Outlook for Nanocrystal LEDs Brightens
A new method for stimulating tiny crystals to emit light puts researchers closer than ever to a versatile technology for brighter, more efficient, and dramatically longer lasting lighting for displays, traffic lights, and eventually interior lighting.
Light-emitting diodes (LEDs) made from semiconductor materials are already used in traffic lights because they last for years and use a fifth of the electricity of normal light bulbs, but they tend to emit blue light that must be converted to white, an inefficient process. Researchers have proposed juicing up LEDs with highly efficient, nanometer-scale crystals of semiconductor tailored to emit light of any color based on their size. (Electrons in smaller crystals emit higher frequency photons when excited.) These so-called nanocrystal quantum dots, however, have a coating of organic molecules that tends to block out the electrons needed to stimulate the dots.
Now a group has found a way around the problem by stimulating the dots indirectly. Victor Klimov and colleagues at Los Alamos National Laboratory assembled their cadmium selenide dots on top of a so-called quantum well, a thin sheet of semiconductor sandwiched between two barrier layers. A quick flash of laser light aimed at the well generates pairs of electrons and positively charged "holes" in the middle layer. Normally the pairs would recombine and emit a photon, but by making the top layer of the well thinner than 30 Angstroms, the researchers forced the recombined pairs to release their energy as a wiggling electric field. This field generated electron-hole pairs in the adjacent dots; these pairs recombine, producing photons, the team reports in the 10 June issue of Nature. "Very crude estimates indicate that potentially we can double the efficiency of current white-light LEDs," says Klimov.
The next step toward practical light sources is to excite the well with electrons rather than laser light. That task is just a matter of engineering, says theoretical physicist and quantum dot researcher Alexander Efros of the Naval Research Laboratory in Washington, D.C. The work represents a completely new way of getting electrons and holes inside quantum dots, says David Norris, a materials scientist at the University of Minnesota, Twin Cities. "I think it makes the chance for having real devices go way up."