Stages Of Crystal Growth Research Articles

Free energy and structure of calcium carbonate nanoparticles during early stages of crystallization

We introduce a metadynamics based scheme for computing the free energy of nanoparticles as a function of their crystalline order. The method is applied to small nanoparticles of the biomineral calcium carbonate to determine the preferred structure during early stages of crystal growth. For particles 2 nm in diameter, we establish a large energetic preference for amorphous particle morphologies. Particles with partial crystalline order consistent with vaterite are also observed with substantially lower probability. The absence of the stable calcite phase and stability of the amorphous state support recent conjectures that calcite formation starts via the deposition of amorphous calcium carbonate.

CVD‐Produced Polycrystalline Silicon Thin Film Prepared by Fluorine‐Mediated Crystal Growth on a Glass Substrate and its Thin Film Transistor Application

Communication: A set of the source gases Si2H6 and F2 in a conventional low‐pressure thermal CVD has enabled the direct preparation of device‐grade poly‐Si films at 450 °C. Field‐effect mobility exceeding 60 cm2/Vs and crystallinity exceeding 90% in bottom‐gate TFTs have been achieved; the high mobility is associated with fast nucleation in the early stage of crystal growth, which has been revealed by AFM and TEM images.

Nascent Crystallization of a Growing Chain on a Catalyst Surface: A Nonequilibrium Molecular Dynamics Simulation Study

The growing chain molecular dynamics (GCMD) simulation method, a new nonequilibrium molecular dynamics code, is proposed to simulate the polymer chain aggregation behavior during polymerization on a catalyst surface. We found that the growing chain crystallizes on the surface in two stages: the nucleation stage and the crystal growth stage. In the first part of the nucleation period, the short polymerizing chain first absorbs on the surface and can be in either an ordered or disordered structure. Still in the nucleation period, when the chain reaches a degree of polymerization, about 100 bonds, the chain folds into a stable nucleus on the substrate with 3-5 stems. In the crystal growth stage where the polymerization also proceeds, we observed a stem elongation process in combination with a chain folding process. In the stem elongation step, the number of stems in the nucleus remains constant, and all the stems expand together to a length of ca. 5-25 ns. In the subsequent chain folding step, the stem length decreases about 20 bonds within a period of ca. 0.1-0.5 ns. During chain growth, the elongation process and the folding process occur in an alternating and repeated fashion. The crystallization mechanism of the polymerizing chain was discussed.

Mid-infrared Cr^2+:ZnSe random powder lasers

We report simple methods of laser active Cr2+:ZnSe powder fabrication with average grain sizes of either approximately 10 or approximately 1 microm without crystal growth stage. Pure, uniformly mixed ZnSe and CrSe powders annealed at 1000 masculineC for 3 days in a sealed evacuated (approximately 10(-4) Torr) quartz ampoule exhibited middle-infrared laser action at room temperature under 1.56 microm excitation of D(2) Raman shifted radiation of Nd:YAG laser. The output-input characteristic clearly demonstrated the threshold-like behavior of the output signal with the threshold energy level of 0.5 and 3 mJ in 2.9 mm spot for 10 and 1 microm grain sizes, respectively.

Numerical study of heat transport and fluid flow during different stages of sapphire Czochralski crystal growth

For different stages of Czochralski crystal growth of sapphire, the flow and temperature field have been studied numerically using the finite element method. The configuration usually used in a real Czochralski crystal growth process consists of a crucible, active afterheater, induction coil with two parts, insulation, crystal, melt and gas. At first the volumetric distribution of heat generation inside the metal crucible and afterheater was calculated. Using this heat distribution as a source the fluid flow and temperature field in the whole system as well as the crystal–melt interface were computed. It was shown that during the last stage of the growth process, the temperature maximum of the system is located at the crucible free surface and the crystal–melt interface has the highest deflection toward the melt.

