bluuebunny/crossref_metadata_2025_split huggingface dataset:
Structure-Mechanical Property Relationships In A Biological Ceramic-Polymer Composite: Nacre
Authors: Gunnison Katie E. , Sarikaya Mehmet , Liu Jun , Aksay Ilhan A. 
Published: March 2011

AbstractThe structure-mechanical property relationships were studied in nacre, a laminated ceramicpolymer biocomposite found in seashell. Four-point bending strength and three-point bend fracture toughness tests were performed, and the results averaged 180 ± 30 MPa and 9 ± 3 MPa·m1/2, respectively, indicating that the composite is many orders of magnitude stronger and tougher than monolithic CaCO3,, which is the primary component of nacre. Fractographic studies conducted with a scanning electron microscope identified two significant toughening mechanisms in the well-known “brick and mortar” microstructure of nacre: (i) sliding of the aragonite platelets and (ii) ligament formation in the organic matrix. These toughening mechanisms allow for high energy absorption and damage tolerance and thereby prevent catastrophic failure of the composite. The structure of the organic matrix and the interfacial structure between the organic and inorganic components were studied with transmission electron microscopy by using both ion milled and ultramicrotomed sections with and without the intact aragonite platelets. We found that the organic matrix is indeed a multilayered composite at the nanometer scale but is thinner (about 100 Å) than reported in the literature. The morphology of the interfacial region between the organic and the inorganic layers suggests the presence of a structural “transitory” region that interlocks the two dissimilar phases.
DNA Binding To Conducting Polymer Films
Authors: Lim Jeong-Ok , Minehan Daniel S. , Kamath M. , Marx Kenneth A. , Tripathy Sukant K. 
Published: March 2011

AbstractThe polycation conducting polymers, oxidized polypyrrole and polyalkylthiophene, possess the ability to form complexes with polyanionic DNA molecules through largely electrostatic interactions. This study demonstrated the solution uptake and binding of 32p radiolabeled DNA by conducting polymer thick films (50–100μm). Polypyrrole (PPy) was synthesized by electrochemical methods and poly(3-hexylthiophene) (PHT) and poly(3-undecylthiophene) (PUT) were synthesized by chemical methods. The DNA binding rates on PPy films were affected by DNA concentration and the oxidation state (measured as conductivity). The DNA kinetics support a diffusion limited model for binding. We measured DNA binding levels onto all three polymer films; PUT, PHT, and PPy. The binding levels increased in the same order as the conductivities of the polymer films. DNA binding onto oxidized PPy film was diminished upon electrochemical reduction. These observations showed, therefore, the binding may be linked with the positive charge sites responsible for conduction in the polymer films.
α-Hemolysin: A Self-Assembling Protein Pore With Potential Applications In The Synthesis of New Materials
Authors: Bayley Hagan , Krishnasastry Musti , Walker Barbara , Kasianowicz John 
Published: March 2011

AbstractA selection of nanoscale membrane pores is being constructed by genetic manipulation of α-hemolysin (αHL), a 33.2 kDa polypeptide secreted by the bacterium Staphylococcus aureus, which can self-assemble into hexameric cylindrical channels -1 to 2 nm In Internal diameter. Ultimately, the new pores will be used to confer novel permeability properties upon materials such as thin films utilizing, for example, monolayer sheets of the hexamer. Recombinant αHL (r-αHL) has now been obtained in multimilligram amounts and purified to homogeneity after overexpression of the αHL gene in Escherichia coli. The properties of r-αHL are closely similar to those of αHL purified from S. aureus. Recent deletion mutagenesis experiments have given us new insight into the assembly mechanism of the pore. Three intermediates have been identified: a membrane-bound monomer; an oligomeric pore precursor; and the hexameric pore itself. Currently, point mutogenesis combined with chemical modification is being used to produce new pores of different internal diameter, with selectivity for the passage of molecules and Ions, and with gating properties (the ability to open and close in response to a physical stimulus, e.g. an electric field or light).
Design and Synthesis of Metals (Tungsten) with Structural Hierarchy for Very High Temperatures
Authors: Welsch G. 
Published: March 2011

