Tag Archives: Angela Herring

Snow reveals the truth about crystalline growth

A Jan. 24, 2014 news item on Nanowerk has a beautiful and timely (given the snowy, frigid weather in Eastern Canada and the US) opening for a story about crystals and metallic nanorods,

This 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 normall – 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.

The Jan. 23, 2014 Northeastern University news release by Angela Herring, which originated the news item, goes on to provide some context and describe this work concerning nanorods,

Hanchen Huang, pro­fessor and chair of the Depart­ment of Mechan­ical and Indus­trial Engi­neering [Northeastern University located in Massachusetts, US], 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.

Here’s a link to and a citation for the paper,

Smallest Metallic Nanorods Using Physical Vapor Deposition by Xiaobin Niu, Stephen P. Stagon, Hanchen Huang, J. Kevin Baldwin, and Amit Misra. Phys. Rev. Lett. 110 (no. 13), 136102 (2013) [5 pages] DoI:
10.1103/PhysRevLett.110.136102

This paper is behind a paywall.

Nature imitates art at Northeastern University (US)

It’s an intriguing mental exercise trying to flip the tables on nature as an inspiration for art to start discussing ‘artmimetics’ as they seem to be doing at Northeastern University (Boston, Massachusetts, US), according to a Dec. 11, 2013 news item on Azonano,

There are exam­ples of art imi­tating nature all around us—whether it’s Monet’s pastel Water Lilies or Chihuly’s glass­blown Seaforms, the human con­cep­tion of nat­ural phe­nomena daz­zles but does not often surprise.

Yet when asso­ciate pro­fessor of physics Latika Menon peered under the elec­tron micro­scope last fall, she dis­cov­ered the exact oppo­site. Instead of art imi­tating nature, she found nature imi­tating art.

The Dec. 10, 2013 Northeastern University news release by Angela Herring, which ‘inspired’ the news item, describes how Menon and her colleagues came to reverse the inspirational direction,

Menon grew up in the eastern region of India and was vaguely familiar with a cul­tural dance from the western state of Rajasthan known as the Bhavai pot dance. Nimble dancers sway their hips as a tall stack of wide-​​bellied pots bal­ances gin­gerly atop their heads. Back in the lab at North­eastern, Menon’s team recently cre­ated  gal­lium nitride nanowires, which bore a striking resem­blance to that stack of pots.

What’s more, a post­doc­toral research asso­ciate in Menon’s lab, Eugen Panaitescu, jumped on the band­wagon with a cul­tural art ref­er­ence of his own. Panaitescu, who hails from Romania, also saw his country’s famous End­less Column reflected in the nanowires. Ded­i­cated to the fallen Romanian heroes of World War I, Con­stantin Brancusi’s 96-​​foot-​​tall mono­lith is con­structed of 17 three-​​dimensional rhom­buses, peri­od­i­cally wavering from a wider cir­cum­fer­ence to a nar­rower one.

The news release goes on to explain more about applications using gallium nitride and why Menon’s insight may prove useful in developing new uses for gallium nitride nanowires,

… Gal­lium nitride is used across a range of tech­nolo­gies, including most ubiq­ui­tously in light emit­ting diodes. The mate­rial also holds great poten­tial for solar cell arrays, mag­netic semi­con­duc­tors, high-​​frequency com­mu­ni­ca­tion devices, and many other things. But these advanced appli­ca­tions are restricted by our lim­ited ability to con­trol the material’s growth on the nanoscale.

The very thing that makes Menon’s nanowires beau­tiful rep­re­sents a break­through in her ability to process them for these novel uses. She deposited onto a sil­icon sub­strate small droplets of liquid gold metal, which act as cat­a­lysts to grab gaseous gal­lium nitride from the atmos­phere of the exper­i­mental system. The net forces between the tiny gold droplet, the solid sub­strate, and the gas cause the nanowire to grow in a par­tic­ular direc­tion, she explained. Depending on the size of the gold cat­a­lyst, she can create wires that exhibit peri­odic serrations.

“It first tries to grow out­ward, but that gives the gold a larger sur­face area,” she said. “So now the wire gets pulled in the inward direc­tion, and then the gold gets a smaller sur­face area, so it grows out­ward again.” This inward and out­ward growth repeated itself again and again to create a peri­odic struc­ture nearly 6 mil­lion times smaller than the end­less column and is sig­nif­i­cantly more promising for its use in advanced devices.

“That there is very little imple­men­ta­tion of nanowire tech­nology in elec­tronics or optical devices is due to the fact that it’s very hard to con­trol their shape and dimen­sions,” said Menon. But now that she has a very simple way of con­trol­ling growth, the next step is to con­trol the size of the cat­alytic droplet with which she starts.

Another advan­tage of Menon’s approach is using what Panaitescu called “macro­scopic tech­niques” to create nanoscale mate­rials, thus making it scal­able and inex­pen­sive. “We just con­trol a few para­me­ters and then leave it, let it do it’s nat­ural thing,” explained Menon.

