Showing papers on "Crystal published in 2007"

Ceramic Materials: Science and Engineering

Abstract: Introduction.- Some History.- What You Already Know.- Bonds and Energy Bonds.- Models, Crystals and Chemistry.- Binary Compounds.- Complex Crystal and Glass Structures.- Equilibrium Phase Diagrams.- Furnaces.- Characterizing Structure, Defects and Chemistry.- Point Defects, Charge and Diffusion.- Are Dislocations Unimportant?- Surfaces, Nanoparticles and Foams.- Interfaces in Polycrystals.- Phase Boundaries, Particles and Pores.- Mechanical Testing.- Deforming: Plastic.- Fracturing: Brittleness.- Raw Materials.- Powders, Fibers, Platelets and Composites.- Glass and Glass-Ceramics.- Gels, Sols and Organic Chemistry.- Sintering and Grain Growth.- Solid-State Phase Transformations and Reactions.- Processing Glass and Glass-Ceramics.- Coatings and Thick Films.- Thin Films and Vapor Deposition.- Growing Single Crystals.- Shaping and Forming.- Conducting Charge or Not.- Locally Redistributing Charge.- Interacting With and Generating Light.- Storing Data Using Magnetic Fields.- Responding to Temperature Changes.- Bioceramics and Biomimetics.- Minerals and Gems.- Industry and the Environment.

Very high-mobility organic single-crystal transistors with in-crystal conduction channels

Abstract: Very high-mobility organic transistors are fabricated with purified rubrene single crystals and high-density organosilane self-assembled monolayers. The interface with minimized surface levels allows carriers to distribute deep into the crystals by more than a few molecular layers under weak gate electric fields, so that the inner channel plays a significant part in the transfer performance. With the in-crystal carriers less affected by scattering mechanisms at the interface, the maximum transistor mobility reaches 18cm2∕Vs and the contact-free intrinsic mobility turned out to be 40cm2∕Vs as the result of four-terminal measurement. These are the highest values ever reported for organic transistors.

Ultracold atoms in a disordered crystal of light: towards a bose glass.

Abstract: We use a bichromatic optical lattice to experimentally realize a disordered system of ultracold strongly interacting $^{87}\mathrm{Rb}$ bosons. In the absence of disorder, the atoms are pinned by repulsive interactions in the sites of an ideal optical crystal, forming one-dimensional Mott-insulator states. We measure the excitation spectrum of the system as a function of disorder strength and characterize its phase-coherence properties with a time-of-flight technique. Increasing disorder, we observe a broadening of the Mott-insulator resonances and the transition to a state with vanishing long-range phase coherence and a flat density of excitations, which suggest the formation of a Bose-glass phase.

Determination of the crystal structure of substrate-induced pentacene polymorphs in fiber structured thin films.

Abstract: It is widely recognized that the intrinsic charge transport properties in organic thin-film transistors (OTFTs) depend strongly on the crystal structure of the organic semiconductor layer. Pentacene, showing one of the highest charge carrier mobilities among organic semiconductors, is known to crystallize in at least four polymorphs, which can be distinguished by their layer periodicity d(001). Only two polymorphs grow as single crystals, and their detailed crystal structure has been solved. The substrate-induced 15.4 A polymorph is the most relevant for OTFT applications; however, its crystal structure has remained incomplete as it only grows as a fiber structured thin film. Here we extend the crystal truncation rod X-ray scattering technique to fiber structured thin films. We determined the complete crystal structure of this polymorph grown on technologically relevant substrates. We found that the molecular arrangement within the unit cell is substrate dependent, which may lead to a controlled fine-tuni...

Nickel-based oxyphosphide superconductor with a layered crystal structure, LaNiOP.

