 In the last three lectures, we have been looking at the photo luminescent properties in organic molecules and we title those lectures as the genie in a molecule. When we look at the photonic applications, not only the organic molecules have an edge over the inorganic phosphorus in terms of large area displays in terms of economic viability, but at the same time we cannot refuse the potential of the inorganic phosphorus, because in the last three decades the inorganic phosphorus have controlled any sort of photonic displays which has been in the market. So, in today's lecture I am going to cover something about inorganic phosphorus, specially I would like to touch on three important issues which has come to prominence in the last 5 to 10 years. One of the issue is of quantum size effect which directly takes us to the issue of nano science and alloying effect in these phosphorus, how do they affect the PL properties and then doping effect in these inorganic phosphorus. Notably if we look at the market and the emerging trends and potential for applications, one would see that three compounds are choice phosphorus which can be used in a variety of environment, one is cadmium sulfide, zinc sulfide and zinc oxide. These are emerging photonic materials, so I am going to take you through a course of slides just to impress upon you these three aspects and also tell why these phosphorus still will hold the market. This is not to underplay the effect of inorganic phosphorus, they are very established technology based compounds, but nevertheless they will be competing with the organic molecules in the years to come. The first and foremost that we cannot deny the potential of this inorganic phosphorus is the application in display material. What you see here is the CRT tube, cathode ray tube and in the cathode ray tube you have the cathode here and you have the anode here, anode is nothing but your screen and this is typically how the old generation computers have performed. So, this is a typical CRT tube where cathode the electron gun goes from here all the way hitting here. I will not go into all these parts because this is a general description of a CRT tube, but quickly I want to take you to the item number 8 in this cartoon. 8 is nothing but the fluorescent screen, a glass which is coated with a very thin graphitic layer, but between the graphitic layer and the glass is coated the phosphorus and the phosphorus are typically like this, they are called triads of red, green and blue. Red, green and blue phosphorus are coated in this fashion. So, when you actually have the cathode ray going and hitting at this triads in different proportion then you would get a continuous image coming out of this screen. So, this is how a typical CRT works, but the issue of color is actually determined by inorganic phosphorus and what are these phosphorus? They are mainly zinc sulfide doped with either silver or copper. If it is doped with copper then you get the green color and if it is doped with the silver then you get the blue color and yttrium oxide doped with the europium is actually used for red. So, you have the zinc sulfide based phosphorus holding key for this CRT applications as you see this is one of the very first TVs which came in picture and this is a full color display. So, for full color display in our TV screens you essentially resort to inorganic phosphorus mainly those are zinc sulfide based ones and typically the spectrum is like this you can see this is the silver doped ZNS, this is the copper doped ZNS and these are the emissions coming from yttria doped with the europium. So, in essence all three phosphorus together control the full color display in a typical CRT tube and what is important here as I told you in the organic molecules we are talking about energy levels and these energy levels are mostly to do with the homo to lumo and the way the homo lumo is spread and based on that you can tune the color this we have seen in the last three talks and in this lecture I would like to take you to the issue of band gap because in inorganic phosphorus we are not talking about discrete energy levels, but we are talking about band gap and this band gap essentially controls the sort of luminescence. As you know that valence band is always fully filled with the electrons and any absorption of radiation will put this electrons in the conduction band and the way the electrons come back will determine how the combination a recombination process occurs. Now, this is typically a semiconductor energy level that you see here the these gap between these energy levels are exaggerated just to make sure that we we talk about this process with some clarity otherwise they are continuous spread of this energy levels forming the valence band and the conduction band. So, in reality and at room temperature there are practically no electrons in the conduction band compared to the number of electrons in the valence band also in reality the distance between the energy levels in the band is practically 0 compared to the size of the band gap. Now, what happens when you have a some light that is shining the electrons can be ejected in the presence of light and they can excite an electron into the conduction band as a result you get a hole and there is a hole and a electron that is present in the valence band and conduction band. Now, this electron cannot stay there it has to come back and that is what will control the emission process and the band gap here will control the recombination process and this is typical for all the semiconductors including zinc sulphide. So, this is just to give you a picture what