Showing papers on "Crystal published in 2009"

Substrate for growing wurtzite type crystal and method for manufacturing the same and semiconductor device

Abstract: A laminated structure comprises a first layer comprising a crystal with six-fold symmetry, and a second layer comprising a metal oxynitride crystal formed on the first layer, wherein the second layer comprises at least one element selected from the group consisting of In, Ga, Si, Ge and Al, N, O and Zn, as main elements, and wherein the second layer has in-plane orientation.

Crystal and electronic band structure of Cu2ZnSnX4 (X=S and Se) photovoltaic absorbers: First-principles insights

Abstract: The structural and electronic properties of Cu2ZnSnS4 and Cu2ZnSnSe4 are studied using first-principles calculations. We find that the low energy crystal structure obeys the octet rule and is the kesterite (KS) structure. However, the stannite or partially disordered KS structures can also exist in synthesized samples due to the small energy cost. We find that the dependence of the band structure on the (Cu,Zn) cation ordering is weak and predict that the band gap of Cu2ZnSnSe4 should be on the order of 1.0 eV and not 1.5 eV as was reported in previous absorption measurements.

Phonon softening and crystallographic orientation of strained graphene studied by Raman spectroscopy

Abstract: We present a systematic study of the Raman spectra of optical phonons in graphene monolayers under tunable uniaxial tensile stress. Both the G and 2D bands exhibit significant red shifts. The G band splits into 2 distinct subbands (G+, G−) because of the strain-induced symmetry breaking. Raman scattering from the G+ and G− bands shows a distinctive polarization dependence that reflects the angle between the axis of the stress and the underlying graphene crystal axes. Polarized Raman spectroscopy therefore constitutes a purely optical method for the determination of the crystallographic orientation of graphene.

Ultrahigh strength single crystalline nanowhiskers grown by physical vapor deposition.

Abstract: The strength of metal crystals is reduced below the theoretical value by the presence of dislocations or by flaws that allow easy nucleation of dislocations. A straightforward method to minimize the number of defects and flaws and to presumably increase its strength is to increase the crystal quality or to reduce the crystal size. Here, we describe the successful fabrication of high aspect ratio nanowhiskers from a variety of face-centered cubic metals using a high temperature molecular beam epitaxy method. The presence of atomically smooth, faceted surfaces and absence of dislocations is confirmed using transmission electron microscopy investigations. Tensile tests performed in situ in a focused-ion beam scanning electron microscope on Cu nanowhiskers reveal strengths close to the theoretical upper limit and confirm that the properties of nanomaterials can be engineered by controlling defect and flaw densities.

Growth and Characterization of BaGa4S7: A New Crystal for Mid-IR Nonlinear Optics

Abstract: The new nonlinear optical (NLO) crystal BaGa4S7 for the mid-infrared (IR) has been grown by a Bridgman-Stockbarger technique. Polycrystalline materials with stoichiometric composition were synthesized from BaS, Ga, and S as the initial materials by solid-state reactions. The ultraviolet (UV) and IR transmittance of the crystal was determined with polished crystal pieces. The UV and IR optical absorption edges were found to be at 350 nm and 13.7 μm, respectively. From optical measurements of second harmonic generation on powders, the NLO coefficient d33 was determined to be 12.6 pm/V. The laser damage threshold of a single crystal reached about 1.2 J/cm2 at 1.064 μm. The Vickers-hardness value of the crystal is 327.5 HV5, which is equivalent to Mohs’ hardness of about 5.

