Researchers at the Honda Research Institute, Purdue University and the University of Louisville have discovered a way to systematically grow carbon nanotubes with either metallic or semiconducting properties, solving a long-standing problem in nanotechnology research and paving the way for the widespread use of nanotubes in electronics.

Nanotubes are one-atom thick sheets of carbon rolled up in a cylindrical form, and come in two types: metallic and semiconducting. Metallic nanotubes are about 100 times stronger than steel while significantly lighter and stronger; semiconducting nanotubes, on the other hand, are among the best electrical conductors ever discovered.

When combined, these two kinds of nanotubes could be used as the basic building blocks to produce the next generation of transistors, as well as bringing important improvements to a number of fields including solar and battery technology.

Carbon nanotubes have been the object of intense research since they were discovered in the early 90s, but they need to be manufactured according to very specific standards in order to be employed successfully — this is particularly true with metallic nanotubes and has been a major obstacle to their effective employment so far.

The researchers have now managed to control almost systematically whether grown carbon nanotubes are metallic rather than semiconducting, with a 91 percent success rate for metallic nanotubes (compared to a previous success rate of 25-50 percent) and room for further improvement.

Nanotubes were "grown" in a vacuum chamber by exposing iron particles to methane gas containing carbon and hydrogen. The particles were heated to 800 degrees Celsius and, with time, the iron became oversaturated with carbon, which precipitated causing the nanotube to begin forming.

The researchers found they could control whether the carbon nanotubes will become metal or semiconductor by using either argon or helium in conjunction with water as the carrier gas that helps flow the methane into the chamber.

But there are still some questions to answer as the exact dynamics aren't entirely clear, particularly when it comes to understanding the role played by water in this process. "The water might promote the formation of the facets, and the argon might somehow prevent the water from doing so, but more research is needed to determine this," Eric Stach, associate professor of materials engineering at Purdue, commented.

Once an answer is found, the researchers will likely be able to fine-tune the process to obtain even higher reliability, meaning an even better control on the nanotube manufacturing process and far better results in all practical applications where nanotubes are to be employed.