Transient time in homogeneous nucleation

The problem of the transient time in nucleation is studied. The size-dependent transient time at a constant temperature is defined and the approximate analytical solution is found and compared with the exact numerical solution for the model Li₂O.2 SiO₂ melt. It is shown that the analytical solution for the size-dependent transient time is in agreement with the numerical result.     

Theory of anomalous critical-cluster content in high-pressure binary nucleation

Nucleation experiments in binary (a-b) mixtures, when component a is supersaturated and b (carrier gas) is undersaturated, reveal that for some mixtures at high pressures the a content of the critical cluster dramatically decreases with pressure contrary to expectations based on classical nucleation theory. We show that this phenomenon is a manifestation of the dominant role of the unlike interactions at high pressures resulting in the negative partial molar volume of component a in the vapor phase beyond the compensation pressure. The analysis is based on the pressure nucleation theorem for multicomponent systems which is invariant to a nucleation model.     

Critical conditions of entropy-driving nucleation in binary colloidal system

In a binary colloidal hard-sphere system, due to the entropic depletion interactions, large spheres may be pushed to pack into clusters. If a cluster is taken as a new kind of component, then the binary colloidal system turns to be a ternary one and it therefore can be dealt in the framework of a multi-component system. It is known that, under certain conditions, a cluster can grow larger and larger and turn into a nucleation, even a kind of phase transition will take place. In this paper, the critical conditions, including the critical size of the nucleating cluster and the corresponding volume fraction of the colloidal system, were determined by analyzing entropy variation through the multi-component of Carnahan–Starling (CS) state equation and the principle of entropy maximum. The results obtained in this way are in good agreement with that of the experiments. In addition, the results also show that, when the critical volume fraction of the system increases from 0.18 to 0.19, the cluster’s density ?$?$ will change from 0.74 to 0.58. Except for the size ratio of large to small spheres, the critical size of the nucleating cluster is almost independent of its density and the volume fraction of the system.     

Effect of diffusion length on the nucleation of chemical vapour deposition diamond

Atomic steps have been suggested as preferential sites for nucleation. However, evidence from recent experiments on diamond growth using faceted sapphire as well as reconstructed silicon substrates shows that atomic steps alone do not always enhance nucleation in the chemical vapour deposition environment. The comparison of the diffusion length of the nucleation precursors and the width of the terraces between the surface steps provides further insights into this nucleation mechanism.     

Homogeneous nucleation and growth in iron-platinum vapour investigated by molecular dynamics simulation

Homogeneous nucleation and growth from binary metal vapour is investigated by molecular dynamics simulation. It is focused here mainly on the iron-platinum system with a mole fraction of 0.5. The simulations are started in the highly supersaturated vapour phase. Argon is added as carrier gas removing the heat of condensation from the forming clusters. The embedded atom method is employed for modelling of the force field of iron and platinum. The simulation runs are evaluated with respect to the nucleation rate, monomer temperature, monomer amount, and with respect to the size of the largest cluster in the system including possible pure metal clusters. It turns out that depending on the composition of the complete system pure platinum clusters with sizes up to 10 to 15 atoms are formed in addition to binary clusters. Due to the high temperature of these clusters iron atoms less likely condense at the beginning of the particle formation simulation. This leads to temporary difference in the temperatures of the platinum and the iron subsystems, which eventually approach each other when only binary clusters are present. In summary, the results obtained from the cluster statistics show that pure platinum nucleation and growth can take place to some extent within the binary system.     

Transient stimulated Raman scattering in binary mixed liquids

Stimulated Raman scattering (SRS) spectra in several mixed liquids with a large optical Kerr constant are studied with nanosecond and picosecond light pulses. These spectra show a dependence of the SRS intensity on the relaxation times T₂ of Raman active modes.     

Primary nucleation processes in binary oxide growth: The case of MgO

The smallest forms of stoichiometric and non-stoichiometric MgO clusters appearing on the MgO(0 0 1) surface during the growth under atomic and/or molecular deposition are investigated from first-principles and empiric potentials. The basic entities (MgO molecule and (MgO)₂ cluster) result from a very exoenergetic and spontaneous redox reaction that involves directly the deposited species (Mg and O atoms, O₂ molecule). The stoichiometric clusters, resulting from the agglomeration of MgO molecules, are very stable under non-polar forms. Their formation energy is modelized, down to very small sizes, within an independent defect model. We point out the specificity of such clusters in the framework of the classical nucleation theory. The high-energy polar isomers are associated to destabilizing macroscopic electric fields and dipoles. These forms may nevertheless be strongly stabilized by incorporating extra Mg adatoms that give part of their valence shell to the cluster and decrease the total dipole in this way, illustrating the delicate coupling between chemistry and electrostatics in growth processes of oxides. Based on these considerations, we propose a scenario describing MgO growth both in the step-flow and in the nucleation regime.     