Shape-Controlled Anatase Titanium(IV) Oxide Particles Prepared by Hydrothermal Treatment of Peroxo Titanic Acid in the Presence of Polyvinyl Alcohol

Abstract: Anatase titanium(IV) oxide (TiO2) particles with specific exposed crystal faces were prepared by hydrothermal treatment of peroxo titanic acid (PTA) solution with polyvinyl alcohol as a shape-control reagent. Crystal phase, shape, and size of TiO2 particles were found to be greatly dependent on pH value of PTA solution and time of hydrothermal treatment. TiO2 particles prepared from PTA solution of pH 7 had {101} and {001} exposed crystal faces, and the shape of TiO2 particles changed with the time of hydrothermal treatment. The prepared TiO2 particles with specific exposed crystal faces showed higher photocatalytic activity for acetaldehyde decomposition than commercial spherical TiO2 particles. This result implies that back reaction was prevented by spatial separation of redox sites in the particles because of selective migration of electrons and positive holes to specific exposed crystal faces and/or different reactivity of electrons and positive holes on the specific exposed crystal face. Furthermore,...

Thermodynamic stability and growth of guest-free clathrate hydrates: a low-density crystal phase of water.

Abstract: We use molecular dynamics simulations with the monatomic water (mW) model to investigate the phase diagram, metastability, and growth of guest-free water clathrates of structure sI and sII. At 1 atm pressure, the simulated guest-free water clathrates are metastable with respect to ice and stable with respect to the liquid up to their melting temperatures, 245 ± 2 and 252 ± 2 K for sI and sII, respectively. We characterize the growth of the sI and sII water crystals from supercooled water and find that the clathrates are unable to nucleate ice, the stable crystal. We computed the phase relations of ice, guest-free sII clathrate, and liquid water from −1500 to 500 atm. The resulting phase diagram indicates that empty sII may be the stable phase of water at pressures lower than approximately −1300 atm and temperatures lower than 275 K. The simulations show that, even in the region of stability of the empty clathrates, supercooled liquid water crystallizes to ice. This suggests that the barrier for nucleation...

Heterogeneously Hybridized Porous Coordination Polymer Crystals: Fabrication of Heterometallic Core-Shell Single Crystals with an In-Plane Rotational Epitaxial Relationship

Abstract: MOF on MOF: Core-shell porous coordination polymer (PCP) crystals are fabricated at the single-crystal level by epitaxial growth in solution. Synchrotron X-ray diffraction measurements unveiled the structural relationship between the shell crystal and the core crystal, where in-plane rotational epitaxial growth compensates the difference in lattice constant.

Visualizing the 3D Internal Structure of Calcite Single Crystals Grown in Agarose Hydrogels

Abstract: Single crystals are usually faceted solids with homogeneous chemical compositions. Biogenic and synthetic calcite single crystals, however, have been found to incorporate macromolecules, spurring investigations of how large molecules are distributed within the crystals without substantially disrupting the crystalline lattice. Here, electron tomography reveals how random, three-dimensional networks of agarose nanofibers are incorporated into single crystals of synthetic calcite by allowing both high- and low-energy fiber/crystal interface facets to satisfy network curvatures. These results suggest that physical entrapment of polymer aggregates is a viable mechanism by which macromolecules can become incorporated inside inorganic single crystals. As such, this work has implications for understanding the structure and formation of biominerals as well as toward the development of new high-surface area, single-crystal composite materials.

High Electron Mobility in Vacuum and Ambient for PDIF-CN2 Single-Crystal Transistors

Abstract: Single-crystal field-effect transistors (FETs) based on a fluorocarbon-substituted dicyanoperylene-3,4:9,10-bis(dicarboximide) [PDIF-CN2] were fabricated by lamination of the semiconductor crystal on Si-SiO2/PMMA-Au gate-dielectric-contact substrates. These devices were characterized both in vacuum and in the air, and they exhibit electron mobilities of ca. 6−3 and ca. 3−1 cm2 V−1 s−1, respectively, Ion:Ioff > 103, and near-zero threshold voltage.

Cloning polymer single crystals through self-seeding.

Abstract: In general, when a crystal is molten, all molecules forget about their mutual correlations and long-range order is lost. Thus, a regrown crystal does not inherit any features from an initially present crystal. Such is true for materials exhibiting a well-defined melting point. However, polymer crystallites have a wide range of melting temperatures, enabling paradoxical phenomena such as the coexistence of melting and crystallization. Here, we report a self-seeding technique that enables the generation of arrays of orientation-correlated polymer crystals of uniform size and shape ('clones') with their orientation inherited from an initial single crystal. Moreover, the number density and locations of these cloned crystals can to some extent be predetermined through the thermal history of the starting crystal. We attribute this unique behaviour of polymers to the coexistence of variable fold lengths in metastable crystalline lamellae, typical for ordering of complex chain-like molecules.

