 Okay, good morning everybody. First of all, I would like to thank Ford organizers for this nice conference. My name is Ebru Jihan. I'm currently a postdoc in Enrico's group in Dresden, Germany. But today, I would like to talk about my previous work that I did when I was in Anders Schirmeister's group in Gießen. So the title of this work, this talk will be a fingerprint of a structural phase transition during superlubric sliding. The main motivation of this talk will be superlubricity. And regarding this, I will... And regarding this, I will first discuss the different efficiency of superlubricity for crystalline and amorphous structures by showing a direct phase transition from an amorphous to a crystalline structure. And secondly, I will briefly touch on the effect of thermal relaxations on sliding friction. So as we all know here, the use of atomic force microscope in fixed measurements has led to opinioning various phenomena of atomic scales, such as stick-stick behavior and superlubricity. So I would like to now talk about a little bit about the concept of this superlubricity. Superlubricity is the ultralow is characterized by the ultralow friction between atomically flat and incommensurate surfaces through collective force cancellations without barrier and plastic deformation during relative motion. Theoretically, if the two surfaces in contact are incommensurate, there is no interlocking state between the atoms. As a result, the atoms can slide easily over each other. And in this, easily over each other. And in this case, if we adapt as the contact area or the number of atoms at the sliding interface increases, the potential energy barrier per atom decreases. And atoms can manage to stay at a higher level of the energy barrier. As a result, it becomes easier to slide. An alternative tool to analyze superlubricity could be nanoparticles with well-defined contact areas manipulated by an atomic force microscope tip on a flat substrate. And this approach has been applied for several years both in ambient conditions, both in ambient and ultralubric conditions for further investigating the interface contamination on superlubric sliding. And then, the next parameter whose effect on friction needs to be investigated would be temperature. And I would like to now talk about two main aspects regarding this temperature, high-temperature measurements. First thing is superlubricity. Atomic scale superlubricity is mainly controlled by the atomic structure of the sliding interface. And therefore, it's important to demonstrate the different efficiency of superlubricity for amorphous, for randomly ordered amorphous and for well-ordered crystal structures. And we can achieve this by a direct phase transition from an amorphous to a crystal structure with the help of temperature. Secondly, our second assumption would be possible contact aging effects in the high-temperature region. Contact aging is defined as increasing friction with time. And for the low-temperature region it has already shown that the presence of pseudo-conventional sub-areas at the sliding interface can contribute to contact aging and result in non-monotonic friction behavior. Based on these results we can argue that perhaps the presence of these pseudo-conventional sub-areas can be accelerated at high temperatures. Coming back to different efficiency of superlubricity I would like to now give an example with the help of amorphous antimony nanoparticles and crystalline gold nanoparticles this study has already shown that superlubricity doesn't work similarly for amorphous and crystalline surfaces. But still, for both types of nanoparticles friction increases top-in-yearly with the contact area and this situation can be formulated according to the number of atoms at the sliding interface number of atoms at the sliding interface and here is the gamma is the scaling factor for superlubric sliding and this scaling factor should be in between 0 and 0.5 for incommensurate crystalline interfaces depending on the shape and orientation or the level of the commensurability between the surfaces on the other hand the scaling factor should be well defined as 0.5 for amorphous structures because in this case shape and orientation don't play a role. So actually this situation already shows that less effective superlubricity occurs for amorphous structures nevertheless these results connected the amorphous and crystalline interfaces with different contact area scaling factor for antimony and gold nanoparticles and therefore open questions remain about the chemical nature of the interface to determine the atomic origins of superlubricity we still need a direct transition a direct switch of the interface from amorphous and crystalline without modifying its chemistry therefore I mean if we could achieve this in our experiments that would be fingerprint please keep that keep that word fingerprint in mind because I will come back to it later okay we now want to make monoparticle manipulation experiments at high temperatures because first of all we want to see a direct transition from amorphous to crystalline structure and antimony nanoparticles could be antimony nanoparticles which are initially amorphous as they're in deposited form could be an excellent candidate for seeing this transition for this purpose we prepare samples by thermal deposition of antimony onto graphite and first of course for providing clean interface conditions graphite subsets were first cleaved in air using an adhesive tape and then they transferred directly to the UHP vacuum chamber and after that evaporation of antimony onto graphite process having performed at 400 degree Celsius for 5 to 10 minutes and this procedure resulted in well defined antimony nanoparticles with atomic flat interfaces with nanoparticles and the graphite substrate after preparation of the sample and without breaking the vacuum the samples were then transferred to the AFM for further for further manipulation experiments manipulation of nanoparticles were performed were performed at different temperatures between 300 Kelvin and 750 Kelvin all manipulation experiments were performed as a three step process first the sample surface was imaged in non-contact mode to avoid scanning induced movement of nanoparticles and nanoparticles suitable for manipulation was chosen and then AFM tape was directly next to the nanoparticle and AFM operation mode switched to contact mode and then particle was pushed by the AFM on the graphite substrate after manipulation AFM operation mode switched back to non-contact mode and the displacement of the manipulated nanoparticle was verified and by the way during all the temperature changes thermal drift was carefully compensated based on continuous acquired topography images and only when the system reached thermal equilibrium the manipulation experiment was performed at a new temperature and here is a real experimental data here the AFM tape getting contact first getting contact with the particle at about 150 nm resulting in an increase in lateral force signal and immediately after that particle began to slide okay manipulation experiments first set of manipulation experiments were performed for a group of three nanoparticles at different temperatures between 300K and 1150K in these experiments temperature progressively increased temperature was progressively increased in 50K having steps resulting in friction forces for 10 different temperatures and these are the manipulation experiments results the first the very first result we can see is that very similar behavior very similar behavior temperature dependent behavior was observed for all three nanoparticles and