Monday, January 27, 2014

Graphene rival 'phosphorene' is born to be a transistor

Graphene rival 'phosphorene' is born to be a transistor

It may have won its discovers the Nobel prize, but graphene now has a serious rival.

"Phosphorene" – which has a similar structure to carbon-based graphene but is made of phosphorus atoms – is a natural semiconductor and so may be better at turbocharging the next generation of computers. The new material has already been used to make rudimentary transistors.

Discovered 10 years ago, graphene is a form of pure carbon just a few atoms thick. This thinness causes electrons to zip across it much faster than they do across silicon, the material at the heart of today's computer chips. So the hope is that graphene chips could eventually replace silicon, leading to much faster computers.

But graphene has a fundamental limitation, says Peide Ye at Purdue University in West Lafayette, Indiana. It conducts electricity a little too well.

Sticky tape

By contrast, silicon is a semiconductor, meaning it can be made to conduct electricity – or block its flow. This switching capability is the defining feature of transistors and makes possible the binary logic at the heart of a computer chip. "Because of that I don't think graphene can really compete with silicon for integrated circuits," says Ye.

Enter phosphorene, which is ultra-thin like graphene but made of phosphorus atoms instead. Most exciting of all, like silicon, it is a natural semi-conductor. Ye, who has spent many years studying graphene, discovered the stuff after a tip-off. "One of my colleagues mentioned that there is a form of phosphorus called black phosphorus that might be a layered structure," he says. "When I looked up its properties on Wikipedia, within about 30 minutes I knew that it had potential."

Ye and his colleagues found that they could use ordinary sticky tape to peel away sheets just a few atoms thick from black phosphorus crystals. The same technique was famously used to peel graphene sheets from chunks of graphite – the material used to make pencil lead. "We call it the Scotch tape technique – it's very low-tech," says Ye.

The researchers have already fashioned simple transistors out of thin layer phosphorene, and shown they can be integrated with other two-dimensional materials and with silicon.

Suicidal silicene

Phosphorene seems to be gaining momentum. Work with phosphorene transistors has also been done by Yuanbo Zhang of Fudan University in Shanghai, China, and colleagues.

Sankar Das Sarma of the University of Maryland in College Park says that phosphorene's natural semiconductor properties give it an edge over graphene. "That's something that cannot yet be easily done using graphene."

Whether phosphorene can compete with silicon has yet to be seen, particularly as it is unclear how to reliably produce the larger sheets that would be required for applications. "Obviously, technology cannot depend on Scotch tape extraction," says Das Sarma.

Phosphorene seems to have advantages over rival thin-layered materials. Silicene, which consists of super-thin layers of silicon for example, is a semi-conductor like phosphorene and in theory may conduct electricity particularly efficiently, like graphene. But it is harder to make than phosphorene or graphene. You can't just peel layers of it off a chunk of silicon – and it has a tendency to self-destruct.

And stanene is a proposed ultra-thin material made of tin atoms that might have similar properties to graphene, but as yet it only exists only in simulation.


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