Showing papers on "Single crystal published in 1997"

Temperature-dependent oxygen electrochemistry on platinum low-index single crystal surfaces in acid solutions

Abstract: Using the rotating ring-disk technique (RRDPt(hkl)E), the oxygen-reduction reaction (ORR) was studied in sulfuric acid solution over the temperature range 298–333 K At the same temperature, the exc...

Synthesis and control of conductivity of ultraviolet transmitting β-Ga2O3 single crystals

Abstract: β-Ga2O3 single crystals were grown by the floating zone method and their conductivity along the b axis was controlled from <10−9 to 38 Ω−1 cm−1 by changing the growth atmosphere. By using feed rods doped with Sn, the grown crystal became highly conductive even under oxidative atmosphere. The optical transmission spectra showed that the β-Ga2O3 single crystal with 0.32 mm was transparent in the visible and ultraviolet region, with 20% transmittance at the fourth-harmonic wave of the Nd:YAG laser (266 nm). The band-gap widening was observed with the increasing of the carrier concentration. It is expected that the light of the KrF laser can be transmitted in the heavily doped β-Ga2O3.

Kinetics of oxygen reduction on Pt(hkl) electrodes : Implications for the crystallite size effect with supported Pt electrocatalysts

Abstract: The kinetics of the oxygen reduction reaction (ORR) on Pt(hkl) surfaces is found to vary with crystal face in a different manner depending on the electrolyte. In perchloric acid, the variation in activity at 0.8 to 0.9 V is relatively small between the three low index faces, with activity increasing in the order (100) < (110) ≈ (111). A similar structure sensitivity was observed in KOH, increasing in the order (100) < (110) < (111), but with larger differences. In sulfuric acid, the variations in activity with crystal face were much larger, with the difference between the most active and the least active being about two orders of magnitude, and increased in the opposite order (111) << (100) < (110). The variations in activity with crystal face in perchloric acid and KOH arise from the structure sensitive inhibiting effect of , i.e., a strongly inhibiting effect on (100) and smaller effects on (110) and (111). The variations in activity with crystal face in sulfuric acid arise from highly structure specific adsorption of sulfate/bisulfate anions in this electrolyte, which has a strongly inhibiting effect on the (111) surface. The crystallite size effect for the ORR reported for supported Pt catalysts in sulfuric acid at ambient temperature is fully explained by applying our single crystal results to classical models of the variation in particle shape with size.

Origin of the Anomalous Magnetic Behavior in Single Crystal Fe 3 O 4 Films

Abstract: Antiphase boundaries (APBs) were observed in ${\mathrm{Fe}}_{3}{\mathrm{O}}_{4}$ single crystal films grown on MgO. The APBs are an intrinsic consequence of the nucleation and growth mechanism in films. Across an APB, the intrasublattice superexchange coupling is greatly strengthened, while the intersublattice superexchange coupling is weakened, reversing the dominant interaction from that found in the bulk. Thus the APB separates oppositely magnetized regions, consistent with Lorentz microscopy measurements. The APBs induce very large saturation fields and nearly random magnetization distribution in zero field.

Optical absorption edge of ZnO thin films: The effect of substrate

Abstract: The optical absorption edge and the near-absorption edge characteristics of undoped ZnO films grown by laser ablation on various substrates have been investigated. The band edge of films on C [(0001)] and R-plane [(1102)] sapphire, 3.29 and 3.32 eV, respectively, are found to be very close to the single crystal value of ZnO (3.3 eV) with the differences being accounted for in terms of the thermal mismatch strain using the known deformation potentials of ZnO. In contrast, films grown on fused silica consistently exhibit a band edge ∼0.1 eV lower than that predicted using the known deformation potential and the thermal mismatch strains. This behavior is attributed to the small grain size (50 nm) realized in these films and the effect of electrostatic potentials that exist at the grain boundaries. Additionally, the spread in the tail (E0) of the band edge for the different films is found to be very sensitive to the defect structure in the films. For films grown on sapphire substrates, values of E0 as low as ...

