Showing papers on "Nucleation published in 1996"
TL;DR: In this paper, isolated single-wall carbon nanotubes (SWNT) were grown by disproportionation of carbon monoxide at 1200°C, catalyzed by molybdenum particles a few nanometers in size.
1,244 citations
TL;DR: Bacteriorhodopsin crystals were obtained from bicontinuous cubic phases, but not from micellar systems, implying a critical role of the continuity of the diffusion space (the bilayer) on crystal growth.
Abstract: Understanding the mechanisms of action of membrane proteins requires the elucidation of their structures to high resolution. The critical step in accomplishing this by x-ray crystallography is the routine availability of well-ordered three-dimensional crystals. We have devised a novel, rational approach to meet this goal using quasisolid lipidic cubic phases. This membrane system, consisting of lipid, water, and protein in appropriate proportions, forms a structured, transparent, and complex three-dimensional lipidic array, which is pervaded by an intercommunicating aqueous channel system. Such matrices provide nucleation sites ("seeding") and support growth by lateral diffusion of protein molecules in the membrane ("feeding"). Bacteriorhodopsin crystals were obtained from bicontinuous cubic phases, but not from micellar systems, implying a critical role of the continuity of the diffusion space (the bilayer) on crystal growth. Hexagonal bacteriorhodopsin crystals diffracted to 3.7 A resolution, with a space group P6(3), and unit cell dimensions of a = b = 62 A, c = 108 A; alpha = beta = 90 degrees and gamma = 120 degrees.
974 citations
TL;DR: The approach provides a powerful means for the quantitative assay of A beta fibrillogenesis and found that the surfactant n-dodecylhexaoxyethylene glycol monoether (C12E6) slowed nucleation and elongation of fibrils in a concentration-dependent manner.
Abstract: We have studied the fibrillogenesis of synthetic amyloid beta-protein-(1-40) fragment (A beta) in 0.1 M HCl. At low pH, A beta formed fibrils at a rate amenable to detailed monitoring by quasi-elastic light-scattering spectroscopy. Examination of the fibrils with circular dichroism spectroscopy and electron microscopy showed them to be highly similar to those found in amyloid plaques. We determined the hydrodynamic radii of A beta aggregates during the entire process of fibril nucleation and growth. Above an A beta concentration of approximately 0.1 mM, the initial rate of elongation and the final size of fibrils were independent of A beta concentration. Below an A beta concentration of 0.1 mM, the initial elongation rate was proportional to the peptide concentration, and the resulting fibrils were significantly longer than those formed at higher concentration. We also found that the surfactant n-dodecylhexaoxyethylene glycol monoether (C12E6) slowed nucleation and elongation of fibrils in a concentration-dependent manner. Our observations are consistent with a model of A beta fibrillogenesis that includes the following key steps: (i) peptide micelles form above a certain critical A beta concentration, (ii) fibrils nucleate within these micelles or on heterogeneous nuclei (seeds), and (iii) fibrils grow by irreversible binding of monomers to fibril ends. Interpretation of our data enabled us to determine the sizes of fibril nuclei and A beta micelles and the rates of fibril nucleation (from micelles) and fibril elongation. Our approach provides a powerful means for the quantitative assay of A beta fibrillogenesis.
790 citations
TL;DR: In this paper, a computer-simulation study of homogeneous crystal nucleation and the structure of crystal nuclei in a Lennard-Jones system at moderate undercooling is presented.
Abstract: We report a computer‐simulation study of the rate of homogeneous crystal nucleation and the structure of crystal nuclei in a Lennard‐Jones system at moderate undercooling. The height of the nucleation barrier has been determined using umbrella sampling, whereas the barrier crossing rate is calculated using molecular dynamics simulation. The simulations clearly show that the barrier crossing is a diffusive process. Nevertheless, the kinetic prefactor in the nucleation rate is found to be some two orders of magnitude larger than predicted by classical nucleation theory. The height of the barrier is in good agreement with the theoretical prediction. Although the Lennard‐Jones system has a stable face‐centered cubic (fcc) phase below the melting line, the precritical nuclei are found to be mainly body‐centered cubic (bcc) ordered. As they grow to their critical size, they become more fcc ordered in the core. However, the critical and postcritical nuclei retain a high degree of bcc ordering in the interface. Furthermore it is found that in the interface the density falls off faster than the structural order parameter, which is in agreement with the predictions of density functional calculations. [P. Harrowell and D. W. Oxtoby, J. Chem. Phys. 80, 1639 (1984)].
