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Single Nanocrystal Spectroscopy
Before semiconductor nanocrystals can be effectively incorporated into LEDs and quantum dot lasers operating at high efficiency, we must first understand the fundamental excitation, relaxation and charge transport properties of nanocrystals. One of the ways our group does this is by probing the interaction of light with individual nanocrystal heterostructures.
To this end, we have recently constructed a single nanocrystal spectrometer capable of collecting fluorescence transients from nanocrystals we design by colloidal synthesis. The new instrumentation was built in collaboration with Dr. Lloyd M. Davis of University of Tennessee Space Institute and Dr. Philip C. Samson and Dr. John P. Wikswo of Vanderbilt University. Our spectrometer can operate in 2 configurations: confocal and widefield. In confocal we can raster scan to collect fluorescence on single photon counting photodiodes for nano- to microsecond dynamics measurements to study exciton and biexciton lifetime and perform photon antibunching to confirm emission from a single emitter. In widefield we can direct the fluorescence to a cooled EM-CCD camera sensitive to single photons to study nanocrystal blinking (fluorescence intermittency).
Using this instrumentation we were able to perform a single nanocrystal spectroscopic study on our ultrasmall white light emitting CdSe to gain insight into the broad spectral distribution of the fluorescence from the nanocrystals. By placing a prism in front of the EM-CCD camera and operating in widefield, we proved that the emission from a single ultrasmall CdSe nanocrystal spanned the entire visible spectrum. This is quite remarkable, and suggested that each nanocrystal contained all surface trap states of appropriate mid-gap energy levels to give rise to the broad emission spectrum.
Orfield, N. J.; McBride, J. R.; Keene, J. D.; Rosenthal, S. J., Correlation of Atomic Structure and Photoluminescence of the Same Quantum Dot: Pinpointing Surface and Internal Defects that Inhibit Photoluminescence. ACS Nano 2015, 9 (1), 831-839.
Dukes, A. D.; Samson, P. C.; Keene, J. D.; Davis, L. M.; Wikswo, J. P.; Rosenthal, S. J., Single-Nanocrystal Spectroscopy of White-Light-Emitting CdSe Nanocrystals. J. Phys. Chem. A 2011, 115 (16), 4076-4081.