 In the past few lectures on photonic materials, we have looked more at the possibility of organic photonics where organic molecules can be used for optoelectronic applications especially we have seen the examples of how organic molecules both small molecule and organic polymers how they can be used for device applications as OLED displays and also in another lecture we have seen the importance of organic molecules in solar cells. Today I am going to talk to you about a group of inorganic compounds which is actually holding a billion dollar industry for more than 20 years now and they are generally categorized as phosphor materials and as you would see in the cartoons in this slide there are variety of applications that you can engineer using the set of molecules or inorganic compounds they are usually oxides or sulphides. So in this lecture I will try to outline the importance of this molecules or oxides and then how this is governing the principle of tuning colors in display devices. Let me start first with what this phosphor history is all about and where it emerged from it actually started as early as 1603. We have documents where an Italian scientist is alchemist actually he found that barite which is nothing but barium sulphate when it is heated with coal on heating he observed a bluish glow which is actually due to barium sulphide which formed. So since then there has been several re-visitation of this issue of glowing materials 1866 first stable zinc sulphide phosphor was described and then calcium sulphide was discovered in 1870 and 1885-87 the concept of bioluminescence from fireflies was also observed. These are some of the milestones in this phosphor research and in the last 20-30 years it has taken a very different turn and we can look at it. Before I define what this phosphor materials are and where we can see them operating just give you a basic definition of the different types of luminescent phosphors that are possible. When we say luminescence it means we are talking about light emission and this can actually come from two processes which are fundamental to photochemistry and those are called as fluorescence and phosphorescence. Fluorescence is a fast decaying process and the decay of this excited molecules happens in the order of 10 power minus 15 seconds. Therefore there is no afterglow it just rapidly quenches. Then you have a spin forbidden transition which is called as phosphorescence and this is because of its spin forbidden nature it is a slow emission process therefore we can call this as a strong afterglow and in both categories we also have a subsets of this process manifested. One is phosphorescence which is excited by light, cathodophosphorescence when it excited by electrons, electrophosphorescence when it is excited by current, same is true for fluorescence you have electrophluorescence, cathodophluorescence and photofluorescence. So as you would see here the category of light emission actually comes from what is the initiating process and the case of phosphores with the cathodophosphorescence or cathodophluorescence is a well established one out of all these possibilities and this is basic to the CRT tube applications which I will come in the next few slides. So when we talk about luminescent materials we should also know that how much ever we engineer the mother nature was also gifted many luminescent materials which are found as it is without even any refining process you can isolate those luminescent materials in nature. For example calcite itself is a blue light emitting one if you can excited with long wave UV. So this is calcite crystal which is glowing under UV radiation and zircon is here and that shows a dominating yellow and then you have eucryphite which is mainly a European doped phosphor and then willmanite is there then we also have a combination of several of these phosphors which is available in nature as it is as willmanite calcite frankly in all these are isolated as minerals. So as you would see here under UV radiation several lights with very strong fluorescence can be obtained but this should also respond to cathodoluminescence. Some of these materials may not be active material for the process by which it is driven either by current or by cathode rays or by photo. So depending on the nature of application these materials can find use in devices and again this is the example of bioluminescence where the firefly actually is glowing. This is the firefly without illuminating the light but when it is actually luminescing it glows like this and the basic compound that is responsible for this chemiluminescence or bioluminescence is this compound luciferin which emits in dark and the basic functional groups here are thiazol groups which becomes very important and to isolate this compound it is a billion dollar work. In fact if you see in commercial catalogs you would see one microgram sold for hundred dollars. So to isolate these compounds are very expensive therefore although it is very highly luminescing with very good quantum efficiency these molecules have not been used per se. But apart from the naturally occurring luminescent materials we also have man made materials and you can see all these sign boards and different ceramic components glowing in the dark mainly because of a variety of compounds that you can prepare and in the UV light they show the same color and in in darkness you can see that they are glowing and it is also possible for us to generate white light emitting phosphorus by combination of these colors. So these are man made phosphorus and this can also find use in device applications and here is another example