722 citations
TL;DR: In this paper, a closed-form solution for the 2D elastic stress field is adapted for reduced loading configurations modeled in a recently constructed fretting fatigue experiment that applies loads relevant to aircraft lap joints.
402 citations
TL;DR: In this paper, the defect structures, including threading dislocations, partial dislocation bounding stacking faults and inversion domains, were investigated by transmission electron microscopy for GaN/Al2O3 epilayers grown by metalorganic chemical vapor deposition using a two-step process.
Abstract: Defect structures were investigated by transmission electron microscopy for GaN/Al2O3 (0001) epilayers grown by metal‐organic chemical vapor deposition using a two‐step process. The defect structures, including threading dislocations, partial dislocation bounding stacking faults, and inversion domains, were analyzed by diffraction contrast, high‐resolution imaging, and convergent beam diffraction. GaN film growth was initiated at 600 °C with a nominal 20 nm nucleation layer. This was followed by high‐temperature growth at 1080 °C. The near‐interfacial region of the films consists of a mixture of cubic and hexagonal GaN, which is characterized by a high density of stacking faults bounded by Shockley and Frank partial dislocations. The near‐interfacial region shows a high density of inversion domains. Above ∼0.5 μm thickness, the film consists of isolated threading dislocations of either pure edge, mixed, or pure screw character with a total density of ∼7×108 cm−2. The threading dislocation reduction in the...
402 citations
TL;DR: An instability of the liquid film against the growth in amplitude of surface waves with a characteristic wavelength is observed, which is believed to be the first observation of spinodal dewetting.
Abstract: We have studied the dewetting of thin liquid metal films (Au, Cu, Ni) on fused silica substrates which occurs after melting with a Q-switched laser pulse. Optical microscopy, scanning electron microscopy, and scanning near field acoustic microscopy reveal two distinctly different modes of the dewetting process: On one hand, we observe heterogeneous nucleation of ``dry'' circular patches, which grow in diameter during the melting period. On the other hand, an instability of the liquid film against the growth in amplitude of surface waves with a characteristic wavelength is observed, which we believe is the first observation of spinodal dewetting. In contrast, the final structure of the ruptured film depends on whether nucleation or spinodal dewetting is dominant.
399 citations
TL;DR: In this article, a model based upon discretized dislocations is proposed for both the initiation of yielding at an upper yield point and the arrest of the indenter at a lower yield point.
383 citations
TL;DR: In this paper, it was shown that atmospheric nucleation may occur by a collision-limited process, rather than by a condensation/evaporation controlled process as is assumed in the classical theory.
Abstract: Measured production rates of tropospheric ultrafine particles (˜3nm diameter) are reported for the first time and are shown to be orders of magnitude greater than nucleation rates predicted by the binary theory of homogeneous nucleation for sulfuric acid and water. Furthermore, the functional dependence of observed particle formation rates on sulfuric acid vapor concentrations is much weaker than predicted by binary theory. We present arguments to show that these discrepancies might be due to the participation of a species such as ammonia which could stabilize subcritical clusters, thereby enhancing nucleation rates. The data suggest that atmospheric nucleation may occur by a collision-limited process, rather than by a condensation/evaporation controlled process as is assumed in the classical theory.
376 citations
TL;DR: In this paper, the Johnson-Mehl-Avrami equation was used to evaluate reaction mechanisms for the hydrothermal synthesis of barium titanate and showed that dissolution-precipitation was the dominant reaction mechanism in the early stages of the process.
