Manipulating Graphene at the Atomic Scale

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Manipulating Graphene at the Atomic Scale
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they sell but you just read my title so that attack about love and so
too to start orders show you this is my favorite graphing was simple the question is why we want to manipulate graphic scale why do we care How about things such as Adam strains and edges and so
this side isn't a sort of motivate them little but this is just a cartoon in and the idea that what we would like to eventually jointly put a lot of in this room would like to do some of us are very do it is to car crash to cut it up into little quantum mechanical Little structures that are designed at the quantum mechanical leveled the perform certain functions and we would like to be able to functional ice the grapheme Adams and molecules and control the edges and wired up With metals so that which can created integrated device on a single layer of graft that we can have better video games for children and so this is this is something that a lot of people are working Towards an outfit showed some nice progress in this direction but there's still a lot of work that we need to do in order to achieve this kind of of the dream I and part of that work involves a better understanding holographic behaves microscopic length skills at the atomic weighing scale in other words what happened around individual out what happens at edges and what happens when he straying a very small lakes scales and so those are the kinds of questions I would talk about today in this talk so here they are
impurities 3 and cunning little never ribbon-shaped that's what I'm focus on our but before I get to that out of policies introductory words talking about techniques that we're using to do this work out which is mainly scanning tunneling microscopy still give us a few introductory words about that technique and then I lost a little bit of also introductory words about the technique of STM spectroscopy which is a technique that we have found to be very useful for exploring these kinds of issues in graphic art so I so 1st such a start with discussing scanning telling my cast appears as a technique works for a small structures a scanning tunneling microscope is a sharp metal Neil which we bring very close to a surface we put a voltage between the surface of a measure the current and the rate at which these electrons come from the service is proportional to the look electronic local density of states of Of the structure beneath the tip break they feared that but and remember giving stuff and the this is a very useful quantity measure because ELECTRONIC local dentist dates gives you a
measure of the probability of finding electron of a certain point in space at a certain point in energy so he can map that quantity out you can learn a lot about electrons are doing in small structures but also by varying this voltage and measuring the differential conductivity of this asked him Junction you can measure what we refer to as the DID and by measuring that as a function of deep voltage between the tip in the sample we can measure the energy dependence of the local density of states beneath yes tips and that's what this little cartoon is meant to show and hear me was 0 Is the Fermi level and these are bill states and Easter they also the STM allows us to measured elastic expectations of small structures and the way we do that is if we put a bias on the tip that's big enough so that electrons only from the the ship the surface have enough energy to create an
inelastic excitation the neck gives those electrons an additional channel to go from the tip to the surface and elastic channel say by making a phone on for example and that they give us said John C. This will step right here that gives us a jump in the DA at at threshold energy for creating those expectations so this technique is been is a lot over the years is to study of graphing in in different
forms in fact people are using SEM to city grafting back in 19 92 loading color back then and it was not isolated Guimond Nova's love I have since isolated is this is grab people you use the study grapheme on metal surfaces a long time ago but more recently people have been studying grapheme with asked him on other substrates such as silicon carbide silicon oxide and more recently love born I tried not possible but above that this is down from the Georgia Tech group the Columbia group from my own group and there's a lot of groups around the world and using it look at graphing characters listed some of the people who were doing nice work but there are others to sorry name was not on the so in my own group were very interested in using asked him to perform local spectroscopy on on graft and so a few years ago we set out to perform as spectroscopy and gave
graphing devices such as the 1 that you see here there's a photograph of a little device we made with Alex held this as a graphic Flake and we can it from the back of a form of assume spectroscopy and what we expected this season with this experiment a few years ago was expected to see a veto in the local density of states in the DI because the local density of states of a mature often X P totalled states which is linear for graphing and so there's drop point and so that's expected to see but when we did experiment that's not what we saw in fact we
saw something that kind of different Suresh weighted down what we actually saw this as a plot of DID verses bias voltage on the surface this is the Fermi level and inferior filled states empty states and each of these curves is measured in different gate voltage there were becoming more and more she doped as we go up each additional curve and so that that's states is sort of shifting to the right as you go up those curves do gaining and end and here we saw some we start to features 1 was this gap like feature that the Fermi level and the others this asymmetric dip off 1 side now this this gap like feature is kinda the formula 4 different gate voltages soaks so so that houses that it's not as a band structure future graphic some kind of expectation what we believe is happening is that this is a sign of is not a true gap but it's a sign of any lasting phone on assisted in elastic Connolly and the idea is that when we when we get where where we come to a bias here about 65 bolts the electrons have enough the common electrons have enough energy to create a phone on another plane phonograph and so that creation that phone and hopes to get electrons and forget a big jump in the current elastic tumbling from here this little dip that featured moves with gate voltage so that tells us that a damn structure future and the fact that that is actually due to the Dirac point so you can see the Dirac point moving as you would expect as you material so this is so I just wanted show you this indirectly through this because it's served at the baseline as
