Showing papers on "Crystal published in 2005"
TL;DR: By using micromechanical cleavage, a variety of 2D crystals including single layers of boron nitride, graphite, several dichalcogenides, and complex oxides are prepared and studied.
Abstract: We report free-standing atomic crystals that are strictly 2D and can be viewed as individual atomic planes pulled out of bulk crystals or as unrolled single-wall nanotubes. By using micromechanical cleavage, we have prepared and studied a variety of 2D crystals including single layers of boron nitride, graphite, several dichalcogenides, and complex oxides. These atomically thin sheets (essentially gigantic 2D molecules unprotected from the immediate environment) are stable under ambient conditions, exhibit high crystal quality, and are continuous on a macroscopic scale.
10,586 citations
TL;DR: In this paper, the authors examined the physical mechanisms governing the formation of snow crystals, treating this problem as a case study of the dynamics of crystal growth from the vapour phase.
Abstract: We examine the physical mechanisms governing the formation of snow crystals, treating this problem as a case study of the dynamics of crystal growth from the vapour phase. Particular attention is given to the basic theoretical underpinnings of the subject, especially the interplay of particle diffusion, heat diffusion and surface attachment kinetics during crystal growth, as well as growth instabilities that have important effects on snow crystal development. The first part of this review focuses on understanding the dramatic variations seen in snow crystal morphology as a function of temperature, a mystery that has remained largely unsolved since its discovery 75 years ago. To this end we examine the growth of simple hexagonal ice prisms in considerable detail, comparing crystal growth theory with laboratory measurements of growth rates under a broad range of conditions. This turns out to be a surprisingly rich problem, which ultimately originates from the unusual molecular structure of the ice surface and its sensitive dependence on temperature. With new clues from precision measurements of attachment kinetics, we are now just beginning to understand these structural changes at the ice surface and how they affect the crystal growth process. We also touch upon the mostly unexplored topic of how dilute chemical impurities can greatly alter the growth of snow crystals. The second part of this review examines pattern formation in snow crystals, with special emphasis on the growth of snow crystal dendrites. Again we treat this as a case study of the more general problem of dendritic growth during diffusion-limited solidification. Since snow crystals grow from the vapour, we can apply dendrite theory in the simplified slowgrowth limit where attachment kinetics dominates over capillarity in selecting the tip velocity. Although faceting is quite pronounced in these structures, many aspects of the formation of snow crystal dendrites are fairly well described using a theoretical treatment that does not explicitly incorporate faceting. We also describe electrically modified ice dendrite growth, which produces some novel needle-like structures.
538 citations
454 citations
TL;DR: In this article, the effect of the crystal size of Ni on the growth of carbon nanofibers (CNFs) was studied in a tapered oscillating element microbalance reactor.
435 citations
TL;DR: The main strength of FINDSYM as mentioned in this paper is its ease of use, which can be run over the internet by filling out a simple and user-friendly html form, no matter what setting the original information is given in.
Abstract: Given the lattice parameters of a crystal and the positions of atoms in the unit cell, it is often useful to identify the space-group symmetry of the crystal in a standard setting along with the Wyckoff positions to which the atoms belong. This can be a very difficult problem to do by hand, especially if the unit cell is given in a non-standard setting or if there are many atoms in the unit cell. The aim of this program is to identify the space-group symmetry and give the lattice parameters and Wyckoff positions of the atoms in a standard setting, no matter what setting the original information is given in. We have compared FINDSYM with other programs that address the same general problem. MISSYM (Le Page, 1987) and its implementation as ADDSYM in the program PLATON (Spek, 2003) searches for possibly missed additional symmetry in a given coordinate set. Two examples of its use are given at the internet site for ADDSYM (1998). FINDSYM failed in both cases to find the additonal symmetry found by ADDSYM. FINDSYM requires that all atomic positions be reasonably close to their idealized positions in the space-group symmetry we are seeking. TRS, RGS and SFND implemented in the program KPLOT (Hannemann et al., 1998; Hundt et al., 1999) use algorithms very similar to those in FINDSYM. Two examples of its function are given by Hannemann et al. (1998) and two more by Hundt et al. (1999). Each of these four examples consists of structures found during the investigation of the energy landscape of a system using global optimization techniques. In each of the four examples, FINDSYM successfully found the same space-group symmetry as that found by the KPLOT routines (this required using a tolerance of 0.03–0.06 Å). FINDSYM was not developed for the purpose of extracting a space-group symmetry from noisey data, although it can accomplish this if the data are not too noisey, as illustrated by its success with the KPLOT examples. Its limitations in this regard are illustrated by its failure with the ADDSYM examples. The main strength of FINDSYM is its ease of use. It can be run over the internet by filling out a simple and user-friendly html form. 2. Method of solution
410 citations
TL;DR: Second-harmonic generation (SHG) in magnetically ordered crystals is reviewed in this article, where the specific degrees of optical experiments -including spectral, spatial, and temporal resolution - lead to the observation of novel physical effects that cannot be revealed by other techniques of probing magnetism.
