Solution processed high aspect ratio ultra-long vertically well-aligned ZnO nano scintillators for potential X-ray imaging applications.

We report the photon (PL), electron (CL) and X-ray (XEL) induced luminescence characteristics of high aspect ratio ultra-long (~ 50 µm) ZnO nanorods (NRs) and discuss the potential for fast X-ray detection based on the consistent and efficient visible emission (~ 580 nm) from ZnO NRs. Nanostructured ZnO scintillators were rearranged to form a vertically well-aligned NR design in order to help light absorption and coupling resulting in luminescent and fast scintillation properties. The design of the nanorod array combines the key advantages of a low-cost growth technique together with environmentally friendly and widely available materials. A low temperature hydrothermal method was adopted to grow ZnO NRs in one cycle growth and their structural, optical and X-ray scintillation properties were investigated. The relatively short (~ 10 µm) ZnO NRs emitting in the near-band-edge region were found to be almost insensitive to X-rays. On the other hand, the higher XEL response of long ZnO NRs, which is a key parameter for evaluation of materials to be used as scintillators for high quality X-ray detection and imaging, along with a decay time response in the order of ns confirmed promising scintillation properties for fast and high-resolution X-ray detector applications.

Vertically Well-Aligned ZnO Nanoscintillator Arrays with Improved Photoluminescence and Scintillation Properties.

ZnO nanoarrays were grown via a low-temperature hydrothermal method. Solutions, each with different additive combinations, were prepared and evaluated. The effects of the additives involved in the growth procedure, i.e., ammonium hydroxide and sodium citrate, were studied in terms of the morphological, optical and scintillation properties of the ZnO nanostructures. Measurement of the nanorod (NR) length, corresponding photoluminescence (PL) and scintillation spectra and their dependence on the additives present in the solution are discussed. ZnO NRs grown on a silica substrate, whose UV transmission was found to be better than glass, showed high-quality structural and optical properties. It was found that the addition of sodium citrate significantly reduced defects and correspondingly increased the intrinsic near-band-edge (NBE) UV emission intensity at ~380 nm. To obtain high-quality nanostructures, samples were annealed in a 10% H2 + 90% N2 atmosphere. The anneal in the forming gas atmosphere enhanced the emission of the UV peak by reducing defects in the nanostructure. NRs are highly tapered towards the end of the structure. The tapering process was monitored using time growth studies, and its effect on PL and reflectance spectra are discussed. A good alpha particle response was obtained for the grown ZnO NRs, confirming its potential to be used as an alpha particle scintillator. After optimizing the reaction parameters, it was concluded that when ammonium hydroxide and sodium citrate were used, vertically well-aligned and long ZnO nanoarrays with highly improved optical and scintillation properties were obtained.

Dampening of positive affect and depression: A meta-analysis of cross-sectional and longitudinal relationships.

Dampening responses to positive affect have been posited to confer vulnerability to depression, but longitudinal studies have not consistently shown dampening tendencies to predict follow-up depression. The cross-sectional, longitudinal, and cross-lagged relationships between dampening and depression were determined using meta-analytic methods. A systematic literature search of the PsycINFO and PubMed databases supplemented by Google Scholar yielded 60 samples suitable for inclusion in the cross-sectional analyses and 12 samples meeting criteria for the longitudinal analyses. In the first meta-analytic study to examine the relationship between dampening and depression, we found dampening to be associated with depression both cross-sectionally (r = .45) and prospectively (r = 0.34). Crucially, dampening at baseline remained a significant predictor of follow-up depression even after controlling for baseline levels of depression in cross-lagged analyses (ß = .09). A bidirectional effect was also found, with baseline levels of depression predicting follow-up tendencies to engage in dampening (r = 0.36). This relationship was again diminished but remained significant after controlling for initial levels of dampening (ß = .14). These results suggest that dampening responses to positive affect are a risk factor for the development of depression and highlight the importance of targeting dampening cognitions in treatment.

Micro Versus Macro Processes: How specific stress exposure impacts sleep, affect, and risk-related behavior on the path to disease in high-risk adults.