Thames spectroscopy of graphic and this is what we see after doing some elaborate tip calibration procedures but what we really interested in is not what happens when you modify graphing What are the local properties of the grapheme when you change it when you manipulate its properties and and 1 of the ways of doing that is to drop Adams and molecules on the graph be in order but impurities in the grass and I just wanna point out that we already have we're ready know something about how grapheme behaves when we went we add adsorbate Stewart from previous measurements for example this is a mere colby's measurement by using scanning SET and you concede electrical poles that Our arise due to impurities in the year derived from the silicon oxide layer also From transport you can also get some idea of what happens when you drop charge impurities underground in the system works from Michael fears laboratory but these measurements actually don't even know they give us a lot of good information they don't tell us what's happening locally in the vicinity of these individual up atomic sites impurities and so that's something that were really interested in studying in 1 of the reasons were interested in it is because if you have a charge Centurion crafting been and predicted that the electrons agraphia will will behave around that impurity it differently than they behaved In other materials of for example if considered cherries and silicon gallium arsenide he expected different kind of behavior for the electrons run and impurity in graphing because electrons in graphing behave as massless Dirac formula so it's been predicted by people such as leather coffin and others as well that if you look at how these relativistic electrons behave around a cool maturity than their different physical readings depending on the strength of the Kumar purity and you can get what is referred trust subcritical behavior further bound states no resonances or or a critical review where there are bound states that have a particular of narrow energy with the spatial distribution so this in the kind of thing that we would like to measured experimentally to better understand and so motivated by this we dropped acts down onto grass
we drop different Adams and and what are the 1st atoms that we looked out is COBOL and so we drop evaporated call Adams down onto a gated graphing device at low temperatures Adams would stick and would not move all around and that we took got STM images of it and so here he see image the Ojeda gravity device on silicon oxide you concede to call Adams there they are and what we did was put RISC intent on top of those cobalt that arms so that we can
measure their electronic structure and security ELECTRONIC structure for of this idyllic charge structure of a single coal Adams sitting on top of a gated graphing device is see IDV as a function of sample biosphere 0 that's the Fermi energy and each of these 1st is were were P doping the sample also were
increasingly P doping as we go up so you can see there's a lot of features there there's a lot of residences there that this is all due to electron structure that Adam just sitting on cobalt and so on Thursday that have complicated but it's not so bad but there's really 3 main features our this this 1st feature here at the center at 0 there's this dip at the Fermi level which is independent and and we believe that that did this simply elastic feature due the vibrations of the Cole album so I don't wanna focus on that in the talk policy forced me to but I would rather focus on is the use of these other features these these features with which change with the gate voltage and so in order to understand those better applied the energy of all these residences here this is energy versus gate voltage and I've also plotted the Dirac point how it moves with a gate voltage so you can see that some of these features move with the drop point and other features don't remove opposite in energy of the drop point now these features that move in red that I've outlined in red was a move with the the drop point because they move with the drop point that implies that they are density of states features 3rd 3rd their features embedded in the back unstructured do that impurities and soap the idea of it which you can see in this little cartoon is that the whole lot Adam if you take the call what adamant that isolated Adam it has atomic energy levels and so if take that Avenue stick it partly drop it onto a piece of graphing those atomic energy levels are going hybridize in some way with the graph announced the lead to defect states which I have drawn schematically here so this is not the result of a calculation is just a and secure a furor our defect defect states in the states of gravity now we conceals defect states with guest in our spectroscopy and something that's kind of nice possibly as kind of no I think is that we can change energy level of those defects states with the back so we can gate gate are Adam which gave the surface and we can move those that stayed down energy and the reason that significant is because that if we put that allows us to kill or deplete bills those these 6 days with charged so here for example I'm showing you a Sonora were repeated that the sample litigate adding he said that these defects states are below the Fermi energy and I have a negative sign there to indicate that they're filled with charge negative charge to their below the Fermi energy and carries can see the actual that is the GATT versus voltage in the leader the impurity states below the Fermi energy now here I'm sure you using the same Adam but but with a different gate voltage so now those states and above the Fermi energy and poor little there because now the charge has flowed out of those states now we've depleted we've removed charge from the deep from that of cobalt defect and here you can see that in the batter share those same states but not pushed above the Fermi energy here now you might also have noticed these other Little States right here where I marked with that those those states are due to induce charging of the defect and the a nice with think about it is that we haven't Adams said on the surface we gave the sample and that and that determines a charge date for the Adam but then we can bring the and the tip is also a gate it's immovable it's another gate and we can bring that and then we can use the tip to change the the the charge on for example we can either for example here in in this case I have electrons in the defect state but I can it this bias voltage yes that is sucking b d electrons out of the Adam and that's where we get this little bump resin this case were I bought I Rory removed the charge from Theada with gate at this bias voltage PSG until is pushing electrons and 2 the adamant that's where we get a little so so this is kind of a nice new capability that we have which is to control on the charge state of this defect using the back gate and and 1 of the things that we would like to do with this new capability is to use it to investigate how cool owned in part of what happens in the vicinity of cool potentials on graphic because now we can change the charge state and what we would like to do is send measure