Single-Photon Emission Behavior of Isolated CdSe/ZnS Quantum Dots Interacting with the Localized Surface Plasmon Resonance of Silver Nanoparticles

To reveal the exciton dynamics, particularly multiexciton dynamics in single colloidal CdSe/ZnS quantum dots (QDs) interacting with the localized surface plasmon resonance (LSPR) of silver nanoparticles (AgNPs), we investigated the single-photon emission behavior in the fluorescence from the single QDs using a single-molecule spectroscopy technique in combination with a femtosecond-pulsed laser excitation. By applying simultaneous measurements of the photon-correlation (photon antibunching), the fluorescence intensity, and the fluorescence lifetime to the single QD near AgNPs, we revealed that the probability of single-photon emission strongly depended on the fluorescence lifetime; that is, the probability of single-photon emission decreased when the lifetime was shorter than subnanoseconds. On the basis of the estimation of both radiative and nonradiative decay rates enhanced by the LSPR, the following mechanism was suggested. In the absence of AgNPs, multiple excitons generated by a high-power excitation lead to single-photon emission via nonradiative Auger recombination process between the excitons. In the presence of AgNPs, even when the excitation power is low, multiple excitons are initially generated in a single QD by the enhanced electromagnetic field of the AgNPs' LSPR Subsequently, a portion of these excitons radiatively decay via plasmon, that is, the radiative decay rate enhanced by LSPR competes with the Auger recombination process. When the enhanced radiative decay rate is faster than that of the Auger process, multiphoton emission can be observed. Therefore, a decrease in the probability of single-photon emission is observed when the fluorescence lifetime is shortened. This result will improve our understanding of fluorescence enhancement by the LSPR of metal nanostructures, assist in the creation of effective single-photon sources, and allow for the utilization of the multiexcitons in QDs for optoelectronic applications.