bluuebunny/crossref_metadata_2025 huggingface dataset:
A Study of the Interface Between SnF2 Particles and Air in Stannous Fluoride Heated in Air
Authors: Denes Georges , Laou E.
Published: April 2011

Stannous fluoride SnF2 and mixed fluorides containing divalent tin are the highest performance fluoride ion conductors known to date. Since the fluoride ion mobility increases with temperature, and that these materials are sensitive to hydrolysis of the Sn-F bonds and oxidation of tin(II) to tin(IV) when heated, there is a significant chance of material degradation on heating. Surface oxidation provides a thin coating of highly refractory Sn02, which passivates against further degradation. The details and limits of this protection in SnF2 have been studied and are presented here.
Direct Determination of Grain Boundary Atomic Structure in SrTiO3
Authors: McGibbon M.M. , Browning N.D. , McGibbon A.J. , Chisholm M.F. , Pennycook S.J.
Published: April 2011

In the electroceramic SrTiO3 the grain boundary atomic structure governs a variety of electrical properties such as non-linear I-V characteristics. An understanding of this atomic structure-property relationship for individual grain boundaries requires a technique which probes both composition and chemical bonding on an atomic scale. Atomic structure models for [001] tilt boundaries in SrTiO3 bicrystals have been determined directly from experimental data, by combining high-resolution Z-contrast imaging to locate the cation columns at the boundary, with simultaneous electron energy loss spectroscopy to examine light element coordination at atomic resolution. In this paper we compare and contrast the grain boundary structure models of symmetric and asymmetric boundaries in SrTiO3.
Investigation of Interfaces in Ceramics by Electron Spectroscopic Imaging
Authors: Mayer J. , Plitzko J. , Steinbach F. , Kohl H.
Published: April 2011

Electron Spectroscopic Imaging (ESI) in an energy filtering TEM is a new analytical technique which allows one to obtain two-dimensional elemental distribution images. In the present paper the detection limits of the new technique will be discussed. Si3N4ceramics with different thicknesses of the amorphous oxide grain boundary layers have been chosen as a model system. Quantitative results have been obtained on three different energy filtering instruments and will be compared with the results of theoretical calculations. The application of ESI for the mapping of near edge fine structure will be discussed.
The Morphology of Cu Clusters on SrTiO3(001) at Initial Stages of Metal Film Growth
Authors: Liang Y. , Carroll D. L. , Bonnell D. A.
Published: April 2011

Copper overlayers deposited on nearly stoichiometric SrTiO3(001) have been investigated with scanning tunneling microscopy (STM). Vapor deposition of Cu on a SrTiO3(001) surface at ambient temperature leads to the formation of three dimensional islands (clusters). The distribution of Cu islands appears to be inhomogeneous with two characteristic morphologies. In regions with a low density of Cu islands the Cu was always associated with step edges or defect sites. In regions with a high density of Cu islands the islands exhibit a random but nearly close packed morphology. The variation of Cu island density is indicative of diffusion of Cu clusters on the SrTiO3(001) surface. Diffusion was further confirmed by annealing the Cu/SrTiO3 at elevated temperatures yielding agglomeration of Cu clusters.
Pressure-Induced Migration of Ferroelectric Interfaces
Authors: Gordon Alex , Dorfman Simon
Published: April 2011

The study of the migration of the first-order paraelectric-ferroelectric interfaces in BaTiO3 under the hydrostatic pressure is provided in the framework of the time-dependent Ginzburg-Landau theory. The analytical solution describing the interphase boundary is applied for the calculations of its width and velocity at different pressures. The calculations are based on the experimental data for BaTiO3. The parameters of the power law of the temperature and pressure dependences of the interface velocity under the temperature and pressure were obtained. Using the BaTiO3 example we illustrate the ability of the suggested approach in the description of the kinetics of the first-order ferroelectric phase transition in perovskites.
The Structure of Spinel/Oxide Reaction Fronts During Spinel-Forming Solid State Reactions
Authors: Hesse D. , Scholz R. , Senz S. , Sieber H. , Werner P. , Heydenreich J.
Published: April 2011

A series of spinels were grown by topotaxial solid state reaction on MgO(001) and sapphire(11.2) substrates. The structure of the various spinel/oxide reaction fronts was investigated by cross-sectional high resolution electron microscopy and other methods. While for extremely low misfit the reaction front is completely coherent, different interfacial defects form in other cases, depending on sign and amount of the spinel/oxide lattice misfit. For a large positive misfit, a network of misfit dislocations occured all running along <100<, with Burgers vectors of types a/2[101] and a/2[011] pointing out of the interface. The perpendicular Burgers vector component along [001] permits these dislocations to glide in order to cope with the advancing reaction front, avoiding kinetically unfavourable climb processes. The latter have, however, been observed in negative misfit, where the interfacial dislocations run along <110>, with their Burgers vectors lying in the interface plane. At the sapphire/MgAl2O4 front the structure is completely different. Here the h.c.p.-type oxygen sublattice of sapphire is reconstructed into the f.c.c-type oxygen sublattice of the spinel, which requires a tilt of the MgAl2O4 lattice and the formation of interfacial ledges.
Oxidation of Coo Into Co3O4 in CoO-ZrO2(CaO) and Co1-xNixO-ZrO2(CaO) Lamellar Eutectic Structures
Authors: Lafleurielle M. , Dhalenne G. , Millot F. , Revcolevschi A.
Published: April 2011