Single-crystal Pb(ZrxTi1−x)O3 thin films prepared by metal-organic chemical vapor deposition: Systematic compositional variation of electronic and optical properties

Abstract: Single-crystal thin films of Pb(ZrxTi1−x)O3 (PZT) covering the full compositional range (0⩽x⩽1) were deposited by metal-organic chemical vapor deposition. Epitaxial SrRuO3(001) thin films grown on SrTiO3(001) substrates by rf-magnetron sputter deposition served as template electrode layers to promote the epitaxial growth of PZT. X-ray diffraction, energy-dispersive x-ray spectroscopy, atomic force microscopy, transmission electron microscopy, and optical waveguiding were used to characterize the crystalline structure, composition, surface morphology, microstructure, refractive index, and film thickness of the deposited films. The PZT films were single crystalline for all compositions exhibiting cube-on-cube growth epitaxy with the substrate and showed very high degrees of crystallinity and orientation. The films exhibited typical root mean square surface roughness of ∼1.0–2.5 nm. For tetragonal films, the surface morphology was dominated by grain tilting resulting from ferroelectric domain formation. We r...

X-ray photoelectron spectroscopy and x-ray diffraction study of the thermal oxide on gallium nitride

Abstract: The oxidation of single crystal gallium nitride in dry air has been investigated. X-ray photoelectron spectroscopy (XPS) revealed minimal oxide growth at 450 and 750 °C for up to 25 h. However, at 900 °C the growth of an oxide approximately 5000 A thick was observed after 25 h. This oxide was determined to be the monoclinic β-Ga2O3 using glancing angle x-ray diffraction. XPS spectra of the Ga 3d and Ga 2p core levels indicated peak shifts of 1.2 and 1.3 eV, respectively, from Ga–O to Ga–N bonding. The Ga L3M45M45 core level binding energy was also investigated and β-Ga2O3 and GaN each presented a characteristic peak shape.

Temperature dependence of elastic, dielectric, and piezoelectric properties of “single crystalline’’ films of vinylidene fluoride trifluoroethylene copolymer

Abstract: Elastic, dielectric, and piezoelectric constant matrix elements of a “single crystalline’’ (SC) film of vinylidene fluoride trifluoroethylene copolymer, P(VDF/TrFE), in which the orthorhombic [001] and [110] axes of fully extended chain crystals are preferentially oriented parallel to the stretching axis and normal to the surface, respectively, were measured at temperatures ranging from 10 K to the Curie point (402–404 K) by using a piezoelectric resonance method. All of the electromechanical coupling factors (k31, k32, k33, k24, and k15) are larger than those of conventional lamellar crystalline films. Some of the matrix elements for a P(VDF/TrFE) single crystal are derived from the measured values of constant matrix elements for the SC film. Some features characteristic of the SC film are revealed. The SC film has a large Young’s modulus for the stretching direction (1/s11) (121 GPa at 10 K). The properties related to the molecular motions along the chain axis, such as 1/s11, shear stiffness constant c55, shear piezoelectric constant e15, etc., exhibit strong relaxations around 250 K. The origin of these relaxations in the crystalline phase is discussed.

Single-crystal Si films for thin-film transistor devices

Abstract: The fact that single-crystal Si would make an ideal material for thin-film transistor devices has long been recognized. Despite this awareness, a viable method by which such a material could be directly produced on a glass substrate has never been formulated. In this letter, it is shown experimentally that location-controlled single-crystal Si regions on a SiO2 surface can be obtained in a glass-substrate compatible manner, via excimer-laser-based sequential lateral solidification of thin Si films using a beamlet shape that self-selects and extends a single grain over an arbitrarily large area. This is accomplished by controlling the locations, shape, and extent of melting induced by the incident excimer-laser pulses, in such a manner as to induce interface-contour-affected sequential super-lateral growth of crystals, during which the tendency of grain boundaries to align approximately orthogonal to the solidifying interface is systematically exploited.

Conformational characteristics of DNA; empirical classifications and a hypothesis for the conformational behaviour of dinucleotide steps

Abstract: This paper is concerned with an investigation of the geometry and structure of DNA as revealed by Xray diffraction of single crystal oligomeric structures. A database of atomic coordinates of 60 na...

Epitaxial carbon nanotube film self-organized by sublimation decomposition of silicon carbide

Abstract: A film of well-oriented carbon nanotubes was produced by sublimation decomposition of silicon carbide at 1700 °C by using YAG laser heating in a transmission electron microscope (TEM). The processes of SiC decomposition and the formation of carbon nanotubes were observed successively by high-resolution electron microscopy (HREM). Carbon nanotubes were mostly oriented along the [111] direction on the (111) surface plane of β-SiC single crystal. The interface between them was observed by HREM.

