Carbon nanotube as electron sources in display devices

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October 9, 2012 by Nakul

Author: Thanveer.B

Electronic displays are one of the fastest growing technologies. There has been a major development in the display technology   with the potential to enable television, handheld computers and mobile phones to be more functional and user friendly.

Currently, most of the electronic devices such as cathode ray tubes used in television ,projectors, in generation of x ray etc make use of electron sources.. Here the major phenomenon that comes into picture is the field emission. In simple words, field emission is liberation of electrons from a surface, on the application of intense field (electric or magnetic).

Old television, cathode ray oscilloscope (CRO) and computers made use of large cathode ray tubes for their displays. In these devices, electrons emitted by CRT’s were allowed to strike phosphor material to produce an image. Though CRT’s were more efficient in terms of viewing angle, color, response speed and temperature range, in the recent past because of their poor display quality and high power consumption, flat panel displays(FPD) overtook  CRT’s and are commonly found in most of present display devices.

One of the Flat panel displays (FPD’s) that replaced CRTs is liquid crystal display (LCD) due to its low power consumption, lightweight and compact structure. Though LCDs were initially created for hand held and portable devices, later it was extended into television and computer monitors. Because of LCD’s seven segment display emerged which led to the development of various types of calculators, watches etc.

Then the plasma display panels (PDP’s) emerged which had an excellent image quality in large scale. PDP’s provided large screen sizes (up to 60 inches) and were more useful in HDTV (high definition television).

The next trend in the display technology is organic light emitting diodes (OLED’s), featuring an ultra-thin structure and more compact design. Polymer LED (PLED), small molecule LED (SMOLED) and dendrimer technology are the variation of OLED’s based on the electroluminescent substance used.

Further research in the display technology led to the development of field emission displays (FED’s). It functions more like the traditional CRT. The only difference is, instead of one electron gun it makes use of many smaller ones to emit electrons. So it provides the combined effect of CRT’s, LCD’s and PDP’s. Because of this technology it is able to produce high quality images which CRT’s provide, along with thin and compact design which LCD’s gave and large screen sizes like PDP’s.

The basic principle of field emission technology is emission of electrons from a material surface using ‘tunneling effect’. Within a conductor, the free electrons are bound by means of surface potential energy barrier. When sufficient field is applied, electrons acquire enough energy and overcome this potential barrier. The simplest way to create such a field is by ‘field enhancement’ at the tip of a sharp object. If the radius of curvature is less, the object is pointier and more charge accumulates at its tip. So now, a material had to be found which on application of a moderate field results in tunneling effect and hence allows electron to escape into free space without heating the cathode as in the traditional CRT technology.

For the emission of electrons traditional FED structure made use of many micro tips, which were blue, red or green. These three together formed one pixel. These micro tips were sharp and were made up of molybdenum. The electrons emitted from these micro tips strikes red, blue and green phosphor and emitted light through the glass display. Unlike traditional CRT, here the color was displayed sequentially. The advantages of the traditional FED included the fact that they only produced light when the pixels were ‘on’, and as a result power consumption was dependent on the display content.

One problem in this kind of display is that the metal molybdenum, used to make the micro tips, would become so heated that local melting results in deforming its sharp tips which is needed to form the electric field used for electron emission.

Further growth in the technology led to carbon nanotube field emission display (CNT FED TV).it is much similar to the previous structure. Here one pixel is composed of three sub pixels where the combination of these sub pixels allows for the intense color manipulation found in case of CRT.

Carbon nanotubes are tubes of carbon atoms of less than 1nm diameter. They can be thought of as a sheet of graphite (a hexagonal lattice of carbon) rolled into cylinder.

Each micro tip is now replaced with many carbon nanotube-based emitters which act as cathodes that produce electrons via field emission. The electric field required for field emission is generated by a gate electrode contained within every sub pixel. Attracted to the positively charged anode placed in between the display glass and the phosphor layer, emitted electrons are swept through a vacuum towards their respective phosphors (red, green, or blue) where light is emitted when the phosphors are struck.

With CNT’s, which have a small tip radius and high aspect ratio, it is possible for electrons emitted by one nanotube to hit the phosphors of one pixel in the display.

The advantage of  this type of display over standard  LCD’s include low power consumption, higher brightness, a wider viewing angle, faster response rate and a large operating temperature as well as lower cost.

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