Chemical Solution Deposition of Protective Er2O3 and Y2O3 Coatings onto Stainless Steel for Molten Metal Casting using Metal-Nitrate Precursors.

Chemical solution deposition (CSD) methods involving the thermal decomposition of 5.0 M Er(NO3)3·5H2O and Y(NO3)3·6H2O precursor solutions were employed to fabricate protective erbia and yttria coatings onto stainless steel (SS304/SS316) coupons. The two techniques tested were dip and spray coating, which were then compared to a commercial yttria spray (ZYP Coatings). It was determined that solution concentration, solvent choice, injection of Er2O3 and Y2O3 micropowder, and the annealing temperature/ramp profile were critical to the coatings' physical properties. For dip coatings, thicknesses were 1-20 µm after two dipping/annealing cycles, and adhesion strength was ∼1000 psi, increasing up to ∼1300 psi if the SS coupons had preliminary sandblasting. Spray coatings from precursor solutions were reported to have thicknesses of 20-80 µm and adhesion strength less than 500 psi (regardless of the coupon surface finish). Cross-sectional views of the coatings confirmed subsurface porosity, and XRD results indicated that the coatings were polycrystalline, with patterns typical to that of cubic Er2O3 and Y2O3.

Electrodeposition and analysis of thick bismuth films.

Due to its unique physical and chemical properties, bismuth is an attractive candidate for a wide range of applications such as battery anodes, radiation shielding, and semiconductors, to name a few. This work presents the electrodeposition of mechanically stable and homogenous bismuth films at micron-scale thicknesses. A simple one-step electrodeposition process using either a pulse/reverse or direct current source yielded thick, homogenous, and mechanically stable bismuth films. Morphology, electrochemical behavior, adhesion, and mechanical stability of bismuth coatings plated with varying parameters were characterized via optical profilometry, cyclic voltammetry, electron microscopy, and tribology. Scratch testing on thick electroplated coatings (> 100 µm) revealed similar wear resistance properties between the pulse/reverse plated and direct current electroplated films. This study presents a versatile bismuth electroplating process with the possibility to replace lead in radiation shields with an inexpensive, non-toxic metal, or to make industrially relevant electrocatalytic devices.

Intrinsic helical twist and chirality in ultrathin tellurium nanowires.

Robust atomic-to-meso-scale chirality is now observed in the one-dimensional form of tellurium. This enables a large and counter-intuitive circular-polarization dependent second harmonic generation response above 0.2 which is not present in two-dimensional tellurium. Orientation variations in 1D tellurium nanowires obtained by four-dimensional scanning transmission electron microscopy (4D-STEM) and their correlation with unconventional non-linear optical properties by second harmonic generation circular dichroism (SHG-CD) uncovers an unexpected circular-polarization dependent SHG response from 1D nanowire bundles - an order-of-magnitude higher than in single-crystal two-dimensional tellurium structures - suggesting the atomic- and meso-scale crystalline structure of the 1D material possesses an inherent chirality not present in its 2D form; and which is strong enough to manifest even in the aggregate non-linear optical (NLO) properties of aggregates.

PbS/CdS Quantum Dot Room-Temperature Single-Emitter Spectroscopy Reaches the Telecom O and S Bands via an Engineered Stability.

We synthesized PbS/CdS core/shell quantum dots (QDs) to have functional single-emitter properties for room-temperature, solid-state operation in the telecom O and S bands. Two shell-growth methods-cation exchange and successive ionic layer adsorption and reaction (SILAR)-were employed to prepare QD heterostructures with shells of 2-16 monolayers. PbS/CdS QDs were sufficiently bright and stable to resolve photoluminescence (PL) spectra representing both bands from single nanocrystals using standard detection methods, and for a QD emitting in the O-band a second-order correlation function showed strong photon antibunching, important steps toward demonstrating the utility of lead chalcogenide QDs as single-photon emitters (SPEs). Irrespective of type, few telecom-SPEs exist that are capable of such room-temperature operation. Access to single-QD spectra enabled a direct assessment of spectral line width, which was ∼70-90 meV compared to much broader ensemble spectra (∼300 meV). We show inhomogeneous broadening results from dispersity in PbS core sizes that increases dramatically with extended cation exchange. Quantum yields (QYs) are negatively impacted at thick shells (>6 monolayers) and, especially, by SILAR-growth conditions. Time-resolved PL measurements revealed that, with SILAR, initially single-exponential PL-decays transition to biexponential, with opening of nonradiative carrier-recombination channels. Radiative decay times are, overall, longer for core/shell QDs compared to PbS cores, which we demonstrate can be partially attributed to some core/shell sizes occupying a quasi-type II electron-hole localization regime. Finally, we demonstrate that shell engineering and the use of lower laser-excitation powers can afford significantly suppressed blinking and photobleaching. However, dependence on shell thickness comes at a cost of less-than-optimal brightness, with implications for both materials and experimental design.

