 I know I'm the last one and I'm holding, I'm between you all and dinner, so I'm going to keep this efficient. Thanks for the opportunity to be here and present, present my work is to be here at my home presenting to you in Europe I appreciate the opportunity. As mentioned, I'm talking about Adamistic insights and friction of MLS to and beyond. This is work done by my former PhD student Muhammad Viser Shirek, and in collaboration with Rob Carpix group at the University of Pennsylvania. So before we go too far I just want to excuse me have a little cold. Before we go too far is want to let you know where I'm speaking to you from. I'm at the University of California Merced. We are located right here in the center of California. The focus is the newest in the UC system opened a little over 15 years ago, and you will be welcome anytime please do visit. All right, let us jump into it, excuse me. The focus or the start of today's talk is going to be a molybdenum disulfide I assume that this audience is very familiar with MLS to, but a brief intro this material is layers of molybdenum and sulfur sandwiches. It appears both naturally and synthetically in a variety of different crystallographic phases and forms that correspondingly have different mechanical thermal and electrical properties. But the most important property from a tribal logical standpoint comes from the layered structure of the material. We have strong bonds within the layers and relatively weak bonds between the layers, which enables or provides the material very low shear strength and correspondingly low friction and where. Actually is nothing new, it was patented for tribal logical applications almost 100 years ago now. And today it's actually used as an additive increases and lubricating oils, particularly those for specialty applications. And most who is also used as a solid lubricant or surface coating for mechanical components like bearings in applications where liquid lubricants are not possible. For example too hot to cold vacuum corrosive etc. One of the prototypical or the prototypical situation for solid lubricants is outer space and shown here on the left is the James component from the James Webb Space Telescope, and a bearing on that telescope that has an MOS to coding. Over here on the far right you see microstructure of the MOS to the demonstrates the Miller structure. So most who is already being used as both an additive and a solid lubricant. The challenge with using most two for such applications is that it's frictional behavior depends on pretty much everything. And that depends on how it synthesize the temperature of the environment, even the substrate roughness showed here on the right of this slide is just one representative example, where we're looking at friction as a function of sliding cycle for order versus amorphous and different temperature and oxygenation conditions. And we see that the friction varies dramatically from case to case. From an engineering design perspective because you really can't predict what the friction is going to be as the properties and conditions vary. Another approach to kind of capturing this is empirical. So here's just again just one example where we're looking at sheer strength as a function of temperature, and we see a monotonic decrease that can be fit to an equation. So this expression here captures the experimental data and some of the parameters indeed can be tied back to physical insights. And this is, again, a reasonable approach compliments compliments design enabling some degree of prediction, but there simply are so many parameters that affect MOS to friction that it's a little bit. It's still a challenge. And here are some empirical approaches. Excuse me, our fundamental studies. And this is what I'm going to be showing you here today. The concept is is that if we look really closely at the surface of an MOS to code a component for example, we see that it comprises many asperities. If we can look at just one single asperity and understand its friction, perhaps we can scale that understanding up to larger, larger components and potentially provide guidance for design. So this approach is realized in research environments using experiments and simulations. Again, I mentioned that the business in collaboration with Rob Carpick's group and of course the AFM results I'm going to be showing you our, our his. So this fundamental approach is realized by combining atomic force microscopy which this session knows all about you've been hearing about it for for a little while now compliment complimented by molecular dynamic simulations, an example of which is shown on the right. I'm not sure about designing experiments and simulations together. And again this could be a whole talk but I just want to point out one key point here that friction has involves two things involves two bodies were sliding in relative motion. So designing a model of an AFM experiment must necessarily capture not only the structure of the sample being measured but also the tip, including its structure shape crystallographic orientation and so on. But once you do that you can really get pretty good matching results. What I'm showing you here on the top. These are friction maps, where there's the AFM experiment on the left and the MD simulation on the right. And down here on the bottom what you're looking at our line traces so this black line across these figures on the top represents these trace the trace and retrace shown here on the bottom. The key feature that you want to look at here is that you can see these stick slip or the sawtooth patterns, which correspond to the tip sticking and slipping across the surface. The reason that's really important from a fundamental study point of view is that the spacing of these features corresponds to the atomic lattice of the sample. This means that we are measuring