Showing papers on "Single crystal published in 1999"

Single-Crystal Colloidal Multilayers of Controlled Thickness

Abstract: Materials whose dielectric constant varies spatially with submicrometer periodicity exhibit diffractive optical properties which are potentially valuable in a number of existing and emerging applications. Here, such systems are fabricated by exploiting the spontaneous crystallization of monodisperse silica spheres into close-packed arrays. By reliance on a vertical deposition technique to pack the spherical colloids into close-packed silica−air arrays, high quality samples can be prepared with thicknesses up to 50 μm. These samples are planar and thus suitable for optical characterization. Scanning electron microscopy (SEM) of these materials illustrates the close-packed ordering of the spherical colloids in planes parallel to the substrate; cross-sectional SEM micrographs of the arrays as well as optical methods are used to measure sample thickness and uniformity. Normal-incidence transmission spectra in the visible and near-infrared regions show distinct peaks due to diffraction from the colloidal layer...

Synthesis of monodisperse cobalt nanocrystals and their assembly into magnetic superlattices (invited)

Abstract: High temperature, solution phase reduction of cobalt chloride in the presence of stabilizing agents was employed to produce magnetic colloids (ferrofluids) of cobalt nanocrystals. We systematically synthesized and isolated nearly monodisperse nanocrystal samples ranging in size from 2 to 11 nm while maintaining better than a 7% std. dev. in diameter. As synthesized cobalt particles are each a single crystal with a complex cubic structure related to the beta phase of elemental manganese (e-Co). Annealing the nanocrystals at 300 °C converts them quantitatively to the more common hexagonal-close-packed crystal form. Deposition of these uniform cobalt particles on solid substrates by evaporation of the carrier solvent results in the spontaneous assembly of two-dimensional and three-dimensional magnetic superlattices (colloidal crystals). A combination of x-ray powder diffraction, transmission electron microscopy, and superconducting quantum interference device magnetometry were used to characterize both the d...

Role of Steps in N 2 Activation on Ru(0001)

Abstract: Using adsorption experiments and density functional calculations we show that ${\mathrm{N}}_{2}$ dissociation on the Ru(0001) surface is totally dominated by steps. The measured adsorption rate at the steps is at least 9 orders of magnitude higher than on the terraces at 500 K, and the corresponding calculated difference in activation energy is 1.5 eV. The low barrier at the step is shown to be due to a combination of electronic and geometrical effects. The consequences for Ru as a catalyst for ammonia synthesis are discussed.

Grain Boundary Migration in Metals: Thermodynamics, Kinetics, Applications

Abstract: Thermodynamics of Grain Boundaries Introductory Remarks Thermodynamics of Surfaces Experiments Applications of Grain Boundary Thermodynamics The Equilibrium Shape of Grain Boundaries Structure of Grain Boundaries Terminology and Definitions Atomic Structure of Grain Boundaries Grain Boundary Motion Fundamentals Driving Forces for Grain Boundary Migration Drag Effects During Grain Boundary Motion Measurement of Grain Boundary Mobility Experimental Results Effect of Wetting Phase Transitions on Grain Boundary Migration Compensation Effect in Grain Boundary Motion Mechanisms of Grain Boundary Migration Thermodynamics and Kinetics of Connected Grain Boundaries Microstructural Elements of Polycrystals Thermodynamics of Triple Junctions Motion of a Grain Boundary System with Triple Junctions Triple Junctions Motion in the Presence of Impurities Experimental Investigations of Triple Junction Motion Triple Junction Drag and Grain Growth in 2D Polycrystals Grain Growth in 3D Systems Kinetics of Grain Growth Inhibited by Vacancy Generation Computer Simulation of Grain Boundary Motion Introduction Driving Force Concepts Migration of [001] Twist Grain Boundaries Motion of Tilt Boundaries Compensation Effect Comparison with Experiments Grain Boundary Diffusion Atomic Mechanisms Simulation of Triple Junction Motion Applications Characterization of Microstructure and Texture Recrystallization and Grain Growth On Precipitation Controlled Grain Size Mechanisms on Retardation of Grain Growth Grain Boundary Junction Engineering Appendices Solutions References

