Photoluminescence (PL) intermittency (blinking), or random switching between states of high- (ON) and low (OFF) emissivities, is a universal property of molecular emitters exhibited by dyes1, polymers2, biological molecules3 and artificial nanostructures such as nanocrystal quantum dots, carbon nanotubes, and nanowires4,5,6. For the past fifteen years, colloidal nanocrystals have been used as a model system for studies of this phenomenon.5,6 The occurrence of OFF periods in nanocrystal emission has been commonly attributed to the presence of an additional charge7, which leads to PL quenching by nonradiative Auger recombination.8 However, the “charging” model was recently challenged in several reports.9,10 Here, to clarify the role of charging in PL intermittency, we perform time-resolved PL studies of individual nanocrystals while controlling electrochemically the degree of their charging. We find that two distinct mechanisms can lead to PL intermittency. We identify conventional blinking (A-type) due to charging/discharging of the nanocrystal core when lower PL intensities correlate with shorter PL lifetimes. Importantly, we observe a different blinking (B-type), when large changes in the PL intensity are not accompanied by significant changes in PL dynamics. We attribute this blinking behavior to charge fluctuations in the electron-accepting surface sites. When unoccupied, these sites intercept hot electrons before they relax into emitting core states. Both blinking mechanisms can be controlled electrochemically and under appropriate potential blinking can be completely suppressed.

We use a simple device architecture based on a poly(3,4-ethylendioxythiophene):poly(styrenesulfonate) (PEDOT:PSS)-coated indium tin oxide anode and a LiF/Al cathode to assess the effects of shell thickness on the properties of light-emitting diodes (LEDs) comprising CdSe/CdS core/shell nanocrystal quantum dots (NQDs) as the emitting layer. Specifically, we are interested in determining whether LEDs based on thick-shell nanocrystals, so-called "giant" NQDs, afford enhanced performance compared to their counterparts incorporating thin-shell systems. We observe significant improvements in device performance as a function of increasing shell thickness. While the turn-on voltage remains approximately constant for all shell thicknesses (from 4 to 16 CdS monolayers), external quantum efficiency and maximum luminance are found to be about one order of magnitude higher for thicker shell nanocrystals (≥13 CdS monolayers) compared to thinner shell structures (<9 CdS monolayers). The thickest-shell nanocrystals (16 monolayers of CdS) afforded an external quantum efficiency and luminance of 0.17% and 2000 Cd/m(2), respectively, with a remarkably low turn-on voltage of ~3.0 V.

Nanocrystal quantum dots are attractive materials for applications as nanoscale light sources. One impediment to these applications is fluctuations of single-dot emission intensity, known as blinking. Recent progress in colloidal synthesis has produced nonblinking nanocrystals; however, the physics underlying blinking suppression remains unclear. Here we find that ultra-thick-shell CdSe/CdS nanocrystals can exhibit pronounced fluctuations in the emission lifetimes (lifetime blinking), despite stable nonblinking emission intensity. We demonstrate that lifetime variations are due to switching between the neutral and negatively charged state of the nanocrystal. Negative charging results in faster radiative decay but does not appreciably change the overall emission intensity because of suppressed nonradiative Auger recombination for negative trions. The Auger process involving excitation of a hole (positive trion pathway) remains efficient and is responsible for charging with excess electrons, which occurs via Auger-assisted ionization of biexcitons accompanied by ejection of holes.

