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.

Fig. 1.

Blinking of quantum dots and diffusion-controlled model. (a) Intensity of fluorescence as a function of time measured from a single CdSe/ZnS core–shell nanocrystal QD. The two tracings show the same data on different time scales, grouped into time bins of 100 msec (Upper) and 5 msec (Lower). (b) Schematic of the states involved in QD blinking. The dot is excited from a lower-energy to a higher-energy state by absorption of a photon. If the dot is in a bright state, relaxation occurs by photon emission; if it is in a dark state, relaxation occurs by a nonradiative mechanism. Blinking corresponds to transitions between the bright and dark states.

Fig. 2.

Probability density of duration of blinking periods for the time trace shown in Fig. 1, using 5-msec time bins. Filled squares indicate dark periods, and open circles indicate bright periods. The absence of points for short bright periods is due to the limited statistics for the finite duration of this data run.

Fig. 3.

Autocorrelation function of fluctuations in fluorescence measured from three individual QDs. The correlation functions are normalized by the mean intensity; however, due to the power-law probability density of blinking periods, this mean depends on the duration of the experiment, and the short-time values of the functions do not approach unity. Average count rates, from top to bottom, are 5,200, 1,700, and 3,000 counts per sec.

Fig. 4.

Power spectral density of fluctuations in fluorescence measured from three individual QDs. Results are for the same time series as for Fig. 3. Solid lines are fitted power laws to low-frequency and high-frequency portions of the power spectra, and horizontal dashed lines are expected shot-noise levels. Statistical errors are expected to be less than the size of the plotted points.

Fig. 5.

Schematic of models for QD blinking. (a) Energy levels in the DCET model. Transitions from |G〉 to |L*〉 or from |D〉 to |D*〉 are driven by incident light. Transitions from |L*〉 to |G〉 are primarily radiative, whereas transitions from |D*〉 to |D〉 are primarily nonradiative. (b) Free energies of energy levels in the DCET model, as a function of reaction coordinate Q. Transitions from |L*〉 to |D〉 occur by electron transfer, at rate k_r, when the system is at reaction coordinate Q^‡. (c) Schematic of trapping mechanism in the Auger-assisted model. Holes are trapped in deep-band states through the promotion of an electron in the conduction band (CB). (d) Schematic of the suggested photo-assisted detrapping mechanism.