Victor I Klimov, PhD, leads a team at Los Alamos National Laboratory that has been working for a number of years on turning NQDs (nanocrystal quantum dots) into lasers. Because the dots are so small, their size influences the energy gap between the conduction and the valence bands. When the recombination of an electron-hole pair causes the release of light in a laser, the color of the light relates to the energy gap. Hence, Klimov and company could change the color of an NQD laser simply by fabricating larger or smaller dots—if they could get the NQDs to be lasers.

However, for reasons that again relate to the size of the dots, you must have more excited electrons than valence-band electrons in a dot to get light amplification. An incoming photon hits one excited electron, an action causing the emission of a photon. However, if you don’t excite most of the valence electrons, odds are that the next unexcited electron will just absorb the emitted photon. You might think the solution is just to excite more of the electrons, but there is a problem with this approach, too.

Again because of quantum effects, electron-hole pairs and neighboring electrons interact to recombine quickly in NQDs—too quickly to permit laser action and without releasing a photon. Recently, however, Klimov and his team have reported that, by fabricating nanocrystals with a cadmium-sulfide core surrounded by a zinc-selenide shell, the physical interface separates the conduction-band electrons from the corresponding holes in the valence band, substantially slowing recombination and making light amplification possible. A conceivable result could be nanosized, color-to-order lasers for sensing and optical interconnect.