Substrate Dependence in the Growth of Three-Dimensional Gold Nanoparticle Superlattices
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Substrate Dependence in the Growth of Three-Dimensional Gold Nanoparticle Superlattices S. Sato, N. Yamamoto, H. Yao and K. Kimura Department of Material Science, Himeji Institute of Technology, 3-2-1 Koto, Kamigori-cho, Ako-gun, Hyogo 678-1297 JAPAN ABSTRACT Three-dimensional superlattices consisting of gold nanoparticles were grown at air/suspension or suspension/solid interfaces. The growth of superlattices was found to be strongly dependent on substrate materials: Micrometer-sized superlattices were grown at air/suspension interfaces and upon silver substrates, whereas no growth was observed on silicon, silicon oxide, or amorphous carbon substrates. To explain the observed substrate dependence, Lifshitz theory was used to calculate the Hamaker constants between gold nanoparticle assemblies and substrates through the suspension. Van der Waals interactions estimated from this calculation fully explain the experimental results.
INTRODUCTION Three-dimensional (3D) superlattices of nanoparticles represent a class of promising new electronic materials whose band structures may be engineered through control of core sizes, of surface coverage thicknesses, and of packing arrangements for the nanoparticles. Recently, high-quality superlattices have been synthesized using the self-assembly process of nanoparticles in liquid phases [1-4]. For superlattices to be introduced into industrial applications, they need to be grown at gas/solution or solution/solid interfaces. However, there have been no reports focusing on superlattice growth at various interfaces. Recently, we have found that gold (Au) nanoparticles, whose surfaces are modified with hydrophilic surfactants, are good components for 3D superlattices since the assembly rate of Au nanoparticles (i.e., the growth rate of superlattices) is widely tunable through pH control [5,6]. By enforcing an extremely slow growth rate (i.e., equilibrium growth), Au nanoparticles self-assemble into high-quality lattice arrangements. In this communication, we report substrate dependence in the growth of Au nanoparticle superlattices in aqueous suspensions.
EXPERIMENTAL
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The preparation of Au nanoparticles is outlined as follows. 4.1 ml of an aqueous solution of 0.12 M hydrogen tetrachloroaurate tetrahydrate (HAu(III)Cl4 4H2O) was mixed with 100 ml of methanol containing 1.5 mM of mercaptosuccinic acid (MSA). 25 ml of an aqueous solution of 0.2 M sodium borohydride (NaBH4) was then added under vigorous stirring. After the reaction, Au nanocrystals were obtained whose surfaces were modified with MSA. The solvent was decanted after centrifugation at 9840 × g, which corresponds to 10,000 rpm for the Kubota 1720 centrifuge. Samples were then washed twice with a 20 % (v/v) water-methanol solution by repeating re-suspension with a sonicator and re-centrifugation, and finally dialyzed to remove inorganic (Na, Cl, and B) and organic impurities. The surfactant affects the dispersion of Au nanoparticles in aqueous suspensions at low ion concentrations. After the Au nanoparticles were dispe
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