Scientists at the Institut d’Electronique de Microélectronique et de Nanotechnologie in Lille, France, and at Utrecht University and the Mesa+ institute at Universiteit Twente in Enschede, both in the Netherlands, have determined that colloidal nanocrystals display a supralinear relation between the rate of decay of their spontaneous emission and the emission frequency. They note that the findings may offer insight into phenomena such as exciton dephasing, radiative recombination of nanocrystals in photonic crystals and Förster energy transfer. In the experiments, the researchers optically excited suspensions of CdSe and CdTe nanocrystals at room temperature using 100-ps pulses of 406 nm light from a PicoQuant laser diode. They performed time-correlated photon counting with a 1350-lines-per-millimeter monochromator and a Hamamatsu photomultiplier tube to obtain luminescence decay curves, which they plotted versus emission frequency.The results support the conclusion that the emission frequency of nanocrystals can be tuned by changing their size, without changes in chemistry. The investigators deduce that the thermal occupation of excitonic states that are close to the ground state and that have low transition probabilities govern the rate of decay at room temperature. Physical Review Letters, Dec. 2, 2005, 236804.