Abstract: Reaction mechanisms for the hydrothermal synthesis of barium titanate are evaluated. Feedstocks of barium hydroxide octahydrate and anatase titania are reacted for varying durations (1–72 h) to provide intermediate-stage samples for characterization by transmission electron microscopy/ energy-dispersive spectroscopy (TEM/EDS), X-ray diffractometry (XRD), and inductively coupled plasma spectroscopy (ICP). Quantitative evaluation of the extent of reaction by ICP and XRD methods permits the analysis of data with the Johnson-Mehl-Avrami equation. This analysis reveals two reaction-rate regimes. Kinetic analysis, based on reaction progress, yields insight into the first reaction-rate regime but is inconclusive in the analysis of the second reaction-rate regime. In the first regime, at the early stage of barium titanate formation, a dissolution-precipitation mechanism dominates. In contrast, in the second regime, at longer reaction times, an in-situ transformation mechanism is probably dominant. However, multiple reaction mechanisms (e.g., in-situ transformation and dissolution-precipitation) may be competing for rate control. Alternatively, dissolution-precipitation may be the dominant mechanism throughout the barium titanate synthesis, with nucleation and growth controlling the first regime and dissolution rate controlling the second regime.
351 citations
TL;DR: In this paper, the authors used the nonlinear Avrami equation and Hoffman-Lauritzen equation to fit poly(L-lactide-co-meso lactide) copolymers.
Abstract: The crystallization kinetics of poly(L-lactide-co-meso-lactide) were determined over a range of 0% to 9% mesolactide. The kinetics were fit to the nonlinear Avrami equation and then to the Hoffman–Lauritzen equation modified for optical copolymers. The theory was found to fit the data well. The crystallization half-time was found to increase about 40% for every 1 wt % meso-lactide in the polymerization mixture. The change in crystallization rate is driven mainly by the reduction in melting point for the copolymers. The copolymer crystallization kinetics were also determined in the presence of talc, a nucleating agent for polylactide. The theory again fit the data well, using the same growth parameters and accounting for the talc only through the nucleation density term. © 1996 John Wiley & Sons, Inc.
TL;DR: The solidification of peritectic alloys, including both nucleation and growth, is reviewed in this paper, where the authors define various compositions associated with binary per-itectic systems, their equilibrium solidification, multicomponent per-architectic system, and metastable phases.
Abstract: The solidification of peritectic alloys is reviewed, including both nucleation and growth. The introduction defines various compositions associated with binary peritectic alloys, their ‘equilibrium’ solidification, multicomponent peritectic systems, and metastable phases in peritectic systems. Nucleation studies are reviewed according to the rate of cooling and the type of specimen. Some comparisons are made with observations in eutectic alloys. At fast cooling rates metastable events can occur, as reviewed for alloys based on the Al-Ti and other systems. Growth studies in peritectic alloys have identified three stages: the ‘reaction’ (three phases in contact); the ‘transformation’ (the peritectic phase thickens by solid state diffusion); and ‘direct solidification’ of the peritectic phase. Models of the stages are outlined and compared with experiment for various binary alloys, followed by a summary of observations of peritectic solidification in multicomponent Fe-Cr-Ni and Fe-C-X alloys. Recent ...
TL;DR: Antiscale magnetic treatment (AMT) as mentioned in this paper has been shown to be more effective above a threshold magnetic field contact time and in flowing systems, and is more effective in treated waters up to 130 h after exposure has ceased.
TL;DR: In this article, pyroclast structures are used to constrain degassing in basaltic lava fountains and the influence of vesiculation rate on magma fragmentation.
TL;DR: In this paper, the authors studied the growth of calcite and aragonite under controlled supersaturation conditions by the counter diffusion of Ca2+ and CO32- ions through a column of silica gel.
Abstract: Morphological development of calcite crystals is related to supersaturation conditions during growth. Crystallization of calcium carbonate (calcite and aragonite) as well as Mg-calcite was studied under controlled supersaturation conditions by the counter diffusion of Ca2+ and CO32- ions through a porous transport medium (a column of silica gel). Under our experimental conditions, where ion transport is constrained to be diffusion controlled, nucleation and growth take place under conditions of high supersaturation, the actual threshold value of the supersaturation depending on the supersaturation gradient. In the pure CaCO3 system, calcite grows at lower supersaturation than aragonite. The calcite develops relatively simple rhombohedra whil the aragonite grows as spherulites. Presence of Mg2+ in the interstitial fluid inhibits nucleation, increasing the threshold supersaturation at which crystallization begins. The resulting Mg-calcite crystals show a range of morphologies depending on the Mg content and the supersaturation at the point of crystallization. At high values of supersaturation, up to 15 mol % MgCO3 is incorporated into the calcite and the crystals form spheres. At lower supersaturations, Mg content decreases and morphologies change progressively through a well-defined and reproducible sequence from spheres to dumbbell-like forms to wheat-sheaf-like bundles and eventually single crystals with steep rhombohedral faces. The crystals are compositionally zoned, showing both sector and oscillatory zoning. The compositional evol tion is related to the supersaturation and interface roughness during crystal growth.
