Effect of titanium powder assisted surface pretreatment process on the nucleation enhancement and surface roughness of ultrananocrystalline diamond thin films

A superior, easy and single-step titanium (Ti) powder assisted surface pretreatment process is demonstrated to enhance the diamond nucleation density of ultrananocrystalline diamond (UNCD) films. It is suggested that the Ti fragments attach to silicon (Si) surface form bond with carbon at a faster rate and therefore facilitates the diamond nucleation. The formation of smaller diamond clusters with higher nucleation density on Ti mixed nanodiamond powder pretreated Si substrate is found to be the main reason for smooth UNCD film surface in comparison to the conventional surface pretreatment by only nanodiamond powder ultrasonic process. The X-ray photoelectron spectroscopic study ascertains the absence of SiC on the Si surface, which suggests that the pits, defects and Ti fragments on the Si surface are the nucleation centers to diamond crystal formation. The glancing-incidence X-ray diffraction measurements from 100 nm thick UNCD films evidently show reflections from diamond crystal planes, suggesting it to be an alternative powerful technique to identify diamond phase of UNCD thin films in the absence of ultra-violet Raman spectroscopy, near-edge X-ray absorption fine structure and transmission electron microscopy techniques.     

Direct visualisation of homogeneous and heterogeneous crystallisation in an ensemble of confined domains of poly(ethylene oxide)

We present a study of homogeneous and heterogeneous nucleation in polymer crystallisation. In bulk samples the crystallization is typically dominated by nucleation from defects (heterogeneous nucleation), and consequently studies must rely on sample preparation to minimize this effect. We present a study of nucleation within discrete droplets of poly(ethylene oxide) that are formed by the dewetting of a thin film on an unfavourable substrate. The samples provide an ensemble of impurity-free droplets, with length scales that can easily be measured. We show that the data for heterogeneous and homogeneous nucleation is qualitatively different, and that the data mirrors the fundamental differences in the underlying mechanisms for the two nucleation processes. The experiments presented here provide a simple method that can be used to study heterogeneous and homogeneous nucleation in great detail.Received: 1 January 2003, Published online: 30 October 2003PACS: 61.41. + e Polymers, elastomers, and plastics - 68.55.-a Thin film structure and morphology - 81.10.-h Methods of crystal growth; physics of crystal growth     

Stimulation of vapor nucleation on perfect and imperfect hexagonal lattice surfaces

Monte Carlo simulations of water vapor nucleation on a perfect crystal surface and on a surface with defects are performed. Mass exchange with the vapor phase is modeled by using an open ensemble. Cluster-substrate interaction is described in terms of conventional atom-atom potentials. The Hamiltonian of the system includes expressions for electrostatic, polarization, exchange, and dispersion interactions. The Gibbs free energy and work of adsorption are calculated by Monte Carlo simulation in the bicano?nical ensemble. The microscopic structure of nuclei is analyzed in terms of pair correlation functions. Periodic boundary conditions are used to simulate an infinite substrate surface. Molecule-substrate and molecule-molecule long-range electrostatic interactions are calculated by summing the Fourier harmonics of the electrostatic potential. Dispersion interactions are calculated by direct summation over layers of unit cells. Nucleation on a surface with matching structure follows a layer-by-layer mechanism. The work of adsorption per molecule of a monolayer on the substrate surface has a maximum as a function of nucleus size. The steady rate of nucleation of islands of supercritical size is evaluated. The work of adsorption per molecule for layer-by-layer film growth is an oscillating function of cluster size. As a function of layer number, it has a minimum depending on the vapor pressure. The electric field generated by a microscopic surface protrusion destroys the layered structure of the condensate and eliminates free-energy nucleation barriers. However, point lattice defects do not stimulate explosive nucleation.     

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.     

Surface nucleation in the crystallisation of polyethylene droplets

The division of semi-crystalline polymeric material into small domains is an effective tool for studying crystal nucleation. The scaling behavior of the nucleation rate as a function of domain size can reveal important information about the mechanism responsible for the birth of a crystal nucleus. We have investigated the process of crystal nucleation in a system of dewetted polyethylene droplets. Through the use of a correlation sample analysis, we are able to differentiate between heterogeneous and homogeneous nucleation mechanisms in a droplet sample. An analysis of the dependence of the nucleation rate on droplet size reveals that the nucleation probability scales with the surface area of the droplet.     

Surface processes in electrocrystallization

A short review of the theories of crystal growth of perfect and imperfect planeS is given. Experimental evidence of two-dimensional nucleation mechanism of growth of perfect crystal planes is provided for the first time and some crystallographic parameters of this process are determined. A quantitative verification of Frank's theory is given by investigating the growth phenomena on nearly perfect planes. The effect of screw dislocations on crystal growth is studied and some phenomena concerning the influence of surface active impurities on crystal growth and dissolution are reported.

The theory of heterogeneous phase formation is briefly discussed and an experimental verification of the basic relation following this theory is given. The effect of the crystallographic structure upon nucleation is closely studied. A method permitting the determination of the activity distribution curve of nucleation sites on the basis of their critical supersaturation is also given.     

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.     

