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Snow falls differently on the nanoscale

Fri, 01/24/2014 - 11:33am
Angela Herring, Northeastern Univ.

Professor and mechanical and industrial engineering chair Hanchen Huang spent 10 years working out his theory for nanorod crystal growth. Image: Brooks CanadayThis time of year it’s not hard to imagine the world buried under a smooth blanket of snow. A picnic table on a flat lawn even­tu­ally van­ishes as tril­lions of snowflakes col­lect around it, a crys­talline sheet obscuring the normally-​​visible peaks and val­leys of our sum­mer­time world.

This is basi­cally how sci­en­tists under­stand the clas­sical theory of crys­talline growth. Height steps grad­u­ally dis­ap­pear as atoms of a given material—be it snow or copper or aluminum—collect on a sur­face and then tumble down to lower heights to fill in the gaps. The only problem with this theory is that it totally falls apart when applied to extremely small situations—i.e., the nanoscale.

Hanchen Huang, pro­fessor and chair of the Depart­ment of Mechan­ical and Indus­trial Engi­neering, has spent the last 10 years revising the clas­sical theory of crystal growth that accounts for his obser­va­tions of nanorod crys­tals. His work has gar­nered the con­tinued sup­port of the U.S, Depart­ment of Energy’s Basic Energy Sci­ence Core Program.

Nanorods are minis­cule fibers grown per­pen­dic­ular to a sub­strate, each one about 100,000 times thinner than a human hair. Sur­face steps, or the minor vari­a­tions in the ver­tical land­scape of that sub­strate, deter­mine how the rods will grow.

“Even if some sur­face steps are closer and others more apart at the start, with time the clas­sical theory pre­dicts they become more equal­ized,” Huang said. “But we found that the clas­sical theory missed a pos­i­tive feed­back mechanism.”

This mech­a­nism, he explained, causes the steps to “cluster,” making it more dif­fi­cult for atoms to fall from a higher step to a lower one. So, instead of filling in the height gaps of a vari­able sur­face, atoms in a nanorod crystal localize to the highest levels.

“The taller region gets taller,” Huang said. “It’s like, if you ever play bas­ket­ball, you know the taller guys will get more rebounds.” That’s basi­cally what hap­pens with nanorod growth.

Huang’s theory, which was pub­lished in the journal Phys­ical Review Let­ters this year, rep­re­sents the first time anyone has pro­vided a the­o­ret­ical frame­work for under­standing nanorod crystal growth. “Lots of money has been spent over the past decades on nanoscience and nan­otech­nology,” Huang said. “But we can only turn that into real-​​world appli­ca­tions if we under­stand the science.”

Indeed, his con­tri­bu­tion to under­standing the sci­ence allowed him and his col­leagues to pre­dict the smallest pos­sible size for copper nanorods and then suc­cess­fully syn­the­size them. Not only are they the smallest nanorods ever pro­duced, but with Huang’s theory he can con­fi­dently say they are the smallest nanorods pos­sible using phys­ical vapor deposition.

The mate­rial has major impli­ca­tions for com­mer­cial appli­ca­tions, including a kind of metallic glue that can fuse two pieces of metal together at room tem­per­a­ture, in ambient envi­ron­ment, and with very little pres­sure input. This tech­nology may enable ambient sol­dering without the need of poi­so­nous lead, and could there­fore be extremely valu­able to the semi­con­ductor industry, which per­vades society through the ubiq­ui­tous use of hand­held and other com­puter devices.

Smallest Metallic Nanorods Using Physical Vapor Deposition

Source: Northeastern Univ.

 
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