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How to form hollow nanotubes?

Posted: 03 May 2010     Print Version  Bookmark and Share

Keywords:Nanotubes  zinc oxide  crystal defect  carbon atoms 

Scientists are successfully making a menagerie of nanometre-sized objects—wires, tubes, belts, and even tree-like structures. However, some find it a challenge to explain the process of how these objects form in the vapour and liquid cauldrons.

Now a team led by University of Wisconsin-Madison chemist Song Jin, in the journal Science, shows that a simple crystal defect known as a "screw dislocation" drives the growth of hollow zinc oxide nanotubes just a few millionths of a centimetre thick.

The finding is important because it provides new insight into the processes that guide the formation of the smallest manufactured structures, a significant challenge in nanoscience and nanotechnology. "We think that this work provides a general theoretical framework for controlling nanowire or nanotube growth without using metal catalysts that can be generally applicable to many materials," says Jin, a UW-Madison professor of chemistry.

Such materials and the Lilliputian structures scientists sculpt have already found broad applications in such things as electronics, solar power, battery and laser technology, and chemical and biological sensing. By further expanding the theory of how the tiny structures form, it should now be possible for scientists to develop new methods to mass produce nano-sized objects using a variety of different materials.

The method described by Jin and his colleagues depends on what scientists call a screw dislocation. Dislocations are fundamental to the growth and characteristics of all crystalline materials. As their name implies, these defects prompt the creation of spiral steps on an otherwise flawless crystal face. As atoms alight on the crystal surface, they form a structure strikingly similar in appearance to the spiral ramps of multi-storey parking structures. In earlier work, Jin and his research group showed that screw dislocations drive the growth of one-dimensional nanowire structures that looked like tiny pine trees. That, says Jin, was a critical clue to understanding the kinetics of spontaneous nanotube growth.

The key to understanding how to harness the defect to make nanostructures in a rational way, Jin explains, is knowing that as atoms collect on a surface of a dislocation spiral, strain associated with screw dislocations builds up in the tiny structures they create.

It turns out that "making the structure hollow and making it twist are two good ways of relieving such strain and stress," Jin explains. "In some cases, the large screw dislocation strain energy contained within the nanomaterial dictates that the material hollow out its centre around the dislocation, thus resulting in the spontaneous formation of nanotubes."

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