Abstract: Second-harmonic generation (SHG) in magnetically ordered crystals is reviewed. The symmetry of such crystals is determined by the arrangement of both the charges and the spins, so their contributions to the crystallographic and the magnetic structures, respectively, must be distinguished. Magnetic SHG is introduced as a probe for magnetic structures and sublattice interactions. The specific degrees of optical experiments - including spectral, spatial, and temporal resolution - lead to the observation of novel physical effects that cannot be revealed by other techniques of probing magnetism. These include local or hidden phase transitions, interacting magnetized and polarized sublattices and domain walls, and magnetic interfaces. SHG in various centrosymmetric and noncentrosymmetric crystal classes of antiferromagnetic oxides such as Cr2O3, hexagonal RMnO3(R=Sc,Y,In,Ho-Lu), magnetic garnet films, CuB2O4, CoO, and NiO, is discussed.
356 citations
348 citations
TL;DR: In this paper, the crystal structure of tobermorite 14 A (plombierite) was solved by means of the application of the order disorder (OD) theory and was refined through synchrotron radiation diffraction data.
Abstract: The crystal structure of tobermorite 14 A (plombierite) was solved by means of the application of the order-disorder (OD) theory and was refined through synchrotron radiation diffraction data. Two polytypes were detected within one very small crystal from Crestmore, together with possibly disordered sequences of layers, giving diffuse streaks along c*. Only one of the two polytypes could be refined: it has B11b space group symmetry and cell parameters a = 6.735(2) A, b = 7.425(2) A, c = 27.987(5) A, γ = 123.25(1)°. The refinement converged to R = 0.152 for 1291 reflections with F o >4σ(F o ). The characteristic reflections of the other polytype, F2dd space group, a 11.2 A, b 7.3 A, c 56 A, were recognized but they were too weak and diffuse to be used in a structure refinement. The structure of tobermorite 14 A is built up of complex layers, formed by sheets of sevenfold coordinated calcium cations, flanked on both sides by wollastonite-like chains. The space between two complex layers contains additional calcium cations and H 2 O molecules; their distribution, as well as the system of hydrogen bonds, are presented and discussed. The crystal chemical formula indicated by the structural results is Ca 5 Si 6 O 16 (OH) ) . 7H 2 O.
346 citations
TL;DR: The great differences of the SERS spectra of rhodamine B at these Ag colloids confirmed that the shapes and crystal planes of silver have great effect on Raman enhancement, especially the crystal planes.
Abstract: By solution-based method, three kinds of silver colloids, self-assembled nanowires, triangular nanoplates and quasispherical nanoparticles, have been synthesized. TEM studies revealed that they exposed different crystal planes, such as {111} crystal planes to triangular nanoplates, mainly {100} and {111} planes to self-assembly nanowires. Hereby, do the distinct shapes and crystal planes have an impact on the surface enhanced Raman scattering (SERS)? The great differences of the SERS spectra of rhodamine B at these Ag colloids confirmed that the shapes and crystal planes of silver have great effect on Raman enhancement, especially the crystal planes.
343 citations
TL;DR: The organic light-emiiting diodes with attractive performance have been achieved using trans-DPDSB as a light-emitting layer, and the amplified spontaneous emission of the needlelike crystals has been observed.
Abstract: Through adding two substituent phenyl groups on distyrylbenzene, we have obtained the cross stacking of 2,5-diphenyl-1,4-distyrylbenzene with two trans double bonds (trans-DPDSB) in crystalline state. In such a cross-stacking mode, the solid-state emission exhibits high-intensity, having characteristics similar to its single molecule. The organic light-emiiting diodes (OLEDs) with attractive performance have been achieved using trans-DPDSB as a light-emitting layer, and the amplified spontaneous emission of the needlelike crystals has been observed.
329 citations
TL;DR: The results of biophysical experiments in solution and computational studies that are inconsistent with the reported crystal structure suggest that the biologically relevant structure of the human telomere quadruplex in K+ solution is not the one determined in the published crystalline state.
