 Okay. Sure. Hello, everyone. My name is Chiang Gao. I'm a postdoc from Tel Aviv University. It's my great honor to be here today. The topic for my talk is the functional properties of a graphene green boundaries. In this talk, I will mainly discuss the dynamics of dislocations, and I will slightly talk about the effect of more superstructures. The superlubricity, roughly speaking, describes the extremely low functional state. And due to the superlubrication behavior, it provides a broad range of applications at different lens scales. And in many of these applications, it will involve the situation of drag lubrication between two solid surfaces. In such conditions, superlubricity may be realized by properly utilizing the structural properties, which give rise to the concept of structural superlubricity. And in specific structural superlubricity, as we know, this refers to the effective cancellation of lateral forces due to the lattice misorientation between two flat and rigid crystal surfaces. And these requirements are relatively easily satisfied by 2D layered materials. Therefore, we see two dimensional materials have been widely studied for structural superlubricity in recent years. And since this concept was proposed, we have seen that experiments have demonstrated structural superlubricity using nano scale, micro scale, homo junctions, and later by hetero junctions under multi-context to realize the so-called robust superlubricity. And these promising results motivate us to further scale up structural superlubricity toward micro scale. However, in previous studies, structural superlubricity is mainly demonstrated by single crystalline samples. And as we know, at present, it is still very challenging to prepare large-sized, high-quality single crystal samples even under life conditions. So in this regard, it is more practical to consider polycrystant two-dimensional materials, which are more feasible to achieve large amounts for real applications. And among the various polycrystant two-dimensional materials, polycrystant graphing can be a good candidate. So what is the polycrystant sample? And so let's take an example of this polycrystant graphing. It contains randomly oriented greens and separated by green boundaries. So although we know that the randomness and orientation of the greens may facilitate the incomersorability of the interface, but the green boundaries on the other hand may produce additional energy dissipation that may impact superlubricity. So to understand the frictional properties of green boundary, it's better to understand the structure of green boundaries. So here I show you the structure of a single green boundary and we can see it consists of a series of dislocations that are shown by these cyan-colored spheres. And if we further zoom in, we see the most common dislocations in polycrystant graphing, which is the pentagon pair dislocation. And one important feature of these dislocations is that they introduce lattice string to the lateral string to the lattice, and which may cause the distortion in topography. And we know the topography properties may be critical to the frictional behaviors we are interested in. So this image shows you the experimental measured polycrystant graphing sample, and it consists of a few green boundaries. And interestingly, we see that the topography of, due to the introduced string by the dislocations, the topographies of the green boundaries are quite non-trivial. It can vary from highly corrugated buckled structure to nearly flat configuration. And to systematically study the topographic properties of green boundaries, we perform AMD simulations. And we find that for small misfeed angle between the greens, the green boundaries are usually corrugated. And for large misfeed angle, the green boundaries are also usually corrugated. But at certain misfeed angles, we do observe nearly flat green boundaries. So here I show you the summarized results for the bump height and the bump density as a function of misfeed angle. So we see that as the misfeed angle increases, the corrugation height decreases. And while for the bump density, it first linearly increases and then drops due to the connection of the neighboring dislocations. And in the middle region of large misfeed angle, we do observe a few flat green boundary cases. And the results are symmetric with respect to 30 degree. And then we perform simulations to study the fractional behavior of these individual protrusions. And here I show you an example of the case where a flake sliding over a single protrusion. And so we can see as the flake approaches this upward protrusion. Blue color means a higher corrugation. It press down this protrusion to the other side. And after this flake leaves this region, this protrusion will buckle back. And this kind of buckling is somewhat similar like the steps through buckling off an arched beam at the micro scale. And the buckling, of course, will generate a kinetic energy pulse and enhance the energy dissipation. So after knowing the fractional behaviors of these individual protrusions, then we consider impractical applications. A more common scenario is that these protrusions materials will under persistent shear and working at different load and temperature conditions. And to investigate the fractional behaviors of green boundaries under these