Athermal heterogeneous nucleation of freezing: numerical modelling for polygonal and polyhedral substrates

In a number of cases of practical interest, nucleation of freezing occurs at small supercooling, on potent nucleant substrates. Nucleation is then deterministic, consisting of the onset of free growth at a supercooling related only to substrate size and shape. Existing analytical treatments have evaluated the critical supercooling only for substrates in the form of a plane circle or a sphere. We develop a numerical treatment for a substrate of arbitrary shape. Coefficients are derived to permit straightforward evaluation of the critical supercooling for shapes typical of actual nucleant particles.     

Surface freezing on patterned substrates

We show that the structure of a substrate pattern drastically influences the nature of surface freezing. By using phenomenological theory and computer simulations of a hard sphere fluid next to a substrate formed by a periodic array of fixed spheres, we find that a pattern which is commensurate with the bulk crystal induces complete surface freezing through a cascade of layering transitions. A rhombic pattern, on the other hand, either generates a crystalline sheet which is unstable as a bulk phase or prohibits surface freezing completely.     

Freezing of fluids confined in a disordered nanoporous structure

Freezing of a simple fluid in a disordered nanoporous carbon is studied using molecular simulations. Only partial crystallization occurs, and the confined phase is composed of crystalline and amorphous nanodomains. This freezing behavior departs strongly from that for nanopores of simple geometry. We present a method for analyzing the freezing in such disordered materials in terms of a transition in the average size and number of crystalline clusters. The results provide a basis for the interpretation of experiments on freezing in such materials, particularly 1H-NMR and scattering experiments.     

Phase diagram of a polydisperse soft-spheres model for liquids and colloids

The phase diagram of soft spheres with size dispersion is studied by means of an optimized Monte Carlo algorithm which allows us to equilibrate below the kinetic glass transition for all size distributions. The system ubiquitously undergoes a first-order freezing transition. While for a small size dispersion the frozen phase has a crystalline structure, large density inhomogeneities appear in the highly disperse systems. Studying the interplay between the equilibrium phase diagram and the kinetic glass transition, we argue that the experimentally found terminal polydispersity of colloids is a purely kinetic phenomenon.     

Rigorous results in the mean-field theory of freezing

The mean-field theory of freezing, of which the Kirkwood-Monroe theory is a special case, is formulated in terms of a variational principle which can be derived from a statistical model. New upper and lower bounds on the free energy are found, and used to prove the existence of and to locate a freezing transition. The Kirkwood-Monroe theory is shown to obey a scaling law which implies that the freezing transition has no critical point and that the pressure-temperature phase diagram is a parabola. For a suitable choice of interaction potential, the free energy and density distribution of a crystalline state are calculated with very small error. The variational principle yields an integral equation found previously by van Kampen. This equation is derived rigorously, and shown to admit solutions of any chosen periodicity. The equation is transformed into a nonlinear integral equation of the Hammerstein type, and some standard uniqueness theorems applied. The equation is also transformed into a set of linear equations closely resembling the Kirkwood-Salsburg equations. Many unsolved problems are raised.     

Grain size and its distribution in NiTi thin films sputter-deposited on heated substrates

Grain size and its distribution in NiTi thin films sputter-deposited on a heated substrate have been investigated using the small angle x-ray scattering technique. The crystalline particles have a small size and are distributed over a small range of sizes for the films grown at substrate temperatures 370 and 420℃. The results show that the sizes of crystalline particles are about the same. From the x-ray diffraction profiles, the sizes of crystalline particles obtained were 2.40nm and 2.81nm at substrate temperatures of 350 and 420℃, respectively. The morphology of NiTi thin films deposited at different substrate temperatures has been studied by atomic force microscopy. The root mean square roughness calculated for the film deposited at ambient temperature and 420℃ are 1.42 and 2.75nm, respectively.     