Unidirectional solidification experiments were carried out at the eutectic composition of the systems CoO-ZrO2(CaO) and Co1-xNixO-ZrO2(CaO) yielding materials made of regularly aligned lamellar structures. These eutectic samples were submitted to a high temperature chemical oxidation treatment under oxygen, in conditions where CoO should be oxidized into Co3O4. The study of partially oxidized samples shows that Co3O4 is formed by two mechanisms which are presented together with the corresponding kinetics.
XPS Studies at the Interface of Ti/AIN Ceramic
Authors: Shen Dian Hong , Bao Changlin , Lu Hua , Lin Zhangda
Published: April 2011

The photoelectron spectra for titanium deposition on a AIN ceramic substrate at room temperature have been measured. Before deposition, the binding energies of Ols and Al 2p show that the substrate contained oxygen as a major impurity and a top layer of degeneration was formed. After deposition of a small amount of titanium, it was found that the Nls separated into two peaks(396.5 eV and 402 eV) and Ti 2p corresponded to the oxide state. With an increase of titanium coverage, the Ti 2p peak shifted toward a lower energy. The peak at 402 eV dominated at a titanium coverage of 0.9 nm, showing the nitrogen-oxygen binding character. For a titanium coverage of 3.0 nm, a new peak at 406 eV was formed. These results suggest that during deposition of Ti onto the AIN substrate which has a top hydrated alumina layer, some of nitrogen atoms tend to be bound with oxygen and to form an interfacial oxynitride layer between the metallic titanium and substrate.
Chemical Bonding, Structure, and Morphology of Mg/∝-Al2O3 and MGO / ∝-Al2O3 Interfaces
Authors: Yu Yan , Lad R.J.
Published: April 2011

Ultra-thin films of Mg and MgO were grown on ∝-Al2O3 (1012) surfaces (r-cut sapphire) and studied using reflection high energy electron diffraction (RHEED) and x-ray photoelectron spectroscopy (XPS). When Mg is deposited at 30°C in ultra-high vacuum (UHV), the first monolayer of Mg atoms chemically bonds to the oxygen anions of the sapphire surface. At Mg coverages above a monolayer, a polycrystalline metallic Mg overlayer is formed. Annealing above 250°C in UHV causes the metallic Mg to desorb from the surface. However, annealing above 250°C in 10−6 torr O2 produces a polycrystalline MgO film. This MgO film recrystallizes after annealing in O2 at 900°C for 60 minutes and exhibits a crystallographic orientation of MgO (100) // ∝-Al2O3 (1012). RHEED indicates that the recrystallized MgO layer dewets the sapphire surface and forms islands. When Mg is deposited at 30°C in 10−6 torr O2, a polycrystalline MgO layer is created. This layer also becomes recrystallized and dewets the sapphire surface after extended annealing in O2 at 900°C. No evidence for a MgAl2O4 spinel phase was observed.
The Adsorption of Liquid and Vapor Water on TiO2 (110) Surfaces: The Role of Defects
Authors: Wang Li-Qiong , Baer Don R. , Engelhard Mark. H.
Published: April 2011

The adsorption of liquid and vapor water on defective and nearly defect-free TiO2 (110) surfaces has been examined using X-ray photoelectron spectroscopy (XPS) and ultraviolet photoemission spectroscopy (UPS). The study has focused on examining electronic defects as created in vacuum and after exposure to both liquid and vapor water. The defective surfaces were prepared by electron-beam exposure and Ar+ bombardment. With exposure up to 104 L low vapor pressure (<10−5 Torr) water to defective surfaces, little or no changes on Ti 3d defect intensities were observed, in agreement with the previous studies. However, defect intensities were greatly reduced after exposing defective surfaces to ∼ 108 L higher vapor pressure (0.2 - 0.6 Torr) water for 5 min. More significantly, XPS and UPS spectra showed that electron-beam induced defects were completely removed upon liquid water exposure, while defects created by Ar+ bombardment were only partially removed. It was found that surface defects created by Ar+ bombardment were removed more readily than sub-surface defects. For a nearly defect-free surface, water coverages were ∼ 0.02 ML at 104 L exposure to low vapor pressure water, ∼ 0.07 ML at 108 L exposure to higher vapor pressure water, and ∼ 0.125 ML with liquid water exposure, respectively.
Magnetic Properties of Heisenberg Antiferromagnetic EuTe/PbTe Superlattices
Authors: Chen J.J. , Dresselhaus G. , Dresselhaus M.S. , Springholz G. , Bauer G.
Published: April 2011

Bulk EuTe is a type II Heisenberg antiferromagnet (AF2) with a single magnetic phase transition temperature at 9.6 K. However, for several isolated EuTe (111) monolayers (MLs) as can be achieved in a superlattice (SL) structure, both ferrimagnetic-like and antiferromagnetic-like phase transitions can take place, depending on the SLs configuration. The temperature-dependent magnetization M(T) of such SLs has been studied near the transition temperature (Tc) by SQUID magnetometry. The functional forms of M(T) at T ≤ Tc can be described by mean-field theory for SLs with 3, 4 and 5 EuTe MLs per SL cell. The magnetic transition temperatures obtained by mean-field analysis, using bulk exchange coupling values, are in close agreement with observed Tc values for SLs with 2, 3, 4 and 5 EuTe MLs. The qualitative behavior of the surface specific heat can be deduced from M(T) data for SLs with three EuTe monolayers.
Effects of Landau Level Coupling on the Magnetic Band Structure for Electrons in a Two-Dimensional Hexagonal Lattice
Authors: Kühn O. , Fessatidis V. , Cui H.L. , Horing N.J.M.
Published: April 2011