Here’s an image the researchers have supplied to illustrate their insights and their work,

Depending on the size of the gold cat­a­lyst used to make them, Latika Menon’s nanowires will exhibit peri­odic grooves that resemble common motifs in art. Images cour­tesy of Latika Menon. - See more at: http://www.northeastern.edu/news/2013/12/menon-nanowires/#sthash.LkgJU4es.dpuf

Depending on the size of the gold cat­a­lyst used to make them, Latika Menon’s nanowires will exhibit peri­odic grooves that resemble common motifs in art. Images cour­tesy of Latika Menon. – See more at: http://www.northeastern.edu/news/2013/12/menon-nanowires/#sthash.LkgJU4es.dpuf

I’m not sure I can connect the  imagery in this pot dance video (it does show some pretty astonishing feats of balance) with any of the images from Menon’s lab but sometimes the source of an inspiration is not readily accessible to those who are not amongst the inspired or perhaps there other versions of the dance that make it more obvious to an untrained eye,

Here’s an image of the other artistic inspiration, Constantin Brancusi’s Endless Column found on Dr. Cătălina Köpetz’s (University of Maryland) webpage featuring Brancusi’s work along with this quote from him “Create like a god, comand like a king, work like a slave.”

The Endless Column, Târgu Jiu, România  [downlaoded from http://terpconnect.umd.edu/~ckopetz/brancusi.htm]

The Endless Column,
Târgu Jiu, România [downlaoded from http://terpconnect.umd.edu/~ckopetz/brancusi.htm]

Interestingly, Dr. Köpetz is a social psychologist working in the university’s Center for Addictions, Personality, and Emotion Research.

For anyone who’d like to read more about Menon’s work, here’s a link to a webpage featuring a PDF selection of her papers and a citation for her latest paper on the work described in the news release,

Vapor–liquid–solid growth of serrated GaN nanowires: shape selection driven by kinetic frustration by Zheng Ma, Dillon McDowell, Eugen Panaitescu, Albert V. Davydov, Moneesh Upmanyu and Latika Menon, Physics Faculty Publications (2013)

Compound semiconducting nanowires are promising building blocks for several nanoelectronic devices yet the inability to…

The paper is open access although you will have to click a few times to retrieve it.

Directed assembly—faster, better, cheaper than 3D printing

Ahmed Bus­naina, director of the NSF (US National Science Foundation) Nanoscale Sci­ence and Engi­neering Center for High-​​rate Nanoman­u­fac­turing at North­eastern University explained current 3D printing technology and how his directed assembly method constitutes a serious upgrade in a Northeastern University Mar. 14, 2013 news release by Angela Herring,

The modern 3-​​D printer is basi­cally a spe­cial­ized ink-​​jet printer. It uses a printer head with spe­cial ink that could con­tain a polymer, par­ti­cles, or nan­otubes sus­pended in solu­tion, or really any­thing. It prints line by line, so prod­ucts requiring higher res­o­lu­tion or large areas take a very long time.

What we have devel­oped at our center is a system that’s like news­paper printing or printing money, where you have a big plate, you put ink on it, and bang: One hit, you’re done. Only here, the ink is made of very small and very sen­si­tive nanopar­ti­cles attracted to the tem­plate using elec­trophoresis, so we have to pick exact dimen­sions and materials.

We put a tem­plate with a pat­tern rep­re­sented by nanowires into a solu­tion that is sim­ilar to ink, but very dilute. Then we apply a couple of volts so that nanopar­ti­cles in the ink are drawn to the nanowires. Then we take out the tem­plate and transfer the assem­bled nanopar­ti­cles to a sur­face of either a hard or flex­ible sub­strate. That would be the first layer of a device, which takes about a minute or two. A sensor may have just a few layers, where advanced elec­tronics may have 10 layers or more.

Busnaina contrasts the speed and range of scales between the current method and his directed assembly method (from the news release),

For low-​​cost, low-​​end prod­ucts, 3-​​D printers are very good but they are slow—it can take days to print a single product. But with directed assembly, we can do low-​​cost, high-​​end prod­ucts, and we can do them very quickly. So, directed assembly will be very valu­able for high-​​value devices like sen­sors, advanced elec­tronics, energy har­vesting, or bat­teries. It might also be used for tissue engi­neering and printing bio­ma­te­rials like cells or proteins.

Directed assembly allows 3-​​D printing to be faster, cheaper, and mul­ti­scale. It can do nano, micro, and macro simul­ta­ne­ously over a large area. No 3-​​D printer can do that; this is beyond the cur­rent 3-​​D printing tech­nology. This will reduce the cost of expen­sive elec­tronics such as an iPhone for less than $10 and sensor sys­tems for a frac­tion of a dollar. These could be sen­sors for health, the envi­ron­ment, infra­struc­ture, water resources, any­thing. They will make advanced prod­ucts afford­able to people in all income classes, not just high-​​income pop­u­la­tions or countries.

What we’re trying to do is make high-​​value things, such as sen­sors, energy-​​harvesting devices, or phone dis­plays, using this tech­nology, which costs 1 per­cent of con­ven­tional man­u­fac­turing. That also means you can make all kinds of devices by design, printing things exactly to specifications—even down to the nanoscale (one thou­sand times smaller than a human hair).

For example, we devel­oped an energy-​​harvesting device that can use any heat source—even body heat—to charge a sensor or a phone. An antenna absorbs heat and con­verts it to cur­rent. We print it using carbon nan­otubes for the ink. This kind of device would not be pos­sible with tra­di­tional 3-​​D printing—it just can’t go that small.

Exciting stuff and you can read more about it at the Northeastern University website or where I first found the item at phys.org.