Self-assembly of polymer-grafted anisotropic nanoparticles
While anisotropic nanoparticles provide unique building blocks for self-assembling useful nanodevices and nanomaterials ranging from plasmonic sensors to chiral metamaterials, controlling their self-assembly process to achieve targeted structure remains challenging. Recently, surface functionalization of nanoparticles with polymer grafts was shown to be a powerful strategy for tuning the orientation-dependent interactions of the nanoparticles. This technique allows modulation of the interaction between nanoparticles as grafted polymers can provide both repulsive interactions arising from their steric hindrance as well as attractive interactions due to their adsorption to the particle surfaces. Utilizing this approach, experiments have successfully assembled nanoparticles into large structures with highly uniform interparticle orientations. However, many challenges remain in fabricating desired nanostructures with the polymer-grafted anisotropic nanoparticles. First, much of the underlying physics governing assembly of such nanoparticles is not well understood and is difficult to discern using experimental techniques due to the nanoscopic nature of the self-assembly process. Second, the relevant parameter space that affects the particle assembly is vast and investigation of such large parameter space is costly in terms of both time and expenses. Third, computationally investigating the behavior of anisotropic nanoparticles is difficult as calculation of their interaction energies is computationally expensive due to the lack of analytical expressions for these energies.In this dissertation, I tackle these challenges in self-assembly of anisotropic nanoparticles through computational modeling, focusing specifically on polymer-grafted nanocubes and DNA-grafted nanorods. For both systems, computational methods and analytical models for efficiently calculating the interaction energies between the anisotropic nanoparticles are first developed. Using such methods as well as advanced Monte Carlo simulations and atomistic calculations, free-energy landscapes describing the assembly of these anisotropic nanoparticles are obtained. Analysis of the free-energy landscapes demonstrates that understanding the interplay between the different interaction components of the systems as well as their dependencies on the relative configurations of the assembled particles is crucial. Specifically for the nanocubes, the competition between the attractive interactions between the inorganic particle cores lead to face-face type of configurations while the repulsive interactions due to the polymer corona induce edge-edge configurations. For the DNA-grafted nanorods, the competition between attractive and repulsive interactions interplay with the chirality of the bridging DNA to induce chiral assembly of the nanorods. Based on these results, material design rules for assembling both the nanocubes and the nanorods into desired configurations are suggested. These results were not only in agreement with many previous experimental studies but also provided the underlying mechanism that explain such assembly behaviors. In summary, the results presented in this dissertation should both aid in fabrication of nanodevices with precisely controlled particle assemblies as well as provide efficient computational methods for future investigation of anisotropic nanoparticles.
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