around these charge impurities with electrons are doing in the graphics that's something that we wanted to but we have problems things ever perfect and what the problems that we've been having been trying to do this when the plant so we have is that he graphing assault in Homewood genius sees this as a minister image of electron hole puddles in graf things that you were you heard about and this electronic in in-home in ideography makes these kinds of measures hard for us because it obscures what's happened to the 1 Adam downstate put Adam right there then if there's some charge distribution around that Adam do the cool impurity gets obscured by all the charge inhomogeneity another issue that we have is that we call this the problem we have a whole lot Adams is an impurity itself has has what we call it and might Coliseum in Homewood genius charge they know what I mean by that is that we can set the charge state of the Adams with the back gate but then and then we won a map with electrons are doing around it in the graph but the problem we have is that sometimes Iast him that will cause charge data Adam to switch mention that to you earlier and so we have trouble there not what's happening around the impurity because sometimes impurities charge will switch and that and you can see that here by this rating this is of a number people have seen other asked him experiments as well this is what we sometimes referred to as an ionization radius and what it means is that when outside of this really is the atoms and 1 charged it but we go inside of the radius it switches to another charge in that due to the fact that the tip is a movable gates for these are problems for us about our alone you know maybe sometimes you try to pretend that there it's all really great but actually it's not bad we have made progress at solving this problems so I want what's some the progress that we've made and we
made a lot of progress because mainly recently through the use of foreign I tried as a substrate so this is a really great thing and and the the 1st people do was the Columbia group and and they showed that if you put Graf born I tried in you can greatly enhanced mobility and so we were inspired by this great result to try this ourselves and so we built some devices in this fashion we put foreign nitrate flakes on still cannot sided with 6 CVD Graafian put it over and we stuck it in Tour scanning tunneling microscope and these devices work out settles group also cultists a fabricate these devices and so we looked at those devices with with S and
I and and it turned out that they look really great sense of this help solve 1 of our problems which is this in Homer Geneva problem because here you can see that Graf in born nitrite complicating fury compare Graf born nite photograph silicon oxide visit the topography the comparison many consider Graf born I tried his orders of magnitude more flat and also down here we compare the charge in home on inhomogeneity for graphing born nitrate and cryptographic silicon just the color skill that graphing on unborn nitrate is is much less is much more homogeneous so that's really great for us that solves 1 of our problems I also want went up at about the same time we're doing this where Brian mean the group also did similar work in collaboration with the Columbia group and they got results that are very similar tars and so that's a great substrate to use so
now were using that substrate
craft for nitrate and it allows us to solve another 1 of our problems because now that we have such a nice flat surface for the graph Pedersen aren't we can perform atomic manipulation money Adams at would put on the grass mean that we can actually engineer our own defects by moving the atoms together and so we can create new defects that have different charging properties than the ones that we have before us for example before we were limited to say monomers that the grabbed the Cobalt monomers but now we can take the Cobalt monomers and move them together you can see move together like that needs to remove together with tipped to create a dimer and we can move this 1 over here to create a trimer so now we can create a small clusters which have different charging properties and that's been that's actually been good for us because when we look at this cobalt primer for example we find that these these clusters have much
nicer charging properties in the monomers and and they can we can put them into stable charged states here for example the call what trimer weakened reaching gave the Cobalt trimer such that when we bring arrest him tips and image electrons around the trimer indifferent chart and in a particular charge state the trial work stays in that charge state so we we can we can put with the Triborough into different charge states and then we can map popular comes around it and and the charge state will not change and so that that's for us that progress and so this is this is a work in progress were not done but we're very excited about this because now were starting to map out with electrons are doing our around school impurities so we can control the charge only school impurities so now we have a chance of actually testing some of these ideas for example here you concede uncharged call what primer charged call charmer in this yellow halo fast electrons in graf rearranging themselves around that charge trimer do that cool impurity so the physics that we won again that is living in that halo so that's a worse that's where the process of analyzing now so OK so so putting Adams down onto grass is 1 way in which you can change the properties of graphing but another way well but you might want to change it in in different ways for example you might want to induce an energy gap in the grassy are you might want to
quantize energy levels in graf and so a way in which you can do that if you want a change in that way 1 1 thing 1 of the many things you can do is turn on a magnetic field and the magnetic field is nice because we try magnetic field it causes the electrons cigar on circles and when you quantized that motion circular motion and get Landau levels and the generously in energy spacing between the and the levels is going to depend on the strength of that magnetic field so this kind of behavior has been seeing a lot for graft and since I'm giving asked him talk about Otto you but some of the nice STM debtors showed that shows these levels and some of the nicest has been taken by a bot Andres group as well as Joe Straus use group in collaboration with Phil 1st and they seem beautiful land levels with with their as him a bit magnetic fields around 5 Tesla but a question that I wanna ask is what happens if you make as the if you take a shit graphing carve out a little tiny device a little tiny submicron device like here like a shown in blue and white What if he wanted to create Mandell levels right there