The kinetic MC modelling of reversible pattern formation in initial stages of thin metallic film growth on crystalline substrates

The results of kinetic MC simulations of the reversible pattern formation during the adsorption of mobile metal atoms on crystalline substrates are discussed. Pattern formation, simulated for submonolayer metal coverage, is characterized in terms of the joint correlation functions for a spatial distribution of adsorbed atoms. A wide range of situations, from the almost irreversible to strongly reversible regimes, is simulated. We demonstrate that the patterns obtained are defined by a key dimensionless parameter: the ratio of the mutual attraction energy between atoms to the substrate temperature. Our ab initio calculations for the nearest Ag-Ag adsorbate atom interaction on an MgO substrate give an attraction energy as large as 1.6 eV, close to that in a free molecule. This is in contrast to the small Ag adhesion and migration energies (0.23 and 0.05 eV, respectively) on a defect-free MgO substrate.     

Growth mechanism of scales of SiC nanowires with/without ZnS powders at varying temperatures

The growth mechanism of scales of crystalline SiC nanowires (SiC-NWs) obtained by directly evaporating solid carbon on silicon wafer with/without ZnS powders at varying temperatures is being discussed. More aligned SiC-NWs of small size and good crystalline structure were formed when ZnS was used. Random SiC-NWs of big size and poor crystalline structure were obtained at conditions free of ZnS. Furthermore, the improved crystalline structure and increased diameter of SiC-NWs were observed when the higher temperature was employed.     

Homogeneous bulk, surface, and edge nucleation in crystalline nanodroplets

The birth of a crystal is initiated by a nucleus from which the crystal grows--a dust grain in a snowflake is a familiar example. These nuclei can be heterogeneous defects, like the dust grain, or homogeneous nuclei which are intrinsic to the material. Here we study homogeneous nucleation in nanoscale polymer droplets on a substrate which itself can be crystalline or amorphous. We observe a large difference in the nucleating ability of the substrate. Furthermore, the scaling dependence of nucleation on the size of the droplets proves that the birth of the crystalline state can be directed to originate predominantly within the bulk, at the substrate surface, or at the droplets' edge, depending on how we tune the substrate.     

Effect of long-range forces on surface freezing

We examine the effect of long-range van der Waals interactions on surface freezing (SF) in linear hydrocarbon chain molecules, and the wetting criteria of the bulk liquid by the crystalline surface phase. We find that although the effect of van der Waals interactions is small for SF of normal alkanes, it is important for SF of dry and hydrated alcohols. We also find that the long-range interactions should not be ignored in the interpretation of wetting phenomena in recent experimental results. The results are in good agreement with recent experiments.     

Deformation of slow liquid and solid clusters upon deposition: A molecular-dynamics study of Al cluster impact on an Al surface

Using molecular-dynamics simulation, we investigate the self-deposition of Al_n clusters (n < 4000) on an Al substrate at velocities below the velocity of sound. Both cold crystalline and hot liquid clusters are studied. We examine the cluster deformation after impact on the surface, which we quantify by its height and base radius. At a given cluster velocity, the shape of deposited crystalline clusters is rather independent of the cluster size; only at small cluster sizes, n ? 40, the clusters are less strongly deformed. With increasing cluster size, liquid clusters are more strongly deformed than crystalline clusters. Faster projectiles become more strongly flattened by the deposition process. Even clusters depositing with vanishing velocity show a finite deformation, which is considerable for smaller clusters. At large cluster speed, clusters penetrate deeper into the (1 0 0) surface than into the (1 1 1) surface and also deform more strongly.     

Bistability in the shape transition of strained islands

The equilibrium shape of a monatomic strained island on a substrate depends on the step free energies and the difference in surface stress between the island and the substrate. For small island sizes the step free energies dominate, resulting in compact islands. Beyond a critical island size, however, the strain energy becomes dominant and the island maximizes its perimeter, resulting in elongated islands. Here we show that for strained islands with force monopoles pointing in opposing directions at neighboring steps, a regime exists near the critical island size where both compact and elongated shapes can coexist.     

