Distance-Engineered Plasmon-Enhanced Light Harvesting in CdSe Quantum Dots

Improvement of light harvesting in semiconductor quantum dots (QDs) is essential for the development of efficient QD-based solar energy conversion systems. In this study, plasmonenhanced light absorption in CdSe QDs sensitized on silver (Ag) nanoparticle (NP) films was examined as a function of interparticle (QD to Ag NP) distance. Up to 24-fold plasmonic enhancement of fluorescence from QDs was observed when the particle separation distance was ≥5 nm. The enhancement effect was observed to largely sustain the exciton lifetimes in QDs and to strongly depend on the incident photon wavelength following the plasmon resonant strength of Ag NPs, confirming that the enhanced photoluminescence was mainly due to the enhancement in photoabsorption in CdSe QDs by the plasmon of Ag NPs. This study suggests applications of Ag NPs in QD-based solar energy conversion for significantly improving light harvesting in QDs. SECTION: Plasmonics, Optical Materials, and Hard Matter S quantum dots (QDs) have received extensive attention because of their attractive chemical and photophysical properties, such as ease of synthesis, tunable band gap, broad absorption spectra, large extinction coefficient, and long photogenerated exciton lifetimes. These properties make QDs a promising class of materials, potentially superior to inorganic or organic molecules, to be utilized as light-harvesting and charge-separation components in solar energy conversion schemes. For example, effective and robust photogeneration of H2 from water facilitated by semiconductor QDs (or hybrid nanostructures) with catalysts (e.g., platinum, nickel) has been previously reported in the literature. The fundamental processes in these photocatalytic reactions normally involve light harvesting by a QD, followed by charge transfer to the catalyst, where the eventual turnover of reactants to fuels occurs. Because most photocatalytic reactions require multiple charges, effective charge accumulation is essential to realize high efficiency. This requires not only schemes with effective charge separation and suppressed charge recombination but also the development of effective approaches for the enhancement of light harvesting by QDs. Utilizing the localized surface plasmon of metal nanoparticles (NPs) or structures to increase the light harvesting in QDs can be a promising strategy to improve the efficiency in a QD-based solar energy conversion system. The localized surface plasmon resonance (LSPR) of metal particles can strongly intensify the optical field in the vicinity of the metal surfaces and thus can improve the absorption of incident photons in the light-harvesting materials nearby. This plasmonic effect has been exploited to improve efficiencies in solar cells. Although plasmon-enhanced photoluminescence in QDs by adjacent gold (Au) or silver (Ag) NPs or nanostructures has been broadly reported, studies focusing on plasmonenhanced light harvesting in QDs, particularly considering their applications in solar energy conversion, have rarely been described. For most previously reported examples, plasmon enhancements of QD photoluminescence were accompanied by striking decreases in exciton lifetime. Thus, metal NP/QD coupling may reduce the exciton lifetime by increasing the rate of radiative relaxation and/or by opening up energy transfer or other nonradiative relaxation pathways. Considering the application of QDs in solar energy conversion, a reduction of exciton lifetime may diminish the benefits of plasmon-enhanced absorption. Briefly, the shorter lifetimes could decrease yields for desired charge transfer or charge separation. They also could reduce the exciton diffusion length. Ideally, a plasmonic enhancement that improves photon absorption by QDs without significantly attenuating exciton lifetimes is preferred for applications in solar energy conversion. Herein, we report the distance-engineered plasmonenhanced photon absorption in CdSe QDs by using Ag NPs Received: August 22, 2013 Accepted: October 3, 2013 Published: October 3, 2013 Letter