in that region but did not want Mandel levels here in these contact regions how how my my duties as well but the answer I think Firfer normal materials is that there's no way that you could do that because you can treat your superconducting magnets submicron dimensions superconducting magnets but this big catching them down that day and so this would normally be impossible but with Graf seen this kind of this kind experiment is actually is sort of possible because crafting has a very peculiar property which is that if he strained bed in a particular way then electrons in graphing will behave as the old there's a magnetic field turned down there in a 1 go round in circles and that's a very kind of peculiar effect I don't know any of the richer that houses and so I just wanna spend 220 to give you a
hand waving explanation of why graphing has this really interesting behavior that when you straining in a particular way behaves as a magnetic field we call it a single magnetic field now the reason that this is that this occurs is because of a very intimate relationship that exists between strain in graf and electro magnetic vector vector potential and 1 way of looking at this as it is to to consider gravity and now this is unstring graphic now consider graphic has undergone some amount of strain he stretch it and what that does is a causes the Adams to change their relative distance between each other that causes beat up the quantum hopping up to change for electron the heart from while carbon and the other and what that does what the mechanical Eigen states electronic items states shift then in reciprocal space like that that little dealt case that's meant to represent the shift of the items of the graph United States in reciprocal space do that strain now if he remembers some basic mechanics that you might remember that of a charged particle is moving to space and you turn on a magnetic field that what that does
is it changes the momentum of the particle by that proportional to the vector potential so take a look at these 2 equations can see that they are very similar and so we can do is we can we can create mapping between this change in momentum and and this vector potential and and if you're smart and through the details as a smart people have done then you find that that for a particular strain for particular strain tensor in graf EU that will lead to a single vector potential that I can you can write down as I've done here in this form vector potential depends on the yacht steel strain or the normal strain components as well as the shear strain in this way you can then take the curl of this this no vector potential and that will create a single magnetic field and so the electrons would then feel that magnetic field which arises purely due to strained even in the absence of an actual magnetic field so so a couple years ago some smart people a really clever paper and here they are it was part will be near Castle said underage 9 honorary case and so but this is a really nice paper they came up with this clever idea of taking this mapping and they figured out a particular strain geometry in which case d resulting Meg magnetic field would be tweaked constant magnetic field which would cause electrons to go around a circle leading to Landau levels suitor Landau levels and what they figured out is that he is that if you create a trigonal strained pattern 1 that has triangular symmetry then if you work through the details of this former and that should lead to a Consett magnetic field the question and and subsequently land levels the question of course those harder test this idea and in that paper they actually suggested an idea of using different thermal contraction and so that's what we did and and so we we tested this idea by by using differential thermal contraction and the idea here that if you put graph on a substrate and call it down grapheme does not shrink that must renew cooler down other materials will shrink more Anakin that can lead to lodge a large buildup of strain in the graph which could then provide possibly highly stream reaches which might show some of these effects of those actually idea that they mention their paper and so we we tried out this idea by epitaxy lead growing gravity
platinum at high temperature and then we pulled down the graphic looked at with arrest him and I just want to mention that other people have grown grapheme on platinum long before us we coffee their recipes for example called shows group and Anselmo groups we just copy their recipes and grower and then we looked at it at
low temperature we saw something kind of interesting this is what we saw this as an STM image of graphing grown on platinum 1 1 1 this this region right here is a patch of graphing if you can see the outline of it and these
bubbles which appear at anatomy scale we refer to a narrow bubbles and we saw these national polls all over the place on these grow on the grass that we grew on platinum and national bubbles are Have a triangular shape they're like little Purim is with 3 sides sort of territory he dropped and what we saw we ended spectroscopy
on the surface and when we did str spectroscopy away from those now bubbles the spectroscopy was pretty featureless and and and pretty boring but when we did spectroscopy right on top of the national bubbles we saw something very interesting we saw these peaks in the local density of states appearing at different energies we saw the very pronounced peaks and what we think these peaks are we believe that these peaks are are due to the landowner levels occurring in these never bubbles as a result of this Little magnetic field that is induced by the strain in these little strain now bubbles and in order to further test that idea we looked a lot of these
never bubbles we perform spectroscopy and a lot of them and we took all of the different peaks in we wanted the distribution of peak energies as you see here we see that this distribution follows it it it follows behavior that we would expect for land levels in graf penis worth and behavior and so from from this behavior we then extract a single magnetic field off from the slope from the spacing of of these peaks Emily extract as magnetic field we get a feel that's very high on the
order of several hundred Tesla here for example I show results from 1 particular enamel bubble here's a picture of that and here you can see that the need to magnetic field of extracted from the snow bubbled up your around 400 Tesla now in order to to further test this interpretation we performed here we we did some theoretical calculations in collaboration with Parker leukemia and Castro NATO that means that they did the calculations and that that at the end and they use continuing last disagree theory and they stimulated now bubbles having roughly the
same size and strain as the ones that we experimentally and this is a magnetic field at calculated which is very comparable to the 1 that we saw experimentally so so this is further evidence To support this interpretation and this is a bit of a result that I'm very excited about it 1 of the reasons is because this because these magnetic field so high several hundred Tessa begins it gives us access to a new physical Rashid for example rather if you consider dc magnetic fields the largest dc magnetic field that anyone has ever created in a laboratory is less than a hundred Tuzla but now we're looking magnetic fields up at around 400 Tuzla so this allows us to potential explore physical regime that's never that could never be accessed by any other means OK so now what I've done it is I told you 2
