Crystal Steer Sapphire plays supporting role for nanotubes

Charles Q. Choi

Carbon nanotubes would make ideal connecting wires in advanced circuits if not for the painstaking effort required to line up each tiny, sticky, floppy strand. Now scientists have found that crystalline sapphire can automatically help guide nanotubes into the patterns needed to build transistors and to make flexible electronics. Electrical signals can flow more quickly through carbon nanotubes than through silicon, which in principle could lead to faster computers, explains Chongwu Zhou, an electrical engineer at the University of Southern California. Moreover, nanotubes can be as small as one-fifth the theoretical minimum size of conventional silicon circuitry.

To make nanotube circuits, scientists scatter nanotubes randomly and attach electrodes wherever they can, or else they try to grow nanotubes toward one another and later fabricate electrodes on them. Such efforts, though, are slow and inefficient, leading scientists to wonder if substrates existed that could naturally orient the tubes. After more than a year of experiments on various crystals, Zhou and his colleagues discovered that sapphire could achieve just that. Sapphire crystal is hexagonal, rising from a flat base, and the researchers found that most vertical slices of sapphire apparently expose constituent aluminum and oxygen atoms in layouts that promote the formation of nanotubes in orderly rows.

In the January Nano Letters, Zhou's team reported the creation of transistors with such aligned nanotubes. The researchers coated commercially available artificial sapphire with a cagelike protein called ferritin and flowed hydrocarbon gas over it while baking it. Iron within the protein catalyzed the growth of single-walled nanotubes from carbon supplied by the gas. Once the sapphire was covered with nanotubes, they could place the metal electrodes of the transistors wherever they wanted and remove the unwanted nanotubes with highly ionized oxygen gas.

Past carbon nanotube transistors were typically constructed atop silicon composites common in the electronics industry. The drawback was that the metal electrodes and the silicon interacted to suck up electrical charge, slowing down performance and raising power consumption. Zhou's strategy eliminates the parasitic drain because sapphire is electrically insulating, not semiconducting like silicon. The method is closely related to the so-called silicon-on-sapphire approach that IBM and other chipmakers have used to build specialized high-performance circuitry, "so we can borrow a lot of knowledge from the semiconductor industry," Zhou remarks.

When compared with other carbon nanotube electronics, these findings display the highest density of aligned nanotubes, at up to 40 per micron. Other methods manage only one to five, Zhou states. Nanotube density is crucial, because the more there are between electrodes, the more signals will be conducted. The scientists can control nanotube density by varying how much iron they employ within the ferritin.

The researchers could readily create flexible electronics from their nanotube transistors by baking a plastic film onto the nanotube transistors and then peeling off the strips, which hold on to the transistors. Carbon nanotube flexible electronics could "relatively easily" outperform the silicon-based versions currently used by industry, Zhou says, and he foresees its use in applications such as large flat-panel displays, vehicle windshields and smart cards. He also notes that such aligned nanotubes could act as sensors: attached molecules could send an electrical signal across the nanotubes if they reacted with cancer markers or other compounds.

These findings are "a very important result in resolving one of the most difficult problems related to carbon nanotube manufacture for integrated circuits," says Kang Wang, director of the Center on Functional Engineered Nano Architectonics at the University of California, Los Angeles. He points to one crucial hurdle to overcome: ensuring that all nanotubes made by this technique are semiconducting, because it currently produces a mix of metallic (fully conducting) and semiconducting ones.