conditions, we built a fully periodic simulation system. And it consists of a three-layer pristine graphing slider, a top one-layer polycrystalline graphing, which is supported by another two pristine graphing layer. And this picture shows you the side view of the system. And for the polycrystalline layer, it contains two periodic green boundaries. And the misfit angle is 8 degrees. After relaxation, we can see a series of upward downward protrusions. The blue color here is the same as upward protrusions and red color is downward protrusions. And now I would like to talk about the main results of our simulation results for the friction. First, for load dependence at zero temperature, we find that dependence is a non-monotonic dependence on normal load. It first increases and reaches a maximum, and then it gradually drops. As we increase the temperature, we see the peak position gradually shift toward a lower normal load regime. And at certain temperature, it becomes a monotonically decrease. And if we further increase temperature, the entire curve will shift down a little bit. And for the temperature dependence of friction, we see somewhat similar behavior as we increase our external normal load. And the peak position changes to lower temperature and finally become a monotonically decreasing like behavior. And also for the load dependence, we see that there is a regime that friction goes down with normal load. And this can be defined as a regime with negative friction coefficients and provides a way to surprise energy dissipation for large-scale sized system. And then to understand the frictional behaviors, we analyze the energy dissipation properties at zero temperature without some more fluctuation. And the first, the dissipation power as a function of normal load shows us that the main dissipation comes from the out-of-plane direction. And if we check the energy dissipation distribution, we find the high energy dissipation size are located near certain dislocations. And so from these results, we can say that the energy dissipation are mainly due to the out-of-plane motions of the items at certain dislocations. And if we check the trajectory of these dislocations in the virtual direction, we find that those dislocations undergo dynamic buckling between upward protrusion state and downward protrusion state. And by increasing the normal load, we find it has two opposite effects. At first, it can promote more dislocations to buckle during sliding while it has surprised the magnitude of buckling, which will reduce the energy dissipation for buckling even. So under high normal load, we see that the dislocation is under smooth-like motion and the energy dissipation is low. So here we can explain that the non-monotonic dependence of friction on load at zero temperature is due to the competitive interplay between the increased fraction of buckling dislocations with the reduction in each buckling event as the normal load is increased. And so to further quantitatively describe the energetic behavior of the two upward and downward state, we extract the free energy profiles from equilibrium simulation. And here this plot shows you the free energy profile for given dislocation and we can see a clear transition energy barrier between the two states. And we find that the average transition energy barrier goes down with normal load and it positively correlates with the protrusion correlation. And with these results, we can also understand roughly or qualitatively explain the buckling at zero temperature. At zero temperature, the transition energy barrier actually varies with the sliding projection and once this transition energy barrier goes to zero, buckling occurs. And with further sliding, if the transition energy barrier becomes zero again and it will buckle back. And with temperature, the buckling can occur at earlier position. So based on these results, we divide two state phenomenological model and in this model, the buckling probability is related to the transition energy barrier. And here I show you the comparison of our AMD simulation results with this model prediction. And you can see the model can well capture the load dependence and the temperature dependence of friction. And here I show you two typical simulation movies. One is at zero temperature and one is at a finite temperature and you can see the thermal activation can facilitate the dynamic buckling of these dislocations. And in the last part, I would like to briefly introduce another very important effect at two-dimensional interface, which is the moral superstructures. Moral superstructures forms also due to the lattice mismatch or lattice misorientation. And to study this effect, we built a similar AMD simulation system where I replaced the graphene slider with hexagonal boronitrate. And here you can see the relaxed moral superstructure on this polycrystalline layer. And because one green in the polycrystalline graphene is aligned with HBIN, so we can see very large-scale moral superstructure on one green. And so this plot shows you our main similar result of friction as a function of normal load at zero temperature and at room temperature. And compared to the results of a homodunction, we find that they show a similar monotonic decrease change in the low normal load regime. While in the high normal load regime, you can see