friction that can be tied back to kind of a ground truth, we're tying this nebulous thing this friction back to something that's well known, the lattice spacing of the service which is really promising for using a fundamental approach to understanding friction were generally shown here as an example of the kind of friction data that Rob was able to get in his group. There are two different friction loops at different. These examples are two different temperatures and these are both on MOS to, and you can see the characteristics stick slip pattern, which again indicates that we're able to correlate friction back to the atomic lattice. Okay, so we've already identified a connection between atomic lattice and friction let's take this one step further, knowing the crystallographic orientation and structure of MOS to. On our left, we see her top down schematic, and one can imagine or hypothesize that you might get higher friction along the armchair direction because effectively if we were drawing this line, we would cross more of these atoms, and if we were drawing a line along this exact direction. So we performed experiments and simulations again, scanning along these two directions on the MOS to service and indeed, we see much higher friction, as well as higher energy dissipation. The map the gap between the friction, the trace and the retrace in the 30 degree direction which corresponds to armchair. So again this is pretty exciting because it's connecting friction to something that we know about the material. The direction dependent study one step further and scan in all of the possible different directions from from a center point on monolayer and bulk MOS to an experiments and the monolayer studies shown for simulations. What these plots are showing here, this is a radar plot, and what this indicates the directions indicate the direction of sliding. Okay, and where when you're further away from the center that indicates higher friction. So, the key point here is that in all three cases, we see two low friction directions, and the rest high friction directions. So we're seeing two fold symmetry and a strong correlation between friction and the direction on which you slide relative to the crystallographic lattice. But this is a little confusing because one would expect six directions where you get low friction. To understand this we first looked at the potential energy landscape of the surface. These colorful fancy figures here. The color represents the energy. So hot red is high energy and blue is low energy. The ideal potential energy surface was measured or calculated in the simulations using a single atom probe. And we found as expected, six low friction directions. However, recall that I told you that friction involves two bodies, right the tip and the substrate. So we repeated the potential energy surface calculation using the tip itself to measure the energy landscape, and it found a completely different picture with low friction in just two directions. The key point here is that the friction we observe is correlated not only to the crystallographic orientation but perhaps as expected, the surface energy. So now let's take this one step further I titled this presentation mo s to and beyond. So let's go one step beyond and try a comparison between mo s to and graphing. So these are the prototypical 2D materials or structures are shown here on the right they're both layered. Can we compare them. And more importantly, can we understand their differences as a means of kind of getting back to the origin of friction. So this study was made possible experimentally by samples synthesized in Charlie Johnson's group at the University of Pennsylvania, showed here on the left. And if I zoom in on this little part here what you can see is that there are regions of graphing. There are regions of mo s to and then there are regions where there's mo s to on top of the graphing. And we're calling that the heterostructure. And what's really cool about these samples is that Rob's group was able to scan all three of these regions graphing mo s to and the heterostructure along the same scan line, such that they could could be compared apples to apples shown here is a result on the left you're looking at the experiment and the right is the simulation. And you can already right away see these are just single scan lines that the friction of the mo s to appears to be a little higher than the friction on the graphing in both the simulations and experiments. And I think that these little step these features here is the sharp peaks correspond to step edges as mentioned by the earlier presenter in this section. To confirm this difference we perform the simulations and experiments again simulations on the left experiments, I'm sorry, experiments on the left simulations on the right were performed at different normal loads. And consistently, the friction of the mo s to is higher than the friction on the graphing grappings and black here. Also, in some of the cases, we see the heterostructure that's in red has lower friction than the mo s to directly on the substrate. So to understand this we turn back to our potential energy landscapes again the color here reflects energy, red is high and blue is low. And we see that the barrier, the energy barrier in other words the barrier that the tip has to move across to slide forward is higher or highest on the mo s to, and this is consistent with the friction that we observe. These were calculated on using the empirical force field. So we asked our colleague Aaron Johnson at Dalhousie University to corroborate them with some DFT calculations. And here you're looking at a comparison of sliding of methane as a just a proxy for a tip in DFT where the blue is the mo s to, and the black is the graphing and we can see again, higher barriers on the mo s to. So we see again that structure