Showing papers by "Wright-Patterson Air Force Base published in 2016"

3.8-MV/cm Breakdown Strength of MOVPE-Grown Sn-Doped $\beta $ -Ga 2 O 3 MOSFETs

Abstract: A Sn-doped (100) $\beta $ -Ga2O3 epitaxial layer was grown via metal–organic vapor phase epitaxy onto a single-crystal, Mg-doped semi-insulating (100) $\beta $ -Ga2O3 substrate. Ga2O3-based metal–oxide–semiconductor field-effect transistors with a 2- $\mu \text{m}$ gate length ( $L_{G})$ , 3.4- $\mu \text{m}$ source–drain spacing ( $L_{\textrm {SD}})$ , and 0.6- $\mu \text{m}$ gate–drain spacing ( $L_{\textrm {GD}})$ were fabricated and characterized. Devices were observed to hold a gate-to-drain voltage of 230 V in the OFF-state. The gate-to-drain electric field corresponds to 3.8 MV/cm, which is the highest reported for any transistor and surpassing bulk GaN and SiC theoretical limits. Further performance projections are made based on layout, process, and material optimizations to be considered in future iterations.

Bilayered Biofoam for Highly Efficient Solar Steam Generation.

Abstract: A novel bilayered hybrid biofoam composed of a bacterial nanocellulose (BNC) layer and a reduced graphene oxide (RGO)-filled BNC layer is introduced for highly efficient solar steam generation. The biofoam exhibits a solar thermal efficiency of ≈83% under simulated solar illumination (10 kW m-2 ). The fabrication method introduced here is highly scalable and cost-efficient.

Photomotility of polymers.

Abstract: The demand for soft robots urges the development of new light-responsive materials for remotely powered actuation. Here, Wie et al. show directional motion over centimeter scales using azobenzene-functionalized liquid crystalline polymer films upon continuous radiation from ultraviolet to visible light.

Photoinduced Topographical Feature Development in Blueprinted Azobenzene-Functionalized Liquid Crystalline Elastomers

Abstract: All-optical deformation and recovery of complex topographical features is demonstrated within elastic sheets composed of main-chain type azobenzene-functionalized liquid crystalline elastomers (azo-LCEs). The azo-LCEs are synthesized via an orthogonal, two-step reaction between commercially available LC monomers and n-butylamine. By employing surface alignment, the local orientation of the nematic director is spatially complex (“blueprinted”). Exposing the blueprinted LCE films to light as an actinic stimulus generates a photomechanical response which yields reversible shape changes between 2D and 3D shapes. The deformation of azo-LCEs strongly depends on the azobenzene concentration as well as the network structure (i.e., crosslink density). Blueprinting complex director profiles within azo-LCEs yield reconfigurable elastic sheets that can be addressed both remotely and selectively which may have benefit in a variety of applications in aerospace, medicine, and optics.

Thermal Conductivity of Wurtzite Zinc-Oxide from First-Principles Lattice Dynamics--a Comparative Study with Gallium Nitride

Abstract: Wurtzite Zinc-Oxide (w-ZnO) is a wide bandgap semiconductor that holds promise in power electronics applications, where heat dissipation is of critical importance. However, large discrepancies exist in the literature on the thermal conductivity of w-ZnO. In this paper, we determine the thermal conductivity of w-ZnO using first-principles lattice dynamics and compare it to that of wurtzite Gallium-Nitride (w-GaN)--another important wide bandgap semiconductor with the same crystal structure and similar atomic masses as w-ZnO. However, the thermal conductivity values show large differences (400 W/mK of w-GaN vs. 50 W/mK of w-ZnO at room temperature). It is found that the much lower thermal conductivity of ZnO originates from the smaller phonon group velocities, larger three-phonon scattering phase space and larger anharmonicity. Compared to w-GaN, w-ZnO has a smaller frequency gap in phonon dispersion, which is responsible for the stronger anharmonic phonon scattering, and the weaker interatomic bonds in w-ZnO leads to smaller phonon group velocities. The thermal conductivity of w-ZnO also shows strong size effect with nano-sized grains or structures. The results from this work help identify the cause of large discrepancies in w-ZnO thermal conductivity and will provide in-depth understanding of phonon dynamics for the design of w-ZnO-based electronics.

