Large Enhancement of Fluorescence Efficiency from CdSe/ZnS Quantum Dots Induced by Resonant Coupling to Spatially Controlled Surface Plasmons Jung-Hoon Song, Tolga Atay, Sufei Shi, Hayato Urabe, and Arto V. Nurmikko* DiVision of Engineering and Department of Physics, Brown UniVersity, ProVidence, Rhode Island 02912 Received May 2, 2005; Revised Manuscript Received June 22, 2005 ABSTRACT Nanoengineered fluorescent response is reported from semiconductor core-shell (CdSe/ZnS) quantum dots in proximity to the surface plasmon polariton field of periodic Ag nanoparticle arrays. Tuning the surface plasmon polariton resonance to the quantum dot exciton emission band results in an enhancement of up to ∼50-fold in the overall fluorescence efficiency, in a design where each Ag nanoparticle is interconnected by a continuous Ag thin film. Propagating modes of surface plasmon resonances have a direct impact on the fluorescence enhancement. The presence of noble metal surfaces can significantly impact the manner in which incident photoexcitation is converted into fluorescence emission from semiconductor nanoparticles. The example of quantum dots (QDs) located within the surface plasmon polariton (SPP) field of the metal is a case in point. 1-5 Excitation of SPPs within metal nanoparticles or on roughened surfaces can create strong local optical fields. Surface-enhanced Raman scattering (SERS), in par- ticular, exploits such large local fields, e.g., in the study of molecules adsorbed on metal nanoparticles 6 or on rough surfaces of Au, Ag, and Cu. 7,8 However, in contrast to the inelastic (instantaneous/coherent) Raman process, the details of the role that SPPs play in photoexcited (incoherent) fluorescence involving surface plasmons and semiconductor nanocrystals is still under debate. The added complexity, in this case, arises from the competition and energy exchange by photon tunneling between plasmons and electronic excitations within the semiconductor QDs and their respec- tive coupling to incoming and outgoing free space photons (which jointly determine the external quantum efficiency). The interplay is expected to depend strongly on the size and shape of the metal and semiconductor nanoparticles, the interparticle distance, concentration, and the spectral relation- ship between the native fluorescence emission of the semiconductor QD and the extinction spectrum of the metal nanoparticles. In fact, experimental reports from many different particle arrangements 1-5,9-13 (some spatially con- trolled, others not) vary widely in terms of reported fluorescence enhancement or quenching and are not always accompanied by fully transparent physical arguments. The relative contribution of absorption and radiative scattering to the overall extinction spectrum of a metal nanostructure is a crucial factor that dictates the conversion efficiency of incident photons to fluorescence emission from the QD/nanoparticle system. If the fluorescent material is in direct physical contact with the metal particles, then quench- ing of the spontaneous emission from the semiconductor nanoparticle is dominant due to nonradiative energy transfer to the metal. 14 Broadly speaking, metal nanostructures whose extinction is dominated by plasmonic absorption tend toward quenching of the QD fluorescence whereas those whose extinction is dominated by scattering can enhance the overall external quantum efficiency. 9 In the case of organic dyes, small (<20 nm) metal colloids are usually efficient fluores- cence quenchers. 10 On the other hand, the use of high spatial resolution lithographic techniques for patterning periodic structures in thin metal films offers more flexibility in spectrally tuning the SPP resonances and controlling the ratio of scattering to absorption cross sections. 15 The spatial profile of the SPP field can be determined a priori, at least semiquantitatively, including the possibility of coupling local SPP modes to propagating modes of the underlying substrate waveguide structure. 16,17 In this paper, we report on engineered nanostructures where colloidal semiconductor QDs are selectively integrated within designed plasmonically active templates with a particular approach to the overall material architecture. On one hand, we have strived for structural control of the spatial relationship between the QDs and metallic nanoparticles to enable the study of local electromagnetic interactions in the NANO LETTERS 2005 Vol. 5, No. 8 1557-1561 10.1021/nl050813r CCC: $30.25 © 2005 American Chemical Society Published on Web 06/30/2005