of how glow in the dark phosphorus can be seen. Now just to satisfy our curiosity I would like to list some of the after glow phosphorus which show phosphorus sense during their emission and some of the candidates are the yttrium oxide doped with sulphur and as you would see this is nothing but Y2O3 one of the oxygen is replaced fully by sulphur and then the dopants here are european magnesium or titanium which will actually generate the excited states that will be responsible for the phosphorus sense here and one thing that I would like to draw your attention is to the visible after glow time in this phosphorus you can see they can radiate light for over 15 hours and that is the specialty of this sort of phosphorus and if you can see the other phosphorus they are all mostly alumina based. So it is mostly yttrium based oxides or it is alumina based oxides and these oxides are usually termed as SAM if it is transom aluminum magnesium oxide or BAM oxides. So depending on the constituents they form a different story in terms of its light output as you would see calcium aluminate with the european is very selective for purplish blue and transom aluminates for blue and bluish green and then transom aluminate the lower analog with european is specially meant for green fluorescence and as you would see here red and yellow is mostly to do with yttrium based phosphorus and these phosphorus materials are mainly used in phosphorus lamps and today the latest generation lights or bulbs are the CFL ones which has a very strong coating of this fluorescence and as you would see these oxides have a variety of applications and we will specifically look into phosphorus which are used in TV screen. So what are these phosphorus? These phosphorus are an established materials technology compared to liquid crystals and other light emitting polymers and they have been used in the cathode ray tube displays for around 100 years now. These phosphorus are also used in avionic tube applications and avionic displays and nothing but all the screens that you see in a cockpit is all totally driven by the CRT tubes and those CRT tubes are usually 6 inch display stuff so all these avionic applications are totally taken over by this CRT phosphorus. Now new research have actually focused on field emission displays and on photo luminescent LCDs which are already coming into market and the phosphorus find a special role in that. When you look at the CRT tube now you would wonder where exactly this phosphorus come into picture but actually the phosphorus are the lightest part of the screen. As somebody put it because the panel that you are facing which is the glass panel is actually containing phosphorus in a very very small units as triads. I will show the contour of how these phosphorus are coated on the phosphorus on the glass screen and this is a very very well established technology now. This technology cannot be substituted by even the nano technology that we are talking about because the sheer temptation for us would be to immediately try to make these phosphorus in nano scale but this particular technology is well proven not for nano but for a micron technology. As I would point out in other slides you would be convinced that CRT phosphorus does not necessarily need a nano phosphorus but you can actually operate with the micron sized phosphorus materials. So in the CRT tube the main display event that is happening is the light that is coming out apart from all the engineering technology that is there and the lightest part is nothing but the screen which has the fluorescent material. Therefore this is a simple contour of your CRT tube this is your cathode ray gun through which you can actually using a deflection system you can move this electron beam up and down therefore it scans 100,000 times per second in such a fashion that you get a complete continuous display of your image and in this place the anode is actually bearing your phosphorus coated screen. So anode is nothing but a graphite coating and on the graphite coating you can pattern this phosphorus. So this is where the phosphorus lie and to the present generation this may be a old time photograph because this sort of monitors existed in the early 80s and these are called monochrome displays. Today nobody has seen or it is redundant nobody knows about a monochrome display but this displays where the first generation displays when the computer came into picture in Indian market and as you would see here we all have lived with just a monochrome display for at least five years and then the technology transcended to full color display so we will see those events more carefully. So when we look at cathode ray tube this is how the simple operation is electron gun actually brings out a cathode ray which is channelized through an add-on so the direction is along this way when it goes through this you can actually deflect with a magnetic field and based on the deflection you can try to scan this electron beam or bend this electron beam wherever you want. So the cathode ray can actually hit the screen million times in a second as per the way the deflection is contoured. Now we can do one more thing we can have a supplement guide in front of the phosphorus screen and in that case you will be able to even channelize through this supplement guide which has lot of holes you can channelize how this electron beam can selectively hit some of the phosphorus material on the screen. So this supplement guide is also called a shadow mask and if you have this shadow mask in front of the phosphorus screen then that means you are talking about a color monitor system but if