The research is available online and slated to appear in an upcoming issue of the Journal of the American Chemical Society.
Like vintners who hope to grow new vineyards from a handful of grape vine cuttings, Rice’s chemists hope their new method of seeded growth for carbon nanotubes will allow them to reproduce their very best samples en masse.
‘Carbon nanotubes come in lots of diameters and types, and our goal is to take a pure sample of just one type and duplicate it in large quantities,’ said corresponding author James Tour, director of Rice’s Carbon Nanotechnology Laboratory (CNL). ‘We’ve shown that the concept can work.’
There are dozens of types of single-walled carbon nanotubes (SWNTs), each with a characteristic atomic arrangement. These variations, though slight, can lead to drastically different properties: some nanotubes are like metals, and others are semiconductors.
While materials scientists are anxious to use SWNTs in structures ranging from bacteria-sized computer chips to geostationary space elevators, most applications require pure compounds. Since all nanotube production methods, including the industrial-scale system invented in the 1990s by the study’s lead author, Richard Smalley, create a variety of 80-odd types, the challenge of making mass quantities of pure tubes – which Smalley referred to as “SWNT amplification” – is one of the major, unachieved goals of nanoscience.
Smalley, who died in October 2005, devoted the last 10 years of his career to studying SWNTs, and pioneered the first method for mass-producing them and many of the techniques scientists use to study them. Tour said Smalley devoted an enormous amount of time and energy to the seeded-growth nanotube amplification research in the final two years of his life.
‘Rick was intent on using nanotechnology to solve the world’s energy problems, and he knew we needed to find a way to make large quantities of pure nanotubes of a particular type in order to rewire power grids and make electrical energy widely available for future needs,’ Tour said. ‘Rick had a way of making things happen, and for six months during 2004, there were no fewer than 50 researchers in four Rice laboratories devoting their effort to this problem. It was unprecedented, and it paid off.’
Tour added, ‘Rick envisioned a revolutionary system like PCR [polymerase chain reaction], where very small samples could be exponentially amplified. We’re not there yet. Our demonstration involves single nanotubes, and our yields are still very low, but the amplified growth route is demonstrated.’
The nanotube seeds are about 200 nanometres long and one nanometre wide. After cutting, the seeds underwent a series of chemical modifications. Bits of iron were attached at each end, and a polymer wrapper was added that allowed the seeds to stick to a smooth piece of silicon oxide. After burning away the polymer and impurities, the seeds were placed inside a pressure-controlled furnace filled with ethylene gas. With the iron acting as a catalyst, the seeds grew spontaneously from both ends, growing to more than 30 times their initial length in just a few minutes.Tour said CNL’s team has yet to prove that the added growth has the same atomic architecture – known as chirality – as the seeds. However, he said the added growth had the same diameter as the original seed, which suggests that the methodology is successful.