Flexible CRT Displays

The unique electronic properties of carbon nanotubes make them promising for a range of applications, including use as ultra-efficient “electron emitters” in bright, low-power displays. Now researchers have found a way to pattern carbon nanotubes in plastic sheets that could lead to flexible versions of these displays – and electronics that you could roll up and put in your pocket.

Several companies, such as Samsung and Motorola, are developing carbon nanotube-based displays that take advantage of the fact that nanotubes can emit electrons extremely efficiently. Like familiar bulky cathode ray tube (CRT) displays, these nanotube versions use electrons to excite phosphors on a screen to produce an image. But unlike standard CRTs, nanotubes displays can be flat, and they use much less energy than other flat-panel technologies.

The new method developed by researchers at Rensselaer Polytechnic Institute (RPI), Northeastern University, and New Mexico State University, could lead to flexible, flat-screen CRTs. The process begins with a pre-patterned surface that controls where multi-walled nanotubes grow. Next, the researchers pour a liquid over the nanotubes and cook it until it forms a polymer. They then peel off the polymer along with the nanotubes. The polymer preserves the nanotube pattern down to the positions of individual nanotubes and keeps them aligned in one direction.

For display applications, where single nanotubes must be isolated from others to get the best efficiencies, the researchers strip off a layer of polymer to expose the tips of nanotubes, then burn off long or tangled nanotubes, leaving only isolated ones. This method has produced very efficient electron emission, the researchers say. “The results we’ve seen are some of the best that have been reported in the literature,” says Swastik Kar, a postdoctoral research in materials science and engineering at RPI and lead author of the paper.

To be sure, the patterned nanotubes are just the first step toward a flexible nanotube display, which, in addition to the nanotube emitters, requires electronics for addressing individual pixels of the display, and a way of making a similarly flexible phosphor layer. The structure will also need to be sturdy enough to maintain a vacuum inside the device. In all, it will likely be at least a few years before a prototype display is ready, says Kar.

The nanotube-plastic composites may lead to other applications. The ability to carefully control patterns of nanotubes may lead to other kinds of flexible, nanotube-based electronics. Also, the plastic-nanotube films can detect small changes in pressure: as the plastic film is compressed, the nanotubes get rearranged, the researchers say, producing a detectable change in the conductivity of the material. This pressure sensitivity is something like the sense of touch, leading the researchers to call their invention “nano-skin.”

The RPI researchers are also working with scientists who used nanotubes as adhesives, mimicking the structures that allow geckos to cling to walls. The extremely high surface area of the nanotubes creates enough friction to hold two surfaces together. One possibility that uses the flexible plastic is a souped-up version of Velcro.

The RPI work is part of a much larger research effort to combine nanotubes with polymers and other flexible materials. “Flexible nanotube-polymer films will find a large range of applications, not only for electronics, but also for sensing applications and even optical applications,” says Liming Dai, professor of materials engineering and chemistry at the University of Dayton in Ohio, who recently developed a chemical sensor using nanotubes embedded in plastic. “It’s an important area. Now is the time for people to push these things toward real applications.”

Home page image courtesy of Yung Joon Jung, Northeastern University, Boston MA. Caption: A sample of the plastic with embedded half-millimeter-wide dots of nanotubes.