In the latest episode of DWDD University on 'Light', Prof. Robbert Dijkgraaf used a number of experiments from the Leiden Physics Practicum Lab. He showed why the sky is blue, that light is a wave phenomenon, how magnetism and electricity More info
are connected and that white light consists of all colors of the rainbow.
Materials are either gas, liquid or solid, based on how their molecules respond to temperature and pressure. But what if the building blocks are self-spinning particles instead of ordinary molecules? Theoretical physicists found out what determines the phase of those More info
When water reaches 100 °C, it turns into a gas phase. At sea level, that is. If you take away some air pressure, water will boil already at colder temperatures. It is clear that materials made up of ordinary molecules take on a phase depending on temperature and pressure. Leiden theoretical physicist Prof. Vincenzo Vitelli wondered what happens if materials have self-spinning dimers as building blocks instead.
To this end, first authors Benny van Zuiden and Jayson Paulose simulate self-spinning dimers on their computer and study how they organize themselves. When they apply a gradually increasing pressure on them, they see the system change from an ordered state to a very chaotic state.
In the figure below we see on the left a beautifully ordered state, with dimers neatly forming a triangular crystal lattice. Moreover, the relative orientation of nearby particles are locked as they spin.
At the far right, the concentration is so high that the system gets stuck in a glassy phase. Remarkably, there is a liquid phase in between. Usually a substance will become more solid as its density increases. Here the opposite happens.
So how can there be a liquid state? With low density, the dimers have plenty of room to move as they wish and stay in sync, like a group of stage dancers. When the stage is too small, dancers will bump in to each other and they chaotically move around, as particles in a liquid. If the stage however gets so tiny that dancers are unable to move, they get stuck in a disordered configuration reminiscent of a glass.
Graphene and other layered materials combine into completely new substances. Leiden physicists establish the ground rules for designing such materials by measuring how the layers in the stack interact. Publication on November 29 in Nature Communications.
of graphene—a single layer of carbon atoms—have been widely celebrated. Not only does graphene exhibit remarkable physics, it also shows great promise for new applications, like flexible display screens and solar cells. But scientists aren’t easily satisfied. The hunt is on for the next generation materials: layered stacks composed of single sheets of ‘flat’ materials like boron nitride (BN), graphene (C) or tungsten disulfide (WS2).
Sum of parts
The trick is that such a layered cake is not just the sum of its parts. You might get properties completely different from those of the individual layers. This even goes for two layers of the same sort; bilayer graphene is in no way like its monolayer cousin. It all depends on how the layers interact. Leiden physicist Sense Jan van der Molen and his group have developed a method to figure out the rules of the game. They can now determine the interaction between layers in each combination of materials.
Using a technique called Low-Energy Electron Microscopy (LEEM) they shine electrons of very low energies at a sample. For every energy level, they record an image of the surface, telling them how many electrons are reflected. This gives them all the necessary information to determine the interlayer interaction and therefore the properties of the newly created material. Their method resolves details 100,000 times smaller than other techniques. This is crucial because novel nanomaterials are typically extremely small—less than the thickness of a human hair.
‘We used our method to prove that boron nitride and graphene do not interact with each other as was only assumed so far,’ says first author and Veni fellow Johannes Jobst. ‘But more importantly, it shows the potential of this novel technique. Now we can study any other combination of layers, like semiconductors on graphene, or two different semiconductors. And once we understand how this interaction works, we can freely design materials that are tailored to specific needs.’
Johannes Jobst is nominated for Discoverer of the Year at Leiden University’s science faculty.
Dr. Sense Jan van der Molen
Principal Investigator, Leiden University
molen [at] physics.leidenuniv.nl
Physics Outreach Officer, Leiden University
arends [at] physics.leidenuniv.nl
+31 (0)71 527 5471
Leiden physicists study stacks of layered materials using a novel technique. They can now answer the question whether a given stack of various materials has properties different from its constituents by probing the interlayer interactions. They employed this method to verify that graphene (grey) interacts strongly with graphene, and boron nitride (purple) interacts strongly with boron nitride, while graphene is not influenced by the presence of boron nitride. We see on the upper right the resulting material: different properties (shades) for combined graphene + graphene and boron nitride + boron nitride, but no interaction between graphene and boron nitride. On the bottom right we see a hypothetical state where all layers interact to form a completely new material, which is not the case in this example.