Anomalous nucleation far from equilibrium

We present precision Monte Carlo data and analytic arguments for an asymmetric exclusion process, involving two species of particles driven in opposite directions on a 2xL lattice. To resolve a stark discrepancy between earlier simulation data and an analytic conjecture, we argue that the presence of a single macroscopic cluster is an intermediate stage of a complex nucleation process: in smaller systems, this cluster is destabilized while larger systems form multiple clusters. Both limits lead to exponential cluster size distributions, controlled by very different length scales.     

Stimulated hyper-Raman emission from sodium vapour

The first observation of tunable stimulated hyper-Raman scattering is reported. A dye laser (ω_p) focussed into sodium vapour produced stimulated hyper-Raman emission (2ω_p?ω_3s-4p) at ～ 2.3 μm when 2ω_p was tuned near the 3s-4d two- photon resonance. Photon conversion efficiencies up to 2% were obtained. The hyper-Raman emission was tuned over a ～ 160 cm^?1 range, and could be observed with pump powers as low as ～ 2kW.     

Nucleation on substrates from the vapour phase

Recent developments in the theory of nucleation of vapour deposits on crystalline substrates are reviewed. To facilitate comparison, the theories are formulated in a common dimensionless notation, and an examination of the major underlying assumptions reveals the basic similarity of many of them. The capture and dissociation rates are expressed in terms of the cluster geometry and the pertinent energy parameters, and from these the ‘chemical’ rate equations are set up. The following types of approximate solutions are discussed: (a) long-time asymptotic solutions, from which the conditions for saturation in the cluster concentration may be deduced, (b) a generalization of the type of approximate solution used by Logan (1969), and (c) numerical solutions employing a minimum number of simplifying assumptions. Based on a simple model, agreement between the latter two seems reasonably good. For any given set of fixed parameters (energies, geometrical constants, and arrival rate from the vapour) several temperature ranges may be distinguished. The main division is between ‘initially complete condensation’ at low temperatures and ‘initially incomplete condensation’ at high temperatures. Within each of the latter cases there are further transitions (a) between different values of the ‘critical size’ i*, and (b) from negligible growth to rapid growth of the supercritical clusters. The influence of all of these factors on the final cluster concentration is described.

The distributions of the clusters in size and spacing are discussed briefly and qualitatively, as are the types of effects that can be induced by defects or other ‘special sites’ on the substrate.

Comparisons are made with some recent experimental studies. In many of these, defects in the substrate seem to play a dominant role, and no detailed comparison with theory seems possible. One notable exception is the nucleation of rare gas crystals on graphite substrates (Venables and Ball 1970), and here, for at least two of the three gases studied, excellent quantitative agreement is obtained.     

Determination of size and concentration of critical clusters of silicon carbide in the vapour phase in homogeneous nucleation process

The concentrations of clusters of various size in the atmosphere during silicon carbide crystal growth have been calculated on the basis of fundamental ideas of homogeneous nucleation theory, taking into account the specific parameters of silicon carbide. It has been shown that the cluster concentration are sufficiently high to conclude that this is the dominant influence during the initial stages of crystal growth. In this way the assumption of the polymer theory of polytypism, namely that the polytype properties of silicon carbide can be determined from the composition of the gas phase, containing sufficiently large clusters with various polytype structures, has been confirmed.     

First nucleation steps during deposition of SiO2 thin films by plasma enhanced chemical vapour deposition

The initial nucleation stages during deposition of SiO₂ by remote plasma enhanced chemical vapour deposition (PECVD) have been monitored by XPS inelastic peak shape analysis. Experiments have been carried out on two substrates, a flat ZrO₂ thin film and a silicon wafer with a native silicon oxide layer on its surface. For the two substrates it is found that PECVD SiO₂ grows in the form of islands. When the SiO₂ particles reach heights close to 10 nm they coalesce and cover completely the substrate surface. The particle formation mechanism has been confirmed by TEM observation of the particles grown on silicon substrates. The kinetic Monte Carlo simulation of the nucleation and growth of the SiO₂ particles has shown that formation of islands is favoured under PECVD conditions because the plasma species may reach the substrate surface according to off-perpendicular directions. The average energy of these species is the main parameter used to describe their angular distribution function, while the reactivity of the surface is another key parameter used in the simulations.     

