[00:00:05]
>> So it's a pleasure to be here. One of my guys pointed out to me that I'm giving a nano talk in the building and because people are. So the other once a petite building. I'll never get that straight that would be my goal from before retirement so I'm.

[00:00:25]
Amused tell you a little bit about myself my background is actually is originally as an inner again a chemist and then over the years I started doing more again in chemistry and so how many people here are chemists Ok so so the answer is not many. And so what I've decided to do is basically give you the bare minimum of chemistry although the chemistry underpins everything that we do Ok so I stripped out a tremendous amount of the hard work that I'll group has done to make new classes of molecules that hopefully do interesting things I'll tell you a little bit about the strategy but all the chemical stuff is going to be kind of just.

[00:01:22]
Summarized in a couple of slides and then the rest will be sort of show and tell. So you mention that I got the class of 1934 a distinguished lecturer Distinguished Professor Ford when trial e.o.t. got it and I nominated him years ago I refer to it as the class of 1904 extinguished professor because basically that's something you get when you're really old Ok.

[00:01:48]
And and I am getting there I still have a few years to catch up to Charlie I haven't made much progress on catching up to him yet but it's been a long time since I've done a substantial amount of work in the lab itself and what I want to do is note that none of the work here has been done by me and that there are a lot of people in my group who have contributed to the design and synthesis of some of the materials that we're using as well as the approaches that we have taken to understand how they work and then we have longstanding collaboration's in particular with the Kiplinger group in the brain our group I mean Benard I've been working with for 24 years now on loop for about 27 years.

[00:02:36]
As well as more recent collaboration's with Eric Bogle's group group now at least England's group and Shanice group and then what you can see is that our group has been pretty well integrated into groups all over the world Ok And so yeah I think quite a few that people mention here you'll see work that was done in collaboration with these people.

[00:03:02]
So I'm going to talk about a little bit about doping and it's going to cover multiple topics it's going to be covering doping at interfaces and also doping of organic electronic materials and. What you see here pictured 3 guys one of whom has passed away Alan McDermott really nice guy from New Zealand as well as Al and he gear and share a car and these folks made the observation in the seventy's that if you take a molecule or get a polymer actually referred to as poly acetylene which is just alternating in single bonds and you treat it with iodine.

[00:03:47]
The conductivity of that material goes up by something like 6 to 9 or to Sebag Ok And so when we mostly think of her again ik materials we think about them as plastics like this is insulators and today they're being used as conduct is there was a report in advanced materials out this year of materials that when doped have conduct a video of 10 to the 5 Siemens percent to meter and that's with the group in straw spread with whom we're working on on aligned polymers and they're also used as semiconductors Ok the most obvious case of these being used as semiconductors that you may be aware of one way or the other is in the case of cell phones Ok so if you have an i Phone 10 or a Samsung phone or an apple watch the display is made out of an old lead or Gammick light emitting diode my t.v. at home is an l.g. $65.00 inch t.v. it's made out of an old display it looks gorgeous other applications that people were playing with pertain to photovoltaics a few years ago the maximum efficiency of a photovoltaic system was on the order of 7 percent conversion efficiency now there are single junction solar cells based on purely organic materials except for the electrodes that are around 18 percent efficient if you had asked me a 7 years ago would that be possible myself and my colleagues to pull in would have really laughed Ok so these things are coming along people are doing printed electronics people are doing bio electronics using these and so it turns out that like other semiconductors and I'll go into this in a little more detail.

[00:05:38]
That you can dope them but the whole concept of doping in organic semiconductors is somewhat different than inner Gannett materials and that when you don't them you can increase their conduct to body by injecting carious you can inject both electrons and holes you can fill traps that improve the mobility You can also have doping near interfaces with electrodes that can lead to either more efficient charge injection into the material or charge collection from the material if it's a solar cell and I have a couple of bullets for how that's done and as I said these a found outlook ations in.