Hydrogenation of Penta-Graphene Leads to Unexpected Large Improvement in Thermal Conductivity

Abstract: Penta-graphene (PG) has been identified as a novel two-dimensional (2D) material with an intrinsic bandgap, which makes it especially promising for electronics applications. In this work, we use first-principles lattice dynamics and iterative solution of the phonon Boltzmann transport equation (BTE) to determine the thermal conductivity of PG and its more stable derivative, hydrogenated penta-graphene (HPG). As a comparison, we also studied the effect of hydrogenation on graphene thermal conductivity. In contrast to hydrogenation of graphene, which leads to a dramatic decrease in thermal conductivity, HPG shows a notable increase in thermal conductivity, which is much higher than that of PG. Considering the necessity of using the same thickness when comparing thermal conductivity values of different 2D materials, hydrogenation leads to a 63% reduction in thermal conductivity for graphene, while it results in a 76% increase for PG. The high thermal conductivity of HPG makes it more thermally conductive tha...

Localized soft elasticity in liquid crystal elastomers

Abstract: Synthetic approaches to prepare designer materials that localize deformation, by combining rigidity and compliance in a single material, have been widely sought. Bottom-up approaches, such as the self-organization of liquid crystals, offer potential advantages over top-down patterning methods such as photolithographic control of crosslink density, relating to the ease of preparation and fidelity of resolution. Here, we report on the directed self-assembly of materials with spatial and hierarchical variation in mechanical anisotropy. The highly nonlinear mechanical properties of the liquid crystalline elastomers examined here enables strain to be locally reduced >15-fold without introducing compositional variation or other heterogeneities. Each domain (⩾0.01 mm(2)) exhibits anisotropic nonlinear response to load based on the alignment of the molecular orientation with the loading axis. Accordingly, we design monoliths that localize deformation in uniaxial and biaxial tension, shear, bending and crack propagation, and subsequently demonstrate substrates for globally deformable yet locally stiff electronics.

Large Perovskite Grain Growth in Low-Temperature Solution-Processed Planar p-i-n Solar Cells by Sodium Addition.

Abstract: Thin-film p-i-n type planar heterojunction perovskite solar cells have the advantage of full low temperature solution processability and can, therefore, be adopted in roll-to-roll production and flexible devices. One of the main challenges with these devices, however, is the ability to finely control the film morphology during the deposition and crystallization of the perovskite layer. Processes suitable for optimization of the perovskite layer film morphology with large grains are highly desirable for reduced recombination of charge carriers. Here, we show how uniform thin films with micron size perovskite grains can be made through the use of a controlled amount of sodium ions in the precursor solution. Large micrometer-size CH3NH3PbI3 perovskite grains are formed during low-temperature thin-film growth by adding sodium ions to the PbI2 precursor solution in a two-step interdiffusion process. By adjusting additive concentration, film morphologies were optimized and the fabricated p-i-n planar perovskite...

Composite batteries: a simple yet universal approach to 3D printable lithium-ion battery electrodes

Abstract: Printable energy storage is anticipated to facilitate innovation in the manufacture of flexible electronics and soft robotics by enabling direct integration of a power source into a system during the fabrication process. To this end, we have established a universal approach to develop 3D printable, free-standing electrodes with an embedded current collector for high-performance Li-ion batteries. This simple approach utilizes a well-dispersed mixture of active material, carbon nanofibers, and polymer to make castable or printable electrode inks. By tuning the ratios of these components in a series of inks, we have observed the effect each parameter had on the resulting rheological, electrochemical, and mechanical properties. Once properly balanced, free-standing electrodes of three common Li-ion battery active materials (i.e., lithium titanate (Li4Ti5O12), lithium iron phosphate (LiFePO4), and lithium cobalt oxide (LiCoO2)) were prepared, each demonstrating excellent cyclability and rate capability. Finally, electrodes were successfully patterned using a direct ink writing method, and a fully-printed, working electrode plus separator electrode assembly were developed.