Time-dependent growth of biomimetic apatite on anodic TiO 2 nanotubes

We report on the initial and later stages of apatite formation from simulated body fluid on titania with different surface morphologies (compact or nanotubular) and different crystal structures (anatase or amorphous). The nanotubular layers were fabricated by electrochemical anodization in fluoride-containing electrolytes. The study investigates the enhanced apatite deposition on titania nanotubes. In the initial stages of apatite growth, more nuclei are formed on the nanotubular surface than on flat compact TiO 2. While the crystallographic structure of the substrate plays a less important role than the morphology in the initial nucleation stages, it is of great importance in the later stages of apatite crystal growth. The nanotubular morphology combined with an anatase structure leads to the formation of apatite layers with a thickness of >6 nm in less than 2 days. No stable apatite layers can be observed on amorphous TiO 2 films, neither on compact nor on nanotubular substrates. XPS, FT-IR and XRD measurements reveal that carbonated hydroxyapatite (CHA) of low crystallinity is formed on annealed nanotubular and compact titania. Electrochemically grown and annealed TiO 2 nanotube arrays having anatase structure are expected to be a good precursor system for the formation of CHA and thus for the preparation of osseointegrative implants.

Thermodynamic and kinetic control of crystal growth in CdS nanomaterials

Download options Please wait... Article information DOI https://doi.org/10.1039/B717656B Article type Paper Submitted 14 Nov 2007 Accepted 18 Jan 2008 First published 18 Feb 2008 Download Citation J. Mater. Chem., 2008,18, 1689-1693 BibTex EndNote MEDLINE ProCite ReferenceManager RefWorks RIS Permissions Request permissions Thermodynamic and kinetic control of crystal growth in CdS nanomaterials P. Christian and P. O'Brien, J. Mater. Chem., 2008, 18, 1689 DOI: 10.1039/B717656B To request permission to reproduce material from this article, please go to the Copyright Clearance Center request page. If you are an author contributing to an RSC publication, you do not need to request permission provided correct acknowledgement is given. If you are the author of this article, you do not need to request permission to reproduce figures and diagrams provided correct acknowledgement is given. If you want to reproduce the whole article in a third-party publication (excluding your thesis/dissertation for which permission is not required) please go to the Copyright Clearance Center request page. Read more about how to correctly acknowledge RSC content. Search articles by author P. Christian P. O'Brien

Initial stages of SiC crystal growth by PVT method

AbstractInitial stages of SiC crystal growth by Physical Vapor Transport method were investigated. The following features were observed: (a) many nucleation crystallization centres appeared on the seed surface during the initial stage of the growth, (b) at the same places many separate flat faces generated on the crystallization front, (c) the number of facets was dependent on the shape of the crystallization front and decreased during growth, (d) appearance of many facets lead to decrease of structural quality of crystals due to degradation of regions where crystallization steps from independent centres met. The results revealed that the optimal crystallization front should be slightly convex, which permits the growth of crystals with single nucleation centre and evolution of single facet on the crystallization front. The subjects of study were the shape and the morphology of growth interface. Defects in the crystallization fronts and wafers cut from the crystals were studied by optical microscopy, atomic force microscopy (AFM) combined with KOH etching and X‐ray diffraction. (© 2007 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

One-step Polyol Synthesis and Langmuir−Blodgett Monolayer Formation of Size-tunable Monodisperse Rhodium Nanocrystals with Catalytically Active (111) Surface Structures

Size-tunable monodisperse Rh nanocrystals can offer unique properties for many heterogeneous catalytic reactions (such as hydrogenation, hydroformylation, and hydrocarbonylation) of both scientific and technological interest. In this article, we report the synthesis of monodisperse, well-shaped Rh nanocrystals in a range of 5−15 nm by a one-step polyol reduction at temperatures of 170−230 °C under Ar, using rhodium(III) acetylacetonate [Rh(acac)3] as the source of metal ions, 1,4-butanediol as the reducing solvent, and poly(vinylpyrrolidone) as the capping agent. Two-dimensional projects of the nanocrystals are polygons, dominated by hexagons, pentagons, and triangles with catalytically active (111) surfaces (>65% yield). Over 45% of the polygons are multiple (111) twinned particles (hexagons and pentagons), favored by thermodynamics. To achieve size uniformity, adjustment of the reduction kinetics of Rh(acac)3 in the nucleation and crystal growth stages has been shown to depend upon several synthetic par...