Epitaxial laser metal forming: analysis of microstructure formation

Abstract: Epitaxial laser metal forming (E-LMF) is presented as a new cladding technique which combines the advantage of near-net-shape manufacturing with a close control of the solidification microstructure. E-LMF is a process where metal powder is injected into a molten pool formed by controlled laser heating. Laser surface treatment has the advantage that heat input is very localised, thus leading to large temperature gradients. This is used, in unison with closely controlled solidification velocities, to stabilise the columnar dendritic growth, thereby avoiding nucleation and growth of equiaxed grains in the laser clad. It is possible with this technique to deposit a single crystal clad by epitaxial growth onto a single crystal substrate. In this paper, the microstructure obtained by E-LMF is analysed by scanning electron microscopy (SEM), optical microscopy (OM) and indexing electron backscattered diffraction (EBSD) patterns. In particular, the grain structure formation in the deposit during the process and the influence of a subsequent heat treatment on precipitation and recrystallisation is characterised.

Excitonic structure and absorption coefficient measurements of ZnO single crystal epitaxial films deposited by pulsed laser deposition

Abstract: The optical properties of high quality single crystal epitaxial zinc oxide thin films grown by pulsed laser deposition on c-plane sapphire substrates were studied. It was found that annealing the films in oxygen dramatically improved the optical and electrical properties. The absorption coefficient, band gap, and exciton binding energies were determined by transmission measurements and photoluminescence. In both the annealed and the as-deposited films excitonic absorption features were observed at both room temperature and 77 K. In the annealed films the excitonic absorption peaks were substantially sharper and deep level photoluminescence was suppressed.

Processing and Characterization of Single-Crystalline Ultrafine Bismuth Nanowires

Abstract: By pressure injecting Bi liquid melt into the nanochannels of an anodic alumina template, we have successfully fabricated Bi nanowire arrays with ultrafine wire diameters and extremely high wire packing densities. Free-standing Bi nanowires with controlled wire diameters and large aspect ratios (length/diameter) were also obtained by subsequent etching of the alumina template. Various techniques such as SEM, TEM, AFM, EFM, HREM, and XRD have been used to investigate the physical characteristics of these nanowires. The Bi nanowires were found to be dense and continuous and had a uniform diameter throughout the length of the wires. Individual Bi nanowires were shown to be single crystals, and all the wires in an array were highly oriented. An interesting metastable phase of Bi was also observed, which can be attributed to a lattice stress-induced high-pressure phase of Bi formed inside the porous anodic alumina template.

Structure of Restacked MoS2 and WS2 Elucidated by Electron Crystallography

Abstract: There has been a lot of confusion about the nature of restacked MoS2 and WS2. The structure has been proposed to be trigonal TiS2 type with octahedral M4+ and called 1T-MoS2. The presence of a distortion in the metal plane that gives rise to a superstructure has been suggested. We have performed electron crystallographic studies on small (submicron) single crystal domains of restacked WS2 and MoS2 to solve their superstructure. We find that what initially seems to be a trigonal crystal is actually a “triplet” of three individual orthorhombic crystals. Using two-dimensional hk0 data from films for both “triple” and “single” crystals we calculated corresponding Patterson projections, which reveal a severe distortion in the Mo/W plane, forming infinite zigzag chains. The projection of the structure suggests M−M distances of 2.92 and 2.74 A for MoS2 and WS2, respectively. Least-squares refinement from the single-crystal data gives R1 = 13.3% for WS2 and R1 = 15.3% for MoS2. Therefore, we submit that restacked...

