Template directed self-assembly of particle monolayers




Zhu, Xilan

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The template directed self-assembly (TDSA) is a promising method for cost-efficient and robust fabrication of nanoscale features. In this dissertation, we study the TDSA of particle monolayers to assess the possibility of its application in manufacturing. The 2D directed self-assembly of micron-sized hard spherical particles within square confinements is studied theoretically and experimentally. Grand Canonical Monte Carlo simulations are used to predict the equilibrium packing structures of particles in square wells of specific dimensions. Spin coating and mechanical rubbing processes are used to direct the self-assembly of a monolayer of particles into the square wells. The precision of the graphoepitaxy decreases as template size increases. Both simulations and experiments show that ordered graphoepitaxy of square lattices vanishes with square templates of side length greater than five times the particle diameter. Missing particles and polydispersity of particles are two main causes disrupting the square packing structure in cells of three to five particle diameter side length. The phase diagram of a monolayer of soft particles described by the Daoud–Cotton model for star polymers is presented. Ground state calculations and grand canonical Monte Carlo simulations are used to determine the phase behavior as a function of the number of arms on the star and the areal coverage of the soft particles. The phase diagram exhibits rich behavior including reentrant melting and freezing and solid–solid transitions with triangular, stripe, honeycomb and kagome phases. These structures in 2D are analogous to the structures observed in 3D. The evolution of the structure factor with density is qualitatively similar to that measured in experiments for polymer grafted nanocrystals [Chen et al., Macromolecules, 2017, 50, 9636]. The formation of particle chains with long range order and orientational control is studied computationally. Monolayers of spherical particle usually self-assemble into isotropic structures. It has been found that some core-shell particles undergo anisotropic compression at elevated density when the particle interaction is piecewise. We expected these particles to self-assemble into aligned particle chains with controlled orientation under the guidance of some parallel confinements. Grand Canonical Monte Carlo simulations are used to predict the equilibrium packing structures of particle monolayers with parallel guidelines. Two different core-shell particles are studied. A graphoepitaxy behavior is observed and the formation of ordered particle chains is found for both particles at certain packing density with widely spaced templates. The orientation of formed particle chains can be controlled with the rigid template and the particle spacing is predicted. The self-assembled particle monolayer is an important alternative for photolithography in nanofabrication. The intolerable high defect rate has been the main challenge before the process can be used in the fabrication of high precision structures. The soft repulsive interaction is introduced for the quality enhancement of square packing structures formed from the template directed self-assembly of particle monolayers. An obvious reduction on the defect rate is observed with the star-polymer-like particles. Also, the soft interaction brings more flexibility in the template design for target structures and the tuning of particle spacing is possible.


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