AbstractThe strength of metals is limited by the weakest link in their microstructures. At high temperature, the weakest link is usually a grain boundary. Mobile dislocations represent another type of weak link. The strength-limitation by this two-level hierarchy of crystal defects can be minimized through microstructure design. Although the strengthening mechanisms differ for grain boundaries and dislocations, it is possible to design microstructural architectures that strengthen both. The design must take the stress state into consideration that the component will encounter during its later use. Processing strategies can then be devised for the synthesis of such components. It is imperative to stabilize the designed microstructure at high temperature because this will determine how well and how long the material will be able to perform. Design, synthesis and stabilization are discussed in the present paper. The lamp filament wire technology is the basis for a new method that enables the synthesis and stabilization of artificial microstructures in high-temperature materials.
Nonlinear Optical Properties of Hierarchical Systems
Authors: Prasad Paras N. 
Published: March 2011

AbstractNonlinear optical processes provide many of the functions needed for Photonics Technology. Molecular materials are hierarchical systems in which the structure can be controlled from the angstrom level to macroscopic scale to optimize the nonlinear optical properties. In this article basic concepts of nonlinear optics and photonics are reviewed. Multifunctional molecular hierarchical systems are discussed which involves heterostructure design at molecular and bulk levels. For optimization of nonlinear response at the molecular level, guidance from theoretical modeling and study of sequentially built and systematically derivatized structures has proven to be useful. The use of Langmuir-Blodgett film approach to control the order and conformation with monolayer resolution is discussed. Examples of our approach to optimize the response at the bulk level involve the use of orientation control by electric field poling and use of composite heterostructures. An important advancement in heterostructure design is the use of sol-gel processing to make organic-inorganic composites which have successfully exhibited electro-optic modulation and femtosecond Kerr gate operations. Finally, novel photorefractive molecular materials are discussed.
Morphology Control Of The Electrochromic Effect In Tungsten Oxide Thin Films
Authors: Witham H. S. , Chindaudom P. , Messier R. , Vedam K. 
Published: March 2011

AbstractElectrochromic tungsten oxide thin films have been prepared by reactive dc-magnetron sputtering under different deposition conditions. Through the use of scanning electron microscopy, spectroscopic ellipsometry, and electrochromic coloration experiments, the effect of preparation conditions on film morphology and electrochromic properties has been studied. The results of this study are consistent with a previous report which found that tungsten oxide thin films are dominated by a hierarchy of dense columnar regions and less dense void regions. This morphology not only dominates the structure of tungsten oxide thin films but also strongly controls the electrochromic properties. From spectroscopic ellipsometry data and Bruggeman effective medium approximation models presented here, it seems reasonable that a cermet model of absorption can accurately describe the tungsten oxide preparationelectrochromic property relationship.
Hierarchical Structures in Liquid Crystalline Polymers
Authors: Sawyer Linda C. , Jaffe Michael 
Published: March 2011

AbstractIt is well known that the structure of highly oriented liquid crystalline polymers (LCPs) can be characterized by a hierarchical fibrillar structural model. Structure models were developed for the lyotropic aramid fibers and the thermotropic aromatic copolyester fibers during the last two decades showing the existence of fibrillar hierarchies. Hierarchies of structure have also been commonly observed for the biological materials. Concepts learned from the latter are useful in materials science studies today. The nature of the smallest nanostructure that aggregates, the combination of these small structures, typically microfibrils, into larger structures and the interaction of these hierarchical entities are important to understanding their behavior. The architecture of the whole of the polymer or the biological material is a further important variable as is the relation of the process with that architecture. This paper discusses details of the structure of LCPs and draws an analogy between the materials science and biological hierarchies.
A Hierarchically Structured Model Composite: A Tem Study of the Hard Tissue of Red Abalone
Authors: Liu Jun , Sarikaya Mehmet , Aksay Ilhan A. 
Published: March 2011