a sharp difference in the heat reduction friction dramatically increases. And here there is another mechanism occurs. And so which we find is due to the, sorry, I won't show maybe this movie for the last. The high energy dissipation at the high normal load we find is due to some stick slip kind of behavior for the moral superstructures at the high normal loads. Okay. So here are the conclusions. I will stop here and I would like to take your questions. Thank you very much. Questions? Just one clarification. So regarding your methods, you didn't say very much about this. Did I understand correctly that you're sliding a rigid flake over your grain boundaries? Yeah, our simulation only the top layer is rigid. We have a six layer system. The top layer is rigid moving with a constant velocity in the sliding direction. While the bottom layer is also rigid fixed at its initial position. Other layers are flexible. Okay. And then another question follow up on the grain boundaries. So the grain boundaries in graphene, I believe are not perfectly terminated. So there will be dangling bonds. Actually, yes, but actually this system, for this system, because we are considering the dislocation relatively far away from the edges. So the edge should not have too much effect on the buckling behavior of the dislocation. I'm not talking about the actual, but this is a grain boundary in the middle, right? Where you have these protrusions. This is a, the protrusions are because there's a grain boundary, right? Yes, yes. Even the grain boundary will have unsaturated atoms. It will have dangling bonds. So it may be chemically reactive. So my question is what influence that would have on friction? I would expect that the friction should go up if you slide over it because you're forming covalent bonds or the system tries to form a covalent bond. I think here each items also have three neighbors. So it's saturated like it will not form easily formed bond with other items in other layers. Just to follow up on that. I think at least for some grain boundaries in graphene or materials like this, you can get just rehybridization of the carbon. So there's some SP3 bonds that show up not just SP2. So they may not be dangling unsaturated bonds but the change in hybridization. And that makes it go out of plane. But that said, I would also expect even with that just chemically it's still distinct. And you may get a friction interaction from that enhancement of friction. Okay. So you mean there may be some chemical ag reaction? Yeah, I think I think there's those are high energy sites. And even even if there were no topography and flexibility from the buckling, just, you know, it's like having a higher energy local site ought to lead to some kind of, you know, pinning interaction, an enhanced barrier to sliding over it. So it's like, I think that's perfectly partially what Lars was getting at us is that factored it. So I have a question. When one graphene surface moves over the other surface, there is also a torque which is produced. So maybe does your, for example, dependence of dissipation on normal force comes from ability to accommodate this movement? Because this superlubric incommensurate state has low friction, but it is high in energy. So surface would like to get aligned. And these defects which are there, they actually accommodate this rotation. And when you increase the pressure, they can move less. And that could be a mechanism which is just accommodated by existence of this grain boundary. So did you think about that? Did you analyze that, these rotations and such stuff? Okay. So first in our simulation, the top layer, so there is no rotation in the top driving layer. And the second actually we find, sorry. So here, we analyzed the energy dissipation and we actually find the dissipation power mainly comes from the out of plane motion. Because during each buckling process, the instantaneous velocities are quite high. And this will generate the kinetic energy process and contribute to the main energy dissipation. And without this buckling event, the energy dissipation is quite low and comparable to the resistance interface. I guess the question was, did you allow for rotation of the flake? No. At the bottom layer, we all have a small rotation, but the top layer is not. Overall the rotation is very small. I want to ask if your dislocation is mobile or is it fixed? It's very stable, even under a thousand Kelvin dislocations will not diffuse this place. Okay, thank you. I have a question myself, so you said in the beginning that friction will apply to the number of bucklings. How does that happen? Because this can be understood from this energy profile. So for some dislocations, if the energy, transition energy barrier is very high, during sliding it will stay in this state. It cannot buckle. If there is a normal load, this will reduce the barrier and it can buckle during sliding. You have a very long slide, let's see. Yes. Captain, yes, please, go ahead. Captain, cut mute. Sorry, I missed the beginning of your talk. I also missed your statement, Chiang. I wonder, you talked about dislocations. You have dislocations to which you can ask the instability. So it's real dislocations. And here, it's like the density of this Pentagon-Hypertagon pair dislocations will increase with a misfeed angle. This way, thank you.