Surface structures of Mg on Mo(1 1 0) surface investigated by RHEED

The surface structures and the growth mode of Mg deposited on Mo(1 1 0) surface have been investigated using a molecular beam epitaxy system with a reflection high-energy electron diffraction. The surface phase diagram of Mg on the Mo(1 1 0) substrate for deposition thickness versus substrate temperature was completed. Three kinds of surface structures, α, β, and γ were observed in the range of 0.1 ML ∼ 4.0 ML. These surface structures are commensurate with the atomic arrangement of the Mo(1 1 0) plane. The dense direction of each structure is parallel to the [0 0 1]Mo orientation and the Mg atoms are commensurate every five, four and six Mo atoms in [ 1 1 ¯ 1 ] Mo orientation in α, β and γ structure, respectively. Moreover, the β structure has the same arrangement of a bcc(1 1 0) plane rotated by 54.7° from the Mo(1 1 0) plane with elongation by 1.17 times. The δ structure, which is three-dimensional crystallites of Mg in initial stage of crystal growth, was observed at the thickness more than 4.0 ML below 200 °C. From this pattern it was found that the crystallites of Mg have a specific epitaxial orientation relationship for the Mo(1 1 0) substrate, [ 1 1 ¯ 0 0 ] Mg ∥ [ 1 1 ¯ 0 ] Mo and (0 0 0 1)Mg∥(1 1 0)Mo, and Mg film grows by Stranski–Krastanov growth mode.

Molecular dynamics study of crystallization of polymer systems confined in small nanodomains

We study the ordering dynamics of polymer crystallization using molecular dynamics simulations, in which the polymers are confined to small nanodomains surrounded by a noncrystalline medium. Although the adsorption process and surface diffusion of polymer chains play a major role in the case of the crystalline interface, the domain interface which consists of noncrystalline medium does not have such a crystal substrate to induce the growth of a crystal layer, which may lead to different ordering dynamics. We found that existence of a noncrystalline domain interface has two opposite effects. In the case of semiflexible polymer systems, the domain interface accelerates crystallization in the initial period, whereas it suppresses crystallization in the intermediate or late period. When the rigidity of polymer chains increases, the acceleration effect of the initial crystallization induced by the domain interface is sometimes hidden by spontaneous homogeneous nucleation. These ordering behaviors can be explained by the restriction on the local chain direction near the domain surface in the crystal nucleation stage and the restriction of large orientation relaxation and coalescence in the crystal growth stage. Simulation results reveal that the confinement by the noncrystalline medium does not have a simple effect on the polymer crystallization dynamics, but has various conflicting effects combined with the time stage, the rigidity of polymer chains, and the strength of the surrounding domain interface.

Air stable, high performance pentacene thin-film transistor fabricated on SiO2 gate insulator treated with β-phenethyltrichlorosilane

A pentacene thin-film transistor (TFT) was fabricated on a SiO2 gate insulator treated with β-phenethyltrichlorosilane (β-PTS). Employing β-PTS for the surface treatment of SiO2, large grains were present in the initial stage of pentacene crystal growth. The field effect mobility was as high as 1.5cm2∕Vs and the on/off ratio was over 106. The surface treatment dramatically improved the stability in air of the pentacene-TFT’s electrical characteristics. A field effect mobility of over 1cm2∕Vs and on/off ratio of over 105 were maintained after scanning the gate voltage 2000 times in air. This result indicates that the surface treatment with β-PTS not only improved TFT performance but also significantly suppressed the device’s degradation.

Structure and chemistry of (111) twin boundaries in MgAl2O4 spinel crystals from Mogok

The atomic scale structure and chemistry of (111) twins in MgAl2O4 spinel crystals from the Pinpyit locality near Mogok (Myanmar, formerly Burma) were analysed using complementary methods of transmission electron microscopy (TEM). To obtain a three-dimensional information on the atomic structure, the twin boundaries were investigated in crystallographic projections $$ [\ifmmode\expandafter\bar\else\expandafter\=\fi{1}10] $$ and $$ [11\ifmmode\expandafter\bar\else\expandafter\=\fi{2}]. $$ Using conventional electron diffraction and high-resolution TEM (HRTEM) analysis we have shown that (111) twins in spinel can be crystallographically described by 180° rotation of the oxygen sublattice normal to the twin composition plane. This operation generates a local hcp stacking in otherwise ccp lattice and maintains a regular sequence of kagome and mixed layers. In addition to rotation, no other translations are present in (111) twins in these spinel crystals. Chemical analysis of the twin boundary was performed by energy-dispersive X-ray spectroscopy (EDS) using a variable beam diameter (VBD) technique, which is perfectly suited for analysing chemical composition of twin boundaries on a sub-nm scale. The VBD/EDS measurements indicated that (111) twin boundary in spinel is Mg-deficient. Quantitative analyses of HRTEM (phase contrast) and HAADF-STEM (Z-contrast) images of (111) twin boundary have confirmed that Mg2+ ions are replaced with Be2+ ions in boundary tetrahedral sites. The Be-rich twin boundary structure is closely related to BeAl2O4 (chrysoberyl) and BeMg3Al8O16 (taaffeite) group of intermediate polysomatic minerals. Based on these results, we conclude that the formation of (111) twins in spinel is a preparatory stage of polytype/polysome formation (taaffeite) and is a result of thermodynamically favourable formation of hcp stacking due to Be incorporation on the {111} planes of the spinel structure in the nucleation stage of crystal growth. The twin structure grows as long as the surrounding geochemical conditions allow its formation. The incorporation of Be induces a 2D-anisotropy and exaggerated growth of the crystal along the (111) twin boundary.

