good morning and thank you for the introduction and for the invitation to speak here and my group works on nanoparticles which actually may contribute I think to the field of solid-state lighting to which professor Nakamura actually initiated you see here a gallery of the type of systems that we can make using colloidal chemistry we have a control of size for example indium arsenide dots tightly tuned and homogeneous achieved by a wet chemical synthesis shape control as can be seen for achieving nanorod shapes with high crystallinity such as the cadmium selenide nanorods and also the ability to increase the functionality of the materials by combining disparate materials on the same nanoparticle such as these gold tipped semiconductor nanorods the first obvious property which is size dependent in these systems and the one which is actually already yielding several applications is the size tunable emission well known by now so you can see these kind of emission spectra from solutions of cadmium selenide based nano crystals starting from red particles at about 6 nanometer in size with three thousand atoms and because of quantum confinement going down to two nanometer in size with about two hundred atoms we have emission at in the blue this property of emission is perhaps the first area in which we see already emerging applications one of them in the area of biological tagging so far mainly used in labs around the world for research checking now potential for employing such materials as in vivo tags and so on exploiting first of all the advantageous photo physical properties and secondly the chemical tunability that allows us to have a lot of flexibility in where we place these materials and those properties are also relevant for their incorporation and various future technologies and lighting and lasers displays and so on one of the key elements for actually improving the optical properties of these materials and tuning them aside from the size and shape is actually the ability to do hetero structuring and I would say that since doping of these materials has not been yet realized fully because of the small size and so on there are central challenges in doping of the main ways actually to tune the properties is through head restructuring so the most common structure is a type 1 structure where we can grow a shell over the core of these small particles a higher bandgap shell which will confine the electron and hole to the central region as as clearly one of the main problems to achieve or to overcome in actually improving the emission properties is related to issues of trapping at the surface so this is the type 1 structure but there is also the opportunity to realize other band offsets by combining different semiconductors in a course shell configuration again this is also realized by the wet chemical synthesis approach and where we have in this example of electron hole separation leading to a longer lifetime in principle reduced quantum yield but that can actually be addressed by a further shell on the surface so it all depends on how good we can passivate the system to avoid non radiative decay today’s talk will focus on interesting systems that we are working on what we call core shell see the draw hetero structures these are systems which combine actually a dot and a rod architecture and they do have very interesting and advantageous optical properties even over spherical particles we have the ability to achieve tunable polarized emission because of this geometry and with high quantum yields and you can see different combinations that I will talk about cadmium selenide cadmium sulfide tuning the length as well also the type 2 system which is zinc selenide on and cadmium sulfide as a shell this docking rod system is interesting also from the viewpoint of combining a zero dimensional quantum dot with eventually a one-dimensional or quasi 1-dimensional rod system I’d like to cover several issues with this including a way to actually extract banned offsets which are as I mentioned before critical for determining the properties of such materials and one of the things that is difficult to measure in such a small system and we use STM and SDS and actually to do that I’ll talk about multi exit ins in these systems about how we can image the exit on directly and located at one brief mentioning about the outlook of possible applications in the topic of the prize today solid-state lighting so this is the first example these are cadmium selenide cat sulfide so what we do here is we start from these hmmm selenide seeds and with the wet

