Lack of robust manufacturing capabilities have limited our ability to make tailored materials with useful optical and thermal properties. For example, traditional methods such as spontaneous self-assembly of spheres cannot generate the complex structures required to produce a full bandgap photonic crystals. The goal of this work was to develop and demonstrate novel methods of directed self-assembly of nanomaterials using optical and electric fields. To achieve this aim, our work employed laser tweezers, a technology that enables non-invasive optical manipulation of particles, from glass microspheres to gold nanoparticles. Laser tweezers were used to create ordered materials with either complex crystal structures or using aspherical building blocks.
This project uses advanced ceramic processes to fabricate large, optical-quality, polycrystalline lanthanum halide scintillators to replace small single crystals produced by the conventional Bridgman growth method. The new approach not only removes the size constraint imposed by the growth method, but also offers the potential advantages of both reducing manufacturing cost and increasing production rate. The project goal is to fabricate dense lanthanum halide ceramics with a preferred crystal orientation by applying texture engineering and solid-state conversion to reduce the thermal mechanical stress in the ceramic and minimize scintillation light scattering at grain boundaries. Ultimately, this method could deliver the sought-after high sensitivity and <3% energy resolution at 662 keV of lanthanum halide scintillators and unleash their full potential for advanced gamma ray detection, enabling rapid identification of radioactive materials in a variety of practical applications. This report documents processing details from powder synthesis, seed particle growth, to final densification and texture development of cerium doped lanthanum bromide (LaBr{sub 3}:Ce{sup +3}) ceramics. This investigation demonstrated that: (1) A rapid, flexible, cost efficient synthesis method of anhydrous lanthanum halides and their solid solutions was developed. Several batches of ultrafine LaBr{sub 3}:Ce{sup +3} powder, free of oxyhalide, were produced by a rigorously controlled process. (2) Micron size ({approx} 5 {micro}m), platelet shape LaBr{sub 3} seed particles of high purity can be synthesized by a vapor phase transport process. (3) High aspect-ratio seed particles can be effectively aligned in the shear direction in the ceramic matrix, using a rotational shear-forming process. (4) Small size, highly translucent LaBr{sub 3} (0.25-inch diameter, 0.08-inch thick) samples were successfully fabricated by the equal channel angular consolidation process. (5) Large size, high density, translucent LaBr{sub 3} ceramics samples (3-inch diameter, > 1/8-inch thick) were fabricated by hot pressing, demonstrating the superior manufacturability of the ceramic approach over single crystal growth methods in terms of size capability and cost. (6) Despite all these advances, evidence has shown that LaBr{sub 3} is thermally unstable at temperatures required for the densification process. This is particularly true for material near the surface where lattice defects and color centers can be created as bromine becomes volatile at high temperatures. Consequently, after densification these samples made using chemically prepared ultrafine powders turned black. An additional thermal treatment in a flowing bromine condition proved able to reduce the darkness of the surface layer for these densified samples. These observations demonstrated that although finer ceramic powders are desirable for densification due to a stronger driving force from their large surface areas, the same desirable factor can lead to lattice defects and color centers when these powders are densified at higher temperatures where material near the surface becomes thermally unstable.
The objective of the experimental effort is to provide a model particle system that will enable modeling of the macroscopic rheology from the interfacial and environmental structure of the particles and solvent or melt as functions of applied shear and volume fraction of the solid particles. This chapter describes the choice of the model particle system, methods for synthesis and characterization, and results from characterization of colloidal dispersion, particle film formation, and the shear and oscillatory rheology in the system. Surface characterization of the grafted PDMS interface, dispersion characterization of the colloids, and rheological characterization of the dispersions as a function of volume fraction were conducted.
Nanoparticle interactions and their impact on particle dispersion and rheology are well known to be functions of the interfacial structure between the particle and the fluid phase. The dispersion and flow properties of a titania nanopowder were evaluated in polydimethylsiloxane fluid using ''grafted to'' surface modification of the titania with short molecular weight PDMS polymers. The interaction energy between particles was modeled using analytical expressions as well as dynamic functional theory for polymer surface chains. Particle dynamics as a function of volume fraction were characterized using light scattering, acoustic spectroscopy, and shear and oscillatory measurements. Autophobic dewetting is a novel short range interaction in this system that may be impacting the maximum packing fraction of particles in a suspension.
The present work demonstrates the use of light to move liquids on a photoresponsive monolayer, providing a new method for delivering analyses in lab-on-chip environments for microfluidic systems. The light-driven motion of liquids was achieved on photoresponsive azobenzene modified surfaces. The surface energy components of azobenzene modified surfaces were calculated by Van Oss theory. The motion of the liquid was achieved by generation of a surface tension gradient by isomerization of azobenzene monolayers using UV and Visible light, thereby establishing a surface energy heterogeneity on the edge of the droplet. Contact angle measurements of various solvents were used to demonstrate the requirement for fluid motion.
This work reports on the grafting of methyl methacrylate polymer brushes containing spirobenzopyran pendant groups from flat silica surfaces and colloidal particles utilizing atom transfer radical polymerization (ATRP). The reaction conditions were optimized with respect to the kind of surface bound initiator, the type of halide and ligand used in the catalytic complex, the presence/absence of untethered initiator, and solvent type. This enabled synthesis of coatings up to 80 {+-} 3 nm thick with controlled spirobenzopyran content. While polymerization kinetics indicate the presence of chain termination reactions, the 'living' character of the process is confirmed by controlled formation of block copolymer brushes. UV/vis spectroscopy was used to characterize the UV-induced isomerization of spirobenzopyran to zwitterionic merocyanine and the thermal back-reaction. Spectral and kinetic analyses of this latter bleaching process points to the existence of free and associated merocyanines in the polymeric brush in both tetrahydrofuran and toluene. However, stabilization of merocyanine species by the polymer matrix is considerably greater in toluene with thermal back-reaction rates approaching those determined for solid dry films.
Colloidal particles were derivatized with end-grafted polymethylmethacryate polymer brushes containing varying concentrations of spirobenzopyran photochromic molecules. The polymers were grown from initiator-functionalized silica partilces by an atom-transfer radical polymerization (ATRP). These core-shell colloids formed stable suspensions in toluene with the spirobenzopyran in its closed, nonpolar form. However, UV-induced photoswitching of the photochrome to its open, polar merocyanine isomer caused rapid aggregation. The nature of this colloidal stability transition was examined with respect to the spirobenzopyran content in the polymeric brush and solvent polarity. Turbidimetry, wettability studies, UV-vis spectroscopy, suspension rheology, SEM, and visual inspection were utilized to characterize the system photoswitchability. It was found that the system exhibiting the greatest transition in toluene was the copolymer brush composed of 20% spirobenzopyran and 80% methyl methacrylate.
Quartz surfaces and colloidal silica particles were derivatized with a poly(methyl methacrylate) copolymer containing spirobenzopyran (SP) photochromic molecules in the pendant groups at a concentration of 20 mol %. Two-photon near-IR excitation ({approx}780 nm) was then used to create chemically distinct patterns on the modified surfaces through a photochromic process of SP transformation to the zwitterionic merocyanine (MC) isomer. The derivatized colloids were approximately 10 times more likely to adsorb onto the photoswitched, MC regions. Surface coverage and adsorption kinetics have been compared to the mean-field model of irreversible monolayer adsorption.