matters but also the surface energetics. So now you might be saying maybe I'll back that well, I'll leave it here. You might be saying you know what, there really could be thousands of differences there are thousands of differences between mo s to graphing that's not really an apples to apples comparison. So we went one step further and we performed a study comparing transition metal disheveled and ice. So this is a class of materials that involves metal transition metal and a child could get chelcogen and mo s to is one such example. So here's again our mo s to structure. And in this case we perform a systematic study, we varied this element between sulfur and mo s to selenium and tellurium. And you can see they all have the characteristic transition metal disheveled and I tmd structure with slightly different lattice spacing. So this is going to enable us to do again that apples to apples comparison with the goal of correlating friction back to fundamental ground truths of the material shown here are the results from the AFM experiments. On the left you can see here a friction map indicating that we're capturing the atomic stick slip. And on the middle part here we're looking at friction traces. And very interestingly, the mo t to has the lowest friction and the mo s to that we've always viewed as the super low friction material actually has higher friction. So, mo t to then mo se to then mo s to what we why could this possibly be. So here are the simulations of the same three materials shown here in the middle is the line trace results, same trend, mo t to smallest mo s to highest to confirm this was representative across all different sliding directions. Here is friction on a radar map that I introduced previously, again recall that data points further towards the perimeter of the circle indicate higher friction. And we can see that the highest friction is the mo s to everywhere except for these two low friction directions where they're really too low and too close to differentiate. We spent also a ton of time trying to explain this using data in the simulations. We looked at contact area deformation all all different things we could possibly think of based on guidance from the literature. There we go. Oops. Sorry about that. I have a little bit of lag. Oh, there we go. Okay, folks. There we go. So what you're looking at here is the color maps are the energy landscape as measured using the AFM tip. The black moving lines here are the trajectory of the tip in the simulations. And this bottom part is the corresponding lateral force or the friction force. And you can see a few interesting things here. First of all, we find that the tip does not move over the energy barriers, but instead goes around them and crosses at the saddle points. So this kind of obviates or, or maybe we'll say confuses at least analyses of the potential energy barrier or the highest point to explain friction. We can also see even from these movies that the path that the MOT2 takes is quite a bit different from the path that the MOS2 takes or rather the path that the tip takes on these materials differs. So let's look at just a still photo instead of the movies. And again we see that the trajectory on the MOT2 is quite a bit different from that on the others. What we did, again, based on the fact that we see the tip moving or crossing at the saddle points is we looked at the energy profile right at those saddle points. So these dashed lines that I'm indicating right here. These dashed lines correspond to what's shown in this figure, which is the saddle point energy profile for the three different materials at three different locations. And what we found, although not that this part isn't shown here, the barrier heights are about the same, but the MOT2 saddle point is wider. And this actually makes sense if you look at the crystallographic structure over here the MOT2 has the largest lattice spacing, and therefore has the largest or the widest saddles by which the tip can traverse and therefore the lowest friction. To generalize these results we used a Prandtl Tomlinson model. This was mentioned in an earlier presentation today. This is mimicking an AFM with a point mass connected by a spring to this moving point and energy barrier with height U and last spacing A. Because we're looking for sort of two-dimensional paths for the tip to move, we used a two-dimensional PT model that we developed previously, that again the key features are the energy, which we showed several times earlier in the talk was important part of friction, and also last spacing as indicated by our study or comparison of the three TMDs shown here are the results of the PT model. This colorful figure is energy on the y-axis lattice constant on the x-axis, and the color represents friction where red is highest. What we see here is that friction decreases with energy barrier as expected as seen many times before, but also I'm sorry I was circling the wrong thing. Increases with energy barrier, excuse me, over here on the right, as shown many times before, if you have higher energy barriers, it's harder to get across and the friction is higher. However, we also see that friction decreases with lattice constant. So what we're finding here is direct connections between energy barriers, which are a function of the two-machines in contact, but also the crystallographic lattice of the substrate, in this case specifically the lattice constant, which affect and its effect may come into play, particularly for surfaces that have similar energy barriers. Okay, so coming into the conclusion here, I know I can hear your stomach scrumbling from all the way from California. I titled this thing Going Beyond, and I wanted to mention just a few key points. I showed you a comparison of three transition metal dishockonides that enabled us to isolate the origins of difference in their