chemical approach with a rather simple reaction you see the absorption of these seeds and their emission you can simply grow the cadmium sulphide shell on top and achieve quite homogeneous nanorods with the seed inside as you see we have the ability to achieve very high quantum yield because of the good passivation there is a slight red shift in the emission and you can see here the absorption tail related to the cadmium selenide States and then strong absorbance below about 500 nanometers related to the cadmium sulphide rod-shaped gel what we wanted to do when we saw this early reaction which by the way was started by tilapia and valor and so on and we followed up on their work was to actually see if we can expand this to other materials and one of the important issues in the shape control of these systems is the fact that they are hexagonal word sight structure and that is the reason the intrinsic asymmetry in such a crystal is the reason for which you can actually achieve shape control so here we actually started with the second selenide which is a semiconductor emitting in the blue but has a cubic crystal structure and the question was is whether or not we will be able to grow a similar seeded rod system and the answer is yes under the suitable conditions it is possible to grow nanorods you do see that they’re not as homogeneous as the type 1 system because of the structural somehow incompatibility between the word site and cubic lattices the cubic lattice of the seed and the word site lattice of the shell nonetheless what you see is that already 40 seconds after the reaction what we see in terms of the emission is already significantly red shifted here in black compared to both the bandgap of zinc selenide and the bandgap of cadmium sulfide namely we already have the type 2 transition in what we see here and eventually at the end of this reaction which only takes about 10 minutes or less than 10 minutes we have again the feature of red emission it is broader as compared to the type 1 system also the the absorbance has lost some of its shape because of this type 2 character but clearly again at 500 nanometers we see the abrupt onset of absorbance of this shell of the cadmium sulphide shell so you see that we have already the ability to tune the hetero structures and the first manifestation I would say of the type 1 versus type 2 characteristics is clearly seen in the lifetime of the emission which decays for a 1 over e a lifetime of a little over 10 nanoseconds for the type 1 system while the trike 2 system has a tenfold increase to 100 nano second decay of the emission because of this reduced overlap between the electron and hole wave functions in the excitonic transition of the emission so the first question that we wanted to address here is actually to try to determine band offsets and optical spectroscopy cannot really give this information because all you see is just transitions between conduction and valence band States so in that sense we have employed scanning tunneling spectroscopy on this unique structure in order to try to directly extract band offset which actually through the level offsets that we will measure and again band opposites in these materials are an important parameter that is really not very easily accessible and what we typically do is simply use parameters from other studies from quantum wells and so on as inputs to what type of band offsets we may expect so this direct approach is actually an approach using tunneling spectroscopy this is work done by my colleague professor who doesn’t belong at the physics Institute in Jerusalem and we’ve been collaborating for over a decade on applying STM and SDS combined with optical spectroscopy to study the electronic structure of such materials so the nice thing that we can do here this is the nanorod placed on a conducting substrate and then taking IV curves at different locations are all along this structure and then looking at the d ID v which is proportional to the density of states so what you see here positions 1 & 2 at the one end and the other extreme of this structure show the in the VI DB clearly a bandgap which is rather broad with conduction band and valence band states appearing in both cases but when we sit on position three we clearly see a narrow gap and we see that moreover that the narrowing is more or less than take a little bit larger in the conduction band side so basically what this means is that we can extract directly at least level offsets by this spatial resolv’d SDS study moreover we can clearly identify the electron wave function if you like we can image it by conducting current imaging tunneling spectroscopy in which the height of the of the tip is fixed and we scan the the and measure of the current at the permanent voltage of 1.2 volts over here which is resonant to the onset of the what we call the cadmium selenide absorption because that would indicate

the position of the core and you see here that indeed this current image manifests or reflects the position of the electron wave function now to actually move from this level offset measurement to band offsets there has to be another step which is actually the requirement of a model a theoretical model for the levels which has its input as the which the pin for which the band offsets are actually an input or the unknowns this is work in collaboration with Fabio della Scala from Italy and it’s an effective mass based model where again these are the unknowns and you see that it is possible with proper tuning of the band offsets namely point 3 volts for the conduction band and point for 4 volts for the valence band to actually reproduce the level structure that is also seen in the the idv spectrum this is the theoretical calculated level structure or spectrum which conforms well with a measured one and also the localization of the electron wave function is seen clearly because of this type 1 band offset so I would say that this issue of type 1 or type 2 was and still is to some extent an ongoing debate namely earlier work by Muller Adal very elegant work using optics