Composition determination of β-(AlxGa1−x)2O3layers coherently grown on (010) β-Ga2O3substrates by high-resolution X-ray diffraction

Abstract: We demonstrate X-ray-diffraction-based composition estimation of β-(Al x Ga1− x )2O3 coherently grown on (010) β-Ga2O3. The relation between the strain along the [010] direction and the Al composition of the β-(Al x Ga1− x )2O3 layer was formulated using the stress–strain relationship in the monoclinic system. This formulation allows us to estimate the Al composition using the out-of-plane lattice spacing determined by conventional X-ray ω–2θ measurements. This method was applied to molecular-beam-epitaxy-grown coherent β-(Al x Ga1− x )2O3/Ga2O3 heterostructures, and the Al composition in β-(Al x Ga1− x )2O3 agrees closely with the composition determined directly by atom probe tomography.

Encoding Gaussian curvature in glassy and elastomeric liquid crystal solids

Abstract: We describe shape transitions of thin, solid nematic sheets with smooth, preprogrammed, in-plane director fields patterned across the surface causing spatially inhomogeneous local deformations. A metric description of the local deformations is used to study the intrinsic geometry of the resulting surfaces upon exposure to stimuli such as light and heat. We highlight specific patterns that encode constant Gaussian curvature of prescribed sign and magnitude. We present the first experimental results for such programmed solids, and they qualitatively support theory for both positive and negative Gaussian curvature morphing from flat sheets on stimulation by light or heat. We review logarithmic spiral patterns that generate cone/anti-cone surfaces, and introduce spiral director fields that encode non-localized positive and negative Gaussian curvature on punctured discs, including spherical caps and spherical spindles. Conditions are derived where these cap-like, photomechanically responsive regions can be anchored in inert substrates by designing solutions that ensure compatibility with the geometric constraints imposed by the surrounding media. This integration of such materials is a precondition for their exploitation in new devices. Finally, we consider the radial extension of such director fields to larger sheets using nematic textures defined on annular domains.

Enhanced fluorescence properties of carbon dots in polymer films

Abstract: Carbon dots of small carbon nanoparticles surface-functionalized with 2,2′-(ethylenedioxy)bis(ethylamine) (EDA) were synthesized, and the as-synthesized sample was separated on an aqueous gel column to obtain fractions of the EDA–carbon dots with different fluorescence quantum yields. As already discussed in the literature, the variations in fluorescence performance among the fractions were attributed to the different levels and/or effectiveness of the surface functionalization–passivation in the carbon dots. These fractions, as well as carbon nanoparticles without any deliberate surface functionalization, were dispersed into poly(vinyl alcohol) (PVA) for composite films. In the PVA film matrix, the carbon dots and nanoparticles exhibited much enhanced fluorescence emissions in comparison with their corresponding aqueous solutions. The increased fluorescence quantum yields in the films were determined quantitatively by using a specifically designed and constructed film sample holder in the emission spectrometer. The observed fluorescence decays of the EDA–carbon dots in the films and in the solution were essentially the same, suggesting that the significant enhancement in fluorescence quantum yields from the solution to films is static in nature. Mechanistic implications of the results, including a rationalization in terms of the compression effect on the surface passivation layer (similar to a soft corona) in carbon dots when embedded in the more restrictive film environment resulting in more favorable radiative recombinations of the carbon particle surface-trapped electrons and holes, and also potential technological applications of the brightly fluorescent composite films are highlighted and discussed.

High-speed, three-dimensional tomographic laser-induced incandescence imaging of soot volume fraction in turbulent flames.

Abstract: High-speed, laser-based tomographic imaging of the three-dimensional time evolution of soot volume fraction in turbulent jet diffusion flames is demonstrated to be feasible at rates of 10 kHz or higher. The fundamental output of a burst-mode Nd:YAG laser with 1 J/pulse is utilized for volumetric impulsive heating of soot particles with a laser fluence of 0.1 J/cm2, enabling signal-to-noise ratios of ~100:1 in images of the resulting incandescence. The three-dimensional morphology of the soot distribution is captured with a spatial resolution of 90% in peak values between the two sets. These data establish parameters for successful high-speed, three-dimensional imaging of the soot volume fraction within highly transient combustion environments.

