2D Nucleation Research Articles

Electrochemical synthesis and photoelectrochemical characterization of copper hexacyanoferrate (II) deposited on n-Si(1 0 0)

In this study, we examined the synthesis and characterization of copper hexacyanoferrate (II) on an n-Si(100) substrate. The copper hexacyanoferrate (II) was formed in the following stages: (i) deposition of metallic copper on an n-Si(100) electrode using an electroless process and (ii) electrooxidation of the copper film in a solution containing Fe(CN)64− ions. In the first stage, a study using a potential step method was performed, and the corresponding nucleation and growth mechanisms were determined. A morphologic analysis of the deposits obtained at different potentials was performed using atomic force microscopy. The results were consistent with a 3D progressive nucleation with diffusion-controlled growth. Electrochemical characterization under illumination demonstrated a charge transfer process due to the photogenerated holes in the valence band of silicon. Afterwards, the diffusion coefficient of the potassium ions and rate constants of the electrochemical processes were determined. Finally, this research is designed to increase the understanding of this system for its possible use in an electrochemical storage device that can be loaded in situ via the photovoltaic action of the doped silicon.

Effect of pH value on the growth morphology of KH2PO4 crystal grown in defined crystallographic direction

KH2PO4 single crystals were grown in aqueous solution at different pH values by using “point seeds” with a defined crystallographic direction at 59 degree to the Z axis. Atomic Force Microscope (AFM) was applied to observe the surface morphology of (100) face. It was found that at the same supersaturation, the larger steps appeared at the lower pH value before appearance of 2D nucleus. We found that 2D nucleus was occurred at σ ≤ 0.04 when pH value is <2.8. The occurrence of 2D nucleus was caused by the decreasing step-edge free energy with the decreasing of pH value in the growth solution. In this paper, we observed the morphologies of (100) faces of KDP crystals which grew in solutions with different pH values. 2D nucleuses appeared on the terrace of growth steps when pH value down to 2.8 and 3.2 at supersaturation of 0.04, while pH value down to 2.4, only 2D nucleation control the growth. Therefore, the pH value can change the growth mechanism of KDP crystals.

Nucleation kinetics of lovastatin in different solvents from metastable zone widths

Cooling crystallization of lovastatin in methanol, ethanol and acetone was carried out and the metastable zone widths in those solutions were determined experimentally by the polythermal method using a Turbidity Monitoring Technique. Two theoretical approaches, the self-consistent approach and classical 3D nucleation theory approach, were then employed to estimate the nucleation kinetics from the measured metastable zone width. The results suggested that nucleation in ethanol and acetone is instantaneous and follows polynuclear (PN) mechanism, while in methanol the nucleation is progressive with a high nucleation order. Because of the high solubility of lovastatin in these solvents, strong solute–solvent interactions exist, which make the activation energy for crystal formation different from that for self-diffusion.

Effect of gluconate ions on electroreduction phenomena during manganese deposition on glassy carbon in acidic chloride and sulfate solutions

Cathodic processes occurring during manganese deposition from acidic baths (pH 3.5) were investigated using glassy carbon rotating disk electrode. Cyclic voltammetry, linear sweep voltammetry and anodic sweep voltammetry were applied. Speciation and buffer capacities of chloride, chloride-gluconate, sulfate and sulfate-gluconate solutions were determined. Two ranges of the potentials (vs. Ag/AgCl) were considered: from −0.8V to −1.56V and below (up to −1.8V) corresponding to hydrogen evolution and manganese deposition, respectively. In the chloride bath, strong adsorption of MnCl+ on the cathode inhibited hydrogen evolution, favored fast metal nucleation and deposition of fine grained coatings, but low buffering action of the bath enhanced self-dissolution of manganese. Addition of gluconate improved buffer capacity of the bath, inhibited metal nucleation and hindered somewhat secondary metal dissolution, but the presence of chloride ions and low ammonium ions concentration resulted in the formation of hydroxide species as a secondary product of metal oxidation. Sulfate bath prevented manganese dissolution, but buffer effect of the solution was too small to stabilize pH at the cathode surface. The best buffering action of the sulfate-gluconate system originated from the cooperative action of gluconate and ammonium species made possible to obtain metallic layer protected from the dissolution by the electrolyte. Weak adsorption of MnSO4 on the cathode in sulfate baths lead to the hydrogen evolution under limiting current in the potential range preceding manganese deposition. Independently on the bath composition, manganese started to deposit at approx. −1.56±0.01V (vs. Ag/AgCl) according to the instantaneous model of 3D nucleation. Average diffusion coefficients of the manganese species of order 10−6cm2/s were found.

