GOOD as modern display screens are, they could be a lot better. Even the best liquid-crystal display (LCD) can produce only about a third of the range of colours which the human eye (in collaboration with the brain) can perceive. But that may soon change, with the deployment in screens of structures called quantum dots.

A quantum dot is a semiconductor crystal a few nanometres (billionths of a metre) across—about 50 atoms wide, in other words. When excited, such crystals emit light. The wavelength, and hence the colour, of this light depends on the size of the dot. Large ones emit long wavelengths (red light). Small ones emit shorter wavelengths (blue). Those in between fill in the spectrum with colours such as green. The plan is to use this property to generate nuances of colour that are beyond the range of existing LCDs.

An LCD employs its liquid crystals, a curious form of matter whose optical polarity can be manipulated electronically, as minuscule shutters that either permit or forbid the passage of white light. In the most modern LCDs this light is generated by light-emitting diodes (LEDs) and is then diffused in a special layer of the screen behind the shutters. White light which passes through a liquid-crystal shutter falls on a filter that imposes one of the primary colours—red, green and blue—on the output. By grouping these shuttered filters in threes, one of each colour, an individual picture element, or pixel, is created. Combining the three primaries in different proportions by varying the amount of light passing through the shutters allows each pixel to produce a range of colours.

Somewhere, over the rainbow

A few years ago even a third of the range of human perception was impressive. But times move on, and Jason Hartlove, the boss of a Californian firm called Nanosys, thinks he can do better. Using quantum dots, he says, Nanosys can increase the number of possible colours still further.

The problem his quantum dots are intended to overcome is that the LEDs preferred by the display-screen industry are not white enough. They emit light which is biased towards the blue end of the spectrum. This colour bias is reflected in the mixture of frequencies that form the picture on the screen. For some viewers, the results can look rather cold.

Nanosys's quantum-dot-enhancement film, as the company calls its product, uses the dots to tweak the spectrum from the LEDs so that it is closer to that of the white light the human eye is used to. It does this, as the product's name suggests, by passing the LED light through a transparent film peppered with quantum dots, which absorb and re-emit some of it.

These dots are of two sizes. The larger re-emits the absorbed energy as red light. The smaller re-emits it as green. The final, filtered image is thus drawn from a broader palette than is permitted to an existing LCD—50% broader, according to Nanosys.

The other advantage Nanosys claims for its technology is that it can be fitted easily into existing manufacturing processes. It is simply a matter of replacing the diffuser layer with a quantum-dot-enhancement film. Making the film itself is easy, too. The dots, composed of a semiconductor called indium phosphide, are sprayed onto a transparent plastic sheet that is then covered with a second sheet. That done, the whole thing is heat-sealed. The film can therefore be manufactured continuously in a reel-to-reel process a bit like printing. This cuts costs enormously.

The result, according to Mr Hartlove, is that cinema-like levels of colour are possible on the small screen, too. That could lead to new applications, such as professional-quality colour photography, being available on tablet computers, mobile phones and the like.

Tweaking traditional LCDs in this way may, moreover, be only the beginning for quantum dots. Some engineers think dots might be used to jump a complete generation of screen technology.

At the moment, many of those in the business believe the next generation of screens will be made using devices called organic light-emitting diodes (OLEDs). Unlike standard LEDs, whose light has to be filtered and processed to get its intensity and colour right, OLEDs would be made into pixels directly, with diodes of different composition providing each of the primary colours. OLED screens can be brighter than LCDs, their colours are richer and deeper, and the screens themselves are thinner and have a lower power consumption—all attractive propositions. Unfortunately, large OLED screens are costly to make and OLEDs tend not to last as long as standard LEDs.

More dotty ideas

Quantum dots may leap to the rescue. Indeed, they promise to be brighter even than OLEDs and may displace them completely. The value of OLEDs, beyond their brightness, is that the electricity which powers and controls them is turned directly into the light the user views. No filtering and processing of that light is required. Exactly the same arrangement is possible, though, with quantum dots, which can also generate light directly from electricity. And, since the circuitry needed to run OLED screens has now been developed, it should be reasonably simple to substitute OLEDs with quantum dots.

Earlier this year researchers at Samsung Electronics did precisely that. They demonstrated an experimental quantum-dot display made of strips of red, green and blue dots on an array of special transistors that powered and controlled them. And Samsung is not alone. Recently QD Vision, a firm based in Massachusetts that uses quantum dots to whiten the output of its LED lighting units, showed off a prototype quantum-dot display which it claims matched the efficiency and colour levels of an OLED screen. Though QD Vision says a lot more work remains to be done before this display can be made commercially, it believes quantum-dot displays should end up being cheaper than OLED screens.

Nor are screens the only use to which quantum dots might be put. Engineers at Nanoco, a British firm, think the dots could help generate solar power. The firm's plan is to increase the efficiency of solar cells by tweaking the incident sunlight so that it matches that best absorbed by such cells. Efficiency is crucial in such cells if they are ever to be a substitute for existing methods of mass power-generation, such as coal and gas.

Quantum dots, then, look as if they have a bright future. Much hype has surrounded the idea of nanotechnology—the thought that manipulating objects on scales of a billionth of a metre will produce useful products. So far, the results have been less than spectacular. Dots, though, may prove an exception.