Multilayer ytterbium‐hafnate/silicate coatings deposited by directed vapor deposition and designed to protect SiC‐based ceramic matrix composites were assessed to determine their thermochemical stability and resistance to attack by molten silicate deposits (CMAS). The study revealed that reactions occurring at the interface between Yb2Si2O7 and Yb4Hf3O12 layers promote coating delamination following isothermal annealing for 100 h/1500°C while coating architectures involving Yb2SiO5 in contact with Yb4Hf3O12 do not experience similar degradation. The outer Yb4Hf3O12 layers, segmented for compliance, were only moderately effective in mitigating CMAS infiltration at 1300°C and 1500°C. The results indicate that the reaction between the melt and coating forms large volumes of a silicate garnet phase at 1300°C, or a cuspidine‐type aluminosilicate at 1500°C, in addition to the apatite and reprecipitated fluorite phases observed in related systems.

Recently we showed large scale fabrication of field-effect transistors from horizontal ZnO nanowires (NWs) on a-plane sapphire. In growth of horizontal ZnO NWs, the substrate experiences a compressive strain of ≈ 7% in its [0001]sap direction (along the width of a NW) to minimize its lattice mismatch with the ZnO NW. Accordingly, ZnO expands along its width to improve its lattice match with the sapphire. Although this system is a highly mismatched one, our results show that horizontal ZnO nanowires grow semicoherently with much fewer misfit dislocations than theoretically expected. We attribute the formation of fewer dislocations to partial strain relaxation of zinc oxide NW to sapphire surface. A critical nanowire thickness is defined beyond which misfit energy largely relaxes to dislocations, changing the growth mode from horizontal to standing.

Understanding the source of variation of the optical properties within an ensemble of nanowires (NWs) is an important step toward fabrication of NWs with more uniform and better-controlled properties. This in turn will facilitate the development of complex architectures for device applications. Analysis of gold- and copper-catalyzed zinc oxide (ZnO) NWs grown in a high temperature tube furnace on a sapphire substrate shows that the structural and optical properties vary with NW growth sites on the substrate. Results show that there are systematic changes in the optical properties of the ZnO NWs from the center to the edge of the substrate, and also from "upstream" to "downstream" along the gas flow direction, implying changes in the availability of the source material and catalyst over the substrate surface during the growth. Photoluminescence microscopy has been used to map out these changes and unravel the growth patterns. We observe two distinct trends that are labeled the "edge" trend and the "gas flow" trend, respectively, and are linked to two growth phases. The first phase starts while the growth tube temperature and gas flow have not yet reached their final steady states. In this phase, due to the limited availability of the source material in gas phase, only metal nanodroplets that are located at the substrate edges have the advantage of early growth. While the edge NWs continue their growth, the second phase starts when the growth tube temperature and gas flow reach reasonable stabilities. Our results show that these two phases are more pronounced in the case of copper-catalyzed NWs. In the first phase, NWs have a better chance to grow at the substrate edges. In the second phase, copper diffuses downstream, causing an interesting variation in the optical properties of NWs. Numerous morphology examinations of NWs show that the variation in emission is related to the change in the ratio of the surface area to the bulk volume of the NWs within an ensemble of NWs. The patterns of the ZnO NW optical properties can be used as an indicator to follow the growth patterns of the NWs across the substrate.

Extended abstract of a paper presented at Microscopy and Microanalysis 2008 in Albuquerque, New Mexico, USA, August 3 – August 7, 2008

ZnO nanowires (NWs) are grown on a bulk copper half-transmission electron microscopy grid by chemical vapor deposition in a high temperature tube furnace. Photoluminescence (PL) microscopy revealed band gap emission at 380 nm and a more intense visible emission around 520 nm due to defect states in these NWs. High-resolution transmission electron microscopy shows that the ZnO NWs are single crystalline with hexagonal structure. Auger electron spectroscopy (AES) and energy dispersive X-ray spectroscopy reveal that copper atoms are present along the length of the NW. AES also found that the surface of the NWs is oxygen rich. The surface concentration of zinc increases moving from the tip toward the base of the NW while the concentration of oxygen decreases. The copper in this system not only remains at the tip of the growing NW but also acts as a dopant along the length of the NW, leading to a decrease in the intensity of the band gap PL of these NWs.

The determination of the statistical distribution of aspect ratios of a nanorod solution is desirable for experimentally synthesized solutions. The traditional method of using transmission electron microscopy (TEM) images for size determination gives statistically incorrect values due to distortions introduced by TEM sample preparation and by difficulties in counting a sufficiently large number of rods. In the present work, we propose a method to obtain the aspect ratio distribution using the observed longitudinal surface plasmon resonance absorption spectrum of gold nanorods in solution. The observed inhomogeneously broadened spectrum is fitted with a collection of homogeneously broadened spectra of nanorods each with a specific aspect ratio and population contribution using Gans extension [Ann. Phys. 47, 270 (1915)] of Mie theory. The fit generates an aspect ratio distribution for the rods in solution from which the median value and the statistical distribution are determined. This method is statistically more accurate, more convenient, and less expensive than the traditional method of TEM analysis on a solid substrate.