BACKGROUND: The stress-to-disease association has been well-accepted for some time. However, the understanding of how stress exposure contributes to psychological disease progression remains unclear. OBJECTIVE: To test the real-time impact of variable stress exposure on risk-related clinical phenomena and affective disease progression in a high-risk sample of active-duty firefighters. METHODS: Participants completed weekly diaries reporting stressful event exposure, affect, sleep, and risk-related and healthy behaviors over six-months and were evaluated for lifetime and current psychiatric disease using clinical interviews before and after the sampling period. RESULTS: Stress exposure impacted clinical phenomena in differing ways. Major personal events and day-to-day hassles predicted health-impairing shifts in sleep and behavior that were associated with increases in symptoms and psychological distress over the 6-month period. In contrast, highly aversive incidents predicted greater adaptive behaviors that were uniquely predictive of symptom decreases over the six-month period. CONCLUSION: These findings shed new light on stress-to-disease processes, demonstrating how variable stress exposure influences critical shifts in behavior and sleep, contributing to psychological adjustment of firefighters over time. These data suggest practical ways to monitor risk in high-risk samples (e.g., monitoring sleep latency) and offer avenues for further explication of disease processes in real time.

Understanding Emotion Inflexibility in Risk for Affective Disease: Integrating Current Research and Finding a Path Forward.

Emotion-related disorders (e.g., depression, anxiety, stress, eating, substance and some personality disorders) include some of the most common, burdensome, and costly diseases worldwide. Central to many, if not all of these disorders, may be patterns of rigid or inflexible emotion responses. Indeed, theorists point to emotion in-flexibility as a potential cause or maintaining factor in emotion-related diseases. Despite the increasing prominence of emotion inflexibility in theories of affective disease, a comprehensive review of the developing empirical literature has not yet been conducted. Accordingly, this review will examine the three dominant lines of inquiry assessing emotion flexibility. These include: (1) the capacity to use and vary deliberate emotion regulation strategies, (2) the context sensitivity of spontaneous emotional responses, and (3) flexibility in the appraisal of emotional events and experiences. Moreover, current evidence suggests that each of these three lines of research may converge to suggest the interplay of two key biological dimensions in emotion inflexibility, threat sensitivity, and cognitive control, known to be impaired in patients with affective disorders. In short, this developing body of work suggests a path by which future research could explicate and even exploit the ties between emotion inflexibility and affective disease, contributing to the development of improved models of risk, assessment, and intervention, with broad implications for psychological health.

Design and fabrication of distributed Bragg reflector multilayers for dynamic pressure sensing.

A novel 2D-surface shock pressure sensor is designed and tested based on 1D-Photonic Crystal, i.e., Distributed Bragg Reflector Multilayer (DBR/ML) structures. The fast opto-mechanical response of these structures to changes in layer thicknesses and refractive indices are ideally suited for dynamic pressure sensing. They offer the potential to minimize acoustic impedance mismatch between the material layers, and most importantly, the potential to monitor both temporal and spatial (lateral) variations during shock compression. In this feasibility study, different materials and device designs are investigated to identify material/device design combinations with optimum response to dynamic loading. Structural and material effects are studied in terms of spectral and mechanical properties, structure stability, and the ease of fabrication process. Structures comprising of different numbers of SiO1.5/SiO1.7 bilayer stacks are modeled, and fabricated. A 10-bilayer structure placed under a dynamic compressive load of ~7.2 GPa, exhibits a blueshift of 29 nm with a response time of ~5 ns which is well within the shock pressure rise time measured with PDV velocimetry. This promising result successfully demonstrates the feasibility of the specifically designed DBR/ML structure as a dynamic pressure sensor.

Low-temperature hydrothermally grown 100 µm vertically well-aligned ultralong and ultradense ZnO nanorod arrays with improved PL property.

The hydrothermal synthesis of ZnO nanorods (NRs) has been investigated using ammonium hydroxide and polyethyleneimine as additives to the conventional nitrate based synthesis route, to obtain thin-films of well-aligned, ultradense and ultralong nanostructures. ZnO NRs longer than 60 µm were obtained in a one-cycle growth run and rod lengths ~ 100 µm by a two-cycle growth. The lengths of the rods were distributed uniformly across the substrate in all samples and highly dense NR arrays were observed. These conditions were obtained by a careful review of the nucleation and growth kinetics for this material system, such that the supersaturation of the solution was only relieved by precipitation on and in the presence of crystalline ZnO, and by the exploitation of a second growth phase due to the chelating behave of PEI and the products of HMTA. Also, the growth behavior was correlated to the solution pH values. The structural and optical data were found to be supportive of the growth conditions. The photoluminescence (PL) spectra from as-grown ultralong ZnO NRs exhibited a strong broad (580-625 nm) visible emission peak. However, annealing in a forming gas atmosphere at 623K (350°C) revealed a PL spectrum with a significantly decreased visible emission and an increased near band gap UV emission at 379 nm. Thus, the mechanisms associated with ammonium hydroxide and PEI addition provide a simple route for synthesizing ultralong and dense arrays of ZnO NRs at low temperature i.e. 368K (95°C).