The growth of ultra-thick inorganic CdS shells over CdSe nanocrystal quantum dot (NQD) cores gives rise to a distinct class of NQD called the "giant" NQD (g-NQD). g-NQDs are characterized by unique photophysical properties compared to their conventional core/shell NQD counterparts, including suppressed fluorescence intermittency (blinking), photobleaching, and nonradiative Auger recombination. Here, we report new insights into the numerous synthetic conditions that influence the complex process of thick-shell growth. We show the individual and collective effects of multiple reaction parameters (noncoordinating solvent and coordinating-ligand identities and concentrations, precursor/NQD ratios, precursor reaction times, etc.) on determining g-NQD shape and crystalline phase, and the relationship between these structural features and optical properties. We find that hexagonally faceted wurzite g-NQDs afford the highest ensemble quantum yields in emission and the most complete suppression of blinking. Significantly, we also reveal a clear correlation between g-NQD particle volume and blinking suppression, such that larger cores afford blinking-suppressed behavior at relatively thinner shells compared to smaller starting core sizes, which require application of thicker shells to realize the same level of blinking suppression. We show that there is a common, threshold g-NQD volume (~750 nm(3)) that is required to observe blinking suppression and that this particle volume corresponds to an NQD radiative lifetime of ~65 ns regardless of starting core size. Combining new understanding of key synthetic parameters with optimized core/shell particle volumes, we demonstrate effectively complete suppression of blinking even for long observation times of ~1 h.

Biexciton photoluminescence (PL) quantum yields (Q(2X)) of individual CdSe/CdS core-shell nanocrystal quantum dots with various shell thicknesses are derived from independent PL saturation and two-photon correlation measurements. We observe a near-unity Q(2X) for some nanocrystals with an ultrathick 19-monolayer shell. High Q(2X)'s are, however, not universal and vary widely among nominally identical nanocrystals indicating a significant dependence of Q(2X) upon subtle structural differences. Interestingly, our measurements indicate that high Q(2X)'s are not required to achieve complete suppression of PL intensity fluctuations in individual nanocrystals.

We recently developed an inorganic shell approach for suppressing blinking in nanocrystal quantum dots (NQDs) that has the potential to dramatically improve the utility of these fluorophores for single-NQD tracking of individual molecules in cell biology. Here, we consider in detail the effect of shell thickness and composition on blinking suppression, focusing on the CdSe/CdS core/shell system. We also discuss the blinking mechanism as understood through profoundly altered blinking statistics. We clarify the dependence of blinking behavior and photostability on shell thickness, as well as on interrogation times. We show that, while the thickest-shell systems afford the greatest advantages in terms of enhanced optical properties, thinner-shell NQDs may be adequate for certain applications requiring relatively shorter interrogation times. Shell thickness also determines the sensitivity of the NQD optical properties to aqueous-phase transfer, a critical step in rendering NQDs compatible with bioimaging applications. Lastly, we provide a proof-of-concept demonstration of the utility of these unique NQDs for fluorescent particle tracking. High-resolution image of an ultra-thick-shell ‘giant’ nanocrystal quantum dot (left). Suppressed blinking behaviour afforded by this class of semiconductor nanocrystal yields new statistical relationships in the probability densities of fluorescence on- and off-time distributions (right).

We demonstrate that photon antibunching observed for individual nanocrystal quantum dots (NQDs) can be transformed into photon bunching characterized by super-Poissonian statistics when they are coupled to metal nanostructures (MNs). This observation indicates that, while the quantum yield of a biexciton (Q(2X)) is lower than that of a single exciton (Q(1X)) in freestanding NQDs, Q(2X) becomes greater than Q(1X) in NQDs coupled to MNs. This unique phenomenon is attributed to metal-induced quenching with a rate that scales more slowly with exciton multiplicity than the radiative decay rate and dominates over other nonradiative decay channels for both single excitons and biexcitons.

In single particle spectroscopy, the degree of observed fluorescence anti-bunching in a second-order cross correlation experiment is indicative of its bi-exciton quantum yield and whether or not a particle is well isolated. Advances in quantum dot synthesis have produced single particles with bi-exciton quantum yields approaching unity. Consequently, this creates uncertainty as to whether a particle has a high bi-exciton quantum yield or if it exists as a cluster. We report on a time-gated anti-bunching technique capable of determining the relative contributions of both multi-exciton emission and clustering effects. In this way, we can now unambiguously determine if a particle is single. Additionally, this time-gated anti-bunching approach provides an accurate way for the determination of bi-exciton lifetime with minimal contribution from higher order multi-exciton states.