As we search for ways to reduce our reliance on fossil fuels, solar cells have become ever more attractive. However, existing solar cells do not convert solar energy to electricity as efficiently as they could. A type of material called “quantum dots” could enable more productive solar energy conversion.Quantum dots are infinitesimal particles. All of them shine brightly, but those made of lead sulfide and lead selenide can be chemically synthesized also to emit light at infrared wavelengths desirable for communications technology. Such quantum dots could encrypt messages that would be hard for a hacker or foreign military code breaker to crack.Infinitesimal particles called quantum dots could form the basis of efficient solar cells.When these quantum dots are blasted with a laser, electrons can jump to higher energy levels, leaving “holes” behind. Almost instantly, each negatively charged electron pairs up with a positively charged hole — a transient pairing called an “exciton.” Within nanoseconds, the electrons may settle down into their original states, emitting photons in the process. Because these photons are released, the quantum dots light up like a Christmas tree — a major reason why Ranojoy Bose and colleagues in Chee Wei Wong’s group at Columbia University in New York City have been using films containing these quantum dots.With this setup, Bose and colleagues investigated the effect of temperature on energy transfer by quantum dots.The researchers can perform another trick that helps them control the amount of light that comes from the dots. In large films or other assemblies of similarly sized dots, the smaller ones can transfer energy directly to larger ones, which then emit photons. The investigators found that they could greatly increase the rate of energy transfer by lowering the temperature of the dots. This physical property allows the flow of energy to be directed. Bose said that, in the future, small quantum dots may be chemically synthesized and artificially assembled so that they are closer to the larger ones, thus increasing the efficiency of the energy transfer.Nano Letters, published online May 31, 2008, doi: 10.1021/nl8011243.