AbstractThe structure and crystallography of the nacre of red abalone, Haliotis rufescens, was studied by transmission electron microscopy imaging and diffraction. We found that the nacre structure is based upon hierarchical {110} twinning in aragonite with the following organization: (i) first generation twins between platelets having incoherent boundaries, (ii) second generation twins between domains having coherent boundaries within a given platelet, and (iii) nanometer-scale third generation twins within domains. Since the aragonite platelets nucleate and grow as separate crystals, this long-range crystallographic relationship between the inorganic units of a biological hard tissue indicates that the nucleation and growth process of crystals may be mediated by the organic matrix and that the organic template structure may also be long-range ordered. We propose a superlattice structure based on the possible twin variants and suggest that the organic matrix structure, or the arrangement of nucleation sites, is compatible with the superlattice. Multiple tiling based upon this superlattice allows all of the crystallographic and morphological platelet configurations observed in nacre.
Photoluminescence in Porous Silicon: Evidence for the Quantum Confinement Model
Authors: Naylor David L. , Lee Sung B. , Pincenti John C. , Bouma Brett E. 
Published: March 2011

ABSTRACTPhotoluminescence spectra have been measured in porous silicon following electrochemical etching in dilute hydrofluoric acid (HF). The effects of HF concentration during etching on the efficiency and peak wavelength of photoluminescence have been investigated. The effects of temperature between 25°C and 200°C on PL spectra have been recorded. Photoluminescence lifetimes as a function of wavelength have been studied following ultrashort UV photoexcitation. A number of lifetime components in the decay are observed the longest in good agreement over the wavelength range of 500 to 600 nm with a silicon quantum wire model. At longer wavelengths a departure from lifetimes of the wire model is observed and two hypotheses for the discrepancy are presented.
Optical Properties of Microcrystalline Silicon in Oxide Matrix through Partial Oxidation of Anodized Porous Silicon
Authors: Ito Toshimichi , Ohta Toshimichi , Arakaki Osamu , Hiraki Akio 
Published: March 2011

ABSTRACTMicrocrystalline silicon embedded in silicon oxide has been prepared by means of partial oxidation of porous silicon produced anodically from degenerate p-Si wafers. Their optical properties such as absorption coefficients and luminescence have been characterized. Results show blue shifts in absorption and photoluminescence spectra in a visible wavelength region with decreasing size of the microcrystalline Si in the Si oxide matrix. The quantum size effect is discussed as well as possible origins of the observed visible luminescence, including light emission from as-anodized (or H-chemisorbed) porous silicon.
Analysis of the Structure of Light-emitting Porous Silicon by Raman Scattering
Authors: Sui Zhifeng , Leong Patrick P. , Herman Irving P. , Higashi Gregg S. , Temkin Henryk 
Published: March 2011

AbastracRaman spectra from a thick porous silicon film (∼100 μm) that strongly emits in the visible (∼ 6350 Å) at room temperature are obtained. An asymmetric peak with a Raman shift of ∼ 508 - 510 cm−1 and a width of ∼ 40 cm−1 is seen in every spectrum. This Raman feature resembles that of μc-Si, suggesting that the local structure of the porous silicon is a network of interconnected crystalline silicon islands with the island size in the nanometer range., and that the, shape of the islands is more sphere-like than rod-like. The characteristic dimension of the islands in these porous silicon films is estimated to be ∼ 2.5 - 3.0 nm on the basis of an empirical model calculation of phonon confinement.
Luminescent Colloidal SI Suspensions from Porous SI
Authors: Heinrich Julie L. , Curtis Corrine L. , Credo Grace M. , Kavanagh Karen L. , Sailor Michael J. 
Published: March 2011

ABSTRACTA procedure for generating colloidal suspensions of Si exhibiting luminescence, attributed to quantum confinement effects, is described. Samples of n- or p-type Si, that have been electrochemically etched to form porous Si, can be ultrasonically dispersed into methylene chloride, acetonitrile, methanol, toluene, or water solvents, forming a suspension of fine Si particles that luminesce. Transmission electron microscopy analyses show the Si particles to have irregular shapes, with diameters ranging from many microns to nanometers. Luminescent, composite polystyrene/Si films can be made by the addition of polystyrene to a toluene suspension of the Si nanoparticles and casting of the resulting solution.
Radiative Recombination Mechanisms in Porous Silicon
Authors: Gardelis S. , Hamilton B. , Kubiak R. A. , Whall T. E , Parker E. C. H. 
Published: March 2011

ABSTRACTThis paper attempts to address the question of the role played by dimensionality in the observed optical transitions of porus silicon and related material systems. The effect of the degree of porosity on the transition energies, line widths and thermal stability. In addition more subtle effects observed in the thermal behaviour of the spectra which may relate to carrier trapping in a random potential of an imperfect localised system are reported. Comparisons with the spectral behaviour which would be predicted by quantum scale confinement of electronic particles are made.
Near IR and Visible Photoluminescence Studies of Porous Silicon
Authors: Perry C. H. , Lu F. , Namavar F. , Kalkhoran N. M. , Soref R. A. 
Published: March 2011