1D to 2D Transition in Tellurium Observed by 4D Electron Microscopy.

A new microwave-enhanced synthesis method for the production of tellurium nanostructures is reported-with control over products from the 1D regime (sub-5 nm diameter nanowires), to nanoribbons, to the 2D tellurene regime-along with a new methodology for local statistical quantification of the crystallographic parameters of these materials at the nanometer scale. Using a direct electron detector and image-corrected microscope, large and robust 4D scanning transmission electron microscopy datasets for accurate structural analysis are obtained. These datasets allow the adaptation of quantitative techniques originally developed for X-ray diffraction (XRD) refinement analyses to transmission electron microscopy, enabling the first demonstration of sub-picometer accuracy lattice parameter extraction while also obtaining both the size of the coherent crystallite domains and the nanostrain, which is observed to decrease as nanowires transition to tellurene. This new local analysis is commensurate with global powder XRD results, indicating the robustness of both the new synthesis approach and new structural analysis methodology for future scalable production of 2D tellurene and characterization of nanomaterials.

Buoyancy driven mixing of miscible fluids by volumetric energy deposition of microwaves.

An experiment that seeks to investigate buoyancy driven mixing of miscible fluids by microwave volumetric energy deposition is presented. The experiment involves the use of a light, non-polar fluid that initially rests on top of a heavier fluid which is more polar. Microwaves preferentially heat the polar fluid, and its density decreases due to thermal expansion. As the microwave heating continues, the density of the lower fluid eventually becomes less than that of the upper, and buoyancy driven Rayleigh-Taylor mixing ensues. The choice of fluids is crucial to the success of the experiment, and a description is given of numerous fluid combinations considered and characterized. After careful consideration, the miscible pair of toluene/tetrahydrofuran (THF) was determined as having the best potential for successful volumetric energy deposition buoyancy driven mixing. Various single fluid calibration experiments were performed to facilitate the development of a heating theory. Thereafter, results from two-fluid mixing experiments are presented that demonstrate the capability of this novel Rayleigh-Taylor driven experiment. Particular interest is paid to the onset of buoyancy driven mixing and unusual aspects of the experiment in the context of typical Rayleigh-Taylor driven mixing.

Suppressed blinking and auger recombination in near-infrared type-II InP/CdS nanocrystal quantum dots.

Nonblinking excitonic emission from near-infrared and type-II nanocrystal quantum dots (NQDs) is reported for the first time. To realize this unusual degree of stability at the single-dot level, novel InP/CdS core/shell NQDs were synthesized for a range of shell thicknesses (~1-11 monolayers of CdS). Ensemble spectroscopy measurements (photoluminescence peak position and radiative lifetimes) and electronic structure calculations established the transition from type-I to type-II band alignment in these heterostructured NQDs. More significantly, single-NQD studies revealed clear evidence for blinking suppression that was not strongly shell-thickness dependent, while photobleaching and biexciton lifetimes trended explicitly with extent of shelling. Specifically, very long biexciton lifetimes-up to >7 ns-were obtained for the thickest-shell structures, indicating dramatic suppression of nonradiative Auger recombination. This new system demonstrates that electronic structure and shell thickness can be employed together to effect control over key single-dot and ensemble NQD photophysical properties.

New insights into the complexities of shell growth and the strong influence of particle volume in nonblinking "giant" core/shell nanocrystal quantum dots.