[00:06:20]
Overheads and other applications I just got a manuscript to work on today some colleagues in Singapore who have been basically using doping to enhance the photo sensitivity of black phosphorus layers. So so sure a cow or Alec Derby and. Alan heater shared the 2000 Nobel Prize for the realization that you can do organic materials and make them highly conductive and then it was later in the eighty's that people really began to focus on the semi conducting properties of materials chain Tang who was at Kodak made the 1st observation of 2 things one is systems that had quote reasonable efficiency as an organic light emitting diode they were getting like one percent and also the efficiency as solar cells where they were getting one percent and that has blossomed between $1086.00 and his reports in Applied Physics Letters Ok to old LEDs being a multi $1000000000.00 a year.

[00:07:30]
Market in these going from something that was not even conceivable to something that might be conceivable for consumer electronics applications are but not likely for large area kitchens. So organic materials are somewhat different than energetic materials and I'll just talk about that in a 2nd 1st of all in a crystal in inner again equate to Rolls and pry the vast majority of people here who work in electronics work with Crystal in organic materials have a really well defined lattice and that they'll have a dope an atom that has either one fewer or one greater electron than the host Ok Which leads to the system being either.

[00:08:14]
P. doped or n. doped and leading it to be a whole or electron transport material or get it materials by comparison are complete mess Ok if you look at the best crystal in organic materials. In organic semiconductor person would look at them as highly disordered impure materials and we don't typically use highly Crystal in materials we use amorphous materials and we use poem Eric materials.

[00:08:43]
Ok and amorphous materials polymer materials can be amorphous but we can also use small molecules Ok that can be evaporated and make a more physical assay films and and then we can have impurities which are dope and then in the context of organic molecules doping is somewhat different in that it involves an oxidation reduction process and I'll show that more on that coming slides and then the way we think about the transport of materials inert again existence is somewhat different in.

[00:09:19]
Inner Gannicus cereals we think about the conduction through Vallance bands and and conduction bands and inorganic materials people somehow use that language but maybe not in the same way that people in inner again ik. Community use them in particular what we have is a manifold of localized transport States on either individual molecules or on a segments of a polymer and that we can have gaps States as you can in a.

[00:09:54]
In organic material and that when we dope what we do is we take one electron out of a molecule that is in the whole manifold of states and we can call that you know paedo ping and when we endo we put in electron in here and we typically don't see a band like transport we see a thermally activated hopping transport we're working toward seeing more band like transport minute and in fact I'm part of an innocent team of people working to do that Ok so that's a bit of the differences so when we look at doping of materials we could talk about endo patent and what we have is a dope and Ok and a host in.

[00:10:43]
What happens when we dope is that the dope and transfers an electron to the host the dope becomes oxidized the host becomes reduced Ok Or we can pee dope and in that case what we have is a material where the dope and becomes reduced and the host becomes oxidized and so this would be a whole carrier of this would be an electron carrier now the requirements for these dope ins are that they can have the we would think but it's not exactly true that they can thermodynamically except an electron from the host if you have been or donate an electron to the host for an doping I'll show you a system where there's an exception to that which may be interesting but fundamentally if we want the dope and to transfer an electron to the host we can basically ask for a system where the ionisation energy of the dope and is less then the electron affinity of the host Ok and converse Lee if we want to Pete Opus system we want the electron affinity of the doping to be greater than the ionization energy of the host.

[00:11:56]
Now as I said before it leads to all these different properties which I mentioned and here is a little. Of molecules that have been n. and p. doped Ok and probably for many of you this just looks like chicken wire but we basically can look at these molecules and think about how we designed them and how their detailed chemical structure will actually pertain to their color their emissivity their electron affinity their ionization potential whether they form Crystal materials whether they form glasses Ok And this is the the hype or or the sales pitch that we often give is that these materials are extremely tunable at the molecular level in such a way that from their molecular constituent properties we can have a guess at many of their bowl properties.

[00:12:55]
So so what if you are going to be a chemist working on doping would you do how would you contribute to this field that has been largely dominated by physicists electrical engineers Ok well what we can do is expand the palette of dope and and make things of They'll built to people that have some unique sets of properties that they can use and so what do you want as a do well what we want is complete and clean electron transfer from the dope into the host or vice versa we want therefore the ion ization energy of the host to be as low as possible if we want to dope something that has a very low electron affinity Ok or we want the electron affinity of the doping to be as high as possible if we want to take an electron from something that's very hard to oxidize.