is the photo luminescence that we are talking about in this game. So, in bulk semiconductors we have a fixed range of energies we have hole, but the electron which has gone into the conduction band has to relax. So, the relaxing electron emits fixed radiation. So, this has to come throwing out some light. So, this causes a fixed emission peak of semiconductors. So, when the electron comes back then this is actually releases light and that is what we call it as phosphorescence or fluorescence. Now, notably we can single out these three compounds which are of interest to us cadmium sulphide is the low back band gap semiconductor and zinc sulphide is the wide band gap semiconductor and in between these two zinc oxide also finds interesting applications. So, all these are classified as wide band gap two six semiconductor materials. So, there are different possibilities that comes out because of the band gap one is since zinc cadmium mercury they fall in the same group. Now, we can think of cadmium zinc cadmium mercury. So, we can think of solid solutions between zinc and cadmium as a result we can go for a new set of wide band gap semiconductors. Now, all this hold a huge potential in wide range of applications as I already told LED solar cell for back light purposes most of this find very useful applications. And because of the possibility of tuning this band gap modulations we can also tune the electrical and optical properties by means of quantum confinement or by alloying or by doping with the transition metal elements tuning properties include generating all colors invisible. We can also produce white light emission in this inorganic phosphorus increase in excitonic binding energy and then room temperature lasing properties all this can be realized in this group of compounds. So, if you control the size of your semiconductor particles which is actually equal or comparable to the exciton bore radius then depending on the size either it can be of this dimension or it can be less than even a exciton bore radius or it can be much less than the exciton bore radius. The bulk this is all in bulk semiconductor only, but still if you can minimize on the size relative to the excitonic bore radius then you are actually manipulating or you are altering the band gap. So, if it is bulk and it is more than the bore radius the band gap changes are very minimal in other words it could be as equal to the bulk value. But if you make it comparable to bore radius then you are opening a band gap considerably higher than what is found in bulk. But if you go further down which is much less than the excitonic bore radius then you can see that you are opening up a band gap which is much much bigger than what you see here. So, what we are finally ending up is a theme on tunable band gap according to the size of quantum dot semiconductor will measurably alter the band gap which means a tunable band gap this is possible as long as the size of the dot is close to or below the exciton bore radius. Now, this is the other picture just to drive home the point that you affect the band gap. So, this is a macroscopic crystal and this macroscopic crystal can be of the order of 0 to 5 millimeter. So, such big crystal and the band gap is confined this way. Now, as you try to reduce this macroscopic crystal by some means either you can break it up or you can build it from the bottom to go to a optimum size. Then you can see as you go down in the size say from 6 nanometer down to 2 nanometer you are actually opening the band gap in this form. As a result when the band gap is getting altered you see the changes coming out both in optical properties, electronic properties as a result you can throw unique variety of applications or variation in the properties just by controlling the band gap. So, this becomes a challenge for both chemists as well as physicists as to understand what is the correlation between the size and the properties. So, in nano size or in quantum confinement you are throwing entirely a different gamete of properties which are not true for the bulk materials. So, in quantum dots which is another important issue specially with cadmium sulphide or cadmium selenide based compounds you are talking about exciton bore radius that is the distance between the electron and the hole and this binding radius is what you call it as a exciton bore radius. So, this exciton bore radius if it is like this and your particle is much much less than the exciton bore radius then you are talking about a quantum confinement. If the size of a semiconductor crystal becomes small enough that it approaches the size of materials exciton bore radius the electron energy levels can no longer be treated as continuous treated as discrete meaning that there is a small and finite separation the situation of discrete energy levels is called a quantum confinement. So, in quantum confinement what are what is that we are looking at suppose you have 3 quantum dots then you are talking about 15 nanometer that is the size of range that we can simulate. If we are talking about several such quantum dots to the order of say 40 lakhs or 4000 of 1000 dots then such small array will only constitute to the order of 2 centimeters. So, so many quantum dots you can try to assemble in just 2 centimeters. So, that is the quantum confinement that we can talk about a feel for the size of a quantum dot each dot is between 2 and 10 nanometers or 10 to 50 atoms in diameter lined end to end 2 million dots would be 1 centimeter long. Now, this can come by suspending the dots in liquid gel or solid matrix. So, we can prepare it in many different ways. So, the