Building Better Nanotubes for Displays

Hank Hogan

Carbon nanotubes could become the basis of improved video displays if manufacturing problems are solved. Now a group of researchers from Tsinghua University in Beijing has demonstrated the fabrication of quality nanotubes on a transparent substrate using a diode laser. The technique could prove useful in manufacturing flat panel displays because it eliminates constraints on substrate geometry and size. It also removes the need of the substrate to withstand high-temperature processing.

Carbon nanotubes spell out letters in these field emission patterns on a fluorescent screen. A voltage is applied across a display device, with a carbon nanotube cathode created using a laser and a commercial graphite inner coating layer as a light absorber. The voltage causes the carbon nanotubes to emit electrons, which strike the screen and create fluorescence. An array of such nanotubes could be used to create the flat panel equivalent of a standard television.

Because of their nanometer-size tips, high aspect ratios and structural stability, carbon nanotubes make good field emission electron sources. Such sources can form part of a display that, in essence, flattens and miniaturizes the cathode-ray tube into a flat panel. However, growing high-quality nanotubes takes elevated temperatures, a requirement that runs up against the roughly 600 °C softening point of glass.

Traditional methods also require putting the entire glass plate into a chemical vapor deposition furnace. For the latest flat panel liquid crystal display generation, that technique would mean accommodating a piece of glass that measures 2160 × 2460 mm, or about 7 × 8 ft, in a furnace.

In their approach, the researchers turned to a different heating source: a low-cost laser diode from BWT Beijing Ltd. operating at 808 nm. They focused the beam down to a 100-μm-diameter spot located atop a glass substrate with a 30-nm-thick layer of the transparent conductor indium tin oxide. Because of the relatively low power density of their light source, they added light-absorption materials, either carbon black or commercial graphite inner coating, a material used in cathode-ray tube technology that is compatible with high vacuum.

A schematic of the researchers’ setup shows the process of using a diode laser to write carbon nanotubes directly onto a transparent substrate. Light from the laser provides local heating, which causes chemical vapor deposition of carbon nanotubes when a catalyst is present. Images reprinted with permission of the American Institute of Physics.

The investigators placed the substrate inside a vacuum chamber with quartz windows, flowed a mixture of acetylene and argon over the substrate and irradiated it with the laser at power levels around 500 mW. With this setup, they produced carbon nanotube spots by leaving the laser at a location for less than 30 s. By moving the laser around, they created a variety of carbon nanotube emitter patterns directly on the glass.

The nanotubes were confined to the laser-induced heat zone. When carbon black was used, the nanotubes were well-aligned, bamboo-shaped and multiwalled. When commercial graphite inner coating was employed, the nanotubes were multiwalled and of high quality but randomly oriented.

After writing the nanotubes and constructing a display device, the scientists applied a voltage to the emitters. They found that the graphite ones produced patterns on a fluorescent screen that were of uniform intensity, which was not the case for those made with carbon black.

They noted that the write time in this proof of principle demonstration was long, with 30 s required to produce each pixel. They anticipate that the problem may be solved by using a dense laser diode array or a high-power laser and a mask. The technique, they also noted, could produce carbon nanotubes for other applications and not just for field emission flat panel displays.

Applied Physics Letters, March 26, 2007, Vol. 90, 133108.