Ahmed, N.; (2010) Computer simulation of dipole-dipole interactions towards understanding nanostructure formation. Doctoral thesis , UCL (University College London).

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Abstract

Chalcogenide nanocrystals, synthesised in solution, have the ability to form larger nanostructures through the association of the nanocrystals. There are a variety of structures that form which range from simple chains to more complex tripod, tetrapod and star morphologies. The association of the nanocrystals has been hypothesised to be the result of dipole-dipole interactions between nanocrystals. This results partially from the observation of the so called “pearl-necklace” type of structures visible in synthesis preceding the formation of chain structures. The Stockmayer fluid potential, comprising a Lennard-Jones potential with additional dipole-dipole interactions, is employed to model computationally the nanocrystals in a binary mixture, where the sizes of the particles can differ. Monte Carlo simulations are performed at a range of reduced densities and various size ratios. A large range of size ratios are examined, reflecting the size distribution of nanocrystals present in synthesis. There are a number of simple models that can describe the formation of simple chain or ring nanostructures. A significant step forward in the understanding of nanoparticle self-assembly is to model the formation of the more complex tetrapod structures. In this context a modified Stockmayer fluid model is developed in which a single nanocrystal is represented by four off-centre Stockmayer fluid particles. The Stockmayer fluid potential highlights that the formation of linear chain structures is in competition with triangular unit structures, with the energy of the triangular units becoming more favourable with increasing size ratio. The modified Stockmayer fluid model is able to produce good quality tetrapod structures over a narrow range of size ratios. There is also greater formation of tripod structures observed by using a novel analysis technique.

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