Bending and stretching 2D materials to change their properties could lead to ultra-small sensors that can help us understand how gravity works at the microscopic scale, according to Professor Kirill Bolotin from Freie Universität Berlin, Germany, who leads the EU-funded Strained2DMaterials project to uncover what happens to 2D materials under strain.
2D materials, which are made from just one or a few layers of atoms, have been hailed as a new age in materials engineering. Why are they so special?
‘Let me highlight two things. Because 2D materials are so thin, they are extremely responsive to external factors. This sounds quite simple but can be very useful. So, for example, when you have the external factor being an electrical field, this material will respond to an electrical field and that’s what we want for a transistor (an electronic switch used to make computers), so that’s why they make great transistors. Or, say, they respond to the landing of external atoms or molecules, well that’s what we want for a sensor; or they respond to light, that’s what we want of an optical device. Once you decrease the material to the extent that every atom in this material is on the surface, that’s the limit of the sensitivity, and that leads to many useful applications.
‘The second reason has to do with how we make materials. It used to be, from the Stone Age, we played with the materials that nature gave us. If you’ve got iron, you can process it but it still remains iron. With 2D materials we can stack them up and essentially build matter atom by atom, layer by layer. And that’s really a big paradigm shift – you can now make things that you want, not what nature gives you. Once we can control the atomic make-up of the material, the possibilities are just limitless.’
You’ve been given funding by the European Research Council to investigate how 2D materials could be used to sense ultra-small forces and weigh ultra-light objects. Can you explain a bit about this?
‘The idea is that you now have material that is the lightest ever, and a light material should mechanically respond when you put a little bit of stuff on it or pull it a little bit. One example would be a sheet of graphene which is clamped on one side and is fully extended. Basically we want to understand how much force does it take to make it bend. It’s like the thinnest possible cantilever (and) our feeling is that it is one of the best force sensors ever.
‘There are many reasons why you want to measure small forces. To give you one example that motivates me, think about gravity. Gravity is extremely well studied, it follows Newton’s law (that it is inversely proportional to the distance between two masses), but it has only been tested at dimensions starting with astronomical down to millimetres. It has not really been tested on the micron scale. Now there are predictions coming from string theory (a mathematical way of thinking about elementary particles as manifestations of tiny vibrating one-dimensional strings) which suggest that if certain models are true, Newton’s law breaks down on the micron scale.
‘There’s actually quite a big quest to find who can measure the smallest possible forces due to gravity down to this kind of dimension. And we feel that this can be one of the ways to look into these extremely small forces and perhaps even test string theory. It’s not my goal to do this kind of measurement, it’s my goal to understand how this can work and see if it’s even a possibility. For me, as a scientist, my role is to find this link in the chain which I can fill and see how my abilities can fit into this chain that others can follow.’
How do you go about changing the strain on a 2D material?
‘It’s like you have thin sheets and you develop little knobs, little levers, to pull it, to bend it. It is almost like you are becoming a car mechanic (but) your car is only a few atoms thick.
‘In one approach, to stretch 2D materials, we use electrostatic forces. One applies voltage between the freely hanging 2D material supported by thin gold electrodes and the substrate (surface material) under it. The electrostatic force “pulls” the material, bending it.
‘In another approach, one can use thermal expansion. (With) any old material, when you change the temperature there is something called thermal expansion - its dimensions expand when you cool the material. So what you can do is you can take your 2D material, put it between two electrodes which are shaped properly, you can cool it down, the gold will contract and it will (stretch the 2D material).
'Finally, you can put your (2D) membrane onto a little hole and you can put a little bit of gas, and the gas will make a balloon.
‘It feels kind of satisfying to really do this stretching, bending, pulling materials which are only a few atoms thick, almost playing with matter at this scale.’
What do you think are some of the most exciting applications for 2D materials?
‘I (work) in relatively fundamental science but I can tell you what I like personally. There are a few very tantalising ideas of how by, for example, straining these 2D materials in just the right way, you can make them superconductive. It could be not just superconductivity, but superconductivity at very high temperatures and even perhaps room temperature. If it were to work, this would be a big revolution.’
Could you explain a bit more about that?
‘Electrical properties of materials are all about electrons. If you take an electron by itself in a vacuum, that is an elementary particle, which weighs this much, has this kind of speed. Now suppose we have a crystal, say graphene, and you put the electron into it. It turns out that because you have this crystalline structure, the properties of electrons change dramatically. To give you the most common example, let’s say the weight of an electron changes. It starts to weigh, say, 10 times less, or in the case of graphene, the weight of an electron can become zero. And it’s not an abstract concept. Electrons behave as if they would have this mass, as if they would have this energy, so the presence of a crystal lattice changes electrons quite a lot.
'It is almost like you are becoming a car mechanic (but) your car is only a few atoms thick.’
Professor Kirill Bolotin, Freie Universität Berlin, Germany
‘And now it’s natural to say, if the presence of this crystal lattice has such a big effect, when you stretch this lattice it should also have a really large effect on how electrons behave. This is not new. For example, transistors, which are normal silicon transistors, are made of strained silicon. It turns out that when you increase the silicon lattice just a little bit, you make electrons lighter a little bit and when they’re lighter, they travel a little bit faster. And in silicon, even if you make an incremental step, you save billions of dollars.
‘With 2D materials everything becomes more interesting. Because the material is 2D, you can do non-uniform application of strain, or you can do periodic application of strain and when that happens you can essentially determine quantum mechanical properties of your electrons on demand. You can really tune its properties. For example, it turns out that when these 2D materials are strained in just the right way, the electrons behave as if they were exposed to very strong magnetic fields. Because it’s as if, it doesn’t have to be a real magnetic field, so you can get this (simulated) magnetic field to levels which could never be achieved in the lab. And this magnetic field can push electrons closer to one another, making them repel less. This is exactly what is needed for superconductivity.’
Any other applications that interest you?
‘Another example that we’re trying to work towards is that we’re trying to put chemical and biological objects on top of 2D materials, and use (the material) as a platform to understand what’s happening in this chemical and biological system nearby. I am trying with colleagues to think of how to, say, visualise propagation of signals inside neurons by looking at the 2D material which is nearby. To me that’s very exciting because you combine different worlds and you could not really have done this before because you really need this atomic thickness to achieve this kind of sensitivity.
‘If I go even one step further I can tell you I have this vision of what would be really, really nice. So now we have different stacks of these 2D materials. We can build very thin devices which are very constrained and have a very small thickness and then let them float in your blood, and essentially we can start thinking of delivering a chemical laboratory inside something that is alive. If you can combine different functionalities of the 2D materials, you can imagine that you can pick up chemical or biological information, you can (translate) it into electrical signals and you can deliver it outside. And to me this kind of idea that you can deliver a lab inside what you want to measure, it’s kind of a big deal.’
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