ways in which we can modify the graph won by dropping Adams down onto it and another fight last reading it locally and now I wanna talk about the 3rd and final part of of this type which is to explore what happens when you modify grabbing by cutting it into little narrow strips that we refer to as now ribbon now it turns out and I think it's obvious that if you cut traffic until little really really get some size quantization show you expect subbands and energy gaps took and indeed that happens but there's also some other physics that occurs which is I think not so obvious which depends on the details of this and you get some very interesting physics depending on on on on the symmetry pageants I wanna talk about that a little bit so so let's talk about the edge on if you take graphing in you could never written out of it and in this particular direction then you'll get edges that call Archer edges so here's the 2 sides of the never revenge is never of an expense in this direction up so if you cut cut and now written out of graft then you it's been predicted and here's some theory the 1st people who've made these predictions it's been predicted that you will get in energy gap and that energy gap should behave as 1 over the wit and I think that makes intuitive sense because as you make the which narrow or you is like particle a box you make the box smaller expect energy little space to get bigger and so for the armchair never ribbons you can really think of this energy gap as a size quantization effect if you turn elect elected interactions it doesn't change things all that much to you just get that gap and there's no edged for the arms at least that's the predictions on the other hand if you take grapheme and if you can't at this other angle 30 degrees from armchair angle right here that angle then you get what is known as a zigzag now ribbons were these edges are zigzag edges and and a zigzag never ribbons have been predicted have very different behavior from arm churned out a ribbons for example is indeed absence of electron electron interactions it was predicted a while ago that you would get our that you should get one-dimensional up metallic bands that are localized edged edge states and decency these Cesar metallic then it was initially predicted that you should not even get an energy gap for his exact never ribbons but then people thought about it some more and they realize that if there's any if if you include electron electron interactions then what happens because this edge state has a big does states at the Fermi energy its print it has been predicted that you would get a magnetic transition that edge would become paramagnetic each it was predicted that each become girl magnetic and that there would be I throw magnetic correlation between the 2 edges and this antiferromagnetic correlation has been predict which is predicted to open up an energy gap and that energy gap was predicted to vary as 1 over with so you so here with his exact you also are expected to get a 1 over with energy gap dependence but for very different reasons very different physics and I just wanna point out that some of these ideas have been about about since the mid nineties and Millie jostle houses on some of these early papers so some of these ideas are or are due in part to her arm so the question is how do we actually loan experimentally measure this kind of effect it's kind of tricky to measure this kind of effect because to really do it right you need to do 2 things simultaneously you need to measure the electron structure of the Napa River while simultaneously measuring the From a geometric structure the atomic point scale but yet to do both of those things and so that's a tall that's a that's a big challenge to do both of those things but people have been trying and had been investigating this system for a number of years here show some parts some work a few years ago done by Philip Kim's group where they use transport measurements to look at ELECTRONIC properties of narrow ribbons that they are defined with the graphically and they were able to see some energy gap behavior and other groups did measurements as well on camera procedures they know Forces Group and others are still transport is a very nice way to measure electron properties of of known structures but 1 of the partners that it doesn't tell us whether a local geometrical structures so it's hard to know what edges are doing in these Namur ribbons for example it's hard to know the level of disorder at the edges whether armchair or or a zigzag and so are because of that that then motivates people like me and people like me who like to use my casket and so luckily transport
doesn't measure everything so there's still some suffered from you measure and other people have been doing microscopy on these these these graphic never structure for many years here some early STM work by 1 the earliest things that people did was a look at the surface of graphite because we can look at the step edge on the surface of graphite that give me some insight into village the behavior of graph edges so that work was done early on more recently people of the looking in graphic national platelets from different materials people she's an army of graphing edges and there's been some very beautiful recent work by Klaus Meulens group making ribbons using elect their precursors I think is really critical and in my own group 1 are our strategies list but what we decided to do was to look it Nanna ribbons that were made by I'm sitting down to and the idea of the hope was that by looking at never ribbons made by zipping to the hope was that this would lead edges pristine and and an hour and so that's what we wanted to do was the Likud never ribbons with nice edges and my group not doing and zipping we don't we don't want to but there are other people who know how to do it for example 1 of the 1st groups to do it was Jim Court Judge James toward group they develop the way back in 2009 unzip now tubes to make never ribbons and more recently dies group has developed a new method for on zipping tubes involves subsonic aiding the nanotubes in a special kind of way and so what I wanna show you now is is some results that we got looking at these national ribbons and here's the recipe for how make them in paper so hung she died gave us some these now ribbons and respond coat them on to clean gold crystals and we look at them with dust here and this is what we saw this as a a
