Three-dimensional Nuclei Research Articles

Incorporation of impurities into macromolecular crystals

The chemical, mechanical and diffraction properties of crystals grown from solution, as well as their growth kinetics and morphological development, depend very much on the types and concentrations of impurities present in their mother liquor. The situation appears vastly more complicated in the case of macromolecular crystals because of the complex nature of the molecules and the biochemical milieu from which they are derived. An attempt is made here to catalog and characterize these various impurities. One class of impurities, large foreign particles (such as dust), microcrystals, misoriented three-dimensional nuclei, and large molecular clusters has been investigated in detail using atomic force microscopy. With this technique we have directly visualized the incorporation of such larger impurities and have delineated some of their more striking consequences. In particular we have found that in some cases such incorporation is accompanied by visible defect formation or dislocations. In other cases of small three-dimensional nuclei, coalescence proceeds in a smooth manner, with alignment and knitting together of the respective lattices. A calculation of the overall defect density in canavalin crystals shows the number of defects to be many orders of magnitude greater than found for most conventional crystals.

Incorporation of microcrystals by growing protein and virus crystals

In the course of time-lapse video and atomic force microscopy (AFM) investigations of macromolecular crystal growth, we frequently observed the sedimentation of microcrystals and three-dimensional nuclei onto the surfaces of much larger, growing protein or virus crystals. This was followed by the direct incorporation over time of the smaller crystals into the bulk of the larger crystals. In some cases, clear indications were present that upon absorption of the small crystal onto the surface of the larger, there was proper alignment of the respective lattices, and consolidation proceeded without observable defect formation, i.e., the two lattices knitted together without discontinuity. In the case of at least one virus crystal, cubic satellite tobacco mosaic virus (STMV), addition of three-dimensional nuclei and subsequent expansion provided the principal growth mechanism at high supersaturation. This process has not been reported for growth from solution of conventional crystals. In numerous other instances, the lattices of the small and larger crystals were obviously misaligned, and incorporation occurred with the formation of some defect. This phenomenon of small crystals physically embedded in larger crystals could only degrade the overall diffraction and materials properties of macromolecular crystals.

Incorporation of microcrystals by growing protein and virus crystals.

In the course of time-lapse video and atomic force microscopy (AFM) investigations of macromolecular crystal growth, we frequently observed the sedimentation of microcrystals and three-dimensional nuclei onto the surfaces of much larger, growing protein or virus crystals. This was followed by the direct incorporation over time of the smaller crystals into the bulk of the larger crystals. In some cases, clear indications were present that upon absorption of the small crystal onto the surface of the larger, there was proper alignment of the respective lattices, and consolidation proceeded without observable defect formation, i.e., the two lattices knitted together without discontinuity. In the case of at least one virus crystal, cubic satellite tobacco mosaic virus (STMV), addition of three-dimensional nuclei and subsequent expansion provided the principal growth mechanism at high supersaturation. This process has not been reported for growth from solution of conventional crystals. In numerous other instances, the lattices of the small and larger crystals were obviously misaligned, and incorporation occurred with the formation of some defect. This phenomenon of small crystals physically embedded in larger crystals could only degrade the overall diffraction and materials properties of macromolecular crystals.

Structure of Relaxed Monolayers of Arachidic Acid Studied by X-ray Reflectometry and Atomic Force Microscopy

Following the constant pressure relaxation on different subphases and the deposition on solid substrates, the structure and morphology of arachidic acid monolayers were studied by X-ray reflectometry and atomic force microscopy. The growth of three-dimensional (3D) phase nuclei was observed from the first minutes of the relaxation process in a monolayer relaxed on a subphase of low pH ≤ 3 with or without metal ions. With an increase in the relaxation time, the granule-like nuclei were transformed into platelike multilayer islands. The structure of 3D islands was commensurate with the B and C forms of bulk arachidic acid, with the B phase being prevalent after a long period of relaxation. During the storage of the samples in air, the granule-like islands were unstable and, after 2 months, transformed into the more stable platelike islands. The relaxation process was much slower in the case of binding the monolayer to cadmium or lead cations in a subphase of pH 5.3. After about 1 h of relaxation, only low folds arrayed with a hexagonal symmetry were observed on the monolayer of cadmium arachidate. On monolayers of lead arachidate, 1-2 bilayer islands emerged with hexagonal boundaries after a relaxation of some 3 h.