Ultrathin Films of Ruthenium on Low Index Platinum Single Crystal Surfaces: An Electrochemical Study

Abstract: We report on electrochemical properties of ultrathin films of ruthenium on platinum single crystal surfaces, Pt(100), Pt(111) and Pt(110), and demonstrate that such films can be obtained by spontaneous deposition. We also show that the spontaneously deposited ruthenium coverage is surface structure dependent. Using the spontaneous deposition process and a constant potential electrolysis, a variety of Pt/Ru surfaces up to ca. 0.4 monolayer of ruthenium were prepared. All such Ru films are stable in the electrode potential range that precedes platinum oxidation. A strong surface structure effect in the electrochemical properties of these thin films was found. On Pt(100)/Ru at a fixed Ru coverage, there is a transition from a reversible to irreversible surface redox behavior that is not observed on other platinum single crystal faces. In contrast to Pt(100)/Ru and Pt(110)/Ru, the individual voltammetric phases of the Pt(111)/Ru electrode are not resolved, and ruthenium surface oxides appear to be the most st...

Conductors with controlled grain boundaries: An approach to the next generation, high temperature superconducting wire

Abstract: Much of the conductor development effort in the last decade has focused on optimizing the processing of (Bi, Pb)2Sr2Ca2Cu3Ox oxide-powder-in-tube conductors and (Bi, Pb)2Sr2CaCu2O8 (Bi-2212) and TlBa2Ca2Cu3Ox thick film conductors. It is demonstrated that in each of these conductors, critical current densities are dictated by the grain boundary misorientation distributions (GBMD’s). Percolative networks of low-angle boundaries with fractions consistent with the active cross-sectional area of the conductor exist in each of these conductors. Further enhancements in the properties require increased numbers of small-angle grain boundaries. Given the processing methods used to fabricate these materials, no clear route employing a simple modification of the established processing method is apparent. To address this need, conductors with controlled or predetermined GBMD’s are necessary. Development of biaxial texture appears to be the only possible way to increase the number of small-angle boundaries in a practical and controllable manner. We summarize in this paper recent results obtained on epitaxial superconducting films on rolling-assisted-biaxially-textured-substrates (RABiTS). This technique uses well established, industrially scalable, thermomechanical processes to impart a strong biaxial texture to a base metal. This is followed by vapor deposition of epitaxial buffer layers (metal and/or ceramic) to yield structurally and chemically compatible surfaces. Epitaxial YBa2Cu3O7–δ films grown using laser ablation on such substrates have critical current densities exceeding 106 A/cm2 at 77 K in zero field and have a field dependence similar to epitaxial films on single crystal ceramic substrates. Deposited conductors made using this technique offer a potential route for the fabrication of the next generation high temperature superconducting (HTS) wire capable of carrying high currents in high magnetic fields and at elevated temperatures.

Reverse Micelle Based Formation of BaCO3 Nanowires

Abstract: A reverse micelle based approach is reported for the synthesis of BaCO3 nanowires with lengths up to 100 μm and diameters as small as 10−30 nm, each of which is a continuous single crystal with the c-axis along the wire length axis. The synthesis is achieved by the reaction of barium and carbonate ions solubilized in the polar cores of nonionic reverse micelles of C12E4 (tetraethylene glycol monododecyl ether) in cyclohexane. The crystal growth of BaCO3 nanowires has been examined by electron microscopy observations, and a directional aggregation process is proposed for the nanowire formation.

Improved Growth of Large Lead Zinc Niobate Titanate Piezoelectric Single Crystals for Medical Ultrasonic Transducers

Abstract: Large piezoelectric single crystals of the Pb[(Zn1/3Nb2/3)0.91Ti0.09]O3 (PZN-PT) binary system have been successfully obtained using a conventional flux method and a suitable ratio of PZN-PT: PbO flux. The largest crystal had dimensions of 43×42×40 mm and weighed 415 g. In order to improve growth behavior and reduce the number of nuclei, the temperature gradient program of the crucible was optimized; that is, the bottom of the crucible was made cooler (>50° C) than the top during cooling. A charged composition of 45 mol% PZN-PT and 55 mol% PbO showed good growth results. A large bar-mode electromechanical coupling factor, k33', (84%) for phased array ultrasonic transducers was obtained. This confirms that large PZN-PT single crystals for phased-array ultrasonic transducers can be grown using a flux method.

