Strengthen through defect

A material formed from a two-dimensional layer of carbon atoms, grapheme combines extraordinary mechanical strength with electrical mobility. Projects spoke to Biplab Sanyal of the University of Uppsala about his work in creating defects in graphene, work designed to enhance its properties.

A material formed from a twodimensional layer of carbon atoms, grapheme has attracted a great deal of attention since it was first described theoretically in Phillip Wallace’s paper of 1947.

Much of the focus has been on developing a method for synthesising the material, and Andre Geim and Konstantin Novoselov were awarded the Nobel Prize for Physics in 2010 for their work in extracting graphene from graphite, bringing its potential to wider attention. “Graphene has many some extraordinary properties – it is the thinnest material possible, just one atom layer thick, and it has extraordinary electrical mobility.

Electrons can travel in graphene without any resistance over a large space and without any scattering. That means it has enormous electronic capabilities, like in silicon electronics; but even with silicon electronics we cannot go down to a one-atom layer thickness, which we can achieve with graphene. It is also very strong, it has extraordinary elastic properties,” says Biplab Sanyal. An Associate Professor at the University of Uppsala in Sweden, Sanyal is a partner in the Functionalization of Graphene project, which aims to engineer defects in graphene so as to enhance the properties of the material. “People from chemistry, physics and engineering sciences are involved in the multidisciplinary project, they all have important contributions to make,” he says.

“We are trying to modify graphene by creating defects in it. We then manipulate the properties in a controlled way to develop the properties we want to achieve.”

Electronic structure

The electronic structure of graphene is crucial to its properties. At certain points in the energy-momentum space in graphene a type of linear cone can be observed, whereas in other materials there is a parabolic shape; this linear dispersion quality is only found in a mono-layer of graphite, i.e., grapheneene. “If you go to a bi-layer you don’t have that linear property, you have a parabolic kind of shape in the energy-momentum space,” explains Sanyal. The carbon atoms in graphene are arranged in a hexagonal lattice within which two sub-lattices, A and B, have been identified. “Despite the fact that these sub-lattices are both comprised of carbon atoms they are not identical. This sub-lattice effect plays an important role in determining many properties of graphene, both in bulk and in nanostructures,” continues Sanyal. “I am a computational materials scientist, so my group calculates the properties of materials using quantum mechanical methods, we use the Schrödinger equation to find the different properties of materials with reasonably good accuracy combined with an efficient predictive power. In doing that we use density-functional theory (DFT), which was developed by Walter Kohn in the ’60s; we’re basically re-casting the Schrödinger equation in a simple form by which we can easily solve the electronic structure problem for solids. Before the work of Walter Kohn people didn’t know how to calculate the properties of solids. There are up to 1023 electrons in a solid, which is beyond the computational power of even the largest supercomputer currently available.”

Particles in graphene move in a way similar to relativistic particles, which can move at the speed of light, and are called massless Ddirac fermions. Although fermions are electrons, they behave in graphene like massless particles, which Sanyal says has real implications for the overall behaviour of the material. “The famous physicist Paul Dirac wrote an equation for relativistic particles, and if you put Mass=0 into that equation then you get the properties which you can see in graphene. That’s why the particles moving in graphene are called massless Ddirac fermions. This can be proved theoretically,” he outlines. This knowledge of the electronic structure of graphene can be used to enhance its properties. “We want to control the properties of graphene, its conductivity for example, which means how the electrons move,” continues Sanyal. “If you treat graphene with a particular acid then certain defects are produced that increase the conductivity. These defects are vacancy defects, small holes in graphene. We first did a theoretical calculation, and we predicted that if you had particular defects in graphene you might get properties that would enhance its overall conductivity. Our experimental colleagues did a lot of research, and they found that the conductivity of graphene is increased if you treat it with acid and create defects. But the defects have to be created in a controlled way.”

The project is now trying to create these defects by bombarding graphene with ions or electrons and trying to make some small holes in the material. This can induce either ionic or electronic interaction. “If you use low-energy ions then it will lead to ionic interaction, it will knock some carbon atoms off the lattice and create a hole. But there are also electrons or ions with very high energy, which will lead to electronic interactions.

The ions may not move so much, but electronic interactions can change some properties of the material. We are talking about knocking off say two or three atoms and making a very small hole in the grapheme lattice. So this requires really precise measurements,” stresses Sanyal.

After these defects were identified Sanyal and his colleagues looked at their effect on the conducting properties of the material. “We found that the conductivity is increased if you have a lower concentration of defects. If you have lots of defects in the lattice then there will be extra scattering and the conductivity will actually go down. You need small concentrations of these vacancy defects in the lattice to really increase the conductivity,” he explains. “We are working closely with our colleagues in organic chemistry, and this effect was already known from research into carbon nano-tubes, the kinds of defects which can be produced by chemical treatment. If you cut a carbon nano-tube and make it flat then you find it is actually graphene – so we can expect similar properties in graphene.”

Enhancing properties

The enhancement of graphene’s conductivity through the creation of defects was predicted by theoretical research, and then reinforced by practical experiments, both producing very similar results. It should be stressed that this area of research on graphene does not concentrate on preserving the Dirac cone in the energy-momentum space, rather on a controlled manipulation of the properties of graphene by deviating from the Dirac physics.

However, the nature of graphene means it isn’t always possible to predict the precise effects of defect manipulation; the project aims to both enhance the existing properties of graphene and introduce new ones entirely.

“Maybe we could bring in some other molecular agents close to graphene and open up a gap in the material. If you want to use graphene as a transistor for electronic devices you need to have a small gap,” explains Sanyal. A number of methods have been suggested for introducing a band gap into graphene; the multi-disciplinary scope of the project takes on real importance in this respect. “We are trying to combine molecular physics with graphene physics. So we are bringing in organic molecules which already exist, and are well-known, but people don’t know how they will interact with graphene to change its properties,” says Sanyal.

“We’re combining two different areas of expertise to affect the properties of graphene and open up a gap in the material. Here the predictive power of quantum simulations will boost the research to identify suitable molecules to chose among thousands of existing ones. Moreover, this research raises new questions– people are even talking about how to introduce magnetism in graphene. How can you incorporate spin in graphene? How will spin propagate in graphene?” This work holds real implications for the commercial sector. Major electronics companies are keen to reduce the size of the transistor tube, but it is impossible to go down to a one-atom layer level of thickness with conventional materials, demonstrating graphene’s wider potential. “Graphene is already a one-atom layer thick material,” points out Sanyal.

Graphene is also very strong, and its mechanical properties mean it could be used in a range of devices. “People are talking about using graphene in sensors, because it can absorb different types of molecules. You could use the properties to develop gas sensors for example, there are many possibilities,” continues Sanyal. “I aim to find out what new functions graphene could have and how to tune its properties to develop new functionalities.

As graphene is one-atom thick, it serves as a ideal playground for surface science, which I will explore in my simulations combined with the research of my experimental colleagues. I want to be able to suggest things to the experimentalists, and say ‘if you do these things in your lab then you can develop some novel properties in graphene.’ In the coming few years at least my research will be focused on graphene.”

Published: Friday, 1st July 2011