Practical devices made of nanostructures are a step closer to being realized. Researchers at Rice University have demonstrated that carbon nanotubes can be chopped into small pieces to form “seeds” that grow more nanotubes of precisely the same type The method could eventually make it possible to grow large amounts of carbon nanotubes with identical structure and properties, which could pave the way for a diverse set of carbon nanotube-based applications, such as vastly improved electrical transmission lines and ultracompact, high-performance computers.
One of the main difficulties of using carbon nanotubes for such applications is that existing manufacturing processes create a mix of nanotubes with a wide range of electronic properties. Indeed, one process for making single-walled carbon nanotubes yields as many as 80 different types of nanotubes. In applications requiring semiconducting nanotubes, such as in transistors for computer chips, highly conductive metallic carbon nanotubes could ruin the device.
The ideal solution would be to grow precisely the type of nanotube needed at exactly the location on a chip where it’s needed. This would make it possible to use nanotubes with consistent semiconducting properties in transistors, for example, and also to connect these transistors with highly conductive metallic versions of carbon nanotubes. Now James Tour, professor of chemistry at Rice University, and his colleagues have taken an important step toward such a system by demonstrating a way to make multiple copies of a single nanotube.
“They have proof now that they’ve been able to grow [a nanotube copy] from a seed,” says Michael Strano, professor of chemistry and biomolecular engineering at the University of Illinois, Urbana-Champaign. “If Jim Tour is ultimately successful, he’ll be able to grow large amounts of just one type of carbon nanotube, and so this will make that one type, or any type, very cheap and affordable.” He adds, “It’s a long road ahead. But it’s an important step forward.”
The Rice researchers used chemical methods to break single-walled carbon nanotubes into smaller pieces. They then chemically attached iron nanoparticles to both ends of these small tubes. When the researchers introduce a source of carbon atoms, in this case ethylene, the iron acts as a catalyst to allow the carbon to attach to the existing nanotube, thereby extending its length. In experiments published in last week’s Journal of the American Chemical Society, the researchers confirmed in two cases that the method allows long nanotubes to grow from iron particle-carbon nanotube seeds. These nanotubes had the same diameter as the seeds, which suggests that they will have the same properties.
These seeds could potentially be placed in specific locations on a computer chip using an existing chemical method, Tour says. Nanotubes with selected electronic properties could then be grown exactly where they are needed.
The Rice method is an outgrowth of research by the late Richard Smalley, who received the Nobel Prize in 1996 for discovering buckyballs, or fullerenes, and pioneered much of today’s work on carbon nanotubes. Smalley, who died just over a year ago, is the first author on the paper.
Many other researchers are developing methods for eliminating unwanted nanotubes from a batch by using ultracentrifuges or electric fields to sort them or by etching them away (see “Nanotube Computing Breakthrough” and “A Step Closer to Nanotube Computers”). These methods, however, aren’t as selective as the iron particle-carbon nanotube seeding technique being developed at Rice.
The main issue with the Rice method is yield. In the current research, only about 3 percent of the chopped up nanotubes grew larger. Tour hopes to improve yield by adjusting the fit between the size of the iron nanoparticles used as catalysts and the diameter of the nanotubes being grown.
The researchers also need to demonstrate that the process can work with a variety of nanotube types and grow nanotubes on a much larger scale, Strano says.
If these obstacles can be overcome, the new method could be a boon to engineers and scientists alike. The variation among nanotubes is so great that there’s “almost a new periodic table of nanotube types,” Strano says. “If [the Rice method] works, it will really enable the field.”
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