size dependent color also goes with the size as you see here if the quantum dot is well below the excitonic bore radius now obviously the color will be in blue now if it is intermediate say around 10 nanometers or so you are talking something about green and if it is beyond the excitonic bore radius then you are talking about red emission. So, one single system, but if you are going to confine it in different ways different sizes then you play around with the whole spectrum of emission. So, quantum does the size of the band gap is controlled simply by adjusting the dot this becomes a challenge for chemist because chemists have n number of ways to produce this quantum dots typically the wet chemical roots and by which you can play around with a wide range of particles which can affect the pl properties because the emission frequency of a dot is dependent on the band gap therefore it is possible to control the output length of the dot. One of the classic example for such a quantum confinement is quantum dots semiconductor nano crystals and the notable ones which are potential for this quantum confinement is cadmium sulphide selenide or telluride this is also called as chalcogenides. Chalcogenides are those which form compounds with sulphur selenium and tellurium. So, cadmium based chalcogenides are a very candid material for quantum dot applications. So, is lead sulphide selenide and telluride and you also have this 3 5 semiconductors like gallium phosphate indium phosphate gallium antimonide gallium arsenide all these are essentially possible to be for quantum confinement. So, this is typically a view graph a T M picture of cadmium selenide nano crystal which can be prepared as you see here this is of the order of 15 nanometers and each dot will be somewhere around 5 to 6 nanometers. Structure of this cadmium sulphide quantum nanodots you can see in a matrix you can see how the quantum dots can be arranged if you just take a high resolution T M image of one of this crystal then you can see that this clearly shows a well ordered quantum dot structure and this quantum dots can be capped with surface stabilizing capping molecules. So, if you want to preserve this cadmium sulphide for specific applications the best way that you can preserve it from getting agglomerated with another quantum dot is to cap it. So, these are all the capping molecules. So, this capping molecules can actually isolate every single quantum dot from the other in other words you can actually separate it out like this. So, each of this is a quantum dot in one sense because they are all capped with a capping molecule of this architecture. So, this is one way that you can keep it and most of this cadmium selenides are not isolated as solid products they are actually as suspensions or colloids. So, if you want to coat it you can actually take a colloid or a suspension and spin coat it you get a quantum dot. So that is the way to do it and typically in a chemistry way you can make this quantum dots by simple chemistry roots you can take cadmium oxide solution and then you can put selenium solution seleniding agents and if you can do it in argon atmosphere cadmium oxide plus selenium will give you cadmium selenide quantum dots or if you want to cap this cadmium selenide with zinc sulphide all you need to do is inside to as you are generating cadmium sulphide you try to put zinc cating molecules and this is a sulphur source. So, you can actually try to coat this cadmium selenide with zinc sulphide. So, this is another way which you call this as cadmium selenide core shell quantum dots and typically you can see that this quantum dots are of this size and it is well spread you can try to isolate it as a colloid or as a suspension and the p l of such a compound shows a very strong red emission. So, this is reported by Pavendis group. So, several such molecules are possible optical properties of this quantum dots you can fine tune like anything. So, if you talk about cadmium selenide the candidates are mainly from blue to red, but if you want a sharp red color then you can go for cadmium telluride and you can see here that the band gap also changes with the size or with the chalcogenites that you are talking about and because of that you can actually generate a wide spread of p l emission controlling the particle size and in most cases you see the absolute quantum yield is that of 60 to 70 percent. So, such highly luminescing suspensions of quantum dots can be acquired and we can play around with several such systems not only cadmium selenide cadmium sulfide you can also play around with zinc sulfide base compounds. Now, the one of the prime application of this quantum dots is in the biotechnological applications what are the requirements for bio applications one it is it should be efficient fluorescence colloidal stability should be there and it should have low nonspecific adsorption. So, if all three are taken care you can use this quantum dots specifically for biological applications main challenges when you try to surface coat with organic compounds then the surface becomes hydrophobic therefore, if you want to tag it with some other molecule this hydrophobic nature will be reluctant to merge with another one therefore, you need to do another surface modification. So, that you can tag it along organophilic ligands should be exchanged with more polar ones to make quantum dots bio comfortable that way you can actually make monolayer shells or multilayer shells and over coating with proteins followed by other layers of bio conjugation can also be achieved in order to enhance the application prospects drawbacks