ABSTRACTWe report photoluminescence emission and photoluminescence excitation studies of porous silicon obtained from p-type Si(111) wafers over the range of 0.9–13.0 eV (∼400–1400 nm). Strong IR emission above and below the bulk silicon band gap at ∼1.09 eV (1135 nm) at 300 K was observed. This luminescence due to intrinsic band-to-band recombination, was found to be enhanced by two orders of magnitude or more over the IR spectrum from an unanodized wafer. The visible luminescence peak that has been attributed to quantum confinement effects in porous silicon was located at ∼1.8 eV (689 nm). We studied the high intensity IR and visible PL in detail as a function of temperature (4–700 K), laser excitation energy (1.96–3.80 eV), and laser power (0.1–1000 mW). The measured shift in the bulk silicon band gap was approximately linear from 300–700 K (∼−0.22 meV/K), while the visible peak red-shifted by ∼− 1.24 meV/K over the same range. Below 300 K they both had comparable temperature dependences. The visible PL was investigated at room temperature in magnetic fields up to 15 T; no discernible shift in the peak position or change in the peak intensity was observed.
Photoluminescence (PL) and Optically Detected Magnetic Resonance (ODMR) Study of Visible Light Emission from Porous Si
Authors: Lane P. A. , Swanson L. S. , Shinar J. , Chumbley S. 
Published: March 2011

ABSTRACTThe photoluminescence (PL) and X-band ODMR of porous Si layers is described and discussed. The layers were prepared by anodizing the (100) face of a Si wafer at 20 mA/cm2 in 20% HF for 5 mai and passively soaking them in 36% HF for up to 10 hrs. The PL was broad and featureless, extending from ˜1.5 to ˜2.1 eV and peaking at 1.68 eV. Its intensity slightly increased upon cooling to 90 K, and then strongly decreased at lower temperatures. A ˜20 G wide asymmetric PL-enhancing ODMR was observed at g ˜2.0031 ±I 0.0009, which could be fit to a sum of two Gaussians. Their g-values were slightly temperature dependent. The ODMR intensity strongly decreased with increasing temperature, and was unobservable above ˜80 K. The results are compared to the optical properties of hydrogenated amorphous Si.
Effect of Surface Coverage on Porous Silicon Photoluminescence: Transmission FTIR Studies
Authors: Robinson M. B. , Dillon A. C. , Haynes D. R. , George S. M. 
Published: March 2011

ABSTRACTTransmission Fourier Transform Infrared (FTIR) Spectroscopy was utilized to monitor the effect of surface coverage on photoluminescent porous silicon. These experiments were performed in situ in an ultrahigh vacuum (UHV) chamber to correlate simultaneously surface coverage and photoluminescence intensity. The goal of these FTIR and photoluminescence studies was to clarify the mechanism of the photoluminescence from porous silicon.
Structural Characterization of Porous Silicon Fabricated by Electrochemical and Chemical Dissolution of Si Wafers
Authors: Miyazaki S. , Yasaka T. , Okamoto K. , Shiba K. , Sakamoto K. , Hirose M. 
Published: March 2011

ABSTRACTThe structure of porous silicon exhibiting efficient visible photoluminescence has been characterized by using Fourier transformed infrared absorption, Raman scattering and x-ray diffraction. It is shown that the lattice spacing in the porous Si layer expands by about 0.3% in the direction perpendicular to the surface and also a partially disordered structure is existing. Electron beam irradiation causes desorption of hydrogen and fluorine bonds which terminate the surface, resulting in the quenching of the visible luminescence. The chemical etching of such layer has led to complete recovery of the luminescence intensity as well as the hydrogen and fluorine bonds termination.
Optical Characteristics of Porous Silicon
Authors: Tischler M. A. , Collins R. T. , Tsang J. C. , Stathis J. H. , Batstone J. L. , Zollner S. 
Published: March 2011