Abstract: A layered oxyphosphide, LaNiOP, was synthesized by solid-state reactions. This crystal was confirmed to have a layered structure composed of an alternating stack of (La3+O2-)+ and (Ni2+P3-)-. We found that the resulting LaNiOP shows a superconducting transition at ∼3 K. This material exhibited metallic conduction and Pauli paramagnetism in the temperature range of 4−300 K. The resistivity sharply dropped to zero and the magnetic susceptibility became negative at <4 K, indicating that a superconducting transition occurs. The volume fraction of the superconducting phase estimated from the diamagnetic susceptibility reached ∼40 vol % at 1.8 K, substantiating that LaNiOP is a bulk superconductor.

Spectroscopy of synthetic Mg-Fe pyroxenes I: Spin-allowed and spin-forbidden crystal field bands in the visible and near-infrared

Abstract: Understanding the fundamental crystal chemical controls on visible and near-infrared reflectance spectra of pyroxenes is critical to quantitatively assessing the mineral chemistry of pyroxenes viewed by remote sensing. This study focuses on the analysis ofspectroscopic measurements of a comprehensive set of synthetic Mg-Fe pyroxenes from the visible through the near-infrared (0.3-2.6 μm) to address the constraints of crystal structure and Fe^2+ content on spin-forbidden and spin-allowed crystal field absorptions in Ca-freeorthopyroxenes. The chemistry and oxidation state of the synthetic pyroxenes are characterized. Coordinated Mssbauer spectroscopy is used to determine site occupancy of Fe^2+ in the M1 and M2 crystallographic sites. Properties of visible and near-infrared absorption bands of the synthetic pyroxenes are quantified using the modified Gaussian model. The 1 and 2 μm spin-allowed crystal field absorption bands move regularly with increasing iron content, defining a much tighter trend than observed previously. A spin-allowed crystal field absorption band at 1.2 μm is explicitly verified, even at low total iron contents, indicating that some portion of Fe^2+ resides in the M1 site. The 1.2 μm band intensifies and shifts to longer wavelengths with increasing iron content. At visible wavelengths, spin-forbidden crystal field absorptions are observed in all iron-bearing samples. The most prominent absorption near 506 nm, attributed to iron in the M2 site, shifts to slightly longer wavelengths with iron content. The purity and extent of this pyroxene series allows visible wavelength absorption bands to be directly assigned to specific transitions of Fe^2+ in the M1 and M2 sites.

Transition of Ionic Liquid [bmim][PF6] from Liquid to High-Melting-Point Crystal When Confined in Multiwalled Carbon Nanotubes

Abstract: In this communication, we report our finding on the transition of ionic liquid [bmim][PF6] from the liquid state to high-melting-point crystal when confined in multiwalled carbon nanotubes. This novel crystallization behavior is explained by the nanosized effect of carbon nanotubes on the structure of ionic liquid.

Particle morphology, crystal orientation, and electrochemical reactivity of LiFePO4 synthesized by the hydrothermal method at 443 K

Abstract: LiFePO4 (space group: Pnma) was synthesized by the hydrothermal method at 443 K. The pH of the precursor solution was systematically changed between 2.5 and 9.5. The particle morphology, crystal orientation, and electrochemical reactivity of the prepared LiFePO4 particles changed depending on the concentration of the Li source and pH of the precursor. The particles obtained from acidic solutions (pH ≈ 3.5) were needle-like particles. On the other hand, plate-like crystals were obtained from weak acidic solutions of 4 < pH < 6.5. At higher pH than 7.2, the particles became randomly shaped. The plate-like crystal had a large facet in the ac-plane, while the needle-like particles had a large facet in the bc-plane. The electrochemical properties of the prepared LiFePO4 were characterized in a mixed solvent of ethylene carbonate and diethyl carbonate with volume ratio of 1 : 1 containing 1.0 mol dm−3 LiClO4 at room temperature. The plate-like crystals exhibited the highest electrochemical reactivity among the prepared samples, and the discharge capacity was 163 mA h g−1 measured at a current density of 17 mA g−1.