Formation of 3D cholesterol crystals from 2D nucleation sites in lipid bilayer membranes: implications for atherosclerosis.

Atherosclerosis is the major precursor of cardiovascular disease. The formation of cholesterol crystals in atherosclerotic plaques is associated with the onset of acute pathology. The cholesterol crystals induce physical injury in the plaque core, promoting cell apoptosis and triggering an increased inflammatory response. Herein we address the question of how cholesterol crystal formation occurs in atherosclerosis. We demonstrate that three-dimensional (3D) cholesterol crystals can undergo directed nucleation from bilayer membranes containing two-dimensional (2D) cholesterol crystalline domains. We studied crystal formation on supported lipid bilayers loaded with exogenous cholesterol and labeled using a monoclonal antibody that specifically recognizes ordered cholesterol arrays. Our findings show that 3D crystals are formed exclusively on the bilayer regions where there are segregated 2D cholesterol crystalline domains and that they form on the domains. This study has potentially significant implications for our understanding of the crucial step in the mechanism by which atherosclerotic lesions form.

Effect of pH value on the growth morphology of KH2PO4 crystal grown in defined crystallographic direction

KH2PO4 single crystals were grown in aqueous solution at different pH values by using “point seeds” with a defined crystallographic direction at 59 degree to the Z axis. Atomic Force Microscope (AFM) was applied to observe the surface morphology of (100) face. It was found that at the same supersaturation, the larger steps appeared at the lower pH value before appearance of 2D nucleus. We found that 2D nucleus was occurred at σ ≤ 0.04 when pH value is &lt;2.8. The occurrence of 2D nucleus was caused by the decreasing step‐edge free energy with the decreasing of pH value in the growth solution. In this paper, we observed the morphologies of (100) faces of KDP crystals which grew in solutions with different pH values. 2D nucleuses appeared on the terrace of growth steps when pH value down to 2.8 and 3.2 at supersaturation of 0.04, while pH value down to 2.4, only 2D nucleation control the growth. Therefore, the pH value can change the growth mechanism of KDP crystals.

Towards an understanding of the nucleation of alpha-para amino benzoic acid from ethanolic solutions: a multi-scale approach.

The molecular assembly and subsequent nucleation of para-amino benzoic acid (PABA) from ethanolic solutions is probed using a multi-scale and multi-technique approach. This is applied by examining and interrelating information regarding the molecular, solution-state, cluster, solid-state and surface structures to understand why the alpha form of PABA is crystallised in preference to its low temperature beta form. Calculations suggest that conformational changes within the solute molecule play little or no role in directing the nucleation of either the alpha or beta crystal forms. Combined ab initio and molecular dynamics calculations of the stability of small clusters in solution suggests that the hydrogen-bonded carboxylic acid dimers, present in the alpha structure, are the most stable in solution and play a major role in the self-assembly and polymorphic expression of the alpha form in ethanol in preference to the beta form. These calculations are in good agreement with X-ray small-angle scattering analysis which reveals the presence of PABA clusters in ethanol which are consistent with the size and shape of a carboxylic acid dimer. SAXS studies also reveal the presence of larger cluster structures in a size range 10-40 nm which appear to grow, perhaps reflecting a change in the balance between monomers and dimers within the solution during the nucleation process. The results of crystallisation-kinetics experiments indicate an instantaneous nucleation mechanism where the number of instantaneously nucleated crystallites is calculated to be 1360-660 nuclei per ml and the subsequent growth is found to be only rate limited by diffusion of the growth unit to the crystallite surface. A linear dependence of growth rate with respect to supersaturation is observed for the (0 1 -1) capping face, which is associated with strong π-π stacking interactions. This is consistent with a solid-on-solid mechanism associated with surface roughened growth and concomitant poor lattice-perfection. Conversely, the side (1 0 -1) surface has a growth mechanism consistent with a 2D nucleation birth and spread mechanism. Hence, these mechanisms result in very fast growth along the b-axis and the needle-like morphology that is observed for alpha-PABA.