Mechanisms of growth for protein and virus crystals

The growth of six protein and virus crystals was investigated in situ using atomic force microscopy. Most of the crystals grew principally on steps generated by two-dimensional nucleation on surfaces though some grew by development of spiral dislocations. Apoferritin grew by a rarely encountered mechanism, normal growth, usually associated only with melt or vapour phase crystallization. Cubic crystals of satellite tobacco mosaic virus (STMV) grew, at moderate to high levels of supersaturation, by the direct addition of three-dimensional nuclei followed by their rapid normal growth and lateral expansion, a mechanism not previously described to promote controlled and reproducible crystal growth from solutions. Biological macromolecules apparently utilize a more diverse range of growth mechanisms in their crystallization than any previously studied materials.

Analysis of the formation of BaTiO3 island deposited on (111) InSb by metalorganic chemical vapor deposition at low temperature

Islands of BaTiO3 in a thin film deposited on a (111) InSb substrate by metalorganic chemical vapor deposition at a temperature of 300°C were investigated. Refractive index measured by ellipsometer using a He-Ne laser was 1.95, which is nearly the same value as that of amorphous BaTiO3 with microcrystals. X-ray diffraction peaks showed the deposit to be mostly amorphous and partly crystalline having the 〈110〉BaTiO3 direction normal to the (111) InSb. Transmission electron microscopy results showed that partially epitaxial BaTiO3 islands with periodic misfit dislocations had been formed at the interface between amorphous BaTiO3 thin layer and the (111) InSb substrate. These BaTiO3 islands on the (111) InSb substrate formed at a low growth temperature were three-dimensional nuclei which were closely associated with surface irregularities of the (111) InSb substrate.

The solid state dehydration ofdlithium potassium tartrate monohydrate is complete in two rate processes II. The nucleation and growth second reaction and dehydration mechanism

A kinetic and mechanistic study has been undertaken of the nucleation and growth reaction that is the second of the two consecutive rate processes that occur during the dehydration ofdlithium potassium tartrate monohydrate. Electron microscopic examinations of the cleaved surfaces of partly reacted crystals show the development of three-dimensional nuclei that are composed of small crystals of the anhydrous product and above 450 K there is evidence of intranuclear melting. Consistent with this model, the second reaction obeys the Avrami-Erofe’ev equation {[ — ln (1 — α)]1/2=kt}. Overall rates of the dehydrations of single crystals and of crushed powder samples were closely similar. The activation energy for dehydration was 150-160 kJ mol-1for both first (reported in part I, preceding paper) and second reactions and for both single crystal and crushed powder reactants. The addition of product crystallites to the reactant reduced sharply, or eliminated, the induction period to the nucleation and growth process. From consideration of the kinetic characteristics, together with the textural changes observed microscopically, we conclude that the following mechanism very satisfactorily accounts for our results. The first reaction proceeds to the dehydration of all crystal surfaces, representing water losses from a layerca. 10 µm thickness. This deceleratory process occurs initially in a structure resembling that of the reactant but later the increasing water site vacancy concentration results in increasing reactant disorder and possibly includes fusion of the outer layer. When the first reaction water evolution has slowed, recrystallization to the structure of the anhydrous product occurs at a limited number of sites to generate germ nuclei that effectively act as seed crystals for nucleus growth. During the second reaction the reactant—product contact interface is identified as a zone of diffusive water loss, similar to that described for the first reaction. Here, however, the product crystallites promote reorganization of dehydrated material, thereby opening channels for water escape and continually exposing new hydrate surfaces at which dehydration continues. This product recrystallization enables advance of the nucleus interface to be maintained, so that rates of both first and second reactions are subject to control by diffusive loss of water from an active boundary of the reactant. Product reorganization removes the inhibiting character of accumulated product layer by introducing escape channels for water loss so that interface advance continues and, although spasmodic, this migrates forward at a constant average linear rate. The work is of interest because kinetic measurements have been obtained for both of the consecutive rate processes that contribute to the overall reaction. The controls of both are shown to be closely similar. The reaction model proposed here provides insight into the structure of the dehydration interface and the mechanism of water release.

