In this work, we investigated the influence of N-polar wet etching on the properties of nitride-based hexagonal pyramids array (HPA) vertical-injection light emitting diodes (V-LEDs). The cathodeluminescence images showed the randomly distribution of hexagonal pyramids with isolated active regions. The transmission electron microscopy images demonstrated the reduced density of threading dislocations. The IQE was estimated by temperature dependence of photoluminescence, which showed 30% increase for HPA V-LEDs compared with broad area (BA) V-LEDs. The improved extraction efficiency was verified by finite difference time domain simulation, which was 20% higher than that of roughened BA V-LEDs. The electrical properties of HPA V-LEDs were measured by conductive atomic force microscopy (CAFM) measurements. HPA V-LEDs exhibited much lower leakage current due to the improved crystal quality.

The effect of the micro-porous layer (MPL) in polymer electrolyte fuel cells (PEFCs) was studied by a combination of in situ visualization of the liquid water distribution and advanced electrochemical analysis using helox and O2 pulses. Four cells with and without MPLs on the anode and cathode side were tested. Visualization studies showed that the significant changes in performance observed when using an MPL on the cathode side cannot be related to a reduction of the water content in the cathode side diffusion layer (GDL). The helox/O2 pulse analysis indicated that two different mechanisms are responsible for the performance loss without an MPL.

A ZnO single crystal is a native substrate for epitaxial growth of high-quality thin films of ZnO-based Group II-oxides (e.g. ZnO, ZnMgO, ZnCdO) for variety of devices, such as UV and visible-light emitting diodes (LEDs), UV laser diodes and solar-blind UV detectors. Currently, commercially available ZnO single crystal wafers are produced using a hydrothermal technique. The main drawback of hydrothermal growth technique is that the ZnO crystals contain large amounts of alkaline metals, such as Li and K. These alkaline metals are electrically active and hence can be detrimental to device performances. In this paper, results from a recently developed novel growth technique for ZnO single crystal boules are presented. Lithium-free ZnO single crystal boules of up to 1 inch in diameter was demonstrated using the novel technique. Results from crystal growth and materials characterization will be discussed.

Melting gels are silica-based hybrid gels with the curious behavior that they are rigid at room temperature, but soften around 110°C. A typical melting gel is prepared by mixing methyltriethoxysilane (MTES) and dimethyldiethoxysilane (DMDES). MTES has one methyl group substituted for an ethoxy, and DMDES has two substitutions. The methyl groups do not hydrolyze, which limits the network-forming capability of the precursors. To gain insight into the molecular structure of the melting gels, differential scanning calorimetry and oscillatory rheometry studies were performed on melting gels before consolidation. According to oscillatory rheometry, at room temperature, the gels behave as viscous fluids, with a viscous modulus, G″(t,ω0) that is larger than the elastic modulus, G′(t,ω0). As the temperature is decreased, gels continue to behave as viscous fluids, with both moduli increasing with decreasing temperature. At some point, the moduli cross over, and this temperature is recorded as the glass transition temperature Tg. The Tg values obtained from both methods are in excellent agreement. The Tg decreases from -0.3oC to -56oC with an increase in the amount of di-substituted siloxane (DMDES) from 30 to 50 mole %. A decrease of the Tg follows an increase of the number of hydrolytically stable groups, meaning a decrease in the number of oxygen bridges between siloxane chains.

Ultra-thin flakes of layered materials have recently been attracting widespread research interest due to their exotic properties. In this work, we study the optoelectronic response of a hybrid of two such materials – graphene and MoS2. Our devices consist of mechanically exfoliated graphene flakes transferred on top of similarly exfoliated MoS2. The electrical response of the hybrid is studied in the presence of white light. We show that the four-point resistance of graphene is modulated in the presence of light. This effect is observed to be a strong function of gate voltage. We have also extended our studies to CVD (chemical vapor deposition) - grown graphene transferred onto MoS2 which show qualitatively similar features, thereby attesting to the scalability of the device architecture.

Increasing awareness of toxicity of halogenated flame retardants (FRs) has resulted in stringent regulations that require the elimination of their use in plastics. Consequently, there is considerable interest for the development of new varieties of high performance FRs. Here, we report the synthesis of new class of polyphenol based FR additives. Preliminary investigations indicate that these FR materials can possibly work through a combination of radical scavenging and char-formation. The thermal stability and char yield of polyphenol samples were investigated by thermogravimetric analysis (TGA). Thermal characterization indicated that these polymers produce significant amount of carbonaceous char upon combustion.