The single-particle energy spectrum of two-dimensional electrons in a lateral surface superlattice of hexagonal symmetry, subject to a normal uniform magnetic field is investigated. Special attention is focused on the coupling of different Landau levels due to the periodic potential. It is shown that the inclusion of this coupling causes strong modifications of the spectrum compared with the limit of no coupling investigated previously. It is found that the importance of Landau level coupling is mainly determined by the relation between the potential amplitude and the cyclotron energy, as well as the Landau level index.
Quantum Confinement in Coated Semiconductor Nano-Particles
Authors: Zhou H.S. , Sasahara H. , Honma I. , Komiyama H. , Sasabe H. , Haus J. W.
Published: April 2011

We previously reported the synthesis and some properties of heterostructure nanoparticles, i.e. CdS coated with PbS (CdS/PbS). We report further characterization and investigation of the infrared photoluminescence (PL) of the CdS and coated CdS/PbS nanoparticles. This allows us to provide a more stringent test of the quantum confinement model. We consider the energy shift of the PL peak and the variation of the PL intensity correlated with the predictions of the quantum confinement model. The experimental results can be explained by this model of coated semiconductor nanoparticles.
Controlled Recrystallization of Hematite from Two Highly Different Phases of Ferric Trihydroxide
Authors: Denes Georges , Kabro P. , Madamba M.C.
Published: April 2011

Hematite Fe2O3, is a semiconductor, and its electrical properties are highly sensitive to the preparation methods and purity. Its precursors can be hydrated ferric oxides; however, these are usually obtained from poorly defined ferric gels, which are obtained by hydrolysis of an aqueous solution of a ferric salt by a base. We have designed a novel synthetic route to ferric hydroxide, by reaction of a peroxo-compound with an aqueous solution of a ferrous salt, which involves simultaneous oxidation of Fe(ll) to Fe(lll), and hydrolysis, in the same reaction process. The two kinds of ferric hydroxide are highly different, however, both give hematite by dehydration/recrystallization, however, the way this occurs for each is different.
Er-Implanted Porous Silicon: a Novel Material for Si-Based Infrared LEDs
Authors: Namavar Fereydoon , Lu F. , Perry C.H. , Cremins A. , Kalkhoran N.M. , Daly J.T. , Soref R.A.
Published: April 2011

We have demonstrated a strong, room-temperature, 1.54 μm emission from erbium-implanted at 190 keV into red-emitting porous silicon. Luminescence data showed that the intensity of infrared (IR) emission from Er implanted porous Si annealed at ≤ 650°C, was a few orders of magnitude stronger than Er implanted quartz produced under identical conditions, and was almost comparable to IR emission from In0.53Ga0.47As material which is used for commercial IR light-emitting diodes (LEDs).

The strong IR emission (much higher than Er in quartz) and the weak temperature dependency of Er in porous Si, which is similar to Er3+ in wide-bandgap semiconductors, suggests that Er is not in SiO2 or Si with bulk properties but, may be confined in Si light-emitting nanostructures. Porous Si is a good substrate for rare earth elements because: 1) a high concentration of optically active Er3+ can be obtained by implanting at about 200 keV, 2) porous Si and bulk Si are transparent to 1.54 μm emission therefore, device fabrication is simplified, and 3) although the external quantum efficiency of visible light from porous Si is compromised because of self-absorption, it can be used to pump Er3+.
Light Controlled Photoluminescence Relaxation in Porous Silicon
Authors: Czaputa R. , Fritzl R. , Popitsch A.
Published: April 2011

We report results of photoluminescence (PL), FTIR and ESR investigations on nanoporous silicon (PS) where a reversible PL intensity relaxation effect in the chemically oxidised material is observed. To be activated the effect needs, however, additional preparation steps including light irradiation and ageing in ambient atmosphere. After illumination with visible light, the PL intensity is remarkably diminished. However it recovers in the dark within the time scale of minutes to hours under ambient atmosphere at room temperature. This cycle can be repeated several times. We show that the variation of the PL intensity is anticorrelated to an ESR signal attributed to silicon dangling bonds. From the IR spectrum, however, no significant change of the pore surface chemical structure can be observed during a cycle. Therefore we conclude that the variation of the PL intensity is rather controlled by a metastable change in the number of dangling bond centers than by modification of the surface chemistry in the porous silicon system.
Investigation Of Efficiency Improvement on Silicon Solar Cells Due to Porous Layers
Authors: Sun Gregory , Li Yuxin , Lu Yicheng , Khan Babar , Tompa Gary S.
Published: April 2011

Observation of light emission from porous Si has demonstrated that the optical properties of Si can be drastically altered by the quantum size effects. We have investigated the improvement of absorption properties of Si material by forming a porous Si layer. Shallow-junction commercial crystalline as well as polycrystalline Si solar cells without anti-reflective coatings have been processed into porous Si solar cells by a wet chemical etching technique. Our best results have demonstrated more than 15% improvement in short-circuit current with no change in open-circuit voltage. The performance of the porous Si solar cells has been found to be sensitive to the porous layer thickness. The efficiency can be reduced when the porous layer is relatively deep, presumably due to the penetration of pores through the shallow junction. We believe porous Si can be optimized for photovoltaic applications by properly controlling its porosity and thickness.
On the Origin of the Electrically-Induced Spectral Shift of Porous Silicon Photo- and Electro- Luminescence
Authors: Bsiesy A. , Hory M.A. , Gaspard F. , Herino R. , Ligeon M. , Muller F. , Romestain R. , Vial J.C.
Published: April 2011