The growth of ultra-thick inorganic CdS shells over CdSe nanocrystal quantum dot (NQD) cores gives rise to a distinct class of NQD called the "giant" NQD (g-NQD). g-NQDs are characterized by unique photophysical properties compared to their conventional core/shell NQD counterparts, including suppressed fluorescence intermittency (blinking), photobleaching, and nonradiative Auger recombination. Here, we report new insights into the numerous synthetic conditions that influence the complex process of thick-shell growth. We show the individual and collective effects of multiple reaction parameters (noncoordinating solvent and coordinating-ligand identities and concentrations, precursor/NQD ratios, precursor reaction times, etc.) on determining g-NQD shape and crystalline phase, and the relationship between these structural features and optical properties. We find that hexagonally faceted wurzite g-NQDs afford the highest ensemble quantum yields in emission and the most complete suppression of blinking. Significantly, we also reveal a clear correlation between g-NQD particle volume and blinking suppression, such that larger cores afford blinking-suppressed behavior at relatively thinner shells compared to smaller starting core sizes, which require application of thicker shells to realize the same level of blinking suppression. We show that there is a common, threshold g-NQD volume (~750 nm(3)) that is required to observe blinking suppression and that this particle volume corresponds to an NQD radiative lifetime of ~65 ns regardless of starting core size. Combining new understanding of key synthetic parameters with optimized core/shell particle volumes, we demonstrate effectively complete suppression of blinking even for long observation times of ~1 h.

Mechanistic study of silver nanoparticle formation on conducting polymer surfaces.

Conducting polymer (polyaniline) sheets are shown to be active substrates to promote the growth of nanostructured silver thin films with highly tunable morphologies. Using the spontaneous electroless deposition of silver, we show that a range of nanostructured metallic features can be controllably and reproducibly formed over large surface areas. The structural morphology of the resulting metal-polymer nanocomposite is demonstrated to be sensitive to experimental parameters such as ion concentration, temperature, and polymer processing and can range from densely packed oblate nanosheets to bulk crystalline metals. The deposition mechanisms are explained using a diffusion-limited aggregation (DLA) model to describe the semi-fractal-like growth of the metal nanostructures. We find these composite films to exhibit strong surface-enhanced Raman (SERS) activity, and the nanostructured features are optimized with respect to SERS activity using a self-assembled monolayer of mercapto-benzoic acid as a model Raman reporter. SERS enhancements are estimated to be on the order of 10(7). Through micro-Raman SERS mapping, these materials are shown to exhibit uniform SERS responses over macroscopic areas. These metal-polymer nanocomposites benefit from the underlying polymer's processability to yield SERS-active materials of almost limitless shape and size and show significant promise for future SERS-based sensing and detection schemes.

Field-assisted synthesis of SERS-active silver nanoparticles using conducting polymers.

A gradient of novel silver nanostructures with widely varying sizes and morphologies is fabricated on a single conducting polyaniline-graphite (P-G) membrane with the assistance of an external electric field. It is believed that the formation of such a silver gradient is a synergetic consequence of the generation of a silver ion concentration gradient along with an electrokinetic flow of silver ions in the field-assisted model, which greatly influences the nucleation and growth mechanism of Ag particles on the P-G membrane. The produced silver dendrites, flowers and microspheres, with sharp edges, intersections and bifurcations, all present strong surface enhanced Raman spectroscopy (SERS) responses toward an organic target molecule, mercaptobenzoic acid (MBA). This facile field-assisted synthesis of Ag nanoparticles via chemical reduction presents an alternative approach to nanomaterial fabrication, which can yield a wide range of unique structures with enhanced optical properties that were previously inaccessible by other synthetic routes.

Solution-liquid-solid growth of ternary Cu-In-Se semiconductor nanowires from multiple- and single-source precursors.

Ternary CuInSe(2) nanowires were synthesized for the first time by the solution-liquid-solid (SLS) mechanism. Here, both metal-organic multiple- and single-source molecular precursors were thermally decomposed in the presence of molten metal nanoparticles and coordinating ligands. The nature of the precursor-multiple- compared to single-source (wherein Cu-Se-In bonds are effectively preformed)-as well as the choice of coordinating ligands, reaction temperature, and reactant order-of-addition strongly affected the morphology and composition of the reaction product obtained. Crystalline, straight, and nearly stoichiometric CuInSe(2) nanowires were most readily achieved using the single-source precursor; however, careful tuning of reaction conditions could also be used to obtain high-quality nanowires from multiple-source precursor systems. The CuInSe(2) nanowires are strong light absorbers from the near-infrared through the visible and ultraviolet spectral regions and, thereby, comprise new soluble and processable "building blocks" for applications in solar-light harvesting.