Abstract: The structure of human telomere DNA is of intense interest because of its role in the biology of both cancer and aging. The sequence [5 0 -AGGG(TTAGGG)3] has been used as a model for telomere DNA in both NMR and X-ray crystallographic studies, the results of which show dramatically different structures. In Na 1 solution, NMR revealed an antiparallel G-quadruplex structure that featured both diagonal and lateral TTA loops. Crystallographic studies in the presence of K 1 revealed a flattened, propeller-shaped structure featuring a parallel-stranded G-quadruplex with symmetrical external TTA loops. We report the results of biophysical experiments in solution and computational studies that are inconsistent with the reported crystal structure, indicating that a different structure exists in K 1 solutions. Sedimentation coefficients were determined experimentally in both Na 1 and K 1 solutions and were compared with values calculated using bead models for the reported NMR and crystal structures. Although the solution NMR structure accurately predicted the observed S-value in Na 1 solution, the crystal structure predicted an S-value that differed dramatically from that experimentally observed in K 1 solution. The environments of loop adenines were probed by quantitative fluorescence studies using strategic and systematic singlesubstitutions of 2-aminopurine for adenine bases. Both fluorescence intensity and quenching experiments in K 1 yielded results at odds with quantitative predictions from the reported crystal structure. Circular dichroism and fluorescence quenching studies in the presence of the crowding agent polyethylene glycol showed dramatic changes in the quadruplex structure in K 1 solutions, but not in Na 1 solutions, suggesting that the crystal environment may have selected for a particular conformational form. Molecular dynamics simulations were performed to yield model structures for the K 1 quadruplex form that are consistent with our biophysical results and with previously reported chemical modification studies. These models suggest that the biologically relevant structure of the human telomere quadruplex in K 1 solution is not the one determined in the published crystalline state.
TL;DR: Evidence for ordering of liquid atoms adjacent to an interface with a crystal is provided, based on real-time high-temperature observations of alumina-aluminum solid-liquid interfaces at the atomic-length scale.
Abstract: Understanding the nature of solid-liquid interfaces is important for many processes of technological interest, such as solidification, liquid-phase epitaxial growth, wetting, liquid-phase joining, crystal growth, and lubrication. Recent studies have reported on indirect evidence of density fluctuations at solid-liquid interfaces on the basis of x-ray scattering methods that have been complemented by atomistic simulations. We provide evidence for ordering of liquid atoms adjacent to an interface with a crystal, based on real-time high-temperature observations of alumina-aluminum solid-liquid interfaces at the atomic-length scale. In addition, crystal growth of alumina into liquid aluminum, facilitated by interfacial transport of oxygen from the microscope column, was observed in situ with the use of high-resolution transmission electron microscopy.
TL;DR: In this article, the authors studied the kinetics of crystal nucleation of an undercooled Lennard-Jones liquid using various path-sampling methods and obtained the rate constant and elucidate the pathways for crystal nucleations, in which critical solid nuclei can be small, compact and face centered cubic, but also large, less ordered, and more body centered cubic.
Abstract: We study the kinetics of crystal nucleation of an undercooled Lennard-Jones liquid using various path-sampling methods. We obtain the rate constant and elucidate the pathways for crystal nucleation. Analysis of the path ensemble reveals that crystal nucleation occurs along many different pathways, in which critical solid nuclei can be small, compact, and face centered cubic, but also large, less ordered, and more body centered cubic. The reaction coordinate thus includes, besides the cluster size, also the quality of the crystal structure.
TL;DR: In this article, a thermodynamically consistent equation for the calculation of the pressure generated during crystal growth in porous materials is provided, which makes use of an equation derived previously (paper I of this series) which is based on the chemical potentials of loaded and unloaded surfaces of confined crystals in porous material.
TL;DR: In this article, two new anil molecules exhibiting photochromism in the crystalline state, N-(4-hydroxy)-salicylidene-amino-4-(methylbenzoate) (2) and N-(3,5-di-tert-butylsaliclyidene)-4-aminopyridine (3), are obtained.
Abstract: Two new anil molecules exhibiting photochromism in the crystalline state, N-(4-hydroxy)-salicylidene-amino-4-(methylbenzoate) (2) and N-(3,5-di-tert-butylsalicylidene)-4-aminopyridine (3), are obtained. Upon irradiation in the UV, the yellow crystals change color to red, owing to enol-keto intramolecular tautomerism. The red color disappears, when crystals are left in the dark or irradiated with visible light. 3 has the most stable keto form among all anil-type photochromic compounds (τ = 460 days at room temperature). Both exhibit nonlinear optical (NLO) properties and show powder second harmonic generation (SHG) of respectively 10 and 3 times vs urea. X-ray diffraction shows acentric structures where molecules line up “head-to-tail” through hydrogen bonds for 2 (space group Pc), or form a chiral helix 3 (space group P32). Evidence of reversible structural change is given for 3, and we demonstrate the functionality of this crystal as an NLO switching material, as SHG can be photomodulated by about 30%.
