Researchers at Rice and Vanderbilt universities in the US have succeeded in controllably doping carbon nanotubes using a technique that simply involves regulating the amount of active nitrogen-doping species during nanotube growth. The result means that it may now be possible to modulate the electronic character of these nanostructures at will - something that could come in useful for a variety of applications, including energy-storage devices and lightweight conducting wires.

The researchers, led by Robert Hauge and Cary Pint, have also found that hydrogen cyanide (HCN) is the active precursor responsible for doping the nanotubes with nitrogen. The structure of HCN is similar to that of C2H2, which is the most active species for growing pure carbon nanotubes. "Understanding how to choose the best precursor to yield the most efficient chemistry is a huge step forward in controllably doping carbon nanomaterials," Pint told nanotechweb.org.

Another exciting result is that the team succeeded in measuring extremely low levels of dopant content in the nanotubes. Until now, existing techniques were only able to measure dopant concentrations of about 0.2 to 0.5 atomic percent – levels that are far too high. "Our new method can measure down to as low as 10–5 atomic percent nitrogen in the nanotube lattice, which I feel is a great step forward because it allows to see nitrogen (and other potential dopants) at the sorts of levels employed when making practical doped devices," said Pint.
 

Nanotubes grow fast

The researchers made aligned single-walled carbon nanotubes using an ultrathin catalytic layer (0.5 nm of iron) using a "supergrowth" method. Here, the nanotubes usually self-assemble into vertically oriented structures that grow upwards from the catalyst layer, which itself remains on the base of the growth substrate. The nanotubes grow fast thanks to a small amount of water vapour present during the process.

"In our system, we simply combined the idea of conventional supergrowth with a precursor that decomposed into an active molecule with an N-C triple bond," explained Pint. "Thus, as the carbon is incorporated into the lattice, the nitrogen gets scooped up into the growing nanotube as well. We are able to control how much nitrogen goes into the tubes by simply regulating the amount of the precursor."

More conducting

The N-doped nanotubes are more conducting than their undoped counterparts because the dopant shifts the Fermi level in these semiconductor materials into the conduction band. This property could be exploited to make tubes that have varying Fermi levels from one end to the other by modulating the amount of N along their lengths. "This means that the nanotubes would then have different conductivities along their lengths," said Pint.

Such a concept is at the heart of bottom-up nanomaterials engineering, he adds.

According to the team, the N-doped nanotubes might be used to build more conductive structures for energy storage devices – such as supercapacitors – or highly conducting lightweight wires of nanotubes. "There is a big market for such components (which are as conductive as copper) in space applications, for instance," said Pint. Other possibilities include energy-harvesting or conductive armour applications, thanks to the fact that nanotubes are 100 times stronger than steel at just one sixth the weight.

The researchers say that they would now like to investigate the electrical properties of their doped nanotubes both at the macroscopic and single-nanotube levels. "The next exciting thing would then be to try and develop 1D nanotemplates using doping as a 'knob' for controlled functionalization of nanotubes," added Pint.