Fracture anisotropy in silicon single crystal

Abstract: In this study the effect of crystallographic orientation on fracture toughness and the fracture path of silicon single crystal was investigated Vickers microhardness indentation was used to introduce cracks along various crystallographic orientations on the (110), (001), and (111) planes The fracture toughness variation was found to follow the symmetry of the indentation axis with no distinct correlation with the elastic constants The observations made suggest that the inclination angle of cleavage planes relative to the indent plane affects the fracture path and toughness significantly Prestraining decreased the hardness and improved the toughness without a modification of fracture path

Giant magnetic-field-induced strains in Heusler alloy NiMnGa with modified composition

Abstract: A giant magnetic-field-induced strain (MFIS) of −3100 ppm has been obtained in Heusler alloy Ni52Mn22.2Ga25.8 single crystal in the [001] direction at a temperature from 23 to 31 °C. This MFIS reaches saturation in an applied field about 6 kOe, and exhibits the same amplitude with an opposite sign while the field is perpendicular to the samples. According to a previous model, this MFIS associates with the twin boundary motion. The martensitic self-strain has been found to be 2%, implying a preferential orientation of martensite variants. Results related to the magnetic properties are discussed.

Refinement of the kaolinite structure from single-crystal synchrotron data

Abstract: The crystal structure of single crystals of kaolinite from Keokuk, Iowa, was refined using data measured at the microfocus X-ray beamline at the ESRE Grenoble, France (X = 0.6883, T - room temperatu(e). The volume of the crystals was 8 and 0.8 txm 3, respectively. Unit-cell parameters are: a = 5.154(9) A, b = 8.942(4) A, c = 7.401(10) A, c~ ~ 91.69(9) ~ , 13 = 104.61(5) ~ "/ - 89.82(4) ~ . Space group C1 is consistent with the observed data. All non-hydrogen atoms were independently refined with anisotropic displacement parameters. The positions and isotropic displacement parameters for the three interlayer H atoms were refined a/so. The position of the intralayer H was found by difference-Fourier methods, although refinement was not possible. Difference-Fourier maps suggested large anisotropic dis- placement vectors of this intralayer H, however, no evidence for a second maximum was found. The diffraction patterns show diffuse scattering in streaks parallel to (001)* through hkl reflections with hk 0, which is caused by stacking faults. No twinning was observed for either of the two crystals.

Temperature dependence of Raman scattering in single crystal GaN films

Abstract: Micro-Raman scattering from single crystal GaN films, both free-standing and attached to Al2O3 substrates, was performed over the temperature range from 78 to 800 K. These measurements reveal that the Raman phonon frequency decreases and the linewidth broadens with increasing temperature. This temperature dependence is well described by an empirical relationship which has proved to be effective for other semiconductors. The experiments also demonstrate that the strain from Al2O3 substrates compresses the epitaxial GaN in the c-axis direction.

Growth, characterization and properties of relaxor ferroelectric PMN-PT single crystals

Abstract: The present paper reviews the recent progress of studies on growth, characterization and properties of relaxor ferroelectric Pb(Mg1/3Nb2/3)O3-PbTiO3 single crystals in Shanghai Institute of Ceramics. Their chemical compositions were selected near the rhombohedral-tetragonal morphotropic phase boundary, more favorable in the rhombohedral phase. The dominant method used for growing the single crystals was the modified Bridgman technique, by which the single crystal boules with the size larger than 25mm diameter and 50 mm long were grown. Different orientation thin plates with thickness less than 1mm were cut from the boules and their domain structures, morphology and properties were characterized. Their phase transition were discussed as well.

GaN single crystal substrate and method of producing same

Abstract: A freestanding GaN single crystal substrate is made by the steps of preparing a (111) GaAs single crystal substrate, forming a mask having periodically arranged windows on the (111) GaAs substrate, making thin GaN buffer layers on the GaAs substrate in the windows of the mask, growing a GaN epitaxial layer on the buffer layers and the mask by an HVPE or an MOC, eliminating the GaAs substrate and the mask away and obtaining a freestanding GaN single crystal substrate. The GaN single crystal has a diameter larger than 20 mm and a thickness more than 0.07 mm, being freestanding and substantially distortion-free.

