A quantum leap for lighting
Consumer electronics: Tiny semiconductor crystals, called quantum dots, enable new forms of energy-efficient lighting
HOW many inventions does it take to change a light bulb? More than you might think. Around the world, many people are switching from traditional incandescent bulbs to compact fluorescent (CFL) bulbs, which require less energy to produce a given amount of light, and therefore save money and reduce carbon emissions. But CFLs
themselves may soon be overhauled by light emitting diodes (LEDs), which are even more energy efficient and have the further advantage that they come on instantly at full brightness, unlike CFLs, which can take a while to warm up. Advocates of LEDs note that the technology is versatile enough to work in almost any situation, from stadium lighting
right down to the tiny light on your phone that flashes to indicate a new message.
But not even LEDs, it seems, are the end of the story. Yet another lighting technology is on the horizon that offers further advantages: even greater power efficiency and softer, warmer light, the colour of which can be precisely controlled. Even though it will be put to rather mundane uses, the technology in question has an exotic name: quantum-dot
Quantum dots are tiny crystals of semiconducting material just a few tens of atoms, or a few nanometres (billionths of a metre), across. They are typically made using some combination of zinc, cadmium, selenium and sulphur atoms. Their origins go back to work published in 1983 by
Louis Brus, then at Bell Labs, in New Jersey, though it was several years before another physicist, Mark Reed at Yale University, described these tiny semiconductor clumps as “quantum dots”. When excited by light or electricity, a quantum dot emits light of a colour determined by the dot's size and the material from which it is made. Light of a particular colour can therefore be produAced by exciting dots of a specific size. Seth Coe-Sullivan, co-founder and chief technology officer of QD Vision, a start-up spun out of the Massachusetts Institute of Technology, likens a quantum dot to a tuning fork: when it is struck, it oscillates at a specific, fixed frequency, producing a note of a particular pitch (or, in the case of a quantum dot, light of a particular colour). This has immediate applications in general lighting, but quantum dots can also be put to many other uses.
Shine a light
In lighting, quantum dots allow the colour of the light from a light source to be precisely controlled, says Jason Hartlove, the chief executive of Nanosys, based in Palo Alto, California—one of a handful of companies making quantum dots and selling lighting components based on them. The first products to come to market use quantum dots to produce warm, white light from blue LEDs. In essence, quantum dots are used to change the colour of the light. The advantage of this approach is that blue LEDs are the brightest, most energy-efficient kind.
Existing white LEDs are also based on blue ones, the light from which is used to excite a phosphor layer made of yttrium aluminium garnet (YAG). The phosphor absorbs some of the blue light and is “pumped” into an excited state. When it relaxes, the energy it has absorbed is re-emitted as yellowish light. The combination of blue and yellow produces a rough approximation of white light. But it contains less red light than is found in natural light, says Mr Hartlove. As a result, the light seems cold and harsh. The same is true of some kinds of fluorescent lighting, which are also deficient in red light. “The light is not very pleasing to the human eye,” says Dr Coe-Sullivan. QD Vision's first product, developed in conjunction with Nexxus Lighting of Charlotte, North Carolina, consists of a film embedded with quantum dots of different sizes in carefully chosen ratios. The film attaches to the front of a bulb containing several blue LEDs, and acts like...
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