Asymmetrical optical microcavity structures for dynamic pressure sensing: design, fabrication, validation.

Optical microcavity (OMC) structures have spectral properties that are directly related to their physical dimensions and material refractive indices. Their intrinsically fast optical response to mechanically-induced changes in these parameters makes OMCs uniquely suited for dynamic sensing when paired with a suitably fast streak camera and spectrograph. Various designs and processes of fabrication for asymmetrical OMC (AOMC) structures were investigated to optimize and assess their feasibility for dynamic sensing. Structural and material effects were studied in terms of spectral properties, structure stabilities and fabrication process. From this study, it was shown that an AOMC structure with a SiO2 cavity layer and Ag mirror layers, fabricated with thin adhesion Al2O3 layers exhibited the best structural stability and spectral properties. Under dynamic compressive loading of ~4 GPa, the structure exhibited a blueshift of 22 nm and a temporal response time of < 3.3 ns, thus demonstrating the potential of AOMC based dynamic pressure sensing.

Synthesis and characterization of a BaGdF5:Tb glass ceramic as a nanocomposite scintillator for x-ray imaging.

Transparent glass ceramics with embedded light-emitting nanocrystals show great potential as low-cost nanocomposite scintillators in comparison to single crystal and transparent ceramic scintillators. In this study, cubic structure BaGdF5:Tb nanocrystals embedded in an aluminosilicate glass matrix are reported for potential high performance MeV imaging applications. Scintillator samples with systematically varied compositions were prepared by a simple conventional melt-quenching method followed by annealing. Optical, structural and scintillation properties were characterized to guide the design and optimization of selected material systems, aiming at the development of a system with higher crystal volume and larger crystal size for improved luminosity. It is observed that enhanced scintillation performance was achieved by tuning the glass matrix composition and using GdF3 in the raw materials, which served as a nucleation agent. A 26% improvement in light output was observed from a BaGdF5:Tb glass ceramic with addition of GdF3.

Plasmon enhancement mechanism for the upconversion processes in NaYF4:Yb(3+),Er(3+) nanoparticles: Maxwell versus Förster.

Rare-earth activated upconversion materials are receiving renewed attention for their potential applications in bioimaging and solar energy conversion. To enhance the upconversion efficiency, surface plasmon has been employed but the reported enhancements vary widely and the exact enhancement mechanisms are not clearly understood. In this study, we synthesized upconversion nanoparticles (UCNPs) coated with amphiphilic polymer which makes UCNPs water soluble and negatively charged. We then designed and fabricated a silver nanograting on which three monolayers of UCNPs were deposited by polyelectrolyte-mediated layer-by-layer deposition technique. The final structures exhibited surface plasmon resonance at the absorption wavelength of UCNP. The green and red photoluminescence intensity of UCNPs on nanograting was up to 16 and 39 times higher than the reference sample deposited on flat silver film, respectively. A thorough analysis of rate equations showed that the enhancement was due entirely to absorption enhancement in the strong excitation regime, while the enhancement of both absorption and Förster energy transfer contribute in the weak excitation regime. The Purcell factor was found to be small and unimportant because the fast nonradiative decay dominates the relaxation process. From the experimentally observed enhancements, we concluded 3.1× and 1.7× enhancements for absorption and Förster energy transfer, respectively. This study clearly shows the plasmon enhancement mechanism and its excitation power dependence. It provides the basis for comparison of the enhancements of various plasmonic UCNP systems in the literature. It also lays the foundation for rational design of optical plasmonic structures for upconversion enhancement.

Synthesis and Luminescence Properties of Transparent Nanocrystalline GdF3:Tb Glass-Ceramic Scintillator.

