Showing papers on "Nucleation published in 1998"

Dislocation nucleation and defect structure during surface indentation

Abstract: We model indentation of a metal surface by combining an atomistic metal with a hard-sphere indenter. This work provides atomistic imaging of dislocation nucleation during displacement controlled indentation on a passivated surface. Dislocations and defects are located and imaged by local deviations from centrosymmetry. For a Au(111) surface, nucleation of partial dislocation loops occurs below the surface inside the indenter contact area. We compare and contrast these observations with empirical criteria for dislocation nucleation and corresponding continuum elasticity solutions.

Plasma assisted molecular beam epitaxy of ZnO on c -plane sapphire: Growth and characterization

Abstract: ZnO single crystal thin films were grown on c-plane sapphire using oxygen microwave plasma assisted molecular beam epitaxy. Atomically flat oxygen-terminated substrate surfaces were obtained by pre-growth cleaning procedures involving an oxygen plasma treatment. A two dimensional nucleation during the initial growth which is followed by a morphology transition to three dimensional nucleation was observed by in situ reflection high energy electron diffraction. X-ray diffraction (XRD) and photoluminescence investigations suggest that the ZnO epilayer consists of a high quality layer on top of a transition layer containing a high density of defects in the interfacial region. A full width at half maximum (FWHM) of 0.005° is obtained for the ZnO(0002) diffraction peak in an XRD rocking curve, while a broad tail extending from the peak can also be observed. The photoluminescence spectra exhibit dominant bound exciton emission with a FWHM of 3 meV at low temperatures and free exciton emission combined with a ver...

Kinetic theory in the earth sciences

Abstract: Preface Rate Laws of Chemical Reactions 2 Transition State Theory 3 Transport Theory 4 Diffusion 5 Irreversible Thermodynamics 6 Nucleation Theory 7 Theory of Crystal Growth and Dissolution References Index

Physics of crystal growth

Abstract: Preface List of symbols 1. Morphology of a crystal surface 2. Surface free energy, step free energy, and chemical potential 3. The equilibrium crystal shape 4. Growth and dissolution crystal shapes: Frank's model 5. Crystal growth: the abc 6. Growth and evaporation of a stepped surface 7. Diffusion 8. Thermal smoothing of a surface 9. Silicon and other semiconducting materials 10. Growth instabilities of a planar front 11. Nucleation and the adatom diffusion length 12. Growth roughness at long lengthscales in the linear approximation 13. The Kardar-Parisi-Zhang equation 14. Growth without evaporation 15. Elastic interactions between defects on a crystal surface 16. General equations of an elastic solid 17. Technology, crystal growth and surface science Appendices References Index.

Self-organized growth of nanostructure arrays on strain-relief patterns

Abstract: The physical and chemical properties of low-dimensional structures depend on their size and shape, and can be very different from those of bulk matter. If such structures have at least one dimension small enough that quantum-mechanical effects prevail, their behaviour can be particularly interesting. In this way, for example, magnetic nanostructures can be made from materials that are non-magnetic in bulk1, catalytic activity can emerge from traditionally inert elements such as gold2, and electronic behaviour useful for device technology can be developed3,4. The controlled fabrication of ordered metal and semiconductor nanostructures at surfaces remains, however, a difficult challenge. Here we describe the fabrication of highly ordered, two-dimensional nanostructure arrays through nucleation of deposited metal atoms on substrates with periodic patterns defined by dislocations that form to relieve strain. The strain-relief patterns are created spontaneously when a monolayer or two of one material is deposited on a substrate with a different lattice constant. Dislocations often repel adsorbed atoms diffusing over the surface, and so they can serve as templates for the confined nucleation of nanostructures from adatoms. We use this technique to prepare ordered arrays of silver and iron nanostructures on metal substrates.