Macroscopic Approach to the Nucleation and Propagation of Foreign Polytype Inclusions during Seeded Sublimation Growth of Silicon Carbide

The nucleation and propagation of foreign polytypes during seeded sublimation growth of silicon carbide is addressed on a macroscopic footing, using a coupled experimental and numerical simulation approach. Experiments are conducted in a contactless growth geometry for different seed substrates with varying polarity, miscut angle, polytype, and crystal shape (concave and convex). The interface shape evolution is tracked by periodic nitrogen marking, which also allows the measurement of lateral and normal growth rates at any time during growth. Classical 2D nucleation theory is included in modeling the full process, and this is compared to experiments. We demonstrate that the occurrence of a foreign polytype is linked to the combined effects of (i) the presence of the natural {0001} facets and (ii) the minimization of 2D nucleation energy. Areas with a high probability of foreign polytype nucleation can thus be predicted. Once formed, the progress or disappearance of foreign inclusions is directly related ...

Characterization of V-shaped Defects in 4H-SiC Homoepitaxial Layers

Synchrotron white beam x-ray topography images show that faint needle-like surface morphological features observed on the Si-face of 4H-SiC homoepitaxial layers using Nomarski optical microscopy are associated with V-shaped stacking faults in the epilayer. KOH etching of the V-shaped defects reveals small oval pits connected by a shallow trench which correspond to the surface intersections of two partial dislocations and the stacking fault connecting them. Transmission electron microscopy (TEM) specimens from regions containing the V-shaped defects prepared using focused ion beam milling show stacking sequences of (85), (50) and (63) at the faulted region using high resolution TEM. In order to study the formation mechanism of the V-shaped defects, low dislocation density 4H-SiC substrates were chosen for epitaxial growth, and the corresponding regions before and after epitaxy growth are compared in SWBXT images. It is found that no defects in the substrate are directly associated with the formation of the V-shaped defects. Simulation results of the contrast from the two partial dislocations associated with V-shaped defects in synchrotron monochromatic beam x-ray topography reveals the opposite sign nature of their Burgers vectors. Therefore, a mechanism of 2D nucleation during epitaxy growth is postulated for the formation of the V-shaped defects, which requires elimination of non-sequential c/4[0001] bilayers from the original structure to create the observed faulted stacking sequence.

Nucleation of ammonium aluminum sulfate dodecahydrate from unseeded aqueous solution

The metastable zone width (MSZW) and the induction times for the cooling crystallization of ammonium aluminum sulfate dodecahydrate from unseeded aqueous solution were measured using focused beam reflectance measurement (FBRM). Dependence of MSZW on cooling rate and saturation temperature and dependence of the induction time on supersaturation and temperature were systematically investigated. Classical 3D nucleation based approach of Sangwal was employed to correlate MSZW with cooling rate. Linear plots of (T⁎/ΔTmax)2 versus lnR gave values of the parameters F1, F, and Z. According to the classical theory, the primary nucleation mechanism was identified and the interfacial energy was calculated.

The effect of a copolymer inhibitor on baryte precipitation

In situ atomic force microscopy (AFM) experiments were used to study the effect of trace amounts of a commercial inhibitor on the (001) baryte surface during growth. The additive tested was a copolymer, used as a scale inhibitor in oil recovery (maleic acid/allyl sulfonic acid copolymer with phosphonate groups, partial sodium salt). The morphology of the growth was used to gain a better understanding of the inhibition mechanism. Without an inhibitor, barium sulfate growth occurred by 2D island nucleation and spreading. The addition of a small amount (0.1 ppm and 0.5 ppm) of copolymer inhibitor enhances 2D nucleation but blocks growth. Just 1 ppm of inhibitor blocks nucleation and growth by adsorption of a copolymer layer onto the baryte surface. Similarly in 3D studies, small amounts of inhibitor seem to act on growth and not on nucleation and larger amounts of copolymer act on both by adsorption of the copolymer to all baryte surfaces keeping the particles in their embryo stage.