Experimental results showing two electrically-induced phenomena, namely the voltage-tunable electroluminescence (VTEL) and the voltage-induced quenching of porous silicon photoluminescence(QPL) are given. In both cases, a spectral shift as large as 300 nm can be recorded for an external bias variation of only 0.5V. This spectral shift is characterised by a blue-shift of the whole EL line in the case of the VTEL whereas it results from a progressive and selective quenching starting by the low-energy part of the luminescence line in the case of the QPL experiments. The origin of this spectral shift is discussed in relation with an electrically-induced selective carrier injection into the silicon nanocrystallites accompanied with an enhancement of the non-radiative recombination which might take place by an Auger relaxation process. Finally, it is shown that a partial oxidation of the porous silicon layer leads to a complete loss of the selectivity of these two phenomena. This result is qualitatively discussed by considering the voltage drop distribution between the substrate and the silicon nanocrystallites. The voltage drops are modified by the growth of the oxide layer on the nanocrystallite surface leading to a modification of the energy barriers at the crystallite boundaries.
Large Grain Size and High Deposition Rate for Microcrystalline Silicon Prepared by VHF-GD
Authors: Hapke P. , Finger F. , Luysberg M. , Carius R. , Wagner H.
Published: April 2011

The growth mechanism and material properties of -type µc-Si:H prepared with plasma enhanced chemical vapour deposition in the very high frequency range is investigated. By increasing the plasma excitation frequency the grain size, deposition rate and Hall mobility can be simultaneously increased without having to adjust other deposition parameters in particular the temperature. This effect is explained by an enhanced selective etching of amorphous tissue and grain boundary regions together with a sufficient supply of growth species at high frequency plasmas.
Spin Resonance Studies on free Electrons and Defects in Microcrystalline Silicon
Authors: Malten C. , Finger F. , Hapke P. , Kulessa T. , Walker C. , Carius R. , Flückiger R. , Wagner H.
Published: April 2011

The effect of micro-doping, defect creation, and non-steady state occupation through optical transitions on the electron spin resonance signals found in undoped and weakly doped microcrystalline silicon with a high degree of crystallinity is investigated. The experimental results are in agreement with the assignment of the resonance at g=1.9983 to conduction electrons in the crystalline grains and the resonanccs around g=2.0052 to dangling bonds in the remaining amorphous phase and at the grain boundaries. The simultaneous presence of both resonances can result from a large conduction band offset between crystalline grains and grain boundaries or the amorphous phase. The presence of conduction electron spin resonance in compensated and even p-type material points also to potential fluctuations. Free electrons in interconnected crystalline grains are in agreement with the weakly activated transport found in µc-Si:H at low temperatures.
Microcrystalline Cubic Boron Nitride/Amorphous Hydrogenated Boron Nitride Mixed Phase Thin Films
Authors: Lin Shu-Han , Feldman Bernard J.
Published: April 2011

Transparent and insulating thin films have been grown by the plasma decomposition of B2H6, NH3, and H2, at a substrate temperature of 250°C. From chemical composition, transmission electron microscopy, infrared absorption, and optical absorption measurements, the thin films are determined to be a mixed phase of crystalline cubic boron nitride and amorphous hydrogenated boron nitride. Also, the films have significantly more boron than nitrogen, a large concentration of hydrogen, a very large bandgap, strong infrared aborption due to both hexagonal boron nitride and boron icosahedra, and good adhesion to various substrates.
Electrical Characteristics of Ultra-Thin Multi-Layers of Poly-Si and Silicon Dioxide
Authors: Chan Kevin K. , Lee Young H. , Stanis Carol L.
Published: April 2011

Rough-surface polycrystalline silicon films have been used for fabrication of electrically erasable-programmable read-only-memories (EEPROM). Silicon-Rich Oxides (SRO), also known as semi-insulating polycrystalline silicon (SIPOS), have been recommended for use as electron injectors. The advantage of SRO as injectors is that both the bottom and top surfaces of the film can be used to obtain high field enhancement. An ultra-thin multi-layer structure of silicon and silicon dioxide has been fabricated by low pressure chemical vapor deposition (LPCVD). High resolution TEM shows alternating layers of 50 A thick SiO2 and polycrystalline Si (grain size ∼ 80 A) films were deposited and MOS capacitors using this multilayer dielectric were studied to understand their electrical characteristics. Both I-V and C-V measurements show that the Fowler-Nordheim tunneling current is proportional to the number of polycrystalline layers. The “turn-on” voltage of the tunneling current is determined by the thickness of first SiO2 layer, and the effective dielectric constant varies from 3.9 to 9.1, proportional to the number of poly-Si layers within the multi-layer structure (MLS). For a 350 Å stack (7 layers), the interface trap density, Dit, was 3×1010 traps/cm2, and the effective dielectric constant was roughly 9.
Effect of the Hetero-Interface on the Photoresponse of A-Si/C-Si Solar Cells
Authors: Jagannathan B. , Yi J. , Wallace R. , Anderson W. A.
Published: April 2011

