Abstract
Near-field plasmonic coupling and local field enhancement in metal nanoarchitectures, such as arrangements of nanoparticle clusters, have application in many technologies from medical diagnostics, solar cells, to sensors. Although nanoparticle-based cluster assemblies have exhibited signal enhancements in surface-enhanced Raman scattering (SERS) sensors, it is challenging to achieve high reproducibility in SERS response using low-cost fabrication methods. Here an innovative method is developed for fabricating self-organized clusters of metal nanoparticles on diblock copolymer thin films as SERS-active structures. Monodisperse, colloidal gold nanoparticles are attached via a crosslinking reaction on self-organized chemically functionalized poly(methyl methacrylate) domains on polystyrene-block-poly(methyl methacrylate) templates. Thereby nanoparticle clusters with sub-10-nanometer interparticle spacing are achieved. Varying the molar concentration of functional chemical groups and crosslinking agent during the assembly process is found to affect the agglomeration of Au nanoparticles into clusters. Samples with a high surface coverage of nanoparticle cluster assemblies yield relative enhancement factors on the order of 10⁹ while simultaneously producing uniform signal enhancements in point-to-point measurements across each sample. High enhancement factors are associated with the narrow gap between nanoparticles assembled in clusters in full-wave electromagnetic simulations. Reusability for small-molecule detection is also demonstrated. Thus it is shown that the combination of high signal enhancement and reproducibility is achievable using a completely non-lithographic fabrication process, thereby producing SERS substrates having high performance at low cost.
Highlights
Planar assemblies of nanometer-spaced metal nanoparticles allow scientists and engineers to harness electromagnetic fields for imaging, photonics, and medicine at the nm-size scale
Imaging at resolutions well below the diffraction limit is possible using 2D arrays of metal nanoparticles,[1] while photovoltaic devices have exhibited an increase in efficiency due to optical field enhancements associated with excitation of the surface plasmon resonance of metal nanoparticle arrays on the surface.[2]
Selective placement of Au nanoparticles on diblock copolymer templates is performed by selective chemical functionalization of poly(methyl methacrylate) (PMMA) domains on PS-b-PMMA
Summary
Planar assemblies of nanometer-spaced metal nanoparticles allow scientists and engineers to harness electromagnetic fields for imaging, photonics, and medicine at the nm-size scale. Au-coated polymer nanopillars fabricated using nanoimprint lithography from electron beam lithography-fabricated silicon molds were used to create high enhancement factors when exposure to solvent caused the pillars to coalesce into pentagons with narrow gap spacings.[5] Facsimiles of Au nanoparticle aggregate-like clusters, prepared with conventional lithographic techniques exhibited 1–5 × 108 signal enhancement.[6] The development of low-cost non-lithographic methods for designing planar metal nanoarchitectures with nanometer scale interparticle spacing allows for greater utilization of the near-field properties in metal nanostructures for device applications. This report presents how metal nanoparticle arrays can be varied by simple changes in the chemical design parameters to optimize nanoparticle spacing and cluster arrangements
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