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Title:Photonic Properties of Self -Assembled Organic Materials: Supramolecular Films, and Nanoribbon -Templated Gels and Polymers
Author(s):Li, Leiming
Doctoral Committee Chair(s):Stupp, Samuel I.
Department / Program:Materials Science and Engineering
Discipline:Materials Science and Engineering
Degree Granting Institution:University of Illinois at Urbana-Champaign
Subject(s):Engineering, Materials Science
Abstract:Photonic properties were studied in two self-assembled systems: supramolecular rodcoil self-assemblies, and materials templated with dendron rodcoil (DRC) nanoribbons. A rodcoil molecule contains covalently connected rod-like and coil-like segments. The aggregation of rodcoils forms supramolecular polar layered structures with infinitymm symmetry. Nonlinear optical properties of these materials are determined by their molecular characteristics. Several sets of architecturally similar but chemically different rodcoils were studied, and their nonlinear susceptibility was found strongly influenced by molecular hyperpolarizability and layer spacing of the supramolecular materials formed by these molecules. The macroscopic polarization of these materials is relatively low, possibly caused by polydomain structures. To improve the polarization, glass substrates were patterned with periodic arrays of squares with lateral dimensions of 7 and 17mum, created by focused ion beams. The average molecular tilt angle was found smaller in the films on patterned substrates, suggesting that substrate micropatterning enhances overall polarization in these supramolecular films. DRC molecules contain covalently connected dendritic, rod-like, and coil-like segments, and form gels with certain monomers and solvents like styrene or 2-ethylhexyl methacrylate (EHMA). Gelation is triggered by the self-assembly of DRC molecules into a network of ribbon-like nanostructures that are 10nm wide, 2nm thick, and up to 10mum long. DRC nanoribbons improve chain orientation in stretched DRC-polystyrene samples polymerized from DRC-styrene gels, compared with pure polystyrene stretched to the same extent. Linear-shape dyes show significantly larger emission polarization when incorporated in stretched DRC-polystyrene than in stretched pure polystyrene. When subject to a DC electric field, DRC-EHMA gels undergo electrophoresis, creating solid films consisting mostly of oriented DRC nanoribbons. The poled nanoribbon film shows uniform birefringence, and the nanoribbons are oriented along the poling field, suggested by TEM studies of ultramicrotomed samples. DRC gelation process disperses inorganic nanocrystals like ZnO, forming a stable nanocrystal-nanoribbon hybrid material. Similar electrophoresis occurs to the gel containing ZnO, orienting both nanoribbons and ZnO nanocrystals along the poling field. The poled DRC-ZnO film generates ultraviolet lasing emissions, with greatly lowered threshold compared with pure ZnO nanocrystals.
Issue Date:2002
Description:158 p.
Thesis (Ph.D.)--University of Illinois at Urbana-Champaign, 2002.
Other Identifier(s):(MiAaPQ)AAI3044159
Date Available in IDEALS:2015-09-25
Date Deposited:2002

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