Secondary Nucleation and Growth of ZnO

Abstract: Recently we discovered that under certain conditions new crystal growth (branch) can be induced on specific crystalline planes of the same material. This is a new phenomenon and is in sharp contrast to typical nucleation and growth in which a crystal will simply grow larger in preferred directions depending on the surface energy of the specific crystalline planes. Based on our observation, we developed a sequential nucleation and growth technique offering the power to assemble complex hierarchical crystals step-by-step. However, the key questions of when and how the secondary nucleation takes place have not been answered. Here we systematically study secondary ZnO crystal growth using organic diamine additives with a range of chain lengths and concentration. We found that ZnO branches form for a narrow diamine concentration range with a critical lower and upper critical nucleation concentration limit, which increases by about a factor of 5 for each additional carbon in the diaminoalkane chain. Our results suggest that the narrow window for secondary growth is dictated by the solubility of the ZnO crystals, where the low critical nucleation concentration is determined by slight etching of the surface to produce new nucleation sites, and the upper critical concentration is determined by the supersaturation concentration. Kinetic measurements show that the induction time and growth rate increase with increasing diamine concentration and follow classical nucleation and growth theory. Observations of branch morphological evolution reveal the mechanisms guiding the tunable crystal size and morphology.

Dynamically induced frustration as a route to a quantum spin ice state in Tb2Ti2O7 via virtual crystal field excitations and quantum many-body effects.

Abstract: The Tb2Ti2O7 pyrochlore magnetic material is attracting much attention for its spin liquid state, failing to develop long-range order down to 50 mK despite a Curie-Weiss temperature thetaCW approximately -14 K. In this Letter we reinvestigate the theoretical description of this material by considering a quantum model of independent tetrahedra to describe its low-temperature properties. The naturally tuned proximity of this system near a Neel to spin ice phase boundary allows for a resurgence of quantum fluctuation effects that lead to an important renormalization of its effective low-energy spin Hamiltonian. As a result, Tb2Ti2O7 is argued to be a quantum spin ice. We put forward an experimental test of this proposal using neutron scattering on a single crystal.

Structure and Electrochemistry of Copper Fluoride Nanocomposites Utilizing Mixed Conducting Matrices

Abstract: Near-theoretical utilization of high-energy-density CuF2 positive electrode materials for lithium batteries was enabled through the use of nanocomposites consisting of 2−30 nm domains of CuF2 within a mixed ionic + electronic conducting matrix of a metal oxide. Small but significant crystallographic changes to the core crystal of the CuF2 were found to occur in all oxide-based matrices. These modifications to the core crystal and the surrounding matrix were investigated through a host of characterization methods, including XRD, XPS, and XAS. This new approach to the enablement of the anhydrous CuF2 is distinctly superior in performance to that of macro CuF2 or CuF2 nanocomposites utilizing carbon as a matrix, the latter of which is also introduced herein for the first time.

Accurate predictions of crystal densities using quantum mechanical molecular volumes.

Abstract: A quantum mechanically based procedure for estimation of crystal densities of neutral and ionic crystals is presented. In this method, volumes within 0.001 electrons/bohr3 isosurfaces of electron density for the constituent isolated neutral and ionic molecules are calculated to define the molecular volume or formula unit volumes used in predicting the crystal density. The B3LYP density functional theory in conjunction with the 6-31G** basis set were employed to generate the electron densities. The suitability of this method of crystal density prediction was assessed by subjecting a large number (289) of molecular and ionic crystals to the procedure and comparing results with experimental information. The results indicate that, for neutral molecular crystals, the root-mean-square (rms) deviation from experiment is within 4%, whereas the rms deviation is somewhat larger for the 71 ionic crystals evaluated (within 5%).

Superconductivity in an Inorganic Electride 12CaO·7Al2O3:e-

Abstract: An inorganic electride, 12CaO·7Al2O3:e-, synthesized by exclusively replacing oxygen ions in the sub-nanometer-sized cages of 12CaO·7Al2O3 crystal with electrons exhibits a superconducting transition at a temperature (Tc) of ∼0.4 K. Tc varies in the range of 0.14−0.4 K with the concentration of anionic electrons, which are primarily distributed over crystallographic spaces without occupying any particular framework ions. The precursor of electride is composed of representative metal oxides, which are electrical insulators. Thus, the exotic crystal structure of electrides provides an insight for a material platform to realize a new superconductor.