Anisotropic Electron Transport Properties in Sumanene Crystal

Abstract: The high electron mobility with large anisotropy was attained in the needle-like single crystal of sumanene, which was indicated by time-resolved microwave conductivity (TRMC) measurement.

Crystal Structure–Ionic Conductivity Relationships in Doped Ceria Systems

Abstract: In the past, it has been suggested that the maximum ionic conductivity is achieved in ceria, when doped with an acceptor cation that causes minimum distortion in the cubic fluorite crystal lattice. In the present work, this hypothesis is tested by measuring both the ionic conductivity and elastic lattice strain of 10 mol% trivalent cation-doped ceria systems at the same temperatures. A consistent set of ionic conductivity data is developed, where the samples are synthesized under similar experimental conditions. On comparing the grain ionic conductivity, Nd 0.10 Ce 0.90 O 2-δ exhibits the highest ionic conductivity among other doped ceria systems. The grain ionic conductivity is around 17% higher than that of Gd 0.10 Ce 0.90 O 2-δ at 500°C, in air. X-ray diffraction profiles are collected on the sintered powder of all the compositions, from room temperature to 600°C, in air. From the lattice expansion data at high temperatures, the minimal elastic strain due to the presence of dopant is observed in Dy 0.10 Ce 0.90 O 2-δ . Nd 0.10 Ce 0.90 O 2-δ exhibits larger elastic lattice strain than Dy 0.10 Ce 0.90 O 2-δ with better ionic conductivity at intermediate temperatures. Therefore, it is shown that the previously proposed crystal structure-ionic conductivity relationship based on minimum elastic strain is not sufficient to explain the ionic conductivity behavior in ceria-based system.

Characterization of high temperature piezoelectric crystals with an ordered langasite structure

Abstract: Piezoelectric single crystals with the ordered langasite structure A3BC3D2O14, including Sr3TaGa3Si2O14, Sr3NbGa3Si2O14, Ca3TaGa3Si2O14, and Ca3TaAl3Si2O14 (CTAS), were studied as a function of temperature, up to 900 °C. The dielectric permittivity e11 and piezoelectric coefficient d11 of the ordered crystals were found to be on the orders of 12–16 and 4–5 pC/N, respectively, slightly lower than langasite (La3Ga5SiO14-LGS) or langanite (La3Ga5.5Nb0.5O14) crystals which possess a disordered structure. The mechanical quality factor Q and electrical resistivity ρ, however, were found to be greatly improved at elevated temperatures ≥500 °C, being one to two orders of magnitude higher, due to cation ordering. Of particular interest is the CTAS crystal, in which, the Ga cations are totally replaced by low cost Al cations. Together with its thermally stable piezoelectric properties and high electrical resistivity, CTAS crystals offer a competitive material for high temperature sensing applications.

Development of crystal texture in Nd-lean amorphous Nd9Fe85B6 under hot deformation

Abstract: A key challenge in synthesizing anisotropic nanocomposite magnets is to form crystallographic texture for R2Fe14B crystals in R-lean alloys (R=rare earth, R<10 at. %). Here, we demonstrate a (00l) texture development for Nd2Fe14B nanocrystals in a Nd-lean amorphous Nd9Fe85B6 under a hot deformation at a large uniaxial stress of ∼310 MPa. The unusual texture formation is attributed to a preferential nucleation of Nd2Fe14B crystals in amorphous matrix at the large stress. The present study provides an opportunity to yield anisotropic nanocomposite magnets from Nd-lean alloys and thus is of wide interest.