Heterojunction solar cells were fabricated by glow discharge deposition of amorphous silicon on p-type crystalline silicon resulting in a n/i/p structure. Dark I-V-T data on the devices show that the conduction in the forward bias regime (<0.4 volts) for better devices agrees with a multi-tunnelling-capture-emission process. The photoresponse was evaluated (under 100 mW/cm2) for various a-Si thicknesses and substrate resistivities. Spectral response tests showed an increased low wavelength absorption as the a-Si thickness was decreased. The blue response of the devices have better fill-factors than the red response indicating defects at the interface. Further, I-V-T and C-V measurements also corroborate the presence of defect states which seem to prevent the spread of the depletion region in crystalline silicon. The photoresponse was found to be very sensitive to the interface defects and the fill-factors ranged from 0.42, for the sample in which the depletion region had spread, to 0.1 in those where the depletion region had been reduced in thickness by the interface states.
Scalable Fabrication and Optical Characterization of Nm Si Structures
Authors: Zaidi Saleem H. , Chu An-Shyang , Brueck S. R. J.
Published: April 2011

Observations of efficient room temperature photoluminescence (PL) from porous Si have generated a great deal of interest in the optical properties of nm-scale Si structures. The stochastic character of porous-Si fabrication results in a distribution of crystal sizes and shapes. We report on a scalable (to large areas) and manufacturable (to high volumes) fabrication technology for uniform, nm-linewidth Si structures providing an important testbed for controlled studies of these optical properties. Large areas ( ∼ 1 cm2) of extreme sub-μm structures (to ∼ 5 nm) are re-producibly fabricated. Both walls (1-D confinement) and wires (2-D confinement) are reported. The fabrication process includes: interferometric lithography, highly anisotropic KOH etching, and structure dependent oxidation. For the walls, nearly perfect <111> crystal planes form the sidewalls and very high width/depth aspect ratios (> 50) have been achieved. Raman scattering results on the walls demonstrate three regimes: 1) lineshapes and cross sections similar to bulk Si for line widths, W > 200 nm; 2) electromagnetic resonance enhancement of the cross section ( to - 100x) for W from 50-200 nm; and 3) highly asymmetric lineshapes and splittings from W < 30 nm. Photoluminescence is observed for the thinnest samples (W < 10 nm) and is as intense as that observed from porous Si with a spectral linewidth ∼ 50 % smaller than that of porous Si.
Luminescence Properties of Periodic Disordered Thin Layer GaAs/AlAs Superlattices
Authors: Zhang Ruth Y. A. , Strozier J. , Horton C. , Ignatiev A.
Published: April 2011

Disordered thin layer GaAs/AlAs superlattices with various disorder periods have been grown using MBE. The disorder was introduced by varying the thickness of GaAs and AlAs layers in the growth direction in various specific but randomly generated finite disorder sequences. The photoluminescence spectra of these disordered superlattice samples showed a sharp peak at the high energy side and a broad peak at the low energy side. The temperature, excitation, and disorder sequence dependence of the photoluminescence spectra indicates that the sharp peak is due to the pseudo-direct exciton emission, and the broad peak is strongly related to the localized states induced by disorder. In addition, the results demonstrate that the luminescence intensity of disordered superlattices can be improved by up to two orders of magnitude over that of ordered short period superlattices. Finally, we propose a kinetic model for the state population and find that the photoluminescence spectra can be well described by this model.
Isolation and Characterization of Two Esr–Active La@C82 Isomers
Authors: Yamamoto Kazunori , Funasaka Hideyuki , Takahashi Takeshi , Akasaka Takeshi , Suzuki Toshiyasu , Maruyama Yusei
Published: April 2011

Two abundant ESR-active lantanofullerene species have been isolated with an efficient HPLC method. Each of isolated species was well identified as a stable metallofullerene molecule. Both mass and ET 9 spectra confirmed that they were La@C82 with 1.159-G and 0.836-G 139La hyperfine splittings. Visible and near-IR absorption spectra, cyclic and differential pulse voltammograms, and 13 C hyperfine splitting structures in ESR spectra for the samples showed features quite different from each other. Their differences were discussed on the basis of their structures.
Properties of Novel Fullerene Tubule Structures: A Computational Study
Authors: Sinnott Susan B. , White Carter T. , Brenner Donald W.
Published: April 2011

Theoretical Young's moduli have been estimated for several fullerene carbon tubule fibers along the tubule axis. These results indicate that carbon fibers composed of nested tubules could have a modulus nearly 1.4 times that of a conventional graphite whisker. The results also indicate that the modulus of the tubule fiber can be further increased by decreasing the distance between the close-packed tubules in the fiber. This is true for both nested and single-shell tubules, but is only significant for the latter. Taking advantage of this property of single-shell tubules, we have examined a hypothetical tubule-diamond composite that, if it could be produced, would yield a modulus greater than diamond while at the same time stabilizing the fiber against shear distortions.
Atomic Force Microscopy of Carbon Nanotubes and Nanoparticles
Authors: Li Ping , Satftler Klaus
Published: April 2011

Very hot carbon vapor was deposited on highly-oriented pyrolytic graphite (HOPG) and on sapphire substrates. Using atomic force microscopy, the obtained carbon structures are analyzed. It is found that carbon nanotubes and cones had formed on the graphite substrate but not on sapphire. Instead, spherical-type nanoparticles are analyzed on sapphire. A particle with double-conical shape is found on the graphite substrate. The two observed cone angles 19° and 60° are explained by perfect network closure of sectors of curled graphene sheets.
Control of Heteroepitaxy in Sol-Gel Derived LiNbO3 Thin Layers
Authors: Clem P.G. , Payne D.A.
Published: April 2011

