Quantum dots: The next big small thing


 

 

 

 

 

Quantum dots – tiny fluorescent crystals that contain just a few dozen molecules of semiconducting metal – are about to transition from an emerging technology to a mainstream product. The leading industrial producers of quantum dots have started delivering the first batches of their product to major electronics manufacturers in Asia, and the first quantum dot televisions and computer displays – which promise both enhanced colors and lower power use than regular LCD and LED-lit screens – are forecast to be on shop shelves within 18 months.

If their promise holds true, quantum dots could soon feature in everything from cell phone displays to digital cinema screens, and quantum dot lighting could soon outstrip even the latest energy-saving fluorescent bulbs and LEDs in terms of power efficiency and better colors.

And yet, quantum dots are still a technology in their infancy. Proponents say they could form the basis of new technologies, including flexible electronic displays, fluorescent textiles and wearable electronics, and even quantum-dot-based wall paints that can capture light and re-emit it into a room. And, if that is not enough, quantum dots might be about to revolutionize many optoelectronic technologies, such as imaging and light-gathering sensors, communications equipment, and solar power cells – including the possibility of a dramatic increase in the electricity produced from silicon solar panels – by enabling them to harvest more of the solar spectrum.

Medical researchers are also investigating the use of non-toxic quantum dots for medical imaging within the human body – potentially replacing some of the radioactive isotopes used in medical common scans. Future biomedical uses could include therapeutic doses of quantum dots that would deliver targeted control over malfunctioning cells within the body – such as cancer cells, brain neurons injured by a stroke, or damaged retinal cells.

Quantum dots could also be the key to entirely new optical technologies, including ultra-fast computers that use light instead of electricity for their logic, and photon-based quantum computers that could solve eldritch calculations beyond the ken of the largest modern supercomputers.

 

Quantum dots sound rather exotic, and indeed they are: each is a tiny semiconducting crystal, a few billionths of a meter across, typically consisting of about 50 or so atoms. As a sort of “small island” of semiconducting atoms, quantum dots have electronic and quantum mechanical properties somewhere between bulk semiconductors and individual molecules.

Quantum dots also have the industrial virtue of being easy to mass-produce – they can now be fabricated as rolled-out films, sprayed onto surfaces, and even manufactured by the bucketful, as fluorescent colloidal liquids. The dots can be made from a number of relatively abundant ingredients, including zinc, cadmium, selenium and sulfur – and even from materials with other special properties, such as graphene. One team of scientists recently described a new wet bulk method of producing high-quality graphene quantum dots, by treating ordinary carbon fiber with acetic acid – a chemical process akin to soaking charcoal in vinegar.

The key technological feature of quantum dots is that they are very fluorescent, and very bright. When a quantum dot is energized, by light or by an electric charge, it immediately re-emits the energy in a small burst of light at a very precise wavelength and color. Quantum dots have been likened to a tuning fork that always makes a particular note when struck – but when a quantum dot is “struck”, it produces a burst of light of a particular color.

The color of the light depends on the size of the dot and the material it is made from: large quantum dots emit red light, the smallest quantum dots emit blue light, and quantum dots of intermediate sizes can produce light in the rest of the spectrum. The colors are very bright, and can be tuned precisely when the quantum dots are made by controlling the proportions of raw ingredients and the temperature of the process, which limits the growth of the quantum dot crystals.

Quantum dots are significantly brighter than the phosphors used in most modern flat screens. They are highly efficient at absorbing light and re-emitting it in their signature color. And quantum dots are also chemically stable, and less prone to fade over time than conventional phosphors.

This makes quantum dots the prime contenders to replace the phosphors currently used in most displays. Some analysts speculate that the introduction of quantum dots in displays could affect demand for the rare earth elements (REEs) essential to many semiconducting display technologies, such as europium, terbium, and yttrium.

Recent shortages of such REEs have driven up production costs for flat screen displays, which have, in turn, driven manufacturers to look for new ways of making them. In 2008, phosphors for displays accounted for around 35 percent of the global demand for REEs – demand that could be expected to decline if quantum dots come into widespread use.

 

Quantum dots will arrive in our homes first as thinner flat-screen televisions with better colors, which are expected to reach the shops by the end of 2013.  The first designs are likely to integrate quantum dot technology into the existing production methods, improving the image quality by reproducing a greater range of colors than existing LCD screens.

A leading California-based nanotechnology company, Nanosys, is producing what it calls a “Quantum Dot Enhancement Film” for Korean electronics manufacturers LG and Samsung. The film is used as a phosphor in front of a blue LED backlight – light from the blue LED excites the quantum dots in the film, and they emit light in a range of colors that combine to form white light.

Blue LEDs are brighter and more energy efficient than white LEDS – and the quantum dot film produces white light that is better adjusted to human vision. Nanosys says the final image has a color range up to fifty percent greater than conventional LED screens, which are currently limited to about a third of the colors that the human eye can see. The Nanosys display uses about half the energy as a screen that uses white LEDs, which should help extend the battery life of mobile devices.

British nanotechnology firm Nanoco Group is also supplying electronics manufacturers in Japan, the USA, Korea and Taiwan with quantum dots for electronic displays. The company’s chief executive has said the first products containing Nanoco’s quantum dots will hit the market next year, and the company has already made two milestone deliveries of quantum dots to one of its customers in Japan. Nanoco has not revealed which companies it is working with, but Sony and Sharp are known to be working on quantum dot display technology.

Nanoco Group began ten years ago as a university spinout, with technology developed at the Manchester University and Imperial College London; it is now one of the leading nanotechnology companies in the UK, and listed on the London Stock Exchange AIM market in 2009.

The company says it is exploring a range of new uses for its quantum dots, including government-funded research into using them to find and kill cancer cells, an agreement with one of the world’s largest lighting companies to develop uses for quantum dots in general lighting, and a development deal with semiconductor firm Tokyo Electronto develop a more efficient type of solar panels using quantum dots.  Both Nanoco and Nanosys have plans to increase the efficiency of solar cells – by using a screen of quantum dots to “tweak” the incoming sunlight, so more light matches the wavelengths absorbed by the silicon solar cells.

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