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“Nanotrees” might help miniaturize gadgetry

May 1, 2008
World Science staff

Some gad­get­s—like iPods and lap­top com­put­er­s—just keep get­ting smaller. And sci­en­tists are try­ing to shrink them even fur­ther, us­ing parts as small as a mil­lionth of a hu­man hair wide. But it’s not easy to cre­ate these.

Re­search­ers stu­dy­ing this prob­lem say they’ve dis­cov­ered a po­ten­tially use­ful pro­cess that al­so cre­ates beau­ti­ful struc­tures: “trees” from wires only a few hun­dred atoms thick. 

Nano­trees in an image created using a scan­ning elec­tron micro­scope. (Cour­tesy AAAS/Sci­ence)

“At the be­gin­ning we saw just a cou­ple of trees, and we said, ‘What the heck is go­ing on here?’” said Uni­ver­s­ity of Wisconsin-Madison chem­ist Song Jin.

The trees turned out to rep­re­sent a new way of grow­ing such “nano­wires,” re­search­ers said, which could lead to new and bet­ter “nano­ma­te­ri­als” for many ap­plica­t­ions. 

Nano­ma­te­ri­als are ma­te­ri­als so min­ute that they have struc­tures de­signed as small as a few hun­d­red atoms or less. They have po­ten­tial uses in de­vices in­clud­ing elec­tric cir­cuits, biosen­sors, so­lar cells, light-emitting diodes and lasers.

The struc­tures grown by Jin and stu­dents look like pine trees, with a trunk and branches swirling around the trunk like a spir­al stair­case. The team grew a whole for­est of the trees, each stand­ing as tall as the width of a few hu­man hairs.

Pre­vi­ously, most nano­wires had been made with cat­a­lysts, sub­stances that pro­mote the growth of nano­ma­te­ri­als along one di­men­sion to form rods. But the trees seem to form on their own, with­out such help, said Jin. The re­search­ers at­trib­ute this abil­ity to a ti­ny ir­reg­u­lar­ity or “dis­loca­t­ion” in the mid­dle of each tree trunk, where the trunk is twisted like a screw. This, they ar­gue, en­cour­ages grow­ing branches to wrap around the trunk.

Another view, with inset showing finer de­tails. The large white bar re­pre­sents 10 na­no­me­ters, or bil­lionths of a me­ter. (Cour­tesy AAAS/Sci­ence)

Dis­loca­t­ions, or lapses in a reg­u­lar crys­tal struc­ture, 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 es­sen­tially one-di­men­sional nano­struc­tures, said Jin. 

En­gi­neer­ing such dis­rup­tions, he added, might not only let sci­en­tists make more elab­o­rate nano­struc­tures, but al­so study the bas­ic prop­er­ties of dis­loca­t­ions.

His team cre­ated its nano­trees ap­ply­ing a slight varia­t­ion of a 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.

“We think these find­ings will mo­ti­vate a lot of peo­ple to do this pur­pose­fully, to de­sign [a] dis­loca­t­ion and try to grow nano­wires around it,” Jin said. “Or per­haps peo­ple who have grown a struc­ture and were puz­zled by it will read our pa­per and say, ‘Hey, we see some­thing si­m­i­lar in our sys­tem, so may­be now we have the so­lu­tion.’”

The find­ings are de­tailed in the May 1 ad­vance on­line edi­tion of the re­search jour­nal Sci­ence. “These are beau­ti­ful, truly in­tri­guing struc­tures, but be­hind them is al­so a really beau­ti­ful, fun­da­ment­al sci­ence,” Jin said.

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Some gadgets—like iPods and laptop computers—just keep getting smaller. And scientists are trying to shrink them even further, using parts as small as a human hair’s width. But in making parts so tiny, it’s hard to create building blocks that stack into neat patterns. Researchers studying this problem say they’ve stumbled on a potentially useful process that also creates beautiful structures: “trees” from wires only a few tens of atoms thick. “At the beginning we saw just a couple of trees, and we said, ‘What the heck is going on here?’” said University of Wisconsin-Madison chemist Song Jin. The trees turned out to represent a new way of growing such “nanowires,” scientists said, which could lead to new and better “nanomaterials” for many applications. Nanomaterials are materials so minute that they have structures designed as small as a few atoms in size. They have potential uses in devices including circuits, lasers, biosensors, solar cells, light-emitting diodes and lasers. The structures grown by Jin and graduate student Matthew Bierman look like pine trees, with a trunk and branches swirling around the trunk like a spiral staircase. The scientists grew a whole forest of these metal trees, each standing as tall as the width of a few human hairs. Previously, most nanowires had been made with catalysts, chemicals that promote the growth of nanomaterials along one dimension to form rods. But the trees seem to form on their own, without such help, said Jin. The researchers attribute this ability to a tiny break in the middle of each tree trunk, where the trunk is twisted like a screw. The twist, they argue, encourages growing branches to wrap around the trunk. The break is known as a screw dislocation. “Dislocations,” or lapses in a regular crystal structure, 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 essentially one-dimensional nanostructures, said the researchers. Engineering such defects, Jin added, may not only let scientists make more elaborate nanostructures, but also study the basic properties of dislocations. His team created its nanotrees applying a slight variation of a 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. “We think these findings will motivate a lot of people to do this purposefully, to design [a] dislocation and try to grow nanowires around it,” Jin said. “Or perhaps people who have grown a structure and were puzzled by it will read our paper and say, ‘Hey, we see something similar in our system, so maybe now we have the solution.’” The findings are detailed in the May 2 issue of the research journal Science and its May 1 advance online edition. “These are beautiful, truly intriguing structures, but behind them is also a really beautiful, interesting science,” Jin said. “Once you understand it, you just feel so…satisfied.”