Self-assembly approaches to nanostructured materials

This research addresses the need to develop simple and versatile approaches to nanostructured materials—that is, materials whose building blocks have at least one dimension in the range of 1 to 100 nm.; The first major goal of this research was to design and fabricate three-dimensional (3D) photonic crystals that can reject a band of optical frequencies in all directions of propagation (i.e., a complete bandgap). Self-assembly of spherical building blocks has been used to fabricate 3D photonic crystals with pseudobandgaps at ultraviolet to near-infrared wavelengths. A versatile technique employing physical confinement provided a tight control over the self-assembled structures (opaline lattices). These opaline lattices have uniform, tunable bandgap properties such as attenuation and midgap position. Defects were also found to have a large influence on the bandgap properties.; The opaline lattices were further explored to fabricate inverse opals with complementary structures. Inverse opals with refractive index contrasts >2.8 have been predicted to exhibit complete bandgaps. Long-range ordered inverse opals with the necessary structures have been fabricated. For example, inverse opals of ceramics and organic polymers were obtained by infiltration of liquid precursors into opaline lattices and selective removal of the template. A field-addressable photonic crystal was also fabricated via hierarchical self-assembly of ∼15-nm magnetite nanoparticles within the opaline lattice.; The second major goal of this research was to develop convenient and versatile approaches for the synthesis of chalcogen and chalcogenide one-dimensional (1D) nanostructures or nanowires. A solution phase chemical method was demonstrated for the large-scale synthesis of single crystalline nanowires of trigonal selenium. Diameters and lengths of the selenium nanowires could be controlled in the range of 10–800 nm, and 0.5–50 μm, respectively. A blue-shift was observed for the electronic transitions of these nanowires when their diameters were reduced from ∼32 to ∼10 nm. Photoconductivity was also measured for individual nanowires.; Chalcogen nanowires were further explored as templates to generate chalcogenide nanostructures with similar dimensions and morphology. For example, silver selenide nanowires have been successfully synthesized through a topotactic transformation utilizing the selenium nanowires as solid templates. This approach may be applicable to transforming selenium nanowires into a variety of other functional materials.