The aim of this research study is to produce high quality TiO2 nanotube arrays. It is shown that sol-gel electrophoresis is a suitable one to obtain vast-area TiO2 nanotube arrays when nanoporous alumina templates are used. To fabricate TiO2 nanotube arrays, alumina templates were produced via a two-step anodizing by a homemade anodizing cell using high purity phosphoric acid as the electrolyte with aluminum and platinum as electrodes. The semiconductor behavior of these templates can also be employed when producing conducting substrates for the grown TiO2 nanotubes. Stabilized titanium sol was prepared by modified hydrolysis of the titanium precursor using acetic acid. In order to produce TiO2 nanotube arrays, the template pores were filled with the precursor sol by applying a DC electric field. Then, the filled template was heat treated to crystallize the desired TiO2 phase. Scanning electron microscopy of TiO2 nanotube arrays showed that the nanotubes have been deposited in the channels of the nanoporous alumina template. X-ray diffraction data confirmed phase structure and composition of TiO2 nanotube arrays after heat treatment. To reach pure anatase phase, the samples were heated at 320°C and 400°C for two hours. To obtain pure rutile phase, the samples were heated at 320°C and 750°C for two hours.

We report the photochromism (PC) in Eu3+ doped Sr2SnO4. The Sr2SnO4:Eu3+ is sensitive to UV light (λ < 350 nm) and turns to purple color. When visible light (λ= 400-700 nm) is irradiated, the colored Sr2SnO4:Eu3+ is bleached. From the results of absorption spectrum, Sr vacancies in the host lattice and charge transfer transition between Eu3+-O2- is responsible for PC process of Sr2SnO4:Eu3+.

Simultaneous use of neutron diffraction and attenuation based transmission Bragg edge imaging for strain measurements is demonstrated in this paper using the pulse neutron source at Los Alamos National Laboratory. Cylindrical samples made from ferritic steel have been subjected to in-situ elastic loading in tension and torsion. Lattice strains were investigated for both deformation modes by time-of-flight (TOF) neutron diffraction using two detector banks at 2θ of ±90°. At the same time, the transmitted portion of the neutron beam was recorded with the goal to analyze the position and shape of the Bragg edges, using a novel time/energy resolved Microchannel Plate (MCP) detector with pixel size of 55 µm and a 28x28 mm2 field of view. Lattice strains obtained using neutron diffraction indicate that the deformation path (tension versus torsion) has important effect on their evolution and related results are summarized.

The emphasis of this paper is to explain the aspects of the experimental setup and data interpretation associated with neutron Bragg edge transmission technique for obtaining through-thickness averaged strain measurements. Implications of using transmission imaging based strain mapping for samples subjected to deformation under tensile loading (where stress at a given cross-section is constant) versus torsional loading (where stress varies linearly from center to outer radius) are discussed. In the case of samples subjected to tensile loading, analysis of the Bragg edge shifts provides the strain value in the direction of the transmitted beam. Thus, three strain components are measured simultaneously when performing Bragg edge imaging in addition to diffraction using two detector banks. For specimens subjected to pure shear by torsion, the Bragg edge transmission technique cannot readily provide quantitative strain information as the mid-point of the Bragg edge does not shift uniformly due to external loading, but results in a broadening of the Bragg edge. Such information can be used to describe the variation of strain distribution along the transmitted beam direction. Spatially resolved Bragg edge maps will be very helpful to detect d-spacing inhomogeneities within the illuminated volume, which may remain undetected when using diffraction only measurements.

Nanostructured Al3+ doped Ni0.75Zn0.25Fe2-xAlxO4 (x = 0.0,0.2,0.4,0.6,0.8, and 1.0) ferrites were synthesized via wet chemical method. X-ray diffraction, transmission electron microscopy, and magnetization measurements have been used to investigate the structural and magnetic properties of spinel ferrites calcined at 950 °C .With the doping of Al3+, the particle size of Ni0.75Zn0.25Fe2-xAlxO4 first increased to 47 nm at x = 0.4 and then decreased down to 37 nm at x = 1. Saturation magnetization decreased linearly with Al3+ due to magnetic dilution. The coercive field showed an inverse dependence on the particle size of ferrites.