Porous silica (SiO2 films and powders), modified with benzophenone (BP), facilitates the formation of stable sliver nanoparticles by taking advantage of the solid supported photosensitizer. The silica serves as a carrier for the BP into an aqueous solution and its subsequent removal. Benzophenone, bound to a silica film, was able to reduce silver ions to generate nanoparticles in solution, while silica powder with bound BP generates silver nanoparticles that are attracted to the silica. Silver nanoparticles are also fabricated in porous silica films by incorporating silver ions into the films before casting and then irradiating the film in a solution containing BP. From pH studies, it is concluded that the ketyl-radicals and anion-radicals of BP and IPA both take part in the reduction of silver ions. These synthetic studies provide a new photochemical reduction method by immobilizing the reactant on a silica surface allowing generation of silver nanoparticles in solution attached to powders or inside a film for catalytic applications or increased conductivity of silica films.

It is found that replacement of the chloride ions in tetrachloroauric acid with bulky bromide ions inhibits the formation of gold nanoparticles in the photochemical reduction in ethylene glycol. However, the addition of silver ions to either the bromide or the chloride auric acid solution is found to enhance the rate of gold nanoparticle formation. These results are found to be accounted for by the previously proposed mechanism (Eustis, S.; Hsu, H.-Y.; El-Sayed, M. A. J. Phys. Chem. B 2005, 109, 4811) which involves disproportionation of the chloroauric complexes to generate free gold atoms and chloride ions. The steric effects of the bulky bromide ions inhibit the formation of the Au-Au bond needed in the electron transfer process of the disproportionation reaction. The addition of Ag+ ions results in the formation of insoluble silver halide, which shifts the disproportionation reaction toward the formation of gold atoms and thus the formation of gold nanoparticles.

The shape anisotropy of nanorods gives rise to two distinct orientational modes by which nanorods can be assembled, i.e., end-to-end and side-by-side, analogous to the well-known H and J aggregation in organic chromophores. Optical absorption spectra of gold nanorods have earlier been observed to show a red-shift of the longitudinal plasmon band for the end-to-end linkage of nanorods, resulting from the plasmon coupling between neighboring nanoparticles, similar to the assembly of gold nanospheres. We observe, however, that side-by-side linkage of nanorods in solution shows a blue-shift of the longitudinal plasmon band and a red-shift of the transverse plasmon band. Optical spectra calculated using the discrete dipole approximation method were used to simulate plasmon coupling in assembled nanorod dimers. The longitudinal plasmon band is found to shift to lower energies for end-to-end assembly, but a shift to higher energies is found for the side-by-side orientation, in agreement with the optical absorption experiments. The strength of plasmon coupling was seen to increase with decreasing internanorod distance and an increase in the number of interacting nanorods. For both side-by-side and end-to-end assemblies, the strength of the longitudinal plasmon coupling increases with increasing nanorod aspect ratio as a result of the increasing dipole moment of the longitudinal plasmon. For both the side-by-side and end-to-end orientation, the simulation of a dimer of nanorods having dissimilar aspect ratios showed a longitudinal plasmon resonance with both a blue-shifted and a red-shifted component, as a result of symmetry breaking. A similar result is observed for a pair of similar aspect ratio nanorods assembled in a nonparallel orientation. The internanorod plasmon coupling scheme concluded from the experimental results and simulations is found to be qualitatively consistent with the molecular exciton coupling theory, which has been used to describe the optical spectra of H and J aggregates of organic molecules. The coupled nanorod plasmons are also suggested to be electromagnetic analogues of molecular orbitals. Investigation of the plasmon coupling in assembled nanorods is important for the characterization of optical excitations and plasmon propagation in these nanostructures. The surface plasmon resonance shift resulting from nanorod assembly also offers a promising alternative for analyte-sensing assays.

Experimental observations and theoretical treatments are carried out for the band shape and relative intensity of the emission from gold nanorods of various aspect ratios in the range between 2.25 (1.5 theory) and 6.0 (9 theory). The calculation of the fluorescence spectra requires knowledge of the nanorod size distribution, the enhancement factors, and the shape of the unenhanced fluorescence spectrum. The size distribution is determined from the fit of the observed absorption spectrum for each value of aspect ratio studied to the theoretical model of Gans. The theory by Boyd and Shen is used for calculating the enhancement of the fluorescence spectrum of the previously observed weak emission of bulk gold, which originates from the interband transition. This is carried out for nanorods of different aspect ratios. To compare theory to the observed nanorod fluorescence spectra, which suffer from self-absorption, the calculated nanorod fluorescence spectra are corrected for this effect using the observed absorption spectra. The comparison between the observed and the calculated fluorescence band shapes is found to be good. The calculated changes in the relative intensities upon changing the aspect ratios are found to be much greater than that observed. This is due to the fact that for the observed emission of all the nanorods studied nonradiative processes dominate the relaxation mechanism of the excited state, a fact that was not included in the theoretical treatments.

Experimental observations and theoretical treatments are carried out for the band shape and relative intensity of the emission from gold nanorods of various aspect ratios in the range between 2.6 (1.5 theory) and 6.3 (9 theory). The calculation of the fluorescence spectra requires knowledge of the nanorod size distribution, the enhancement factors and the shape of the un-enhanced fluorescence spectrum. The comparison between the observed and calculated fluorescence band shapes is found to be good. The calculated changes in the relative intensities with aspect ratios are found to be much greater than that observed experimentally. This is due to the fact that for the observed emission of all the rods studied, nonradiative processes dominate the relaxation mechanism of the excited state, a fact that was not included in the theoretical treatments. Experimental results and theoretical treatments will be presented.