tends to aggregate and adsorb nonespecifically especially when you try to play down on the surfactant then the issue of agglomeration comes into picture. So, quantum dot surface coating for biocompatibility usually goes through this you can actually have this coating with the cadmium sulfide the quantum dots you can coat the outer shell with zinc sulfide and then you can have the capping agent. So, essentially this is one way encapsulating this with the hydrophobic core of a micelle and these quantum dots are encapsulated with the phospholipid micelles and depending on the range and the architecture of your core shell then you can see different taggings can be generated. So, this is one way that you can increase the biocompatibility and mainly the carrier that you would like to tag this compounds with are the PEG PE or PEG these are polyethylene glycols because these are biocompatible and they would form a micelle of these shells. So, it is very easy then to drive it into the biological system just to give a example whether this can be used in a safe way in vivo cell has cell imaging has been done using this quantum dots. For example, if you can take a live rat and or a mouse that quantum dot can be injected which will go and target the tumor zone. So, it is possible to see where the cancerous cells are. So, you can use this quantum dots in vivo to predict the malignancy also quantum dots can be used for breast tumor cells here we have green quantum dots filled vesicles which they move towards the nucleus to spot the areas where breast tumor cells can be located. This was proved by Alivis Tos as early as 2002 since then this in vivo studies on quantum dots have been of active interest. So, you can actually now play with the any sort of compounds and all it needs is a quantum confinement. So, you can actually modulate the band gap by modulating the size you can get a wide spectrum of colors and the colors are also very specific. So, if you want any combination you can go for such specific colors using quantum confinement. Now, another example of alloying that we have seen in the literature is that of zinc sulfide, cadmium sulfide. This is not the quantum confinement issue, but this is another issue in phosphorescence that is alloying. So, you take zinc sulfide you put a low band gap sulfide there what really happens you can see here as you are doping the cadmium sulfide A is cadmium more and as you decrease the cadmium content you see that the absorption is going towards the blue color and typically your emission also goes from red to blue. So, from green to blue. So, you clearly see with addition of cadmium 0.9 or 0.75, 0.64, 0.5 again you can see that the absorption edges changes and it directly affects the photo luminescence. So, this is one way of trying to tune the band gap and we call this alloying because zinc and cadmium they are in the same group and therefore, they are structurally similar they crystallize in the same symmetry and it is possible for you to keep on loading cadmium into zinc sides. So, this is one way of alloying the phosphor. This is another view graph that shows suppose you have cadmium instead of zinc and you keep adding zinc what really happens. So, as you decrease the zinc content from 90 percent to 5 percent you can clearly see that the PL intensity varies and also the maxima keeps shifting from here to here the maxima shifts to lower side as you keep doping it with more and more of zinc compound. So, either way you can do reverse engineering either you can try to blue shift it or red shift it depending on the size of the atom and such tuning is possible. We can also tune the properties by going for metallic and substitutions for example, if you take zinc selenide it shows emission in near blue region. So, if you start doping with transition metals these are all activators in this band gap. So, depending on the nature of the transition metal that you are doping you can get specific colors you can play around between 400 to 550 nanometer if you are doping with copper and specifically if I am interested in red then I would go for manganese doping. So, just by doping a particular transition metal you can actually play around with the color coordinates and this was proposed as early as 2005 by Pradhan and co-workers. Another important thing is the photo luminescence is very sensitive to the crystal field environment and as a result even if you are going to dope just 2 percent of manganese or 6 percent of manganese you can tune the band gap considerably D. D. Sharma's group have reported quite a bit on this where you dope manganese in zinc sulphide or doped manganese in cadmium sulphide. You can clearly see with this is the undoped zinc sulphide PL emission which is centered around 450 nanometers or so. The moment you try to dope manganese then you see the shift is more towards 600 and this is peculiar for MN 2 plus crystal field site and therefore you even with 1 or 2 3 percent you can immediately shift the PL emission. Same is true for manganese doped cadmium sulphide this is the undoped PL of cadmium sulphide. The moment you put manganese you can shift it to red region. So, manganese is very specific to these semiconductors so you can tune the band with minimum quantity of manganese doping and this is what happens in both these cases you can see when you have smaller ratios say 0.10 percent 0.19 percent you can clearly change the PL compared to larger doping concentration. So, even with small amount of manganese we can fine tune and in this case you can see each of these colors are specific to the CAE coordinates. So, if you want something near