Adding aluminum to propellants, pyrotechnics, and explosives is a common way to boost their energy density. A number of approaches have been investigated that shorten aluminum ignition delay, increase combustion rate, and decrease the tendency of aluminum droplets to agglomerate. Previous work showed that particles of mechanically alloyed Al-Mg powders burn faster than similarly sized particles of pure aluminum. However, preparation of mechanically alloyed powders with particle sizes matching those of fine aluminum used in energetic formulations was not achieved. This work is focused on preparation of mechanically alloyed, composite Al-Mg powders in which both internal structures and particle size distributions are adjusted. Binary powders with compositions in the range of 50 - 90 at. % Al were prepared and characterized. Milling protocol is optimized to prepare equiaxial, micron-scale particles suitable for laboratory evaluations of their oxidation, ignition, and combustion characteristics. Quantitative particle size analyses are done using low-angle laser light scattering. Electron microscopy and x-ray diffraction are used to examine particle morphology and phase makeup, respectively. Combustion of aerosolized powder clouds is studied using a constant volume explosion setup. For all materials, ignition and combustion characteristics are compared to each other and to those of pure Al. Compositions with improved performance (i.e., shorter ignition delay and faster pressurization rate) compared to pure Al are identified.

In this contribution we explored the impact of irradiation intensity on the fluorescence of single quantum dots and ensembles of single quantum dots over time scales much greater than the blinking of the individual quantum dots. We also investigated how blinking and photo enhancement are influenced by photo-oxidation when the intensity of the irradiating laser was increased. Analysis of the emission photographs shows the presence of large time scale individual and ensemble blinking behavior. Analysis of the data indicates that photoinduced fluorescence enhancement occurs even when the inter-dot spacing of the quantum dots precludes the formation of a Coulomb blockade. Additionally, irradiation of ensembles of CdSe/ZnS SQDs at higher source intensities lead to a more accelerated suppression of quantum dots emission via photo-oxidation than for ensembles of closely packed quantum dots.

Natural polymers, used for hydrogel fabrication, are generally bioactive and provide good environment for cell growth and proliferation. However, these polymers have low mechanical strength. Several approaches have been attempted to improve their mechanical properties such as fabrication of interpenetrating polymer network (IPN) and semi-IPN hydrogels, and also addition of a nano sized fibers or nano-particles. The aim of this study was to investigate the feasibility of using naturally derived nano-fillers such as cellulose nanocrystallines to enhance the mechanical properties of hydrogels. Gelatin methacrylate (GelMA) was used as a protein model for preparation of photo-crosslinked hydrogel. The effects of concentrations of photo initiator and cellulose nanocrystallines (CNC) on the characteristics of hydrogels were examined. In vitro studies showed negligible cytotoxic effect of CNC on human osteosarcoma cell growth when using less than 20 mg/ml CNC. Therefore, it is viable to use this nano-filler for biomedical applications. It was found that the compression modulus of gelatin hydrogel was increased 1.5 fold by addition of 10 mg/ml of CNC. These results demonstrate the high potential of using CNC for tissue engineering applications to enhance the mechanical strength of hydrogels.

The transient lattice changes on the Ag(111) crystal due to acoustic wave propagation after excitation with femtosecond pulses was studied by means of time resolved X-ray diffraction. The lattice disorder after UV irradiation is detected by changes of the XRD rocking curve shift, broadening, and total diffraction intensity as a function of time. We have observed a blast force formed within two picoseconds after fs UV irradiation. Experimental results show an initial lattice contraction followed by lattice expansion that propagates with sound velocity.

Here we present new materials obtained using, either fungal or isolated tobacco cells in association with different percentages of carbon nanotubes (CNTs). As a proof of concept, we used either Candida albicans or a non-green Tobacco BY-2 cell line combined with multi-walled CNTs. The electrical, mechanical, and conductivity vs temperature properties for some of these materials have been determined. C. albicans-based tissues have high conductivity and are stable at elevated temperatures. By lowering the CNTs content, we obtained a stable, electrically conductive optical transparent film, though with a relatively high sheet resistance. Further, we produced, using tobacco cells, a material that exhibits good electrical as well as mechanical properties.

Plasmon waveguide resonance (PWR) Raman spectroscopy provides chemical content information with interface or thin film selectivity. Near the plasmon waveguide interface, large increases in the interfacial optical energy density are generated at incident angles where plasmon waveguide resonances are excited. When a polymer of sufficient thickness is deposited on a gold film, the interface acts as a plasmon waveguide and large enhancements in the Raman signal can be achieved. This paper presents calculations to show how polymer thickness and excitation wavelength are predicted to influence PWR Raman spectroscopy measurements. The results show the optical energy density (OED) integrated over the entire polymer film using 785 nm excitation are 1.7× (400 nm film), 2.17× (500 nm film), 2.48× (600 nm film), 3.08× (700 nm film) and 3.62× (800 nm film) higher compared to a 300 nm film. Accounting for the integrated OED and frequency to the fourth power dependence of the Raman scatter, a 532 nm excitation wavelength is predicted to generate the largest PWR Raman signal at the polymer waveguide interface. This work develops a foundation for chemical measurements of numerous devices, such as solar energy capturing devices that utilize conducting metals coated with thin polymer films.

