Tunable electrical properties of polystyrene/gold core-shell structure by in situ metallization of cationic gold complex
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Hyoukryeol Choi School of Mechanical Engineering, Sungkyunkwan University, Jangan-gu, Suwon, 440-746, South Korea
Jae-Do Nama) Department of Polymer Science and Engineering, Sungkyunkwan University, Jangan-gu, Suwon, 440-746, South Korea (Received 7 April 2008; accepted 16 October 2008)
Gold-coated polystyrene (PS) beads were fabricated by an in situ metallization route involving a cationic-gold complex with a controlled amount of sulfonic acid groups formed on the PS bead surface. The interaction ratio of SO3 to [Au(phen)Cl2]+ may be estimated to be 2.4, which means that 2.4 sulfonated groups will interact with one gold cationic ligand based on geometric considerations. A modeling methodology was developed to predict the mechanical deformation, conductivity, and contact surface area of a spherical bead under compression.
I. INTRODUCTION
The controlled surface modification of polymers allows the fabrication of polymer–metal core-shell beads for various applications, such as chemical and biological sensing,1,2 multiplexed optical bar coding,3 photonic crystal,4 and microelectronic packaging technology.5 Especially in electronic device packaging, the electrical percolation of the core-shell beads through the metallic shell layer can be used for adhesive interconnection systems in high-accuracy flip-chip processing.6 The polymer-cored metal shell structure has been made by the physical deposition of metal particles on the charged polymer surfaces through an electrostatic interaction.7–9 The physical bonding of metal particles on the polymer surface has been accomplished by the modification of the polymer surface through the introduction of carboxyl, amine, hydroxyl, and sulfonate groups10 using various surface modification techniques, such as grafting copolymerization,11 UV irradiation,12 plasma treatment,10 chemical oxidation,13 and so forth. However, it should be mentioned that the physical bonding between the metal and the organic surface is not strong enough to sustain the external stresses commonly exerted in most fabrication processes, and, in particular, such attractive forces as electrostatic or hydrogen bonding can easily be broken in salt solutions.7 a)
Address all correspondence to this author. e-mail: [email protected] DOI: 10.1557/JMR.2009.0028 J. Mater. Res., Vol. 24, No. 1, Jan 2009
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It has been reported that a strong bond can be formed between the polymer surface and gold nanoparticles by the in situ chemical reaction of a gold complex in the vicinity of the charged polymer surface,14 due to the soft nature of gold and sulfur atoms.15 A similar route provided strong metal–polymer bonding in a platinum– polyelectrolyte layered composite system, which sustained the interlaminar bonding under a large amount of bending deformation.16 In this in situ direct metallization route, the countercharge interaction of the ionic metal complex and the charged polymer surface seems to ultimately determine the metal loading density and final morphology of the m
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