A kinetic study of the dehydrations of the alums KCr(SO4)2. 12H2O and KAl(SO4)2. 12H2O

A kinetic study of the dehydrations of the alums KCr(SO4)2. 12H2O and KAl(SO4)2. 12H2O in water vapour atmospheres is reported. The product-yield-time data are described by the Avrami-Erofe’ev (n= 2) and contracting cube equations. The observed kinetic behaviour differed from expectation for this reaction, which is known to proceed through the growth of three-dimensional nuclei. The retention of the volatile product (water) in the vicinity of the reactant solid influences the course of interface generation and development during the early stages of dehydration. Later there is a ‘first order’ approach to the establishment of equilibrium between water vapour and the solid reactant/product mixture. This pattern of kinetic behaviour is confirmed by a parallel thermal analysis (DSC) study in dry nitrogen. Evidence was obtained that an appreciable quantity of water was retained by the solids throughout these reactions. This is consistent with the view that water, temporarily retained at the active reactant-product interface promotes the difficult recrystallization step. Photoacoustic studies on partly thermally dehydrated samples of KCr(SO4)2. 12H2O showed the formation of a temporary water depleted surface layer that readily rehydrated either by adsorption from the atmosphere or by facile intracrystalline water diffusion. All the present observations can be explained by a reaction mechanism based on two main features: (i) the dehydration reaction is initiated across the total surface but is limited in extent and cannot be considered topotactic; (ii) the rate determining step for the subsequent unlimited advance of dehydration into the solid bulk is product crystallization, possibly topotactic, and this confers apparent topotacticity to the whole processes.

Effect of impurities on the growth of KDP crystals: Mechanism of adsorption on (101) faces

The results of the dependence of growth rate, R 001, along 〈001〉 directions of KDP crystals in aqueous solutions at 30°C on the concentration c i, up to 4.0×10 -6 mol/mol, of Fe 3+ and Cr 3+ impurities in aqueous solutions at different supersaturations are presented and discussed. It is observed that with an increase in c i, in the case of Fe 3+ impurity R 001 first increases and then decreases at 4.2% supersaturation. For Fe 3+ impurity at 6.3% supersaturation and for Cr 3+ impurity at 6.3% and 8.55% supersaturations, R 001 regularly decreases with an increase in c i. At 13% supersaturation, R 001 increases with an increase i n the concentration of both Fe 3+ and Cr 3+ impurities. The effect of impurities on growth rates at 4.2%, 6.3% and 8.5% supersaturations is discussed in terms of changes in the thermodynamic and kinetic parameters involved in the crystal growth theories. The increase in R 001 at 13% supersaturation is, however, explained by considering the formation of three-dimensional impurity nuclei on the pyramidal faces. The analysis suggests that adsorption of both Fe 3+ and Cr 3+ impurity takes place at kink sites on (100) and (101) faces.

Aluminum-electrocrystallization from metal—organic electrolytes

Aluminum electrodeposition from aprotic solutions of the organic complex KF. 2Al(C 2H 5) 3 on polycrystalline aluminum and vitreous carbon is described for potentiostatic and voltage pulse conditions. Near the equilibrium of the aluminum electrode, below curents of about −3 mA cm −2, electron transfer controls the deposition rate. Stationary polarizations over −0.1 V generate a material transport limitation which was controlled and eliminated by the rotating disk electrode technique. Extending the overvoltage to −0.1 to −0.3 V a slow chemical process characterized by a reaction current of ca −5 mA cm −2 precedes charge transfer. Above −0.3 and −0.7 V two further deposition mechanisms possibly involving slow chemical and crystallization steps were observed. On a foreign substrate, ie vitreous carbon, it was found that peripheral kinetically controlled growth of instantaneously formed two-dimensional plates takes place on top of plates so as to produce three-dimensional nuclei which then grow by a transport limited mechanism.

