8. Display technology

In its broadest sense, information technology is not just about processing, conveying and storing information, but displaying it. The most immediate and perhaps the most important interface in desktop computer systems is that between the machine and the eye, mediated by the computer screen. Display technology-the conversion of electronic data to a visual display-is a vast industrial concern, of which computer screens are but one aspect. Television screens are of course basically the same devices; but traffic signals are a very different kind of visual display, and increasingly electronic media are replacing ink and paper as the vehicle for all kinds of written information.

Standard-sized television screens today employ the same technological principles as the earliest of cathode-ray tubes, dating from before the discovery of the electron. A beam of electrons is scanned rapidly over a pixellated array of phosphor dots, which glow in a particular color when irradiated. Separate red, green and blue phosphors at each pixel suffice to generate the colors of most of the visible spectrum.

This is a cumbersome system, since it requires an electron gun placed some distance behind the screen. The TV terminal is therefore the most bulky part of many personal computers. Laptop computers use a different display medium, which is more expensive to produce, but less wasteful of space and has a lower power consumption. These flat screens make use of liquid-crystal light shutters, which can be switched electronically between a transparent and an opaque black state. In the transparent state, a liquid-crystal pixel element lets through light from a background source, which also passes through a color filter to generate the three primaries of each pixel. Some of the challenges for liquid-crystal display technology include faster switching speeds and development of display systems that permit a wide viewing angle, so that the picture does not vanish or become bizarrely colored when not seen face-on.

But still better than this shutter-and-filter method, would be a display in which each element is an intrinsic light emitter that is electronically switchable, robust, flat and cheap to produce. Most efforts in this direction are focused on the fabrication of banks of light-emitting diodes-the challenge is to make them cheap, reliable and bright enough that the flat screen becomes an economically viable product. Light-emitting diodes based on the same kind of inorganic semiconducting materials used in laser diodes have long been in use: gallium arsenide doped with phosphorus, for example, offers light emission in the visible range. But whereas such materials cover the spectrum from red to green, efficient blue LEDs were not available until the advent of gallium nitride. LEDs made from this material were in fact marketed several years before the corresponding blue-light lasers, bringing full-color bright LED displays and TV screens within reach for the first time. An indium-doped version of gallium nitride is an efficient emitter of green light, and is used in LED-based traffic lights, which are not only brighter than those that use incandescent bulbs but also have much lower power consumption and longer lifetimes. Applications like these seem set to secure gallium nitride as a major technological material in the next several decades.

Figure 17. A Traffic Light that uses Red, Amber and Green Light-emitting
Diodes Rather than Incandescent Bulbs

But for flat-screen color displays, inorganic semiconductors now face stiff competition from organic materials: light-emitting polymers. These work on much the same principles: the emitter is a semiconductor, which luminesces when charge is injected (although the mechanisms of charge transport and recombination are quite different). The polymers acquire an electrical conductivity from the presence of delocalized electron orbitals along their chains. The first polymer LED, fashioned in 1990, was made from the hydrocarbon polymer poly(p-phenylene vinylene), which emits in the yellow part of the spectrum. Conducting polymers have since been devised that glow right across the visible range, so that full color displays are now possible in principle from polymer LEDs. The advantages are that these materials are very lightweight, flexible (a polymer LED can be rolled up like a sheet of paper), easy to process and fabricate into patterns (this can be done using a kind of printing process) and are amenable to fine-tuning of the emission wavelength by chemical modification of the polymer chain. But the drawbacks are that the emission efficiencies (power out relative to power in) can be very low, and that the polymers may be susceptible to chemical degradation after many hours of use. These shortcomings are being overcome, however, and a full-color polymer LED display is already commercially available.

Figure 18. A Display Device based on a Polymeric ("Plastic") Light-emitting Diode

But polymers are not the only class of organic materials capable of providing electroluminescent devices. These have also been fabricated from thin films of small molecules, notably the complex of aluminum with three tris-(8-hydroxyquinoline) molecules, which emits light in the green region of the spectrum. Both this material and electroluminescent polymers have also been used as the active medium in "organic" thin-film diode lasers.

A completely different approach to full-color flat-screen displays is to miniaturize the old cathode-ray tube technology in so-called field-emission devices. Here, as in tube-based screens, a stream of electrons excites a colored phosphor; but the electron beam is generated very close to the phosphor dot by using an intense electric field to pull the electrons from a microfabricated needle-like tip, a "field emitter." The emitter is charged to a negative potential with respect to a plate above it; at the very apex of the tip, the field is then strong enough to pull electrons out into the empty space, where they are accelerated towards the plate, but pass right through a hole-just as the electrons in a cathode-ray tube fly past the positively charged accelerator plates-to strike the phosphor.

In a cathode-ray tube, emission of electrons from the cathode is promoted by heating it to a high temperature. But in the miniaturized vacuum tubes, making the cathode from a material in which "free" (conduction) electrons already have a higher energy than they would in the vacuum (that is, materials with a negative electron affinity) means that emission can take place even when the cathode is cold. Diamond doped with elements that provide "excess" electrons has this property, and so diamond thin films, shaped into arrays of pyramid-like tips, are being investigated as potential cold-cathode displays.

Figure 19. A Diamond-based Field-emission Device for Display Technology
The diamond acts as a "cold cathode" emitting electrons
even at room temperature

The centuries-old "display technology" of ink on paper is still popular today. Most people still find documents easier to read on paper than on screen, no matter how prettily colored the latter can be. Paper is also a much more convenient and portable medium for information. But a book can weigh as much as a laptop computer capable of holding an entire library's-worth of data. To combine the advantages of both, researchers are developing "electronic ink": materials that, in the form of thin films, can resemble the stark black-on-white of ink on paper and yet which can be reconfigured electronically into new pages. One version, called E-Ink, consists of micrometer-sized clear plastic capsules containing black and white pigment particles, which can be rearranged by applying an electric field across the capsule. If the black particles are drawn to the top, the capsule appears dark when seen from above; but switching the field can allow the white particles to be presented instead.

Figure 20. E-Ink is an Electronically Switchable Ink Comprised of Clear Plastic Microspheres Containing Black and White Pigment Particles
Which of them is displayed at the upper face of the capsule is determined by an electric field

One can regard this microstructured device as a kind of smart composite material. A layer of E-Ink laminated between two sheets of clear, electrically conducting material patterned into tiny pixels provides a page whose text can be electronically controlled. In this way, the computer might not so much replace the book as reinvent it in a new, lightweight form.

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