Revolutionizing the Touch Screen?
Using nanotechnology, researchers fashion a new kind of transparent electrode for flat-panel displays
By Patrick Glynn
Image courtesy of Yi Cui
Illustrated are fabrication and transfer processes for nanotroughs.
Link to larger image
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 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 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.
Three supplementary video clips accompanying the Nature Nanotechnology
article dramatize the properties of the electrode (the clips are available free of charge in the Nature Nanotechnology, June 2013 issue
Patrick Glynn is at the DOE Office of Science, Patrick.Glynn@science.doe.gov