it is a monochrome material you do not need this supplement screen or a supplemental guide or the shadow mask so we will see how the shadow marks will work and in case you have a shadow mask like this which is in the gray image then in a typical CRT color monitor you would not use just one gun but you would use three guns and each one of this gun is maintained at a different potential to excite the corresponding color. So the colors are actually made like triads if you can follow here I can draw a triad like this at any point any given place so each triad forms a pair and each of this triad can actually be injected with a appropriate cathode ray which has a different energy and depending on that the mixing of this colors occurs. So if you want more of red color dominance then you need to pump in more cathode ray energy so that you get this on a brighter proportion compared to blue and green. So in a color monitor you actually have three electron guns doing the job and each of this gun will go through one single orifice in the shadow mask which will in turn do the right color mixing. So when all these three hit at three different colors then the actual color mixing will happen and in a typical TV screen you would see this side of a panel and each one is separated by few angstrom thickness so in essence you would actually see a triad of this sort because this will be finally chipped into micro pixels so each pixel would have a combination of triad. So this is how the phosphor coating is actually made on the glass electrode and the color mixing as you know the principle three colors are here and these colors when they are mixed in equal proportion then you get white and this white is a combination of 30 percent of green 33 percent of blue 33 percent of red. So actually there is a way to address to this color mixing or to the color purity so those in the CRT trade never try to talk about phosphor in terms of red blue and green but they have coded it such a way they know what is the color purity or what is the dominant color that will emerge out of a particular phosphor. So the color coordinate that is recommended for pure white is 0.33 0.33 which is actually defined by a CAE diagram this is an international standard this is an international standard is called CAE XY diagram so you can actually plot the components of blue red and green in terms of a two dimensional plot and by this way your x axis value and your y axis value will tell you where exactly you stand for example the one which we spoke about white should necessarily come here. So this is your white emission and if the coordinate is somewhere here then you talk more about red dominant one if your XY coordinate is somewhere here then you talk about green or blueish green and so on. So this is a very useful parameter to evaluate what set of a phosphor you make. So whenever you make a phosphor and you see there is a potential then necessarily you need to talk about CAE coordinates then the color purity is actually defined. Typically these oxides are colorless those which are used are colorless in ordinary light but in the presence of UV radiation you would see that they are glowing and in cathodoluminescence actually this will become much more brighter therefore it will give a dominant green blue and red and what are the phosphors that are used for cathode ray tube application to single out some example which are as of now used in CRT applications as I told you yttrium oxide or yttrium oxysulfide with the European doping is supposed to be the most preferred one for red. In fact yttrium oxide with European also gives the same 625 nanometer emission but a proportion of sulphur into oxygen is always recommended because it improves on the sharpness and it restricts the degradation. Therefore yttrium oxysulfide is preferred and as you would see here the other group which is really taken the show in CRT application is zinc sulphide the base material or the host is the same zinc sulphide but if you actually dope it with the choice quantity of copper and aluminum these are actually doped less than 5% and if you make the right proportion then you get green for copper but if you dope it with silver then you get a clear blue emission. So in the whole issue of CRT phosphors as you would see in the further slides numbers a number of examples are given many compounds have been traded commercially and the most important phosphor of all is the zinc sulphide based one because zinc sulphide phosphor is actually able to take care of both blue as well as green emission. The red is always unique to yttrium oxysulfide. So for improved contrast we have pigmented variations that are available I will come to this in one of the slides and then particle size and color coordinate variations are available to meet individual customer specification. As I told you in one of the earlier slide I talked about the strands of red, green and blue phosphors which are actually coated on the front panel therefore if you want to make such sleek and very narrow width of this phosphor stripes then the particle size of the starting material is important because you need to make a blend and then you need to burn it carefully to make such very sharp strands of phosphors. Therefore particle size becomes a very unique issue and it is preferred by different customers who have different technology to dope this phosphors on the glass light. As you see in this slide again the whole thing is given and one of the unique stuff that we see in this CRT phosphors is that they are coded with specific numbers. So depending on the numbers it is possible for us to categorize what sort of