Lithium niobáte (LiNDO3) in single crystal form is useful for the fabrication of acoustooptic and active waveguide devices. In this paper, the feasibility of integrating LiNbO3 thin layers on sapphire is reported. The performance of signal modulators, surface acoustic wave devices, and second harmonic generators relies on control of crystallographic orientation, so a thin-layer deposition method must meet high standards of crystallographic perfection and optical quality. Solution deposition of lithium niobium ethoxide was evaluated on (110) and (006) sapphire substrates for heteroepitaxy. Atomic force microscopy was used to determine the development of microstructure during the transition from the amorphous to crystalline state. Slab waveguides were formed and evaluated for optical quality and loss. Optical losses in the TEo mode of 500nm (110) LiNbO3 thin layers were determined to be 6 dB/cm. Preliminary results are given for the heteroepitaxial growth of α-Ga2O3 buffer layers.
Growth and Properties of PbTiO3/PLT Heterostructures
Authors: Kim Yeongkwan , Han Y.H. , Erbil A. , Boatner L.A.
Published: April 2011

Ferroelectric superlattice structures composed of three-dimensionally epitaxial PbTiO3 and PLT thin films have been successfully grown on SrTiO3 substrates by metalorganic chemical vapor deposition. The modulation structures were confirmed by θ-2θ XRD, and the excellent in-plane orientational relationship between the superlattice film and the substrate by (100), (110), and (111)-pole figures. The φ-scans through the (110) and (111) reflections were used as additional evidence for three-dimensional epitaxy. The substrate dependence of the epitaxial orientation of PLT and PbTiO3 single-layered thin films was investigated. PbTiO3 thin films with very high crystalline perfection can be successfully grown on KTaO3 substrates.
A Low Temperature Vaporization of Pb(THD)2 with NH3
Authors: Lee June Key , Lee Wan In , Desu Seshu B.
Published: April 2011

Pb(THD)2 is one of the most popular CVD precursors for lead. It was found in this study that the volatility of Pb(THD)2 can be considerably improved by using ammonia as a carrier gas. An adduct between Pb(THD)2 and NH3 is generated in-situ, when the NH3 carrier gas is passed through the bubbler containing Pb(THD)2. The ammoniated precursor was isolated and its molecular structure was characterized by elemental analysis, infrared and NM.R. PbO thin films could be prepared with this precursor system while maintaining the bubbler temperature at 110° C, which is considerably lower than the temperature used with the Pb(THD)2 precursor in the commonly used carrier gas.
Preparation and Use of Mosaic Target for the Deposition of Ferroelectric Bismuth Titanate Thin Films
Authors: Ghosh P.K. , Azimi M.E.
Published: April 2011

Ion beam sputter deposition using a Kaufman source is a well-established technique for fabricating thin films. This method has the capability for yielding smooth surfaces and depositing uniform films over a large area. We have used this technique, in the past, with a 5″ diameter target to deposit smooth ferroelectric bismuth titanate (Bi4Ti3O12) films. One of the major obstacles was fabricating the large target with the desired composition. To reduce cost by simplifying the target preparation process we developed a ‘mosaic’ target to deposit ferroelectric Bi4Ti3O12films. Films were deposited at different substrate temperatures. Measurements show that the post-annealing of the deposited films results in measurable ferroelectric properties and enhanced low field dielectric behavior in the films. The discussion will include the target fabrication, the observed properties of the resultant films, and the effect of the deposition and post-annealing temperatures on the film properties.
Preparation of PbZrxTi1−xO3 Films on Si Substrates Using SrTiO3 Buffer Layers
Authors: Tokumitsu Eisuke , Itani Kensuke , Moon Bum-Ki , Ishiwara Hiroshi
Published: April 2011

We report the preparation of PbZrxTi1−xO3 (PZT) films on Si substrates with a SrTiO3 (STO) buffer layer. STO buffer layers and PZT films were formed on Si substrates by the electron-beam assisted vacuum evaporation technique and sol-gel technique, respectively. By evaporating a thin (8nm) metal Sr layer prior to the STO deposition, which deoxidizes the SiO2 layer at the Si surface, (100)- and (111)-oriented STO thin films can be grown on Si(100) and (111) substrates, respectively. It is shown that a strongly (100)-oriented PZT film is grown on STO(100)/Si(100), whereas a strongly (111)-oriented PZT film is obtained on STO(111)/Si(111). It is also found that the STO buffer layer remains intact even after the PZT deposition. Secondary ion mass spectrometry (SIMS) analysis showed that the STO barrier layer was effective in preventing diffusion of Pb into the Si substrate.
Growth of Ferroelectric PLT Thin Films on Various Single Crystal Substrates
Authors: Kang Y.M. , Ku J.K. , Baik S.
Published: April 2011

Ferroelectric Pb1−xLaxTi1−x/4(x = 0 ∼ 0.28) epitaxial thin films were prepared on MgO(001), SrTiO3(001), and LaAlO3(001) single crystalline substrates using pulsed laser deposition. The change in lattice constants of PLT films by La concentration, x, was investigated systematically for each substrate. RBS studies revealed that the composition of PLT films is consistent with that of the targets. Lattice constants, degree of c-axis orientation, crystal quality of the PLT films were characterized by symmetric and asymmetric X-ray scans. The strain which occurred during the cubic to tetragonal phase transition seemed to be the major factor determining the c-axis orientation configuration, i.e., 90° domain structure and the crystal quality of PLT films.
Anisotropic Dynamic Strain Sensors Based on Ferroelectric PZT Films
Authors: Bakirov A.A. , Sviridov E.V. , Dudkevich V.P.
Published: April 2011