[00:13:50]
We don't want the dope and themselves to the molecules to have side reactions we don't want the ions that we create once we do to do side reactions. We may want to if we have a multilayered device be able to spatially localize these materials. In the layer that we want.

[00:14:10]
We may want these things not to act as traps although you can imagine situations where you want to control the number of traps and we'd like to be able to deposit them both from solution and from vacuum and ideally we'd like to handle these materials and air Ok so for peed opens things that are oxidants in general except for the most oxidizing of materials that's relatively straightforward some materials will actually interact with the water in air and become reduced and that could be problematic for endo Pince it turns out that that's a much more challenging situation because if you look at the materials that are you want to dope Typically it will require something to dope them with a sufficiently low ionization energy that oxygen can oxidize the dope and Ok and we set out for ourselves the challenge of identifying dope ins that would be relatively stable in air and yet be able to stay to dope things that are very very difficult to dope i.e. have very low electron affinities there so just mention that there are a whole slew of different kinds of Pete opens that we've worked with over the years some of which were developed in our group others in other groups Ok And these are materials that have relatively high electron affinities electron affinities on the order of in the case of some of these around 5.4 k. electron volts McCain.

[00:15:46]
For endo being a lot of work has been done with alkali metals that's not surprising they're very highly reducing they can be evaporated. They are often mobile ions after they transfer the electrons which can be problematic Lee They can also act electrostatically as traps and they're going to be pretty air sensitive.

[00:16:11]
So let me give you an idea about that Ok so here we consider reaction and I'm going to be focusing today on end doping Ok And the reason I'm going to be focusing on and doping is that developing air stable and dope and opens is much more challenging than Pete opens Ok so here what we have is a donor that is the end open and accept or that is the host Ok and we do an electron transfer reaction so if you look at the kind of materials that we might examine which simply give up an electron in much the way that sodium gives up an electron to become a sodium ion there's a molecule called cobalt the scene it's a well known molecule it's been synthesize probably are around when I was born if not earlier Ok And it has around the metal 19 electrons what that means is that there's one electron that's in a very high lying anti bonding orbital and the molecule wants to do whatever it can to get rid of it and so it will donate it to something else.

[00:17:13]
So if you look at the oxidation potential or the couple for cobalt the scene and it's cat high and call cobalt the Sydney m it occurs at about negative $1.00 volts 1st is fairest scene Ok now that's fine to dope something which would be an except or in a photovoltaic system and maybe a transistor Ok but it would not even come close to touching something that you would want to go for an organic light emitting diode and then we can go and make compounds that are even more reducing we can decorate the the rings with metal groups we can make compost like this compounds like this and now this guy is already quite sensitive and these guys you can even touch them in here Ok.

[00:18:03]
And this is where we need to be for all it's so then the question is how do we get around that. And the idea that we had and it's shown schematically here but I'll show you examples on for the slides is can we have a system where we have something that is very reactive and wants to give an electron which itself has an odd number of electrons and then when it gives an electron it makes it stable capital and can we have that odd number electron masked when we use it as a bond so to let trans on 2 of these things come together to make a dime or and so we have a situation where instead of having one thing that's highly reactive with this electron it wants to give up we form a bond but we don't want that bond to be so stable that it loses all the reducing force Ok And so then the idea is that we have that dime or a Korea act with 2 of these except as to give 2 cabins to enter it's Ok And so what we're doing is we're kinetically stabilizing the dope and at the cost of also thermal nomic Lee weakening it a bit but we might be able to make it a sensitive.

[00:19:20]
And so our initial idea was to basically have a system where we could take the diameter and using heat or a light we can just cleave the die or into 2 things just home a little break the bond and then generate the reactive species and then it will go so one manifestation is shown here here's a radical which doesn't exist for any appreciable time Ok and this radical can go away and leave a positive charge and you'll see this later Ok but if 2 of these come together we can form a bond Ok And the idea was to do that and then be able to somehow Cleave that bond and access the dope it's.