A reduction pathway in the synthesis of PbSe nanocrystal quantum dots.

Colloidal nanocrystal quantum dots (NQDs) of narrow band gap materials are of substantial general interest because of their unparalleled potential as infrared fluorophores. While PbSe NQDs are a promising class of infrared-active nanocrystals due to high emission quantum yields and a wide useful spectral range, typical synthetic methods are sensitive to a variety of factors, including the influence of solvent/ligand impurities that render reproducibility difficult. In this work, we specifically examine the effects of diphenylphosphine and 1,2-hexadecanediol, as surrogates for putative trioctylphosphine-based reducing impurities, on the synthesis of PbSe NQDs. Specifically, we compare their influence on NQD size, chemical yield, and photoluminescence quantum yield. While both additives substantially increase the chemical yield of the synthesis, they demonstrate markedly different effects on emission quantum yield of the product NQDs. We further examine the effects of reaction temperature and oleic acid concentration on the diol-assisted synthesis. Increased oleic acid concentration led to somewhat higher growth rates and larger NQDs but at the expense of lower chemical yield. Temperature was found to have an even greater effect on growth rate and NQD size. Neither temperature nor oleic acid concentration was found to have noticeable effects on NQD emission quantum yield. Finally, we use numerical simulations to support the conjecture that the increased yield is likely a result of faster monomer formation, consistent with the activation of an additional reaction pathway by the reducing species.

Utilizing the lability of lead selenide to produce heterostructured nanocrystals with bright, stable infrared emission.

Infrared-emitting nanocrystal quantum dots (NQDs) have enormous potential as an enabling technology for applications ranging from tunable infrared lasers to biological labels. Notably, lead chalcogenide NQDs, especially PbSe NQDs, provide efficient emission over a large spectral range in the infrared, but their application has been limited by instability in emission quantum yield and peak position on exposure to ambient conditions. Conventional methods for improving NQD stability by applying a shell of a more stable, wider band gap semiconductor material are frustrated by the tendency of lead chalcogenide NQDs toward Ostwald ripening at even moderate reaction temperatures. Here, we describe a partial cation-exchange method in which we take advantage of this lability to controllably synthesize PbSe/CdSe core/shell NQDs. Critically, these NQDs are stable against fading and spectral shifting. Further, these NQDs can undergo additional shell growth to produce PbSe/CdSe/ZnS core/shell/shell NQDs that represent initial steps toward bright, biocompatible near-infrared optical labels.

Atomic distributions in the gamma-brass structure of the Cu-Zn system: a structural and theoretical study.

The crystal structures, atomic distributions, and theoretical electronic structures of five different Cu5-xZn8+x gamma-brass compounds (x = -0.59(3), -0.31(3), 0.00(3), 0.44(3), and 0.79(3)) are reported with the goal of identifying chemical influences on the observed phase width. These structures have been refined by both neutron and X-ray powder diffraction to obtain accurate crystal chemical parameters. All compounds crystallize in the space group Iz3m (No. 217) (Z = 4), and the unit cell parameters are a = 8.8565(4), 8.8612(5), 8.8664(3), 8.8745(4), and 8.8829(7) A, respectively, for Cu5.59Zn7.41, Cu5.31Zn7.69, Cu5.00Zn8.00, Cu4.56Zn8.44, and Cu4.21Zn8.79. The results indicate specific site substitutions on both sides of the ideal composition "Cu5Zn8". In all cases, the 26-atom cluster building up the -brass structure shows a constant inner [Cu4Zn4] tetrahedral star with compositional variation occurring at the outer octahedron and cuboctahedron. First principles and semiempirical electronic structure calculations using both a COHP and Mulliken population analysis were performed to understand the observed compositional range and to address the "coloring problem" for the site preferences of Cu and Zn atoms for this series of compounds.