using lifetime measurements along with a model actually extracted for a similar structure of somewhat different dimensions 0 volt band offset in the conduction band however with our study we clearly see that we have here a small but actually a type one band offset in this system still we still wanted to revisit this issue in light of this difference and to that end we have applied a study and went into a study of multi exit ons in this system so to discuss multi exit ons we go back to the optical properties and here we see a kind of summary of the important processes in quantum dots excited optically you can form an exit on which will decay very quickly to the conduction and valence band states on the time scale of a picosecond or so and then it’s possible also to create track certain states by populating further exit tones in these levels this is actually a critical point because it in principle is the position of population inversion threshold in a quantum dot the difference between the exit on and by exit on state now it turns out that this violet on state can decay actually radiatively with the timescale that would be not so different than the radiative decay rate of the exit on so about 10 nanoseconds however it was found that there are much faster decay processes related to OJ recombination as you see here in this cartoon whereby the annihilation of one of these exit tones provides energy and excites the electron here for example so it’s a 3-body collision and by this og relaxation pathway the lifetimes are limited to about 10 to 100 picoseconds so this is what you have to compete with if you want to actually think about lazing in these kind of systems and indeed multi exit cons are very relevant for lazing and this is data from older work from 2002 in which we demonstrated lazing optically pumped from nanorods in a cylindrical micro cavity as you see here we have Whispering Gallery modes of this lazing mode and we see clearly the threshold blazing behavior of the Nano roads the main point here were that we actually achieved a significantly lower threshold and spherical dots attributed partially also to the reduction in the og rates which I’ve mentioned earlier it’s also possible to achieve polarized lazing because of this rod architecture which provides followed polarized emission so now we go into looking at the multi exit ons in the seated nanorods and with a methodology that we employ a methodology that was initiated by dr. Don Juan was a postdoc with me several years ago and now a very successful faculty member at Weitzman was a methodology which is basically a state filling methodology what we call quasi CW multi exit on spectroscopy so it’s employing nanosecond pulses which means that at five nanoseconds we are significantly longer than the OG relaxation time so over this five nanosecond pulse time we can transiently populate our system gradually by increasing the intensity of the pump pulse through the exit down state and then if we increase it further by exit on Fri exit on and so on and therefore this quasi CW experiment is analogous to a state filling approach in which we can then map the emission of the exit on bikes it contracts it on respectively so these are the results for looking at the emission of these multi exit on States

for the Taekwon see that nanorod so at lower the excitation powers you see that we mostly have just the exit on state and when we increase the power we have the emergence of this blue peak which is the bi exit on peak and note that in this case it’s actually red shifted compared to the excitonic transition namely it’s a binding by exit on interaction in this type 1 system and even higher powers we can observe a third peak here appearing which is the tri-axial tonic transition here shifted to the blue already because the degeneracy of this first state is twofold it’s a nest-like state and we also see the appearance of a state which is assigned to emission from the academy of sulfide states directly the power dependence of these transitions is basically conformal with exit on invites atomic transitions so now if we look at the type 2 system again you see the exit onic transition clearly then when we increase the intensity the blue peak which is the bi exit on in this case is actually repulsive it’s blue shifted compared to the X Nanak transition another difference from the type 1 system is that we hardly see evidence for a tracksuit on t we do see it eventually but it’s not appearing and not well resolved it’s a rather small and weak transitions which basically is consistent with the fact that in this type 2 system the whole will be localized in the seed and the electron is delocalized into the shell so the overlap is weak so that can bring me back to reiterate the differences in multi exit tonic transitions in type 1 and type 2 quantum dots we refer to a quantum dots but similar behavior appears in our see that rods in the type 1 system basically the electron and hole of the two pairs are forced to be in the same core region which leads to a dominance of the correlation binding interaction and a redshift or binding interaction of the by exit on in the type 2 case however we force the electrons and holes into separate regions in this example electrons in the shell holes in the core and therefore the repulsion the Coulomb repulsion