Radiation-induced synthesis of mono- and multi-metallic clusters and nanocolloids

Abstract: This review is devoted to metal cluster synthesis in solution via radiolytic reduction of ionic precursors under the proper conditions. The size and structure of the final particles are described in relation to the nucleation and growth mechanism of the process with a special interest in oligomers and nanometric-sized particles. The influence of either a polymeric surfactant or a ligand or a support is described. The role of a chemical electron donor in the development of cluster size is explained. Particular attention is paid to the formation of bimetallic clusters and to the synthesis conditions required to obtain either a core-shell or an alloyed structure in relation to a possible intermetal electron transfer.

Decompression-induced melting of ice IV and the liquid–liquid transition in water

Abstract: Although liquid water has been the focus of intensive research for over 100 years, a coherent physical picture that unifies all of the known anomalies of this liquid1,2,3, is still lacking Some of these anomalies occur in the supercooled region, and have been rationalized on the grounds of a possible retracing of the liquid–gas spinodal (metastability limit) line into the supercooled liquid region4,5,6,7, or alternatively the presence of a line of first-order liquid–liquid phase transitions in this region which ends in a critical point8,9,10,11,12,13,14, But these ideas remain untested experimentally, in part because supercooled water can be probed only above the homogeneous nucleation temperature TH at which water spontaneously crystallizes Here we report an experimental approach that is not restricted by the barrier imposed by TH, involving measurement of the decompression-induced melting curves of several high-pressure phases of ice in small emulsified droplets We find that the melting curve for ice IV seems to undergo a discontinuity at precisely the location proposed for the line of liquid–liquid phase transitions8 This is consistent with, but does not prove, the coexistence of two different phases of (supercooled) liquid water From the experimental data we calculate a possible Gibbs potential surface and a corresponding equation of state for water, from the forms of which we estimate the coordinates of the liquid–liquid critical point to be at pressure Pc ≈ 01 GPa and temperature Tc ≈ 220 K

Thermodynamics of Calcite Growth: Baseline for Understanding Biomineral Formation

Abstract: The complexity of biomineralized structures suggests the potential of organic constituents for controlling energetic factors during crystal synthesis. Atomic force microscopy was used to investigate the thermodynamic controls on carbonate growth and to measure the dependence of step speed on step length and the dependence of critical step length on supersaturation in precisely controlled solutions. These data were used to test the classic Gibbs-Thomson relationship and provided the step edge free energies and free energy barriers to one-dimension nucleation for calcite. Addition of aspartic acid, a common component in biomineralizing systems, dramatically affected growth morphology and altered the magnitude of the surface energy.

Computer simulation study of gas–liquid nucleation in a Lennard-Jones system

Abstract: We report a computer-simulation study of homogeneous gas–liquid nucleation in a Lennard-Jones system. Using umbrella sampling, we compute the free energy of a cluster as a function of its size. A thermodynamic integration scheme is employed to determine the height of the nucleation barrier as a function of supersaturation. Our simulations illustrate that the mechanical and the thermodynamical surfaces of tension and surface tension differ significantly. In particular, we show that the mechanical definition of the surface tension cannot be used to compute this barrier height. We find that the relations recently proposed by McGraw and Laaksonen [J. Chem. Phys. 106, 5284 (1997)] for the height of the barrier and for the size of the critical nucleus are obeyed.

Parameterizations for sulfuric acid/water nucleation rates

Abstract: We present parameterized equations for calculation of sulfuric acid/water critical nucleus compositions and homogeneous nucleation rates. The parameterizations are in agreement with the thermodynamically consistent version of classical binary homogeneous nucleation theory [Wilemski, 1984] incorporating the hydration effect. The new parameterizations produce nucleation rates that differ by several orders of magnitude from the rates predicted by other parameterizations available in the literature. Model simulations of atmospheric aerosol formation show that the use of the new parameterizations may in some cases result in simulated nucleation mode particle number densities that are by a factor of 1000 lower than those obtained using the old parameterizations.