Graphene holds the promise of impressive applications such as wear-resistant, friction-free coatings. But first manufacturers have to be able to produce large sheets of graphene under precisely controlled conditions. Dirk van Baarle studied how graphene grows at the atomic scale More info
and what determines the friction with other materials. PhD defence on 29 November.
An almost perfectly friction-free, wear-resistant coating in machinery could generate enormous savings in fuel and maintenance. In the world of nano-technology such coatings will probably even have applications that we are currently not able to predict. In his PhD research Dirk van Baarle studied a candidate for such coatings: graphene. Van Baarle: 'It's quite a challenge to produce graphene of a predictable quality.'
Graphene is only super strong if the wire mesh of carbon atoms that make up the material are perfectly regular in form. But with the present production methods, a sheet of graphene is in practice almost always made up of a patchwork of small pieces that have been grafted onto one another. Van Baarle was able to observe almost per carbon atom live how islands of graphene grow towards one another and how this process is influenced by temperature and substrate. This is the first step towards a production method for making larger, flawless sheets of graphene.
Chicken wire pattern
Graphene occurs spontaneously when a very clean surface of iridium comes into contact with ethylene (C2H4, a hydrocarbon) at a temperature of around 700 degrees Celsius. The gas molecules disintegrate on the hot surface, leaving behind the carbon atoms, which spontaneously form a network of linked hexagons, in a chicken wire pattern.
For his research Van Baarle used a unique piece of equipment in the Huygens-Kamerlingh Onnes Laboratory, the VT-STM (Variable Temperature Scanning Tunneling Microscope). This apparatus comprises a minuscule stylus with a point that is just a few atoms thick. It can be used to systematically scan a surface with such a high degree of precision (what you are in fact doing is measuring the flow of electricity between the stylus and the surface) that even individual atoms can be distinguished. What makes the Leiden instrument unique is that it can do this even at high and variable temperatures.
A remarkable finding is that atomic processes occur not only in the growing layer of graphene. In practice, the surface of the iridium does not match the atomic layers in the substrate perfectly. The iridium forms broad steps on the surface, where the graphene grows over it. But these steps can continue to grow underneath the graphene or can withdraw as a result of the iridium atoms in the substrate realigning themselves. This process, too, has to be closely controlled in order to allow perfect sheets of graphene to form.
In the theoretical part of his research, Van Baarle developed a model of how friction occurs at atomic level. When two surfaces slide over one another, the actual contact points are only nanometres in size, just a very few atoms. The friction is at its maximum when the stiffness of the nano-protrusions is roughly average: not too soft, but also not too stiff.
Van Baarle: 'One of my colleagues is currently coating an object with nano-needles using a lithography technique (a technique that is also used for computer chips). These needles vary in stiffness, depending on the direction in which they bend. This means that the friction of the surface is different in different directions.' This can be useful, for example, for a coating on a revolving axis, to prevent it moving laterally.
'Internally we are already using graphene coatings in our equipment to reduce friction without using lubricants,' Van Baarle explains. 'It has already resulted in a patent and a start-up, Applied Nanolayers. No wonder our professor, Joost Frenken, has already won a valorisation prize.'
Casper van der Wel and Daniela J Kraft (2016) Automated tracking of colloidal clusters with sub-pixel accuracy and precision, Journal of Physics: Condensed Matter, Nr. 4 Special Issue: Emerging Leaders, 29, 044001. [DOI]
Vera Meester and Daniela J. Kraft (2016) Spherical, Dimpled, and Crumpled Hybrid Colloids with Tunable Surface Morphology, Langmuir, 32 (41), 10668-10677. [DOI]
9 Dec, 16:00
Van der Waals Colloquium TBA:
14 Dec, 12:30, Academy building, Rapenburg 73
Thesis Defense Marija Mučibabić (BSM): “Intricacies of α-Synuclein Aggregation“ Promotores: Prof.dr. T.J. Aartsma & Prof.dr. G.W. Canters.