“The control we have achieved in building these structures is spectacular,” said Pulickel Ajayan of Rensselaer. “They resemble polymer dendrimers in morphology and it is quite impressive that we can controllably create such complexity in individual nanowire/nanotube units.”
Ajayan, Guowen Meng and colleagues made the alumina templates by adjusting the anodizing voltage. Other researchers have created Y-shaped pores by lowering the anodizing voltage by a factor of 1/√ 2. The Rensselaer team built on this by lowering the voltage by the same factor up to four times in succession to create four generations of Y-branching.
The scientists also made pores split into n multiple smaller branches by reducing the voltage by a factor of 1/√ n. They formed up to 16 branches in this way.
By combining Y-shapes and multiple branches the scientists created a wide variety of nanoscale templates.
To make carbon structures based on the templates, the scientists deposited multi-walled carbon nanotubes inside the pores by the pyrolysis of acetylene. Then they etched away the template to reveal a hierarchically branched nanotube formation. They also made nickel nanowire structures.
“Since we are following a template approach we can create this type of structure for a wide range of material systems - metals, semiconductors, carbon, polymers [or] insulators - any structure that can be deposited via electrodeposition, vapour phase deposition and infiltration inside the pores of the templates we have developed,” said Ajayan.
The researchers reckon they will also be able to grow single-walled carbon nanotubes inside the templates. And they believe that they can create heterostructures containing two or more materials, for example by electrodepositing metal in the stems and then growing nanotubes in the branches.
“I can think of several applications involving separation of size-selective molecules, sensors [with] far better flexibility since the different branches of these complex nanowires could be chemically functionalized individually, complex electronic circuitry with multiple junctions and interconnects, and drug delivery systems similar to dendrimers,” said Ajayan. “In addition, the templates could be powerful in creating novel nanoscale architectures much more complex than [those that] exist today.”
The researchers reported their work in the Proceedings of the National Academy of Sciences of the USA (PNAS).