25 March 2011 by Chris Edwards
At the meeting of the American Physical Society this week in Dallas, two groups of researchers are there to describe their work on a new form of silicon that seems to mimic graphene. It was first proposed theoretically in 2007 - nothing like silicene appears in nature. That is in stark in contrast to graphene, which you can make out of pencil leads and sticky tape - and some patience. As a result, silicene may turn out to be a chemical curio.
At the meeting, Antoine Fleurence of the Japan Advanced Institute of Technology, yesterday described how that team managed to grow flat sheets of silicene on top of a crystal of zirconium diboride. At the same meeting, Guy Le Lay's team from the CNRS research institute in France will today (Friday, 25 March) talk about their work in confirming the graphene-like structure of silicene having made the material for the first time last year on sheets of silver.
The question was not so much: can we make silicene? That's simply a matter of chemical curiosity. Synthetic chemists delight in making the improbable to test out theories of molecular structure. Fullerene, the forerunner of the carbon nanotube, was one of those curiosities that turned out to have a real-world use. The question was, having made it, does it behave like graphene?
According to the most recent data from the American Physical Sciences meeting in Dallas, the answer is: probably. The films made by Le Lay and colleagues suggest that the outer electron structure of silicene is quite close to that of graphene. They have evaluated this indirectly rather than passing a current through it. One strong hint is the way that the material looks similar to graphene when imaged with a scanning tunnelling microscope.
Another hint is that, normally, silicon oxidises very easily whereas this material does not. The silicon-silicon bond is somewhat weaker than the one between silicon and oxygen, in contrast to the situation with carbon, which has a much stronger, in relative terms, bond to itself. It's the reason why silicon lifeforms are the stuff of Star Trek. However, the silicon-oxygen bond can make for a very bouncy, inert combination when it's part of silicone.
Because this film does not oxidise so well, it indicates that the silicene has a similar bond structure to graphene where multiple electrons take part in bonding to neighbouring silicon atoms, strengthening the bond. The result is a material with the Dirac cones or delocalised electrons that make graphene such a good conductor of electricity.
So far so good. However, before we write off graphene as a contender for transistor material of the future it's worth bearing in mind that the selection of the substrate material for these silicene sheets is, so far, crucial. The nanoribbons may be even harder to deploy than those made with graphene, which is by comparison much more readily synthesisable. It could be the ultimate in ultra-thin silicon-on-insulator transistor materials - just one atomic layer thick. Or it could prove to be an interesting but confounding material that never quite delivers on the promise.
Although the fact it's the same core material as that used by a $300bn industry today, the manufacturing techniques needed to deploy it may turn out to be even more exotic than those needed to put graphene onto conventional wafers. Just as soon as the industry has worked out how best to use graphene: as a great conductor or a new wave of devices that exploit quantum mechanics.
Posted By: Chris Edwards @ 25 March 2011 10:06 AM General
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