The pseudobinary phase diagram cis-/trans-azobenzene and the cis → trans isomerization in various states of aggregation

The pseudobinary phase diagram of trans-azobenzene and the thermally instable cis-isomer was examined by thermomicroscopical and DSC-measurements. Two eutectic temperatures have been determined (at 41.4 C and 36.7 C). The latter occurs only in the presence of a metastable polymorph of cis-azobenzene after quenching a melt, that contained as well cis-as trans-azobenzene, presumable under conditions of heterogeneous nucleation. In the melt and in solution the thermally induced cis-trans-isomerization follows first order kinetics. Below the eutectic temperature the rate is controlled by three-dimensional nuclei growth to reaction extent 0.20. Between the eutectic temperature and the melting points of product respectively educt the kinetics follow the PROUT-TOMPKINS law. The reason is the increasing isomerization rate with increasing fraction of melt.

Growth kinetics of Cd 3As 2 films on sodium chloride substrates

The growth morphology of thin Cd 3As 2 films on NaCl substrates has been described. It was found that growth of the films begins with the formation of three-dimensional nuclei. The dependence of both number density and size of such nuclei on condensation parameters was investigated. In addition, nucleus size distributions, number densities of nuclei, saturation densities and initial nucleation rates were analysed on the basis of theoretical expressions. Values for the activation energies of adsorption and surface diffusion and for the binding energy of a pair of atoms were estimated.

The influence of the concentration and form of the complex ion of cadmium with ethylenediamine on the electrocrystallization of Cu-Cd alloy

Tests were conducted on the electrocrystallization of Cu-Cd alloy on a single-crystal Cu(111) substrate as a function of the concentration and form of the complex ion of the more electronegative constituent with ethylenediamine (en) and ammonia. The concentrations and forms of the complex ions were found from a numerically determined diagram of the ionic equilibria in a Cu 2+-Cd 2+-en-NH + 4 system. The potentiostatic impulse method was used for these tests. It was shown that, independently of the form and concentration of the complex ion of cadmium with ethylenediamine, the mechanism of electrocrystallization of the Cu-Cd alloy depends on the formation of three-dimensional crystallization nuclei in the form of cones constructed of superimposed layers and is identical with that previously described by the present author and coworkers. It was found that with an increase in the degree of coordination of cadmium with ethylenediamine at comparable alloy electrocrystallization potentials the layers of alloy formed have an increasingly fine-grained structure.

Epitaxy of MgO on alkali halides with NaCl-type structure

Reflection high-energy electron diffraction has been used to examine the growth of MgO on (001) surfaces of LiF, NaCl, and KCl in the temperature range 25–250 °C. MgO grew initially with parallel orientation, i.e., (001)MgO∥ (001)s and [100]MgO∥[100]s. The substrate temperature at which parallel orientation occurred depended on the nature of the substrate, increasing in the sequence LiF, NaCl, and KCl. The mode of MgO growth did not depend on the substrate temperature and the lattice misfit. MgO initially formed three-dimensional nuclei. At high substrate temperatures (250 °C) the range of film thickness over which parallel orientation persisted depended on the nature of the substrate, decreasing in the sequence LiF, NaCl, and KCl. For a given substrate this range of film thickness increased with increasing substrate temperature. Infrared spectra obtained for films deposited on substrates at 25–250 °C showed the presence of hydroxyl groups only in films deposited on substrates at low temperatures (25 °C). These results suggest that (1) the range of film thickness over which parallel orientation persists is influenced by the misfit between the MgO and the substrate lattice, decreasing with increasing lattice misfit and (2) the parallel orientation is also influenced by the incorporation of impurities such as hydroxyl groups into the growing film at later stages of growth on substrates at low temperatures.

The anodic behaviour of lead amalgam electrodes in HCl solution

The anodic behaviour of the lead amalgam electrode has been investigated in aqueous hydrochloric acid solutions. Both voltammetric and potential pulse results are described. The mechanism of passivation is shown to be the nucleation and growth of three-dimensional nuclei of PbCl 2 which progressively block the electrode. The nuclei are considered to be right circular cones, distributed at random on the electrode surface. As in the case of solid lead electrodes, a simultaneous dissolution of PbCl n 2− n complexes is observed, which diffuse away from the electrode under mass transport control. Evidence is also presented that the first stage in the growth of the anodic film is the two-dimensional nucleation and growth of a monolayer of PhCl 2. Unfortunately, this process is partially obscured by the dissolution reaction. A reaction scheme is proposed.

