University of Wisconsin-Madison chemists have accidentally created nano pine trees with trunks and branches. As said the lead researcher, 'At the beginning we saw just a couple of trees, and we said, 'What the heck is going on here?' They were so curious.' In fact, this could lead to an entirely different way of growing nanowires. The researchers think that it will give them 'powerful means to create new and better nanomaterials for all sorts of applications, including high-performance integrated circuits, biosensors, solar cells, LEDs and lasers.' But read more...
You can see on the left some of these spiraling pine tree-like nanowires. (Credit: Song Jin, UW-Madison) Here is a link to a larger version of this photo and another one to more pictures.
These nanotrees have been developed in chemistry professor Song Jin's
research group. The two other main contributors to this study are graduate students Matthew Bierman and Albert Yue-Kong Lau.
Now, let's discover why this way of growing nanowires is new. "Until now, most nanowires have been made with metal catalysts, which promote the growth of nanomaterials along one dimension to form long rods. While the branches on Jin’s trees also elongate in this way, growth of the trunks is driven by a 'screw' dislocation, or defect, in their crystal structure. At the top of the trunk, the defect provides a spiral step for atoms to settle on an otherwise perfect crystal face, causing them stack together in a spiral parking ramp-type structure that quickly lengthens the tip."
Here is a more detailed explanation. "Dislocations are fundamental to the growth and characteristics of all crystalline materials, but this is the first time they’ve been shown to aid the growth of one-dimensional nanostructures. Engineering these defects, says Jin, may not only allow scientists to create more elaborate nanostructures, but also to investigate the fundamental mechanical, thermal and electronic properties of dislocations in materials. His team created its nanotrees specifically by applying a slight variation of a synthesis technique called chemical vapor deposition to the material lead sulfide. But the chemists believe the new mechanism will be applicable to many other materials, as well."
Besides the beauty of their nanotrees, the chemists were surprised by two other facts. First, the trunks of the nanotrees have a very long length when compared with the branches, which indicates that the trunks were growing much faster. "Another oddity was the twist to the trunks, which sent the branches spiraling. 'The long and twisting trunks were telling us we had a new growth mode,' says Jin. Suspecting dislocation, the team set about refining their technique for growing the pine trees -- they soon learned to produce entire forests with ease -- and then confirmed the presence of dislocations with a special type of transmission electron microscopy."
This research work has been published online by Science on May 1, 2008, under the name "Dislocation-Driven Nanowire Growth and Eshelby Twist." Here is the beginning of the abstract. "Hierarchical nanostructures of lead sulfide nanowires resembling pine trees were synthesized via chemical vapor deposition. Structural characterization reveals a screw-like dislocation in the nanowire trunks with helically rotating epitaxial branch nanowires. It is suggested that the screw component of an axial dislocation provides the self-perpetuating steps to enable one-dimensional crystal growth, in contrast to mechanisms that require metal catalysts."
For more information and additional pictures, you can read the Science's supporting online material for this article (PDF format, 10 pages, 2.88 MB).
Finally, here is the refreshing conclusion of Song Jin about these nanotrees. "These are beautiful, truly intriguing structures, but behind them is also a really beautiful, interesting science. Once you understand it, you just feel so ... satisfied." Now it remains to be seen if this discovery might have practical applications as think the researchers.
Sources: University of Wisconsin-Madison news release, May 1, 2008; and various websites
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