[00:20:01]
Well my background as I mentioned earlier was in or Ghana chemistry and in particular in organic chemistry that. Involved the combination of inner Gammick and organically Guinness which is known as organic metallic chemistry and what we knew is that there were molecules like Cobalt to see which I showed you a couple of slides earlier that have $1000.00 electrons but they will actually react with each other in the manner that I had just mentioned to make divers and that one classic example of that is a diver that's based on a Rhodium and there rhodium is the element that's just under cobalt in the periodic table and what was also known as that you could heat this material Ok the diameter of this material and on a cold finger at $77.00 degrees Kelvin you can basically isolate the 1000 electron compound which is highly reducing But if you want it up it will dime rise again Ok so there are a series of these things some based on the elements of Rhodium and radium some based on iron in Ruthin iyam and there are even some that are based on osmium Ok and then we discovered something actually by reading the literature on some of these compounds and that was which was not surprising that if you take a cad high end of a rhodium compound and you reduce it and then you look at in a electro chemical cell the reverse wave you can see there's more current here than there is there Ok Which means that when we reduce this to the 19 electron compound it's actually going away before we can re oxidize it.

[00:21:51]
And then when we look back we can see there's a blip over here and that blip over here is due to the diameter of that compound Ok Now the other thing which we noted and we had not really considered is that if you start with the dimer and now you go and you oxidize it Ok And this is just a standard internal stand it when you come back you don't see the reverse wave of the cat ion of the dime or what happens is once you take one electron out of this thing it falls apart and you get one equivalent of a cat eye and one equivalent of a very very reducing material that itself Ok is very quickly oxidized Now you might get some insight into this by looking at the crystal structures which we have and what I'll point out is that the bar in the lanes the single Bon lanes in these molecules are very very long for normal organic molecules in normal organic molecule have a bond lane of about.

[00:22:56]
$1.00 angstroms Ok And these are almost a 10th of an extra longer and that may be associated with the fact that the bond is weaker The other thing that you can do is look at the molecular orbitals Ok and the highest occupied molecular orbital has character in that single bond in most molecules that bond is the single bonds or who much much lower in energy for the filled orbitals then the double bonds or the metal orbitals Ok so that's also unusual.

[00:23:31]
So what that showed us is that potentially these class of diamonds can really act by 2 different mechanisms one is the one that we 1st thought of where the Dharma Cleaves and then gives 2 very reactive materials that transfer electrons the 2nd one is where the dime are Ok as was the case in the electro chemical experiment.

[00:23:56]
And in a process that may be uphill transfers one electron to a host and then you make the Catalan radical of that dire and then as I said before that immediately falls apart to give your cat I'm in an Am I in and now you have that highly reactive auto electron species which immediately finds another except or and then you get 2 of these cat Aryans into an audience so.

[00:24:27]
What I will tell you is that our initial intent was to do this and the vast majority of the things that we have ended up making go by the 2nd mechanism and I will not bore you with about 2 men years of work at least of how we went and figured this out but I can tell you that the kinetics of those 2 mechanisms are very different Ok they have different rate laws.

[00:24:55]
One will involve things where they can come apart and come back together and we can do experiments with ice topic labeling to look for that we can see whether the 1st step has a dependence on the Redux potential difference between the donor and the accept or which is indicative of the 2nd mechanism and we can look at things like the entropy of activation and with all of that we figured out how these worked and then that gave us insight and how to go and design new materials.

[00:25:27]
So that's basically it for the chemistry more or less so now we're going to use just a little show and tell about some of the applications this is a small list these things are now being used by a 2030 groups around the world to do various different things and I'll just walk you through a few the idea being to give you an idea of what could be done so that it may stimulate you to think about occasions.

[00:25:53]
So. If you have a material that is made up over again ics because the materials are amorphous and in homogeneous Ok the Lumos the unoccupied states are not all at one level Ok they're all in slightly different environments and that leads them occurring at slightly different levels there Kate and so what you have is a distribution of energies to reduce them and it may be relatively narrow but then you could have outliers Ok And those can either be too impurities in the system they can be 2 very odd configurations of the molecule in the system and in those cases for example these empty orbitals may be much lower so if you're an electron that's sort of hopping along from here to here to here to here and then you get down there to get back up into this manifold is going to require energy it's going to slow down the electron but what if we could fill these low lying States with electrons then we would only have to have the electron go from here to here to here to here and the mobility would be higher.

