Optical Properties of Nanoparticle Pair Structures
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Optical Properties of Nanoparticle Pair Structures Marie L. Sandrock, Mahnaz El-Kouedi, Maryann Gluodenis and Colby A. Foss, Jr.1 Department of Chemistry, Georgetown University, Washington, District of Columbia 20057. 1 Current address: Trex Enterprises Corporation, 3038 Aukele Street, Lihue, Hawaii, 96766. ABSTRACT The synthesis of nanoparticle pair structures via porous host electrochemical template synthesis reviewed. Electrochemical template synthesis offers two advantages over solution methods, namely: 1) control over particle pair structure and orientation; and 2) control over geometry, size and composition of each member of the particle pair. These features of electrochemical template synthesis allow for straightforward comparison of experimental and theoretical spectra. Orientation control allows for the evaluation of second order nonlinear optical properties of centrosymmetric and non-centrosymmetric nanoparticle pair systems. The dependence of Second Harmonic Generation intensity on particle pair shape size and orientation is discussed. The synthesis and linear spectra of metal-semiconductor nanoparticle pair structures are also discussed, with emphasis on interparticle physical and electromagnetic interactions. INTRODUCTION There are numerous methods for synthesizing nanoscopic metal particles, such as bulk solution reduction of metal salts [1,2], electric discharge [3], vacuum deposition [4], and template synthesis [5,6]. Template synthesis involves the use of a host material whose pores define the geometry of the structures grown within it, for example via electrochemical deposition of metals. In the last decade, there has been an increasing interest in the optical properties of nanoscopic metal particle structures, with a keen emphasis on how optical properties depend on particle size, shape, composition, and orientation in the incident field. Electrochemical template synthesis is well-suited for fundamental optical studies on nanoparticle structures because it allows for the control of particle size, shape and orientation. A common porous host material is anodic aluminum oxide, which possesses not only well-defined parallel arrays of cylindrical pores, but is also transparent through most of the visible and near-infrared spectrum. A number of previous studies have focused on the optical properties of metal spheres and rods grown in porous aluminum oxide [5-9]. In our laboratories, we have extended the electrochemical template synthesis method to metal nanoparticle pair structures, where both members of the pair are spheres or rods of equivalent dimensions, or where the pair structure is non-centrosymmetric (e.g., composed of a sphere and a rod, or two rods of different dimensions). We have been able to examine the linear spectra as a function of particle shape and interparticle spacing [10], and compare the second harmonic generation (SHG) signals produced by centrosymmetric and non-centrosymmetric structures [11]. We have also extended the template synthesis method to pair structures where one member
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