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  • Technology: A Billion Points of Light – and More Coming

Photonics Spectra
Jan 2009
Lynn Savage,

Quantum dots, more generically known as semiconductor nanoparticles, are hiding in the shadows no longer. The tiny crystals, attractive because they exhibit large quantum yields up and down the visible spectrum when they are energized, are being used as substitutes for fluorescent dyes in biological imaging. They also are being explored for use in diode lasers, LEDs, display technologies and solar panels, among other applications.

Typically made in a core-shell configuration – such as cadmium selenide centers covered by zinc sulfide coatings – quantum dots are attractive to researchers and engineers especially because the particles’ photonic emissions are directly related to their size. Larger quantum dots – still only tens of nanometers across – emit in the red end of the spectrum; particles only a few nanometers in diameter emit in the blue. They can be energized by a number of sources, including lasers and electrical power.

QD Vision Inc. of Watertown, Mass., is attempting to take advantage of quantum dots’ high brightness and narrow emission bandwidth (aka color purity) by developing thin and flexible displays that incorporate the particles. The company is working out cost-effective ways to manufacture the arrays of pixels that would comprise the displays, which could be scaled from cell phone size to big-screen televisions. Compared with LCDs, LEDs and organic LEDS, displays based on quantum dots would require far less power and would be more readily viewed in direct light.

Work is continuing on finding the best way to integrate quantum dots into photovoltaic (PV) cells. The interest lies in the fact that the bandgap tunability of the particles will enable PV cells to reach efficiencies approaching 65 percent and possibly beyond. Also in the particles’ favor is that those made with some materials, such as lead selenide, produce far more electrons when struck by a photon than can basic silicon-based photovoltaics. Expect continuing research into candidate materials that can exploit the sun’s output in the ultraviolet and infrared ranges.

In addition, computing may get a boost from quantum dots because the particles can act as tiny carriers of binary information. But instead of supplying the binary states that one obtains with electrons (either a zero or a one), quantum dots act as qubits – the “quantum bits” that can simultaneously act as ones and zeroes as well. Research into creating nanoparticles that reliably act as qubits is ongoing at the University of Michigan in Ann Arbor, at the US Naval Research Laboratory in Washington and at the University of California, San Diego, in La Jolla, among others.

All of the interest in quantum dots also is driving a growing number of researchers to look at using the same semiconductor materials in atypical nanoscale shapes, including rods, wires and cones. Changes in the contours of these particles are leading to discoveries of further unusual optical properties as well as to novel routes to tunability.

quantum dots
Also known as QDs. Nanocrystals of semiconductor materials that fluoresce when excited by external light sources, primarily in narrow visible and near-infrared regions; they are commonly used as alternatives to organic dyes.  
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