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11/29/10

Carbon nanotubes as field Emitters

Since carbon nanotubes have a high electrical conductivity and fine tip, they are really good field emitters. However, their aspect ratio has made it difficult to fit them into triodes without losing this advantage. They are also relatively difficult to produce in large uniform batches, are very sensitive to adsorbed gases, and are rather difficult to bind securely within the device.
CNT based field Emission Cold Cathode
Carbon nanotubes are the best field emitters of any known material. They can be metallic as well as semi conducting, depending on the tube geometry. They are mechanically extremely stiff and resistant to bending with high electrical conductivity and sharp tip. If the tip is placed close to another electrode and a voltage is applied between the tube and electrode, a large electric field builds up near the tip of the tube and emits an electron beam. The magnitude of the electric field is inversely proportional to the radius of curvature of the tip. Thus the sharper the tip is, the larger the electric field. Even with only a few volts applied to an electrode a few microns away from the nanotube tip, electric fields in the range of a millions of Volts per centimeter will build up near the tip. These fields are large enough to pull a substantial number of electrons out of the tip. As "cold cathode" electron emitters, carbon nanotube films have been shown to be capable of emitting over 4 Amperes per square centimeter. Furthermore, the current is extremely stable.
Carbon nanotubes (CNTs) based field emission (FE) cathodes are used in the area of electric propulsion (EP) which need an efficient electron source to neutralize the exhaust plume in low-power EP devices. Since most low-power EP systems have limited power capacity, any expenditure in power that does not directly generate thrust is a source of inefficiency. Hence rather than using a hollow cathode, CNT cathodes are used for plume neutralization. Researchers have demonstrated the potential of FE cathodes not only in the areas of low power EP, but also in the areas of propellant-less propulsion such as space tethers, and spacecraft charge control.
Lateral field emitters, or emitters parallel to a substrate, have attracted considerable attention in vacuum microelectronics due to ease of fabrication and packaging, design versatility of electrode geometry, and precise control of spacing between electrodes by fine photolithography. Researchers have found that lateral emitter arrays can be fabricated based on the lateral arrays of CNTs assembled using dielectrophoresis.
Highly graphitic carbon nanotubes of diameter of 5−8 nm can be obtained from a thermal reaction of activated carbon and Bi2O3 powders, which was grown via a carbon-thermal reduction and Bi catalyzed vapor−liquid−solid process. Individual carbon nanotubes terminated with an onion-shaped carbon can be used to make field emitters having excellent emission properties. They can be operated at a very large current density, low voltage for a long period of high emission stability at a high emission current density under a constant applied voltage. Hence, as-fabricated CNTs could act as excellent electron emitters and have great potential application in field emission panel displays.
Researchers at the Naval Research Laboratory (NRL), US, have made carbon nanotube arrays that can produce a field emission. The devices contained arrays of carbon nanotubes grown by chemical vapour deposition on micro fabricated template structures with integral gates. The researchers claim that the current densities are more than adequate for applications such as micro satellite thrusters, portable x-ray sources, miniature gas analyzers, general cold-cathode lighting sources, high-performance microwave tubes, lightning arrestors, electron microscope cathodes and field emission displays. The carbon nanotube field emitter arrays are also robust, can be regenerated and do not suffer from destructive arcing.
Film field emitter (FFE) s is very useful in many applications like accelerator and high intensity X-ray sources for medical and security examinations.
However, carbon nanotubes can lead to the creation of new types of flat-panel displays. In contrast to traditional cathode ray tube monitor, where one electron gun creates the entire picture, using carbon nanotubes can instead create individual electron guns for each pixel, thus increasing the resolution of the monitor.
MWNT based field emitters
Multi-walled nanotubes (MWNTs)-based conducting polymer/metal-oxide/metal/MWNTs composites (polyaniline (PANI)/SnO2/Sn/MWNTs) have been synthesised. MWNTs can be made by chemical vapour deposition technique. SnO2/Sn/MWNTs can be prepared by using chemical reduction followed by calcinations. By in situ polymerisation method, surface of SnO2/Sn/MWNTs were coated with PANI. PANI/SnO2/Sn/MWNTs field emitters were fabricated over flexible graphitised carbon fabric substrate by spin coating technique. The fabricated PANI/SnO2/Sn/MWNTs field emitters exhibit excellent field emission properties with a turn on field of 1.83 V ┬Ám-1 and a field enhancement factor of 4800.
Samsung and Motorola besides others are using the above materials for display applications.

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