 Well, Lisa's getting set up a couple announcements, we'll move from here to the posters, and then the posters will transition that'll be posters will be six to seven is that correct or well will be it will start a little late, but we'll move to posters then at seven o'clock. At the terrace there'll be a photograph group photograph at seven, and then a reception after that. So be there to be photographed, so we can all get a nice shot of everyone together. And we should capture one of the we should tell everybody with their camera on and get one of these online shots to. And, and then we'll have the reception after so. Okay, I see Lisa is there and she's getting set up. Alisa. Hello everyone. We can hear you and see you. All right. Thank you very much. Let me hide the floating MIDI control. Okay, very good. So thank you very much for this invitation and, like I meant I'm sorry not to be there in beautiful Italy. I realize that the weather is finally good also in New York took a while. Okay, so good morning from me and good evening from you to everyone. For the ones of you who do not know my work. I'm an experimentalist and my group is heavily involved in scanning probe microscopy for nano mechanic study from friction to elasticity and also solid liquid interfaces. But we also have a quite a lot of research going on on thermal scanning probe lithography, which I will not talk today but you know just to have an overview of what we do in our lab. So, lately, a really a focus in my lab has been on Van der Waals materials. Van der Waals materials like graphene that everybody knows or transition metal decalcogenite or boronitride. They are characterized by very strong in plain bonds. So, for example, for graphite this gives try to a very high elastic constant C11 in plain, which is about one terra Pascal. And on the other hand, on the transverse side, the elasticity is much softer. So in fact, the C33 perpendicular to the plane elastic constant is only 36 giga Pascal. So this high amizotropy in the mechanical properties and also in the electronic properties gives right to many interesting properties in this material. While quite a lot of studies have been done on the in plain elasticity of this materials. Think about all the elasticity measurement done for for example graphene or other to the materials deposited on membrane where atomic force microscopy tips are used to indent and then measure the elastic properties. Van der Waals is known on the transverse properties and in particular what is happening in between the layers right so if you have your two layer three layer for example to the film on a solid substrate. You don't investigate the Van der Waals interaction between the layer or the transverse elasticity and then understand how this is for example related to other properties like friction and dissipation. So you need to have access first of all to these properties, but this part of of the work required also some novel experimental development development, because as you can easily see here right. When you bend a graphene on top of or another to the film on top of a membrane, your indentation death of or are of the order of 100 nanometers, which is something let's say relatively easy to measure with an atomic force microscope. On the other hand, if you have a two layer film of say three layers or two layers on a substrate on a rigid substrate, and you want to indent and measure the elasticity between the film. So your indentation needs to be very small, otherwise you don't measure anymore the interlayer elasticity but you measure a coupling with, for example, the substrate elasticity. So ideally, your indentation need to be of the order of the Armstrong. So then you are in a very different regime and you need a different type of methodology. So, because of this, we have developed what we called Armstrong indentation just to maintain the tradition of micro indentation nano indentation now we are into the Armstrong indentation regime, always using atomic force microscopes. This is a method inspired by the work of carpet and salmone in 1997. And I'm not going to enter too much into details on the methodology. But this methodology, as you will see will be very similar to another methodology that I will talk more extensively in this seminar here. So the general idea is to use a modulation and to first indent in the material and then by very tiny sub Armstrong modulation, we've draw the tip and then measure the change in force as a function of the changing load. And by integrating this curve you have force versus indentation curve that as you can see here, give rise to a resolution that is smaller than one Armstrong, and this allows us to measure very stiff materials and more importantly, to measure to the materials on rigid and this. So this is a little bit. This is the reference to the article that I was saying and this is. Sorry, I can't see anymore. My mouse here. Not sure why this up here. Okay. Are you still hearing what I'm saying. We see you and hear you, but we also don't see your mouse. I wonder if. Very sorry for this. I don't know what happened so let me maybe. It looks like my computer is having a problem. Yeah, it's like it's completely frozen. Okay. Okay, let's see if I can. So we just unshared your screen. Okay, very good. Now I think at least this frozen. Okay, I think she's had a computer crash. As my guests. Lisa, if you can hear us, you are now frozen. She's still smiling. That's true. We're guessing she may need to reboot. We made it through the whole day without any major technical glitches. So unfortunately we got cursed at the very end, but my. Yeah, my guess is she's, she's just needs to restart with things are working fine on our side. Okay, now she's dropped. Okay, she'll, she'll reconnect in a minute. So for those of you online, please bear with us. We think we'll be back online momentarily. Okay. I'm very sorry. I don't know what happened. No, no problem. We see you and welcome back. Okay. I'm very, very sorry. Okay. Hopefully now we'll be good. Okay, so we are, we arrived here. And so this is the modulated the instrumentation method that where I was describing before, where we use a low keen to apply a very tiny modulation or to the piezo tube. And