Blinking Statistics Correlated with Nanoparticle Number

Abstract

We report fluorescence of single semiconductor nanorods (NRs) and few-NR clusters, correlated with transmission electron microscopy for direct determination of the number of NRs present in a single fluorescent source. For samples drop-cast from dilute solutions, we show that the majority of the blinking sources (approximately 75%) are individual NRs while the remaining sources are small clusters consisting of up to 15 NRs. Clusters containing two or three NRs exhibit intermittent fluorescence intensity trajectories, I(t), similar to those of individual NRs. The associated statistical parameters of on- and off-time probability densities for two- and three-NR clusters are indistinguishable from those of individual NRs. In contrast, statistically distinguishable blinking parameters are observed for clusters of five or more particles. In particular, the "truncation time" of the on-time probability density, i.e., the time characterizing the transition from a power law to an exponential decay, was found to increase superlinearly with the number of particles. Our long (2.4 x 10(4) s) blinking measurements also directly reveal the previously unobserved truncation of the power law distribution of the off-times for single nanoparticles.

Collective fluorescence enhancement in nanoparticle clusters

“…3a). Although it has been shown that blinking statistics are threshold dependent when the threshold is varied significantly 16,20,24 , our analysis confirms that the trends reported here hold for a wide range of reasonable thresholds (see detailed analysis of on-times for different thresholds in Methods). All results reported in the main paper were determined using a threshold of m dark + 7σ dark (red line in Fig.…”

“…2) by setting a threshold distinguishing the fluorescence intensity as 'on' or 'off, ' digitizing the trace according to that threshold, and then converting the digitized trace to sequences of 'on' and 'off ' times 6,20 . We identify the threshold above which a fluorescence signal is considered 'on' using the mean, m dark , and standard deviation, σ dark , of the background intensity measured from a nearby bare region of the substrate.…”

“…One likely explanation for some of the scatter is that, for the combined independent NRs and the Monte-Carlo simulation, N is the number of emitting NRs; for the clusters, N is the number of NRs in the cluster counted from the TEM images, and may therefore be greater than the number of emitting NRs in each cluster, because a significant fraction (up to ~50-70%) of NRs may be permanently dark 20,21,25 . For the clusters, N therefore represents an upper bound on the number of emitting NRs.…”

“…Increasing the excitation power decreases τ c,on 13,14,23 . The PDF of off-times similarly follows a truncated power law with the corresponding power-law exponent α off~1 .3 and the truncation time τ c,off~1 ,000 s in NRs 20 .…”

“…1a) allows us to directly image both the number of nanoparticles and their location within a cluster, and to measure cluster fluorescence over more than five orders of magnitude in time, from an elementary time step of 0.1 s to several hours. We achieve this by correlating fluorescence microscopy and transmission electron microscopy (TEM) 20,21 .…”