Carbon Catabolite Repression and Impranil Polyurethane Degradation in Pseudomonas protegens Strain Pf-5

Abstract: Polyester polyurethane (PU) coatings are widely used to help protect underlying structural surfaces but are susceptible to biological degradation. PUs are susceptible to degradation by Pseudomonas species, due in part to the degradative activity of secreted hydrolytic enzymes. Microorganisms often respond to environmental cues by secreting enzymes or secondary metabolites to benefit their survival. This study investigated the impact of exposing several Pseudomonas strains to select carbon sources on the degradation of the colloidal polyester polyurethane Impranil DLN (Impranil). The prototypic Pseudomonas protegens strain Pf-5 exhibited Impranil-degrading activities when grown in sodium citrate but not in glucose-containing medium. Glucose also inhibited the induction of Impranil-degrading activity by citrate-fed Pf-5 in a dose-dependent manner. Biochemical and mutational analyses identified two extracellular lipases present in the Pf-5 culture supernatant (PueA and PueB) that were involved in degradation of Impranil. Deletion of the pueA gene reduced Impranil-clearing activities, while pueB deletion exhibited little effect. Removal of both genes was necessary to stop degradation of the polyurethane. Bioinformatic analysis showed that putative Cbr/Hfq/Crc-mediated regulatory elements were present in the intergenic sequences upstream of both pueA and pueB genes. Our results confirmed that both PueA and PueB extracellular enzymes act in concert to degrade Impranil. Furthermore, our data showed that carbon sources in the growth medium directly affected the levels of Impranil-degrading activity but that carbon source effects varied among Pseudomonas strains. This study uncovered an intricate and complicated regulation of P. protegens PU degradation activity controlled by carbon catabolite repression. IMPORTANCE Polyurethane (PU) coatings are commonly used to protect metals from corrosion. Microbiologically induced PU degradation might pose a substantial problem for the integrity of these coatings. Microorganisms from diverse genera, including pseudomonads, possess the ability to degrade PUs via various means. This work identified two extracellular lipases, PueA and PueB, secreted by P. protegens strain Pf-5, to be responsible for the degradation of a colloidal polyester PU, Impranil. This study also revealed that the expression of the degradative activity by strain Pf-5 is controlled by glucose carbon catabolite repression. Furthermore, this study showed that the Impranil-degrading activity of many other Pseudomonas strains could be influenced by different carbon sources. This work shed light on the carbon source regulation of PU degradation activity among pseudomonads and identified the polyurethane lipases in P. protegens.

Directed Self-Assembly of Block Copolymers for High Breakdown Strength Polymer Film Capacitors

Abstract: Emerging needs for fast charge/discharge yet high-power, lightweight, and flexible electronics requires the use of polymer-film-based solid-state capacitors with high energy densities. Fast charge/discharge rates of film capacitors on the order of microseconds are not achievable with slower charging conventional batteries, supercapacitors and related hybrid technologies. However, the current energy densities of polymer film capacitors fall short of rising demand, and could be significantly enhanced by increasing the breakdown strength (EBD) and dielectric permittivity (er) of the polymer films. Co-extruded two-homopolymer component multilayered films have demonstrated much promise in this regard showing higher EBD over that of component polymers. Multilayered films can also help incorporate functional features besides energy storage, such as enhanced optical, mechanical, thermal and barrier properties. In this work, we report accomplishing multilayer, multicomponent block copolymer dielectric films (BCDF)...

Selective two-photon absorptive resonance femtosecond-laser electronic-excitation tagging velocimetry.

