Preparation and Optical Properties of Au-shell Submicron Polystyrene Particles
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Preparation and Optical Properties of Au-shell Submicron Polystyrene Particles Tianhao Ji1,2, Yair Avny2 and Dan Davidov1 Racah Institute of Physics, 2Department of Organic Chemistry, The Hebrew University of Jerusalem, Jerusalem 91904, Israel 1
INTRODUCTION Recently, there has been an immense interest in the fabrication of composite nanoparticles, such as core-shell structural materials [1-6]. These core-shell composite particles may show special properties different from those of the core or shell particles. Thus, Au or Ag nanoparticcles were coated by silica. The silica shell not only stabilizes the colloidal Au nanoparticles, but also influences their optical property [3,6]. Furthermore, Halas et al. [5] have demonstrated that by varying the ratio of gold-shell to silica-core, the plasmon optical resonance of the Au shell and silica core strongly shifts across the visible spectrum and into the infrared region. Core-shell particles can be used as building blocks for different devices including tunable Photonic-Band-Gap (PBG) structures. Herein, we report on the fabrication of such particles with emphasis on the preparation of the Au nano-shells coated polystyrene (PS) with a diameter of about 700 nm.
Preparation and Characterization Several methods to prepare core-shell structural material have been suggested including surface precipitation reaction [3], surface reaction [7], self-assembly [4], and surface seeding [8]. In the present paper we combine the self-assembly and the surface seeding methods to prepare Au shell on PS latex beads. Reagent-grade polyethylenimine (PEI), HAuCl4.3H2O, NH2OH and trisodium citrate dihydrate were purchased from Aldrich. All chemicals were used as received without further purification. The colloidal Au nanoparticles were synthesized according to a wellknown procedure [9] by the reaction between HAuCl4.3H2O and trisodium citrate dihydrate. The size of the Au particles was 3 nm or 15 nm. The procedure of preparation of the core-shell particles is described in scheme 1. First, the positively charged polyethylenimine (PEI) polyelectrolyte is adsorbed onto the surface of the (negatively charged) PS microspheres by both electrostatic and hydrogenbonded attraction (step1). This is followed by adsorption of the negatively charged Au nanoparticles into the PEI polymers via the amine groups (step2). We found, however, that the 3nm Au particles on the coated PS surfaces are more disperse and tend to cluster less than the 15nm colloidal particles. In the next step (step3), the coverage of the Au coating on the PEI surface is increased by a gold seeding procedure involving NH2OH and HAuCl4 reagents. Similar seeding procedure has been exploited in the past to obtain Au-shell silica nanoparticles by the reaction of chloroauric acid and sodium borohydride D9.56.1
on the surface of gold nanoparticles [8]. However, there is evidence that the use of the NH2OH and HAuCl4 reagents in scheme 1 increases Au coverage on glass substrate [10]. The seeding step is repeated to yield higher coverage of go
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