is the dominant term which leads to a blue shift of the binding interaction so this is actually quite a sensitive measure of if you like of type 1 versus type 2 behavior in these systems I’ll mention in passing that we also saw in this work a different scaling law for the og lifetimes which has been found by others who scaled by volume for type 1 systems but actually scales by volume times the radiative lifetime of the exit on to the type 2 system which means that in a type 2 case it is actually elongated of possible relevance for this problem of achieving more efficient gain in in type 2 systems so now we go back to our seeded nanorods which are cadmium selenide cap sulfide in principle type 1 but now we we use a seed which is much smaller 2.2 nanometers instead of 4 nanometers in diameter and here for this system when we do the same experiment note that we see clearly that we see the exit on peak but then the by exit on peak is again blue-shifted which is a clear signature of type 2 behavior as i mentioned earlier in addition you see that the try exit on peak is hardly resolved in this in this case of a smaller seed so we have here a type 1 band offset but with type 2 behavior and looking into a size dependence of the core and how it affects these multi hexitonic characteristics you can see that there is a crossover here between the excitonic transition and the by excitonic transition which occurs at about 2.7 nanometers in diameter namely we go from a an interaction which is binding so this would be a type 1 case to a repulsive interaction which we call quasi type 2 because the electron state is now basically spilling out from the seed location into the shell because of the confinement in the smaller seed so the signature is clearly seen both in the transfer from a binding interaction to a repulsive interaction and also in the relative intensity of the Tri excitonic transition so this also was correlated with computational results again by fabio de la sala and also here you can see the effect of localization at larger seed sizes here this is the the position of the state would be inside the cadmium selenide but when we go to a smaller and smaller seed also in the calculation there is a deal of localization of the first state which basically exits core region and as a result we move from type 1 over here to a quasi type 2 behavior when we go to a smaller seed so the next challenge that we wanted to address after resolving the band offset issues is actually to try and apply a direct method to image the exit on in these systems and this is a big challenge because yes it’s possible to see single nano crystal emission this is for spherical dots of course this is a far field measurement and our not allowed on this spot looks like that so

we want to resolve the exit on within this mode so we have to revert obviously to high resolution microscopy such as Near Field approaches and there are two approaches here either Near Field through a an aperture or aperture less approach which is what we have taken namely we base our contrast mechanism here on the interaction of a scanning AFM tip with our nano crystal so we illuminate and collect in the far field but resolution will be determined by the interaction of our AFM tip we the nano crystal and the inspiration for the first type of interaction scheme that we want to take here is one of tip quenching microscopy is actually taken from this inspiring and the older work starting from the work of drug stage from the 60s looking at the effect of um of a metallic mirror as it approaches the surface with European complexes and the emission is eventually at the very short ranges is quenched because of energy transfer effects this was later modeled very nicely by chance proc and Silby so how can we use this quenching effect for a nano scale imaging is by actually employing a correlated measurement where we have our AFM scanning on top of the illuminated nanoparticle and basing the interaction of the AFM tip to serve as the quencher or is the interaction object with our nanoparticles so now the resolution is actually dictated by the sharp AFM tip and not by the far field resolution of the optical microscope so just to demonstrate how the method works this would be the topography image and in parallel to that as we scan the tip we can see the intensity image which is actually taking all the photons that we see and we are using tapping mode for the AFM we have to use tapping mode of the AFM because otherwise we drag our nanoparticle so obviously because of the fact that during the tapping we have different distances here you don’t see too much you see streaking which is related to blinking of the nanoparticle which is a known effect but we can’t really resolve the position of this nanoparticle if we are at position one what we do here is that we have the ability to actually measure and correlate the arrival of our photons with the position of the tip this is a the method that we developed and when we are far away the intensity actually doesn’t depend on distance as you would expect but when you are on the particle you’d clearly see this quenching effect and that allows us to actually slice an image taking only the photons which arrive in the close proximity to the surface and here you already start to see and resolve the quenching spots in