[00:27:06]
So you can do that and the way we've done that is by having one of our dope ins that's a strong reducing agent and by just putting in tiny tiny amounts of it such that there's only enough of the dope and in the system to fill the trap States but not enough to go and start filling this actual manifold of transport States Ok And when you do that.

[00:27:35]
The example that this was done on was c 60 and the diameter was one of these diaries based on everything me I'm cat on what you can do is if you heavily dope you can just reduce and make the Ruthin e m an ion Ok. But if you very lightly dope then you'll just fill the trap States and in fact what you see is that when you look at the work function of the material in the homo position you'll see that there are 2 regions for the work function and the homo position one which has a steep slope of one which has a narrow slope so what's happening is that in both cases there Kate what's going on is that in this low doping region we're filling the traps States there are very few of those traps States so a very small number of electrons is going to lead to a large change in work function but then when we actually begin to fill the manifold of the states that are where we're really reducing the molecule Ok then it takes many more electrons to lead to a change in the work function Ok and that's what we see here so one of the things that you can do is ask yourself what kind of effect this that have on Gammick field effect transistor and one effect that it has on is in the threshold energy that is required to turn on and off a transistor and this is work that was done in collaboration with Anton Khan and Bernard Kipling.

[00:29:06]
So you can buy a c 60 from Aldrich and it comes as 99.9 percent pure. And if you make a transistor out of it it has a turn on voltage of about 18 volts Ok Now if you go and you purify it 5 times by taking the pure c 6 that you get and sublime it 5 times that drops down to about 6 volts if you now take that material and add 10 to the minus 3 more percent of dope of dope and Ok what happens is the threshold voltage drops down to 6 volts which is equivalent without any substantial change in the on off ratio so what we're doing is before we even turn on the the bias to get this thing to work as a transistor we're filling the trap States and then we would bias it then we just immediately start dumping electrons into the transport levels and so in this manner we can take a material that was a little bit crappy clean it up electronically and have it behave better Ok Now there are a couple of things that I want to mention to you this paper was published in physics reflect a few years ago and our collaborator in Princeton Antwan Khan who's an electrical engineer called this ultra low doping now for those of you who work in semiconductor world Ok putting 10 to the minus 3 mo persisted you would consider either own high duping or you would even call it an alloy Ok but in the organic world you need to understand that when we don't things we typically put in one to 10 percent Ok so this is a cultural difference in what ultra low doping means the 2nd thing is you might say to me as I have to many people.

[00:31:02]
Organic electronics is based on pure materials why would you take an impure material and then add yet another thing to it to get it to sort of behave like this when you can just go and sublime it Ok I mean silicon supplanted much of times why you just sublime and get really pure and the answer is that an awful lot of the organic materials that are used Ok are polymers and you can't purify them that way Ok so you have to take what you're given and try to make the best of them and this is a mechanism by which you may be able to take a material that doesn't have great electronic properties improve them another thing that you can do is impact the levels that are required for injecting or collecting charges Ok so if you don't have your firmly level aligned with the homo to inject the whole you'll have a substantial barrier to inject the whole but if you Pete dope the material what you can do for the electrode itself as well what you can do is is shift the Fermi level relative to the homo and then lower that barrier.

[00:32:15]
And so and that can make injection a lot easier so this is another collaborative effort between Bernard's group and our lives Ok where what was examined was c 60 electrodes and we had sausage drain electrodes and one case we had the sausage drain electrode on top of the c 60 and then on the other case we put in 3 down on meters of so there is something nano here 3 nanometers you'll see that a few times of a dope and in here and what happened was the measured ofat mobility which is not an intrinsic material property but it's a device characteristic from which people extract material properties.

[00:32:59]
Basically triple Ok Why is that because here the current was not limited only by the resistance of the 60 but it was limited by the contact resistance at the electrode and for many of you who might be thinking about working with graphene and other 2 d. materials I have been told by the people who have worked on them that dealing with the issue of contact resistance is one of the challenges that you have to make good materials so here what happens is the contact resistance goes down by a factor of 2 and a half Ok and therefore the measured mobility gets closer to something that is representative of the intrinsic mobility.