then we detect the change in force. And essentially, we, for every load, we measure Delta F over Delta Z, and then by integrating, we really decrease quite a lot the signal to noise ratio. By doing this type of studies, we have recently discovered something very interesting, which is a phase transition of epitaxial graphene, which is a graphene grown on silicon carbide. And under pressure into a diamond phase. So the, the transition occurs at room temperature. And under the action of the AFM tip, we were able to detect a change as you can see here. So this is a typical image of epitaxial graphene. So you have a silicon carbide, then you have a buffer layer, and then you have one layer or two layer on top. So our results have indicated that by doing, as I was saying, this indentation curves on silicon carbide bear silicon carbide, you know, we get an elasticity, a young modulus of 400 about giga Pascal, which is what we expect for silicon carbide. And for graphite, we get a transverse modulus of 36 giga Pascal, which is exactly the transverse elastic modulus of graphite, which is the same as the 10 layer graphite 10 layer graphene on silicon carbide, five layer graphene on silicon carbide the stiffness is a little bit higher because probably we start to feel more the substrate. We measure the buffer layer so only this layer here so there is nothing on top, we essentially measure the young modulus of the silicon carbide again nothing unexpected. What really was unexpected was what happened when you have one buffer layer plus one layer. So we call this two layer graphene, it may be a little bit misleading because depending on the community or in is really buffer layer plus one layer, it's not an exact situation but we call it two layer graphene to indicate that you really have two layer of carbon on top of silicon carbide. So when we measure this type of system, as you can see here, we get a stiff net that is much higher than silicon carbide higher than sapphire and close to the stiffness of bulk CVD diamond. So in order to explain this, we work with theorists who run some molecular dynamic simulation, DFT simulation, and they indeed actually the simulation here proved the formation of a single layer diamond. We are undergoing more and more studies about this very interesting phase transition, also another system like hexagonal boronitride. We are, we also started to run current AFM measurement to understand at what load you really see this phase transition and we realize that the load is about 250 nano Newton. So based on all these type of studies, we also started to be more interested into the dissipation mechanism and that is something that is the second part of my seminar today and that I really it's also the more new part of our work, which came a little bit as a surprise, because the idea was for us really to investigate more the dissipation due to the phase transition. Before entering to that, we started to say, okay, before studying the dissipation after or through the phase transition, let's get a plateau of what happened much before so for loads much smaller. So when the phase transition still does not occur. So this is the work that we started to do. So we were interested now not to study the transverse elasticity in the direction perpendicular to the plane, but we were interested in studying the shear elasticity. So the shear elasticity. The way we started to look at was by using very small loads that give rise to extremely small indentation and when I say small indentation I'm really talking about. Sub Armstrong so maybe 0.1 0.2 type of indentation so the you essentially almost I would say zero indentation right and but the tip is still in contact so with the graph in film. And underneath we have the silicon carbide structure or other type of structure that I will talk in a moment. And then we slide off a very small amount and we stay in the elastic regime. In this way, we measure the, what we call the interfacial shear modulus of the top atomic layer in respect to everything that you have underneath. We started to play with this type of experiments with two different system. One is a regular epitaxial graphene system, where like I saw, I described before you have freestanding graphene and then you have this famous buffer layer, which is a mix of SP to SP three bonding with the silicon carbide substrate. So it's really an interfacial carbon layer. And then we have this system where hydrogen has been intercalated between the buffer layer and the silicon carbide. And so this buffer layer is released and becomes a was a freestanding monolayer graphene, as you can see from the name here. So then again, you can have one quasi freestanding to quasi freestanding monolayer and so on, and you have hydrogen here that saturates the dangling bond of silicon carbide. So these are the older system that we have studied. So we have one layer graphene plus buffer layer on silicon carbide, we have two layer plus buffer layer, we have quasi freestanding one layer with the hydrogen intercalation, and then we have two layer with the hydrogen intercalation. Then we have a different system which is a 10 layer graphene, but now grown on the carbon phase of C H silicon carbide. And because of this, the stacking is a very interesting stacking with essentially twisted layer, where the layers are alternating 30 degree and two degree, compared to the silicon carbide axis. And then we work on bulk graphite, and we performed interfacial share models experiment on each of this sample. So always keep in mind that what we do we share of a tiny bit, the top layer compared to what you have underneath. So in this case, for example, you have to think about sharing this top layer, compare to all this, some straight underneath the same here the same here and so on. Okay, so what we have found. Well, first of all, this is an overview of the methodology so very similar to the modulated answering dentition I show before, except then now we share laterally. So essentially what we do we move in this area where you know this is the classical lateral force versus displacement that we have seen many times in this conference right that this will give rise to our static friction and then our kinetic friction and the