Large-Scale Hierarchical Self-Assembly Structures from Gold Nanoparticles
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1059-KK12-11
Large-Scale Hierarchical Self-Assembly Structures from Gold Nanoparticles Nam-Jung Kim, and Hao Li Mechanical & Aerospace Engineering, University of Missouri, columbia, MO, 65211 ABSTRACT Metallic nanoparticles may form hierarchical dendrites in the presence of ionic surfactant through self-assembly upon solvent drying at room temperature. With nanoparticle density varying in the drying area on the supporting solid substrate, the morphology and relevant size of the dendrites evolve in different structures. At the region where the nanoparticle density is high, the large dendrite can develop with manifest crystal symmetry. At the low density region, many small sizes of compact crystals can be found, indicating that particle nucleation dominates over the long-range crystal growth. SEM image reveals the ordered stacking of gold nanoplates over the long dendrite branches, resembling the liquid crystal array. We present the possible physical origins to explain the various structures of the assembled dendrites during the solvent evaporation at the interface of solid and air. INTRODUCTION Dendrite is an attractive structure exhibiting hierarchical secondary branches and arms which make the structure look like a tree with complexity in shape. It is relatively difficult to simulate the dendrite formation to get the correct and complete picture because of the intrinsic nonequilibrium process and the relevant instability [1]. Conventionally the metallic dendrite is frequently observed at the melting/freezing process of metal alloys involving high temperature gradient profile and it is well accepted that the thermal fluctuation at the front line of solidification is the key factor to cause the complex dendrite pattern [2]. Recently metallic nanoparticles have been employed to produce the various dendrite patterns at relatively mild experimental conditions [3]. Those works, however, synthesized the dendrite via the slow solution-based chemical reaction. In this paper, we demonstrate the fast formation of metallic dendrite developing directly on the solid substrate on the course of droplet evaporation. Diluted gold nanoparticles in water tend to aggregate in the presence of a small quantity of ionic surfactant as the water evaporates from a microliter drop of the nanoparticle solution on solid substrate. The aggregation among gold nanoparticles eventually results in the dendrite pattern showing the remarkable crystal symmetry on the branched structure. It is interesting to note that both six-armed (like snow-flake) and fourarmed (like holy-cross) dendrite can co-exist nearby in one sample of droplet. From the optical and electron microscope images, we identify the various sizes and shapes of dendrites and investigate the possible driving forces to control and influence the complex pattern. Nanoscale aggregate with well-maintained crystal symmetry and microscale dendrite formation may play important roles simultaneously to produce the variety of self-assembly phenomena on the fast development timescale.
EXPERIMENT G
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