Cryogenic Single Nanocrystal Spectroscopy: Reading the Spectral Fingerprint of Individual CdSe Quantum Dots

Spectroscopically resolved emission from single nanocrystals at cryogenic temperatures provides unique insight into photophysical processes that occur within these materials. At low temperatures the emission spectra collapse to narrow lines revealing a rich spectroscopic landscape and unexpected properties, completely hidden at the ensemble level. Since these techniques were first used, the technology of nanocrystal synthesis has matured significantly and new materials with outstanding photophysical stability have been reported. Here we review our recent work that shows how cryogenic spectroscopy of single nanocrystals probes the fundamental excitonic structure of the band edge, revealing spectral fingerprints that are highly sensitive to a range of photophysical properties as well as nanocrystal morphology. In particular, spectral and temporal signatures of biexciton and trion emission are revealed and their relevance to emerging technologies discussed. In addition, we show how high resolution excitation spectroscopy can provide information on external processes that ultimately limit the coherence of the nanocrystal band-edge states. Overall we demonstrate how cryogenic single nanocrystal spectroscopy can be used as a vital tool for understanding fundamental photophysics and guiding the synthesis of new nanocrystal materials.