17 January 2012
New technique paves way for mass production of high quality graphene nanosheets and quantum dots
Researchers have discovered a simple, low cost and environmentally friendly way to turn common graphite flakes into bulk amounts of either high quality graphene nanosheets or quantum dots.
According to a team from the University of Ulster lab of carbon based nanomaterials, such structures could lead to new nanoelectronics and energy conversion technologies.
The project was led by Pagona Papakonstantinou, professor of Advanced Materials at UU, who claims the simple process is quicker and greener than currently established techniques for industrial scale production.
Quantum dots are tiny islands of electrons which can be used as building blocks for controlling the flow of electrons at the single electron level. Graphene quantum dots can be created by cutting sheets of graphene into small islands of the desired shape. Past attempts to create high quality graphene quantum dots, have involved sophisticated equipment or expensive raw materials, which have resulted in low yields. Solution processes to produce graphene nanosheets and quantum dots in high yields have involved the use of strong acids or prolonged sonication, which introduce defects on the graphene nanocrystal.
The UU researchers' solution was to ground cheap graphite flakes with a small quantity of ionic liquid to produce a gel and subsequently clean the ionic liquid. According to Prof Papakonstantinou, the grinding in ionic liquid helps to simultaneously fragment and exfoliate graphite flakes into graphene nanosheets. Their size could be tailored by applying different the grinding times. When longer grinding times are used, graphene quantum dots with an average diameter of 10nm and a thickness of two to five graphene layers are the dominant products.
The most important attribute of the produced graphene nanosheets and quantum dots is that they are clean from any solvent contamination and possess a low concentration of oxygen, which is inherited from the starting graphite powder. Supported by other microstructural investigations, the researchers believe this suggests that the centre of graphene nanosheets and quantum dots is free of defects and therefore it would be possible to maintain very high mobilities suitable for nanoelectronic devices.
Prof Papakonstantinou said: "Our procedure is mild and relies on pure shear forces to detach the graphene layers from the graphite flakes. Therefore, in contrast to other techniques reported so far, severe defect formation on the crystalline plane of graphene is avoided. No acids or prolonged sonication are used, resulting in high quality material. Moreover our method has the potential to be applied to other layered materials such as MoS2 or BN in addition to graphite. "
Dr Nai-Gui Shang, a researcher at UU, added: "Grinding is a Chinese traditional way of making ink for calligraphy and painting for over two thousand years, where the ink is produced by grinding the ink stick in a ink slab, mixed with a small amount of water. We thought why not try it with graphite flakes? Here, ionic liquid used as a novel green grinding agent, plays a critical role in the both good quality and high yield of graphene nanostructures. We believe that graphene nanostructures produced in this way can be applied successfully to inkjet printing of nanoelectronics."
The UU researchers are currently exploiting the large amount of catalytic edges for energy conversion applications.
The results have been published online this month in the Royal Chemical Society's journal Chemical Communications .