Giant Kerr nonlinearities and solitons in a crystal of molecular magnets

Abstract: The authors show the formation of microwave solitons in a crystal of molecular magnets via an electromagnetically induced transparency and have the giant cross-phase modulation phase shifts with the advantages of low pump powers, low absorptions, high sensitivities, and certain frequency tunability.

Self-construction of core-shell and hollow zeolite analcime icositetrahedra: a reversed crystal growth process via oriented aggregation of nanocrystallites and recrystallization from surface to core.

Abstract: Zeolite analcime with a core−shell and hollow icositetrahedron architecture was prepared by a one-pot hydrothermal route in the presence of ethylamine and Raney Ni. Detailed investigations on samples at different preparation stages revealed that the growth of the complex single crystalline geometrical structure did not follow the classic crystal growth route, i.e., a crystal with a highly symmetric morphology (such as polyhedra) is normally developed by attachment of atoms or ions to a nucleus. A reversed crystal growth process through oriented aggregation of nanocrystallites and surface recrystallization was observed. The whole process can be described by the following four successive steps. (1) Primary analcime nanoplatelets undergo oriented aggregation to yield discus-shaped particles. (2) These disci further assemble into polycrystalline microspheres. (3) The relatively large platelets grow into nanorods by consuming the smaller ones, and meanwhile, the surface of the microspheres recrystallizes into ...

Single nitrogen vacancy centers in chemical vapor deposited diamond nanocrystals

Abstract: Nanodiamond crystals containing single color centers have been grown by chemical vapor deposition (CVD). The fluorescence from individual crystallites was directly correlated with crystallite size using a combined atomic force and scanning confocal fluorescence microscope. Under the conditions employed, the optimal size for single optically active nitrogen-vacancy (NV) center incorporation was measured to be 60 to 70 nm. The findings highlight a strong dependence of NV incorporation on crystal size, particularly with crystals less than 50 nm in size.

Thermal response in crystalline Iβ cellulose : A molecular dynamics study

Abstract: The influence of temperature on structure and properties of the cellulose Iβ crystal was studied by molecular dynamics simulations with the GROMOS 45a4 force-field. At 300 K, the modeled crystal ag ...

Dielectric Anisotropy of a Homochiral Trinuclear Nickel(II) Complex

Abstract: Hydrothermal reaction of (S)-1,1‘1‘ ‘-2,4,6-trimethylbenzene-1,3,5-triyl-tris(methylene)-tris-pyrrolidine-2-carboxylic acid (TBPLA) with Ni(ClO4)2·6H2O gave pale green block crystals of 1. X-ray crystal structure analysis showed the complex to be a trinuclear descrete homochiral molecule with each Ni center sitting in distorted octahedron geometries. Anisotropic permittivity measurements reveal that 1 exhibits huge dielectric anisotropy along its three different crystal axes that is ca. 3.5 times of er (E//c)/er (E//b) with temperature and frequency independence.

Ductile titanium alloy with low Poisson's ratio.

Abstract: We report a ductile beta-type titanium alloy with body-centered cubic (bcc) crystal structure having a low Poisson's ratio of 0.14. The almost identical ultralow bulk and shear moduli of similar to 24 GPa combined with an ultrahigh strength of similar to 0.9 GPa contribute to easy crystal distortion due to much-weakened chemical bonding of atoms in the crystal, leading to significant elastic softening in tension and elastic hardening in compression. The peculiar elastic and plastic deformation behaviors of the alloy are interpreted as a result of approaching the elastic limit of the bcc crystal under applied stress.

Intra- and intermolecular band dispersion in an organic crystal.

