08 November 2011
UK graphene research leading the field
The UK is setting the pace in the race to develop and commercialise graphene and this lead looks set to continue with Chancellor George Osborne pledging £50million for a 'Global Research and Technology Hub'. Indeed, Universities and Science Minister David Willetts believes the 'wonder material' has the potential to drive UK economic growth.
Graphene was discovered at the University of Manchester by Professors Andre Geim and Konstatin Novoselov. As a result, the two were awarded the 2010 Nobel Peace Prize in Physics. The one atom thick material is considered by many to be a natural successor to silicon and, since Profs Geim and Novoselov's discovery, the University of Manchester has intensified its research.
Dr Leonid Ponomarenko, from the University of Manchester's School of Physics and Astronomy, specialises in the electronic properties of graphene and is working on a new technique to control the material in a way previously considered impossible.
By sandwiching two sheets of graphene with boron nitride, another two dimensional material, his team has developed a four layered structure. Because two layers are completely surrounded by boron nitride, it's been possible for the first time to observe how graphene behaves when unaffected by the environment and how it reacts when encapsulated by another material. As a result, new phenomena such as metal insulator transition can be observed.
"We observed that graphene's properties don't change over time," said Ponomarenko. "And that's the beauty of it. But work still needs to be done to see if other materials can do the same job. Graphene is sensitive to its environment and protecting it from both sides is important if we want to keep its properties under control."
The team worked with boron nitride due to its similarity to graphite and the fact it can be peeled with sticky tape down to a single atomic layer. Dr Ponomarenko noted: "Boron nitride is a very good insulator, while graphene is an extremely good conductor of electricity.
The surface of boron nitride is atomically flat, which, in combination with its insulating properties, makes it a perfect material as a substrate for graphene. In my opinion, if graphene electronics become a reality, there is no better substrate material than boron nitride on which to fabricate a graphene chip."
Nanoribbons have great potential
Nanoribbon research is the focus of Dr Andrei Khlobystov from the University of Nottingham's School of Chemistry, who believes nanoribbons have great potential in the production of nanomaterials for use in next generation computers and data storage devices. "One of the real problems with graphene is that it has no electronic band gap; if you want to make a transistor, you should be able to turn it on and off," he said. "Although you can manipulate graphene to some extent, it is always 'on' so it's not a good transistor. Once you start cutting a 2d sheet of graphene into ribbons, it's possible to develop a real electronic band gap, so nanoribbons provide realistic opportunities to introduce graphene into electronics."
Currently, nanoribbon preparation involves taking a piece of graphene and cutting it with an electron beam or chemical etching. However, this means it's only possible to make one nanoribbon at a time and results in poorly defined edges. "We needed a new technique to enable mass production and create atomically smooth edges," said Dr Khlobystov. "Our new method addresses both issues."
The team put molecules containing carbon and other elements in a nanotube and used this as a template to limit the growth structure in two dimensions, while allowing growth in one dimension. "You can't form a 2d sheet of graphene," noted Dr Khlobystov, "If you simply put carbon atoms in a nanotube, provide lots of energy and let them rearrange into a ribbon, you only get another nanotube within the first nanotube. So another element must be added to attach itself to."
The team tried a number of elements, but the only one to work was sulphur. "Sulphur attaches itself along the edge of the ribbon, stabilises it and allows nanoribbon formation instead of the 'tube within the tube' which would normally happen." Once the molecules containing carbon and sulphur break down into individual atoms and reassemble into the ribbons, the nanoribbons have atomically smooth edges.
Catalysing the process
Robert Weatherup, part of the Hofmann research group within the University of Cambridge's Department of Engineering, is looking to grow larger areas of graphene using chemical vapour deposition. With this, a catalyst film is exposed to a carbon-containing gas at elevated temperatures. Graphene assembles on the catalyst surface.
"We use alloys as the catalyst film," said Weatherup. "This means we can tune the graphene growth by tuning the catalyst alloy and thus achieve high quality monolayer graphene growth at low temperatures."
By adding a tiny amount of gold to the surface of a nickel film, graphene could be grown at 450°C, rather than the 1000°C normally required. At the higher temperature, many of the materials used in electronics manufacturing can be damaged, so graphene can't be integrated directly. By using gold, the number of places where graphene grows on the film is reduced because the alloy blocks its growth. As each graphene flake emerges, it grows larger and for longer before it joins with another flake. The conductivity of the graphene is improved because electrons travelling through it are not disturbed as often by joins between flakes.
"Obtaining growth at this temperature is a big step forward," Weatherup observed. "The main benefit of reducing the growth temperature is that we can then grow graphene directly on to materials such as plastics, which are damaged by higher growth temperatures. This could open the gateway to flexible electronics."
Nothing less than revolutionary
So, once commercialised, how do the researchers believe graphene will change the electronics industry? Dr Ponomarenko describes the material as nothing less than 'revolutionary'. "The size of an individual transistor can be drastically reduced, probably down to 10 atoms across if it's made of graphene," he said. "This will revolutionise electronics, making it more powerful and much faster."
According to Dr Khlobystov, the biggest hurdle is the transition from cutting edge research into mass production. "It's a job for engineers to think about," he noted. "Carbon nanostructures have a whole spectrum of exploitable properties. Carbon is cheap, abundant and, when we get the technology right, it could replace almost all metal in electronics – critical, as we are running out of rare metals."
Weatherup believes graphene's first large scale commercial use will be as a transparent conductor for flexible touch screens – replacing the expensive and brittle indium tin oxide. "I then expect it will see use for individual high performance electronic components such as high frequency transistors used in microwave electronics which could be integrated with existing silicon electronics," he added. "Given the proven track record of silicon electronics it seems unlikely graphene will replace it, but more likely that graphene's amazing properties will be integrated with existing silicon based devices to add new functionality and performance."
Author
Chris Shaw
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