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Today's touch screen and flat-panel displays require ultrathin sheets made from an unusual compound containing the element indium. Researchers have developed a replacement fabricated from common, earth-abundant materials using nanotechnology.
We seem to be living in the age of the touch screen. Everywhere you look these days, people are peering into and poking away at the screens of their favorite gadgets—smartphones, tablets, music players, etc. What's interesting is that this pervasive technology depends critically on a natural element—"indium"—that is almost rare, and certainly less than abundant around the planet. Nearly all touch screens—and in fact virtually every flat-panel display, from laptops to contemporary flat screen TVs—contain a couple of ultrathin sheets made of an indium compound. The reason? The compound—called indium tin oxide, or ITO—has an uncommon pair of properties: it's a pretty good conductor of electricity, and it also happens to be transparent (at least in very thin layers).
When you look at your flat-screen computer monitor or smartphone, the light you are seeing is actually passing through two of these transparent ITO sheets—and they are absolutely vital to the screen's operation. One carries a positive charge while the other carries a negative one. The transparent ITO sheets are in fact the screen's electrodes. Together, in combination with transistors and some other components, they generate the electric fields that in turn manipulate the liquid crystals that form the pixels that compose the images you are seeing. (Touch screens require some additional components beyond passive flat-panel displays, which enable the electric field to be altered when the screen is touched.)
The widespread use of indium in these devices has long raised a couple of concerns. First, as mentioned, the element is not very abundant, and as global demand has skyrocketed in recent years, there have been worries about price. The lion's share of extracted indium is produced outside the United States, as a byproduct of zinc mining. China is a major producer, for example, as are South Korea, Canada, Japan, and a number of other countries. So access to indium is dependent on a global supply chain. There's even been concern expressed about the supply of indium being exhausted, perhaps in a matter of a couple of decades.
Second, while the indium compound ITO seems to be the best material available for the job at the moment, it does have its drawbacks. To keep it transparent, it must be made very thin, but thinning it reduces its conductivity, so there's a tradeoff. Also, it's brittle—which complicates industrial production. And it turns out to be almost opaque to the infrared end of the spectrum—a limitation when it comes to photovoltaic applications.
So for some years researchers have been seeking substitutes for ITO. Many alternatives have been proposed, but nearly all to date have been shown to have clear disadvantages. (For example, one alternative might have greater physical flexibility, but low electrical conductivity—while another might have requirements for purity that would make industrial production prohibitively difficult and expensive).
Recently, however, researchers at a DOE Energy Frontier Research Center (EFRC), using nanotechnology, have devised a new kind of transparent electrode that shows real promise as an ITO substitute. The electrode is highly transparent. It has electrical conductivity essentially equal to commercial-grade ITO. It is not brittle, but rather extremely flexible and also fairly physically robust overall. And, perhaps best of all, it can be fabricated using earth-abundant materials and well-known fabrication processes, which in theory could be scaled up to an industrial level.
The researchers subjected the electrode to stress tests and found that repeated bending did little to diminish performance.”
The electrode was developed at the Center on Nanostructuring for Efficient Energy Conversion (CNEEC), an EFRC at Stanford University, one of 46 such EFRCs established by the DOE Office of Science in 2009. Leading the research team was Stanford Associate Professor Yi Cui, a researcher at the Stanford Institute for Materials and Energy Sciences, a joint institute of Stanford University and DOE's SLAC National Accelerator Laboratory. The research, supported by the DOE Office of Science, was reported in the journal Nature Nanotechnology.
Essentially, the CNEEC electrode consists of a crisscrossed network of metal "nanotroughs," or specially shaped nanowires, affixed to a transparent plastic sheet. The combination of the tiny size and the shape of the nanotroughs make them invisible to the eye. So the sheet looks transparent. Yet the network of nanotroughs provides a level of electrical conductivity comparable to that of ITO.