Controlled Synthesis of Ln3+ (Ln = Tb, Eu, Dy) and V5+ Ion-Doped YPO4 Nano-/Microstructures with Tunable Luminescent Colors

Abstract: YPO4 nano/microcrystals with multiform crystal phases and morphologies, such as hexagonal nano/submicroprisms, spherical-like nanoparticles, and nanorods with different length/diameter ratios as well as tetragonal nanospindles, have been synthesized via a facile hydrothermal route. A series of controlled experiments indicate that the pH values in the initial solution, phosphorus sources, and the organic additive trisodium citrate (Cit3−) are responsible for crystal phase and shape determination of final products. It is found that Cit3− as a ligand and shape modifier has the dynamic effect by adjusting the growth rate of different facets under different experimental conditions, resulting in the formation of various geometries of the final products. The possible formation mechanisms for products with diverse architectures have been presented. More importantly, a systematic study on the photoluminescence of Ln3+ (Ln = Tb, Eu, Dy) and V5+ ion-doped samples annealed at 500 °C has been explored in order to obta...

Isolation of calcium-phosphate crystals of bone by non-aqueous methods at low temperature.

Abstract: We have developed low temperature nonaqueous solution methods as well as low power plasma ashing for the degradation of the organic matrix of bone power which have permitted us to obtain bone crystals essentially free of organic matrix constituents without any significant change in their composition, overall structure, or internal short-range order. We have also been able to disperse the crystals, which has made it possible to examine the isolated crystals by X-ray diffraction and resolution-enhanced Fourier transform infrared (FTIR) spectroscopy and isolated single crystals by high resolution transmission electron microscopy (TEM) and electron diffraction. TEM of isolated single crystals of fish, chicken, mouse and bovine bone have clearly demonstrated that the crystals are very thin plates. No rod or needle-like crystals were observed in any of the bone samples in the four species studied including the earliest crystals deposited. Although there were variations in the size distribution of the crystals in the different species studied, in general the average crystal dimensions were very similar.

Nanodiamond Photoemitters Based on Strong Narrow‐Band Luminescence from Silicon‐Vacancy Defects

Abstract: 2009 WILEY-VCH Verlag Gm Single-photon emission (SPE) was first observed from single atoms and ions in traps and from molecules. More recently, semiconductor quantum dots and photoactive point defects, such as nitrogen-vacancy (N-V), silicon-vacancy (Si-V), and nickel-nitrogen complexes in diamond have been used in SPE experiments. Among the array of luminescent nanomaterials, point defects (color centers) in diamond seem to be the most-promising single-photon source for quantum-physics applications, such as optics, information processing, and cryptography. This is due to the remarkable photoemission properties of the defects, such as their extraordinary stability at room temperature and their high quantum efficiency (typically >0.1), which are combined with the unique chemical and mechanical properties of the diamond material itself. In recent years, in addition to quantum emitters, classical light sources based on luminescence from the color centers in diamond have generated great interest for biomedical applications such as optical labels. Up to now, research into diamond photoactivity has focused largely on photoluminescence (PL) from N-V defects. N-V complexes are commonly formed by diamond doping with nitrogen, during its synthesis by high pressure–high temperature (HPHT) or chemical vapor deposition (CVD) techniques, or by implantation of nitrogen ions into diamond. In both cases, a large number of the N-V centers are created in the diamond bulk, and a spatial selection is required to detect single-photon emission from individual defects. This complication can be partially alleviated if individual defects do not occupy the same host matrix, and hence the production of isolated diamond nanoparticles with single photoactive defects would be a revolution for practical SPE applications. For instance, this would allow for the composition of sets of optical emitters of any desirable configuration, and facilitate the convenient integration of the nanoemitters into larger structures. Moreover, the use of nanoparticles to house individual qubits may offer new advances in the development of quantum computers. The use of diamond nanoparticles with strong and narrow-band luminescence as optical labels would be very applicable for biotechnology and medicine. However, the thermodynamic stability of the point defects in the diamond matrix can change dramatically whenmoving from bulk crystal to diamond nanoparticles with a lateral size of a few lattice constants. In particular, density functional tight binding (DFTB) simulations predict the preferable positioning of nitrogen as being at the surface of the nanodiamonds, where the formation of N-V defects is unlikely to occur. This theoretical conclusion is confirmed by a number of experimental works. Rabeau et al. found that the luminescent activity of N-V centers in CVD diamond nanocrystals strongly depends on the crystal size, with no N-V emission being observed from nanocrystals smaller than 40 nm. Also, no nitrogen-related defects were detected in a PL study of detonation nanodiamond with typical grain sizes below 10 nm. Therefore, to develop efficient photoemitters on a base of nanodiamond (ND) with a size of < 10 nm (characteristic for nanotechnology), a Si-V emitting source is suggested here as an alternative to N-V sources. Structurally, both defects are expected to be a single atom of an impurity (nitrogen or silicon) and a vacancy located in a neighboring site of the diamond lattice. However, the Si-V center possesses a number of advantages in its optical properties over N-V centers in diamond bulk. Zerophonon (ZP) emission from Si-V defects is observed at wavelengths of 738 nm and 757 nm, which are away from the characteristic broad band PL of ND, spread between 450 nm and 650 nm, whereas the 637 nm ZP emission from N-V defects (in its negative-charge state, which is usually used for single-photon observation) overlaps this PL. Moreover, the very-weak vibronic sideband of the 738 nm electronic transition