stick and sleep. So in this part where we have just the the contact right where you essentially just pull your contact the the change in lateral force divided by the change in displacement gives you the total effective stiffness, which is a a stiffness given by a two springs in series. One is the cantilever lateral spring constant. And the other one is really the stiffness of your tip substrate contact. So the shear modulus, the interfacial shear modulus is inside this quantity. On the other hand that we know exactly what is the lateral spring constant of our cantilever so the torsional spring constant of our cantilever so we can. found from the experiment we can found this value and then we use a locking amplifier to deeply really decrease to dramatically decrease the signal to noise ratio. So we go in contact we apply a certain load as I say before quite a small load but enough to keep the contact and then we move essentially along this line here for some displacement that is smaller than one and strong. I think is 0.3 m strong. Okay. Next, we obtain the lateral spring the lateral contact stiffness has a function of the load. And this contact stiffness based on contact mechanics is given by eight times the interfacial effective shear modulus times a where a is the contact radius. Since the contact radius is a function of the load addition young modulus which we can measure with the modulated nano indentation method that I shown to you before. So we really can measure the young modulus of the interface right we know the tip radius so all this part is known, we can obtain the interfacial effective shear modulus. How we obtain this we just plot the experimental lateral contact. The stiffness as a function of the load as you can see here, and then we use this same equation to fit the experimental data. And these are the results. So for the different sample we obtain values of the order of 100 of mega Pascal. So first observation is that the modulus that we are measuring is indeed not a shear modulus of in playing graphene. Because the shear modulus of in playing graphene so if we would stretch the graphene layers so we would get more values that are more of the order of the Terra Pascal. On the other hand we measure very low values so this very low values indicate that what we are measuring is more the out of playing shear modulus which is what I anticipated at the beginning. So it's really the sliding off the top layer compared to the substrate. And this makes sense right because again, it's like having two springs and one it's extremely stiff and the other one it's much softer so what we measure is the soft spring. After that, let's try to understand right what we have measured and let's try to see how this interfacial shear modulus is which is an elastic constant is related to the dissipation and to the atomic scale friction. So first of all, let's try to understand the origin of this numbers. So this is something that took us quite a lot of work and I must say is really not finished because I think we will need really to work with theorists. So I'm also asking help from the audience if there are anybody here interested in a collaboration because we have some how to say ideas of where these numbers are coming from, but of course these are more possibilities that there are no proof that this is really the origin of these numbers. So I plotted here the interfacial shear modulus values for the different system. This here we have bulk graphite. Here we have two layer on hydrogen. So this is the, let me see, put here. So this is two layer graphene on hydrogen intercalated silicon carbide. So it's this type of structure but with two layer. This is one layer. So this is exactly this structure. Then we have two layer on buffer layer. So this is this structure plus one layer so two layer plus buffer. Then we have twisted the 10 layer graphene, which is the structure that I was telling you so it's 10 layer but the layer are twisted in terms of stacking. And then we have one layer on buffer layer silicon carbide, which is exactly the structure you see here. So why we get this type of change in interfacial shear modulus? We think that there are three main factors here. One is the stacking. I put it, I listed here. One is the stacking. One is the substrate interaction. And the third one is the substrate shear stiffness. So let's start from the stacking. So the stacking actually is the one that has the most motivation because there are studies performed by Annalisa Fazolino, DFT studies that indicate that the C44 elastic constant in graphite, which is essentially the interfacial shear models that we are measuring, strongly depend on the stacking. And she demonstrated that, for example, the AB stacking, which I put it here, the AB stacking is the most stable stacking in graphite, and is also giving rise to the largest number of interfacial shear modulus. For this reason, I indicated here the three structure that have AB stacking. So bulk graphite here has AB stacking, two layer on hydrogenated silicon carbide, AB stacking, two layer on buffer layer, also AB stacking. As you can see, this is not enough to explain all our results because this structure here, which does not have AB stacking. In fact, one layer on silicon carbide doesn't have any stacking at all because it's just one layer graphene, right? It still has a shear modulus that is much larger than one layer, for example, on buffer layer, layer graphene on buffer layer. So why this one layer on hydrogen, which is here, is much larger than one layer on buffer layer. So the explanation that we have here is given by the presence of this hydrogen. So what we believe is happening is that this hydrogen is actually hindering the motion. Whereas the one layer on buffer layer, it's a non-commensurate structure. And we think that also it's a mixed, TM studies indicate that they have a mix of AB and AC stacking. So it's a disorder if you want interface, carbon-carbon interface. And so we think that that's the reason why this has such a low shear modulus, interfacial shear modulus, compared to this structure here that is on the other hand, hindered by the presence of hydrogen. We also think that hydrogen could be attached to the top graphene. And