Spirals on Si(1 1 1) at sublimation and growth: REM and LODREM observations

Using recently proposed improvements of reflection electron microsopy (REM) we study, in perfectly controlled thermodynamics conditions, spiral and spirals shapes on Si(1 1 1) surface. It is shown that the new method named low distortion reflection electron microscopy (LODREM) is a powerful instrument, resolving in much more details (compared with REM) growth or evaporation spirals at the crystal surface. More precisely, we examine the distance between two successive steps of a spiral at growth (or evaporation) with respect to the supersaturation (or undersaturation). It is found that this distance scales with an exponent close to −1/2. This result, which deviates from the BCF theory, originates from a non-local behavior with a slow kinetic of attachment of the adatoms at the steps.     

Spiral growth without dislocations: molecular beam epitaxy of the topological insulator Bi2Se3 on epitaxial graphene/SiC(0001)

We report a new mechanism that does not require the formation of interfacial dislocations to mediate spiral growth during molecular beam epitaxy of Bi2Se3. Based on in situ scanning tunneling microscopy observations, we find that Bi2Se3 growth on epitaxial graphene/SiC(0001) initiates with two-dimensional (2D) nucleation, and that the spiral growth ensues with the pinning of the 2D growth fronts at jagged steps of the substrate or at domain boundaries created during the coalescence of the 2D islands. Winding of the as-created growth fronts around these pinning centers leads to spirals. The mechanism can be broadly applied to the growth of other van der Waals materials on weakly interacting substrates. We further confirm, using scanning tunneling spectroscopy, that the one-dimensional helical mode of a line defect is not supported in strong topological insulators such as Bi2Se3.     

New developments on size-dependent growth applied to the crystallization of sucrose

The effect of crystal size on the growth rate of sucrose (C₁₂H₂₂O₁₁) at 40 °C is investigated from a theoretical and an experimental point of view. Based on new perspectives resulting from the recently introduced spiral nucleation model [P.M. Martins, F. Rocha, Surf. Sci. 601 (2007) 3400], crystal growth rates are expressed in terms of mass deposition per time and crystal volume units. This alternative definition is demonstrated to be size-independent over the considered supersaturation range. The conventional overall growth rate expressed per surface area units is found to be linearly dependent on crystal size. The advantages of the “volumetric” growth rate concept are discussed. Sucrose dissolution rates were measured under reciprocal conditions of the growth experiments in order to investigate the two-way effect of crystal size on mass transfer rates and on the integration kinetics. Both effects are adequately described by combining a well-established diffusion-integration model and the spiral nucleation mechanism.     

Sublimation of atomic layers from a chromium surface

We employ low-energy electron microscopy to study the kinetics of thermal etching, or sublimation, of Cr(001) at approximately 1100 K. Atomic layers are removed from the surface by spontaneous nucleation and growth of two-dimensional vacancy islands, by rotation of spiral steps, and by island decay. The growth rates of vacancy islands and the rotation frequencies of double spirals are measured as a function of temperature, and the results are correlated with activation barriers of surface processes. Mass transport between the surface and bulk is shown to be unimportant.     

Adsorption and nucleation on smooth surfaces

The nucleation and growth of condensate nuclei on smooth surfaces, e.g., an immiscible liquid or a smooth solid, can occur both by the direct addition of molecules from the vapor and from those adsorbed on the substrate. We show how to generalize nucleation theory to allow for the simultaneous occurrence of both mechanisms. The vapor-condensate-substrate interfacial forces, the contact angle, the critical supersaturation, and the coefficient in the adsorption isotherm are different ways of expressing the affinity between vapor molecules and the substrate surface. The critical supersaturations for nucleation on the surface of an immiscible liquid and nucleation on the surface of a perfectly smooth solid are predicted in terms of these parameters and the relationships among them. For most values of these parameters we find that adsorbed molecules are usually far more important to the nucleation process than those in the vapor phase.     

Melting, freezing and nucleation in nanoclusters of potassium chloride

Molecular dynamics simulations of the melting, freezing and nucleation are presented for unconstrained nanoclusters of KCl with a number of ions between 512 and 10648. The maximum extent of the probed liquid supercooling is analysed to the light of theoretical predictions and compared with experimental data. The fraction of the solid-like ions in the supercooled liquid is used as an indicator of heterogeneities within the liquid. Induced nucleation by seeding the supercooled liquid indicates that solid-liquid coexistence is stable, and sustained during the lifetime of the clusters, relatively to the supercooled liquid. A phenomenological analysis on the relaxation times of the crystal growth process is made. Critical nuclei sizes computed from the effectiveness of the seeds in the heterogeneous nucleation of the supercooled liquid, and from the residual crystallites in clusters not totally melted, are presented as a function of the temperature. The behavior of the systems is followed through various properties such as liquid and solid molar fractions, enthalpies of melting, heat capacities, self-diffusion coefficients and relaxation times related to the freezing process. The consistency of the simulation results for the heterogeneous nucleation is assessed by means of a classical nucleation model, from which an estimate of the interfacial surface tension is also worked out and compared with experimental data.     

