Self-organization of nanoscopic building blocks into ordered assemblies
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Self-organization of nanoscopic building blocks into ordered assemblies Mark A. Horsch1, Christopher R. Iacovella1, Zhenli Zhang1 and Sharon C. Glotzer1,2,i 1 Department of Chemical Engineering and 2Department of Materials Science & Engineering, University of Michigan, Ann Arbor, Michigan 48109-2136 Abstract We studied the self-assembly of nanoscopic building blocks comprised of polymer-tethered nanoparticles using computer simulation and predict that these building blocks can assemble into mono- and multi-layer sheets and shells. The simulations further demonstrate that for some nanoparticle geometries and tethered nanoparticle topologies, ideas from block copolymers, surfactants and liquid crystals can be used to predict the ordered morphologies attained via selfassembly and that for specific cases the morphologies are consistent with Israelachvili packing rules. Introduction Many nanoscopic building blocks have been synthesized including nanospheres[1], nanorods[2], nanocubes[3], nanoplates[4, 5], nanoprisms[6], etc., and it is anticipated that selfassembly may provide a promising means for manipulating these building blocks into functional and useful materials. One increasingly popular method for self-assembly involves functionalizing nanoparticles and nanostructured molecules with “tethers” of organic polymers[7, 8] or biomolecules[9, 10] with specific or nonspecific interactions to facilitate their directed assembly. We propose that limiting the tethers to a countable number and attaching them to specific locations on nanoparticles or nanostructured molecules should lead to predictable and tunable control over the types of structures that can assemble when the tethers and nanoparticles are immiscible[11]. Schematic examples of tethered nanospheres and nanorods arranged according to geometry are illustrated in Fig. 1. Figure 1. Examples of tethered nano building blocks. Top to bottom: tethered spheres and tethered rods.
In a broad sense, tethered nano building blocks can behave like amphiphiles and share features in common with block copolymers and liquid crystals; in the case of the tethered nano building blocks the thermodynamic immiscibility is coupled with liquid crystal ordering. We therefore expect the appearance of ordered microphases to be governed by similar physics, whereby two or more incompatible species seek to minimize their free energy by aggregating with species of their own kind, subject to the topological constraint of being covalently linked to the incompatible species[12]. We also expect that the nano building blocks have features in common with liquid crystals where their rigidity and geometry influences the packing and can result in complex ordering. To investigate self-assembly of tethered nano building blocks we performed molecular simulations via Brownian dynamics[13] of “minimal” models to study how thermodynamics and architecture can be manipulated to achieve certain target structures[11, 14]. Details of the model and simulation method may be found in Ref. [11]. Results We u
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