[00:33:41]
Here's another example that we did in collaboration with Mark her Sims who is at Northwestern Mark has done a lot of work on making electronics out of a raise of carbon nanotubes. And typically when you get carbon nanotubes you end up making peach channel transistors and we had the idea and we talked to Mark and a meeting and he said yes let's try it that if we just put a very very very light amount of dope and on there such that we fill all the traps states in the gap and we move the Fermi level near the conduction band rather than right near the bell and span could we convert these things from being peach channel transistors to entangle transistors and he tried different conditions and you'll see here curves in purple Ok green and blue and this is the on off ratio in the bottom line is that.

[00:34:39]
You can get good on off ratios for n. channel transistors Ok reasonable turn on voltages and here mobility is that are between 2 and 3 centimeters squared to both 2nd which for Again it materials for an internal materials not bad then a colleague on assumes all has looked at another one of our di Merrick dope ins this case based on an organic material with somewhat different architecture and slightly different nano tubes and what she's been able to do is again take peach channel transistors and dope them and now what they're seeing are mobility is on the order of 10 to 15 centimeters squared provoked 2nd so what we've done is we've taken of perfectly good enchanting which hero and we've used a trace just a trace of doping to change its characteristics getting a little closer to home we collaborated with Eric Vogel and examine the doping of Molly dye sulfide Molly dye selenite tungsten dye sulfide tungsten by selenite and you'll see 2 slides on these materials or at least one of them.

[00:35:45]
Ok And so with the End opens we can basically dope the conduction band with one of these dope ins that we have we can actually in the case of punks and they sell the 9 dope the balance band and when you do that you can dramatically shift the the transistor characteristics of the dope and of the of the transistor Ok And you can tune that Ok now here's another example that was done by way Chen and his colleagues with our dope ins in Singapore and now instead of we did this in the case with Erik with solution processing they vacuum deposited dope ins on by layers of these by childhood unites Ok And here is the case with tungsten Ok And again when they when they do they they make these things pretty heavily traveled dope the conduct Dinse of the material goes up by a 5 orders of magnitude and the interesting thing is that when you're exposed this to either oxygen or air even after you know 10 hours basically you don't lose very much of the conductance.

[00:36:53]
Another example which is of maybe less interest to this audience is that of endo being a polymer So there are many Pito polymers and the Peto polymer is as I have noted can give rise to conduct of it is retained quite as I routinely Ok as high as a 1000 seaman's percent a meter in that in some recent cases as high as 10 to the 5 seamers percent for endo materials if you get 10 to the minus 3 seaman's percent to meter.

[00:37:25]
You're not surprised you can see that Ok if you get 10 to the minus one you're feeling good if you get one seamer percent a meteor feeling very good Ok And so now with these dope ins what we can do is don't materials and get up to about 8 percent to meter and also very high or reasonably for endo polymers power factors for thermal electrics what I hope to be able to talk about within a few months is a paper that we're working on now with this group in China Diane pe who developed these polymers where they're now seeing end channel conduct of idiots on the order of 90 Siemens percent to meter and I'll just tell you that to our knowledge for a solution process material 8 Siemens for said to meter is unprecedented and 90 percent to meter people might have raised their eyebrows as to whether you could do.

[00:38:16]
Another example of this is the use of these dope ns to make electronic structured layers in vacuum deposited solar cells and this is work done in collaboration with him bowling at the University of Illinois so they have a system where they make they basically deposit I to you then they don't very thin layer of c 60 with our or thin e m compound to make an only contact then they have a little buffer layer of c 60 then they have the process then they have a whole transport material a dope whole transport material to make eye contact Ok with the whole conducting a layer and what I can say is that in the absence of the dope and you get these odd shaped curves we're calling them s. shaped curves in the community and that is indicative of a barrier to get the electrons out of the device you put in just a tiny tiny bit of well actually it's about 6 weight percent of this into this material.

[00:39:16]
And you see here the curves which are classic curves for a piece of system Ok so we've basically killed the charge barrier. For collection of the electrons and one of the things that's also important in the in the world of para is something called History cysts and that is when you look at the scan going up in potential and then you look at the going down of potential there are different Ok And here what you are looking at is the scan going up and down in potential Ok here you can see the history says in the blue curve one that goes up one that comes down Ok Actually it's probably one that goes up and the other one comes down here they're super imposable which means that the issue of history says it's not something to worry about.