The researchers used an interesting fabrication process to fashion the electrode. The first step was to generate a ring of crisscrossed polymer fibers, using a well-known process called electrospinning. Electrospinning uses an electrical charge to deposit a nanoscale thread of polymer on a spinning receptor, such as a copper ring. The effect is to wind the fiber around the copper ring until the ring is crisscrossed with a web of intersecting nanoscale plastic thread.
Then the researchers used of one of several "deposition" processes, such as thermal evaporation, to deposit a thin coating of metal on the polymer threads. At different points, the researchers tried silver, copper, platinum, aluminum, chromium, nickel and their alloys. They even tried ITO. Copper performed the best in terms of conductivity and transparency.
Then using a lamination process, the researchers transferred the metal-coated polymer to various kinds of substrates, including a glass slide, paper, textile, a curved glass, and a plastic sheet. (The plastic sheet would be the best candidate for the flat-panel screen application.) They submerged the electrode in water or other solvents to dissolve away the polymer, so that only the network of metal and the substrate remained.
When the polymer dissolved away, the nanowires were left with a "dent" or trough running along the bottom. Hence the term "nanotrough." It turns out that this concave shape is important: it reduces the wire's electromagnetic cross-section, permitting more light to past through the substrate, thereby enhancing the invisibility of the nanowires and the transparency of the electrode.
Typically, nanowires have only a small fraction of the electrical conductivity of their normal-size counterparts. The researchers in this case were able to achieve conductivity levels at roughly half the level of bulk wires, which is unusually high. The researchers theorized that the deposition process probably produced fairly pure metals, which may have helped boost conductivity; in addition the networked configuration of the nanotroughs—and the fact that they were continuous rather than in some way "spliced" together—also may have lowered resistance.
The researchers characterized the electrodes using electron microscopy. They also subjected the electrode to stress tests and found that repeated bending did little to diminish performance.
Three supplementary video clips accompanying the Nature Nanotechnology article dramatize the properties of the electrode (the clips are available free of charge at http://www.nature.com/nnano/journal/vaop/ncurrent/full/nnano.2013.84.html).
Image courtesy of Yi Cui
Researchers used a technique called electrospinning to create a network of nanoscale polymer threads, on which they deposited a thin metal coating. The network of metal nanowires was then attached to a substrate and the assembly soaked in a solvent to dissolve away the polymer.
In the first clip, a researcher attaches a piece of tape to the electrode and then tears it away—the electrode continues to function, unharmed, showing the strength of the bond between the metal and the substrate. In the second clip, the researchers show how affixing the metal to the adhesive side of transparent Scotch-like tape transforms the tape into an electrical conductor. The third clip shows that the plastic version of the electrode is extremely flexible and can be bent without deterioration in performance—and also that it is flexible enough to function as a touch screen. In the clip, a researcher writes the word "Stanford" on the electrode, and as the researcher writes, the electrode makes contact with another electrode to complete the circuit, and the writing simultaneously appears on a computer monitor to which the electrode is connected.
The one big issue that the current study didn't address is the chemical stability of the electrode—that is, how it would stand up in relation to moisture and other environmental stresses. The researchers suggest that the chemical stability of the nanotroughs could be enhanced through a process known as "passivation," which involves coating a material with an anti-corrosive layer.
ITO is fairly entrenched as a technology, and whether the CNEEC electrode could make inroads into the touch screen and flat-screen market remains to be seen. But the work illustrates how control of matter at the nanoscale enables fabrication of artificial materials that cannot only rival but also surpass the properties of rare natural ones.
—Patrick Glynn, DOE Office of Science, Patrick.Glynn@science.doe.gov
Hui Wu, Desheng Kong, Zhichao Ruan, Po-Chun Hsu, Shuang Wang, Zongfu Yu, Thomas J. Carney, Liangbing Hu, Shanhui Fan, and Yi Cui, "A transparent electrode based on a metal nanotrough network," Nature Nanotechnology, published online May 19, 2013
DOE Office of Science, Office of Basic Energy Sciences
National Basic Research of China
Center on Nanostructuring for Efficient Energy Conversion
DOE Energy Frontier Research Centers