WHAT ARE QUANTUM DOTS?



Quantum dots are semiconductor nanoparticles that glow a particular color after being illuminated by light. The color they glow depends on the size of the nanoparticle. When the quantum dots are illuminated by UV light, some of the electrons receive enough energy to break free from the atoms. This capability allows them to move around the nanoparticle, creating a conductance band in which electrons are free to move through a material and conduct electricity. When these electrons drop back into the outer orbit around the atom (the valence band), as illustrated in the following figure, they emit light. The color of that light depends on the energy difference between the conductance band and the valence band.


What can you use quantum dots for?

Optical applications



So far, quantum dots have attracted most interest because of their interesting optical properties: they're being used for all sorts of applications where precise control of colored light is important. In one simple and relatively trivial application, a thin filter made of quantum dots has been developed so it can be fitted on top of a fluorescent or LED lamp and convert its light from a blueish color to a warmer, redder, more attractive shade similar to the light produced by old-fashioned incandescent lamps. Quantum dots can also be used instead of pigments and dyes. Embedded in other materials, they absorb incoming light of one color and give out light of an entirely different color; they're brighter and more controllable than organic dyes (artificial dyes made from synthetic chemicals).
Still in the world of optics, quantum dots are being hailed as a breakthrough technology in the development of more efficient solar cells. In a traditional solar cell, photons of sunlight knock electrons out of a semiconductor into a circuit, making useful electric power, but the efficiency of the process is quite low. Quantum dots produce more electrons (or holes) for each photon that strikes them, potentially offering a boost in efficiency of perhaps 10 percent over conventional semiconductors. CCDs (charge-coupled devices) and CMOS sensors, which are the image-detecting chips in such things as digital cameras and webcams, work in a similar way to solar cells, by converting incoming light into patterns of electrical signals; efficient quantum dots could be used to make smaller and more efficient image sensors for applications where conventional devices are too big and clumsy.
Quantum dots are also finding their way into computer screens and displays, where they offer three important advantages. First, in a typical LCD (liquid crystal display screen), the image you see is made by tiny combinations of red, blue, and green crystals (effectively color filters that switch on and off under electronic control) that are illuminated from behind by a very bright backlight. Quantum dots can be tuned to give off light of any color, so the colors of a quantum dot display are likely to be much more realistic. Second, quantum dots produce light themselves so they need no backlight, making them much more energy efficient (an important consideration in portable devices such as cellphones where battery life is very important). Third, quantum dots are much smaller than liquid crystals so they'd give a much higher-resolution image. Quantum dots are also brighter than a rival technology known as organic LEDs (OLEDs) and could potentially make OLED displays obsolete.


Quantum computing


Computers get faster and smaller every year, but a time will come when the physical limits of materials prevent them advancing any further, unless we develop entirely different technologies. One possibility would be to store and transmit information with light instead of electrons—a technology broadly known as photonics. Optical computers could use quantum dots in much the same way that electronic computers use transistors (electronic switching devices)—as the basic components in memory chips and logic gates.
Optical computers may or may not take off, depending on how much progress computer scientists make with rival technologies, including quantum computers. In a quantum computer, bits (binary digits) are stored not by transistors but by individual atoms, ions, electrons, or photons linked together ("entangled") and acting as quantum bits called qubits. These quantum-scale "switches" can store multiple values simultaneously and work on different problems in parallel. Individual atoms and so on are hard to control in this way, but quantum dots (on a considerably larger scale) would be much easier to work with.

Biological and chemical applications



Quantum dots are also finding important medical applications, including potential cancer treatments. Dots can be designed so they accumulate in particular parts of the body and then deliver anti-cancer drugs bound to them. Their big advantage is that they can be targeted at single organs, such as the liver, much more precisely than conventional drugs, so reducing the unpleasant side effects that are characteristic of untargeted, traditional chemotherapy.
Quantum dots are also being used in place of organic dyes in biological research; for example, they can be used like nanoscopic light bulbs to light up and color specific cells that need to be studied under a microscope. They're also being tested as sensors for chemical and biological warfare agents such as anthrax. Unlike organic dyes, which operate over a limited range of colors and degrade relatively quickly, quantum dyes are very bright, can be made to produce any color of visible light, and theoretically last indefinitely (they are said to be photostable).

Who invented quantum dots?



Quantum dots were discovered in solids (glass crystals) in 1980 by Russian physicist Alexei Ekimov while working at the Vavilov State Optical Institute. In late 1982, American chemist Louis E. Brus, then working at Bell Laboratories (and now a professor at Columbia University), discovered the same phenomenon in colloidal solutions (where small particles of one substance are dispersed throughout another; milk is a familiar example). He discovered that the wavelength of light emitted or absorbed by a quantum dot changed over a period of days as the crystal grew, and concluded that the confinement of electrons was giving the particle quantum properties. These two scientists shared the Optical Society of America's 2006 R.W. Wood Prize for their pioneering work.

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