Graphene Gets Some Competition

New building blocks for 2-D circuits

“Flatland” has never looked so good. A little less than a decade ago, physicists showed they could pull away loosely bound layers of graphite to reveal graphene, a 2-D carbon structure. The material was shown to have very promising electronic properties. But graphene isn’t the only game in town. A whole host of 2-D structures are attracting attention. Many can be formed just as graphene is, from layered 3-D materials; one such material, molybdenum disulfide, has been used in recent months to form flexible, transparent transistors and some of the basic building blocks of logic chips. Others are flattened forms of naturally 3-D structures. In April, for example, a team based at Ohio State University reported they had wrangled germanium, a mainstay of the semiconductor industry, into a 2-D structure that transports electrons faster than its 3-D counterpart does.

Rounded up here are some of the most promising materials on the 2-D scene. Some offer smaller and less power-hungry transistors that could be used for future logic and memory chips. Others could be ideal for computing with light and for other far-out applications. Some may work in concert with one another or with graphene, while others are direct competitors.

This list is by no means complete. It does not include, for example, a host of metal oxides that have shown a lot of potential as nanoscale electrical insulators. “People have been trying to figure out how many layered materials there are. A rough count would give you [more than] 100,” says Jonathan Coleman, a physicist at Trinity College Dublin. “There is literally an abundance of these things, and we’re only just scratching the surface.”


GRAPHENE (ATOMIC ELEMENT C)

First isolated in 2004, graphene is a single layer of carbon that boasts extraordinary properties. At low temperature, for example, electrons can zip through the material 100 times as fast as they do in silicon for a given voltage. Among 2-D materials, graphene manufacturing is the most mature. Chipmakers can now coat entire wafers with the stuff and have used those to construct speedy RF transistors. But making logic will be trickier. Silicon and other semiconductors have a bandgap, the energy barrier that atom-bound electrons must jump over to freely move around the material. Natural graphene has none, so there is no way to turn the current flow off in order to make a switch. There are ways to induce a small bandgap, such as by stacking one layer of graphene on top of another or by cutting the material into nanoribbons, but these approaches tend to reduce electron speed. But the promise of the material for electronics, optics, and energy storage was still enough for the European Commission to announce that it will pump as much as €1 billion into graphene R&D earlier this year.


BORON NITRIDE (BN)

Conductors wouldn’t be anywhere without insulators. That’s where boron nitride comes in. Two-dimensional boron nitride—a mix of boron and nitrogen atoms that is commonly arranged in hexagonal form—is a semiconductor. But its bandgap is too big to be practical for switches, so the material is often tapped to function as an insulator in devices made from other 2-D materials. Researchers have demonstrated they can form a 2-D structure composed of alternating strips of graphene and boron nitride, which could potentially be stacked to form complex circuitry.


MOLYBDENUM DISULFIDE (MoS2) AND COMPANY

Layers made of molybdenum disulfide, a molecular mix of molybdenum and sulfur, have some advantages over both graphene and silicon. As a semiconductor, the material boasts a bandgap, making it a natural choice for logic. What’s more, MoS2’s bandgap is “direct,” which means that unlike silicon it can readily emit and absorb light. It would pay to watch this space. MoS2 is just a fairly well-studied member of a larger group called transition metal dichalcogenides. This family, which also includes tungsten diselenide, consists of materials that combine one of 15 transition metals with one of three members of the chalcogen family: sulfur, selenium, or tellurium. So far, only a handful have been studied for their 2-D electronic properties.


SILICENE (Si) AND GERMANENE (Ge)

As exciting as materials like graphene may be, chipmakers would probably prefer a 2-D alternative that would allow them to continue using common semiconducting materials without fear of contaminating their fabs. Enter the 2-D versions of silicon and germanium: silicene and germanene. Versions of both materials were isolated in only the last two years, and research on them is just beginning. Other 2-D materials, such as graphene, can be readily pulled from bulk material that consists of a 3-D stack of loosely bound 2-D layers. But silicon and germanium don’t come in that form; their atoms form tight bonds in all three dimensions. As a result, flattened versions of the materials tend to buckle, which can lead to inconsistent electronic properties.

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