Transparent Conductive Films
One of the more amazing attributes of carbon nanotubes is that they can form films that are highly electrically conductive, but almost completely transparent. The film is only about 50 nanometers thick, and very porous. Under an electron microscope, the film is seen to be a just a few layers of endless carbon nanotube ropes.
The films have an ideal conductivity for multiple types of touch screens which have applications including point-of-sale terminals, games, portable computers, cell phones, personal digital assistants and many others. The transparent films used initially for touch screens also reach any application that requires a large-area transparent conductor, including LCD displays, plastic solar cells, and organic LED lighting, and transparent carbon nanotube films have been demonstrated in the laboratory to be effective in all these areas.
The semiconducting properties of carbon nanotubes can be exploited to create printable transistors with extremely high performance. Specifically, researchers have shown CNT-based transistors employing a sparse nanotube network to achieve mobilities of 1 cm2/V-s (Schindler et al., Physica E (2006), while those using an aligned array of single-walled nanotubes can reach as high as 480 cm2/V-s [Kang et al., Nature Nanotech. 2, 230 (2007)]. Nanotubes also prove to be useful additives to polymer-based TFTs and help to overcome some of the shortcomings of those devices. Beyond their performance, such devices are compatible with solution-based printing techniques, which enable dramatic cost savings in such devices as LCDs and OLED-based displays.
Carbon nanotubes are the best field emitters of any known material. This is understandable, given their high electrical conductivity, and the unbeatable sharpness of their 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. 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 [B.Q. Wei, et al. Appl. Phys. Lett. 79 1172 (2001)]. An immediate application of this behavior receiving considerable interest is in field-emission flat-panel displays. Instead of a single electron gun, as in a traditional cathode ray tube display, there is a separate electron gun for each pixel in the display. The high current density, low turn-on and operating voltage, and steady, long-lived behavior make carbon nanotubes ideal field emitters for this application. Other applications utilizing the field-emission characteristics of carbon nanotubes include: high-resolution x-ray sources, general cold-cathode lighting sources, high-performance microwave tubes, lightning arrestors, and electron microscope cathodes.
Nanotubes might also represent a solution to thermal management problems plaguing the semiconductor industry. As more and more transistors are packed on chips, microprocessors are getting hotter and noisier. The industry is searching for new types of heat sinks to control temperatures on chips. Nanotubes have tremendous thermal conductivity, and a number of firms are developing nanotube-based heat sinks. Due to the unique conducting and semiconducting properties of nanotubes, devices based on individual carbon nanotubes may eventually replace existing silicon devices. For example, several prototypes for future memory devices based on nanotubes have been demonstrated. In light of their high carrying capacity, nanotubes might replace copper interconnects in integrated circuits. Additionally, individual nanotubes have been shown to be superior to existing silicon transistors and diodes.