Single crystal superconductor nanowires by electrodeposition

Abstract: Superconducting Pb wires (diameter∼50 nm) have been prepared by pulse electrodeposition in nanoporous membranes. Single crystal or polycrystalline nanowires may be grown selectively and reproducibly depending on the pulse parameters. Unexpectedly, the growth of single crystal wires requires a greater departure from equilibrium conditions (greater overpotential) than the growth of polycrystalline ones. The importance of controlling the crystal texture is demonstrated by measurements of the superconducting transition temperature Tc which give significantly different results for polycrystalline and single crystal nanowires.

Molecular beam epitaxy growth of ferromagnetic single crystal (001) Ni2MnGa on (001) GaAs

Abstract: The ferromagnetic shape memory alloy Ni2MnGa has been grown on GaAs by molecular beam epitaxy. In situ reflection high energy electron diffraction, ex situ x-ray diffraction, and transmission electron microscopy selective area electron diffraction indicate the single crystal growth of a pseudomorphic tetragonal phase of Ni2MnGa on (001) GaAs. Both vibrating sample magnetometry and superconducting quantum interference device magnetometry measurements show that the Ni2MnGa film is ferromagnetic with in-plane magnetization and has a Curie temperature of ∼320 K.

Photochromism of 1,2-Bis(2-methyl-6-nitro-1-benzothiophen-3-yl)perfluorocyclopentene in a Single-Crystalline Phase: Dichroism of the Closed-Ring Form Isomer

Abstract: 1,2-Bis(2-methyl-6-nitro-1-benzothiophen-3-yl)perfluorocyclopentene was found to undergo a reversible photochromic reaction in the single-crystalline phase. Upon irradiation with 366-nm light, the single crystal turned green with keeping the crystal shape. The colored crystal showed dichroism. When the crystal was rotated under polarized light, the color changed from yellow to blue. The yellow and blue colors were attributed to two perpendicular electronic transitions at 465 and 600 nm of the closed-ring form. The two transition moments coincide with the directions of short and long axes of the closed-ring form isomers, which were regularly packed in the crystal lattice.

Oxide ionic conductivity of apatite type Nd9·33(SiO4)6O2 single crystal

Abstract: Single crystal of hexagonal apatite type Nd 9·33 (SiO 4 ) 6 O 2 which is an oxide ionic conductor was prepared by the FZ method and an anisotropy of its conductivity was investigated. The conductivity of a parallel component to a c-axis (2·1×10 −8 S cm −1 at 30°C) was higher about one order of magnitude, compared with that of perpendicular component.

Method of fabricating a high reliable SOI substrate

Abstract: A semiconductor device with high reliability is provided using an SOI substrate. When the SOI substrate is fabricated by using a technique typified by SIMOX, ELTRAN, or Smart-Cut, a single crystal semiconductor substrate having a main surface (crystal face) of a { 110 } plane is used. In such an SOI substrate, adhesion between a buried insulating layer as an under layer and a single crystal silicon layer is high, and it becomes possible to realize a semiconductor device with high reliability.