also three different regimes distinguished in each case for the first as you can see from the gray background color for the first friction transition corresponds to the phase transition of nanoscale antimony and this regime can be extended up to approximately 650 Kelvin and for the second transition we can mark the very high temperature regime with increasing friction values because in this temperature the system because in this temperature in this temperature range temperatures exceed the evaporation temperature of antimony at about 400 degrees Celsius and the system is approaching the solid-liquid transition also here the lines in red and blue were calculated based on the concept of the highly-activated protonism model and these different lines represent the different energy barriers for low and high temperature regimes in addition to that this red data point shown here was recorded after the sample was cooled back from 750 Kelvin to room temperature and interestingly the friction was found to decrease by a factor of 1.76 I would like to now detail the seemingly irreversible friction behavior on this slide and let's take a look at these two different friction traces recorded at room temperature before and after high temperature treatments again I would like to say that a significant reduction is noticeable by a factor of 1.76 however we cannot say the same thing for the shape and size of the particles apparently they didn't change much when exposed to high temperatures in fact this result already suggests a mechanism that refers to an irreversible change at the interface while such changes are negligible for thermal stable graphite can we talk about the phase transition of antimony at this point literature gives us useful information about it and because many people have studied phase transitions in another scale antimony and accordingly temperatures above 420 Kelvin can be used as a control parameter to directly trigger phase transition in another scale antimony and in such a case phase transition should be naturally expected for our antimony nanoparticles as well because our transition temperature the transition temperatures we observed in our experiments around 450 Kelvin seems to be in the right temperature range so assuming such a phase transition and allows us for a more quantitative analysis of temperature dependent friction as we may remember we have previously divided this friction evolution with temperature into three different regimes let's consider these three regimes again low temperature regime, high temperature regime and highest temperature regime in general we explain the friction reduction friction reduction by increasing with increasing temperature by a thermally activated from Tomlinson model and we should be able to see this effect exactly when the temperature is increased from 300 Kelvin to 350 Kelvin in the second range 300 Kelvin to 700 Kelvin range to get a little bit complicated here for the more or less stable friction reduction we need to take into account the phase changes of antimony in addition to the thermally activated friction reduction because we cannot ignore the effect of enhanced force cancellations at the sliding interface which becomes more regular as a result of the amorphous crystalline phase transition and finally for the last highest temperature regime we should emphasize that these aforementioned temperatures exceed the evaporation temperature of antimony and the system approaches the solid liquid transition furthermore we have to also consider that heating the normal particles does not necessarily result in a single crystal structure as a consequence as a consequence we they can they may allow us more structural irregularities at the sliding interface such as grain boundaries and dislocations which can further increase friction by counteracting the force cancellations at this point at this point I also mentioned before that this red data points refers to an irreversible reversal friction change but at this point we are still not sure whether this irreversible friction change is caused by the phase transition itself or the exposure to the high temperatures therefore we made additional manipulation experiments on a freshly prepared sample to analyze the effect of high temperatures or whether the high temperatures affect high temperatures have an effect on the resulting irreversible friction behavior we prepared a new sample and we analyzed a group of nanoparticles at room temperature before any heat treatment and then the sample was heated up to 500 Kelvin and another group of nanoparticle was analyzed at this temperature after that sample was cooled back to room temperature and another third group of nanoparticles was analyzed here and as a result these experiments also confirmed previous observations namely at the lowest temperature friction was highest and it decreases by a factor of roughly 2 when the sample is heated up to 500 Kelvin and then this friction change is found to be irreversible because more or less the same level of friction is measured after cooling back to room temperature this experiment this results also confirmed the phase transition occurs in antimony and this phase transition causes an irreversible change in friction but also confirmed that the role of interface structure without the same material with the same chemistry on superlubric friction and lastly if we correlate this result with the scaling low of superlubricity and if we assume that our nanoparticles are amorphous and the scaling factor for amorphous structures is fixed as 0.5 and if we fix the data we obtain at room temperature before any heat treatment with the fixed scaling factor of 0.5 we achieve a reduced scaling factor of 0.42 with the data obtained at room temperature after heat treatment and this must be attributed to the increased efficiency of forced cancellations after the amorphous crystalline phase transition and this is the fingerprint that I mentioned before give it I'm coming to end so did you actually do it both ways heating up and then cooling down cooling back yeah now what happened I mean I made temperatures after 2 or 3 days I mean when I was sure that it's cooled back at room temperature and the friction I mean we found that friction is changed and at room temperature before and after heat treatment friction results are not the same we found this so the friction was dropping when you were and this was we also find the same behavior when we cooled the sample back to room temperature because it's surprising that it should go amorphous it should stay crystalline when you cool down the amorphous state is not in the beginning our nonparticles amorphous and when we cool when we heat stop this it became more or less crystalline and after this I'm cooling back to room temperature we argued that the nonparticles stayed crystalline so the friction should not go go up again go up again it doesn't go up again any questions from the remote audience have any raised hands here the phase transition is induced by the increasing of temperature basically do you have any idea if the sliding and the heat that you push into the system with the sliding tip would be enough to induce the same crystallization at some point because the temperature without increasing the temperature too much just by pushing the particle this will inject some heat maybe we can define this with the I because I mean in the beginning but I think I don't know but I don't think so the speed during the manipulation it was several hundreds of nanometers per second so it's another low speed I think it makes it cycling that I need it will take a very long time thank you very much