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Antibunching and Excitation Lifetimes in CdSe Nanocrystals

Margaret P. Cosgriff

Published Jan 1, 2009• 0 citations
OpenAlex

Abstract

Using a beam-splitter and two photon detectors, we measured the cross-correlation function of a series of single nanocrystals, and found that the nanocrystals exhibit antibunching (zero correlation at zero time delay) as expected, indicating that the nanocrystals were indeed emitting only one photon at a time. Modeling the nanocrystals as a three-level energy system (ground, excited, and "dark"), we measured the characteristic time delay between photons emitted by a nanocrystal, which corresponds to the inverse of the transition rate from the excited state and the ground state. We performed this measurement using 532nm and 405nm wavelength light to excite the nanocrystals. Nanocrystals excited by 532nm wavelength light had a mean time constant of 22.7 ± 5.3ns, and those excited by 405nm wavelength light had a mean time constant of 22.4 ± 6.3ns.

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Using a beam-splitter and two photon detectors, we measured the cross-correlation function of a series of single nanocrystals, and found that the nanocrystals exhibit antibunching (zero correlation at zero time delay) as expected, indicating that the nanocrystals were indeed emitting only one photon at a time. Modeling the nanocrystals as a three-level energy system (ground, excited, and "dark"), we measured the characteristic time delay between photons emitted by a nanocrystal, which corresponds to the inverse of the transition rate from the excited state and the ground state. We performed this measurement using 532nm and 405nm wavelength light to excite the nanocrystals. Nanocrystals excited by 532nm wavelength light had a mean time constant of 22.7 ± 5.3ns, and those excited by 405nm wavelength light had a mean time constant of 22.4 ± 6.3ns.
The fluorescence of single colloidal CdSe quantum dots is investigated at room temperature by means of the autocorrelation function over a time scale of almost 12 orders of magnitude. At short time, the autocorrelation function shows a complete antibunching indicating single photon emission and atomic-like behavior. On longer time scales (up to tens of seconds), we measure a bunching effect due to fluorescence intermittency that can not be described by fluctuations between two states with constant rates. The autocorrelation function also exhibits a non-stationary behavior related to power-law distributions of "On" and "Off" times.
The fluorescence of single-colloidal CdSe quantum dots is investigated at room temperature by means of the autocorrelation function over a time scale of almost 12 orders of magnitude. Over a short time scale, the autocorrelation function shows complete antibunching, indicating single-photon emission and atomiclike behavior. Over longer time scales (up to tens of seconds), we measure a bunching effect that is due to fluorescence intermittency and that cannot be described by fluctuations between two states with constant rates. The autocorrelation function also exhibits nonstationary behavior related to power-law distributions of On and Off times.
In this paper, precise measurement of the time coherence of CdSe nanocrystals fluorescence is reported. At low temperature, where emission linewidth is expected to be below 0.2 meV, direct linewidth measurements are not possible, due to insufficient spectrometer resolution and spectral diffusion of the emission wavelength. To obtain a higher resolution, Michelson interferometer is used. An original highly sensitive method, based on the time-correlation between the photons detected at the two interferometer outputs is developed.
We investigate the fluorescence intensity correlation function of a single CdSe quantum dot (QD) using a start–stop experiment. We observe strong photon antibunching, a signature of non-classical light emission, over a large range of intensities (0.1–100kW/cm2). The lack of coincidence at zero time delay indicates a highly efficient Auger ionization process, which suppresses multi-photon emission in these colloidal QDs. Using careful analysis of the saturation behavior of the coincidence histograms, the absorption cross-section of a single QD has also been derived.