TL;DR: A model for crystallization over long timescales is described that can be applied to a wide range of impure water ices under typical astrophysical conditions if the fragility factor D, which describes the viscosity behavior, can be estimated.
Abstract: Electron diffraction studies of vapor-deposited water ice have characterized the dynamical structural changes during crystallization that affect volatile retention in cometary materials. Crystallization is found to occur by nucleation of small domains, while leaving a significant part of the amorphous material in a slightly more relaxed amorphous state that coexists metastably with cubic crystalline ice. The onset of the amorphous relaxation is prior to crystallization and coincides with the glass transition. Above the glass transition temperature, the crystallization kinetics are consistent with the amorphous solid becoming a "strong" viscous liquid. The amorphous component can effectively retain volatiles during crystallization if the volatile concentration is approximately 10% or less. For higher initial impurity concentrations, a significant amount of impurities is released during crystallization, probably because the impurities are trapped on the surfaces of micropores. A model for crystallization over long timescales is described that can be applied to a wide range of impure water ices under typical astrophysical conditions if the fragility factor D, which describes the viscosity behavior, can be estimated.
TL;DR: In this paper, the authors examined the evolution of six California foreshock sequences, the 1986 Mount Lewis (ML = 5.5), the 1986 Chalfant (ML= 6.4) and the 1986 Stone Canyon (SL = 4.7), and found that the foreshocks seem to concentrate near structural discontinuities in the fault and may themselves be a product of an aseismic nucleation process.
Abstract: We find that foreshocks provide clear evidence for an extended nucleation process before some earthquakes. In this study, we examine in detail the evolution of six California foreshock sequences, the 1986 Mount Lewis (ML = 5.5), the 1986 Chalfant (ML = 6.4), the 1986 Stone Canyon (ML = 4.7), the 1990 Upland (ML = 5.2), the 1992 Joshua Tree (MW = 6.1), and the 1992 Landers (MW = 7.3) sequence. Typically, uncertainties in hypocentral parameters are too large to establish the geometry of foreshock sequences and hence to understand their evolution. However, the similarity of location and focal mechanisms for the events in these sequences leads to similar foreshock waveforms that we cross correlate to obtain extremely accurate relative locations. We use these results to identify small-scale fault zone structures that could influence nucleation and to determine the stress evolution leading up to the mainshock. In general, these foreshock sequences are not compatible with a cascading failure nucleation model in which the foreshocks all occur on a single fault plane and trigger the mainshock by static stress transfer. Instead, the foreshocks seem to concentrate near structural discontinuities in the fault and may themselves be a product of an aseismic nucleation process. Fault zone heterogeneity may also be important in controlling the number of foreshocks, i.e., the stronger the heterogeneity, the greater the number of foreshocks. The size of the nucleation region, as measured by the extent of the foreshock sequence, appears to scale with mainshock moment in the same manner as determined independently by measurements of the seismic nucleation phase. We also find evidence for slip localization as predicted by some models of earthquake nucleation.
TL;DR: The role of Ag in quaternary alloys is to effectively trap Mg atoms, resulting in Mg Ag co-clusters, which provide favorable sites for nucleation of the Ω phase as mentioned in this paper.
TL;DR: In this article, two different growth mechanisms for CoNi and FeNi particles are shown for monodisperse, spherical particles, homogeneous in composition, obtained in the submicron size range in both cases.
TL;DR: The structure and morphology of low growth temperature GaN nucleation layers have been studied using AFM, reflection high energy electron diffraction (RHEED), and transmission electron microscopy (TEM).