compound is used for that particular application. As you would see here in all this blue emitting phosphors it is silver but along with silver you also see a chloride ion that is doped which is actually considered to be a co-activator. So if you want a very strong fluorescence not only you dope with silver which is the actual dopant and the actual activator you also use chloride as a co-activator which improves on the efficiency of this compounds. Green ones are those with copper and aluminum and again we have yttrium and in all this classes you also see that there is a tag attached to it pigmented and non-pigmented because pigmented brings a different effect in terms of the contrast of these phosphors. One thing is the bright glow that is luminescence another thing is the contrast. Contrast is actually inversely related to luminescence therefore we need to have a very sharp control on the contrast also it is not just a stray light that is coming out but it has to be very sharp therefore when we talk about contrast the issue of pigmentation comes into picture I will show some examples on those lines. And in this view graph it may not be very easy for you to read through all the numbers but I am just picking out this column for discussion in this slide as you would see here these are all the candidates used over the years for cathode right tubes and for those who are engineering this CRT monitors they usually are very sensitive to the choice of the particle size of this phosphors. So if we are going to make using a chemical route or by some other route it is not just important to get the emission peak which is decided for example 525 you would anyway get when you dope manganese in zinc silicate but you need to achieve the right micron size and it has to be a narrow distribution. If it is a narrow distribution then such phosphor particles can be easily engineered for making this panels. Suppose you have a very wide distribution then it is very difficult for putting that as a strand in the front panel therefore one of the thing that I want to point out to you is the average size that is preferred is actually 8 micron because we are living in a nano world there is always a temptation for us to talk about nanometer but one should also realize that the micron technology is proven and it cannot be shaken that easily therefore when you talk about CRT we are not the issue is not about nano size but the issue is about a very narrow distribution and what is good for processing is a average micron range of 8 micron. So if you have a 8 micron particle you do not have to really feel bad but you can only be happy because that is what exactly is preferred for developing the front panel and you also see here on the left slide I just want to pick out on a few things for example a phosphor which is highlighted in white. These are the candidates for getting white light phosphors and what are these candidates those are a mixture of zinc sulfide doped with silver and zinc cadmium sulfide doped with copper such combinations actually give you white light phosphors so you can go for that because if you are looking for applications in liquid crystal displays as a backlight emission then you would rather prefer a white light rather than red green or blue and therefore there are other combinations for example if you take yttrium oxy sulfide which is doped with terbium from 2 to 5 percent then you would end up with white light which has a dominant emission at 545. So by varying the doped and concentration it is possible for us to engineer a variety of phosphors and as you would see here the range is mainly between zinc sulfide or yttrium base oxides or aluminates in this case for example this is yttrium aluminum garnet doped with terbium then you should be able to get a 544 nanometer green emission and we can also gamble with several other substitutions in case of zinc sulfide base compounds. There is one compound which is quite peculiar in this table that is gadolinium oxy sulfide doped with terbium again shows a 545 nanometer emission and just want to make a correlation between this and this in this case this is a Yag compound that is yttrium aluminum garnet whereas in this case this is only simply a gadolinium oxide but both are giving same emission at 545 nanometer and the peculiarity is because of the dopant which is the same that is terbium 3 plus. So as you would see it is not the host that would control the light emission but the dopant because of the particular excitations in the D2F transition or F2D transition which will determine whether this is unique of the dopant or not and as you would see from the red emission it is always European which gives you emission at 620 nanometers. So this is the range of compounds that we have now the applications for this materials include CRTs which is the major player in the displays and then in the last 10 years several groups have come or several applications have emerged one is FED that is field emission displays which are known for the contrast that it can bring in the image resolution or plasma display panels which has revolutionized today's modern living room because you do not need a space it can be wall mounted therefore plasma display panels are really replacing the conventional CRT tubes but for a higher cost the processing of plasma display panel is 3 times more costlier than a cathode ray tube but for from the aesthetic point of view and for making a large panel display still plasma display panels are counted to be one of the modern technologies and then of course these same phosphors can be used in vacuum fluorescent displays fluorescent lamps