to white you can see here for 0.10 percent or 0.19 percent you can clearly get nearly white light emitting cadmium sulphide nanoparticles. So, this is one way that we can tune the color. Just before I go to specific examples from our own work I would like to list some of the important semiconductors which are already in market. There is a established technology already for the semiconductors and these are the list of materials that we can see which is already used in our optoelectronic applications. For example, we are talking now about zinc sulphide, zinc selenide they are already in LEDs which they are used in LEDs and you also have cadmium telluride used in detectors. Mostly you can see the gallium based indium based ones which are traditionally used and this is one of the reason why organic molecules are challenging because these are very expensive routes to make LEDs. So, if you can use a rugged organic molecule for both large area displays as well as for minimizing on the threshold voltage then you can actually bring down the cost of this LEDs by orders of magnitude. So, this is one of the reason why organic displays are more prominent in today's application, but nevertheless these are proven technology where the chemistry or the physics of these semiconductors are already understood. So, with this in view let me go to some of the examples from our own group just to show that how the chemical routes can actually modify the sizes and depending on the size how the PL can vary. So, just one example on the quantum size effect how we can make this out of quantum dots without capping because once you cap it then you try to modify or you try to restrict the applications you cannot use it as it is. So, one of the main thrust is to prepare uncapped cadmium sulphide. One of the way that you can do that is using sonochemical synthesis the principle of sonochemical synthesis I have already discussed in the first module on preparation. Therefore, the simplest way to work on it is take cadmium acetate and elemental sulphur in DMSO you can just sonicate it for 3 to 5 hours you can get well dispersed cadmium sulphide nanoparticles. How do we know that we have nanoparticles we can look at the properties and how we can achieve this nanoparticles by simple cavitation technique and this cavitation will break into a hot spot and this hot spot will essentially will try to take out the organic moiety it will sort of cleave the organic moiety and then it will release reactive cadmium and this will react with your sulphur and form cadmium sulphur nanoparticles. So, this is a simple way one of the reason why we can resort to sonochemical stuff is you do not have any side products that will affect the cadmium sulphide nanoparticles everything will go into the solution therefore, you can easily preserve it. Just to give a comparison between the properties of cadmium sulphide prepared by two different methods for example, you can prepare it by microwave where you can take cadmium acetate and sulphur elemental sulphur expose it to microwave you will get cadmium sulphide. You can also use it use sonochemistry to prepare and just to give you a idea how they look the particle size distribution of the sonochemically prepared one clearly shows the average agglomerate size is well below one micron whereas, in the microwave reaction you see a spread like this. So, if you are actually looking for specific application it is not possible to use a cadmium sulphide particle which has such a wide range you need to have a narrow range and in such cases you can see that the sonochemical approach is very selective. One of the way you can also answer this question whether it is nano or not or whether it is below 10 nanometers is from the broadening of the peak. You can see the x-ray broadening of the peak clearly suggest that they are in nano range although this is also in nano range but still it is much narrower compared to sonochemically prepared compound and typically if you look at the T M pictures you can see clearly the variation. This is clearly in the amorphous range because your electron diffraction pattern of cadmium sulphide shows a blood pattern which means it is amorphous but this one is crystalline the one prepared by microwave is crystalline it gives a dot pattern and the size of the clusters are well below 10 nanometers. So each of these are particles and if you look at this they are below 10 nanometers whereas this is roughly around 20 to 30 nanometers. So, depending on the preparative route you can actually prepare a wide size of range and these ones although they are agglomerated you can see specifically they are individual particles just fuse together without any capping that is important and once we prepare particles less than 10 nanometers you can also see that they are blue shifted. So, you can see that the blue shift for cadmium sulphide compared to the microwave prepared cadmium sulphate particle. So, another example of how we can modify this compounds by chemical route let us take the microwave route and take specific example of cadmium sulphide which is doped with zinc. So, cadmium if you keep on increasing from say 1 percent to 5 percent 1 to 5 percent of zinc you can clearly say even with such low doping concentration this is your cadmium sulphide typical hexagonal structure and once you start doping with zinc you can see that these x-ray peaks are slowly coming down and they fade and immediately transforms into a cubic phase which is typical of zinc sulphide. So, from a hexagonal to a cubic pattern you can just simply transform in just with less than 5 percent of zinc doping. So, the