Abstract: The structure and morphology of low growth temperature GaN nucleation layers have been studied using atomic force microscopy (AFM), reflection high energy electron diffraction (RHEED), and transmission electron microscopy (TEM). The nucleation layers were grown at 600 °C by atmospheric pressure metalorganic chemical vapor deposition (MOCVD) on c‐plane sapphire. The layers consist of predominantly cubic GaN (c‐GaN) with a high density of stacking faults and twins parallel to the film/substrate interface. The average grain size increases with increasing layer thickness and during the transition from low temperature (600 °C) to the high temperatures (1080 °C) necessary for the growth of device quality GaN. Upon heating to 1080 °C the nucleation layer partially converts to hexagonal GaN (h‐GaN) while retaining a high stacking fault density. The mixed cubic‐hexagonal character of the nucleation layer region is sustained after subsequent high‐temperature GaN growth.
TL;DR: In this paper, it was shown that the abnormal grain growth (AGG) behavior is not linearly dependent on the driving force arising from the grain size difference, but the growth rate is expected to increase abruptly at a critical supersaturation.
Abstract: If the grains dispersed in a liquid matrix are spherical, their surface atomic structure is expected to be rough (diffuse), and their coarsening has been observed to be controlled by diffusion in the matrix. They do not, furthermore, undergo abnormal growth. On the other hand, in some compound material systems, the grains in liquid matrices are faceted and often show abnormal coarsening behavior. Their faceted surface planes are expected to be singular (atomically flat) and therefore grow by a defect-assisted process and two-dimensional (2-D) nucleation. Contrary to the usual coarsening the-ories, their growth velocity is not linearly dependent on the driving force arising from the grain size difference. If the growth of the faceted grains occurs by 2-D nucleation, the rate is expected to increase abruptly at a critical supersaturation, as has been observed in crystal growth in melts and solutions. It is proposed that this growth mechanism leads to the abnormal grain coarsening. The 2-D nucleation theory predicts that there is a threshold initial grain size for the abnormal grain growth (AGG), and the propensity for AGG will increase with the heat-treatment temperature. The AGG behavior will also vary with the defects in the grains. These predictions are qualitatively confirmed in the sintered WC-Co alloy prepared from fine (0.85-Μm) and coarse (5.48-Μm) WC powders and their mixtures. The observed dependence of the AGG behavior on the sintering temperature and the milling of the WC powder is also qualitatively consistent with the predicted behavior.
TL;DR: In this article, the microcellular processing of poly(ethylene terephthalate) in the amorphous and semicrystalline states is studied in order to quantify the processing differences.
Abstract: Microcellular semicrystalline polymers such as poly(ethylene terephthalate) show great promise for engineering applications because of their unique properties, particularly at higher densities. Recent studies reveal some high density microcellular polymers have longer fatigue lives and/or equal strengths to the neat polymer. Relatively few microcellular processing studies of semicrystalline polymers have been presented. In general, semicrystalline polymers are relatively difficult to microcellular process compared to amorphous polymers. In this paper and a companion paper, the microcellular processing of poly(ethylene terephthalate) in the amorphous and semicrystalline states is studied in order to quantify the processing differences. The microcellular processing steps addressed in this paper include gas/polymer solution formation and microvoid nucleation. Particular emphasis is given to microvoid nucleation comparing the processing characteristics of semicrystalline and amorphous materials. Moreover, this study identifies a number of critical process parameters.
In general, the semicrystalline materials exhibit ten to one thousand times higher cell nucleation densities compared with the amorphous materials, resulting from heterogeneous nucleation contributions. The amorphous materials show a strong dependence on cell density, while the semicrystalline materials show a weaker dependence. Moreover, classical nucleation theory is not adequate to quantitatively predict the effects of saturation pressure on cell nucleation for either the amorphous or semicrystalline polyesters. Both the semicrystalline and amorphous materials exhibit constant nucleation cell densities with increasing foaming time. Foaming temperatures near the glass transition are found to influence the cell density of the amorphous polyesters, indicating some degree of thermally activated nucleation. Furthermore, classical nucleation theory is not adequate to predict the cell density dependence on foaming temperature. Similar to the amorphous polyesters above the glass transition temperature, nucleation in the semicrystalline materials is found to be independent of the foaming temperature.