Architecture and assembly of the Bacillus subtilis spore coat.

Bacillus spores are encased in a multilayer, proteinaceous self-assembled coat structure that assists in protecting the bacterial genome from stresses and consists of at least 70 proteins. The elucidation of Bacillus spore coat assembly, architecture, and function is critical to determining mechanisms of spore pathogenesis, environmental resistance, immune response, and physicochemical properties. Recently, genetic, biochemical and microscopy methods have provided new insight into spore coat architecture, assembly, structure and function. However, detailed spore coat architecture and assembly, comprehensive understanding of the proteomic composition of coat layers, and specific roles of coat proteins in coat assembly and their precise localization within the coat remain in question. In this study, atomic force microscopy was used to probe the coat structure of Bacillus subtilis wild type and cotA, cotB, safA, cotH, cotO, cotE, gerE, and cotE gerE spores. This approach provided high-resolution visualization of the various spore coat structures, new insight into the function of specific coat proteins, and enabled the development of a detailed model of spore coat architecture. This model is consistent with a recently reported four-layer coat assembly and further adds several coat layers not reported previously. The coat is organized starting from the outside into an outermost amorphous (crust) layer, a rodlet layer, a honeycomb layer, a fibrous layer, a layer of “nanodot” particles, a multilayer assembly, and finally the undercoat/basement layer. We propose that the assembly of the previously unreported fibrous layer, which we link to the darkly stained outer coat seen by electron microscopy, and the nanodot layer are cotH- and cotE- dependent and cotE-specific respectively. We further propose that the inner coat multilayer structure is crystalline with its apparent two-dimensional (2D) nuclei being the first example of a non-mineral 2D nucleation crystallization pattern in a biological organism.

Hyperpolarizability calculation and kinetic effect of impurities on LVP

In order to investigate microscopic second order nonlinear optical properties of L-valinium picrate (LVP) crystals, the molecular dipole moment, polarizability, and first hyperpolarizability are estimated by density functional theory (DFT). Apart from that, a quantitative study using Atomic force microscopy (AFM) combined with crystal growth experiments on the effects of impurities (magnesium and copper) on LVP is presented. Two-dimensional (2D) nucleation growth mechanism and typical surface morphologies are described and discussed. In the presence of impurities, the decreasing of the size of 2D islands is probably incurred by the reducing of the step kinetic coefficient. From the supersaturation dependence of the normal growth rate, the effective step edge free energies are evaluated as γ0=2.90kT, γMg=2.76kT, γCu=2.95kT.

The Influence of the Interaction between Silicon and Hydroxyl Radicals before the Copper Electrodeposition. Activity Changes of the Silicon Surface

Several studies have emerged in an attempt to explain the aging process which occurs in the human body, where free radicals (FR) are species involved in these alterations. Considering this subject, there is a great interest in being able to detect, quantify and control the amount of FR that human body produces at certain periods, and especially the hydroxyl radical (–OH) [[i],[ii]].In this study we examined the changes in the surface activity of n-Si after its exposition to a solution containing hydroxyl radicals. Changes in the surface activity caused by the interaction between silicon and –OH radicals were characterized by analyzing the nucleation and growth mechanism (NGM) of copper on silicon electrodes. For this aim, we studied the copper deposition on: i) n-Si without exposure to hydroxyl radicals and ii) n-Si exposes at different periods of time to hydroxyl radical. It was observed a change in the copper NGM on n-Si, from 3D progressive nucleation diffusion-controlled growth (PN3DDIFF), for the system without exposure to the –OH radical, toward 3D instantaneous nucleation diffusion-controlled growth (IN3DDIFF), when the semiconductor substrate was exposed to the –OH radicals. In both cases analysis by the Atomic Force Microscopy (AFM) technique was performed confirming these mechanisms. [[i]] Tomasz Rapecki, Anna M. Nowicka, Mikolaj Donten, Fritz Scholz, Zbigniew Stojek, Electrochem. Commun. 12, (2010), 1531-1534. [[ii]] Anna Maria Nowicka, Ulrich Hasse, Mikolaj Donten, Michael Hermes, Zbigniew Jan Stojek, Fritz Scholz, J Solid State Electrochem., 15, (2011), 2141–2147.

