Novel Phases In Polyhedral Nanoparticles: Mixtures And Spatial Confinement

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Colloids present an interesting experimental system to study fundamental scientific problems as well as to tackle technological challenges, through novel material design. Several control parameters like size, shape, inter-particle interactions, assembly-geometry (including dimensionality) and external fields (among many others) can result in a great variety in morphologies and material properties. In particular, polyhedral nanoparticles are potentially powerful candidates with a rich phase behavior and availability of robust experimental methods for their synthesis. Our aim in this thesis is to understand, through computer simulations, various aspects of polyhedra phase behavior. In particular, we study a specific case of binary mixtures of polyhedra called binary tessellating mixtures in Chapter 2. The motivation here is to study if these superstructures are generated from the geometrical condition of spacefilling i.e. tessellation, without the use of any enthalpic interactions (which tend to be harder to control in experiments). As we see in Chapter 2, pure entropic self-assembly of these mixtures fails to reach tessellated phase due to kinetic barriers, which can be alleviated by small targeted enthalpic interactions. We further explore the wider problem of self-assembly of binary polyhedral mixtures in Chapter 3 to understand the generic predictive rules that can help guide experimental efforts. We find that the mixture miscibility (a critical criterion for novel superstructures) is strongly determined, among other factors, by the difference between order-disorder transition pressure ([INCREMENT]ODP) of the individual polyhedra in the mixture. We also propose a general qualitative roadmap for the mixture phase behavior. In chapter 4, using the guiding rules discovered while studying mixtures, in combination with novel plastic crystalline 'mesophases' exhibited by a subset of polyhedra, we develop a design scheme that allows for the formation of ordered mixtures without introducing any enthalpic interactions. These so-called Mixed Rotator mesophases (MRMs) form purely from an entropic self-assembly and are stable for a large range of volume fractions. Apart from shape bi-dispersity (mixtures), we also investigate the effect of geometrical confinement on polyhedral self-assembly in Chapter 5 and show that a parallel-plate confinement leads to many novel phases that are not seen in bulk, through the case of four representative polyhedra from the truncated cube family. We conclude with a summary of our findings and a discussion of currently prevalent research directions.

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Self-assembly, colloids,confinement; Monte Carlo simulations; polyhedral nanoparticles, mixtures


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Ph. D., Physics

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Doctor of Philosophy

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dissertation or thesis

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