes are forming you can clearly see there is a jump and then it goes now to a different magnetization. So, the growth certainly affects the morphology and the morphology affects the crystal symmetry and the crystal symmetry affects the magnetization. So, this goes hand in hand therefore, when you are dreaming of making a nano structure you should know in your mind that with every additional thickness that you are adding to your film something could be happening to its structure or to the magnetic property. Now, I want to come to the second part of the story where I will try to show you instead of taking copper 111 or copper 100 if you suppose use two different techniques how you can change the morphology. Now, what is the fundamental difference between thermal deposition and pulse laser deposition? In thermal deposition you have the depositing species arrive with low kinetic energy nearly two orders less than the kinetic energy of the PLD grown films whereas, in this case the ions carry very high kinetic energy and in this case with continuous arrival in steady state, but it comes as a short burst of 10 to 100 microseconds. Now, this is the fundamental difference between this. So, in one sense you are using a soft route in other sense in another approach you are using a harder route. Now, we can see how you can modify the growth mode for example, we take the same example which we have seen. You are already familiar now with this STM micrographs of ion of ion grown on copper 111 copper 111 where we have seen this stripes are forming if you are going to use thermal evaporation at 300 k, but if you are going to use the same one using PLD pulse laser deposition at 300 k you can see the growth mode is completely modified. There are no decorations along the stepages which was the case in thermally grown film whereas, in the pulse laser deposited films you see the kinetic energy of the incoming ions are going to be very rapid and they would actually form islands to coalesce as a result you have a two dimensional growth, a two dimensional growth even from 0.08 monolayer. So, even before you cover one full atomic plane you can enforce a two dimensional growth and that is the case if you go nearly to 0.9 monolayer you see it is a flat growth there is no island growth it is nearly flat this is annealed at room temperature now if you go to 1.5 monolayer you can clearly see this white patches whatever you see is the second layer is the second monolayer which is growing and this black regions whatever you see is nothing but your first monolayer which is already grown. So, first monolayer is fully covered and the patches are the second monolayer and you can see at 1.5 monolayer all these islands are invariably percolated here. So, they have a connectivity this is called the threshold of percolation and that is exactly happening at 1.5 monolayer. Now, if you go further you can still see that this two dimensional growth is maintained as long as you are growing it in pulse laser deposition, but I want to again refer you back if it is iron grown on thermal deposition we already saw for say 2.5 monolayer it has transformed from one dimensional to two dimensional growth. I will also take you now through this route where I will try to show you if I grow PLD grown films on iron on copper 100 what would happen. Now, how do I know that I am growing such a layer we can use this sort of redoscillations which I told you this redoscillations are very critical because you start at this point where the substrate is very clean. So, you have a red spot if you map the 00 spot of your copper 100 crystal now this will show the intensity here, but as you are depositing you see that the redoscillation is now falling down and it is picking up here and it is going down picking up here going down. So, it is showing some oscillation and this oscillation clearly shows it is growing in a atomic layer by layer growth otherwise you would not see this periodicity in the oscillation, but you should also understand that this is the redoscillation for thermally grown film where you can see in the first few monolayers it is actually growing as stripes or as islands as a result you are not going to see any sort of oscillation here. What does that mean if you are planning to make 1 nanometer or 2 nanometer thin iron films now thermally grown film is not the solution you have to go for PLD grown film because that is the one which is showing a very nice oscillation in the lower thickness level whereas if you are going for 5 nanometer or 6 nanometer thick films you can see here there is a nice 2 dimensional growth that is why you see the oscillations in a regular fashion. So, this is not bad what happens is first it grows using island mode, but then it recovers to a 2 dimensional mode whereas in this case in PLD grown films it starts with 2 dimensional growth and then it becomes more rough to a 3 dimensional growth that is why you see this oscillation is actually fading out. So, there is a compromise if you want to grow high thicknesses then you can resort to this. So, plenty of changes happen with respect to thickness and with respect to the growth mode which is important for nano structuring and this is the beautiful STM image of iron grown on copper 100 where you can see that this iron islands are spreading on the copper 100 surface this is for 0.18 monolayer this is for 0.56 monolayer. Now, when you go come to 1 layer there is a absolute coverage of the first