Self-Assembly and Near-Field Interactions between QDs and PNPs

This research aims to tailor the photoluminescence of quantum dots (QDs), from enhancement to quenching, by utilizing localized surface plasmons supported by plasmonic nanoparticles (PNPs). To this end, we are currently working on (1) control of the photoluminescence characteristics of QDs by changing the concentration of plasmonic nanoparticles and (2) self-assembly of QDs and PNPs to maximize the photoluminescence. Recent studies have found that mixing QDs and PNPs could enhance the photoluminescence although the near-field interactions could not occur based on the estimated interparticle distance according to particle concentrations (Xu et al., Appl. Phys. Lett., 96, 174101, 2010). This unexpected enhancement is due to Brownian motions of particles, leading to continuous trespass of QDs into the strong evanescent field region around PNPs upon the excitation of the localized surface plsamon. If the excitation or emission wavelength of QDs are tuned with the localized surface plasmon wavelength, strong near-field interactions between QDs and PNPs are expected. Our hypothesis is that the concentration of PNPs should significantly affect the photoluminescence characteristics of QDs, from strong enhancement for lower concentrations to quenching for higher concentrations. While Brownian motion is a critical mechanism of the dynamic near-field interactions, aggregation and sedimentation of nanoparticles may adversely affect the pholuminescence engineering. The best way for the reliable photoluminescence control would be to self-assemble QDs around a PNP with the optimal QD-PNP gap distance. Figure 1(a) shows that the fluorescence rate is strongly enhanced when a light-emitting dipole is placed ~10 nm distant from a gold nanoparticle, but it is quenched if the distance becomes smaller (Anger et al., Phys. Rev. Lett., 96, p.113002, 2006). Thus, if the PNP-QD gap distance is precisely controlled, we can control the photoluminescence depending on our needs. We will assemble silica-coated gold nanoparticles and quantum dots using the layer-by-layer assembly with oppositely charged nanoparticles (Sun et al., J. Mat. Chem., 12, p.1775, 2002). Figure 1(b) illustrates the self-assembled nanoprobe, where the silica layer thickness is precisely adjusted to achieve the optimum QD-PNP gap distance. This research will (1) synthesize self-assembled QD-PNP nanoprobes by optimizing the self-assembly conditions, (2) conduct the optical characterization to validate the hypothesis on the enhancement of photoluminescence, and (3) fundamentally understand the near-field interactions between QDs and PNPs by developing a theoretical model and comparing the theory with the measurement. The success of this research will be readily applied to the photovoltaic energy conversion and various biomarker detection. For example, we can electrospray the self-assembled nanoprobes on top of a commercially available Si solar cell to make a down-conversion layer. With simple modifications,  the  same self-assembly technology can be used for different types of solar  cells,  such  as thin-film solar cells and dye-sensitized solar cells, by embedding the QD-PNP nanoprobes into the solar cell structure. Another application of this nanoprobe is the biophotonic sensing. Since QDs in the excited state can create surface plasmons at PNPs that, in turn, radiate into the far field, and QDs in the ground state can be excited by surface plasmons at PNPs (Lakowicz, Plasmonics, 1, p. 5, 2006), a combination of fluorescence and plasmonics in QD-PNP self-assembled nanoprobes could be beneficially used to enhance the photoluminescence for extremely low level detection, such as a single cell detection for example.

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