TL;DR: In this article, phase and size controlled CoO nanocrystals of hexagonal and cubic phases have been prepared by thermal decomposition of Co(acac)3 in oleylamine under an inert atmosphere.
Abstract: Highly crystalline, phase- and size-controlled CoO nanocrystals of hexagonal and cubic phases have been prepared by thermal decomposition of Co(acac)3 in oleylamine under an inert atmosphere. Kinetic and thermodynamic control for the precursor formation leads to two different seeds of hexagonal and cubic phases at higher temperatures. The crystal size of both CoO phases can be easily manipulated by changing the precursor concentration and reaction temperature.
TL;DR: Use of a new ionic liquid crystal, 1-dodecyl-3-methylimidazolium iodide, and iodine as an electrolyte of dye-sensitized solar cells leads to a high short circuit photocurrent density and a high light-to-electricity conversion efficiency, due to a self-assembled structure of the imidazlium cations, resulting in high conductivity of the electrolyte.
TL;DR: It is found that the nature of a polaron in this material is dependent upon both the crystal structure and distribution of oxygen vacancies, which may be explained in terms of O2p-W5d mixing, including the formation of long W-W dimer bonds.
Abstract: The oxygen vacancy in WO3 has previously been implicated in the electrochromism mechanism in this material. Previous theoretical calculations on the oxygen vacancy in WO3 have not considered the full range of crystal structures adopted by the material. Here we report studies of the oxygen vacancy in seven crystal phases. The use of a very accurate tungsten plane-wave pseudopotential means that a byproduct of this study is a more detailed and complete picture of undefected WO3 than previously available. Electronic structures of the crystal phases in both undefected and defected systems have been calculated and are discussed. The band gap in WO3 is dependent upon bonding−antibonding interactions, these being dependent upon overlap in each direction. The effect of an oxygen vacancy is dependent upon the availability of both Op and Wd electrons, this being different for the various phases. A variety of behavior is predicted, which may be explained in terms of O2p−W5d mixing, including the formation of long W−...
TL;DR: This article explains the different hysteresis activities caused by different types of antifreeze proteins at equimolar concentrations as a consequence of their solubility features during the phase of reversible association between the proteins and the ice, i.e., at the melting point.
TL;DR: The urea–solvent interface at the nanometre scale is studied and kinetic Monte Carlo simulations of the micrometre-scale three-dimensional growth of urea crystals are reported, offering insights into the role of the solvent, the degree of supersaturation, and the contribution that extended defects make to crystal growth.
Abstract: Understanding crystal growth is essential for controlling the crystallization used in industrial separation and purification processes. Because solids interact through their surfaces, crystal shape can influence both chemical and physical properties. The thermodynamic morphology can readily be predicted, but most particle shapes are actually controlled by the kinetics of the atomic growth processes through which assembly occurs. Here we study the urea-solvent interface at the nanometre scale and report kinetic Monte Carlo simulations of the micrometre-scale three-dimensional growth of urea crystals. These simulations accurately reproduce experimentally observed crystal growth. Unlike previous models of crystal growth, no assumption is made that the morphology can be constructed from the results for independently growing surfaces or from an a priori specification of surface defect concentration. This approach offers insights into the role of the solvent, the degree of supersaturation, and the contribution that extended defects (such as screw dislocations) make to crystal growth. It also connects observations made at the nanometre scale, through in situ atomic force microscopy, with those made at the macroscopic level. If extended to include additives, the technique could lead to the computer-aided design of crystals.
TL;DR: Empirical guidelines for steering molecular assembly into prescribed crystal architectures via hydrogen bonding continue to emerge, with recent developments demonstrating that charge-assisted hydrogen bonds introduce both strength and compliance that can facilitate solid state design.
TL;DR: In this article, a novel crystal growth method has been established for the growth of single crystal with selective orientation at room temperature using volatile solvent, the saturated solution containing the material to be crystallized was taken in an ampoule and allowed to crystallize by slow solvent evaporation assisted with a ring heater.
TL;DR: The crystal structure of 4-nitrobenzamide·4-iodobenzamide has amide dimer tape and iodo⋯nitro interaction in orthogonal directions as discussed by the authors.
