Carbon nanotube growth from ‘nanorings’ provides diameter control

Nanoring templates can set carbon nanotube diameters, an ability whose previous absence has held back nanotube use in electronic applications. A Japanese team led by Nagoya University’s Kenichiro Itami has controlled nanotube structures in syntheses starting from cycloparaphenylene (CPP) or ‘carbon nanoring’ templates.1 ‘The carbon nanotube diameter can be predictably controlled by the carbon nanoring,’ Itami tells Chemistry World.

Throughout their 22-year history, single-walled nanotubes have been made as mixtures that exhibit varying properties, such as metallic or semi-conducting electronic structures. Chemical vapour deposition (CVD) is a common commercial approach, heating ethanol or methane beyond 1000°C. That gives multiple diameters and sidewall structures (or chirality) far from the atomic-level precision electronic applications usually call for. 

But recently, scientists have influenced these properties by growing nanotubes from templates. In 2009, a Chinese team extended short nanotubes while controlling their chirality.2 ‘They put one tube on a quartz surface, treated it with methane heated to 900°C, and saw growth at either end,’ Itami explains. ‘This inspired our experiments.’

Itami’s team thought they could use CPP nanorings as templates. After overcoming the challenge of synthesising these and other related molecules, they separately coated nanomolar amounts of two CPPs onto tiny fragments of sapphire. They grew nanotubes from these templates using high-temperature ethanol vapour in conditions close to those found in CVD. ‘Though heating at 500°C is unusual for organic chemists, we tried lower temperatures, and it turns out that it’s the critical temperature,’ Itami says.

The chemists used CPP templates with nine and 12 para-linked benzene rings, which have 1.2nm and 1.7nm diameters respectively. In both cases, they made nanotubes with a tight range of mainly slightly wider diameters. But Raman spectroscopy also showed multiple sidewall structures, while the amount they produced was too low to calculate yields.

 Nevertheless, Ramesh Jasti of Boston University, US, whose team has also produced CPPs, calls the result ‘quite important’. ‘This is the promising result many of us have been waiting for,’ he says.  ‘Although there is still much to be done, the potential to grow CNTs of predetermined chirality using organic templates is huge.’

‘This is only the first step,’ Itami adds. ‘For real production we need years. In the ideal situation, carbon nanotube production will be completely changed over to this template method.’