so there could be almost like a Velcro type of sliding, which increase even more the shear elasticity. Okay, so the other low values is given by twisted 10 layer graphene. And again, we think this is due to the fact that the stacking is really not the preferential stacking. And so the structure is loser. Finally, the substrate shear stiffness is important. So for example, if you look at the two green here, they should have the same values. Because this one is on top of a stiffer substrate, which is essentially this structure here, compared to this one, which is on top of this substrate. Then you really have an increase. And indeed, the ratio between this green and this red is exactly the same ratio as this green and this red, indicating really that the substrate, simply shift the value of the shear modulus of the top layer. That said, we moved on to understand the relationship with friction and dissipation. So we perform friction studies, atomic force microscopy friction studies on the different sample. So we then calculating the, you know, we plotted the friction versus normal load, and we look into the friction coefficient. We also, and we started to see all sorts of differences for all the different materials. And to compare them, since they have a different stiffness, we plotted the friction as a function of the contact area. And as you can see the data gives quite good linear curves. And so then we extracted the slope of this curve, which is the friction per unit contact area. In this way, we were able to compare all the different materials, even if they have a different stiffness. And then we plotted the friction per unit contact area has a function of the measure interfacial shear modulus for all the material measured. And with a very nice surprise, they were all sitting on top of a of the same general curve. And this is a simple reciprocal curve with is general for all the different materials. And interestingly, what you can see here is that, for example, for the two materials that have the two layer and one layer this to structure here. So these two material by coincidence they have a very similar shear modulus. And they also have a very similar friction per unit area. So it really looks like that the interfacial shear modulus is an extremely important parameter to control interfacial to control nano scale friction. Of course, we know that there are many parameters that control friction. But what is important here to consider is that the material is always a graphic material. So we didn't change the chemistry. And we, how to say, we eliminated the impact also of the contact area because we normalize per contact area. That's saying that we try to understand. Let me jump in for a second. Accounting for the delays, I think we're getting close to the time allotted and we are also getting close to running up against the need for our poster session. I'm wondering how quickly can you wrap this up. We just have two slides. And then it finished. Okay, so, so this is the model. So this is the familiar Tomlin some model, and by looking into, you know, by running see very simple simulation of the Tomlin some model, and then plotting the friction as a function of the interfacial shear modulus. As you can see here, we really get a very similar result obtained from our experiment and we even get the same type of fitting function which is a simple reciprocal function which as you can see here, is better than a exponential decay. We also work with the group of area to Zatti to do a little bit more complex type of simulation. This is a framework contour oval simulation where the tip is sliding on top of a series of springs, which are a top periodic potential. And the periodic potential here mimic the substrate interface, while the springs mimic the the graphene structure. And again, what we obtain is something similar. So it's the the strong coupling of the friction force with the interfacial shear elasticity. Okay, with that, I'm sorry to rush a little bit, but I would like to thank you everybody, my group and the funding agency. Thank you very much. Thank you. All right, we do have time for a couple questions. Thank you. Okay, a way, perhaps simple question. How did this PT model fits your experiment because the samples are not points like yeah. The sample is not point like. But this PT model is, yes, yes, for points like samples. I can say what do you mean point like so the, it's a one dimensional. So it's, you know, you have a interatomic potential right in interaction potential and then you have Yeah, in this model, the tip is, is a single particle. Yes, yes, it's a single particle. Yes, it's a single particle moving on top of the potential. Extended sample. Mm hmm. Yes. It does fit it because of course it doesn't fit the exact numbers right. But it fits the relationship indicating so it's a friction per unit area. Okay. So that's what we have plotted. And in this terms, it fits the relationship between the friction and the interfacial shear modulus. Yeah. Okay, thanks. Anyway, Ali is here around, I think, and I will discuss with him later on. Last quick question. Do you hear me? Can I ask a question? Yes. Go ahead. Okay, so thank you very much for this splendid talk. I have a very, it's not a simple question I think it's a very curious to know if you had compared. You can measure the bulk models or shear modulus which is absolutely fantastic. Did you try to compare this value to the bulk value mean did you, do you have the possibility to check that the personal ratio is okay or it's not but it's a question more or less. That's a very good question actually because we don't, so I would say that's really the question that we also have is, is the, no, so the short question is at the moment, no, we don't have any way to really measure the personal ratio. So, so the, we just assume that the personal ratio is the personal ratio of graphite, but it may not be exactly. Yeah, I agree with you. Because it's of the interface right is the personal ratio of that particular interface. Yeah. Thank you. Okay, so with that, let's thank Lisa again for this wonderful talk and thank all the speakers this afternoon. We will now move to the poster session. We have 11 posters and area advises me we will