Portland, Ore.- Chemists at Central Michigan University have grown carbon nanotubes at a record-low 175 degrees C using a polymer type known as dendrimers as the substrate.
"This is the first time anyone has grown carbon nanotubes directly from a dendrimer catalyst at temperatures low enough that the dendrimer is not destroyed," said CMU professor Bradley Fahlman, lead researcher on the project.
Whereas conventional polymers grow in long, tangled chains, dendrimers branch out from a core in a symmetrical, tree-like arrangement. In the growth process, the ends of one generation of branches sprout new polymer chains, creating a next-generation shell.
"The dendrimer we are using is a generation-four branched polymer, meaning it's a sphere with four [concentric] layers of branching. Ours has iron bonded to the outside of the dendrimer acting as the catalyst to which the carbon bonds and grows our nanotubes," said Fahlman.
The process grows nanotubes from the ends of the dendrimer's branches. Functionalizing the tubes with metal, semiconducting or photoelectric molecules will materials with varied properties to be created, the researchers said.
"Most of the other approaches to growing carbon nanotubes require temperatures on the order of 700 degrees C to 1,000 degrees C, but we have been able to grow them at temperatures low enough that it does not decompose the dendrimer," Fahlman said. The researchers have not pinned down the exact mechanisms that allow the nanotubes to grow at a lower temperature. Experiments have shown that a CO2 gas under high pressure or certain organic solvents under ambient pressure are required to produce such results.
Fahlman performed the work with fellow CMU professors Bob Howell and Dillip Mohanty; dendrimer pioneer Donald Tomalia, president and chief technology officer of Dendritic NanoTechnologies Inc. (Mount Pleasant, Mich.; dnanotech.com); and a team of undergrad, graduate and postgraduate researchers from CMU.
The team predicts that, once perfected, the polymer nanotube-based materials could enable plastic solar cells; plastic electronics; easier hydrogen storage; disposable field-emission flat-panel displays; and reinforced materials such as plastics as strong as steel and super-strong rubber; and nanotube-enabled application-specific polymers.
Dendrimers are the most costly polymers; Fahlman's team bought its raw materials at cost and still paid about $200 per gram. But if the same results could be achieved with the inexpensive polymers used for commercial products, then the era of plastic electronics could be around the corner. Fahlman's team will next seek to prove that dendrimers can be imbued with semiconducting properties and to show that inexpensive polymers can also sprout nanotubes.
"We think we can encapsulate metals inside or outside the dendrimer to give them semiconducting properties," said Fahlman. "We are also trying this on a hyper-branched polymer that is just like a dendrimer but is less spherically perfect and costs only $10 a kilogram."