TL;DR: In this paper, a potentiostatic pulse method was employed to electrochemically deposit silver nanocrystallites on the atomically smooth graphite basal plane surface, where the current increased in approximate proportion to (time)1/2 as expected for an instantaneous nucleation and three-dimensional growth mode of deposition.
Abstract: A potentiostatic pulse method has been employed to electrochemically deposit silver nanocrystallites on the atomically smooth graphite basal plane surface. Voltage pulses having amplitudes of 100, 250, and 500 mV vs Ag0 and durations of 10 or 50 ms were applied to graphite surfaces immersed in dilute (≈1.0 mM) aqueous silver nitrate. During the deposition pulse, the current increased in approximate proportion to (time)1/2 as expected for an instantaneous nucleation and three-dimensional growth mode of deposition. Consistent with this growth mode, noncontact atomic force microscopy (NC-AFM) examination of graphite surfaces following silver deposition revealed the existence of silver particles at a coverage of near 1010 cm-2 which were well-separated from one another on atomically smooth regions of the graphite basal plane surface. These particles were disk-shaped having a height of 15−50 A and an apparent diameter which varied from 200 to 600 A; particle dimensions increased smoothly with the coulometric l...
TL;DR: In this paper, the formation of the so-called γ α 2 lamellar structure was studied in two-phase TiAl-based alloys, which is the most typical of the alloys of this category.
TL;DR: In this paper, a single-layer germanium-antimony-tellurium (Ge-Sb-Te) ternary amorphous thin film as a single layer or sandwiched between various dielectric films, such as silicon dioxide (SiO2), siricon nitride (Si3N4), tantalum oxide (Ta2O5), zinc sulfide (ZnS), and ZnS−20 m...
Abstract: Crystallization processes were studied for germanium–antimony–tellurium (Ge–Sb–Te) ternary amorphous thin film as a single layer or sandwiched between various dielectric films, such as silicon dioxide (SiO2), siricon nitride (Si3N4), tantalum oxide (Ta2O5), zinc sulfide (ZnS), and ZnS–20 mol % SiO2. The processes were analyzed quantitatively, based on transmittance changes in Ge–Sb–Te films heated either exothermally or isothermally. Both Kissinger equation and Johnson–Mehl–Avrami kinetic analysis were adopted to estimate activation energy and the reaction order of the processes. Ge–Sb–Te single‐layer amorphous film crystallized in two stages, nucleation and crystal growth. These two processes can be distinguished by exothermal crystallization patterns. By sandwiching this film into dielectric films, crystallization activation energy increases and the nucleation processes are affected. The Si3N4 and Ta2O5 dielectric films accelerate the nucleation, while the SiO2 films inhibit it, and the ZnS and ZnS–20 m...
TL;DR: In this paper, the formation and decomposition of hydrate are discussed at a conceptual level and key issues for research in this area are identified and some possible directions for future work are suggested.
TL;DR: In this article, it was shown that 1.06 mm pulses from a Q-switched Nd:YAG laser induce crystallization in supersaturated solutions of urea in water.