the intensity image however a much powerful much more powerful germán and more much more improved is actually by looking at lifetime so again at position one we measure the lifetime and of course it’s going to be the same for the tip when it’s close or far from the surface but in position two we see the quenching effect by actually manifested by a shortening of this lifetime and this allows us again to create an image this is a lifetime or rate decay rate image in which we clearly see very nicely the position of our nanocrystal well resolved much more improved signal-to-noise compared to intensity and this is related both to theoretical consideration and also to practical considerations for the advantage of lifetime imaging over intensity imaging so now let’s apply this method to our seated nanorod case so you see here the topography again the lifetime you see is very localized this is a hundred nanometer see the Nano Road and we see very clearly 50 nanometer full width half max of the position of the emission from this seed of nano Road the intensity as I mentioned this is broader as was seen for the quantum dots as well so now we can actually merge the images and you can actually locate and kind of see where is the image emission emanating from and this would in principle be the seed location we wanted to apply this also to the quasi type 2 systems and the type 2 case which was more challenging because in those systems the emission lifetimes are much longer they are less stable somewhat and this is a demanding experiment because you’re sitting on a single particle for a few minutes and what we have done here is actually move into a radial polarization scheme which allows for a component of the polarization which is perpendicular to the surface and therefore can also yield not only quenching effects but potentially enhancement effects between the tip and the emitting chromophore and indeed with this approach you see here a type-2 system namely a quasi type 2 system with a small seed as I mentioned before and also here we see our nano load in topography but also the emission lifetime image clearly shows a localized emission which we assign even in this type 2 case most likely to the position of the seat or near the seed because that’s where

the exit on will actually emit from from the region of overlap between the electron and hole wave functions at the interface between the seed and the shell material so again we can create this merged image by this method so we’ve actually went through about 40 nano rods here we could actually locate the seed and we found the correlation a rough correlation it’s a noisy experiment here but we did find a correlation that for shorter particles you see that the sieve is at about 0.3 of the length from one end and as you go to longer nano roads about 120 140 nano meter in length you move to about point two of the length and we can actually correlate this with seat position measurements with an independent means these are experiments in which we grow gold on this system and we found that the gold with a thin shell actually localizes on the seed location as you see in this image in this dark field TEM image the reason is that we have localization of electrons or a sink of electrons here and those electrons are the ones contributing to reduction of gold on this side and from that experiment we also can locate the seed and we also see this trend of the seed being closer to one end and went for short for long rods while for short rods it actually goes farther from from that same end so this is a summary of these of my talk focusing on the seeded nanorods where we demonstrated that with them we can actually measure directly band offsets in colloidal nano crystals and the same numbers I think can be also used for spherical systems in principle because the chemistry is more or less similar between the seeded nanorods and the course shell systems the multi exit on study allowed us also to investigate type 1 2 quasi type 2 behavior depending on the the size of the seed in cabinets element cadmium sulfide seeds and we were able also to image directly the exit on using this Near Field aperture this approach of lifetime imaging last but not least really we are celebrating today the invention of what is enabling now solid-state lighting and it is a possibility and it’s actually being developed throughout the world and in several labs including my own and also commercially hopefully that nano crystals could find themselves as useful for the conversion layer required to transform the blue LED into the desirable color that we want in a white yellow and so on light taking an advantage of the tunable emission of the flexibility in chemistry and the tailored properties that I’ve introduced today and actually there is also a spinoff from assume our technology transfer company that is trying to contribute to this area of solid-state lighting with nano crystals so let me now thank my group and especially note the work of a meet seat who has done the multi accident study a yellow storage the imaging lab be men again a lot of the synthesis and I’ve indicated the collaborators or dead mellow and his group fabio de la sala and also I’d like to mention our ongoing ties with neil tesla and his group and these are the funding agencies and thank you for your attention