Nanotubes Enable Flat Panel Displays
EVANSTON, Ill. -- Flat panel displays are desirable where space is at a premium: in portable electronic devices and as computer monitors and television sets. While the demand is high for the displays, so are the prices. The process technology used to create flat panel displays is complex, and the high voltage required to drive the pixels increases the complexity. A new application of carbon nanotube technology promises to reduce the complexity and thus the cost.
In a flat panel display technology under development at Northwestern University, molybdenum strips act as gates for carbon nanotube emitters embedded in the glass substrate. The perpendicular cathode strips and the gates enable the researchers to address the individual pixels in the display. Courtesy of Northwestern University.
Robert P.H. Chang and his colleagues in Northwestern University's materials science and engineering department have developed a flat panel display that uses carbon nanotubes as field emitters. They etched a series of 250-µm-thick glass substrates with 30- to 100-µm-diameter holes, at pitch distances of 100 µm to 1 mm. They plated the top surface of the glass with a 300-nm-thick layer of molybdenum strips and covered the sides of the holes with an insulating layer.
The researchers filled these holes with a paste of carbon nanotubes and etched the surface to form a 4-µm gap between the molybdenum metal gates and the paste field emitters. They grounded the back of the emitter gate structure to conductive cathode strips and mounted a ZnO-phosphor-coated glass plate 0.2 mm away as the anode. They found that they could control the individual pixels by applying voltage to the cathode strips on the back of the substrate and the molybdenum gate strips on the front.
At first, the emission across the array varied, but as the gate-pulse voltage was gradually increased, the stability and uniformity of the emitter array improved. This "training" process burned away impurities in the emitter-gate gap. The researchers suggest that the same steps could be applied to a malfunctioning emitter to bring it back into service.
They investigated several geometries, including a 1 x 1-cm structure with emitters in a 100 x 100 grid, 0.1 mm apart. In this structure, when they biased the phosphor screen anode at 500 V, a gate voltage of 50 V was sufficient to control emission.
The initial experiments, which the group described in the Feb. 26 issue of Applied Physics Letters, demonstrate that a design that does not require expensive submicron process technology can provide a structure that operates at a gate voltage as low as 20 V. Chang is continuing development of the carbon nanotube display technology, with emphasis on devising an inexpensive method of producing high-quality carbon nanotubes in large quantities. When that process is refined – in about a year, he expects – he will seek industrial partners to continue the work.
Chang, who is the director of the university's Materials Research Center, is excited about the potential of the technique. "The ability to apply carbon nanotubes in an active device ... might turn into a real product," he said.
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