Flexible design of hybrid polymer nanoassemblies consisting of nonlinear optical (NLO) polymer nanosheets and gold nanoparticle alignment was done to elucidate near-field effects of localized surface plasmon (LSP) coupling, which was generated from coupled gold nanoparticles, on enhanced second harmonic generation (SHG) from nonlinear optical (NLO) dyes in hybrid nanoassemblies. Structurally well-defined hybrid polymer nanoassemblies comprising NLO polymer nanosheets and aligned gold nanoparticles were fabricated using bottom-up approaches: Langmuir-Blodgett (LB) technique and nanoparticle adsorption. Two hybrid polymer nanoassembled structures were particularly examined: a single-layer NLO polymer nanosheet and gold nanoparticle monolayer (single-layer structure) exhibiting intralayer LSP coupling, and a single-layer NLO polymer nanosheet sandwiched between two-layer gold nanoparticle monolayers (sandwich structure). The latter enables interlayer LSP coupling between the two gold nanoparticle monolayers. Dependence of SHG intensity on the distance between the NLO layer and nanoparticle layer was examined according to the LB layer structure and gold nanoparticle size variation. The SH light intensity from the NLO polymer nanosheet decreased almost exponentially with increasing spacer distance between the NLO polymer nanosheet and gold nanoparticle monolayer in both single-layer and sandwich structures. The decay length depends strongly on the gold nanoparticle size, indicating effective spatial distance for enhanced SHG from NLO polymer nanosheets. Theoretical calculations were used to study the enhancement mechanism. Finite difference time domain (FDTD) calculations reproduced the exponential behavior of SH light intensity as a function of separation distance, which confirmed the importance of coupled gold nanoparticle formation and parallel geometry of near-field coupling of the coupled gold nanoparticles with NLO polymer nanosheets for efficient SHG enhancement. Dipole-type LSP coupling along the long axis of adjacent gold nanoparticles at the fundamental frequency dominates enhancement of SHG from NLO dyes oriented parallel to the long axis of LSP coupling, which occurs at the center of the Au NPs.

Effective utilization of coupled surface plasmon resonance from gold nanoparticles was demonstrated experimentally for optoelectronic applications based on second-order nonlinear optics. Hybrid polymer nanoassemblies were constructed by manipulating gold nanoparticle arrays with nonlinear optical active polymer nanosheets to investigate the second harmonic generation. The gold nanoparticle arrays were assembled on heterodeposited polymer nanosheets. The second harmonic light intensity was enhanced by a factor of 8. The observed enhancement was attributed to coupling of surface plasmons between two adjacent gold nanoparticles, thereby enhancing the surface electromagnetic field around the nanoparticles at the fundamental light wavelength (1064nm).

ABSTRACT:The paper describes fabrication of hybrid polymer nanoassemblies possessing polymer nanosheets and metal nanoparticle arrays. Based on a bottom-up approach with ultrathin polymer Langmuir-Blodgett films (polymer nanosheets) and metal nanoparticles, we constructed hybrid nanoassemblies for opto/electric nanodevice applications: nanoscale positioning of metal nanoparticles, free-standing hybrid polymer nanosheets, hybrid polymer nanoassemblies for luminescence sensor application, and enhancement of nonlinear optical (NLO) efficiency via surface plasmon coupling. Assembling nanomaterials such as nanoparticles and organic functional molecules through a bottom-up approach makes it possible to strengthen free-standing nanofilm stability, enhance luminescence of luminescent molecules, and enhance second harmonic light from NLO molecules under the influence of localized surface plasmons. The high potential of hybrid polymer nanoassemblies for precise nanoscale design is also demonstrated.[doi:10.1295/polymj.PJ2006235] KEY WORDS Polymer / LB Film / Metal Nanoparticle / Surface Plasmon / Hybrid / Design of nanostructures using polymer materials has been widely explored and has now become prevalent. Bottom-up approaches such as layer-by-layer (LbL) adsorption, 1 surface-initiated polymerization exemplified by atom transfer radical polymerization (ATRP), 2 and Langmuir-Blodgett (LB) technique [3][4][5][6][7] make it possible to develop myriad nanoarchitectures on solid substrates. These approaches and their combinations offer a promising route to polymer nanoarchitectures for a wide variety of functional materials and surfaces. Using these nanostructures, for instance, the surface properties can be controlled uniformly, even on a nanoscale. These compelling qualities of ultrathin polymer films have been expanded to broad scientific fields. It is important to intensify the interdisciplinary interactions, for example, between polymer materials and inorganic nanoparticles in basic and application-oriented work.We have investigated functionalization of polymer LB films consisting of poly(alkylacrylamide)s. Poly-(N-dodecylacrylamide) (pDDA) proved to be excellent for LB assembly; the material provides a highly oriented and densely packed monolayer at the airwater interface. 8,9 Varying the side chain length, the film thickness is tuned at a nanometer scale (1-2 nm in length). Widely various functional molecules were incorporated and distributed uniformly in a two-dimensional (2D) field. The key factor is the existence of a 2D hydrogen-bonding network between polymer backbones, which enhances the monolayer stability at the air-water interface, leading to quantitative deposition onto solid substrates via vertical dipping. We recently designated polymer LB films consisting of alkyl acrylamide polymer as polymer nanosheets. Using these advantages, directional (vectorial) electron transfer between several redox components was achieved in multilayered polymer nanosheets, 10,11 extending to logic gate operations.12, 13 We have...

We report enhanced second harmonic generation from nanostructured multilayers in combination with nonlinear optical polymer nanosheets and gold nanoparticles.

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