Victor Y. Lee

Preparation and characterization of neutral and oxidized polypyrrole films

Abstract Oxidized and neutral films of polypyrrole have been prepared electrochemically in the absence of oxygen and water. The neutral films are insulating and can be readily oxidized by chemical oxidizing agents to give films of greater conductivity than can be achieved by electrochemical oxidation. Optical spectroscopy provides evidence for the similarity of the polymeric carbonium ion produced by both types of oxidation. NMR studies are consistent with the α,α’ bonding in these polymers; they also show the expected downfield shifts relative to the neutral polymer on both chemical and electrochemical oxidation. ESR studies of both the electrochemically oxidized and the neutral polymer suggest the presence of highly mobile spins.

Superconducting Tl-Ca-Ba-Cu-O thin films with zero resistance at temperatures of up to 120 K

We have prepared superconducting Tl‐Ca‐Ba‐Cu‐O thin films on a variety of substrates with transition temperatures as high as ≂120 K, confirmed by sharp onsets of substantial Meissner and shielding signals at the same temperatures. The properties of the films are found to depend sensitively on the post‐annealing conditions. Highly textured c‐axis‐oriented films comprised mostly of Tl2Ca1Ba2Cu2Ox, Tl1Ca2Ba2Cu3Ox, and Tl2Ca2Ba2Cu3Ox were synthesized by varying the annealing procedure with corresponding maximum superconducting transition temperatures of ≂100, 110, and 120 K, respectively.

Application of Complex Macromolecular Architectures for Advanced Microelectronic Materials

The distinctive features of well-defined, three-dimensional macromolecules with topologies designed to enhance solubility and amplify end-group functionality facilitated nanophase morphologies in mixtures with organosilicates and ultimately nanoporous organosilicate networks. Novel macromolecular architectures including dendritic and star-shaped polymers and organic nanoparticles were prepared by a modular approach from several libraries of building blocks including various generations of dendritic initiators and dendrons, selectively placed to amplify functionality and/or arm number, coupled with living polymerization techniques. Mixtures of an organosilicate and the macromolecular template were deposited, cured, and the phase separation of the organic component, organized the vitrifying organosilicate into nanostructures. Removal of the sacrificial macromolecular template, also denoted as porogen, by thermolysis, yielded the desired nanoporous organosilicate, and the size scale of phase separation was strongly dependent on the chain topology. These materials were designed for use as interlayer, ultra-low dielectric insulators for on-chip applications with dielectric constant values as low as 1.5. The porogen design, chemistry and role of polymer architecture on hybrid and pore morphology will be emphasized.

Efficient, blue light-emitting diodes using cross-linked layers of polymeric arylamine and fluorene

Abstract Single-, double- and triple-layer polymer light-emitting diodes (PLEDs) with indium-tin oxide (ITO) anodes and Ca cathodes were fabricated, based on solution processable and thermally cross-linkable hole transporting polyarylamine and blue luminescent polyfluorene; and an electron transporting oxadiazole trimer. Triple-layer PLEDs show the highest efficiency, brightness and saturation of blue emission. These results are interpreted in terms of charge injection balance and electron-hole recombination efficiency. The maximum external quantum efficiencies achieved (>1%) are comparable to the best polymer blue PLEDs.

Crystallography and microstructure of Tl-Ca-Ba-Cu-O superconducting oxides

We report on the crystallography and microstructure of six oxides in the Tl‐Ca‐Ba‐Cu‐O system with nominal compositions Tl1Ba2Cu1O5, Tl1Ca1Ba2Cu2O7, Tl1Ca2Ba2Cu3O9, Tl2Ba2Cu1O6, Tl2Ca1Ba2Cu2O8, and Tl2Ca2Ba2Cu3O10. The structures consist of one, two, or three Cu perovskite‐like units sandwiched between Tl‐O monolayers or bilayers. The predominant defects in the crystals with bilayer and trilayer Cu perovskite‐like units are stacking faults that produce local intergrowths of related structures. The density of stacking defects in these oxides correlates with changes in the superconducting transition temperatures.