Abstract: Molecular tapes mediated via strong O–H⋯N hydrogen bonds and weak C–I⋯O interactions are present in complexes of 4-nitrobenzoic acid·4-iodopyridine and 3,5-dinitrobenzoic acid·4-iodopyridine. Crystal structure of 4-nitrobenzamide·4-iodobenzamide has amide dimer tape and iodo⋯nitro interaction in orthogonal directions. There is good structural insulation in the hydrogen bonding and halogen bonding domains in these crystal structures. However, the less polarizable bromine and chlorine atoms show cross-pairing interactions that are less predictable. These preliminary results show that both carboxylic acid⋯pyridine and iodo⋯nitro heterosynthons may be simultaneously exploited for crystal design.
TL;DR: In this paper, it was shown that the [N-V]− center in diamond is not photochemically stable and that with intense excitation, a proportion of the centers in a crystal are converted to [N −V]0 centers.
TL;DR: The critical nucleus formed during the nucleation process has the crystal structure of bulk NaCl, and it is observed that the critical nucleus is clearly faceted, the nuclei have a cubical shape.
Abstract: We report a numerical simulation of the rate of crystal nucleation of sodium chloride from its melt at moderate supercooling. In this regime nucleation is too slow to be studied with “brute force” molecular-dynamics simulations. The melting temperature of s“Tosi Fumi” d NaCl is ,1060 K. We studied crystal nucleation at T= 800 and 825 K. We observe that the critical nucleus formed during the nucleation process has the crystal structure of bulk NaCl. Interestingly, the critical nucleus is clearly faceted, the nuclei have a cubical shape. We have computed the crystal-nucleation rate using two completely different approaches, one based on an estimate of the rate of diffusive crossing of the nucleation barrier, the other based on the forward flux sampling and transition interface sampling methods. We find that the two methods yield the same result within an order of magnitude. However, when we compare the extrapolated simulation data with the only available experimental results for NaCl nucleation, we observe a discrepancy of nearly five orders of magnitude. We discuss the possible causes for this discrepancy. © 2005 American Institute of Physics . fDOI: 10.1063/1.1896348g
TL;DR: Colloidal, organic solvent-soluble Ln3+-doped LaVO4 nanoparticles have been synthesized by a precipitation reaction in the presence of (C18H37O)2PS2- as ligand, that coordinates to the surface of the nanoparticles.
Abstract: Colloidal, organic solvent-soluble Ln3+-doped LaVO4 nanoparticles have been synthesized by a precipitation reaction in the presence of (C18H37O)2PS2- as ligand, that coordinates to the surface of the nanoparticles The materials are well soluble in chlorinated solvent such as chloroform Energy transfer of excited vanadate groups has been observed for Ln3+ ions that emit in the visible and the near-infrared (Eu3+, Tm3+, Nd3+, Er3+, Ho3+, Dy3+, Sm3+, Pr3+), thus making it a very generic sensitization mechanism The LaVO4 nanoparticles have a different crystal structure than bulk LaVO4 ones (xenotime instead of monazite), similar to YVO4 nanoparticles This xenotime crystal structure results in a more asymmetric crystal field around the Ln3+ ions that is advantageous to their luminescence, for it increases the radiative rate constant, thus reducing quenching processes
Patent•
06 Dec 2005TL;DR: In this paper, the authors proposed a method to produce a resonant cavity light emitting device using a seed gallium nitride crystal and a source material in a nitrogen-containing superheated fluid.
Abstract: A method may produce a resonant cavity light emitting device. A seed gallium nitride crystal and a source material in a nitrogen-containing superheated fluid may provide a medium for mass transport of gallium nitride precursors therebetween. A seed crystal surface may be prepared by applying a first thermal profile between the seed gallium nitride crystal and the source material. Gallium nitride material may be grown on the prepared surface of the seed gallium nitride crystal by applying a second thermal profile between the seed gallium nitride crystal and the source material while the seed gallium nitride crystal and the source material are in the nitrogen-containing superheated fluid. A stack of group III-nitride layers may be deposited on the single-crystal gallium nitride substrate. The stack may include a first mirror sub-stack and an active region adaptable for fabrication into one or more resonant cavity light emitting devices.
TL;DR: In this paper, the authors proposed two-dimensional coupled photonic crystal microcavity arrays as a route to achieve a slow group velocity of light (flat band) in all crystal directions.
Abstract: We recently proposed two-dimensional coupled photonic crystal microcavity arrays as a route to achieve a slow-group velocity of light (flat band) in all crystal directions. In this letter we present the experimental demonstration of such structures with a measured group velocity below 0.008c and discuss the feasibility of applications such as low-threshold photonic crystal lasers with increased output powers, optical delay components, and sensors.