room-temperature still image of a single national written on gold and I and and it's a beautiful never really believe we looked at a lot of fun beautiful and his but but using this technique Banana River a rock are almost always really nice so you can see that it has very nice straight edges what we take appears a cross-sectional slices and his little black fly straight there is the cross-sectional slices and he conceded that nearly edges we see this curvature which we did not expect unexpected feature and at that price throws off at 1st because we thought that maybe we were staying crushed Namur tubes or folded or graphic folded under so we will we put a lot of effort into looking at different samples and fold that we actually investigated folded down ribbons to really see what's happening at the end we determined that we said we determine that this edges not at this stage we see here is actually not folded under it's not a question about who not folded under we believe very strongly now that what we're seeing is actually is a terminal edge as a show in this little cartoon but we just happen have some curvature right here at the edge off and so the Enzo now that we know that we have these nice terminal edges of our graphing never ribbons we can look at them with higher energy higher resolution at low temperature and and and an investigator atomic structure and the electron structure simultaneously and that's what we did so here I show a higher resolution passed him image take a low temperature of edge of a list of enamel ribbon that has a width of 29 meters and I'm just I'm just zooming in on edge so this is edge this is gold and this is now a ribbons greenish In this Orange means that here and and this is edge so 1st I want to see that we have a nice straight well ordered edge and because we have we can get a comic resolution for for the offer this region near the edge that allows us to determine the curry reality of hedge so now we can actually directly determine her relative yet but simply looking at this angle rest with respect that angle and that allows us to determine that this never returned for example has an 8 1 edge which means that it goes zigzag zigzags 88 times in Anzac zigzags exudes exact and and that's about a 20 carried city and so on and so what we now that we know the car rally we can measure the local electron structure and we do that using asked
him spectroscopy as I described before the technique I will perform as spectroscopy act he said and then points as we move inward toward the center of the never ribbon as moved in from the edge and here show the Daoud the the plot of the ITT versus voltage the 1st point I can barely see but there's a blacked out there on the gold that refers to this Top curves on the Goldie Cedars specter Miss featureless but here as we go in the edge what we I want wonderful from low-energy regime which I refer to as elastic what we see are these peaks suddenly sprouting up here it low energy and those peaks are those bills peaks Paul exponentially amplitude as we move away from the edge you see that here a lot of the amplitude of the peaks and part because those peaks exponentially that indicates
that they are due to an edge state because that's what the characteristics of an edge state is that as altitude falls exponentially away from the edge that's what an edge and so here so here were seen an edge they never Riverdance but also significantly it is another significant factors is that we we see to Little sometimes they're asymmetrical but we see these 2 little peaks here you can't get closer concede 2 peaks and we believe that this indicates that there is an energy gap in edge state worthy energy gap is energy difference between those 2 peaks and so we measured these this behavior on many different narrow ribbons and we saw that offered different Savannah rebels regarded different energy and security show that Dad we see that energy gap gets bigger as decrease the width of the Napa River so we see 1 over with dependence on the energy gap of this ad state so now also are we measured how edge state Varese parallel to the edge and here in this batter and we see that get state possibly goes up and down up and down so
there's a pure disk city
amplitude state which is approximately which is approximately the same period a city of of of the 8th 1 edge so now we have the atomic scale structure and electronic structure the so now we can start to compare results to theoretical calculations and so these calculators were done by Stephen William ole Garcia than their co-workers and they used a tight binding model too stimulate ELECTRONIC structure of now ribbons that have precisely the same symmetry of the same age structure and the same way as the as never ribbons that we measured and here I should results of their calculations electronic structure in the density states of calculated for this narrow ribbon and 1 never written with a 20 meter with art and you can stand and this calculations done in absence of electron elected interactions and what you see here is a metallic state at 0 which leads to keep the density of states that that's not what we saw and so in order to simulate what we saw they had turned on electoral actually interactions and they did that using may be covered model using a mean-field solution of a Harvard model and when they turn on election elected interactions that gap open often that state and that gap that opened up was of an order of magnitude very close to what we saw experimentally so so there's some agreement here between their theory and are down and is 1 of Medici with the physics is what's happening as eternally electro lectern interaction that's causing that edge date to become magnetic so the 2 be edged into the edges becomes magnetic compact draw magnetically correlated antiferromagnetic correlations causes this gap right there open up so that that's what happens in the model so weak we actually saw the same gap at our experiment but we and in the model arise magnetism but we have not actually met scene we have not measured magnetism and experiment we just see the gap so we we can also compare now that we have this nice theory we capture compare our results the spatial dependence of the edge state to their theory so here's this spatial tenants as we move forward perpendicular from the edge parallel to the edge and the wit the tenants of the edge state energy gap that we measured experimentally and shares with a calculated using their tight binding model red is the intensity considers pretty good agreement between experiment theory here so I feel that this is very strong evidence that we are seeing an edge state for finished graphing never ribbons and this edge state has an energy gap that as with the tender so that's where I wanna
stop on a nonstop just by concluding that I think are still has a few surprises left I told you a little bit about what happens and we sprinkle Adams demographically how were able to measure still feel the effects of gravity and how we have discovered the presence of an edge data pyrographic narrow ribbons I think there's still a lot of things to do in the future and will talk about them in the future but for now I just want to tell you my collaborators worth a