Theoretical Equilibrium Morphology and Twin Energy of Monoclinic Hydroxyapatite

Microsized hydroxyapatite (HAp), we synthesized under mild hydrothermal conditions, shows both monoclinic and hexagonal polymorphs made by rod-shaped single crystals along with simple and multiple twins. The theoretical morphology, obtained through the Hartman-Perdok analysis on the P21/c polymorph, allowed to predict the stable surfaces of the main {hkl} forms. Their surface energies have been evaluated, at 0 K, by ab initio quantum-mechanical calculations. The resulting HAp equilibrium shape (ES) [1] is dominated by a pseudo-hexagonal prism formed by the the {001}, {100} and {-102} forms (having close energy values), all developing around the OH channels of the structure. The basal {010} form truncates the pseudo-hexagonal prism perpendicularly to the OH channels, while three other less important prisms {012}, {110} and {11-2}, lying in between the pseudo-hexagonal prism and the basal pinacoid, enter the ES. A simple kinetic model based on 2D nucleation is proposed to explain the morphology of the single crystals [1]. Assuming [2, 3] that the simplest twin law for the monoclinic phase is generated by a threefold rotation about the 21axis, we found that the three original composition planes (OCP) of the twin coincide with the faces of the {001}, {100} and {-102} forms. For each OCP there are two different surface terminations, according to whether the frontier between parent (P) and twinned (T) crystals intercepts (or not) the centers of mass of PO4and Ca ions and of the O atoms of the OH groups. We examined, as an example, the two kinds of interfaces for the twin (100)P/(001)T. The calculated twin energies result to be very close and low (3.3 and 4.2 erg cm-2), when compared to the value of the HAp surface energy in the [010] zone (≍ 1500 erg cm-2). This means that the probability of formation of a 2D-nucleus oriented in twin position on all the [010] zone is practically equivalent to that needed to generate a normally oriented 2D nucleus in the same zone

High pressure crystallization and crystallography of glucose isomerase

Crystallization of protein is still a "bottleneck" for the three-dimensional (3D) structure analysis of a protein molecule. Therefore, the study on the acceleration of the crystallization is important. Visuri et al. repoted, for the first time, that the crystallization of glucose isomerase (GI) crystals was significantly enhanced with increasing pressure [1], while they did not discuss further effects of pressure on the crystallization. We measured solubilities, growth rates of a crystal face, step velocities and two-dimensional (2D) nucleation rates, three-dimensional (3D) nucleation rates of the GI crystal under high pressure. From these results, we found the negative molar volume change of crystallization delta V, the decrease in a step ledge surface energy and the increase in a step kinetic coefficient with increasing pressure. Activation volume of crystallization delta V‡ was discussed and the absolute value of it was almost half of that of delta V [2]. X-ray crystallography is also very important to study how the three dimensional structure of protein is related to its function under high pressure at atomic level. Kundrot et al. analyzed the crystal structure of lysozyme at 100 MPa using a beryllium (Be) cell. The crystal was prepared under 0.1 MPa. Charron et al. reported that thaumatin crystals grown under 150 MPa were analyzed under 0.1 MPa. Although Kundrot et al. concluded that the protein structure under high pressure was compressed, they could not estimate the influence of the bond structure of the crystal at 0.1 MPa on the compressed structure. In Charron's case, the thaumatin structures in crystals prepared under 0.1 MPa and 150 MPa were almost identical, when they did X-ray analyses of both crystals at 0.1 MPa. From these two results, we considered that compression of the crystals was reversible. We conducted both Kundrot's and Charron's methods for GI using a stand-alone type Be vessel [3]. We found that (1) GI molecule was reversibly compressed under high pressure, (2) the distributions of water sites around GI molecules were almost identical in the range of pressures from 0.1 to 100 MPa.