Atomic beam diffraction from solid surfaces

Abstract: Atomic beam techniques are presently being used in many branches of surface physics such as studies of the particle-surface physisorption potential, surface structure, surface phonons, nucleation and growth on metal and insulator surfaces, surface diffusion and accommodation and sticking of molecules. This review concentrates on diffractive phenomena from surfaces, which up to now were investigated mainly with helium. The theoretical background for diffraction calculations is outlined and representative examples of different applications are given. The main subjects covered are: structural determinations of chemisorbed and physisorbed systems, investigations of disordered surfaces, selective adsorption resonances, diffusion and nucleation studies and investigations of growth and phase transitions on surfaces. Diffraction results obtained with Ne, Ar, and are also summarized.

On the Interpretation of Crystal Size Distributions in Magmatic Systems

Abstract: holocrystalline under a wide spectrum of cooling regimes implies batch system. Instead, the CSDs of each system reflect a combination that cooling and crystallization can be uncoupled and considered of kinetic and dynamic influences on crystallization. Heterogeneous separately. This is tantamount to realizing that the Avrami number nucleation and annexation of small crystals by larger ones, enis large in most igneous systems. Crystallization automatically trainment of earlier grown and ripened crystals, rate of solidification adjusts through nucleation and growth to the cooling regime, and front advance, and protracted transit of a well-established mush all aspects of the ensuing crystal population reflect the relative roles column are some of the eVects revealed in the observed CSDs. There of nucleation and growth, which reflect the cooling regime. The may be an overall CSD evolution, reflecting the maturity of characteristic scales of crystal size, crystal number, and crys- the magmatic system, from simple straight nonkinked CSDs in tallization time are intimately tied to the characteristic rates of monogenetic systems to multiply kinked, piecewise continuous CSDs nucleation and growth, but it is the crystal size distributions (CSDs) in well-established systems such as Hawaii and Mount Etna. This that provide fundamental insight on the time variations of nucleation is not unlike the evolution of CSDs in some industrial systems. and growth and also on the dynamics of magmatic systems. Crystal Finally, the fact that comagmatic CSDs are not often captured size distributions for batch systems are calculated by employing evolving systematically through large changes in nucleation rates, the Johnson‐Mehl‐Avrami equation for crystallinity related to even in low crystallinity systems, may suggest that magma is always exponential variations in time of both nucleation and growth. The laced with high population densities of nuclei, supernuclei, and slope of the CSD is set by the diVerence a ‐ b, where a and b crystallites or clusters that together set the initial CSD at high are exponential constants describing, respectively, nucleation and characteristic population densities. Further evolution of the CSD growth. The batch CSD has constant slope and systematically occurs through sustained heterogeneous nucleation and rapid anmigrates to larger crystal size (L) with increasing crystallinity. The nealing at all crystallinities beginning at the liquidus itself and diminution in nucleation with loss of melt is reflected in the CSD operating under more or less steady (not exponentially increasing) at late times by a strong decrease in population density at small rates of nucleation. crystal sizes, which is rarely seen in igneous rocks themselves. Observed CSDs suggest that a ‐ b ~6‐10 and that b ~0. That is, growth rate is approximately constant and nucleation rate apparently increases exponentially with time. Correlations among CSD slope, intercept, and maximum crystal size for both batch and open systems suggest that certain diagnostic relations may be useful in interpreting the CSD of comagmatic sequences. These systematics are explored heuristically and through the detailed

Crystal grain nucleation in amorphous silicon

Abstract: The solid phase crystallization of chemical vapor deposited amorphous silicon films onto oxidized silicon wafers, induced either by thermal annealing or by ion beam irradiation at high substrate temperatures, has been extensively developed and it is reviewed here. We report and discuss a large variety of processing conditions. The structural and thermodynamical properties of the starting phase are emphasized. The morphological evolution of the amorphous towards the polycrystalline phase is investigated by transmission electron microscopy and it is interpreted in terms of a physical model containing few free parameters related to the thermodynamical properties of amorphous silicon and to the kinetical mechanisms of crystal grain growth. A direct extension of this model explains also the data concerning the ion-assisted crystal grain nucleation.

