Quantum optics with nanocrystal quantum dots in solution: Quantitative study of clustering

Applying a combination of traditional fluorescence correlation spectroscopy and antibunching measurements to solutions of nanocrystal quantum dots (NQDs), we can reliably establish the regime where only one dot or less is present in the detection volume. Under these conditions, it is possible to probe various photophysical properties of colloidal nanocrystals with single-dot sensitivity in their "native" solution environment. We apply this method to quantitative studies of NQD aggregation. By first measuring dilute Rhodamine 590 solutions that have no aggregation, we find that the clustering parameter, <n> (the average number of quantum emitters per diffusing cluster), can be determined with better than 5% accuracy. We then use this technique to quantify clustering of CdSe NQDs prepared either as toluene or aqueous solutions. On the basis of the correlation data, NQDs exhibit minimal aggregation (<n> is less than 1.1-1.2) in fresh as-prepared solutions for both aqueous and nonaqueous systems. On the other hand, sample aging leads to considerable increase in the degree of aggregation as indicated by increased values of <n>. For example, in a sample of biotinylated NQDs aged for 120 days the number of two-dot aggregates becomes approximately equal to that of isolated NQDs. The ability to study single dots in solutions, demonstrated here, opens interesting opportunities for both biorelated research and also for studies of fundamental photophysics of nanocrystals, especially the effects of environment on electronic structures and carrier relaxation behaviors.