Core concept: silicene.

In 2003, physicists from the University of Manchester isolated flakes of graphene, a single layer of carbon atoms arranged in a honeycomb. The researchers described their methods in 2004 (1), and in 2010 they were awarded the Nobel Prize in Physics for discovering the wonder substance. Since then, researchers have shown that the 2D carbon material conducts electricity and is stronger than steel (2), suggesting it could transform the future of small and flexible electronics devices. However, the modern electronics industry is built on transistors fashioned from silicon, not carbon. The surging interest in graphene led physicists to wonder if other materials, including silicon, might be formed into usable, flat sheets. Theory said yes: In 2007, researchers at Wright State University in Dayton, Ohio, published a theoretical basis for 2D silicon sheets, calling the material “silicene.” (3) The name stuck, and subsequent investigations of silicene suggest that the electronic properties of the material can rival those of graphene. Experimental work caught up with theory in 2010 when French physicists reported growing silicene nanoribbons—flat strips of silicon atoms—on a silver substrate under high vacuum conditions (4). X-ray images of the 1.6-nanometer-wide nanoribbons showed the silicon atoms formed into a honeycombed structure, like the carbon atoms in graphene. In 2012, Chinese researchers analyzed silicene using scanning tunnel spectroscopy and found experimental evidence that like graphene, the silicon-based material had Dirac cones. These unusual structures allow electrons to flow quickly across a material, boosting electrical conductivity (5). Theoretical physicist Abdelkader Kara at the University of Central Florida in Orlando, a coauthor on many pioneering silicene studies, says interest in the material has exploded in recent years. Kara notes that only a few dozen papers had been published by 2012, when he helped author the first scientific review of the field (6). Now, Kara says, there are thousands of papers. He adds that experimental groups all over the world— including Japan, China, France, and the Czech Republic—are racing to manufacture usable silicene. The Fourth International Meeting on Silicene will be held in Beijing in June 2014. Researchers still face steep hurdles before silicene becomes useful, much less displace graphene as the king of the 2D wonderstuffs. One hurdle is the issue of structure: Carbon atoms naturally join to form 2D sheets because that is the lowest energy state for the element. (Three-dimensional diamond crystals form only under extreme conditions.) Silicon’s lowest energy state, however, is the crystal, 3D state. To coax silicon atoms into a flat formation takes extra energy. “If you throw silicon atoms in the air and they meet, they will form a big solid crystal,” says Kara. “If you throw carbon atoms in the air, they’ll make something that is planar.” The slightest perturbation during the manufacturing process will cause the silicon atoms to form a bulky ball rather than a smooth sheet. Perhaps the most daunting obstacle ahead is the substrate on which the silicene is grown. To date, silicene has only been grown on metal, but metal robs the material of its electrical advantages. Researchers want to grow silicene on an insulator, which will interact only weakly, if at all, with the material. “Grow it on an insulator and you can use it right away,” Kara says. However, that will take time: The first silicene-based devices, he estimates, are probably a decade away if research continues at its current pace.