Fluid-Mediated Assembly of Nanocrystalline Silicon

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CBET - 1603445PI: Hobbie, Erik K.Many electronic devices such as color displays and solar cells are produced using semiconductor nanocrystals that are composed of elements with potential toxicity. It may be possible instead to produce these and other devices using silicon nanocrystals (SiNCs). However, for SiNCs to be useful they must form structures called superlattices where the distance between the crystals is carefully controlled to yield desired material properties. SiNCs tend to pack too tightly owing to the forces between the crystals, which diminishes the performance of the resulting superlattice. The investigators will explore the possibility of using polymers to alter the packing of SiNCs, increasing the distance between crystals without making the distance so large that the unique properties of the material are lost. Experiments will be conducted to explore effects of several parameters on superlattice formation, and data will be interpreted using theoretical models and numerical simulations. The project has the potential to demonstrate that useful silicon-based superlattices can be formed, which would provide an earth-abundant, non-toxic alternative material for electronic devices. The research team will participate as mentors in NATURE, Nurturing American Tribal Undergraduate Research and Education, which engages tribal high school and college students throughout North Dakota in science and engineering projects. The first objective of the project is to form superlattices of SiNCs by evaporative drying and controlling the spacing of nanocrystals by adding polymers of various molecular weights. In evaporative assembly, SiNCs of known size are suspended in an organic solvent, and droplets of the suspension are deposited on glass and allowed to dry. During drying, nanocrystals accumulate at the edge of the droplet and the polymer may phase separate from the SiNCs. Parameters that influence the resulting structures include polymer molecular weight, processing temperature, SiNC concentration, geometry, solvent vapor pressure and shear rate. The influence of long-range order on the luminescent properties of the assemblies will then be investigated. The second objective is to develop modified equilibrium theories of the phase behavior of nanocrystal suspensions using effective interparticle potentials tested in molecular simulations of colloidal crystallization. Lattice-Boltzmann simulations will also be generated to help analyze and control the multiphase flow associated with slow evaporation.