Principal Investigator Moungi Bawendi
By looking at individual quantum dots, we have learned about phenomena never expected from ensemble measurements. Blinking, the tendency for dots to turn on and off with peculiar statistics, was discovered in our group in 1996. That same year, we saw spectral diffusion, where the narrow emission spectra of a dot changes position over the course of a measurement.
These two properties are related. Spectral diffusion is believed to be caused by fluctuations in the local electric field and can be induced by placing a single dot between two electrodes. Often times, these spectral changes will be preceded by a blinking event. Such a blinking event could be caused by the dot becoming charged and decaying nonradiatively. When the charge leaves, the dot returns on but the local electric fields have changed, causing a Stark effect in the dot. Despite over 10 years of study and many phenomenological studies showing the universality of blinking, the underlying mechanism remains an open question. Blinking has important implications for the utility of dots in applications such as single particle tracking and single photon sources. Even though blinking was not expected from ensemble studies, future research in our group demonstrated that this property does in fact also influence ensemble luminescence.
We continue to do spectroscopic studies at the single dot level to better understand properties such as multiexciton formation, radiative decay, and ordered photon emission. Such studies allow us to study fundamental physics of nanocrystals without the inhomogeneities inherent to ensemble measurements.