A three-step seed-mediated growth method was used to make gold nanoparticles. Different surfactants, alkyltrimethylammonium bromides (CnTAB, n ) 10, 12, 14, 16, and 18) and cetylpyridinium chloride (C16PC), were chosen as stabilizers. In general, it was found that as the length of the surfactant chain increased, the resulting gold nanoparticles' aspect ratio increased: the aspect ratio was 1 (for C10TAB), 5 ( 2 (C12TAB), 17 ( 3 (C14TAB), and 23 ( 4 (C16TAB). The plasmon absorption maxima for the gold nanoparticles varied as a function of the shape, from 520 nm (spheres) to beyond 2000 nm (high aspect ratio nanorods). We propose that the surfactant binds as a bilayer to the growing nanoparticle and assists in nanoparticle elongation via a "zipping" mechanism.

We report a method to make crystalline silver nanowires in water, in the absence of a surfactant or polymer to direct nanoparticle growth, and without externally added seed crystallites. The reaction is one in which silver salt is reduced to silver metal, at 100 °C, by sodium citrate, in the presence of NaOH. Hydroxide ion concentration is key to producing nanowires, which are up to 12 microns long, instead of nanospheres.

Reverse microemulsions were used to synthesize barium fluoride doped with 0−65 mol % neodymium. Although the products were polydisperse, average particle sizes below 100 nm were achieved. XRD analysis showed that powders with 0−10 mol % Nd were single phase, while samples with dopant levels of 10−50 mol % contained two phases. Products with more than 50 mol % Nd were amorphous by XRD. Fluorescence of Nd:BaF2 showed an unusually high threshold for concentration quenching as well as very short lifetimes compared to those of bulk samples. The use of a cosurfactant and variation in reaction conditions provided control over particle size; smaller particles resulted by limiting the aqueous volume while simultaneously increasing the amount of cosurfactant for a given concentration of reactants.

In this work, we develop a new approach to generative density estimation for exchangeable, non-i.i.d. data. The proposed framework, FlowScan, combines invertible flow transformations with a sorted scan to flexibly model the data while preserving exchangeability. Unlike most existing methods, FlowScan exploits the intradependencies within sets to learn both global and local structure. FlowScan represents the first approach that is able to apply sequential methods to exchangeable density estimation without resorting to averaging over all possible permutations. We achieve new state-of-the-art performance on point cloud and image set modeling.

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A perturbative inversion scheme [S. D. Rajan et al., J. Acoust. Soc. Am. 82, 998-1017Am. 82, 998- (1987] is applied to estimate the water-column sound-speed field in a three-dimensional volume of the shallow ocean. The input data to the inversion are estimates of modal travel time data calculated from measurements from a distributed network of sources and receivers. The differences between the data and modal travel times calculated for an assumed background profile are used as the basis for updating, or perturbing, the background model through the inversion algorithm to arrive at a solution. According to this technique, the horizontal plane is divided into a grid of range-independent regions and an estimate of the depth-dependent sound-speed profile within each region is obtained. The linearized perturbative technique is particularly well-suited for the three-dimensional inverse problem as the solution is typically obtained in less than 20 iterations. In this paper, the method is validated using synthetic data which are representative of oceanographic range-dependence in shallow water environments. [Work supported by ARL:UT IRD.]

The overall goal of this work is to quantify the effects of environmental variability and spatial sampling on the accuracy and uncertainty of estimates of the three-dimensional ocean sound-speed field. In this work, ocean sound speed estimates are obtained with acoustic data measured by a sparse autonomous observing system using a perturbative inversion scheme [Rajan, Lynch, and Frisk, J. Acoust. Soc. Am. 82, 998-1017 (1987)]. The vertical and horizontal resolution of the solution depends on the bandwidth of acoustic data and on the quantity of sources and receivers, respectively. Thus, for a simple, range-independent ocean sound speed profile, a single source-receiver pair is sufficient to estimate the water-column sound-speed field. On the other hand, an environment with significant variability may not be fully characterized by a large number of sources and receivers, resulting in uncertainty in the solution. This work explores the interrelated effects of environmental variability and spatial sampling on the accuracy and uncertainty of the inversion solution though a set of case studies. Synthetic data representative of the ocean variability on the New Jersey shelf are used.

The overall goal of this work is to develop a sparse autonomous observing system to sample the four-dimensional ocean. For this purpose, a perturbative inversion scheme [S.D. Rajan, et. al., J. Acoust. Soc. Am., 82, pp. 998-1017 (1987) ] is applied to estimate water-column sound-speed in all three spatial dimensions at a single “snapshot” in time. The input data to the inversion are estimates of modal travel time calculated from measurements from a distributed network of acoustic sources and receivers. Temporal variability is assessed by carrying out repeated inversions for new realizations of the input data. In initial applications of the inversion scheme, out-of-plane propagation effects were ignored and the solution was obtained by assuming straight-line paths in the horizontal plane. The effect of neglecting horizontal refraction on solution accuracy is quantified. The vertical and horizontal resolution of the solution depends on the quantity of input data and on the quantity of sources and receivers, respectively. Thus, for a particular environment, the available data may be insufficient to quantify the given variability, resulting in uncertainty in the solution. This talk explores the effects of environmental variability and spatial resolution on the uncertainty of the solution. [Work supported by ARL:UT IR&D.]