Nitrogen and Hydrogen Adsorption by an Organic Microporous Crystal

Abstract: Quick on the uptake: Following its identification during a targeted search, the intriguing crystal structure of 3,3′,4,4′-tetra(trimethylsilylethynyl)biphenyl was investigated. Simple removal of the included solvent provides an organic crystal with an open microporous structure that has a striking similarity to that of zeolite A (see picture). Reversible adsorption of nitrogen and hydrogen gases at 77 K confirms that the microporosity is permanent.

Effective Nonlinear GaSe Crystal. Optical Properties and Applications

Abstract: We present an overview of the current state of the literature and research performed by the authors of the present paper on the experimental and theoretical results on the structural-, optical-, nonlinear optical (NLO)-properties (including two-photon absorption (TPA) and the terahertz (THz) range of spectra) and practical applications of a highly anisotropic Gallium Selenide (GaSe) semiconductor with emphasis on the ɛ-GaSe Physical properties of ɛ-GaSe are important to researchers and designers developing different devices by using this material This crystal possesses an outstanding NLO properties: high optical birefringence Δn ∼ 03 at 700 nm; high transparency range (07−180 μm) with low absorption coefficient (α ≤ 03 cm−1); very high nonlinear susceptibility χ(2) (d 22 ≈ 86 ± 17 pm/V, corresponding to (20 ± 04) × 10−7 esu) that is used for phase matched second harmonic generation (SHG) in a wide transparency range; high power threshold for optical damage; possibility to perform optical frequency conversion under phase-matching conditions in the near- to mid-IR and THz range of spectra, etc The domain structure of crystal in connection with the NLO properties is discussed as studied by confocal Raman microscopy experiments Perspectives for future research of GaSe are considered in the present article, which does not pretend to be one reflecting all existing papers on GaSe crystal and discussed subjects

Crystal Structure of A-amylose: A Revisit from Synchrotron Microdiffraction Analysis of Single Crystals

Abstract: The three-dimensional structure of A-amylose crystals, as a model of the crystal domains of A-starch granules, was revised using synchrotron radiation microdiffraction data collected from individual micron-sized single crystals. The resulting data sets allowed a determination of the structure with conventional X-ray structure determination techniques normally used for small molecules and not for polymers. Whereas the gross features of this improved structure do not differ extensively from previous structure determination, the high resolution of the diffraction diagrams, which is unusual for a crystalline polymer, allowed the resolution of important new fine details. These include a distortion of the amylose double helices resulting from the occurrence of two intracrystalline molecules of water and a tight network of hydrogen bonds involving each of the primary and secondary hydroxyl groups of the glucosyl moieties. Water molecules are located in discrete pockets that do not interfere with one another. In ...