Luminescent quantum dots of semiconducting materials could eventually help to identify tumors, illuminate large flat-panel monitors, or make optical data processing a reality. Quantum dots are nanoscopic crystals so small that their chemical properties are similar to those of individual molecules. Researchers at MIT have now developed a microfluidic technique that delivers tiny crystals of particularly uniform size - and thus excellent optical quality.

Cadmium selenide quantum dots are usually obtained by injection of precursor compounds into a hot solvent. Many factors, including local temperatures in the reaction vessel, concentration gradients, as well as the rates of mixing and the final cooling process, substantially influence the results, but are difficult to control. Relief is promised by microfluidic technology, a miniature reactor system made of long, very narrow channels on a platform in the form of a chip. The extremely small dimensions allow for very exact control of substance and heat transport. Conventional microfluidic reactors have drawbacks, however. The reactants diffuse slowly. In addition, the particles do not move through the channels at the same speed; those in the middle move faster than those alongside the slowing channel walls. The resulting nanocrystals thus spend different amounts of time in the reactor. These two phenomena lead to quantum dots with a wide range of diameters.

It shouldn't have to be this way, thought the team of scientists headed by Moungi G. Bawendi and Klavs Jensen . The solution: a two-phase microfluidic system in which gas bubbles divide the stream of liquid in the channels into individual, very regular segments. Within these segments, back-mixing results in a constant exchange of material between the walls and center of the channels - all particles spend roughly equal time in the reactor. In addition, in order to accelerate the diffusion of the reactants, the mixing zone of the channel is made with tight curves. The subsequent reaction zone reaches the necessary high temperature of 260 °C and is thermally isolated from the third zone, in which the reaction is stopped at temperatures under 70 °C. With their microfluidic reactor, the researchers attain quantum dots of uniform size in significantly higher yields than with previous microfluidic techniques. In addition, reaction times can be shortened without lowering yields - an important criterion for commercial processes.

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