Abstract: The high crystallinity of many inorganic materials allows their band structures to be determined through angle-resolved photoemission spectroscopy (ARPES). Similar studies of conjugated organic molecules of interest in optoelectronics are often hampered by difficulties in growing well-ordered and well-oriented crystals or films. We have grown crystalline films of uniaxially oriented sexiphenyl molecules and obtained ARPES data. Supported by density-functional calculations, we show that, in the direction parallel to the principal molecular axis, a quasi-one-dimensional band structure of a system of well-defined finite size develops out of individual molecular orbitals. In contrast, perpendicular to the molecules, the band structure reflects the periodicity of the molecular crystal, and continuous bands with a large dispersion were observed.

Single nitrogen vacancy centers in chemical vapor deposited diamond nanocrystals

Abstract: Nanodiamond crystals containing single color centers have been grown by chemical vapor deposition (CVD). The fluorescence from individual crystallites was directly correlated with crystallite size using a combined atomic force and scanning confocal fluorescence microscope. Under the conditions employed, the optimal size for single optically active nitrogen-vacancy (NV) center incorporation was measured to be 60-70 nm. The findings highlight a strong dependence of NV incorporation on crystal size, particularly with crystals less than 50 nm in size.

Textural and capacitive characteristics of hydrothermally derived RuO2·xH2O nanocrystallites : Independent control of crystal size and water content

Abstract: RuO2·xH2O nanoparticulates in different crystal sizes with various water contents were prepared via a hydrothermal synthesis route, successfully demonstrating the independent control of crystal size and water content of RuO2·xH2O. The crystalline and hydrous nature of hydrothermally derived RuO2·xH2O particulates not only reduces the proton diffusion resistance but also enhances the electronic conductivity for the redox transitions of active species. Novel and unique properties of hydrothermally derived RuO2·xH2O, i.e., effective inhibition of crystallite coalescence upon annealing, relatively high thermal stability, and maintenance of the original nanostructure, are attributable to the coalescence barrier of RuO2·xH2O crystallites due to the lattice energy. Maintaining/fine-tuning the original nanostructure of annealed RuO2·xH2O crystallites with high mesoporosity favors the penetration of electrolytes into the whole oxide matrix. This effect of not only reducing the proton diffusion resistance but also ...

Potassium intercalation in graphite: A van der Waals density-functional study

Abstract: Potassium intercalation in graphite is investigated by first-principles theory. The bonding in the potassium-graphite compound is reasonably well accounted for by traditional semilocal density functional theory (DFT) calculations. However, to investigate the intercalate formation energy from pure potassium atoms and graphite requires use of a description of the graphite interlayer binding and thus a consistent account of the nonlocal dispersive interactions. This is included seamlessly with ordinary DFT by a van der Waals density functional (vdW-DF) approach [Phys. Rev. Lett. 92, 246401 (2004)]. The use of the vdW-DF is found to stabilize the graphite crystal, with crystal parameters in fair agreement with experiments. For graphite and potassium-intercalated graphite structural parameters such as binding separation, layer binding energy, formation energy, and bulk modulus are reported. Also the adsorption and sub-surface potassium absorption energies are reported. The vdW-DF description, compared with the traditional semilocal approach, is found to weakly soften the elastic response.

Three-dimensional Si/Ge quantum dot crystals.