It is frequently observed that as-grown single-walled carbon nanotubes (SWCNTs) contain defects. Controlling the defect density is a key issue for the control of nanotube properties. However, little is known about the influence of the growth conditions on the formation of nanotube defects. In addition, SWCNT samples frequently contain carbonaceous by-products which affect their ensemble properties. Raman spectroscopy is commonly used to characterize both features from the measurement of the defect-induced D band. However, the contribution of each carbonaceous species to the D band is usually not known making it difficult to separately extract the defect density and relative abundance of each. Here, we report on the correlated evolution of the D and G’ bands of SWCNT samples with increasing growth temperature. In the general case, three to four Lorentzian components are required to fit them. Coupled with HRTEM characterization, the low frequency components of the D and G’ can be attributed to the contribution of SWCNTs while high frequency components are associated with defective carbonaceous by-products. The nature of these defective by-products varies with the type of catalysts and with the growth conditions.

There are several significant challenges that must be overcome for PEM fuel cell commercialization such as electrode flooding, carbon corrosion, and significant cost due to the high loading of the platinum catalyst. Thus, a new structure is proposed for the cathode catalyst support consisting of Si/TiOx core/shell nanowires with branched structures, which has the potential to reduce electrode flooding, increase stability, and dramatically reduce the required Pt loading. In this study, Pt-coated Si/TiOx core/shell nanowires with and without branches are compared. The Pt surface area on supports with branch structures was calculated to be more than 4 times larger than on supports without branch structures, while keeping the Pt loading at only about 0.1 mg/cm2 (for the samples with branched structures). SEM, XRD, AES, and TEM were used to characterize the morphologies and structures of the as-prepared samples. Branched Si/TiOx core/shell nanowire structures may be a promising catalyst support to enable commercialization of highly cost-efficient PEM fuel cells and to promote an era of clean energy usage.

Although additive layer manufacturing is well established for rapid prototyping the low throughput and historic costs have prevented mass-scale adoption. The recent development of the RepRap, an open source self-replicating rapid prototyper, has made low-cost 3-D printers readily available to the public at reasonable prices (<$1,000). The RepRap (Prusa Mendell variant) currently prints 3-D objects in a 200x200x140 square millimeters build envelope from acrylonitrile butadiene styrene (ABS) and polylactic acid (PLA). ABS and PLA are both thermoplastics that can be injection-molded, each with their own benefits, as ABS is rigid and durable, while PLA is plant-based and can be recycled and composted. The melting temperature of ABS and PLA enable use in low-cost 3-D printers, as these temperature are low enough to use in melt extrusion in the home, while high enough for prints to retain their shape at average use temperatures. Using 3-D printers to manufacture provides the ability to both change the fill composition by printing voids and fabricate shapes that are impossible to make using tradition methods like injection molding. This allows more complicated shapes to be created while using less material, which could reduce environmental impact.

As the open source 3-D printers continue to evolve and improve in both cost and performance, the potential for economically-viable distributed manufacturing of products increases. Thus, products and components could be customized and printed on-site by individual consumers as needed, reversing the historical trend towards centrally mass-manufactured and shipped products. Distributed manufacturing reduces embodied transportation energy from the distribution of conventional centralized manufacturing, but questions remain concerning the potential for increases in the overall embodied energy of the manufacturing due to reduction in scale. In order to quantify the environmental impact of distributed manufacturing using 3-D printers, a life cycle analysis was performed on a plastic juicer. The energy consumed and emissions produced from conventional large-scale production overseas are compared to experimental measurements on a RepRap producing identical products with ABS and PLA. The results of this LCA are discussed in relation to the environmental impact of distributed manufacturing with 3-D printers and polymer selection for 3-D printing to reduce this impact. The results of this study show that distributed manufacturing uses less energy than conventional manufacturing due to the RepRap's unique ability to reduce fill composition. Distributed manufacturing also has less emissions than conventional manufacturing when using PLA and when using ABS with solar photovoltaic power. The results of this study indicate that open-source additive layer distributed manufacturing is both technically viable and beneficial from an ecological perspective.