Schematic of the TPCVD method.Temperature could hold the key to growing carbon nanotubes of a particular chirality – or handedness – that is an important determiner of properties.
Producing single-walled nanotubes (SWNTs) of a specific chirality is essential for many potential applications. Over the years, different approaches to chirality-controlled growth have been tried. Now researchers from Peking University, Hong Kong Polytechnic University, and Jiangsu University have come up with a chemical vapor deposition (CVD) process that could hold the answer [Zhao, et al., Sci. Adv. 2 (2016) e1501729].
“We have developed a new CVD system that can vary the temperature of SWNT growth periodically and therefore the chirality of the SWNTs,” explains Jin Zhang of Peking University.
Catalyst particles such as Fe are used to initiate growth. In the early stages of growth, a cap forms over the catalyst surface. But once an infant nanotube starts to grow, it is very difficult to change – or control – the chirality.
So the new process introduces temperature changes during the initial growth stage. The variation in temperature is accomplished by simply changing the position of the furnace relative to the sample.By oscillating the furnace,the temperature can be changed from 820?C to 880?C, and the perturbation process can be repeated up to 90 times in 9 minutes. During this stage,the structure of SWNTs can change many times – along with the chirality – leading to a systematic variation in SWNT helical angle. The end result is enrichment of small helix angle (less than 10?) SWNTs up to 72%.
“To our best knowledge, this is the first [method] that leads to the growth of small helical angle SWNTs and it paves the way for further SWNT chirality control by rational catalyst design and experimental control,” says Zhang.
The researchers dub their new process ‘tandem plate chemical vapor deposition’ (or TPCVD).
“We have known for years that the catalyst-tube interface energy is lower for non-chiral, either armchair A or zigzag Z type (depending on the catalyst details),” says Boris I. Yakobson of Rice University [1]. “Solid catalysts often yield near-armchair tubes, while zigzag have seemed elusive.”
Zhang and his team have found a clever way to perturb a growing tube, so that it sequentially ‘mutates’ into other types towards the lower energy zigzag state, he explains. “This feels very satisfying. As in biology, to promote mutations a stress-factor is needed, and here it is temperature jumps. In fact, Zhang has explored temperature modulation approach for a long time, and now it bears fruit remarkably. Rick Smalley would love to see this!” Yakobson says.
The TPCVD method could be widely applied in normal tube furnace CVD systems, believes Zhang. The only major limitation being that the rapid temperature variation can lead to the poisoning of some catalysts, which would decrease the enrichment efficiency. But Zhang says that the team is now working on solutions.
Reference
[1] Liu, et al., Phys. Rev. Lett. 105 (2010) 235502.
This article was originally published in Nano Today (2016), doi:10.1016/j.nantod.2016.06.004