Imaging of surface morphologies of l-arginine trifluoroacetate crystals

The growth mechanism and defect-mediated surface morphologies are investigated by atomic force microscopy (AFM). Both screw-dislocation-controlled growth and 2D nucleation growth operate simultaneously during growth. Hollow core locating at the top of spiral hillock validates that the dislocation has a large Burgers vector. 2D nuclei introduce growth islands and steps with wavy fronts are observed. Impurity-induced step bunching is presumably formed as a result of the imbalance of step speed. The formation of the hollow channels is due to instability of the interface and these may ultimately cause other defects such as liquid inclusions come into being.     

Modeling of the surface nucleation and growth on active centers from the vapor phase

The formation of nuclei of a new phase and their growth on the inhomogeneous substrate from a vapor phase are studied. Basic kinetic equations describing such a process are solved numerically. The effect of the depletion of active sites during the growth of nuclei is taken into account. Basic characteristics of the nucleation process, such as size distribution function, nucleation rate and number of nuclei formed on the unit surface are determined. It is shown that the size distribution of nuclei evolves by a nontrivial way as a function of time. This process is fully nonstationary from the viewpoint of nucleation rate. The total number of nuclei reaches the number of active centers for a sufficiently long time. Presented at the VIII-th Symposium on Surface Physics, Třešt’ Castle, Czech Republic, June 28 – July 2, 1999. This work was supported by Grant No. 202/99/0403 of the Grant Agency of the Czech Republic.     

Spontaneous onion-structure formation from planar lamellar nuclei

Nucleation and growth are a basic elementary process of ordering. The nucleation process is controlled by a competition between interfacial and bulk energy. Thus an ordered structure of a nucleus at its birth is not necessarily the most stable thermodynamically: Ostwald step rule. In addition to this, we found the topological transformation of nuclei from the most stable bulk structure (planar lamella) to a metastable one (onion) in a lyotropic liquid crystal. This indicates that the fate of nuclei of low-dimensional internal order can also be seriously affected by an additional competition between interfacial and elastic deformation energy.     

Characterization of local structures with bond-order parameters and graphs of the nearest neighbors,a comparison

We compare two methods for the characterization of local order in samples undergoing crystal nucleation and growth. Particles with a crystal-like surrounding need to be identified to follow the nucleation process. Both methods are based on the knowledge of the particle positions in a small volume of the sample. (i) Local bond-order parameters are used to quantify the orientation of the nearest neighbors of a particle, while (ii) the graph method determines the topological arrangement of the nearest neighbors and the bonds between them. Both methods are used to detect crystal-like particles and crystal nuclei in a supercooled fluid surrounding and to determine the structure of small crystal nuclei. The properties of these nuclei are of great interest for a deeper understanding of crystal nucleation, and they can be studied in detail in colloidal model systems that allow to follow the evolution of the nuclei with single particle resolution.     

Imitative modeling of the evolution of surface structures during crystal growth from a molecular beam

An imitation model of layered crystal growth from a molecular beam is presented which takes account of the random character of the formation of nuclei and the dependence of their growth rate on their mutual positions. The dependence of the form of the oscillations in the intensity of specular reflection of a fast electron beam on the nucleation rate has been studied within the framework of the model presented. The possible causes of oscillation damping have been analyzed in connection with the time evolution of the surface structure. V. D. Kuznetsov Siberian Physicotechnical Institute at Tomsk State University. Translated from Izvestiya Vysshikh Uchebnykh Zavedenii, Fizika, No. 8, pp. 103–109, August, 1997.     

Growth spiral activity and step velocities on a crystal surface

The normal growth rates and slopes of the dominant growth spirals on a crystal face were measured using a novel application of interferometry and atomic force microscopy. The dislocations responsible for the spirals were also observed directly. In this way it was shown that the velocities of similar steps on growth spirals of different activity, growing simultaneously on the same crystal face, were the same, and hence independent of the growth spirals to which they belonged. With this information the relative activities of the spirals were quantified and discussed in terms of hollow dislocation cores and the back-stress effect.     

Etch figures on silicon iron

Steps, which in some points of the (110)planes become spiral pits, were obtained on the surface of silicon iron by electrolytic etching. The height of the steps, which was from tens to 1000 Å,was measured by multiple beam interference. An explanation of the step-like etching is proposed and some of the mechanisms, which can lead to the production of etch spirals with the above height of the steps, are given.Delivered at 1st Congress of Czechoslovak Physicists in Prague on September 23rd, 1957.     

Preparation of carbon fiber/ZnO core-shell structure by a simple method

Carbon fiber/ZnO was prepared by surface modification precipitation in aqueous solution. Corresponding nucleation and crystal growth model was proposed for this structure. The effects of annealing temperature on the structure and absorptive properties of the composites were investigated. Results showed that obtained ZnO shell was ca. 200nm on the surface of carbon fiber. The ZnO coating can protect the CFs from oxidation at a relatively high temperature. Energy bandgap calculated from the absorptive spectra was about 3.30eV.     