Kinetic characteristics of the formation of thin films by a mechanism based on the growth and merging of three-dimensional nuclei formed on active centers of the substrate

The kinetic characteristics of the growth and merging of three-dimensional hemispherical nuclei formed on randomly situated active point centers of the substrate are considered. For a specified density of the active centers λ and an average nucleus waiting time τ, the time dependences of the average film thickness, the dispersion of the thickness, the rate of film growth, and the number of active centers occupied by the developing nuclei are all calculated. These relationships agree qualitatively with experimental data. The size distribution of the nuclei is considered. It is shown that the theoretical size distribution of the nuclei corresponds to the experimental distribution if allowance is made for the period of transience of the nucleus.

Epitaxial growth on MgO on (001) surfaces of silver single-crystal film substrates

The epitaxial growth of evaporated MgO films (<25–4000 Å) on smooth (001) surfaces of silver single-crystal film substrates at a temperature inthe range 25°–200°C has been studied by reflection high energy electron diffraction. At substrate temperatures below 150°C, MgO is oriented with its (111) plane parallel to the surface of the substrate. The (111) orientation is fibrous. At 150°C a major fraction of the deposit is oriented in parallel and at 200°C only the parallel orientation is observed. The deposit initially grows as oriented three-dimensional nuclei. As the film thickness increases to 500 Å, randomly oriented MgO is simultaneously formed at the surface. With further increase in thickness the degree of epitaxial orientation decreases and at 1000 Å a small amount of one- degree (111) orientation appears at the surface. At 4000 Å the surface orientation becomes one-degree (111); the (111) orientation is interpreted as a final growth orientation.

Influence of the supersaturation on the mode of thin film growth

AbstractThe thermodynamic criterion of the mechanism of growth of thin films is reconsidered. It is shown that the supersaturation plays a significant role in determining the growth mode in addition to already discussed interrelation between the specific surface energies of the substrate, deposit and the substrate‐deposit interface. The conclusions are in good agreement with experimental observations in deposition of f.c.c. metals on f.c.c. metal substrates that at positive surface energy change the island growth is favoured at supersaturations lower than some critical one while at higher supersaturations a layer growth takes place. The STRANSKI‐KRASTANOV mechanism consisting in formation of threedimensional nuclei on top of previously deposited layers of the overgrowth is considered in more details. It is concluded that its occurrence depends on the sign and the value of the lattice misfit between the substrate and the overgrowth, on the interaction forces between them and on the thermodynamic stability of the deposit. The validity of the classical nucleation theory for explanation of the growth mode transition is also discussed.

Nucleation and growth of GeTe films on KCl and SnTe substrates

The nucleation and growth of GeTe films on KCl or SnTe substrates is found to follow the same mechanism on both substrates. Nucleation occurs through the formation of three-dimensional nuclei of almost equal sizes, bounded by edges along the [110] and [1 1 0] directions. The growth continues through the lateral extension of the nuclei. The overgrowth layer is anisotropic strainwise, due to the difference in lattice misfit along [110] and [1 1 0]. This angular misfit is responsible for an increase in the equilibrium energy by 5%. The excess strain is accommodated by misfit dislocations forming a network parallel to the [110] and [1 1 0] directions, the dislocations being of the edge orientation with Burger's vector equal to 1 2 a<110> and with a different linear density in the two directions. Annealing the system GeTe/SnTe results in a substantial decrease in the density of dislocations due to cross diffusion of Ge and Sn respectively, which forces the dislocations to climb to the surface.

The effect of substrate surface contamination on the electrolytic growth of epitaxial nickel films

Thin (001) single-crystal copper substrates were contaminated by exposure to the atmosphere. Electrodeposition of nickel took place upon the contaminated surface. Duplex films as well as stripped deposits were examined by transmission electron microscopy and electron diffraction. In contrast to deposition onto clean substrates, small three-dimensional (3-D) nuclei formed at the start of deposition. Deposits became continuous at an average thickness of ≈50Å. Apart from the small initial nuclei, larger block-like growths were observed. Contrast phenomena as well as the results of misfit measurements suggested that the lattices were initially strained to give a coherent nickel-copper interface.