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FRONTIER RESEARCH
INTERVIEW
2D materials could expand our
understanding of gravity – Prof.
Kirill Bolotin
01 September 2016
by Joanna Roberts
Prof. Kirill Bolotin, Freie Universität Berlin, Germany
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
See also
Atom-scale sensors and
quantum bits – scaling the
possibilities of 2D tech
Tales of the second
dimension
Making materials just one
atom thick
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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.
Atom-thick solution to energy
storage conundrum
Molecules rev up for world’s
tiniest race
‘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?
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‘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.
‘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.
'It is almost like you
are becoming a car
mechanic (but) your
car is only a few
atoms thick.’
Professor Kirill Bolotin,
‘With 2D materials everything becomes more
Freie Universität Berlin,
interesting. Because the material is 2D, you can do
Germany
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.’
More info
Strained2DMaterials
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