The present paper deals with anisotropie dynamic strain sensors based on polycrystalline Pb(Zr0.53Ti0.45W0.01Cd0.01)O3 films. The films were deposited on ceramic substrates designed to operate at radiofrequencies. The planar interdigitated Al electrodes were deposited on the film surfaces. To pole the films the sawtooth monopolar pulses of the amplitude ranging from 5 · 107 to 8 · 107 V/m were used during the heating of the films from room temperature to 300 °C and the following cooling down to room temperature (Tc = 360 °C).
Processing Path and The Evolution of Crystallinity in Rapidly Solidified Glassy Alloys
Authors: Otooni M. A.
Published: April 2011

Atomic transport properties in rapidly solidified glassy alloys are not fully understood in spite of much experimental data on the subject. Nevertheless, a thorough understanding of these properties is of fundamental importance when studying the practical limits of the application of rapidly solidified glassy alloys.

Several attempts are made in this paper to explain our experimental results on the mechanical flow properties near the transition temperature, Tr. These results are interpreted in the context of possible operating mechanisms such as diffusion and/or relaxation processes. Some inferences have been made on the nature of viscosity and its fluctuation near the transition temperature. The linear temperature dependence of the viscosity near the transition temperature is explained by invoking the free-volume concept for the viscosity of the glassy alloys.

These results are used to provide appropriate data for the selection of feasible processing paths which will produce alloys with advanced thermomechanical properties.
Rapidly Solidified Nb Base Alloys With High Carbide Content
Authors: Sheftel Elena N. , Kaputkin Dmitry E.
Published: April 2011

Nb base alloys with high Zr and C contents possess high strengths at up to 1400°C, but their low-temperature ductilities are unacceptably low due to nearly continuous rough eutectic carbide net along grain boundaries. The net has been broken by rapid solidification (RS). The structure and microhardness of these alloys after RS as well as after consequent compacting and/or annealing have been studied.
Modelling of Two-Dimensional Grain Growth and Crystallization
Authors: Krakow William
Published: April 2011

PC based computer programs have been developed to simulate twodimensional microstructural grain growth either by nucleation or transformation from an amorphous phase. By controlling the parameters of the rate of grain growth, nucleation rate, depletion zone size, and changes in these latter two variables most grain growth conditions can be achieved in practice. Several cases have been explored which include: constant nucleation rates, both increasing and decreasing rates, and the instantaneous saturation of nucleation sites. The evolution of domain sizes can be investigated which is dependent on the effective rate of nucleation during various stages of grain growth. Real time graphics displays are present the instantaneous growth parameters and their changes as well as information on fractional area growth.
Hall-Petch Basis for Assessing Alloy Strengthening
Authors: Armstrong Ronald W. , Douthwaite R. Michael
Published: April 2011

The Hall-Petch relation σ = σo + kl−½, provides for the separate consideration of friction stress strengthening within the polycrystal grain volumes through σo and grain boundary strengthening through the product of the microstructural stress intensity k and the reciprocal square root of the grain diameter l Smaller grain diameters are normally obtained at higher alloy contents as illustrated for yield strength results reported for different face-centered-cubic Al-Mg alloys. Results on Al-Li alloy give an interesting example of substantial grain boundary strengthening that is associated with reduced ductility of the material. More complete results reported for the Cu-Al system, allow an evaluation of the strengthening component dependencies on alloy composition, in particular, connecting with a predicted square root of grain boundary obstacle stress in k. The much studied Cu-Zn alloys bring out subtle changes in σo and k
Hall-Petch Analysis of Yield, Flow and Fracturing
Authors: Armstrong Ronald W.
Published: April 2011

The explanation for the grain size dependence of the polycrystal yield stress or cleavage stress of steel, investigated in complementary studies by Hall and Petch, is rooted in dislocation pile-up theory first described by Eshelby, Frank and Nabarro just a step away from previous theoretical work on the stress concentrating properties of cracks. Beginning from Cottrelllocking of dislocations at grain boundaries by carbon that is responsible for the pronounced yield point behavior and the grain size dependence of the yield strength of steel, the same concepts have been shown to apply for the complete stress/strain behavior of steel and other materials. Other mechanical properties such as fatigue strength show a similar dependence on grain size. A notable application of such grain size based considerations has been to the ductile-brittle transition and fracture toughness properties of steel and related materials — because refinement of grain size both strengthens a material and improves its fracture toughness. Early results for the polycrystal hardness dependence on grain size gave evidence that the strength benefits of grain size refinement ought to extend to ultrafine grain sizes.
Microsrtuctures and Electrical Characterization of CVD-W and Mo Films as Contacts in Photocells
Authors: Gesheva Kostadinka A. , Stoyanov G. I. , Stefanov R. , Vlakhov E. S.
Published: April 2011

