Nanotubes inch closer to electronic applications

	Slava V. Rotkin

Researchers at Lehigh and two other universities have fabricated dense arrays of single-walled carbon nanotubes into a thin-film semiconductor material, moving a step closer to the integration of the tiny tubes into electronic devices.

The keys to their success are two-fold, the researchers say. A new growth method allows parallel and linear alignment of nanotubes. And a rational design of the device geometry, guided by theoretical research, overcomes non-uniformities in the density and distribution of the tubes in devices.

The researchers, who represent the University of Illinois at Urbana-Champaign and Lehigh and Purdue universities, published their results in the April issue of Nature Nanotechnology. Their work is supported by the National Science Foundation and the U.S. Department of Energy.

Carbon nanotubes, discovered in 1991, are strong and easy to shape without breaking, and can act as metals or semiconductors. They show great potential in nanoelectronics, medicine, sensing and optoelectronics, and as strengthening elements in composite materials.

But incorporating them into scalable integrated circuits has been challenging because the tubes are difficult to manipulate and because single-tube devices do not readily generate large current outputs.

An important step toward large-scale integration

The arrays of nanotubes fabricated by researchers at Illinois can be transferred to plastic and other unusual substrates, and used in flexible displays, heads-up displays, structural health monitors and other applications. The arrays can also be used to enhance the performance of devices built with conventional silicon-based chip technology.

“The aligned arrays represent an important step toward large-scale integrated nanotube electronics,” says John A. Rogers, a Founder Professor of materials science and engineering at Illinois and corresponding author of the Nature Nanotechnology article.

The new arrays consist of hundreds of thousands of nanotubes, each measuring about 1 nanometer in diameter (1 nm is a billionth of a meter, or about the width of a dozen atoms) and as much as 300 microns in length (1 micron is a millionth of a meter or 1,000 nm). The tubes are spaced about 100 nm apart.

The arrays act as a useful thin-film semiconductor material in which electric charge moves independently through each nanotube. The tubes can be integrated into electronic devices by conventional chip-processing techniques. Many devices can be built from one array, with a typical device containing about 1,000 nanotubes and producing 1,000 times more current than that produced by earlier devices containing a single nanotube.

Lehigh’s role in the breakthrough

The breakthrough in fabricating more effective nanotube devices was aided by researchers at Lehigh, who used computer modeling and analytical calculations.

Slava V. Rotkin, Frank J. Feigl Junior Faculty Scholar and assistant professor of physics at Lehigh and co-author of the Nature Nanotechnology article, says an optimal density of nanotubes in an array depends on the geometry of the device and can be predicted theoretically.

But the arrays are not completely uniform, says Rotkin, who is a primary member of Lehigh’s Center for Advanced Materials and Nanotechnology and a member of the Center for Optical Technologies. The nanotubes in the arrays are not perfectly parallel and they are situated at varying distances from each other.

Nonetheless, says Rotkin, the properties of single tubes within an array “average out,” enabling the properties in the overall array to obtain uniformity from one device to the next—despite the non-uniform distribution of nanotubes in devices.

Thus, says Rotkin, although each device looks different under the microscope, the electronic properties of all the devices are almost identical.

“The location of nanotubes in the array changes from device to device,” says Rotkin, “but [the properties] self-average because of the electrostatic coupling between the neighboring tubes in a device. In a sense, we have found that nanotubes ‘talk’ to their neighbors, both near and far.”

The researchers create their nanotube arrays by depositing thin strips of iron nanoparticles on a wafer of single-crystal quartz. The iron acts as a catalyst for the growth of carbon nanotubes by chemical vapor deposition. The nanotubes grow past the strips of iron and lock onto the quartz crystal, which aligns their growth.

“This is a revolutionary approach in the fabrication of new devices,” says Rotkin, who joined Lehigh’s faculty in 2003 after serving as a Beckman Fellow and later teaching at Illinois’s Beckman Institute for Advanced Science and Technology. Rogers is also a researcher at the Beckman Institute.

“Researchers had previously learned how to fabricate random networks of nanotubes, but these were not so useful for electronics because the interaction between the tubes is not constructive there,” says Rotkin. “The parallel arrays that John Rogers has produced are much more effective because of the electromagnetic coherence between the tubes.”

Using the arrays, the researchers built and tested a number of transistors and logic gates, and compared the properties of the nanotube-array devices with the properties of individual-tube devices. Interestingly, says Rotkin, in a nanotube-array device in which nanotubes are coherent and coupled to each other, the quantum properties of single tubes are even stronger than they are in the single tubes of individual-tube devices.

“This is the first study that shows properties in scalable-device configurations that approach the intrinsic properties of the tubes themselves, as inferred from single-tube studies,” says Rogers.

--Kurt Pfitzer