30 August 2012—Researchers at Cornell University, in Ithaca, N.Y., are doing work that suggests electronic circuits could continue to be miniaturized until they are just a few nanometers thick. The Cornell team reports in this week’s Nature that it has developed a technique for manufacturing the components of electronic circuits on a single one-atom-thick sheet. The result, they say, will be flexible and transparent electronics that are as thin as they can possibly be. Stacking these sheets could someday yield complex, three-dimensional integrated circuits that are still slimmer than any of today’s chips.
The team grew a layer of graphene from a chemical vapor on a copper substrate, a method they call patterned regrowth. They used lithography to etch away the graphene areas they didn’t want, and then in the vacated spaces they grew a layer of a second material that is electronically distinct from the graphene but mechanically connected to it. Just as significant, say the researchers, the structure and electrical behavior of the graphene remains unchanged by the process that creates the new material growing right up to its borders.
The Cornell scientists showed that they could grow hexagonal boron nitride, a single-atom-thick insulator, or graphene chemically modified to make it behave as an n-type semiconductor, alongside the original graphene.
When they examined the hybrid materials’ electrical properties, they saw that the boron nitride acted as an insulator between neighboring graphene regions and, even more important, that those regions maintained the two-dimensional carbon’s amazing electrical characteristics. By alternating graphene and boron nitride regions, they could create an array of wires.
The junctions where the different materials meet showed no visible breaks, says Mark P. Levendorf, a graduate student who is part of the Cornell team. This was true even at a resolution of about 10 nanometers, where the materials’ molecular structures are visible as geometric shapes fitted together like a microscopic jigsaw puzzle.
Achieving this seamless fit, he says, requires a highly reactive environment for the boron nitride and graphene to grow in—one that yields small grains with boundaries that connect well. (Imagine the difference between a bucket filled with rocks and the same bucket filled with sand.) In a less-reactive environment, says Levendorf, “boron nitride doesn’t even want to stitch with itself.”
Levendorf notes that, as far as he and his colleagues were able to resolve, the disparate materials didn’t break down along junctions where graphene met boron nitride (or where one type of graphene met another), and there was minimal electrical resistance along borders between different types of graphene. “There may be faults at the atomic scale, but we’re not sure,” he says. The group is currently investigating the atomic-scale phenomena that make this heterogeneous sheet possible, including the mechanism for knitting the adjacent areas together.
Levendorf says that the team is also attempting to expand the palette of materials they can merge on single sheets. By adding p-type graphene to the mix, for example, the researchers will be able to create p-n junctions that are just one atom thick. But the first such semiconductor they are investigating for use with their regrowth technique is molybdenum disulfide—which, unlike graphene, has an inherent bandgap, making it easier to form into a transistor.
Experts in flexible electronics say they can immediately see how the breakthrough could contribute to the field. The Cornell team’s method “may have huge potential in flexible electronics, where thinner is often better, due to improved mechanics of bendability,” says John A. Rogers, a flexible and stretchable electronics expert at the University of Illinois in Urbana-Champaign.