Physicists think that there may be around 500 2D materials, including not just graphene and TMDCs (transition-metal dichalcogenides), but also single layers of metal oxides, and single-element materials such as silicene and phosphorene. “If you want a 2D material with a given set of properties,” says Jonathan Coleman, a physicist at Trinity College Dublin, “you will find one.”
Transition-metal dichalcogenides (TMDCs) are a single sheet of transition-metal atoms such as molybdenum or tungsten that was sandwiched between equally thin layers of chalcogens: elements, such as sulfur and selenium, that lie below oxygen in the periodic table. TMDCs are almost as thin, transparent and flexible as graphene.
Different combinations of the basic ingredients can produce TMDCs with a wide range of electronic and optical properties. Unlike graphene, for example, many TMDCs are semiconductors, meaning that they have the potential to be made into molecular-scale digital processors that are much more energy efficient than anything possible with silicon.
Ironically, one of the most exciting frontiers in 2D materials is stacking them into structures that are still very thin, but definitely 3D. By taking advantage of the vastly different properties of various super-flat materials, it should be possible to build entire digital circuits out of atomically thick components, creating previously unimagined devices. Applications are already being touted in fields from energy harvesting to quantum communications — even though physicists are just beginning to learn the materials' potential.
“Each one is like a Lego brick,” says Kis. “If you put them together, maybe you can build something completely new.”
Others are exploring different parts of the periodic table. Zhang's team and another led by Peide Ye at Purdue University in West Lafayette, Indiana, last year described stripping 2D layers of phosphorene from black phosphorus, a bulk form of the element that has been studied for a century. Like graphene, phosphorene conducts electrons swiftly. But unlike graphene, it has a natural band gap — and it is more stable than silicene.
Both Zhang and Ye succeeded in making phosphorene transistors. This year, the first transistor from silicene emerged14, although it survived for only a few minutes. Still, Le Lay is optimistic that these issues are not insurmountable. Just two years ago, he points out, Geim and other physicists were saying that a silicene transistor could not be made with current technology. “So it's always dangerous to predict the future,” laughs Le Lay.
Even as some physicists search for new 2D materials and try to understand their properties, others are already sandwiching them together. “Instead of trying to pick one and say this is the best, maybe the best thing to do is to combine them in such a way that all their different advantages are properly utilized,” says Kis.
This could mean stacking components made of different 2D materials to make tiny, dense 3D circuits. Materials could also be layered inside components — something that chip designers already do when they grow layers of different semiconductors on top of one another to make devices such as the lasers inside DVD players