lot of people collaborated on this work here are the different Shiites who collaborated I walk their names but you can't beat them yourself right there but sheer below more importantly other students start who collaborated on this work and for the Adam spectroscopy in this project was done by Victor bra reaches Decker weight France Michael soul 1 Charlie here China can we will reunion will Gannett single field to magnetic field work with them by his Levy Sarah Burke he seemed Meeker among the supremacy in addition the help from these guys the graphic now ribbon work with them by changing how yen Chan being gel joint on Shane came only yards in real time and the funding for for the work this work was funded by the Department of Energy Office of Naval Research and the U.S. National Science Foundation said how we sell some work to do but I'm gearing up to do it we at least we have this sample processing down that that's good that that's you the hardest at some level part we gotta prepare these samples but now we have to start preparing tips and and there and part of the problem is that the system were doing the ad Adam moving it on depletion of the Adams as the same system but I would use for the employer's STM so I'd have you know wanna finish a that project before we do this poor because you know depend on ENI such be energetic not expect a strong high such big energy for the cobalt and this is at 4 degrees Kelvin but I don't have a magnetic field I know I'm the only guy around but the figures Islamic Hicksville I have no magnetic field I was still magnetic field tirelessly magnetic 5th so far this I have that acts Zvi should have letters of a real 1st I want to know that we do not know what is the actual termination you know everything that I know everything I know right there right here on the slide in you know him to do so we so this is our heads so so we don't really know the tree rally of energy we know this structure the actual termination like is attorney by hydrogen atoms or carboxylic acid or oxygen and that we don't know so we don't know the absolute edge from Asia however despite Novak is even if it's terminated with different Adams at some level you still expect these as state to exist because if if termination is just bonded to that signal on sticking out as long as the network is not all messed up and we still expect this edge they took her so that would lead by that big of and that means you
notice that is a bomb digits this curvature discourage that structurally and natural consequence of the Lisbon will dictate is also a reminder inside Red but a lot of the observations saying last fall In this I'm just wondering when you sold trimer culture whether they need Riddell selections see didn't notice yes I I didn't
show all of the data there but there are freedom we have seen some feel oscillations in certain press premier so they should
people present in the nunnery but also you're saying you're saying that you could have a free dealt like a free
Dell oscillation decayed son but 1 thing is is that it I don't see exactly how that would lead to an energy gap especially with the
dog get it there voyage of some sort interaction in a while but it would be correction died and all or you mean
crossed the width you go out OK I mean we have to think about ahead of as interesting observation you use of the 2
opposite edges former Delaware antiferromagnetic a lot that that's the prediction we have not forced magnetism because learned about the possible in the prediction that must be judo why interaction of course depends on the sleeves and as with so much club are not
sure you're not sure asking What
are you know they are take care interaction between magnetic impurities in a metal you get a most of the tree and dry yes please
In in in growth seen at 0 density as a reserve Kauai interaction which DP is has 1 more heart you unexpectedly for two-dimensional systems sold away how you can figure this out you take the distance between here it is then they're going to ration will be carried by electrons was typical memento of the water off
course of this distance it takes a still stays Corus pointers is memento which is nonzero and then plug it into the usual I formula which will give you 1 of are you that the the
White forward the best In the bans were exactly you pulled endurance is about is on the east side of the yes In this case yes in this case original firms wasn't the case there is a typical range of momentum which contributes to the polarization which is of the order of the distance between magnetic and these are the details the were exactly the letters you put them in the article if you would have decided that will oscillate between indecently close to where
the oscillation from without the
oscillation counts From that you have to sublet and that you have the year corners of release which is and it meant I don't know their due to and if it really generates so hard but that the high just using glad to through your it that thing is that
it's so it's not actual magnetic field it's it's an orbital back so it's it's just how the electrons are moving their moving as they feel that field but there's no real magnetic field there for example there's knows a month the
stands are not feeling it it's purely an orbital so it's not
so I don't think that you know I don't think there's a magnetic field and that's really affect us it's it's just affecting electrons moving McGrath does that it's also relate to feel be
expected quantum whole effects as a result of it I say visual to effects Contel effects
but yeah I mean but in these early but were just looking at the these very small regional just
yet AI you're saying if he could perform transfer measurement on and on I'm just strained Regency I think in principle you should feel that you should expect an
effect on transport from the sea but by measure no effect on transport for reasonable wrinkle so if you not only that does a sort of certainly critical angle a typical scales get filled over 1 Cecil require a strain of us they want to them away what my might system so it the fields are so watch because Michael soda bubbles train with those bubbles relief at the start what it about you know what you want Tesla field it's probably typical sub oxide or it can
also depend on the symmetry of the strain field visits we kind of got lucky here really has this trigonal symmetry because the 1 1 1 yes if you just have a
homogeneous rate it will be plus minus cancel but that additional capped brothels the that
kind lucky Yong nunnery
all I don't go gold presumably it should it so you can't seem to be using the others that just as bad the is what you're asking
a degree what is it that I don't think about what why don't use what you'd
expect from got the model by gold OK so yes I mean take a
look Indian see building I mean migrate here let's see others offset from 0 make the comments and at 1st but I want a player that people have grown Graf feed on gold 1 1 1 and they see that the gold one-on-one does not interfere too much at ELECTRONIC structure of the graphing still has the Dirac Collins and there's not a lot of charge transfer from a perfect system however here we have an imperfect system Kozeyah these never ribbons and there it is not a perfectly clean system there is there's also adds or beats down there it was transported to care and so what we see our our our fluctuations on the order of plus or minus 20 millivolt from ribbon to rebound variations and these variations are almost precisely what people have seen in the