fact that it affects the crystal structure clearly shows that you can engineer this compound you can make solid solutions. Similarly, we do not have to confine just with the lower doping concentration of zinc we can go 10 percent, 20 percent, 30, 40 and 50 percent you can clearly see compared to your parent cadmium sulphide you can almost make a amorphous or weakly crystalline zinc sulphide based cubic pattern. So, definitely using chemical routes we can successfully dope and this is the view graph of how the zinc sulphide dope the cadmium sulphide nanoparticles are and you can clearly see that these are all the embedded particles and they clearly shows the crystallinity and the lattice planar spacings are specific to the zinc concentration and this is your SID pattern showing that these are crystalline and we can index that to specific planes. Now, when we look at the absorption spectra and the PL spectra one can clearly understand how we can alloy this compound and if alloying is really true then it has to show in the optical bandage. Cadmium sulphide as I told you is a low band gap material compared to zinc sulphide. So, if you actually dope cadmium sulphide with zinc immediately you will see the shift in the absorption edge. So, typically you can see say from 550 it comes down to nearly 420 nanometers. So, this is one way that you can understand whether your chemical doping or chemical process is really useful for fine tuning your band gap and if that is so then your PL spectra should also show changes and you can see here zinc sulphide shows very large bandwidth mainly because it is turning more amorphous and it is cubic in pattern and because of that you have a very broad PL emission compared to cadmium sulphide. So, any sort of zinc incorporation will widen your PL emission and this is noted in the emission spectra. Now, we can also try to see how the zinc doping really affects. So, you take the cadmium sulphide case and start doping it with zinc you can see as you go from 1 to 5 percent of zinc doping you can notice that there is a peak that is propping up from zinc doped once say around 550 nanometers and this is typical for the influence of zinc on cadmium sulphide. So, this is your cadmium sulphide emission and once you start doping with zinc you see this hump is coming and this is mainly coming from defect emission because in zinc sulphide if your sulphur concentration is less then immediately it will induce a defect concentration. So, zinc seems to be more sensitive to sulphur deficiency or sulphur defects compared to cadmium sulphide. So, this is one thing that you can note and also as you go further from 0 to 50 percent you can see that the PL is shifting towards blue and these defect induced concentrations or emission is more pronounced as you keep on doping higher amount of zinc and once you go more towards cadmium sulphide you can see that you almost get a defect free emission that is possible. Now, if you have broad emission then one of the thing that we would like to do is see whether we can get a broad emission that would amount to white light emission. So, how can we do that? If we try to take this composition which is 50-50 in other words those should be slightly cadmium rich compositions 0.54 or 0.51, 0.54, 0.58 always keeping cadmium slightly rich compared to zinc in other words they are nearly 50-50, but intentionally we keep cadmium on the higher side then if we try to dope 1 percent, 2 percent and 3 percent of manganese we can try to see what is the influence. If you look at the absorption spectra you can see that this is the zinc sulphide band gap and this is your cadmium sulphide band gap and the moment you try to dope with manganese 1 percent in cadmium rich situation you can see that the bandage is immediately shifting to red. So, even with the little incorporation of manganese it is very sensitive. So, you can push this to higher band gap and how do we know that manganese is clearly doped in all these concentrations if you take the EPR spectra that is electron paramagnetic resonance spectra you will see this sextant that is six line spectra. So, this six line spectra is typical for Mn 2 plus concentration why because this is a d phi system and this is supposed to show six lines and you can clearly see that the six line spectra is pronounced which means manganese is actually isolated. So, you can selectively dope manganese in the zinc sulphide suppose there is manganese manganese 2 plus interaction this six line spectra will not come it will come out as a broad spectra which means manganese are clustering together it is not doped clearly. So, one can go for very elemental range of doping and precise way you can dope this co-activators and you can fine tune the band. The emission that is mainly responsible for this 600 nanometer emission comes from the manganese 2 plus concentration and this is mainly because of the 4 at t 1 g to 4 a 2 excitation as a result you can do this get this pronounced 600 nanometer peak that is coming. So, if you see 600 nanometer peak it is purely because of the co-activator that is responsible which is nothing but your manganese ion. So, manganese 2 plus will induce such a selective emission and that is what you see here. So, I start with cadmium sulphide cadmium zinc sulphide now you see here that this is your nearly equiautomic composition and in this case the broadening is of the order of 100 nanometer. Now, if you go to manganese doped once 1 2 or 3 percent you can clearly see for optimum of 3 percent concentration which is your red one you can see almost you are full with that half maxima is 155 which means