Discovered two years ago by researchers at the University of Manchester, UK, and created by extracting individual atomic planes from bulk crystals, a new class of 2D one-atom thick materials has rapidly become a hot topic in physics. Now scientists at Manchester and the Max-Planck Institute in Germany have gone one step further to prove that a 2D gauze of carbon atoms, called graphene, can exist in the free state, with potential for use in a range of applications.
The team, led by Professor Andre Geim and Dr Jannik Meyer, manufactured the free-hanging graphene membrane by placing several golden wires across the material using lithography techniques. These layers were then put onto a silicon oxide chip, which was etched away leaving the graphene hanging freely in the air, or a vacuum, from the metallic scaffold. ‘We made membranes as an important starting point to study the [material's] mechanical properties,' explains Geim.
Using transmission electron microscopy (TEM), it was discovered that graphene's ‘intrinsic corrugation' produces stability in the usually unstable ultra-thin matter in the free state. Geim believes that the membranes, which ‘behave more like silk tissue than a solid material', could therefore exist even without the support of the scaffold.
Possible uses of the material include sieves to filter light gases, and the manufacturing of micromechanical switches and electronic transistors. The most immediate application, however, is to aid TEM. Geim says, ‘Transmission electron microscopy allows you to study very complex molecules, but you have to put them on a substrate that is transparent. This is usually an organic material, [which] gives a lot of signatures that are hard to distinguish from the intrinsic signatures of the sample. Graphene offers an incredible advantage. Not only is it the thinnest material possible, it is crystalline, so it has very well defined diffraction spots that you can easily rule out [in your analysis]. This is the most perfect substrate.'
He adds, ‘In the long term, these materials can be used in micromechanical devices. Graphene is about 1,000 times lighter and 10 times more rigid [than silicon], which allows you to increase all switching times by a factor of at least several thousand. One group has used [it] as a micromechanical resonator [Cornell University, USA].'
The team at Manchester has developed a range of proof-of-concept devices (see box below on ‘Transistor technology') and propose that the challenge now lies in ensuring that the membranes are cheap to manufacture and suitable for industrial production. ‘Our approach is not scaleable,' explains Geim. ‘The pieces of graphene we get are excellent quality, but they are only sub-millimetres [in size and] not good for mass production. But there are groups who have demonstrated graphene wafers that are scaleable. It remains to be seen if the quality is good enough for real [products]. We will know more in a couple of years.'
Given the rapid rate of progress in the field during the last two years, Geim suggests that reproducibility may well prove to be one of the main advantages that graphene has over the use of 1D carbon nanotubes.