ABSTRACTThe luminescent characteristics of porous silicon are affected by the preparation conditions as well as subsequent treatment. Oxidation, which easily occurs in ambient conditions under illumination, greatly reduces the luminescence efficiency. Vertical inhomogeneity, which is inherent from the preparation procedure, can complicate interpretation of results, especially following treatments which non-uniformly affect the porous material. These results point out the need to pay careful attention to the sample's structure and history, as well as the necessity of controlling this degradation in order to realize practical luminescent devices.
Heating, Laser Irradiation and Passivation Study on the Light-Emitting Porous Silicon
Authors: Chen H. C. , Wang W. , Manjularani K. N. , Snyder L. C. , Zheng X. L. 
Published: March 2011

ABSTRACTFor light-emitting porous Si there has been a severe problem with instability and degradation of the light emission. We report that a stabilization of the emission intensity and the peak energy can be achieved in air by a proper laser irradiation, In-situ photoluminescence measurements were performed to monitor the degradation and stabilization process under different conditions and parameters, such as laser power, laser wavelength and environment (ambient atmosphere of certain gas or ultra high vacuum). We found oxygen is the major cause for the emission degradation in this laser enhanced adsorption process, and the laser heating effect can be excluded. For a comparison we study the reversible thermal heating and quenching process. We also discuss microwave and ECR plasma passivation results.
The Role of Silicon Monohydride and Dihydride in the Photoluminescence of Porous Silicon and Photoluminescence of Porous Silicon Buried Underneath Epitaxial GaP
Authors: Tsai C. , Li K.-H. , Sarathy J. , Jung K. , Shih S. , Hance B. K. , White J. M. , Kwong D.-L. , Sharps P. R. , Timmons M. L. , Venkatasubramanian R. , Hutchby J. A. , Campbell J. C. 
Published: March 2011

ABSTRACTThermal annealing studies of the photoluminescence (PL) intensity and Fourier-transform infrared (FTIR) spectroscopy have been performed concurrently on porous Si. A sharp reduction in the PL intensity is observed for annealing temperatures > 300 °C and this coincides with desorption of hydrogen from the SiH2 surface species. The role of silicon hydride species on the photoluminescence intensity has been studied. The surfaces of luminescent porous Si samples were converted to a predominate SiH termination using a remote H-plasma. The as-passivated samples were then immersed in various concentrations of hydrofluouric solutions to regulate the recovery of SiH2 termination on the surface. Photoluminescence measurements and transmission Fourier-transform infrared spectroscopy have shown that predominant silicon monohydride (SiH) termination results in weak photoluminescence. In contrast, it has been observed that the appearance of silicon dihydride (SiH2) coincides with an increase in the photoluminescence intensity. To achieve electroluminescence it will be beneficial to generate carriers with sufficient energy to populate the states of the quantum-confined Si structures. A viable method to accomplish this is to utilize a wide-bandgap heterojunction injector such as GaP. Toward that end we report the successful formation of porous Si buried underneath GaP islands and we demonstrate that the buried porous Si layer exhibits strong photoluminescence.
Electroluminescent Properties of Porous Silicon Films under Electrochemical Oxidation
Authors: Billat S. , Bsiesy A. , Gaspard F. , Herino R. , Ligeon M. , Muller F. , Romestain R. , Vial J. C. 
Published: March 2011

ABSTRACTBright visible electroluminescence has been evidenced during the anodic oxidation of ptype porous silicon films in aqueous electrolytes. It is shown that in galvanostatic conditions, two very different light emission regimes can be obtained depending on the anodic potential. At low potentials, during the progressive oxidation of the pore walls, the emission spectra present a maximum in the red-orange wavelengths : the emitted intensity increases with the oxidation level, while the maximum is shifted towards shorter wavelengths. This emission vanishes when the anodic potential starts to increase more rapidly. Upon increasing polarizations, a second regime of light emission with a spectrum centered in the green wavelength range is observed. In this case, the spectrum and the intensity are both stable under extended anodization.This last regime which is explained by carrier injection into the anodic oxide is not specific of the porous character of samples. On the contrary, the red-orange emission is shown to be closely related to the photoluminescence properties of the porous layer. A phenomenologic model is proposed, which analyses the electroluminescence characteristic dependence upon the oxidation level in terms of competition between the enhancement of the radiative process and the progressive electrical cut-off of the emitting crystallites.