it is a very good candidate now for fine tuning white light. So, what you essentially do is you try to stretch the emission both towards the blue towards the red by just altering your composition with the manganese. So, in other words you can actually use manganese for white light emission this already I also showed from one of the examples of D. D. Sharma's group that it is very specific even if you put 0.10 percent or 0.4 percent then you should be able to bring about a candid change in the PL spectra. So, this one example by which you can fine tune to get a white light, but at the same time this is very specific because instead of taking cadmium excess suppose I take zinc excess in other words zinc is more in this equiautomic composition now if I take zinc and still doped manganese immediately you see that broad emission what you see here this broad emission immediately goes to a narrow emission. So, such is the sensitivity of your bandgap you know even if you put manganese there now it is only a green emission because it is the cadmium rich compositions which are important so one thing is the host lattice another one is the guest lattice. So, your host lattice is as much important as your guest so in other words to sum it up we can say cadmium rich ones are good for broad emission and zinc based ones are good for narrow emission and you can fine tune that with manganese doping. So, this is one example that I wanted to show and this is how the bandgap can be looked at you have a white bandgap zinc sulphide and then you try to make a solid solution with your cadmium sulphide now you are actually altering the bandgap and in between if you put co-activator like manganese 2 plus you are essentially inducing a broad emission. So, this is the schematic diagram showing all possible transitions involved in the emission feature as I told you we have this peculiar emission which is responsible for the broad emission now I will try to show one more example of alloying effect in zinc oxide. Zinc oxide is very interesting because it is used as a UV sensor now if you want to alloy it you can try to do that either with manganese or you can try to do that with the cadmium doping. So, typically the zinc oxide bandgap is 3.3 ev but you can actually dope it with magnesium and cadmium magnesium is a dopant which has a very high bandgap that is 7.5 it is called a white bandgap insulator now cadmium sulphide is a low bandgap stuff. So, this is having 2.3 and because they have comparable these two have comparable size it is possible although they have very high bandgap variation still you can try to put as much of magnesium into zinc. So, this is one way we can try to alloy this compound and then what becomes important is you cannot keep on doping magnesium there because it will be restricted since magnesium oxide and cadmium oxide they throw a cubic phase compared to wood side phase. So, what is the solubility limit? So, magnesium actually has a solubility limit of 4 percent this has been widely suggested and one can actually try to put zinc into magnesium phase up to 40 percent but it is not possible to put magnesium into zinc phase which is very critical. So, is it possible to increase the concentration of magnesium by means of some other weight chemistry route which can give a metastable phase that is one question that we can try to answer cadmium solubility is only 2 percent can we increase it using any chemical procedures. So, many studies have gone through and I will just show some of our own results on this zinc oxide this can be tuned by alloying up to 6 percent to 16 percent people have achieved it by first depositing gold on silicon and then over the gold layer you try to deposit it because of the mismatch in the lattice parameter you can try to do that as you can see these are all the rods that you can grow and the tip of the rod is actually a gold which will keep coming as a flux. So, it is more like a surfactant your as you keep growing your zinc oxide the gold will come on the tip that is what you see here the from this SEM picture now is it possible to grow cadmium doped zinc oxide yes you can clearly see that the bandage is shifting and the lambda max is also shifting. Therefore, by means of wave phase deposition it is possible to dope cadmium this is proposed by Wang in 2005 and if that is the case then what happens to the bandage and to the p l you can see here in this case 30 percent cadmium when you dope then it is shifting to a lower band gap and the absorption also shifts to the lower band gap. So, with increase in cadmium you can push this to a lower band gap emission. So, this has been done by MBE grown zinc cadmium oxide films and this is reported in APL in year 2006. So, one can use MBE technique for making such heterostructures and this is the view graph of your zinc magnesium oxide nano needles which are grown on silicon you can see these these are the grown needle shaped structures of this zinc magnesium oxide that you can grow and they clearly show the lattice fringes which is typical of the zinc magnesium oxide and as you would see here with the different concentration of magnesium you can actually shift the p l emission and therefore, it is possible for you to achieve in this case even up to 20 percent of magnesium can be doped skillfully. We can also try to allow it by doping it with copper and in this case you can see how the p l varies the maxima shifts towards the red and p l also changes considerably with copper doping. One other way that we can prepare this compound is by using microwave poly all synthesis which we will try to discuss in the next lecture.