Abstract: Selective two-photon absorptive resonance femtosecond-laser electronic-excitation tagging (STARFLEET), a nonseeded ultrafast-laser-based velocimetry technique, is demonstrated in reactive and nonreactive flows. STARFLEET is pumped via a two-photon resonance in N2 using 202.25 nm 100 fs light. STARFLEET greatly reduces the per-pulse energy required (30 μJ/pulse) to generate the signature FLEET emission compared to the conventional FLEET technique (1.1 mJ/pulse). This reduction in laser energy results in less energy deposited in the flow, which allows for reduced flow perturbations (reactive and nonreactive), increased thermometric accuracy, and less severe damage to materials. Velocity measurements conducted in a free jet of N2 and in a premixed flame show good agreement with theoretical velocities, and further demonstrate the significantly less intrusive nature of STARFLEET.

Bacterial Nanocellulose-Based Flexible Surface Enhanced Raman Scattering Substrate

Abstract: Owing to high purity, simple surface chemistry, and 3D nanofibrous structure, biosynthesized bacterial nanocellulose (BNC) is a highly attractive biomaterial for a wide range of applications. Conventional cellulose-based laboratory filter paper, adsorbed with plasmonic nanostructures can be employed as a flexible surface enhanced Raman scattering (SERS) substrate. In this work, a BNC film-based SERS substrate fabricated by gravity-assisted filtration method is reported. The 3D porous structure of BNC facilitates uniform and dense adsorption of plasmonic nanostructures on the surface and in subsurface regions, which results in large SERS enhancement. Furthermore, significantly lower surface roughness of BNC compared to conventional filter paper results in an excellent uniformity of SERS activity across the entire substrate. Harnessing the smooth surface of BNC, it is shown that BNC-based SERS substrate serves as an ideal platform for collection, detection, and recognition of bacteria. The 3D plasmonic BNC composites demonstrated here are highly attractive for a broad range of applications including sensing, catalysis, and energy harvesting.

Theoretical analysis of the combined effects of sulfur vacancies and analyte adsorption on the electronic properties of single-layer MoS2.

Abstract: We report a first-principles theoretical investigation on the electronic structure and electron transport of defective single-layer (SL) MoS2, as well as of corresponding structures adsorbed with benzyl viologen (BV), which was shown to provide improved performance of a field effect transistor. O2 adsorption was included to gain an understanding of the response upon air-exposure. Following analysis of the structure and stability of sulfur single vacancy and line defects in SL MoS2, we investigated the local transport at the adsorbed sites via a transport model that mimics a scanning tunneling spectroscopy experiment. Distinct current-voltage characteristics were indicated for adsorbed oxygen species at a sulfur vacancy. The electronic structures of defective MoS2 indicated the emergence of impurity states in the bandgap due to sulfur defects and oxygen adsorption. Electron transport calculations for the MoS2 surface with an extended defect in a device setting demonstrated that physisorption of BV enhances the output current, while facile chemisorption by O2 upon air-exposure causes degradation of electron transport.

Simultaneous 10 kHz TPIV, OH PLIF, and CH2O PLIF measurements of turbulent flame structure and dynamics

Abstract: Simultaneous 10 kHz repetition-rate tomographic particle image velocimetry, hydroxyl planar laser-induced fluorescence (OH PLIF), and formaldehyde (CH $$_2$$ O) PLIF were used to study the structure and dynamics of turbulent premixed flames. The flames investigated span from the classically defined corrugated flamelet regime to conditions at which broadened and/or broken flamelets are expected. Methods are presented for determining 3D flame topologies from the Mie scattering tomography and for tracking features through space and time using theoretical Lagrangian particles. Substantial broadening of the CH $$_2$$ O region is observed with increasing turbulence intensity. However, OH production remains rapid, and the region of OH and CH $$_2$$ O overlap remains thin. Local flame speeds exceeding three times the laminar flame speed are observed in regions of flame–flame interaction. Furthermore, a method of tracking fluid residence time within the CH $$_2$$ O layer is presented and shows that residence time decreases at higher turbulence intensity despite the broader distribution of the CH $$_2$$ O, indicating an increase in local reaction rate.

