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

Abstract

Fluorescence blinking in nanocrystal quantum dots is known to exhibit power-law dynamics, and several different mechanisms have been proposed to explain this behavior. We have extended the measurement of quantum-dot blinking by characterizing fluctuations in the fluorescence of single dots over time scales from microseconds to seconds. The power spectral density of these fluctuations indicates a change in the power-law statistics that occurs at a time scale of several milliseconds, providing an important constraint on possible mechanisms for the blinking. In particular, the observations are consistent with the predictions of models wherein blinking is controlled by diffusion of the energies of electron or hole trap states.fluorescence intermittency ͉ power spectrum ͉ nanocrystals H igh-quality, monodisperse semiconductor nanocrystals can be produced in large quantities by colloidal-synthesis techniques (1). These nanocrystals, known as quantum dots (QDs), can exhibit bright luminescence, whose wavelength is controlled by the size of the nanocrystals (2). This property makes colloidal QDs attractive candidates for several applications, including light-emitting diodes (3), solid-state lasers (4), and biological labeling (5, 6). Such applications, however, may be compromised by fluctuations in the QD luminescence. In particular, individual QDs emit light intermittently, switching irregularly between bright (''on'') and dark (''off'') states (7). Widespread interest in this blinking phenomenon was stimulated by the surprising observation that the durations of bright and dark periods follow power-law statistics (8, 9). Specifically, the blinking periods are described by probability densities of the formwith a value of between 0.4 and 1.0. 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. A resolution of 200 s was achieved in one of the earliest measurements (8), and a small number of later experimental studies have included analysis of submicrosecond blinking dynamics (14,16,17,41). The remainder of the quantitative characterizations have been restricted to time scales of several milliseconds or longer. In this paper, we report measurements of fluctuations in QD fluorescence on time scales from microseconds to tens of seconds. We observe a change in the fluctuation dynamics for time scales less than several milliseconds.This observation is consistent with the predictions of a class of models where blinking is controlled by slow diffusion of the energies of electron or hole trap states. In these models, the fluorescence of a QD is quenched through the trapping of a carrier, which occurs when the energy of the trap state fulfills a resonance condition. The duration of the blinking periods is determined by the diffusion of the energy of the QD-trap system about this resonance condition. This diffusion-controlled mechanism ...

Quantitative modeling of the role of surface traps in CdSe/CdS/ZnS nanocrystal photoluminescence decay dynamics

“…Our analysis shows that NCs with very-low-energy trap states ( Ͻ Ϫ50 meV) are rare, yet several of the models that account for single NC PL intermittency require the presence of deeply trapped states (45). Models including spectral diffusion of acceptor energies (46,48) have previously been used to explain power-law blinking statistics. However, we find that trap energies in our samples are typically higher energy than would be required for blinking.…”

Three-Dimensional Optical Control of Individual Quantum Dots

“…The fluorescent blinking of nanocrystal quantum dots is the result of a bistability between an emitting state where the quantum dot is described as on and the nonemitting off state. 2 The extreme brightness and photostability of QDs make them excellent choices as markers to visualize biological systems. For instance they have been used to mark individual receptors in cell membranes 3 or to label living embryos at different stages.…”

1/fαpower spectrum in the Kardar–Parisi–Zhang universality class

“…Since this simplifies analysis, and also because other kinds of noise such as Johnson-Nyquist noise and shot noise [6] have been successfully described under the assumption of stationarity, the same assumption has also been often adopted in models of 1/f α noise for solid-state systems [1,2]. On the other hand, recent experiments have shown examples of 1/f α noise in intermittent systems, such as blinking quantum dots [7][8][9][10] and nanoscale electrodes [11], which were shown to be non-stationary, or aging. The 1/f α noise in those systems was accounted for by models based on the intermittent dynamics [10][11][12], and the same line of analysis was also applied to turbulent fluid [13,14] and the fluctuating electroconvection of liquid crystal [15].…”

“…with t n = t min + (n − 1)∆t (t min = ∆t for the simulations) 8 , t N = T, and ω being a multiple of 2π/N∆t. Figure 4 shows the power spectra S h (ω; T) (panels (a)-(d)) and S q (ω; T) (panels (e)-(h)) with different T, for both the experiments ((a), (b), (e) and (f)) and the simulations ((c), (d), (g) and (h)) and for both the circular case ((a), (c), (e) and (g)) and the flat case ((b), (d), (f) and (h)).…”