Controlled Growth of Gold Nanorod Arrays from Polyethylenimine-coated Alumina Templates
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Controlled Growth of Gold Nanorod Arrays from Polyethylenimine-coated Alumina Templates Jeong-Mi Moon and Alexander Wei ([email protected]) Department of Chemistry, Purdue University, West Lafayette, IN 47907-1393 ABSTRACT Au nanorod arrays were grown by electrodeposition in Au-backed nanoporous alumina templates modified with polyethylenimine (PEI) as an adhesion layer. By varying the concentration and molecular weight of PEI, the length of nanorod arrays could be finely controlled. The local length distribution was extremely narrow with relative standard deviations on the order of 2% for rod lengths from 700 nm to 17 microns. The uniform growth rate appears to be determined by the adsorbed PEI matrix, which controls the growth kinetics of the grains comprising the nanorods. Templates coated with poly(acrylic acid) did not impart fine control in nanorod growth. The nanorods could also be thermally annealed within the template and released as monodisperse particles of uniform size. INTRODUCTION Periodic arrays of plasmonic nanostructures are candidates for the development of nanophotonic materials and devices, such as photonic band gaps [1,2] and near-field optical waveguides [2-5]. Also plasmonic nanostructures arranged in close proximity to one another often experience a strong electromagnetic coupling and become capable of generating intense but highly localized electromagnetic fields, which are desirable for enhancing a variety of nonlinear optical effects and spectroscopic sensing modalities such as surface-enhanced Raman scattering (SERS) [6-8]. Designing metal nanorod arrays for applications requires a suitable fabrication method, one which enables control over particle size, aspect ratio, and interparticle spacing [9,10]. Top-down methods such as electron-beam lithography impart excellent spatial control, but at present these are limited in sample throughput and cannot easily provide particle separations of less than 25 nm [2-5]. Self-assembly has been used successfully to organize spherical nanoparticles into close-packed 2D and 3D arrays [11-15], but organizing anisotropic particles such as nanorods into axially oriented 2D arrays is a far more challenging problem [16-20]. Size is another important factor: plasmonic responses scale roughly with particle volume but are also limited by size-dependent retardation and damping effects, so the optimal enhancements are most likely to be produced by arrays of nanorods having diameters between 40 and 100 nm, depending on the wavelength of interest [9,10]. We have recently demonstrated that nanoporous anodized aluminum oxide (AAO) membranes coated with high molecular weight polyethylenimine (PEI; Mw = 750,000) can provide significant benefits for the controlled growth of monolithic 2D arrays of Au nanorods via templated electrodeposition [21]. Here we investigate the controlled electrochemical growth of Au nanorods in AAO templates coated with PEI of different molecular weights. The electrodeposition of metal in nanoporous AAO templates is an establi
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