Direct Templating of Semiconductor Nanostructures With Lyotropic Liquid Crystals
Braun, Paul Vannest
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https://hdl.handle.net/2142/82910
Description
Title
Direct Templating of Semiconductor Nanostructures With Lyotropic Liquid Crystals
Author(s)
Braun, Paul Vannest
Issue Date
1998
Doctoral Committee Chair(s)
Stupp, Samuel I.
Department of Study
Materials Science and Engineering
Discipline
Materials Science and Engineering
Degree Granting Institution
University of Illinois at Urbana-Champaign
Degree Name
Ph.D.
Degree Level
Dissertation
Keyword(s)
Chemistry, Polymer
Language
eng
Abstract
Material growth in lyotropic liquid crystals has proven to be a very powerful methodology for the creation of nanoscopic features in mineral phases. Two distinct methodologies were studied---bulk templating and thin film templating. Bulk templating yields a powder of material containing the expected nanoscale features and thin film templating, a thin film with a defined nanostructure. In both systems, the structure and symmetry of the features in the resultant material are an exact match with the characteristic dimension of the precursor liquid crystal. Various nanoscale semiconducting superlattices were generated by direct templating with lyotropic liquid crystals including superlattices of CdS, CdSe, ZnS, and CdTe. The semiconductor growth process copied the symmetry and characteristic dimensions of the original hexagonal mesophase by avoiding growth of mineral within regularly spaced hydrophobic regions. The final product was a superlattice structure in which a mineral continuum was featured with hexagonally arranged cylindrical pores 2--3 nanometers in diameter, 5 nanometers apart. As required for direct templating, we found that both the features in the nanostructured solids, and their dimension can be directly controlled through the mesophases structure and water content. Several other metal sulfides such as PbS, Ag2S. CuS and HgS were produced only as nonfeatured solids using identical synthetic methodologies. The sensitivity of the methodologies to the chemistry of precursors has allowed us to develop a model that takes into account both the nature of the liquid crystal and the chemistry of the final mineral product to explain the mechanism and conditions necessary for direct templating of nanoscale features. Semiconductor growth in a cubic liquid crystal (consisting of spherical micelles) yielded a hollow, 20--100 nm in diameter spherical product. The diameter of this product is 1--5 times the diameter of the micelles making up the cubic liquid crystal, so it does not appear the hollow spheres are a directly templated product. Instead, some sort of a co-assembly process must be operating here.
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