then X-ray screens and storage plates for medical images and radiography and off late it is also used as taggons in several documentation events I will show one or two examples on that and for immunological assay it can be tagged with organic molecules and of course the issue of phosphorescence is used now in every bit of application including toys and other safety devices so as you would see the range of compounds it is not limited and several possibilities are there but chief among them is the application to CRT tubes now in the CRT tube how do we make it as you would see from the cartoon that is given below the CRT tube is actually mounted on the top down position and you can see the evaporation process is actually happening this way so it is a very involved technology and to make one CRT tube it takes several hours starting from the simple glass panel to making the phosphor and then to bring in all the electronic component it takes more than 10 hours to bring out one picture tube so it is a very involved issue and as you would see here the way you try to bring about this triad coating is the specialty of the CRT tube so if you if you can make the right choice and if you can make the right approach to coat this material then the color purity and the clarity of the pictures will be extremely impressive as far as the CRT tube is concerned and coatings can also be made using this sort of vacuum technology setup and therefore this also brings in another issue that you cannot go for a very large area so CRT tubes are confined only maximum to 24 29 29 inch or 32 inch monitors and bigger than that it is very very difficult therefore people are trying to transcend from a CRT application to plasma display panel because the way that is operating the principle behind plasma display is very different from CRT and to manage this deflection of cathode rays in a in a very focused way it is very important as a result the CRT application is actually confined to a small area display this is how the current induced photo luminescence occurs in a typical phosphor where depending on the dopant and co-dopant you can actually get the light output from from this displays and the spectra from the phosphors actually look like this the silver doped ones show blue light here and the copper doped ones show blueish to blueish green so these two candidates give the blue and green component and as you see here yttrium oxide phosphors doped with europium usually give a very very strong red light and if you look at the full width at half maxima it is only ranging to two less than two nanometers which shows how selective this luminescence can be compared to the other two phosphors and this is mainly because of the mechanism of photo luminescence that is operating in the zinc oxide base phosphors compared to yttrium base phosphors and if we are looking for full color display then what we desire is that it should not be narrow but it should also have a wide mixing between the corresponding colors for example blue should nicely mix with the green component in this fashion so that the overlap is more more the overlap then you can try to get the white light emission or the full color display in a better way therefore not just getting a narrow band emission but mixing of these colors to get the full spectrum is very important therefore the amount of dopant and code open that you add becomes very very important and lastly on the mechanism of how this zinc sulphide based phosphors operate there are two three things that can be dominant one is the electrons are excited to from state 1 to state 2 that is the conduction band and this can this can dwell in the conduction band for a short span and then this can give out green emission and come back to the copper ground state level and then this can return back to the ground state so this is one process there are other things that are also accompanied with it because of several processing issues one is if it is a defect induced one then the electron gets trapped momentarily in this trap level and because of the because of the influence of the cathode ray they do get ejected out and finally they return giving a green emission so there are trap levels and the number of trap densities or trap levels do control the quantum efficiency and the purity to some extent if we have very little amount of iron that is coming out as impurity then this can actually act like a killer so this is called a killer level because any other impurities which are in same comparable atomic level can actually bring about quenching of this light in the case of emission that is happening in the red light based on European oxide based phosphorus there is a unique issue that is involved the European 3 plus can actually go into two different sites in Yttria one is there is nearly 70% of sites have C2 symmetry and then 30% of sites have C3I symmetry therefore the European which is actually doped in Yttria can either go to C3I site or it can go to C2 site now if because there is a inversion symmetry all the European ions which are occupying the C3I site actually do not contribute to fluorescence so only if they occupy the C2 sites then they are more productive in terms of emission and therefore this has to be carefully monitored that the occupancy of the European site is dominated by a C2 based occupancy and in fact it is possible even to reconvert the C3 occupied sites to C2 and the conversion that is possible is up to 85% to C2 symmetry and those C2 occupied European sites actually are responsible for the red emission this same phosphorus can also be used for plasma display panels and as you would see the