Phosphorene introduced as graphene alternative

Sheets of ultrathin phosphorus could lead to faster semiconductor electronics

CLOSER LOOK AT PHOSPHORENE  Sheets of phosphorous just one atom thick have a ridged structure, as shown in this illustration. Graphene sheets, which are made of carbon, are flat.

P. Ye

DENVER — Phosphorus has joined carbon as the only elements to be separated into sheets each a single atom thick, researchers announced March 7 at a meeting of the American Physical Society. The newly fabricated ultrathin material, dubbed phosphorene, could prove superior to its popular carbon counterpart for use in next-generation electronics.

Graphene, which consists of a flat honeycomb lattice of carbon atoms, has been the darling of materials scientists since 2004. That’s when Andre Geim and Kostya Novoselov peeled off sheets of graphite with Scotch tape, a simple step that produced graphene and was rewarded with the 2010 Nobel Prize in Physics (SN Online: 10/5/10). Among graphene’s remarkable properties is that it can shuttle electrons rapidly while hardly heating up, which led some scientists to predict that it could replace silicon in computers and other electronics.

However, graphene has one major and perhaps fatal flaw: It’s difficult to turn off the electron flow. Silicon is a semiconductor, allowing transistors in computer chips to switch electric current on and off; graphene is a conductor through and through. As a result, some physicists say it’s time to look for other thin materials that share the attractive properties of graphene but are semiconductors. “Graphene is beautiful, interesting and useless,” says David Tománek, a condensed matter physicist at Michigan State University in East Lansing.

While some scientists have moved on to compounds such as molybdenum disulfide, Tománek’s colleague Peide Ye, a condensed matter physicist at Purdue University in West Lafayette, Ind., kept it simple. He decided to explore black phosphorus, a stable variety of the element that forms under high temperature and pressure. He began by purchasing a chunk on eBay.

Ye, Tománek and their team measured the sample’s properties and plugged them into equations to predict how thin layers of the material would behave. The analysis suggested that phosphorene would transport charge better than silicon (though not as well as graphene); most importantly, it would be a semiconductor.

Then the researchers moved from theory to experiment. Borrowing the Scotch tape method from graphene manufacturing, the researchers isolated phosphorus layers just a few atoms thick and used them to fashion transistors. Then in December, Ye managed to exfoliate sheets just one atom thick.

Researchers seem intrigued but skeptical about the new material. “Nothing really comes close to graphene” in a variety of properties, says MIT chemical engineer Michael Strano, emphasizing phosphorene’s inferior charge transport speeds. But he adds that any new material with a unique combination of properties is exciting. Phosphorene could be useful as a two-dimensional Lego block that can stack with graphene, molybdenum sulfide or other thin materials to achieve distinct electronic properties.

In the meantime, Ye and Tománek are still studying the new material and working out how it might be useful. Their one-atom-thick sheets feature phosphorus atoms arrayed in hexagonal ridges, unlike graphene’s totally flat honeycombs. During a March 5 presentation, Tománek’s graduate student Zhen Zhu shared research predicting that phosphorus can exist in another stable, layered structure called blue phosphorus, offering yet another material to play with. Ye is planning to produce blue phosphorus, while also running experiments probing the performance of phosphorene made from black phosphorus.

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