Transparent glass-ceramic containing rare-earth doped halide nanocrystals exhibits enhanced luminescence performance. In this study, a glass-ceramic with Tb doped gadolinium fluoride nanocrystals embedded in an aluminosilicate glass matrix is investigated for X-ray imaging applications. The nanocrystalline glass-ceramic scintillator was prepared by a melt-quench method followed by an anneal. The GdF3:Tb nanocrystals precipitated within the oxide glass matrix during the processing and their luminescence and scintillation properties were investigated. In this nanocomposite scintillator system, the incorporation of high atomic number Gd compound into the glass matrix increases the X-ray stopping power of the glass scintillator, and effective energy transfer between Gd3+ and Tb3+ ions in the nanocrystals enhances the scintillation efficiency.

CdTe quantum dots and polymer nanocomposites for x-ray scintillation and imaging.

Investigations are reported on the x-ray scintillation and imaging application of CdTe quantum dots (QDs) and their polymer nanocomposites. Aqueous CdTe QDs with emissions ranging between 510 and 680 nm were prepared and incorporated into polyvinyl alcohol or polymethyl methacrylate polymer matrices. The x-ray luminescent properties were evaluated and a resolution of 5 lines∕mm was obtained from the nanocomposite films. Additionally, the fast decay time, nonafterglow, and superior spectral match to conventional charge coupled devices, show that CdTe QD nanocomposites have high promise for x-ray imaging applications.

Dispersion characteristics of silicon nanorod based carpet cloaks.

A wide range of transformation media designed with conformal mapping are currently being studied extensively due to their favorable properties: isotropy, moderate index requirements, low loss and broad bandwidth. For optical frequency operation, the transformation media are commonly fabricated on high index semiconductor thin films. These 2D implementations, however, inevitably introduces waveguide dispersion, which affects the bandwidth and loss behavior. In this paper, for carpet cloaks implemented by a silicon nanorod array, we have confirmed that waveguide dispersion limits the bandwidth of the transformation medium by direct visualizing the cut-off conditions with near-field scanning optical microscopy (NSOM). Furthermore, we have experimentally demonstrated the extension of cut-off wavelength by depositing a conformal dielectric layer. This study illustrates the constraints on the 2D transformation media imposed by the waveguide dispersion and suggests a general technique to tune and modify their optical properties.

Composite organic-inorganic butterfly scales: production of photonic structures with atomic layer deposition.

Recent advances in the photonics and optics industries have produced great demand for ever more sophisticated optical devices, such as photonic crystals. However, photonic crystals are notoriously difficult to manufacture. Increasingly, therefore, researchers have turned towards naturally occurring photonic structures for inspiration and a wide variety of elaborate techniques have been attempted to copy and harness biological processes to manufacture artificial photonic structures. Here, we describe a simple, direct process for producing an artificial photonic device by using a naturally occurring structure from the wings of the butterfly Papilio blumei as a template and low-temperature atomic layer deposition of TiO2 to create a faithful cast of the structure. The optical properties of the organic-inorganic diffraction structures produced are assessed by normal-incidence specular reflectance and found to be well described by multilayer computation method using a two-dimensional photonic crystal model. Depending on the structural integrity of the initially sealed scale, it was found possible not only to replicate the outer but also the inner and more complex surfaces of the structure, each resulting in distinct multicolor optical behavior as revealed by experimental and theoretical data. In this paper, we also explore tailoring the process to design composite skeleton architectures with desired optical properties and integrated multifunctional (mechanical, thermal, optical, fluidic) properties.

Chemical reduction of three-dimensional silica micro-assemblies into microporous silicon replicas.

The carbothermal reduction of silica into silicon requires the use of temperatures well above the silicon melting point (> or =2,000 degrees C). Solid silicon has recently been generated directly from silica at much lower temperatures (< or =850 degrees C) via electrochemical reduction in molten salts. However, the silicon products of such electrochemical reduction did not retain the microscale morphology of the starting silica reactants. Here we demonstrate a low-temperature (650 degrees C) magnesiothermic reduction process for converting three-dimensional nanostructured silica micro-assemblies into microporous nanocrystalline silicon replicas. The intricate nanostructured silica microshells (frustules) of diatoms (unicellular algae) were converted into co-continuous, nanocrystalline mixtures of silicon and magnesia by reaction with magnesium gas. Selective magnesia dissolution then yielded an interconnected network of silicon nanocrystals that retained the starting three-dimensional frustule morphology. The silicon replicas possessed a high specific surface area (>500 m(2) g(-1)), and contained a significant population of micropores (< or =20 A). The silicon replicas were photoluminescent, and exhibited rapid changes in impedance upon exposure to gaseous nitric oxide (suggesting a possible application in microscale gas sensing). This process enables the syntheses of microporous nanocrystalline silicon micro-assemblies with multifarious three-dimensional shapes inherited from biological or synthetic silica templates for sensor, electronic, optical or biomedical applications.