display mechanism is quite different compared to the CRT applications and in this case what you see is a plasma discharge and this plasma discharge is actually brought about by the UV light and once UV light is generated in the back panel that UV light will shine on the respective ones red green and blue and as a result you will get a plasma which is generated that will account for the emission of full color display so even in this case the way the red green and blue are mapped is of this fashion as a sub pixel and this will constitute one plasma cell therefore the plasma that is generated will have to have a different mixing co-efficiency between each of this panel and as a result you will get the proper color output so in plasma display panel the main mechanism is not the cathode ray tube or the cathode rays it is the UV light emission which will bring about the color display so here again you have choice materials which are used for plasma display and we have a set of such phosphorus which have been proven and as you know that continuous exposure to UV can also result in degradation of this phosphorus or the phosphorus can get choked and as a result a nice display of this sort can become as disfigured like this so a distorted display like this clearly shows that the phosphorus are getting degraded in the plasma display panel so this is one of the main problem that you encounter in practical application compared to CRT tube you have a problem of the phosphor degradation because of a continuous exposure to plasma so this has to be taken care some of the candidates for plasma display panels are borates aluminates silicates this is slightly different from the phosphorus that are used in CRT tubes main reason is these silicates and aluminates are preferred over the conventional zinc sulfide based phosphorus is mainly because of the thermal properties they are much more rugged for very high heating effect which can happen in the plasma therefore to withstand the internal heating effects we actually use a high temperature ceramics which also have same color efficiency therefore most of this oxides if you see they are all insulating or these are high temperature materials so this is very unique for PDP applications and when we come to fluorescent lamps you again see that the major contributor is cerium doping because cerium doped phosphorus they respond immediately to mercury plasma that is generated in all the fluorescent tubes so it has to respond and cerium is very unique for green emission you also have magnesium tungstate and then of course europium based yttria these are used for blue and red the fluorescent color of a sine tube is a combination of emission from phosphor and the discharge that is taking place so if it is a mercury discharge then the emission will be mostly dominantly blue in color suppose it is a neon discharge alone emits red and adds red in combination with phosphorus for example green emission color is gold with neon so if you have a neon discharge then you can get a different emission for the same phosphor compared to mercury discharge therefore these are being worked out as combinations for mixing and now when I talk about the contrast of a TV screen we should understand the contrast is nothing but the luminescence contrast performance this is defined as a ratio of your luminescence versus diffuse reflectance diffuse reflectance is actually coming from the stray radiation to improve the contrast of a TV screen not only the phosphor brightness is important just because it glows bright doesn't make it a good candidate and because we need to combat with the daylight reflectivity of the screen so the contrast is actually a gamble between the brightness of the phosphor and the daylight reflectivity and as a result it is generally agreed that this has to be pigmented so that the daylight reflectivity can be controlled when the phosphors are glowing and in order to reduce the reflectivity of the white phosphor powders for example these are all candidates for white phosphor each phosphor particle is covered with a pigment for example cobalt aluminate is a good pigment for blue emitting zinc sulphide which is doped with silver and aluminum similarly you can use iron oxide it could be alpha iron oxide powder which is actually used to control the reflectivity as far as the red emitting yttria compounds are concerned so each of this phosphors have to be controlled in order to combat with the daylight reflectivity as a result we have a new generation called the pigmented phosphors which are used and this again depends on the company which is making and the requirements so pigmented phosphors are a very important issue and here again the pigmentation as you see comes from several colored inorganic oxides for example cobalt aluminate is mostly a peacock greenish blue combination iron oxide is predominantly a yellow brown stuff which is added to control the reflectivity the pigment particles have to be quite small in size because they should not be of a comparable size with the stuff otherwise the adhesion between the actual phosphor and the pigment will not be say will become equal as a result the narrow range that is preferred for pigmented for particles is of the order of 80 to 120 nanometer in addition to other ingredients like inorganic oxides you also have organic polymers which can with a good adhesion which can actually coat this phosphors for getting more contrast another example of this phosphors are upconversion phosphors as we know when we try to use a UV light then you will get a emission of a higher wavelength in other words lesser energy this is actually