Characterizing Quantum-Dot Blinking Using Noise Power Spectra

Colloidal semiconductor nanocrystals, or quantum dots (QDs), can exhibit bright, long-lived fluorescence, with an emission wavelength that is simply tuned by changing the size of the nanocrystal. This property makes them promising, for example, as biological labels, and as the active medium in light-emitting diodes or lasers. However, such applications may be compromised by fluctuations in the intensity of the QD fluorescence. Optical microscopy of single immobilized QDs has shown a blinking behavior, where the dots alternate between “on” (fluorescing) and “off” (non-fluorescing) states, with power-law distributions for on and off periods. Such measurements, though, are relatively difficult and time-consuming, making it difficult to study large numbers of different samples in different environments. This Letter shows that complementary information is provided by measuring the power spectral density of fluctuations in the fluorescence from macroscopic ensembles of QDs. The power spectrum has the form of 1/f noise, which can be understood as the incoherent sum of the identical noise spectra of individual QDs. The ensemble noise measurements thus provide a simple, rapid technique to obtain information about the blinking statistics of individual QDs. It can be applied to a wide range of environments, including those not previously accessible to experiment, which should simplify the task of identifying and possibly eliminating the causes of blinking.