Graphene-like form of silicon proves hard to handle.
In 2011, physicist Guy Le Lay stood before a half-filled room on the last day of the American Physical Society’s March meeting in Dallas, Texas, and presented data on a new form of silicon. In his laboratory at Aix-Marseille University in France, Le Lay had grown sheets of honeycombed silicon with layers just one atom thick. He had only preliminary evidence that was unpublished at the time. “It was a risk, you know?” he says now of his decision to present the data.
At this year’s meeting, on 18–22 March in Baltimore, Maryland, scientists will deliver about two dozen talks on silicene (see ‘Speaking of silicene’), the material that Le Lay tentatively described two years ago.
The name recalls graphene, the current darling of the materials-science world — and the flurry of interest suggests that silicene could be the next one. But for that to happen, Le Lay and others will have to overcome silicene’s unfortunate tendency to stick to practically everything it touches.
Structurally, silicene looks a lot like graphene, which is also a honeycombed sheet, but of carbon atoms rather than silicon. Silicene’s two-dimensional structure should lead to strange quantum effects and allow electrons to streak across it at incredible speed — properties that, in graphene, have entranced physicists and builders of electronic devices since it was first characterized in 2004. In 2010, work on graphene won a Nobel prize, and earlier this year, graphene research was selected by the European Commission as one of its billion-euro flagship projects (see Nature 493, 585–586; 2013).
Silicene could even have some extra attractions. It is predicted to have characteristics similar to topological insulators — materials that conduct electrons only on their outer surfaces — another trendy area of research.
Above all, silicene is made of silicon, the same material that drives the modern electronics industry. Bringing it all together could lead to “a new era” in silicon electronics, says Kehui Wu, a physicist at the Chinese Academy of Sciences’ Institute of Physics in Beijing.
There’s just one problem: silicene is super sticky. “Graphene is a very stable material,” says François Peeters, a condensed-matter theorist at the University of Antwerp in Belgium. But silicene reacts easily with the environment — oxidizing in the air and bonding chemically with other materials. And unlike graphene, which lies flat, silicene crinkles into bumps and ridges as a result of the way neighbouring silicon atoms bond with each other. That makes it more likely to stick to surfaces.
Silicene’s reactivity makes it much harder to produce than graphene. The Nobel prizewinning work on graphene began by peeling sheets from a block of graphite with a piece of sticky tape. Silicene, by contrast, can only be grown in an ultra-high vacuum on top of a material that matches its natural structure (see ‘Making silicene’).
Crystalline silver has proved to be the best fit because its atomic structure allows it to lock together with silicene’s wavy ridges, and the silver’s non-reactive surface means that it doesn’t pull the silicene apart, Le Lay says. Reversing a technique he honed for depositing silver onto silicon, Le Lay grew the first samples of silicene on silver1.
Only two other materials have been found to support silicene up to now. One, zirconium diboride, has the advantage of naturally sucking silicene onto its surface from a block of silicon positioned below2. The other, crystalline iridium, was reported as a possibility only in January this year3.
Unfortunately, all three of these materials conduct electricity, says Yukiko Yamada-Takamura, a materials scientist at the Japan Advanced Institute of Science and Technology in Nomi. The bulky conductors mask silicene’s delicate electrical properties, making it impossible to check whether the theoretical predictions of strange quantum effects are correct.
To see if silicene performs as expected, experimentalists will have to find a semiconducting or insulating surface on which to grow it. Better still would be to develop a technique to create free-standing sheets of silicene, Yamada-Takamura says. It’s not entirely clear how that would be done, but given the increasingly competitive nature of the field, she says, “I will not tell you even if I had an idea.”
As Peeters isn’t racing to grow silicene himself, he’s more willing to speculate. He thinks that sandwiching silicene between two sheets of another material, such as graphene, could stabilize it and prevent it from reacting with the outside world.
“I think if it will be used, it will be used in sandwich form, because that’s the way in which you can stabilize it,” he says. The outer sheets “can be any material; it really depends on what you want to do with it”.
Despite its troubles, silicene’s future looks bright. It has been included as part of Europe’s massive graphene programme, and is catching on in the United States, Le Lay says.
The talks on it at the March meeting are likely to be more popular than they were two years ago, but Le Lay won’t be there to find out. He’s too busy giving seminars at departments everywhere from Hawaii to Austria to Japan. “It’s crazy,” he says. “But it’s good!”
Vogt, P. et al. Phys. Rev. Lett. 108, 155501 (2012).
Fleurence, A. et al. Phys. Rev. Lett. 108, 245501 (2012).
Meng, L. et al. Nano Lett. 13, 685–690 (2013).
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