A NEW range of televisions from Sony is the first to use minuscule devices known as quantum dots to produce colours which are more vibrant than those which appear on a conventional liquid-crystal display (LCD). Quantum dots are crystals of semiconductor material just a few nanometres (billionths of a metre) in size. They could have a big future in lighting and display technologies, but are difficult and expensive to manufacture, and use toxic materials. However, Geoffrey Ozin, from the University of Toronto, Uli Lemmer, from the Karlsruhe Institute of Technology, in Germany, and their colleagues believe they have found a way to deal with these problems.
An LCD screen works with a backlight shining through red, blue or green filters to produce the pixels which make up an image. Many televisions use light-emitting diodes (LEDs) as the backlight because they are brighter and use less power than fluorescent bulbs. Sony’s new televisions uses quantum dots with conventional LEDs to produce a hybrid backlight of greater intensity. In time, though, quantum dots might be used directly as the coloured pixels on screens.
When a voltage is applied to a quantum dot it causes electrons contained in the crystal to release energy in the form of light. Changing the size of the dots changes the amount of energy released, which in turn determines the wavelength, and therefore the colour of the emitted light. This means they can be made into nanoscopic LEDs and, in principle, be tailored to generate any colour of the rainbow from red (long wavelenghts) to violet (short wavelengths).
The most advanced quantum dots are made of materials like cadmium selenide, whose crystal structure makes it easy to create dots of different sizes. In contrast, controlling the size of silicon dots, especially those meant to emit shorter wavelengths, has proved a challenge. Silicon-based devices tended to be a mélange of different-sized dots. As a result, they did not emit pure colours but an unpredictable mix. In fact, most of them, if they glowed at all, emitted red or reddish light. Which is a pity because the material is cheap and already used in huge quantities by the electronics industry. Moreover, recent research has shown silicon nanoparticles are not toxic.