Crystal structure and biodegradation of aliphatic polyester crystals

Abstract: The biodegradability of aliphatic polyesters, which are produced by biosynthesis and chemo-synthesis, strongly correlates to the polymer morphology such as crystallinity, molecular orientation, chain packing and crystal surface, in addition to the chemical structure. For elucidation of biodegradation mechanism of crystal regions on an atomic level, lamellar single crystals of poly([R]-3-hydroxybutyrate) (P(3HB)) and its copolymers, poly([R]-3-hydroxyvalerate) (P(3HV)), poly(4-hydroxybutyrate) (P(4HB)), poly(L-lactic acid) (PLLA) and poly(ethylene succinate) (PES) were prepared from dilute solution by isothermal crystallization, and the crystal structures and morphologies were investigated by means of mainly transmission electron microscopy and atomic force microscopy. All single crystals were intramolecular single crystals, with spiral growth in some cases, and these crystals gave well-resolved electron diffractograms. The enzymatic degradation of lamellar crystals of P(3HB) and its copolymers was carried out with extracellular PHB depolymerases, and it was revealed that enzymatic degradation of lamellar crystals progressed from crystal edges and ends rather than the chain-folding surfaces of crystals, in spite of the homogeneous adsorption of enzyme molecules on the crystal surfaces by substrate-binding domain. Furthermore, in the case of PLLA single crystals, single crystals were also degraded from the edges to yield a rounded shape without decreasing the molecular weights and lamellar thickness. These results suggest that the attack by the active site of enzyme takes place at disordered regions of the crystals, that is, disordered lateral sites with high mobility of molecular chains are preferentially degraded.

Large magnetoresistance and finite-size effects in electrodeposited single-crystal bi thin films

Abstract: Trigonal-axis oriented single-crystal Bi thin films have been made by electrodeposition followed by suitable annealing. Very large magnetoresistance with ratios as much as 1500 at 5 K and 2.9 at 300 K under 5 T, Shubnikov--de Haas oscillations, and finite-size effects have been observed.

Epitaxial Electrodeposition of Copper(I) Oxide on Single-Crystal Gold(100)

Abstract: Epitaxial electrodeposition was used to crystallize a 0.5 μm thick film of Cu2O onto single-crystal Au(100) from an alkaline copper lactate solution at room temperature. The film showed strong out-of-plane and in-plane orientation, with no rotation of the respective crystallographic directions of the Cu2O overlayer relative to the Au substrate. The technique should be amenable to the deposition of other metal oxides, allowing the simple and inexpensive selection of surfaces that show high catalytic activity or crystallographic directions that have desired electrical or magnetic properties.

Reinvestigation of the crystal structure of dehydrated sodium zeolite x

Abstract: The structure of a single crystal of dehydrated zeolite Na−X selected from a batch grown by Petranovskii in Russia, of composition Na92Si100Al92O384 per unit cell, was determined by X-ray diffraction methods in the cubic space group Fd3; a = 25.077(4) A at 21 °C. R1 = 0.054 for the 320 reflections for which Fo > 4σ(Fo); wR2 based on F2 and all data is 0.140. Na+ ions are found at four crystallographic sites. Sites I‘ and II are fully occupied with 32 Na+ ions each. The remaining 28 Na+ ions partially occupy two 12-ring (site III‘) positions with 10(4) near an O−Al−O sequence and 18(4) near an O−Si−O sequence. These results differ from those of the two previous investigations of dehydrated Na−X, which, in turn, disagree substantially with each other. The near energy equivalence of two (or more) III‘ sites, the presence of an impurity cation in one determination, and a consideration of the differing qualities of the diffraction data are sufficient to reconcile all results. The substitution of four silicon ...

Role of interfacial structure on exchange-biased FeF 2 − Fe

Abstract: We have studied the effect of the interface structure on the exchange bias in the ${\mathrm{FeF}}_{2}\ensuremath{-}\mathrm{Fe}$ system, for ${\mathrm{FeF}}_{2}$ bulk single crystals or thin films. The exchange bias depends very strongly on the crystalline orientation of the antiferromagnet for both films and crystals. However, the interface roughness seems to have a strong effect mainly on the film systems. These results indicate that the exchange bias depends strongly on the spin structure at the interface, especially on the angle between the ferromagnetic and antiferromagnetic spins. We have also found a strong dependence of the hysteresis loops shape on the cooling field direction with respect to the antiferromagnetic anisotropy axis, induced by a rotation of the ferromagnetic easy axis as the sample is cooled through ${T}_{N}.$ For the single crystal systems the results imply the existence of a perpendicular coupling between the antiferromagnetic and ferromagnetic spins at low temperatures.