Abstract: We report a new photophysical phenomenon in which 1.06 mm pulses from a Q-switched Nd:YAG laser induce crystallization in supersaturated solutions of urea in water. Because the solutions are transparent at the incident wavelength, a photochemical mechanism is unlikely. The needle-shaped crystals that initially form tend to be aligned parallel to the electric field vector of the light, suggesting a Kerr-like field-induced alignment of urea molecules that aids in organizing prenucleating clusters. The effect has application to pump-probe nucleation studies and to clean nucleation in sealed systems. [S0031-9007(96)01456-1] PACS numbers: 81.10.Dn, 42.50.Vk The study of the light-induced condensation of supersaturated vapors dates back to the work of Tyndall in 1869 [1]. More recent interest has centered on the formation of atmospheric aerosols [2], laser-induced chemical vapor deposition [3], and laser-induced clustering in atomic and molecular systems [4]. The mechanism typically involves the photochemical generation of a nonvolatile product that acts as a nucleus for the growth of the condensed phase [5]. Nucleation in liquid solutions is a more complex problem involving two components, and, to our knowledge, there have been no reports of light-induced nucleation from supersaturated solutions. Nevertheless, this problem is of great theoretical and commercial importance, such as in industrial crystallization processes [6]. Recently, while attempting to observe second harmonic generation in supersaturated solutions of urea in water, we have noticed that pulses from a Q-switched Nd:YAG laser can induce nucleation in such solutions. Because the incident light source is near infrared, a photochemical mechanism is unlikely. The orientation of the crystallites that are formed depends on the plane of polarization of the incident radiation, suggesting an electric-field-induced effect. Aqueous urea solutions, with concentrations, c, in the range of 11.5 -13.5M, were prepared by combining solid urea and water in a 1.3-cm diameter pyrex test tube, which was then heat sealed with a torch. Great care was taken to exclude dust from samples. Supersaturated solutions were generated and regenerated by heating the tubes to 45 ‐ C and holding them at that temperature for
TL;DR: In this paper, the photoluminescence properties of GaN thin films were investigated for different nitridation times, and it was found that the band edge and the yellow-band luminescences are strongly dependent on the nitridated starting surface.
Abstract: GaN layers were grown by gas‐source molecular beam epitaxy on sapphire substrates using ammonia as a nitrogen source. The nitridation of an Al2O3 surface prior to the GaN growth was followed in situ by reflection high‐energy electron diffraction. A strong variation of the surface lattice parameter was observed corresponding to the formation of an AlN relaxed layer. The nucleation of GaN on such a nitridated surface is facilitated, as checked by atomic force microscopy. Optimization of the nitridation procedure is achieved by investigating the photoluminescence properties of GaN thin films grown for different nitridation times. It is found that the band‐edge and the yellow‐band luminescences are strongly dependent on the nitridated starting surface. Finally, the optimized nitridation process is used to grow high‐quality GaN epitaxial layers.
Patent•
03 Sep 1996
TL;DR: In this paper, a thermal reactor system that produces nanoscale powders by ultra-rapid thermal quench processing of high-temperature vapors through a boundary-layer converging-diverging nozzle is described.
Abstract: A thermal reactor system that produces nanoscale powders by ultra-rapid thermal quench processing of high-temperature vapors through a boundary-layer converging-diverging nozzle. A gas suspension of precursor material is continuously fed to a thermal reaction chamber and vaporized under conditions that minimize superheating and favor nucleation of the resulting vapor. According to one aspect of the invention, the high temperature vapor is quenched using the principle of Joule-Thompson adiabatic expansion. Immediately after the initial nucleation stages, the vapor stream is passed through the nozzle and rapidly quenched through expansion at rates of at least 1,000° C. per second, preferably greater than 1,000,000° C. per second, to block the continued growth of the nucleated particles and produce a nanosize powder suspension of narrow particle-size distribution. According to another aspect of the invention, a gaseous boundary-layer stream is injected to form a blanket over the internal surface of the nozzle to prevent vapor condensation in the throat of the nozzle and its potential failure.
TL;DR: In this article, the oxide mediated epitaxy (OME) technique was used to grow single-crystal CoSi2 layers on Si surfaces by placing a thin layer of cobalt (1-3 nm) onto surfaces covered with a thin silicon oxide layer and annealing at 500-700 °C.
Abstract: Uniform, single‐crystal CoSi2 layers have been grown on Si by the technique of oxide mediated epitaxy (OME). Deposition of a thin layer of cobalt (1–3 nm) onto surfaces covered with a thin silicon oxide layer and annealing at 500–700 °C led to the growth of epitaxial, essentially uniform, CoSi2 layers on the (100), (110), and (111) surfaces of Si. The nucleation and growth of silicide apparently occurred subsurface, leaving the silicon oxide layer largely on the surface of the silicide after the growth. On all surfaces, thicker (10–30 nm), excellent quality, CoSi2 single‐crystal thin films have been grown by repeated growth sequences. Experimental results are presented along with a discussion on the possible roles played by the thin oxide layer in promoting the epitaxial growth of silicide.