Breakdown of classical nucleation theory near isostructural phase transitions

We report simulations of crystal nucleation in binary mixtures of hard spherical colloids with a size ratio of 1:10. The stable crystal phase of this system can be either dense or expanded. We find that, in the vicinity of the solid-solid critical point where the crystallites are highly compressible, small crystal nuclei are less dense than large nuclei. This phenomenon cannot be accounted for by either classical nucleation theory or by the Gibbsian droplet model. We argue that the observed behavior is due to the surface stress of the crystal nuclei. The observed effect highlights a general deficiency of the most frequently used thermodynamic theories for crystal nucleation. Surface stress should lead to an experimentally observable expansion of crystal nuclei of colloids with short-ranged attraction and of globular proteins.     

The effects of temperature upon NiO formation and oxygen removal on Ni(110)

Oxygen chemisorption and NiO nucleation and growth on Ni(110) have been studied with low energy electron diffraction and Auger electron spectroscopy. Changes in the Auger peak energies and shapes were shown to occur only upon NiO formation. The effects of step-changes in temperature upon NiO nucleation and growth were studied and it was shown that temperature steps or annealing during the chemisorption regime did not significantly affect either chemisorption or NiO formation. During NiO growth, temperature steps to a higher temperature caused reduced growth rates, while steps to lower temperature caused increased growth rates. The reaction rate constant from the island growth model was calculated and shown to agree with literature data. The values obtained from temperature step measurements agreed within a factor of two with those obtained for reactions without temperature steps. Therefore, no systematic temperature effect upon the NiO nuclei density was observed for Ni(110). The activation energy for growth of NiO was found to be 5.5 kcal/mole. Dissolution of oxygen into bulk nickel was also studied and it was shown that bulk diffusion of oxygen in nickel was not rate controlling. Rather, surface phase transitions were observed which allowed incorporation of oxygen over the temperature range of 150°C to greater than 800°C, depending on the quantity of oxygen already incorporated.     

On the theory of transient nucleation at the intermediate stage of phase transitions

The evolution of a system of growing aggregates in a macroscopically homogeneous medium with account of both the reduction in metastability and the continuing initiation of new nuclei is studied. The corresponding integro-differential model describing the intermediate stage of phase transitions is solved analytically for arbitrary nucleation kinetics and growth rates of nuclei. An exact solution of the Fokker–Planck equation is found with allowance for the diffusivity along the axis of nucleus radii. In limiting cases of purely kinetic and mixed kinetic-diffusion rates of crystal growth for a special form of diffusivity, the obtained solutions transform to earlier known expressions.     

Modelling and simulation of dynamic recrystallisation based on multi-phase-field and dislocation-based crystal plasticity models

ABSTRACT

The mechanical properties of metals used as structural materials are significantly affected by hot (or warm) plastic working. Therefore, it is industrially important to predict the microscopic behaviour of materials in the deformation process during heat treatment. In this process, a number of nuclei are generated in the vicinity of grain boundaries owing to thermal fluctuation or the coalescence of subgrains, and dynamic recrystallisation (DRX) occurs along with the deformation. In this paper, we develop a DRX model by coupling a dislocation-based crystal plasticity model and a multi-phase-field (MPF) model through the dislocation density. Then, the temperature dependence of the hardening tendency in the recrystallisation process is introduced into the DRX model. A multiphysics simulation for pure Ni is conducted, and then the validity of the DRX model is investigated by comparing the numerical results of microstructure formation and the nominal stress–strain curve during DRX with experimental results. The obtained results indicate that in the process of DRX, nucleation and grain growth occur mainly around grain boundaries with high dislocation density. As deformation progresses, new dislocations pile up and subsequent nucleation occurs in the recrystallised grains. The influence of such microstructural evolution appears as oscillation in the stress–strain curve. From the stress–strain curves, the temperature dependence in DRX is observed mainly in terms of the yield stress, the hardening ratio, and the change in the hardening tendency after nucleation occurs.     

Stable nonlinear excitations of the spiral type in 2D magnetic models

A new class of stable topological excitations having the shape of small-amplitude spirals is considered in a 2D isotropic Heisenberg ferromagnet model. An exact solution generalizing some previous results is obtained. The main characteristics of small-amplitude spirals are determined using the conservation laws. The possibilities of experimental realization of such spiral structures are considered.     

BCC枝晶生长原子堆垛过程的晶体相场研究

通过在自由能泛函中引入各向异性参数得到了一个基于高斯内核的改进晶体相场模型, 并采用该模型研究了体心立方结构(BCC)枝晶生长的原子堆垛过程. 结果表明, 在BCC由正十二面体平衡形貌演化为枝晶组织过程中, 形核位置经历了由面心({110}面)到尖端(<100>取向)的转移, 进而发生界面失稳形成枝晶组织; 枝晶生长过程中, 新的固相原子首先在枝晶尖端附近形核, 并快速向尖端及根部生长, 枝晶尖端被新原子完全包覆后将再次诱发液相原子附着形核及生长; 随初始液相密度的增加, 固-液界面移动速率增加, 速率系数的各向异性也增强.