layer. Now, the second layers are already starting to grow this white islands are nothing but the second layer which has started growing. So, you can nearly enforce a 2 dimensional growth if you are trying to grow iron on copper 100 which is not the case if you are going to grow an iron copper 111. Lastly, I would like to leave with another classic example of how to use this approach to make artificial alloys artificially ordered alloys by the way iron and copper cannot exist as alloy because iron and copper are immiscible they are immiscible therefore, it is miscible. Therefore, this cannot form an alloy in mother nature you would not see iron copper alloy therefore, is it possible for me to create a artificial alloy because I have the capability of using molecular beam epitaxy to force growth. So, in such case if I want iron copper artificial alloy let me take a single crystal copper 100 substrate which is arranged in a cubic close packed way and this is a FCC lattice. Now, I can try to put one atomic plane of iron and I can put one atomic plane of copper and then I can put what atomic plane of iron so on. Now, you can see as I increase this periodicity I can immediately come across a FCC lattice. So, this is the way to grow or to build the blocks by going for a 2 dimensional growth. So, how is it possible how critical it is to fabricate a layer by layer atomic growth especially between 2 immiscible systems. This is appeared in this journal if one is interested we can read that and I will again tell you to make this layers in alternate fashion you need to first deposit iron and this is using read oscillation you can control the maxima and then you can stop there and go for the second layer which is nothing but copper iron and then iron copper and so on. As you can see the intensity whenever you are depositing iron it is growing rough therefore, the intensity is less whenever you are growing copper the intensity is increasing. So, copper is able to grow nicely on iron, but iron is not able to grow nicely on copper this balance will be there, but nevertheless we can try to keep going for stackings of this order. This is the STM image of this iron copper iron copper layers. So, copper iron copper iron copper layers so you can see for 2 monolays which is nothing but iron and copper all this black holes are the first layer that is iron and this grey region whatever you see here this is nothing but copper and you can also see that the third layer has just started falling on the top. So, you can clearly make such a iron copper interface at 2 monolays then at 8 monolays and you can make up to 18 monolays, but if you see as you go from 2 to 50 monolays the surface has become rough and you can see this sort of stripes this sort of 90 degree stripes are signature for a FCC iron copper to go to BCC iron copper. So, as you build this bilayers at a critical thickness you can see there is a transformation from FCC to BCC and you will also see the changes in the magnetic property. For example, if you look at the roughness if it is FCC you see a roughness almost of this fashion and once it transforms to BCC somewhere here you see suddenly the roughness jumps somewhere here indicating that there is a clear transformation between FCC and BCC lattice. And you can also see that from the lead pattern this is copper 100 which is a clean FCC P 1 cross 1 pattern and if you go to a 15 monolayers it is the same way, but if you go to 50 monolays as you see from these two you can see this 3 cross 1 pattern which is called pitch pattern and this is indicative that from FCC the multi layer iron copper multi layer has transformed to BCC. So, you can keep on monitoring both from STM scanning tunneling microscopic images as well as this one typically this magnetic features can be recorded this way the way we evaluate whether it is magnetic or not you can go down and look at the loop and as the saturation magnetization disappears you decide that is the T C. So, you can make a plot of T C versus this monolayers so if you just put the first layer iron it is like this then the second layer copper then the third layer iron then fourth layer copper iron copper iron you can see that the T C increases and then it becomes almost steady. In other words even if you go for 100 repeats the T C is going to remain the same in other words the iron copper 50 50 alloy will have T C in bulk to be somewhere around approximately 4 10 Kelvin. So, what you are what you are trying to say is if this iron copper alloy if it is artificially made which is 410 Kelvin you do not need to go for a very thick film you can achieve the same T C in a thickness of just 8 monolayers or so. So, this way you can control the nano structuring therefore, I will wrap up here by saying that we looked at the 3 growth modes island growth layer by layer growth and layer by layer and island growth where I have shown you example if you have I if you have thermally grown film it comes as a stripe if you have pulse laser deposition it comes out like this you can easily map this transformation using LED and you by you can also indulge in making artificial alloys because this is the nicest way to make artificial alloys because you have control over atomic layer by layer growth and also you are able to study the magnetic property of this individual layers and the bilayers as you proceed with the layer thicknesses. So, with this I conclude and then we can take some examples in the next few lectures.