Article pubs.acs.org/JPCC
Blinking Statistics of Small Clusters of Semiconductor Nanocrystals Kevin J. Whitcomb,† Duncan P. Ryan,‡ Martin P. Gelfand,‡ and Alan Van Orden*,† †
Department of Chemistry and ‡Department of Physics, Colorado State University, Fort Collins, Colorado 80523, United States S Supporting Information *
ABSTRACT: Individual and small clusters of CdSe/ZnS core/shell semiconductor nanocrystals are studied with single-molecule, time-correlated, singlephoton counting confocal fluorescence microscopy to obtain fluorescence intermittency (blinking) statistics and correlations. Clusters were distinguished from individual nanocrystals using intensity autocorrelations and decay histograms. Because clusters do not exhibit on/of f blinking, we have constructed objective prescriptions to distinguish between high and low emission intervals using lifetime as well as intensity data. The high/low blinking of clusters is statistically similar to the on/of f blinking of individual nanocrystals. These results support a model for the distinctive fluorescence properties of small nanocrystal clusters based on nonradiative energy transfer, in which the high/low blinking of a cluster is a consequence of on/off blinking of the “acceptor” (smallest energy gap) nanocrystal in the cluster.
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INTRODUCTION Colloidal semiconductor nanocrystals (NCs) are nanometerscale semiconductor particles that have found use in many applications due to their ease of synthesis and their sizedependent spectroscopic and electronic properties. Such applications include photovoltaic cells,1−3 solar hydrogen generation,4,5 photon detectors,6,7 lasers,8−11 and bioimaging.12 In many applications, it is necessary to have a high density of NCs to maximize light absorption and to facilitate charge transport.13−17 The question arises: how does the close proximity of the NCs affect the properties of the material? Although the fluorescence properties of individual NCs have been well studied,18−25 the interactions between close-packed NCs are not as well understood.26−31 To address these issues, we and others have undertaken studies applying singlemolecule microscopy techniques to small clusters of NCs as model systems.32−36 With a few exceptions,37,38 the fluorescence intensity of an individual NC under constant illumination exhibits intermittency:19−21,39−46 it is either in an on state, corresponding to high quantum efficiency, or in an of f state, consistent with zero emission within experimental uncertainty. This behavior is often termed blinking. In the off state NCs continue to absorb light,47 and the absence of fluorescence is due to fast nonradiative decay; however, the details of that process (or processes) are a subject of active investigation.19,39,48−52 One feature of NC blinking, which is distinct from the blinking behavior of other fluorophores such as molecular dyes, is that the on/of f event times follow power law probability distributions, usually with an exponent of about 1.5, over a broad range of times.41 At the longest times deviations from power law behavior, particularly for the on intervals, are sometimes observed and termed “truncation”.21,40 Another characteristic of NC blinking is correlation between event durations.44,46 The durations of on events show positive © 2013 American Chemical Society
correlation with nearby on event times and negative correlation with nearby off event times. Likewise, off event durations show analogous correlations. These correlations are termed blinking memory and decay over the course of many events. Previous work using confocal fluorescence microscopy correlated with atomic force microscopy (AFM) revealed that small clusters of close proximity ( 16% or with τR < 2.0 s and f S > 16%: thus, our classification scheme is robust. Clusters are approximately 3−7 NCs in size based on fluorescence intensity compared to individual NCs. Clusters and individual NCs also exhibit distinct behavior in fluorescence lifetime−intensity distributions (FLIDs). Figure 2
either an individual NC or a small cluster of NCs in the confocal region. In previous work we found that the clusters of NCs, in comparison to isolated NCs, showed faster decay of the intensity autocorrelation function and enhancement of the short lifetime component in a biexponential fit to the fluorescence decay histogram. These effects were only observed in NC clusters that were close packed (
