Silver nanowires could be key to printable electronics
March 3, 2017
Every smart phone, computer and electronic device relies on conducting circuits: loops of material that funnel electrons through a device. As electronics get more sophisticated, however, engineers are looking for new ways to make circuits that are more flexible and that can work on different surfaces.
Circuits can now be produced with an ink jet printer. Nanoparticles of silver dissolved in water make a sort of ink that conducts electricity. Now researchers from Duke University have found that by changing the shape of the nanoparticles, they can make a highly conductive ink that can be used on almost any surface.
When a metal is broken down into nanoparticles—when either its length, width or height are thinner than one-hundredth of a percent of a millimeter—some of its properties change, including its ability to conduct electricity.
Silver by itself is a great electrical conductor. However, Duke chemistry professor Benjamin Wiley, senior author of this study, says that electricity moves more slowly through a collection of nanoparticles because electrons have trouble jumping from one nanoparticle to another. This is a property of circuits that physicists call "contact resistance."
The researchers experimented with several different nanoparticle shapes to see how the nanoparticle shapes affected electrical conductivity. So they did a side-by-side comparison of nanoparticle shapes—one of the first comparisons of its kind in published science—in which they tightly controlled the amount of silver and added ingredients in the ink, so the only variable was the shape of the nanoparticles.
Ian Stewart, lead author of the study and Duke graduate student, made granular and flake-shaped nanoparticles, as well as long and short nanowires. He created inks by dissolving the particles in distilled water and deposited equal amounts of each ink on glass surfaces. After heating the ink, the researchers measured the electrical resistance—how much a material limits charges from flowing through it—in each ink.
The long nanowires conducted electricity up to 4,000 times better than the other nanoparticle shapes. The electrons were able to quickly travel a relatively long distance along one before having to jump to another. In fact, a solid silver wire was only 10 times more conductive than the long nanowires, which may sound like a big difference but is remarkably small for a printable conductor.
The long nanowires have a few further advantages over the other shapes. First, silver nanoparticles found in some of today’s printable electronics need to be sintered, a process that involves heating a powder just enough for the grains to fuse together without completely melting. You can see a video of sintering silver nanoparticles here. Sintering the silver decreases the number of jumps the electrons have to make as they run through the nanoparticles, but it often requires heating the material to more than 200 degrees Celsius (392 degrees Fahrenheit). Plenty of materials like heavy plastics can take the heat. New DVDs, for example, have circuits printed on the back to prevent theft. The high temperature required for these, however, limits the materials that can hold a printable circuit.
The long nanowires, however, do their best conducting with only enough heat to dry the ink. In fact, they were more conductive on their own than the granular nanoparticles were after sintering. That opens up lighter plastics and even paper as materials for printable circuits. Paper and plastic circuits could allow for smart packaging, more inexpensive solar cell electronics and touchscreens. In the medical field, an increased selection of materials could even open the door to printed electronics implanted in the body.
There is also an economic upside to the long nanowires. Because they perform better than the other shapes using the same amount of silver, there is an opportunity to get the same performance as current printed circuits using less silver. Given that silver is not a cheap material, this could present meaningful savings over millions of circuits. The researchers think they might even be able to find further savings by using silver-plated copper nanowires, but more research would be needed to determine whether those cheaper wires could conduct as well.
Electronics are becoming an increasingly dominant part of daily life. Studies like this one broaden the scope of what electronics can be, where they can go and how well they can work. The study was published in the journal ACS Applied Materials and Interfaces.
Daniel Lane covers science, medicine, engineering and the environment in North Carolina.