ARTICLE pubs.acs.org/JPCC
Single-Photon Emission Behavior of Isolated CdSe/ZnS Quantum Dots Interacting with the Localized Surface Plasmon Resonance of Silver Nanoparticles Hiroyuki Naiki,† Sadahiro Masuo,*,‡ Shinjiro Machida, and Akira Itaya* Department of Macromolecular Science and Engineering, Kyoto Institute of Technology, Matsugasaki, Sakyo-ku, Kyoto 606-8585, Japan
bS Supporting Information ABSTRACT: 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.
1. INTRODUCTION The use of colloidal quantum dots (QDs) as a material for the optoelectronic applications15 and biolabeling69 has been the focus of great attention. One interesting optical property of individual QDs is their single-photon emission behavior at room temperature.1013 Because single-photon sources capable of emitting only one photon at any point in time are essential for quantum information processing, the development and creation of single-photon sources are actively in progress. In this context, QDs are good candidates because of their high absorption cross section, exceptional photostability, wide excitation bandwidth, and narrow emission bandwidth. However, single QDs show strong fluorescence blinking,14,15 which inhibits stable singlephoton emission. To suppress the blinking, Wang et al.16 synthesized colloidal QDs consisting of graded alloy, and Lakowicz et al.,1719 Kanemitsu et al.,20,21 and Biju et al.22 investigated the fluorescence behaviors of QDs interacting with localized surface plasmon resonance (LSPR) of metal nanostructures. Recently, we used a photon correlation technique in combination with picosecond-pulsed laser excitation and reported r 2011 American Chemical Society
single-photon emission behavior from single CdSe/ZnS QDs that exhibit enhanced fluorescence by interacting with silver nanoparticles’ (AgNPs) LSPR.23 The probability of singlephoton emission decreased with an increase in fluorescence intensity with a decrease in the fluorescence lifetime (
