Tag Archives: Jiaxing Huang

Silver nanowires have a surprising ability to self-heal

It seems there could be a new member of the flexible electronics materials community, silver nanowires, according to a Jan. 23, 2015 news item on ScienceDaily,

Wth its high electrical conductivity and optical transparency, indium tin oxide is one of the most widely used materials for touchscreens, plasma displays, and flexible electronics. But its rapidly escalating price has forced the electronics industry to search for other alternatives.

One potential and more cost-effective alternative is a film made with silver nanowires–wires so extremely thin that they are one-dimensional–embedded in flexible polymers. Like indium tin oxide, this material is transparent and conductive. But development has stalled because scientists lack a fundamental understanding of its mechanical properties.

A Jan. 23, 2015 Northwestern University news release (also on EurekAlert), which originated the news item, explains what makes silver nanowires a candidate as an alternative to indium tin oxide for use in flexible electronics,

… Horacio Espinosa, the James N. and Nancy J. Farley Professor in Manufacturing and Entrepreneurship at Northwestern University’s McCormick School of Engineering, has led research that expands the understanding of silver nanowires’ behavior in electronics.

Espinosa and his team investigated the material’s cyclic loading, which is an important part of fatigue analysis because it shows how the material reacts to fluctuating loads of stress.

“Cyclic loading is an important material behavior that must be investigated for realizing the potential applications of using silver nanowires in electronics,” Espinosa said. “Knowledge of such behavior allows designers to understand how these conductive films fail and how to improve their durability.”

By varying the tension on silver nanowires thinner than 120 nanometers and monitoring their deformation with electron microscopy, the research team characterized the cyclic mechanical behavior. They found that permanent deformation was partially recoverable in the studied nanowires, meaning that some of the material’s defects actually self-healed and disappeared upon cyclic loading. These results indicate that silver nanowires could potentially withstand strong cyclic loads for long periods of time, which is a key attribute needed for flexible electronics.

“These silver nanowires show mechanical properties that are quite unexpected,” Espinosa said. “We had to develop new experimental techniques to be able to measure this novel material property.”

The findings were recently featured on the cover of the journal Nano Letters. Other Northwestern coauthors on the paper are Rodrigo Bernal, a recently graduated PhD student in Espinosa’s lab, and Jiaxing Huang, associate professor of materials science and engineering in McCormick.

“The next step is to understand how this recovery influences the behavior of these materials when they are flexed millions of times,” said Bernal, first author of the paper.

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

Intrinsic Bauschinger Effect and Recoverable Plasticity in Pentatwinned Silver Nanowires Tested in Tension by Rodrigo A. Bernal, Amin Aghaei, Sangjun Lee, Seunghwa Ryu, Kwonnam Sohn, Jiaxing Huang, Wei Cai, and Horacio Espinosa. Nano Lett., 2015, 15 (1), pp 139–146 DOI: 10.1021/nl503237t Publication Date (Web): October 3, 2014
Copyright © 2014 American Chemical Society

This particular version of the paper is behind a paywall. However, access to the paper is possible although I make no claims as to which version it is or whether it will continue to be freely accessible.

Nano crafts class: get out your ‘paper’ and scissors

It’s not all atomic force microscopy and nanotweezers as scientists keep reminding us that the techniques we learned in kindergarten can be all the high technology we need even when working at the nanoscale. From the Nov. 14, 2012 news item on ScienceDaily,

Two Northwestern University researchers have discovered a remarkably easy way to make nanofluidic devices: using paper and scissors. And they can cut a device into any shape and size they want, adding to the method’s versatility.

The Nov. 14, 2012 Northwestern University news release by Megan Fellman explains both nanofluidic devices and the new technique,

Nanofluidic devices are attractive because their thin channels can transport ions — and with them a higher than normal electric current — making the devices promising for use in batteries and new systems for water purification, harvesting energy and DNA sorting.

The “paper-and-scissors” method one day could be used to manufacture large-scale nanofluidic devices without relying on expensive lithography techniques.

The Northwestern duo found that simply stacking up sheets of the inexpensive material graphene oxide creates flexible “paper” with tens of thousands of very useful channels. A tiny gap forms naturally between neighboring sheets, and each gap is a channel through which ions can flow.

Using a pair of regular scissors, the researchers simply cut the paper into a desired shape, which, in the case of their experiments, was a rectangle.

“In a way, we were surprised that these nanochannels actually worked, because creating the device was so easy,” said Jiaxing Huang, who conducted the research with postdoctoral fellow Kalyan Raidongia. “No one had thought about the space between sheet-like materials before. Using the space as a flow channel was a wild idea. We ran our experiment at least 10 times to be sure we were right.”

The process is a little more complex than kindergarten crafts (from Fellman’s news release),

To create a working device, the researchers took a pair of scissors and cut a piece of their graphene oxide paper into a centimeter-long rectangle. They then encased the paper in a polymer, drilled holes to expose the ends of the rectangular piece and filled up the holes with an electrolyte solution (a liquid containing ions) to complete the device.

Next they put electrodes at both ends and tested the electrical conductivity of the device. Huang and Raidongia observed higher than normal current, and the device worked whether flat or bent.

The nanochannels have significantly different — and desirable — properties from their bulk channel counterparts, Huang said. The nanochannels have a concentrating effect, resulting in an electric current much higher than those in bulk solutions.

Graphene oxide is basically graphene sheets decorated with oxygen-containing groups. It is made from inexpensive graphite powders by chemical reactions known for more than a century.

Scaling up the size of the device is simple. Tens of thousands of sheets or layers create tens of thousands of nanochannels, each channel approximately one nanometer high. There is no limit to the number of layers — and thus channels — one can have in a piece of paper.

To manufacture very massive arrays of channels, one only needs to put more graphene oxide sheets in the paper or to stack up many pieces of paper. A larger device, of course, can handle larger quantities of electrolyte.

Kindergarten techniques worked well for Andre Geim and Konstantin Novoselov who received Nobel prizes for their work on graphene (from my Oct. 7,2010 posting),

The technique that Geim and Novoselov used to create the first graphene sheets both amuses and fascinates me (from the article by Kit Eaton on the Fast Company website),

The two scientists came up with the technique that first resulted in samples of graphene–peeling individual atoms-deep sheets of the material from a bigger block of pure graphite. The science here seems almost foolishly simple, but it took a lot of lateral thinking to dream up, and then some serious science to investigate: Geim and Novoselo literally “ripped” single sheets off the graphite by using regular adhesive tape.

Then, there’s the ‘Shrinky Dinks’ nanopatterning technique (from my Aug. 16,2010 posting),

Scientists at a Northwestern University laboratory have taken to using a children’s arts and crafts product, Shrinky Dinks, for a new way to create large area nanoscale patterns on the cheap.

It’s good to be reminded that science at its heart is not about expensive equipment and complicated techniques but a means of exploring the world around us with the means at hand.