[00:40:06]
You can use these dope to modify the work function of materials so let's assume you have something like indium 10 oxide that has a work function and then we react with something that is a strong and dope and at the interface what we can do is dump electrons into the i.t.o.e. when we do that we are the material and then what we end up putting is Catalans right on the surface Ok And what that does is it creates something that's called an interface dipole that shifts the vacuum level Ok and that's a consequence Ok the work function goes down so let's see what you can do at this in will show a few examples Ok so here a bunch of different materials you dirty gold dirty silver p. Doc p.s.s. which is an organic material clean gold Molly oxide and here all their work functions in the absence of doping and then we put down literally a banana metre or so of dope and on top by and large I'm on a layer.

[00:41:14]
And now what you can see is that all these work functions get below 3 so we've taken materials that would normally be good anodes and we've turned them into materials Ok and then you expose them to air Kate and you can see that there is some loss of that decrease of work function but it stays pretty constant Ok And the point being that we can make low work function materials that are by and large air stable without having to use things like calcium a very extreme example of that is shown here.

[00:41:49]
Ok this is actually a single crystal zinc oxide that was grown in u.h.c. And when you expose this this one to on the order of 5 and a half anatomy of this roof in ium is that link compound the work function drops from $3.00 to $2.00 So as a reference calcium has a work function of $2.00 So now what we've done again is we've made it extremely low work functional electrode Now there's another interesting thing here and I won't go into all the details Ok but what these folks were interested in doing was making a hybrid light emitting diode device where they created x. Aton in this ink oxide and then we're going to transfer that x. of time to this material which is highly highly a missive.

[00:42:44]
And they designed this material such that it's homo gap was just slightly smaller then the bandgap of the zinc oxide However instead of getting a type one interface they got a type 2 it to face and as a consequence when they did the energy transfer to this material to create a hole here an electron there they get a back electron transfer to the conduction band of the zinc oxide which quinces the luminescence when they put down a mano layer of this Ruthin iyam compound in between what it does is it creates just a touch of band bending and it shifts those bands up such that now the conduction band is well lined and the back electron transfer doesn't occur and what you can see here in this inset is here is the photo luminescence in the absence of the dope and and here is the photo luminescence with one monolayer of doping.

[00:43:43]
Ok now you could do the same thing with other materials this is an example with graphene we can pee dope graphene Ok and we can end up graphene when we end up grapheme we begin to fill the conduction band when we pee dope we get to deplete the conduction the balance bend and we can modify the work function of graphene by I show here 1.8 volts but we can now modify it by well over 2 volts and we can convert graphene into a very good cathode for injecting electrons into materials for example for again a cloud emitting diodes.

[00:44:18]
Ok a couple of more things I want to talk about and this is a paper that was published it's getting close to 2 years ago in nature materials about dumping doping materials for all it's Ok so when you want to do materials for Oleg's Ok it turns out that they have very very low electron affinities and as a consequence it's very hard to dope them Ok And so here is a material that's used in all it's called p o p y 2 it's reduction potential versus fairest seen as negative 2 volts Ok and it's electron affinity is also 2.2.

[00:45:02]
Ok now it turns out that these dog dimer is if we broke this pond and made a radical which we can't just cleanly Ok would be plenty strong to dope this if we have this to transfer the electron here is very much a pill but we can calculate through calculations and the experimental parameters that we have.

[00:45:28]
What the effective reducing strain of this diamond would be which would be this reducing strain minus one half of the energy required to break that bond and the long story short is that we shouldn't be able to do this it's uphill per electron point 2 volts and for the overall reaction of one of this plus 2 of these a pill by point 4 electron volts Ok so we should be able to do that but we tried nonetheless and the fact that we couldn't do it thermally because it was so uphill here Ok we got around doing a technique that's called Photo induced electron transfer in that technique what we do is we take this molecule we photo excited and what that does is it puts one electron from the homo into the Lumo in a very simple picture and now this can dump an electron into the half filled homo and now we have it doped is soon as we now have it doped we make that cat ion it falls apart Ok to give one cat I and one and I and and then now we do have transiently this very reduced in material which dopes and other system Ok now.