Two-dimensional nucleation of ice from supercooled water

We report the temperature dependent nucleation rates of ice from single water drops supporting aliphatic alcohol Langmuir films. Analysis in the context of a classical theory of heterogeneous nucleation suggests that the critical nucleus is essentially a monolayer, and that the rate-limiting steps in these nucleation processes are therefore not merely influenced by, but instead dictated by, the physics of the water-alcohol interface. Consequently, reduced dimensionality may be much more important in heterogeneous nucleation than has previously been believed.     

Structure of Be films quenched from the vapour

The crystallite boundary mismatch in crystalline films leads to tensile stresses which act on the substrate. Due to these stresses the substrate is bended. In the dense random packing of atoms in an amorphous film local tensile- and compressive stresses compensate and thus the films seem to be free of stresses with regard to the substrate. From this point of view the measurement of mechanical stresses produced by vapour quenched films can be used for structural analysis. Thus an apparatus is described which allows in situ measurements of mechanical stresses and electrical resistivity of vapour quenched films from 1.2 K to 400 K.An investigation of beryllium films revealed that stresses are produced even by films with a superconducting transition temperature of 9.6 K. From this fact it can be concluded that vapour quenched Be is not completely amorphous. Furthermore, it is shown that inhomogeneous films exist with a two phase double layer structure which is responsible for some peculiarities reported in the literature.     

Level-crossing experiments in selective reflection from mercury vapour

Doppler-broadened level-crossing curves have been observed in selective reflection, showing an additional crossing effect between Zeeman-sublevels of different isotopes. Pressure-broadening by foreign gases has also been studied.     

Influence of dispersive long-range interactions on properties of vapour–liquid equilibria and interfaces of binary Lennard-Jones mixtures

The influence of dispersive long-range interactions on properties of vapour–liquid equilibria and interfaces of six binary Lennard-Jones (LJ) mixtures was studied by molecular dynamics (MD) simulations and density gradient theory (DGT). The mixtures were investigated at a constant temperature T, at which the low-boiling component, which is the same in all mixtures, is subcritical. Two different high-boiling components were considered: one is subcritical, the other is supercritical at T. Furthermore, the unlike dispersive interaction was varied such that mixtures with three different types of phase behaviour were obtained: ideal, low-boiling azeotrope, and high-boiling azeotrope. In a first series of simulations, the full LJ potential was used to describe these mixtures. To assess the influence of the long-range interactions, these results were compared with simulations carried out with the LJ truncated and shifted (LJTS) potential applying the corresponding states principle. The dispersive long-range interactions have a significant influence on the surface tension and the interfacial thickness of the studied mixtures, whereas the relative adsorption and the enrichment are hardly affected. Furthermore, the influence of the long-range interactions on Henry's law constants and the phase envelopes of the vapour–liquid equilibrium was investigated. The long-range interactions have practically no influence on the composition dependency of the investigated mixture properties.     

Laser-assisted deposition of thin films from photoexcited vapour phases

Laser-assisted chemical vapour deposition (LCVD) has been extensively studied in the last two decades. A vast range of applications encompass various areas such as microelectronics, micromechanics, microelectromechanics and integrated optics, and a variety of metals, semiconductors and insulators have been grown by LCVD. In this article, we review briefly the LCVD process and present two case studies of thin film deposition related to laser thermal excitation (e.g., boron carbide) and non-thermal excitation (e.g., CrO₂) of the gas phase. PACS 81.15.Fg; 81.15.Kk     

Atomistic mechanism of NaCl nucleation from an aqueous solution

Despite great technological relevance, the initial steps of nucleation and crystal growth from solution are still poorly understood. While experimentally difficult to access, simulations in principle may provide insight at the atomic level. However, in most cases the computational demand dramatically exceeds the scope of current hardware. Since crystallization usually occurs on time scales much larger than the few ns of a molecular dynamics simulation, special techniques for the study of rare events are of particular interest. In the present work the nucleation of sodium chloride aggregates from aqueous solution is investigated from path sampling molecular dynamics simulation. The introduced simulation schemes appear to be widely applicable.