FLUORESCENCE INTERMITTENCY IN SINGLE CADMIUM SELENIDE NANOCRYSTALS

SEMICONDUCTOR nanocrystals offer the opportunity to study the evolutio n of hulk materials properties as the size of a system increases from the molecular scale(1,2). In addition, their strongly size-dependent o ptical properties render them attractive candidates as tunable light a bsorbers and emitters in optoelectronic devices such as light-emitting diodes(3,4) and quantum-dot lasers(5,6), and as optical probes of bio logical systems(7). Here we show that light emission from single fluor escing nanocrystals of cadmium selenide under continuous excitation tu rns on and off intermittently with a characteristic timescale of about 0.5 seconds. This intermittency is not apparent from ensemble measure ments on many nanocrystals. The dependence on excitation intensity and the change in on/off times when a passivating, high-bandgap shell of zinc sulphide encapsulates the nanocrystal(8,9) suggests that the abru pt turning off of luminescence is caused by photoionization of the nan ocrystal. Thus spectroscopic measurements on single nanocrystals can r eveal hitherto unknown aspects of their photophysics.