[00:46:44]
Here is a movie visit showing this. Visible spectrum in this peek here is that growing in we know it Ok trust me now here's the interesting thing I told you that that reaction was uphill by 4 tenths of a bolt so we turn on the light and it goes from a conduct to video of 10 to the minus 9 or 10 to minus 10 seaman's percent to mean up to a few by 10 to minus 3 that's not that surprising with the light on what was surprising is that when we turn the light off Ok it stays for 100 hours Ok and fact we've now looked at this over 9600 hours Ok and we only lose about 2 orders of magnitude of the 5 or 6 in conduct to Vittie after a year so one of the things I joke with people about is that again a chemist look at photo induced electron transfer and systems and photovoltaic systems and synthetic systems for the looking at protein.

[00:47:44]
That do photosynthesis and if they get charge separated states through photo induced electron transfer that last micro seconds they considered that a long time so I did a back the envelope calculation and it turns out that these things last 10 to 13 microseconds so from the spam point of Long Live Photo induced electron transfer these are pretty long lived now I will not get into a lot of the details but what's interesting is that when you don't these materials Ok.

[00:48:15]
The the ability to inject charge in from the electrode becomes much much easier Ok And these things turn on very well and they give very high quality. That's Ok. There's another story here but I don't have time to talk about it and that is that we can turn these things on electrically and not only using light but that's another story this slide I already showed finishing up why.

[00:48:43]
We want to do for the community is give people dope and that have a variety of different doping strengths Ok so that maybe we could do everything or maybe we can only feel trapped levels that we want to give them different shapes who k. so that if they want something that's really large and doesn't move around we have that we also want to be able to control the kinetics there may be cases where you want to mix things together make a film and then and only then have them dope.

[00:49:12]
And so right now our group has made these things and we have a whole slew of them that are available for people to use people think of these things as esoteric we can make these things on you know many grams scale and I would just point out that when you're putting down mana layers or doping at a few percent Ok when we can make them 100 gram scale in my lab that needs that they're basically infinitely available So finally what I want to show you is that by understanding how to do chemistry we can make compounds that are very clean to Opens that are extremely strong they can dope things that with electron affinities as low as $2.00 e d And there are stable you can handle them an area like this pizza I would like this pizza I'll handle it in there Ok but I would not store it for months in air.

[00:50:10]
But you can you can handle it you can transfer it. That we can tune their properties we consume the size the shape of the dope and and compared to other approaches to having a stable. They're more reducing they don't have side reactions and so one of the holy grails for us was to be able to adopt these materials for old LEDs and we've shown that that works and what we didn't expect was that they would stay dope for a period of.

[00:50:39]
Hundreds of hours if not years Ok so what I hope to show you today is that there are a variety of things that you can do and that the materials that you can play with are easy to play with Ok they're not going to go and up in flames when you deal with them and that you can modify the into facial properties for doing a lot of different things and you can modify conduct So with that I will stop and thank you for listening and if we have a minute for questions I can take questions or not people can talk to me later.

[00:51:20]
Yeah. Yeah. So. What happens you do too much as you don't have a transistor you start filling the you start putting enough carriers in that you don't have it go off and just becomes conductive So if you just put a few carriers in it remains as a semiconductor if you put a ton of carriers we can we can Could we can heavily dope c 60 and get conduct to Vittie is up to 10 Siemens percent a meter Ok but that that would be fine if we wanted to be for example and into a layer.

[00:52:11]
For extracting charge but if you want to make it into a transistor you just want to have x. just enough carriers to fill the trap levels. So it so this is done by vacuum deposition Ok you can do it you can do solution thing so so for example we've worked with folks Ted sergeant in arm a mosque in for and we're doing this now and it's where you just put tiny tiny tiny amounts of dope in on in the case of.

[00:52:48]
A sergeant in a Masi in. Quantum dots and you can pass of a truck States Ok but if you start throwing tons of dope in there then you start really heavily doping the quantum down so what you get what you need to do is basically. Do some of Perkel work to find out how what concentration of doping or how much doping you need and then if you have material of the same quality Ok then you can go and you do that and for you know for Antwan cause and they can control that extremely precisely in their vacuum deposition chamber.

[00:53:36]
Thank you.