Experimental analysis of temperature and crystallinity profiles of wood sawdust/polypropylene composites during cooling

AbstractThe temperature profiles of wood sawdust/polypropylene (PP) composite melts during cooling were experimentally investigated with a cooling jacket apparatus connected to the end of an injection‐molding machine. Real‐time melting temperature was measured with an unsheathed thermocouple array coupled with a high‐speed data‐acquisition unit. The crystallinity level of the solidified composites was evaluated with an X‐ray diffractometer. Before the crystallization temperature (Tc) was reached, the cooling rate of the melt layer near the wall was greater than that near the center. After Tc was reached, the opposite behavior was observed. Wood sawdust content did not affect the general temperature and crystallinity profiles across the duct diameter but led to more nonuniform temperature profiles across the duct diameter. The sawdust particles could act as a nucleating agent during the nucleation stage to increase Tc of the PP and as an interfering agent during the crystal growth stage to decrease the overall crystallinity level of the PP. The temperature and crystallinity profiles were not affected by the coolant flow rate. The normalized induction time changed with reduced radius (r/R, where r is the distance between the central duct to any point along the cross‐section of the duct and R is the duct radius) positions and coolant flow rate, especially for neat PP and PP composites with a low sawdust content (10 wt %). © 2006 Wiley Periodicals, Inc. J Appl Polym Sci 102: 1896–1905, 2006

Formation Mechanism of Porous Single-Crystal Cr2O3 and Co3O4 Templated by Mesoporous Silica

Porous single crystals of Cr2O3 and Co3O4 templated by mesoporous silicas, SBA-15 and KIT-6, have been fabricated. Specimens at different stages of crystal growth were characterized by using XRD an...

SANS study of the early stages of crystallization in polyethylene solutions

Crystallization of low molecular weight polyethylene from solution has been studied using small angle neutron scattering (SANS). The detection sensitivity of the volume fraction degree of crystallinity is estimated to be 10−5, allowing for the measurement of the very early stages of crystal growth. SANS spectra for both the early and late stages of crystallization can be satisfactorily interpreted with a lamellar crystal model; there is no evidence of diverging or spinodal-decomposition-like density fluctuations during the early stage of crystallization in polyethylene solutions. A possible explanation of the dominant wavevector in small angle X-ray scattering that led to the proposal of ‘spinodal decomposition’ mechanism for early stage crystallization is suggested.

Crystal Growth Mechanism of Spherical Silicon Fabricated by Dropping Method

We have investigated the crystal growth mechanism of spherical Si fabricated by a dropping method. The Si spheres were classified into two categories by surface morphology. The grain size of a Si sphere with a smooth surface is larger than that with a rough surface. To investigate the crystal growth mechanism of the spheres, Si samples with various crystal sizes were observed. Si sample size is related to Si droplet size of, which influences the ease of solidification. Large Si droplets take longer to solidify than small Si droplets. Si samples 4, 2 and 1 mm in diameter correspond to the initial, intermediate and final stages of crystal growth, respectively. In the Si sample 4 mm in diameter, a disk of (111) plane crystals is observed. This result suggests that the initial crystal growth of the Si spheres consisting of large grains involves the formation of a disk of (111) plane Si crystals. The crystal growth mechanism for a Si sphere 1 mm in diameter is proposed.

