A better understanding of graphene
Story By Ruth Beran
“We make power bits run around in dark stuff” was the title of Chun Y Cheah’s winning entry in the Up-Goer Five Challenge* held at The MacDiarmid Institute’s ninth student and postdoc symposium held in November 2013. He was describing his research using only words from a list of the ten hundred most common ones.
“I couldn’t use words like pencil or carbon, so I said ‘dark stuff’, and instead of electricity, I said ‘power bits’. The ‘bits’ is the way I explained electrons,” says Cheah.
He also couldn’t use the word ‘graphene’, the two-dimensional form of carbon that is the topic of his research.
Graphene was first produced in the lab in 2004, when British scientists put graphite (the same material that is used in ordinary pencils) in between pieces of adhesive tape and repeatedly stuck the tape together and then pulled it apart, until eventually they were left with a layer of carbon that was one atom thick.
While the extraction of graphene resulted in a Nobel Prize in Physics in 2010 for its creators, it is no longer produced by sticking pieces of adhesive tape together. And as a completely new material, graphene’s physical properties are still intriguing scientists like Cheah and his supervisor Professor Alan Kaiser, an Emeritus Investigator of the MacDiarmid Institute for Advanced Materials and Nanotechnology.
“Graphene has turned out to be a wonder material. It conducts electricity as well as copper,” says Cheah. It is also nearly transparent (as it is only one atom thick), is remarkably strong for its weight (being 100 times stronger than steel), and outperforms any other material as a conductor of heat.
|Fig 1: The red line shows the regime where conductance is dependent on temperature, the green line shows the regime where conductance is dependent on the applied electric field, the two red arrows illustrate the crossover between the two regimes at 40 K.|
To study the electronic properties of graphene, collaborator Cristina Gómez-Navarro (who is currently based at the Universidad Autónoma de Madrid) created chemically-derived graphene, where graphite was oxidised to form layers of graphite oxide.
This material was then floated in water, and very, very thin layers of graphene oxide were separated out. Hydrogen was then flowed over the layers of graphene oxide to eliminate the oxygen, with the hydrogen combining with the oxygen to form water. Electrodes were then attached to the resulting microscopic piece of graphene and electricity flowed through it at different temperatures and voltages.
“This chemical procedure results in graphene where the quality is not perfect throughout the whole material,” says Cheah. “So you have places where it is more perfect for electrons to flow from A to B but there are also regions where we have defects or disorder, where the resistance is a bit higher because the crystal structure is not arranged properly.”
When electrons travel through this so-called ‘partially disordered graphene’, the imperfections act like roadblocks or obstacles but the electrons do not jump over these areas, they tunnel through in a process known as quantum tunnelling.
“This is a quantum phenomenon, so in the world of the very small we see strange things happen,” says Cheah.
On top of this, electrons will not necessarily travel to the nearest energy state, they may travel to an area that is further away in order to find an energy state that is more or less equal to it. This is a phenomenon known as ‘variable range hopping’.
|Fig 4: The previously reported experimental
data from a three-dimensional carbon network shows a remarkable similarity to the results
found for disordered graphene.
Variable range hopping in conducting materials has been studied previously and two separate theoretical frameworks about how electricity flows through the material have been published separately in the literature.
What Kaiser and his team have done is created a unified model of electronic transport through this type of graphene, allowing better predictions in the future.
“We had papers saying that conductivity depends on temperature and we had other papers saying that it depends on the electric field, but we have never seen both of these considered together,” says Cheah. “What we’ve done is brought both of these theoretical frameworks together.”
At temperatures close to room temperature (220 K), conductivity through the disordered graphene is temperature dependent.
“As you crank the temperature downwards to 40 K, at a certain point conductivity ceases to depend on temperature anymore,” says Cheah.
There is a crossover point, where conductivity in the graphene depends solely on the electric field (ie. the drain-source voltage applied to it).
“What is even more interesting is that if we crank the temperature even further down to 2 K, which is close to absolute-zero temperature, we no longer see any sign of conductance depending on temperature. The whole thing depends solely on the applied electric field,” says Cheah.
What is more, this ‘crossover scenario’ also accounts for other experimental data published over a decade ago by other scientists studying another three-dimensional carbon material.
“We included their data to show that our model doesn’t just apply for graphene but also applies for three-dimensional carbon networks as well.”
Cheah believes that this new model of electronic transport, which shows that there are in fact two regimes of variable range hopping at play with a crossover point at 40 K, ‘will allow a lot of physicists and chemists to understand their data in the future’.
“What we’ve done is increased the fundamental knowledge,” he says. “It will not necessarily spill over into practical applications yet.”
Cheah expects to submit his PhD thesis around the end of the year, and for the past three years he has received a MacDiarmid Institute scholarship. His research extends on work done previously by Lina Jaurigue, an honours student who was also supervised by Emeritus Professor Alan Kaiser.
C Y Cheah, C Gomez-Navarro, L C Jaurigue and A B Kaiser “Conductance of partially disordered graphene: crossover from temperature-dependent to field-dependent variable-range hopping” J.Phys.: Condens. Matter25 (2013) 465303 (5pp)
Fig 1 & 4 © 2013 IOP Publishing Ltd
Portrait courtesy of Image Services, Victoria University
*The Upgoer Five challenge got it’s name from this comic!
Find out more about the MacDiarmid Emerging Scientist Association.