Total internal reflection photonic crystal prism.

An integrated total internal reflection prism is demonstrated that generates a transversely localized evanescent wave along the boundary between a photonic crystal and an etched out trench. The reflection can be described by either the odd symmetry of the Bloch wave or a tangential momentum matching condition. In addition, the Bloch wave propagates through the photonic crystal in a negative refraction regime, which manages diffraction within the prism. A device with three input channels has been fabricated and tested that illuminates different regions of the reflection interface. The reflected wave is then sampled by a photonic wire array, where the individual channels are resolved. Heterodyne near field scanning optical microscopy is used to characterize the spatial phase variation of the evanescent wave and its decay constant.

Density-controlled growth of aligned ZnO nanowires sharing a common contact: a simple, low-cost, and mask-free technique for large-scale applications.

An effective, low cost, simple, and mask-free pathway is demonstrated for achieving density control of the aligned ZnO nanowires grown for large-scale applications. By a slight variation of the thickness of the thermally evaporated gold catalyst film, a significant change in the density of aligned ZnO nanowires has been controlled. The growth processes of the nanowires on an Al(0.5)Ga(0.5)N substrate has been studied based on the wetting behavior of gold catalyst with or without source vapor, and the results classify the growth processes into three categories: separated dots initiated growth, continuous layer initiated growth, and scattered particle initiated growth. This study presents an approach for growing aligned nanowire arrays on a ceramic substrate with the simultaneous formation of a continuous conducting electrode at the roots, which is important for device applications, such as field emission.

Large-size liftable inverted-nanobowl sheets as reusable masks for nanolithiography.

A low-cost procedure is introduced for fabricating large-area, liftable, ordered TiO2 nanobowl sheets. The sheet is made using the template of self-assembled polystyrene spheres, followed by atomic layer deposition (ALD), ion milling, and etching. By introducing a thin organic layer between the nanobowls and the substrate, the whole sheet can be lifted-off in full size. The dimension of the holes at the bottom of the nanobowls is controlled by additional ALD; thus, the sheet has been applied as a reusable mask for producing nanodot patterns with designed sizes. This technique demonstrates a simple and economic nanolithiography approach for producing various designed patterns without using a clean room, and it has a great potential for scale-up, mass production, and commercial applications.

Growth of uniformly aligned ZnO nanowire heterojunction arrays on GaN, AlN, and Al0.5Ga0.5N substrates.

Vertically aligned single-crystal ZnO nanorods have been successfully fabricated on semiconducting GaN, Al0.5Ga0.5N, and AlN substrates through a vapor-liquid-solid process. Near-perfect alignment was observed for all substrates without lateral growth. Room-temperature photoluminescence measurements revealed a strong luminescence peak at approximately 378 nm. This work demonstrates the possibility of growing heterojunction arrays of ZnO nanorods on AlxGa1-xN, which has a tunable band gap from 3.44 to 6.20 eV by changing the Al composition from 0 to 1, and opens a new channel for building vertically aligned heterojunction device arrays with tunable optical properties and the realization of a new class of nanoheterojunction devices.

A photonic crystal superlattice based on triangular lattice.

A two-dimensional superlattice photonic crystal structure is investigated in which the holes in adjacent rows of a triangular lattice alternate between two different radii. The superimposition of a superlattice on a triangular lattice is shown to reduce the photonic bandgap, introduce band splitting, and change the dispersion contours so that dramatic effects are seen in the propagation, refraction, and dispersion properties of the structure. For single mode propagation, the superlattice shows regions of both positive and negative refraction as well as refraction at normal incidence. The physical mechanisms responsible for these effects are directly related to Brillouin Zone folding effects on the triangular lattice that lowers the lattice symmetry and introduces anisotropy in the lattice.