Abstract: Modern nanotechnology offers routes to create new artificial materials, widening the functionality of devices in physics, chemistry, and biology. Templated self-organization has been recognized as a possible route to achieve exact positioning of quantum dots to create quantum dot arrays, molecules, and crystals. Here we employ extreme ultraviolet interference lithography (EUV-IL) at a wavelength of I ) 13.5 nm for fast, large-area exposure of templates with perfect periodicity. Si(001) substrates have been patterned with two-dimensional hole arrays using EUV-IL and reactive ion etching. On these substrates, three-dimensionally ordered SiGe quantum dot crystals with the so far smallest quantum dot sizes and periods both in lateral and vertical directions have been grown by molecular beam epitaxy. X-ray diffractometry from a sample volume corresponding to about 3.6 × 107 dots and atomic force microscopy (AFM) reveal an up to now unmatched structural perfection of the quantum dot crystal and a narrow quantum dot size distribution. Intense interband photoluminescence has been observed up to room temperature, indicating a low defect density in the three-dimensional (3D) SiGe quantum dot crystals. Using the Ge concentration and dot shapes determined by X-ray and AFM measurements as input parameters for 3D band structure calculations, an excellent quantitative agreement between measured and calculated PL energies is obtained. The calculations show that the band structure of the 3D ordered quantum dot crystal is significantly modified by the artificial periodicity. A calculation of the variation of the eigenenergies based on the statistical variation in the dot dimensions as determined experimentally ( ±10% in linear dimensions) shows that the calculated electronic coupling between neighboring dots is not destroyed due to the quantum dot size variations. Thus, not only from a structural point of view but also with respect to the band structure, the 3D ordered quantum dots can be regarded as artificial crystal.

Microwave effect in the fast synthesis of microporous materials: which stage between nucleation and crystal growth is accelerated by microwave irradiation?

Abstract: Microporous materials, such as silicalite-1 and VSB-5 molecular sieves, have been synthesized by both microwave irradiation (MW) and conventional electric heating (CE). The accelerated syntheses by microwave irradiation can be quantitatively investigated by various heating modes conducted in two steps such as MW-MW, MW-CE, CE-MW, and CE-CE (in the order of nucleation-crystal growth). In the case of synthesis by MW-CE or CE-MW, the heating modes were changed for the second step just after the appearance of X-ray diffraction peaks in the first step. We have quantitatively demonstrated that the microwave irradiation accelerates not only the nucleation but also crystal growth. However, the contribution to decrease the synthesis time by microwave irradiation is larger in the nucleation stage than in the step of crystal growth. The crystal size increases in the order of MW-MW

Crystal Phase Control of Luminescing α‐NaGdF4:Eu3+and β‐NaGdF4:Eu3+ Nanocrystals

Abstract: NaGdF4:Eu3+, NaEuF4, and NaGdF4 nanocrystals were synthesized in the high-boiling coordinating solvent N-(2-hydroxyethyl)-ethylenediamine (HEEDA). Phase pure nanomaterials, crystallizing either in the cubic α-phase or the hexagonal β-phase, were obtained by adjusting one reaction parameter only, i.e., the molar ratio between metal and fluoride ions in the synthesis. The hexagonal β-phase is formed, if this molar ratio is close to stoichiometric, whereas the cubic α-phase is obtained in the presence of excess metal ions. The optical properties of the Eu3+ doped samples are different for the two crystal phases. The results indicate an increased number of oxygen impurities close to Eu3+ ions, if excess metal ions are used in the synthesis.

Waveguiding in two-dimensional piezoelectric phononic crystal plates

Abstract: We investigate the possibility of designing phononic crystal-based devices for telecommunication applications using materials commonly employed in microfabrication. We focus our attention on a phononic crystal made of a square array of cylindrical holes drilled in an active piezoelectric PZT5A matrix. Two different structures are considered, namely, a freestanding phononic crystal plate and a plate deposited on a silicon substrate. The geometrical characteristics of the phononic crystal plates (lattice parameter and thickness) were chosen to ensure the existence of an absolute band gap around 1.5GHz; a common frequency in radio frequency telecommunications. Computations of the dispersion curves of these active structures were conducted with the help of the finite element method. We demonstrate the existence of absolute band gaps in the band structure of the phononic crystal plates and, then, the possibility of guided modes inside a linear defect created by removing one row of air holes in the phononic crystal. In the case of the supported phononic crystal plates, we show the existence of an absolute forbidden band in the plate modes when the thickness of the substrate significantly exceeds the plate thickness. We discuss the conditions to realize waveguiding through a linear defect inside the supported plate. The present work provides evidences that phononic crystal properties can be integrated with existing silicon based microdevice technology