friction. I think that looking at other TMDs is really a promising area for friction studies, because there are so many different possible combinations of elements. If we can understand how these different combinations of elements make create for materials, and then underscoring that to their friction, we might be able to ultimately produce or provide guidelines for quote unquote designer TMDs. Another dial that can be turned in MOS2 and similar such studies is our dopants. So with dopants, you can add our very small amounts, very small elements added in very small quantities to the material that can completely change their properties. For MOS2 specifically, it's been shown that dopants can drastically affect their friction and particularly their temperature performance. So this is a study where at high temperature, we see one of the best materials is this gold based dopant right here, the screen one. Excuse me. But at lower temperatures, a nickel dopant was found to be best. So again adding very small quantities of dopants, you're completely changing the properties. So we performed another study on nickel dopant and found that particularly at low contact pressures, it can dramatically increase the wear life of MOS2 coatings. Lastly, I introduced the idea of a heterostructure when I showed you the graphing MOS2 work, but this is kind of just the tip of the iceberg right. There's so many different possible 2D materials. Conceptually, we may be able to combine them, layer them, group them to customize or tune the properties of two dimensional low friction materials. So wrapping up, MOS2, which is what I started with is already being used as an additive as a coating, it's limited in that it's hard to predict. So from a design perspective, it's hard to predict how it's going to perform under different conditions. I also propose that a fundamental approach to understanding the connections between ground truth like lattice constant and friction is really needed to perform the baseline to ultimately get us to where we need to be for design guidelines. I'm doing single disparity studies as I showed you with AFM and complementary molecular dynamics. And ultimately, I hope that these kinds of studies can provide the fundamental understanding that we need to design 2D materials with tunable or optimize properties. Thank you again for this opportunity to share our research with you from California. I know I know we're going to be short on time and you all are hungry. Please do ask questions now or my email is here and you may follow up with me remotely later. Thank you again. See. Very nice. Do you have questions. Thank you. Thank you for very nice talk. I just wanted to image the friction map across heterostructure. I think you said the friction map on the heterostructure of graphene, which was this, this one. Yeah, this is a trace. Do you have also the map of it or let's say business? I don't have that in front of me. We do have, we do have friction maps that I don't have that in this presentation. Okay, okay. Thank you very much. May I ask something? You started saying that within things made covered with the most as to the where differences, or did I understand wrongly so that the preparation changes the properties. The introduction. That's absolutely right. How it's deposited under what conditions it's deposited. Now this work was done at Sandia, and they found that MOS to frictional behavior is very dependent, not just on the conditions in which it operates, but the conditions in which it's synthesized. So it's a pretty strange or something that is not perfect material in all these cases. And it's partially due to that. Another, another explanation that's been proposed a few times is that they, you know, according to me just come back here, you know this this concept of MOS to, you know, there's these ideal layers that you're sliding relative to one another, but an as synthesized coding doesn't quite look like this. Actually, maybe this here's another example. I don't see the lamella aligned with the direction of sliding. So there's expected to be some kind of a run in period, where you're getting from the as synthesized structure to something like this locally. And so it's believed that how you synthesize the material affects the, the reorientation of the material to get you to that low friction behavior. I have just a question. If you change sliding direction on mobile than this or fight. Are you going also to change the slope of stick slip characteristic. Oh gosh, that's a good question. I don't know if we have anything. I can't tell. It's a very good question. I'll have to follow up on that. Next question like for the doping, do you know where in which site, you think to the dopants will be included if we'll be like interlayer or if there is some substitution depending on the element. That's a, that's a really good question as well and the answer is going to be that I don't know. So shown here is a schematic of the possible positions where nickel could be in this study was nickel but I think this is a more general maybe I'll just say it generally. There's the possible position so we can have an S substitution. We can have a molybdenum substitution here we have two different possible interpolation sites between layers. The reality is that it depends on quite a few things it depends on which element it is. It depends on the concentration of the element. And again it also depends on how it's synthesized. When when the materials deposited if there's a lot of energy available you may or may not have a atoms may or may not have access to certain sites. So, the reality is that it's probably a mix and the preferred position depends on what's in there and how it's made. Thank you very much. And with this we close the session and we close for everybody. And then at the bus and Adriatic shuttle is at 6 30 so.