the Stokes law or Stokes lines which we which we have learnt from the spectroscopy but upconversion phosphors are actually anti Stokes phosphors because they take energy in the lower lower nanometer range but they actually emit a higher energy emission and therefore these upconversion phosphors are also called as anti Stokes phosphors this upconversion phosphors are actually used as a anti counterfeit phosphor and it's a luminescent material that converts different invisible infrared light into visible light the best example is when we try to go through a security zone usually our identity cards are scanned through some light and those are usually infrared lights because that is safer and as you would see here in the normal light tag like this a monogram might have two different lights so person who is trying to do may try to put a counterfeit like this but in the infrared actually it has to glow fully same so these are special phosphors which will actually take low energy but will emit in higher energy so this is nearly green which is actually emitting in blue and these are actually called as two photon process or more than two photon process so what happens you have a activator which actually takes it to this level and there is another co-activator which will push it to this level therefore the emission will actually be a two photon emission against one photon that you are giving so this is a called upconversion phosphors since stable anti Stoke phosphors are not generally available and are difficult to manufacture they are actually attractive candidates for security applications one or two examples of that yttrium fluoride and then sodium lanthanum tungstate these are very critical to prepare people who want to do can't easily make it therefore these are special chemicals which have a controlled emission and these are also called as anti Stokes phosphors in all normal fluorescence we know that this is behaves based on Stokes law however in anti Stoke phosphor we have two or three photon absorption and they emit a single photon of visible light this is another example of how the IR excited phosphors behave you can see here this is the yttrium ytturbium dopant which actually takes care of the emission from this state 2f7 by 2 state to 2f5 by 2 state but then you also have a co-doping in the form of ytturbium which will actually translate this further to higher energy levels as a result the emission that we see here from f4 9 by 2 to 4 I 15 by 2 is nothing but your red light emission and this is actually engineered by the doping of both ytturbium and erbium as activator and co-activator and there are also other examples of up conversion not necessarily the examples that I showed there here you are actually using a combination of gallium arsenide base semiconductors and in combination with this phosphors they can actually do a up conversion and as you see here this is your gallium arsenide based panels and here you can put your phosphor and then you can get through suitable filters either red green or blue colors but in this set of conversions you would see the phosphors that are used has a much better gamete of color for example the triangle that we see on the outer side this is corresponding to the up converters what has been engineered here so they have a much better gamete of colors compared to OLEDs which are highly pure but at the same time they have a very limited color resource and then comes your NTSC based phosphors which are in the middle so when you think of several hybrid combinations you see this up conversion phosphors seem to have a larger scope than the conventional ones we can also work out for up conversion phosphors using simple lanthanum oxide which is doped with erbium and ytturbium you can see the color purity that you can get out of such substitutions and we can also look for red emitting cerium based phosphors which are reported and the color emission in each case will differ depending on this side symmetry for example in a cubic crystal field you will get for the same doping a blue color but in a distort in a distorted cubic lattice you will expect a red light and suppose you are going to add with the nitrogen replacing oxygen atoms then you can modify the color emission to green so such substitutions are also possible and lastly I would like to leave with one more thought that LCD whenever we handle LCD laptop computers which we normally use the color emission or the bright color that comes out is actually because of the phosphors and you have a very sleek fluorescent lamp like this and this pencil is kept there for comparison so it is as small as that and this can bring out the white light emission that is needed for LCD displays so apart from the liquid crystal panel that you have the background emission is actually moderated by this phosphors which are coated in this tube fluorescent tube so the range of applications of this phosphors are not limited only to CRT but to a variety of other display materials and as you would see it is just a simple doping in a host matrix which brings about all these fascinations so here the chemistry becomes very important chemistry in terms of the choice of dopant and the chemistry of up conversion comes into picture then the chemistry of controlling the size comes into picture the chemistry of pigmentation to improve the contrast comes into picture therefore there is plenty of chemistry principles that are involved in this technology which we need to bear in mind therefore a useful chemist chemist who understands all these ideas can successfully produce an engineer a variety of new phosphors for future applications.