Controlling Factors for the Brittle-to-Ductile Transition in Tungsten Single Crystals

Abstract: Materials performance in structural applications is often restricted by a transition from ductile response to brittle fracture with decreasing temperature. This transition is currently viewed as being controlled either by dislocation mobility or by the nucleation of dislocations. Fracture experiments on tungsten single crystals reported here provide evidence for the importance of dislocation nucleation for the fracture toughness in the semibrittle regime. However, it is shown that the transition itself, in general, is controlled by dislocation mobility rather than by nucleation.

Electrochemical Preparation of Platinum Nanocrystallites with Size Selectivity on Basal Plane Oriented Graphite Surfaces

Abstract: Platinum nanocrystals were deposited on basal plane oriented graphite surfaces from dilute (1.0 mM) PtCl62--containing electrolytes using a pulsed potentiostatic method. The deposition of platinum nanocrystals occurred via an instantaneous nucleation and diffusion-limited growth mechanism which resulted in narrow particle size distributions (relative standard deviation <35%) for mean crystallite diameters smaller than 40 A. The number of particles per unit area on these surfaces was 109−1010 cm-2. Noncontact atomic force microscopy images reveal that platinum nanocrystals nucleated both at defect sitessuch as step edgesand on apparently defect-free regions of the atomically smooth graphite basal plane. Using electron transparent graphite surfaces, selected area electron diffraction analyses revealed that the structure of deposited platinum nanocrystals was fcc with a lattice constant that was indistinguishable from bulk fcc platinum. Platinum nanocrystals were not epitaxially oriented on the graphite basa...

Nucleation and growth of Si nanowires from silicon oxide

Abstract: Nucleation and growth of Si nanowires by laser ablation and thermal evaporation of Si powder sources mixed with ${\mathrm{SiO}}_{2}$ have been investigated by means of transmission electron microscopy. At the initial nucleation stage, Si oxide vapor condensed on the substrate and formed Si nanoparticles (the nuclei of nanowires). Each Si nanowire nucleus consisted of a polycrystalline Si core containing a high density of defects and a Si oxide shell. A growth mechanism was proposed based on the microstructure and different morphologies of the Si nanowires observed.

Texture control of PbTiO3 and Pb(Zr,Ti)O3 thin films with TiO2 seeding

Abstract: The nature and the role of 1 to 5 nm thick TiO2 seed layers for the growth of textured PbTiO3 and Pb(Zr, Ti)O-3 thin films on textured Pt(111) thin film substrates have been studied. Under otherwise identical in situ sputter deposition process conditions, the PbTiO3 texture could be turned from (100) to (111) orientation by adding the seed layer. This is demonstrated by patterning the TiO2 film. Auger electron spectroscopy and x-ray photoemission spectroscopy showed that the seed layer was a continuous TiO2 film. X-ray photoelectron diffraction measurements revealed epitaxial ordering in the seed layer. As there is no azimutal order among the Pt grains, the reduced information of azimutally averaged polar cuts is obtained. These give strong evidence for a strained rutile (110) structure. Various deposition experiments indicated that the TiO2 is effective only when it is ordered before the PbTiO3 nucleation starts. The epitaxial relationship between PbTiO3(111) and Pt(111) is thus mediated by the intermediate, epitaxial TiO2 film, which is dissolved of transformed to PbTiO3 afterwards. The observed growth behavior is discussed in terms of surface and interface energies. (C) 1998 American Institute of Physics. [S0021-8979(98)03607-X].