Two-Dimensional Crystal Growth and Stacking of Bis(phthalocyaninato) Rare Earth Sandwich Complexes at the 1-Phenyloctane/Graphite Interface

Initial stages of two-dimensional crystal growth of the double-decker sandwich complex Lu(Pc*)2 [Pc* = 2,3,9,10,16,17,23,24-octakis(octyloxy)phthalocyaninato] have been studied by scanning tunneling microscopy at the liquid/solid interface between 1-phenyloctane and highly oriented pyrolytic graphite. High-resolution images strongly suggest alignment of the double-decker molecules into monolayers with the phthalocyanine rings parallel to the surface. Domains were observed with either hexagonal or quadrate packing motifs, and the growing interface of the layer was imaged. Molecular resolution was achieved, and the face of the phthalocyanine rings appeared as somewhat diffuse circular features. The alkyl chains are proposed to be interdigitating to maintain planar side-by-side packing.

Mollusk shell formation: Mapping the distribution of organic matrix components underlying a single aragonitic tablet in nacre

Control over mineral formation in mollusk shells is exerted by the macromolecules of the organic matrix. Using histochemical methods, we mapped the carboxylates and sulfates of proteins and polysaccharides on the surfaces of decalcified interlamellar matrices from the nacreous shell layer of the cephalopod Nautilus pompilius, expanding upon an earlier study by Crenshaw and Ristedt [Crenshaw, M.A., Ristedt, H., 1976. The histochemical localization of reactive groups in septal nacre from Nautilus pompilius. In: Watabe, N., Wilbur, K.M. (Ed.), The Mechanisms of Mineralization in the Invertebrates and Plants. University of South Carolina Press, Colombia, pp. 355–367]. We observed four different zones underlying a single crystal: (1) a central spot rich in carboxylates; (2) a central ring-shaped area rich in sulfates; (3) an area between the central nucleation region and the imprint periphery containing carboxylates, and (4) the intertabular matrix, rich in carboxylates and sulfates. We also mapped matrix functional groups on the nacreous matrix surfaces of the bivalve Atrina rigida, but did not identify well-defined zones. Immuno-mapping of the constituents of the aragonite-nucleating protein fraction from Atrina nacre showed that these macromolecules are located both in the intertabular matrix and in the center of the crystal imprints for both Atrina and Nautilus matrix surfaces. Their presence at the latter location is consistent with their purported role in aragonite nucleation. The observed differentiation in the distribution of matrix components and their functional groups shows that the different stages of single crystal growth are highly controlled by the matrix.

Columnar calcites as testimony of diagenetic overprinting at the boundary between Upper Tournaisian dolomites and limestones (Belgium): multiple origins for apparently similar features

AbstractIn topographic flat areas, sedimentary settings may vary from one outcrop to another. In these settings, calcite precipitates may yield macroscopically similar columnar features, although they are products of different sedimentary or diagenetic processes. Three columnar calcite crystal fabrics, i.e. rosettes, palisade crusts and macro‐columnar crystal fans, have been differentiated near and at the contact between Upper Tournaisian dolomites and limestones along the southern margin of the Brabant‐Wales Palaeohigh. Their petrographic characteristics, and geochemical and fluid inclusion data provide information on the (dia)genetic processes involved. Rosettes composed of non‐luminescent columnar calcite crystal fans (1–5 cm in diameter) developed on top of one another, forming discrete horizons in repetitive sedimentary cycles. The cycles consist of three horizons: (I) a basal horizon with fragments from the underlying horizon, (II) a micrite/microspar horizon with incipient glaebules, (III) an upper horizon consisting of calcite rosettes, with desiccation features. The petrographical features and δ18O signatures of −10·0 to −5·5‰ and δ13C values of −5·5 to −3·2‰ support either evaporative growth, an evaporative pedogenic origin, or overprinting of marine precipitates. Palisade crusts, composed of a few to 10 mm long non‐luminescent calcite crystals, coat palaeokarst cavities. Successive palisade growth‐stages occur which are separated by thin laminae of micrite or detrital quartz, displaying a geopetal arrangement. Palisade crusts are interpreted as intra‐Mississippian speleothems. This interpretation is supported by their petrographic characteristics and isotopic signature (δ18O = −8·7 to −6·5‰ and δ13C = −4·8 to −2·5‰). Macro‐columnar crystals, 1–50 cm long, developed mainly perpendicular to cavity walls and dolomite clasts. Crystal growth stages in the macro‐columnar crystals are missing. δ18O values vary between −16·4 and −6·8‰ and δ13C values between −5·2 and −0·9‰. These features possibly support a late diagenetic high temperature precipitation in relation to hydrothermal karstification.