Self-terminating growth in hexagonal boron nitride by metal organic chemical vapor deposition

In this work, we demonstrate the growth of atomically smooth few-layer hexagonal boron nitride (h-BN) on sapphire substrates by metal organic chemical vapor deposition using triethylboron (TEB) and NH3 as precursors. Changing the V/III ratio in a certain temperature and pressure range was found to change the growth mode from random 3D nucleation to self-terminating growth. Infrared reflectance and Raman spectroscopy were used to identify the h-BN phase of these films. Atomic force microscopy measurements confirm that the surfaces are smooth and continuous even over atomic steps on the surface of the substrate. Using X-ray reflectance measurements, the thicknesses of films grown under self-terminating conditions were determined to be ∼1.6 nm and independent of the growth time (1 to 60 min) and TEB flux rate. On the basis of the results of this study, a possible mechanism for the observed self-terminating growth behavior is discussed.

Changes in the surface activity of n-Si after interaction with hydroxyl radicals

Changes in the surface activity of n-Si after being exposed to a solution containing hydroxyl (OH) radicals were approached. These changes caused by the interaction between silicon and the OH radicals were characterized by analyzing the nucleation and growth mechanisms of copper electrodeposited on silicon. Thus, both copper depositions on n-Si without prior exposure to hydroxyl radicals and with it at different exposure periods were analyzed. By means of j/t transients analysis a change in the nucleation and growth mechanisms of copper on n-Si was observed, from 3D progressive nucleation diffusion-controlled growth for the system unexposed to the radical to 3D instantaneous nucleation diffusion-controlled growth, when the semiconductor substrate was exposed to the radicals OH. These changes were corroborated through ex situ Atomic Force Microscopy.

TFOx: A versatile kinetic Monte Carlo program for simulations of island growth in three dimensions

A three dimensional (3D) kinetic Monte Carlo (KMC) code has been developed that simulates the general behavior of the 3D irreversible nucleation and growth of epitaxial islands, as motivated by experimental observations of oxide nuclei formation and growth during the early stages of copper oxidation. This package was originally a versatile two dimensional (2D) KMC code [Thin Film Oxidation (TFOx)] that considered a variety of elementary steps, including deposition, adsorption, surface diffusion, aggregation, desorption, and substrate-mediated indirect interactions between static adatoms. We extended TFOx to describe 3D island growth. This new version of TFOx is composed of a C++ console program and Python graphical user interface (GUI), such that parameterized simulation, parallel execution, and 3D growth capabilities are feasible. We examined the effects of the potential gradient and the Ehrlich–Schwöbel barrier and found that the 3D island morphology is significantly influenced by the incorporation of these two factors.

Dislocations in CdTe heteroepitaxial structures on GaAs(301) and Si(301) substrates

Initial stages of ZnTe growth on GaAs(301) and Si(301) substrates by the method of molecular beam epitaxy are investigated by means of in situ one-wave ellipsometry. Layer-by-layer growth of the ZnTe film is observed on GaAs(301) substrates, whereas 3D nucleation occurs during epitaxy on Si(301) substrates. Misfit dislocations (MDs) are inserted into the ZnTe film during the growth of the first monolayers. Owing to MDs, the film lattice turns with respect to the substrate lattice, which is confirmed by X-ray measurements. Threading segments of MDs in CdTe/ZnTe/GaAs(301) and CdTe/ZnTe/Si(301) heterostructures are subjected to etching. The etch dislocation pits are found to have different shapes, which testifies to different types of threading dislocations. In the case of layer-by-layer etching, the dislocation density is found to increase inward the CdTe film, which testifies to annihilation of dislocations in the course of growth of CdTe films. The dislocation annihilation rate is higher in films grown on GaAs(301) than in those grown on Si(301). A possible reason is the higher mobility of dislocations in CdTe films on GaAs(301) substrates.