Nanowires for Displays

Researchers at the University of Illinois in Urbana Champaign have developed a simple process to grow upright copper nanowires on different surfaces. The nanowire arrays could find use in field-emission displays, a new type of display technology that promises to provide brighter, more vivid pictures than existing flat-panel displays. In such an application, the nanowires would be used to fire electrons at phosphor particles on a screen, lighting them up.

The new manufacturing method, developed by Kyekyoon Kim and Hyungsoo Choi, leads to copper nanowires between 70 nanometers and 250 nanometers wide. The researchers can use the process to grow the nanowires on various surfaces, including silicon, glass, metal, and plastic. They describe the nanowire array and demonstrate a prototype field-emission display in an online Advanced Materials paper.

Vertical arrays of metal nanowire hold promise for making chemical and biological sensors in addition to electron emitters in field-emission displays (FEDs). But the difficulty of growing well-defined arrays has kept these technologies at bay, says Yugang Sun, a scientist at Argonne National Laboratory’s Center for Nanoscale Materials. Controlling the vertical growth of nanowires typically involves growing them in a template made from another material. Fabricating and then washing away the template is time-consuming. Moreover, many of these methods involve transferring the nanowires to the desired surface.

The new method does not require a template. The researchers use a common synthesis method called chemical vapor deposition. They expose the substrate to vapors of a specially made copper-containing compound at 200 to 300 degrees Celsius. The resulting copper nanowires that grow on the substrate are five-sided with a sharp pentagonal tip. “The challenge is to design and synthesize a precursor and proper conditions under which nice wires will grow,” Kim says.

The copper nanowires are suitable for use in FEDs because they are uniform and have a very pointed tip. “The smaller the tip size the stronger the electric field,” Kim says. “That is why even with a very small voltage…they will become very efficient electron emitters.” The nanowires emit electrons at low voltages of 100 Volt, unlike the tungsten filament used in conventional, bulky cathode-ray-tube televisions (CRTs), which require many kilovolts.

Field-emission displays promise to be less power-hungry than plasma screens and liquid crystal displays, while keeping a CRTs brightness and sharpness. They work on a similar principle as CRTs, but are only a few millimeters thick. Instead of using a single electron gun, they use millions of tiny electron emitters to shoot electrons at red, green and blue phosphors coated on a screen.

Companies such as Sony and Motorola first tried to commercialize field-emission displays about 10 years ago. Those displays used micrometer-sized metal tips as electron emitters. But the tips required high voltages and couldn’t be made on large areas. Some manufacturers then shifted attention to carbon nanotubes. Both Samsung and Motorola have developed carbon nanotube-based FED technology (see “Nanotech on Display” and “High-Definition Carbon Nanotube TVs”). Field Emission Technologies, a Sony spin-off, is taking a different approach. They are using metal nanotips as emitters. The company plans to ship professional FED video monitors based on this technology in 2009.

But all of these displays are expensive and are still not ready for the commercial TV market. The reason for that is both economical and technical, says David Barnes, an analyst at DisplaySearch, a consultancy in Austin, TX. One of the key technological barriersis to create and maintain a vacuum between the electron emitters and the phosphor-coated glass. The emitters can also degrade over time due to the extremely high energies forming at their tips. Maintaining both the vacuum and the emitters for a TV’s 10-year life is a challenge.

Field-emission displays that use copper nanowires will face the same problems. However, says Barnes, copper “.might be a little more robust.”

Chris Chinock, founder and president of Insight Media, a Norwalk, CT-based consulting firm that focuses on the display industry, calls the new development a promising research result, although too early-stage “to put on our radar just yet.” He points out that the nanowires will have to be thinner than 70- to 250-nanometers. Carbon nanotubes and metal nanotips are only a few nanometers big, resulting in 10,000 or more emitters at each pixel. Even if half of them do not work, there are still enough to light up the pixel.

While the industry is not expecting commercial FED displays out any time soon, Barnes says that more research on different new technologies is warranted. “When people have made lab prototypes it’s pretty compelling,” he says. “There is this bright aliveness that you would get from watching a traditional CRT.”