Tungsten and molybdenum hexacarbonyls were used as precursors in chemical vapour deposition process for preparation of W and Mo thin films. Pyrolitical decomposition of these precursors proceeds at temperatures of 250–400°C. Thin films with thicknesses in the range of 0,02–1 μm were deposited on different substrates - bare or covered with CdTe glass, and monocrystalline Si. Microstructural studies performed by Reflection High Energy Electron Diffraction (RHEED) method showed that films deposited tend to grow textured. This is discussed as probably due to differences in the growth rate for various crystal planes. The sheet resistances of the as-deposited W and Mo films are in the range of 20–30 Ω/□ for thicknesses of 0.15 μm. After thermal annealing the resistance of W films drops to about 2 Ω/□ and for Mo films to about 9Ω/□. Decreasing in the resistivity of the films is tightly connected with the decreasing in the impurities concentration. These impurities are considered to be in the base of the observed behaviour of the temperature dependence of the electrical resistance of the films. The CVD-W and Mo films are studied as back contacts on CdTe layer in CdS/CdTe photocells. In the paper some preliminary results are presented for the sheet and contact resistances when CVD W and Mo films are deposited at lower temperatures on the surface of CdTe layers, deposited by closespaced sublimation method. The thin film materials, produced by CVD technology look promising with respect to the required high deposition rates and extremely wide deposition areas in the mass production of solar cells.
Tin Dioxide Films Prepared by a Rapid Chemical Vapor Deposition Method
Authors: Nishino Junichi , Ohshio Shigeo , Saitoh Hidetoshi , Kamata Kiichiro
Published: April 2011

A rapid chemical vapor deposition method was studied for the preparation of tin dioxide films. The films were prepared under atmospheric-pressure on fused quartz and Si substrates at 200° to 600°C using bis(2,4-pentanedionato)tin as a source material. SnO2 films obtained at 300° to 600°C had polycrystalline rutile structures, while the film deposited at 200°C was amorphous judging from the XRD patterns. The deposition rate of SnO2 increased with increasing substrate temperature and vaporization temperature of the source material. The maximum deposition rate was 440 nm min−1. The transmittance of SnO2, deposited at Ts = 600°C, was more than 70% in the wavelength range 400–800 nm.
Chemical Vapor Deposited β-SiC for Optics Applications
Authors: Goela Jitendra S. , Pickering Michael A. , Taylor Raymond L.
Published: April 2011

The fabrication, properties and optics applications of transparent and opaque Chemical Vapor Deposited (CVD) β-SiC are reviewed. CVD-SiC is fabricated by the pyrolysis of methyltrichlorosilane, in excess H2, in a low-pressure CVD reactor. The CVD process has been successfully scaled to produce monolithic SiC parts of diameter up to 1.5 m and thickness 2.5 cm. The characterization of CVD-SiC for important physical, optical, mechanical and thermal properties indicate that it is a superior material for optics applications. CVD-SiC properties are compared with those of the other candidate mirror and window materials. SiC process/property relationships are discussed, emphasizing the differences in process conditions, microstructure, and properties between transparent and opaque CVD-SiC.
Frictional Forces in Thin Liquid Films
Authors: Urbakh Michael , Klafter Joseph , Daikhin Leonid
Published: April 2011

Shear thinning of confined liquids is studied in the framework of the time dependent Ginzburg-Landau equation coupled to a shear-induced velocity field. Scaling relationship between the effective viscosity and the shear rate is analytically derived with an exponent which depends on the velocity profile within the liquid and on the boundary conditions. The velocity profile is derived in the limit of low shear rate. Thinning is observed for shear rates faster than typical liquid relaxation rates. Relevance to existing systems and predictions amenable to new experiments are discussed.
Surface Crystallization and Thin Film Melting in Normal Alkanes
Authors: Wu X. Z. , Shag H. H. , Ocko B. M. , Deutsch M. , Sinha S. K. , Kim M. W. , King H. E. , Sirota E. B.
Published: April 2011

Normal alkanes of carbon number n>14 exhibit surface crystallization at their liquid-vapor interface. This has been investigated with x-ray reflectivity, grazing incidence scattering and surface tension measurements. The structure and thermodynamics of the surface layer is consistent with a monolayer of the bulk rotator phase occurring at the surface above the bulk melting temperature. On the other hand, thin films of alkanes on SiO2, exhibit a reduction of the melting temperature. The surface crystalline phase is observed for carbon number n>14. The vanishing of surface phase for small n may be due to a transition from surface freezing to surface melting behavior. These measurements can yield the relative surface energies of the various phases.
Dynamics of Nanometer SiO2 Particles and their Coalescence Characteristics
Authors: Blaisten-Barojas Estela , Liu Ling , Zachariah Michael
Published: April 2011

Various mechanisms of glassy transformations involving computational annealing were investigated by Molecular Dynamics simulations. Large clusters of silicon dioxide ranging from sub to nanometer size regime were considered. Silica is both a prototype ceramics and glassy material. Silica particles are fabricated in flow and flame reactors to design novel granular materials which depend strongly on the heating and cooling processes. During these processes extensive thermally driven relaxation in growing clusters allow for configurational changes from a liquid-like cluster to a glassy cluster. Crystal-like structures were investigated as well. Cooling rates comparable to experimental rates were achieved in these simulations. We find that the glass transition temperature decreases with decreasing cluster size. Calculations were performed by implementing a massively parallel particle decomposition schema of Molecular Dynamics with an excellent speedup and a significant decrease of complexity.
Molecular Dynamics Computer Simulations of Diffusion in Porous Silicates
Authors: Heffelfinger Grant S. , Pohl Phillip I. , Frink Laura J. D.
Published: April 2011

In this work a newly developed dual control volume grand canonical molecular dynamics technique simulates the diffusion of gas in a cylindrical pore. This allows spatial variation of chemical potential and hence an accurate simulation of steady state pressure driven diffusion. The molecular sieving nature of imicroporous imogolite models and the Knudsen effect are discussed and compared with experimental data.