Method for fabricating matrix made up of oxide single crystal and method for manufacturing functional device

Abstract: PROBLEM TO BE SOLVED: To provide a new method for dicing a matrix made up of an oxide single crystal by a dry process. SOLUTION: Grooves 3, 4 are formed in a matrix 1 made up of an oxide single crystal by emitting a laser beam 6 and dissociating/evaporating the molecule of the oxide single crystal through an optical reaction to remove the molecule and fabricate the matrix 1. The matrix is caused to undergo a dicing process by cleaving the matrix 1 along the grooves 3, 4 to obtain a cut piece of a specified shape. Preferably the matrix 1 is wafer 1, on which functional devices are formed at positions corresponding to the cut pieces. The functional devices are such as an optical device and a piezoelectric vibrator.

Observation of a Metallic Antiferromagnetic Phase and Metal to Nonmetal Transition in Ca 3 Ru 2 O 7

Abstract: Single crystal ${\mathrm{Ca}}_{3}{\mathrm{Ru}}_{2}{\mathrm{O}}_{7}$ shows a metallic antiferromagnetic phase intermediate between a first-order metal to nonmetal transition at ${T}_{M}\phantom{\rule{0ex}{0ex}}=\phantom{\rule{0ex}{0ex}}48\mathrm{K}$ and the antiferromagnetic ordering (N\'eel) temperature, ${T}_{N}\phantom{\rule{0ex}{0ex}}=\phantom{\rule{0ex}{0ex}}56\mathrm{K}$. The metallic antiferromagnetic phase is predicted within various Mott-Hubbard models. Magnetization and electrical resistivity reveal strongly anisotropic metamagnetism in the nonmetallic antiferromagnetic phase. The charge and spin excitations are strongly coupled: The $H\ensuremath{-}T$ phase diagrams determined by magnetization and magnetoresistivity are indistinguishable and reveal a multicritical point. The heat capacity of ${\mathrm{Ca}}_{3}{\mathrm{Ru}}_{2}{\mathrm{O}}_{7}$ suggests it is a highly correlated electron system.

Synthesis of PbS nanocrystallites using a novel single molecule precursors approach : X-ray single-crystal structure of Pb(S2CNEtPri)2

Abstract: The synthesis and characterisation of some lead(ii) dithiocarbamato complexes Pb(S 2 CNRR′) 2 is reported. These compounds were used as single molecule precursors to produce nanocrystalline PbS by their thermolysis in trioctylphosphine oxide. The optical and morphological properties of the resulting PbS nanocrystallites were investigated; the influence of experimental parameters, such as precursor, growth time and temperature, on the final nanodispersed materials is also reported.

Lattice engineered compliant substrate for defect-free heteroepitaxial growth

Abstract: Presented here is proof-of-principle that a thin single crystal semiconductor film—when twist-wafer bonded to a bulk single crystal substrate (of the same material)—will comply to the lattice constant of a different single crystal semiconductor thick film grown on its surface. In our experiment, a 100 A film of GaAs was wafer bonded to a GaAs bulk substrate, with a large twist angle between their 〈110〉 directions. The resultant twist boundary ensures high flexibility in the thin film. Dislocation-free films of In0.35Ga0.65P(∼1% strain) were grown with thicknesses of 3000 A, thirty times the Matthews–Blakeslee critical thickness, on twist-wafer-bonded films of GaAs.

Optical properties of single-crystal anatase TiO2

Abstract: Polarized Reflection spectra of single-crystal anatase TiO 2 were measured in a photon energy range from 2 to 25 eV using synchrotron orbital radiation. Dielectric constants and absorption spectra were obtained from reflection spectra by Kramers-Kronig transformations. To interpret the structures that were observed in the spectra, the electronic states of anatase were calculated by using a cluster model with a size of Ti 5 O 14 . The structures observed in the energy region from 3 to 12 eV can be identified primarily due to the transition from O(2 p ) orbitals to Ti(3 d ) ones. The polarization dependence of the spectra in the above energy region is also effectively exhibited by the calculations from this model. The vague structures in the higher energy region (>12 eV) which can be clearly distinguished in spectra from the low energy part are due to the transition from O(2 p ) orbitals to Ti(4 s , 4 p ) ones.

Theoretical and experimental study of super-high electromechanical coupling surface acoustic wave propagation in KNbO3 single crystal

Abstract: The propagation characteristics of surface acoustic waves (SAWs) and piezoelectric leaky surface waves in single crystal KNbO/sub 3/ were investigated theoretically and experimentally. The results show that the electromechanical coupling coefficient of the surface wave propagating along the X-axis of the rotated Y-cut plane is very large, K/sup 2/=0.53, compared to K/sup 2/ of 0.055 for LiNbO/sub 3/.