High Tc YBa2Cu3O7−x thin films on Si substrates by dc magnetron sputtering from a stoichiometric oxide target

Thin films of YBa2Cu3O7−x were deposited on Si substrates at 600–700 °C by dc magnetron sputtering from a stoichiometric oxide target. Resistivity measurement results indicate that these films are superconducting with a zero resistance Tc as high as 76 K, without further high‐temperature post‐annealing treatments. These films give both core and valence‐band x‐ray photoemission, and x‐ray diffraction spectra similar to those for superconducting films prepared with a high‐temperature post‐annealing step. No significant diffusion of Si from the substrate into the film was detected for the films deposited at 650 °C or lower, according to depth profiles obtained using secondary ion mass spectrometry.

Aerosol flow reactor production of fine Y1Ba2Cu3O7 powder: Fabrication of superconducting ceramics

An aerosol flow reactor operating at 900–1000 °C is used to prepare high‐purity Y1Ba2Cu3O7 powders with a uniform chemical composition and a submicron to micron average particle size by thermally decomposing aerosol droplets of a solution consisting of the nitrate salts of Y, Ba, and Cu in a 1:2:3 ratio. The powders were at least 99% reacted based on thermogravimetric analysis, and the x‐ray diffraction pattern is essentially that of Y1Ba2Cu3O7. Magnetic susceptibility measurements showed the powders to be superconducting with a transition at 90 K even for average reactor residence times as short as 20 s. Sintering cold‐pressed pellets between 900 and 1000 °C provides dense, fine grained (average size on the order of 1 μm) superconducting ceramics with sharp 90 K transitions. The grain size and shape of a final sintered part could be varied depending on powder production, processing, and sintering conditions.

Interfacial glass transition profiles in ultrathin, spin cast polymer films

Interfacial glass transition temperature (T(g)) profiles in spin cast, ultrathin films of polystyrene and derivatives were investigated using shear-modulated scanning force microscopy. The transitions were measured as a function of film thickness (delta), molecular weight, and crosslinking density. The T(g)(delta) profiles were nonmonotonic and exhibited two regimes: (a) a sublayer extending about 10 nm from the substrate, with T(g) values lowered up to approximately 10 degrees C below the bulk value, and (b) an intermediate regime extending over 200 nm beyond the sublayer, with T(g) values exceeding the bulk value by up to 10 degrees C. Increasing the molecular weight was found to shift the T(g)(delta) profiles further from the substrate interface, on the order of 10 nm/kDa. Crosslinking the precast films elevated the absolute T(g) values, but had no effect on the spatial length scale of the T(g)(delta) profiles. These results are explained in the context of film preparation history and its influence on molecular mobility. Specifically, the observed rheological anisotropy is interpreted based on the combined effects of shear-induced structuring and thermally activated interdiffusion.

Annealing treatment effects on structure and superconductivity in Y1Ba2Cu3O9−x

We report the effects of heat treatment and ambient on the structure and superconducting properties of Y1Ba2Cu3O9−x. The structure undergoes an orthorhombic‐to‐tetragonal transition on heating at about 700 °C, caused by oxygen loss and disordering of oxygen vacancies on the copper plane between the barium layers. Heat treatments that promote maximum ordering of the oxygen vacancies result in superior superconducting properties.

Correlation between polymer architecture and sub-glass-transition-temperature light-induced molecular movement in azo-polyimide polymers: influence on linear and second- and third-order nonlinear optical processes

We report on light-induced linear and second- and third-order nonlinear optical effects in high-glass-transition-temperature (Tg) photosensitive nonlinear optical azo polyimides. We present evidence of light-induced orientation of azo chromophores at room temperature in very high-Tg polyimides (Tg up to 350 °C) even though the chromophore is firmly embedded into the polymer backbone. We show that the isomerization reaction and the light-induced polar and nonpolar orientation depend on the molecular structure of the unit building blocks of the polymer. The mechanism of the photoassisted poling process is clarified, and it is shown how the linear and second- and third-order nonlinear optical effects can be controlled by light.