Tuning the Optical Properties of Large Gold Nanoparticle Arrays
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Tuning the Optical Properties of Large Gold Nanoparticle Arrays Beomseok Kim, Steven L. Tripp, and Alexander Wei ([email protected]) Department of Chemistry, Purdue University, West Lafayette, IN 47907-1393 ABSTRACT Gold nanoparticles in the mid-nanometer size regime can undergo self-organization into densely packed monoparticulate films at the air-water interface under appropriate passivation conditions. Films could be transferred onto hydrophilic Formvar-coated Cu grids by horizontal (LangmuirSchaefer) deposition or by vertical retraction of immersed substrates. The latter method produced monoparticulate films with variable extinction and reflectance properties. Transmission electron microscopy revealed hexagonally close-packed arrays on the micron length scale. The extinction bands of these arrays shifted by hundreds of nanometers to near-infrared wavelengths and broadened enormously with increasing periodicity. Large particle arrays also demonstrated extremely high surface-enhanced Raman scattering (SERS), with enhancement factors greater than 107. Signal enhancements could be correlated with increasing periodicity and are in accord with earlier theoretical and experimental investigations involving nanoparticle aggregate structures. INTRODUCTION The synthesis of nanostructured materials with useful and tunable properties is central to developments in nanoscale science and technology. Nonlithographic bottom-up approaches based on self-assembly and self-organization are especially appealing because of their intrinsically low overhead for large-scale production. This approach has been useful in the selforganization of monolayer-protected metal nanoparticles into periodic two-dimensional (2D) arrays, with many of these assemblies demonstrating novel optical or electronic properties as a function of particle size or interparticle spacing.1 Interestingly, numerous examples of 2D arrays comprised of small (15 nm) gold nanoparticles to selforganize into well-ordered 2D arrays at the air-water interface. The choice of surfactant is critical in the formation and physical properties of these nanoparticle ensembles: the surfactant layer is required to be hydrophobic and highly repulsive at close range but thin enough to maintain short interparticle separations, a crucial factor in the electronic and optical properties of metal nanoparticle assemblies.1,6 Simple extension of established surfactant methodologies does not provide adequate control during particle aggregation; for example, passivating 20-nm colloidal gold particles with a surfactant monolayer of dodecanethiol results in the formation of multilayered aggregates at the air-water interface (see Figure 1a). We therefore sought to design a surfactant monolayer with greater spacing between chains, with the premise that their relatively high conformational entropies (greater exclusion volumes) would increase the barrier against steric compression.7 Y6.1.1
surfactant layer structure
20-nm colloidal Au at air-water interface
a dodecanethiol
S
S
S
S
S
S 50 nm
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