past 4 that ought to on gold 1 1 1 with people have for spectroscopy of Namur
ribbons on gold 1 1 1 they also sought shifts in the electronic structure of the same order so I so it I can't really tell you what is causing so we do so we do see some charge transfer on the same order as the nanotube guys but I cannot distinguish
whether this charge transfer from the Cold War from absorbate social
France remote gold your 3 Micro which is huge that is efficient to transfer charged bitter pretty Jack lately
from 4 my question must be sure that the bubbles are as the substrate restructured that the reason I ask question in the seventies In my group we we did a lot to work on forming graphing graphing forms as stable monolayer phase on nickel 1 1 1 of the key epitaxy effort is excellent but if you go off that orientation and you still get the same phase transformations but it's accompanied by the formation of a on the substrate other words of the all substrate restructures with a performed to expose 1 1 1 facets on which the graphing growth and so my question was other bottles are religious that perimeter of the substrate covered with graphic well I mean we tried
tearing them apart you notice is underneath them but it got kind of
messy so that was that would be the obvious thing to do so we weren't
able to we were not able to look back I think that there is not a pyramid underneath 1 reason I I I think that is because 1 of things that we notice is that when the graphing is touching a metal in actually in full body contact like on the platinum on the gold then the features this this electron phone on featured the Dirac drive features the drug point future they they go away they get swallowed up by the electron wave function and were not and now bubbles were seeing as soon as as soon as we get to the edge of the National mobile all of these features comeback in a we start seeing the future that we expect further service suspended Graf whereas I would expect to start seeing you know the metallic the signs of the metal underneath and we don't see that I so so that sort of like a little kind of experimental cancer but other than that I don't really know know what the energy of formation is for these that that I don't suppose that actually it
was restructured still see said as and deceit but you're saying
would that be that then the graph would have to provide enough strain took 2 cars Theo but 2 of them well if you go up if it
if you do these former graphing at high temperatures and where the there's enough mobility dont ability of substrate bid to restructure perimeter yet yes but
but but remember people have been looking in this at room temperature for many years and so they've so got from high temperature room temperature it's not seeing was really something that was that we saw as we went from room temperature low-temperature
BOC magnetic field will be washed out the temperature report from the slaughterhouse while on his way to observes in Japan but it's too
big but the energy staging it is hundreds of Miller vaults so I think that we would need temperatures on the orders of Katie on the order of hundreds of mobile washed so I see it as a means of normal the
to put on the vast amount from walked out of a 1 carat gathered
these but we have these but we have these separations now they big you know 100 what separation 100 mil Baltimore a few hundreds available so that would be a gigantic enormous temperatures
but you should buy see bikini of some of as
we start moving from 4 K to room temperature yes I hadn't thought of that have seen anything said
Felicity onto it would we see
around the distribution of her realities but we would very much like to see the armchair because then there should not be in a state that would be very important observation we have I can't remember the closest that we got good but we haven't we haven't got me charity is the armchairs are very rare the most rare because it requires a zigzag now to you know and those are the most rare ones a perfect exec
because we believe they cut back silly so I think it makes sense that there's not a
lot of them down on the surface but we would like to observe that 1 way actually that I wanna try to do it is you know this guy class Meulens doing his of molecular precursor growth he only grows armchairs so this selective precursor grows only armchair so we're actually trying to do that to get there to then I think if
you vote and a small role at some point that out the box that kind of and in your life when you get into the small role will eventually balked at his peak enough so that you can pick up the jet features from new spectroscopic decided not only did I wonder whether you can pick up those both get something like that it faces a look at the Croats a leading a game source of the problem all get becomes operational so that you should be able to detect using that spectroscopic I see John I
mean that something 1st that that's a big 1st look for blood I tell you a problem that we have 1 of the problems that we have is that as we go into the middle the a river and its in close contact with the goal and so then we we don't see the graphing features a witty spectroscopy when that when that never Robina's is directly in contact with the gold so as you go into the ball then it is it's washed out so we need to take an hour rivers and put them on a different kind of a different surface luckily this curvature takes the number River foot the gold and so we're able to see the exponential decay before before the before the
graphing reaches the gold so I believe that the exponential decay is an intrinsic affected not do the gold but as we go further and we cannot do the
measurement uncle and that makes you
just wondered how the festival yield manage not to make it a comeback again can't say well the
problems at once we want for this particulars system we do not have the capability to warm up and pulled down and find the same microscope location so I'm pretty sure that we take time 90 per cent sure that we've taken the
same sample and warmed up and pulled it down 19
95 percent share of that we stay the same sample of warmed up and called it down the whole try backing chamber room temperature and
called a full it down and then look at others regions of the surface
we did not do do a careful study at room temperature so I expect that they disappear based on the measures at other people have done but we did not do a careful study at room temperature we did a lot of our measurements at low-temperature so
I expect that they disappear because of the differential thermal expansion is not so strong
but but but I don't know I'm pretty sure we did not that measurement I have to ask my again as they did the room temperature but I don't think we did might sex very much so none the evasion question if


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