多孔硅的电荷存贮特性与光电压滞后衰减

自从Canham[1]在1990年报道了多孔硅的光致发光以来,人们便开始了多孔硅的发光特性及其发光机理的研究[2,3].我们在获得多孔硅材料的基础上,曾首次报道了多孔硅光电压的滞后衰减现象[4].在本文中主要对如上实验结果进行了分析和探讨.     

Absence of an abrupt phase change from polycrystalline to amorphous in silicon with deposition temperature.

Using fluctuation electron microscopy, we have observed an increase in the mesoscopic spatial fluctuations in the diffracted intensity from vapor-deposited silicon thin films as a function of substrate temperature from the amorphous to polycrystalline regimes. We interpret this increase as an increase in paracrystalline medium-range order in the sample. A paracrystal consists of topologically crystalline grains in a disordered matrix; in this model the increase in ordering is caused by an increase in the grain size or density. Our observations are counter to the previous belief that the amorphous to polycrystalline transition is a discontinuous disorder-order phase transition.     

Surface nucleation in the freezing of gold nanoparticles

We use molecular simulation to calculate the nucleation free energy barrier for the freezing of a 456 atom gold cluster over a range of temperatures. The results show that the embryo of the solid cluster grows at the vapor-surface interface for all temperatures studied and that the usual classical nucleation model, with the embryo growing in the core of the cluster, is unable to predict the shape of the free energy barrier. We use a simple partial wetting model that treats the crystal as a lens-shaped nucleus at the liquid-vapor interface and find that the line tension plays an important role in the freezing of gold nanoparticles.     

Adsorption on amorphous semiconductors: A modified Haldane-Anderson model

The Haldane-Anderson model previously used for describing the adsorption on a crystalline substrate is generalized to the case of an amorphous substrate. It is demonstrated that the main difference between the occupation numbers of the adatom is observed in the case where the atomic level overlaps with the band gap (for a crystalline semiconductor) and the mobility gap (for an amorphous semiconductor). The adatom charge on the amorphous substrate is smaller than that on the crystalline substrate.     

Morphology of amorphous polymers

It has been suggested that amorphous polymers consist of small domains (30 to 100 A) in which there is local ordering or alignment of neighboring segments. Although free volume remains the primary physical parameter useful in characterizing the properties of amorphous polymers (as percent crystallinity for crystalline polymers), the distribution of the free volume, as determined by the size, interconnection, internal order, etc. of the domains is proposed to also be of importance. The paper reviews electron microscope and electron diffraction evidence for the presence of the domain structure, while also pointing out significant remaining problems.     

Mobility transition of solid rare gases in confined environments

We report the results of x-ray diffraction and small angle scattering studies of Ar and Kr confined in sol-gel and Vycor glasses. The confined liquid crystallizes in a disordered hcp structure on freezing. Upon further cooling a sharp transition occurs at a reduced temperature of T/T(m) approximately 0.65, where the crystalline structure disappears and the total scattering decreases. This behavior marks the onset of a well-defined mobility transition, where the confined sample migrates out of the pore space.     

Amorphous-like Raman spectra of semiconductor microcrystals

The size dependence of Raman scattering from gas-evaporated Si, Ge and GaP microcrystals, for which a free boundary condition can be assumed, has been investigated. As the microcrystals become sufficiently small (smaller than about 100A), spectra very similar to those of amorphous materials are observed, even though electron microscopy proves that they are crystalline. The amorphous-like Raman signals are believed to come from the surface layers of microcrystals.     

Phase transitions in simple models of rod-like and disc-like micelles

The interplay of interactions between micelles, and the aggregation of these micelles into large, highly anisotropic micelles, is studied. Simple, hard-body, models of rod-like and disc-like micelles are used, which allows us to apply fundamental measure theory to determine the free energy. Then we study the phase transition from the fluid phase to a liquid crystalline phase. We find that aggregation induces a strongly first order transition from a fluid phase of small micelles to a close packed liquid crystalline phase of infinitely large micelles. Received: 3 December 1997