Molecular dynamics of homogeneous nucleation in the vapor phase. I. Lennard-Jones fluid

Abstract: Molecular dynamics computer simulation was carried out to investigate the dynamics of vapor phase homogeneous nucleation at the triple point temperature under supersaturation ratio 6.8 for a Lennard-Jones fluid. To control the system temperature, the 5000 target particles were mixed with 5000 soft-core carrier gas particles. The observed nucleation rate is seven orders of magnitude larger than prediction of a classical nucleation theory. The kinetically defined critical nucleus size, at which the growth and decay rates are balanced, is 30–40, as large as the thermodynamically defined value of 25.4 estimated with the classical theory. From the cluster size distribution in the steady state region, the free energy of cluster formation is estimated, which diminishes the difference between the theoretical prediction and the simulational result concerning the nucleation rate.

Thin liquid polymer films rupture via defects

Abstract: The standard rupture scenario of thin polymer films on solid substrates is investigated in detail. The geometry of the dewetting structure is analyzed by means of the Minkowski functionals in two dimensions. Furthermore, the temporal behavior and the impact of ambient conditions are considered. In contrast to what has been frequently asserted, we show that spinodal dewetting does very probably not play any significant role in the rupture process. On the contrary, it is demonstrated that rupture is dominated by nucleation from defects, which are intrinsic to the polymer due to its peculiar morphology. It is concluded that polymer films on solid surfaces are either stable (in the case of complete wetting) or unstable, but never metastable.

Dewetting of an Evaporating Thin Liquid Film: Heterogeneous Nucleation and Surface Instability

Abstract: Film rupture as the initial stage of dewetting is investigated for a volatile, spin-coated nonwetting film. During structure formation in the liquid film the film thickness is continuously reduced via evaporation. The dynamical character of the experiment allows the study of hole formation caused by distinct rupture mechanisms occurring at different film thicknesses. Both heterogeneous nucleation for thick films as well as spinodal dewetting for film thickness below 10 nm have been observed. The balance between both processes can be shifted by controlling the ambient humidity. The structures resulting from film rupture are quantified with respect to their different geometrical properties. For the first time we find that spinodal dewetting is caused by destabilizing polar interactions. [S0031-9007(98)05676-2] PACS numbers: 68.15. + e, 68.45. ‐ v, 68.55. ‐ a The wetting behavior of films is of central importance for thin film technology since it determines the homogeneity of thin films and coatings [1 ‐ 3]. Controlled dewetting can be employed to pattern thin films on the nanometer scale as was recently demonstrated by the production of biomolecular coatings with a defined structure for medical applications via spin coating of protein solutions [4]. Investigations of thin nonvolatile liquid films on nonwetting substrates have shown that dewetting takes place in three successive phases: rupture of the film, growth of the holes resulting in the formation of a polygonal network of straight liquid rims, and the decay of rims via a Rayleigh instability. The growth dynamics of single holes is well understood [5], whereas the understanding of film rupture as the initial stage of dewetting remains in

Control Over Aragonite Crystal Nucleation and Growth: An In Vitro Study of Biomineralization

Abstract: Control over calcium carbonate deposition in an in vitro system containing the major macromolecular components present in mollusk shell is shown to depend on the presence of a specific protein fraction in the appropriate microenvironment, in this case, silk fibroin adsorbed on β-chitin. Experiments with synthetic polypeptides elucidate aspects of the sequence dependency of nucleation of the aragonite polymorph, shown here growing in the nacreous layer of the bivalve Atrina serrata (note the orientation of the small, newly formed crystals and their merging together in more mature areas).

Homogeneous Nucleation Catastrophe as a Kinetic Stability Limit for Superheated Crystal

Abstract: Homogeneous nucleation kinetics for melting in superheated crystals is analyzed in order to derive a kinetic stability limit for the crystal lattice above its equilibrium melting point (T-m). It is found that at a critical temperature (T-m(K), which is about 1.2T(m) for various elemental metals) a massive homogeneous nucleation of melting occurs in the superheated crystal. Such a homogeneous nucleation catastrophe, which occurs before other proposed (rigidity, volume, and entropy) catastrophes can intervene, provides us with a kinetic stability limit for superheated crystals.