Electrically tunable infrared reflector with adjustable bandwidth broadening up to 1100 nm

Abstract: A tunable infrared reflector has been fabricated using polymer stabilized cholesteric liquid crystals containing a negative dielectric, anisotropic liquid crystal and a long and flexible ethylene glycol twin crosslinker. The reflection bandwidth of this prototype smart window can be tuned from 120 nm to an unprecedented 1100 nm in the infrared region upon application of only a small DC electric field, without interfering with the incident visible solar light. Bandwidth broadening was induced using very low operational power with acceptable switching speeds but only takes place in cells with particular gap thicknesses. Calculations reveal that between 8% and 45% of incident solar infrared light can be reflected with a single cell. The infrared reflector can potentially be used as a smart window to maintain the indoor temperature throughout the year, thereby reducing reliance on artificial lighting, heating and cooling, resulting in more than 12% reduction of building operation costs.

Distribution and accumulation of 10 nm silver nanoparticles in maternal tissues and visceral yolk sac of pregnant mice, and a potential effect on embryo growth.

Abstract: We examined the distribution of silver in pregnant mice and embryos/fetuses following intravenous injections of 10 nm silver nanoparticles (AgNPs) or soluble silver nitrate (AgNO3) at dose levels of 0 (citrate buffer control) or 66 µg Ag/mouse to pregnant mice on gestation days (GDs) 7, 8 and 9. Selected maternal tissues and all embryos/fetuses from control, AgNP- and AgNO3-treated groups on GD10 and control and AgNP-treated groups on GD16 were processed for the measurement of silver concentrations, intracellular AgNP localization, histopathology and gross examination of tissue morphology. Inductively-coupled plasma mass spectrometry revealed silver in all examined tissues following either AgNP or AgNO3 treatment, with highest concentrations of silver in maternal liver, spleen and visceral yolk sac (VYS), and lowest concentrations in embryos/fetuses. For VYS, mean silver concentration following AgNO3 treatment (4.87 ng Ag/mg tissue) was approximately two-fold that following AgNP treatment (2.31 ng...

Storage stability of exhaled breath on Tenax TA.

Abstract: Exhaled breath is coming to the forefront of non-invasive biomarker discovery efforts. Concentration of exhaled breath volatile organic compounds (VOCs) on thermal desorption (TD) tubes with subsequent analysis by gas chromatography-mass spectrometry (GC-MS) has dominated this field. As discovery experimentation increases in frequency, the need to evaluate the long-term storage stability of exhaled breath VOCs on thermal desorption adsorbent material is critical. To address this gap, exhaled breath was loaded on Tenax TA thermal desorption tubes and stored at various temperature conditions. 74 VOCs, 56 of which have been previously uncharacterized, were monitored using GC-MS over a period of 31 d. The results suggest that storage of exhaled breath at cold temperatures (4 °C) provides the most consistent retention of exhaled breath VOCs temporally. Samples were determined to be stable up to 14 d across storage conditions prior to gaining or losing 1-2 standard deviations in abundance. Through gene set enrichment analysis (GSEA), certain chemical classes were found to be positively (acids) or negatively (sulfur-containing) enriched temporally. By means of field sample collections, the effect of storage and shipping was found to be similar to those studies preformed in the laboratory at 4 °C. Collectively this study not only provides recommendations for proper storage conditions and storage length, but also illustrates the use of GSEA to exhaled breath based GC-MS data.

Core/Alloyed-Shell Quantum Dot Robust Solid Films with High Optical Gains

Abstract: We report high optical gain from freestanding, optically stable, and mechanically robust films that are loaded with cross-linked CdSe/Cd1–xZnxSe1–ySy core/alloyed shell quantum dots (QD). These solid films display very high net optical gain as high as 650 cm–1 combined with a low pump excitation gain threshold of 44 μJ/cm2. The functionalization of the QDs using short-chain bifunctional cross-linkers not only significantly improves the net optical gain by allowing for a nearly 2-fold increase in QD loading but also provides stable passivation of the QDs which imparts excellent thermal stability, mechanical robustness, and stability under harsh chemical environments. The gain achieved here is up to 3-fold higher than that typically reported for traditional drop-cast QD films. Moreover, stable photoluminescence over long shelf storage time is a distinguished characteristic of the films. The QD films fabricated here span large areas (several cm2), can be readily micropatterned and sustain multiple harsh chem...