Fluorescence Blinking Statistics from CdSe Core and Core/Shell Nanorods

Abstract

We report fluorescence blinking statistics measured from single CdSe nanorods (NRs) of seven different sizes with aspect ratios ranging from 3 to 11. This study also included core/shell CdSe/ZnSe NRs and core NRs with two different surface ligands producing different degrees of surface passivation. We compare the findings for NRs to our measurements of blinking statistics from spherical CdSe core and CdSe/ZnS core/shell nanocrystals (NCs). We find that, for both NRs and spherical NCs, the off-time probability distributions are well described by a power law, while the on-time probability distributions are best described by a truncated power law, P(tau(on)) approximately tau(on)(-alpha)e((-tau)(on)/(tau)(c)). The measured crossover time, tau(c), is indistinguishable within experimental uncertainty for core and core/shell NRs, as well as for core NRs with different ligands, for the same core size, indicating that surface passivation does not affect the blinking statistics significantly. We find that, at fixed excitation intensity, 1/tau(c) increases approximately linearly with increasing NR aspect ratio; for a given sample, 1/tau(c) increases very gradually with increasing excitation intensity. Examining 1/tau(c)versus the single-particle photon absorption rate for all samples indicates that the change in NR absorption cross section with sample size can account for some but not all of the differences in crossover time. This suggests that the degree of quantum confinement may be partially responsible for the aspect ratio dependence of the crossover time.

Collective fluorescence enhancement in nanoparticle clusters

“…The small mechanical drift was accounted for by offsetting the position of the recorded frames appropriately during data analysis. The analysis of fluorescence statistics, previously described in detail 6,20 , is performed after thresholding and subsequent digitizing of a thresholded signal and the conclusions in this paper hold qualitatively for a range of thresholds. Fluorescence data collected from independent measurements on two different chips match very well (Supplementary Figs S8 and S9).…”

“…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.…”

“…The fluorescence intensity stochastically changes between bright and dark regimes, commonly referred to as onand off-states. Furthermore, this blinking does not follow a simple two-level quantum jump model, but instead displays approximately power-law (Lévy) statistics over many decades in time [3][4][5][6][7][8] . Similar non-Poissonian statistics have been observed in electron transport through colloidal nanocrystal (NC) arrays 9 .…”

“…Previous studies of blinking dynamics in a broad range of nanoscale systems, including individual NCs and nanorods (NRs) [3][4][5][6][7][8] , showed that most such systems obey power-law or modified powerlaw dynamics. Specifically, in NCs and NRs, the probability density function (PDF) of on-times follows a truncated power law as a function of on-time, t on 6,22,23 , where 1 < α on < 2 is the power-law exponent.…”

Evidence for a diffusion-controlled mechanism for fluorescence blinking of colloidal quantum dots

“…The power-law behavior holds regardless of sample temperature (9), QD size or composition (10), nanoparticle shape (11), or excitation intensity (12). So far, though, experimental studies of QD blinking have been limited in their temporal resolution.…”