Multiscale simulations of damage of perfect crystal Cu at high strain rates

We use the molecular dynamics code, large-scale atomic/molecular massively parallel simulator (LAMMPS), to simulate high strain rate triaxial deformation of crystal copper to understand void nucleation and growth (NAG) within the framework of an experimentally fitted macroscopic NAG model for polycrystals (also known as DFRACT model). It is seen that void NAG at the atomistic scales for crystal copper (Cu) has the same qualitative behaviour as the DFRACT model, albeit with a different set of parameters. The effect of material temperature on the nucleation and growth of voids is studied. As the temperature increases, there is a steady decrease in the void NAG thresholds and close to the melting point of Cu, a double-dip in the pressure–time profile is observed. Analysis of this double-dip shows disappearance of the long-range order due to the creation of stacking faults and the system no longer has a face centred cubic (fcc) structure. Molecular dynamics simulation of shock in crystal Cu at strain rates high enough to cause spallation of crystal Cu are then carried out to validate the void NAG parameters. We show that the pre-history of the material affects the void nucleation threshold of the material. We also simulate high-strain-rate triaxial deformation of crystal Cu with defects and obtain void NAG parameters. The parameters are then used in a macroscale hydrodynamic simulation to obtain spallation threshold of realistic crystal Cu. It is seen that our results match experimental results within the limit of 20% error.     

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.     

The state of the surface of martensitically transforming cobalt single crystals

The surface structure of single crystal cobalt has been monitored by low-energy electron-diffraction (LEED) in order to determine the existence of residual surface phases or “frozen in” surface embryos responsible for the nucleation of the martensitic transformation in cobalt. It has been determined that no such residual surface embryos exist and that the structural phase of the surface is equivalent to the respective allotropic phase of the bulk. The transformation at the surface is also observed to be strongly first-order with significant temperature hysteresis.     

Nucleation of cracks near the free surface in deformed metallic nanomaterials with a bimodal structure

A theoretical model that effectively describes the nucleation of cracks in stress fields of dislocation pile-ups near the free surface in metallic nanomaterials with a bimodal structure has been developed. The dependences of the critical shear stress τ_c (for the formation of a crack with an equilibrium length of 10 nm on a dislocation pile-up near the surface) on the size d of a grain containing the dislocation pile-up have been calculated for copper with a bimodal structure. Theoretically, it has been found that the critical shear stress τ_c for the nucleation of a crack near the free surface in a nanomaterial with a bimodal structure is approximately 30% higher than that for the crack nucleation within the nanomaterial at a distance from the free surface.     

Roughening of a Si(100) surface induced by the adsorption of oxygen near the solid-oxide nucleation threshold

An investigation of the processes on a Si(100) surface interacting with oxygen near the solid-oxide nucleation threshold using x-ray photoelectron spectroscopy and atomic-force microscopy is described. The nucleation threshold is the boundary between the temperature and oxygen pressure regions where a phase transition with the formation of a submonolayer oxide and a roughening transition caused by oxygen adsorption occur. Near the nucleation threshold, either a random rough relief or a quasiperiodic structure is formed on a surface coated with chemisorbed oxygen. The formation of the rough relief due to oxygen adsorption has been interpreted within the theory of phase transitions as a result of vacancy clustering. A model that allows one to describe the dynamics of processes on the surface near the nucleation threshold in qualitative and in some cases in quantitative terms has been suggested. Zh. éksp. Teor. Fiz. 114, 239–262 (July 1998)     

Evidence for out-of-equilibrium crystal nucleation in suspensions of oppositely charged colloids

We report a numerical study of the rate of crystal nucleation in a binary suspension of oppositely charged colloids. Two different crystal structures compete in the thermodynamic conditions under study. We find that the crystal phase that nucleates is metastable and, more surprisingly, its nucleation free-energy barrier is not the lowest one. This implies that, during nucleation, there is insufficient time for subcritical nuclei to relax to their lowest free-energy structure. Such behavior is in direct contradiction with the common assumption that the phase that crystallizes most readily is the one with the lowest free-energy barrier for nucleation. The phenomenon that we describe should be relevant for crystallization experiments where competing solid structures are not connected by an easy transformation.     

Optical and structural investigation of a-plane GaN layers on r-plane sapphire with nucleation layer optimization

Nonpolar a-plane GaN epilayers are grown on several r-plane sapphire substrates by metal organic chemical vapour deposition using different nucleation layers:(A) a GaN nucleation layer deposited at low temperature(LT);(B) an AlN nucleation layer deposited at high temperature;or(C) an LT thin AlN nucleation layer with an AlN layer and an AlN/AlGaN superlattice both subsequently deposited at high temperature.The samples have been characterized by Xray diffraction(XRD),atomic force microscopy and photoluminescence.The GaN layers grown using nucleation layers B and C show narrower XRD rocking curves than that using nucleation layer A,indicating a reduction in crystal defect density.Furthermore,the GaN layer grown using nucleation layer C exhibits a surface morphology with triangular defect pits eliminated completely.The improved optical property,corresponding to the enhanced crystal quality,is also confirmed by temperature-dependent and excitation power-dependent photoluminescence measurements.     

噪声对均质形核过程影响的晶体相场法研究

采用晶体相场模型,模拟了有色噪声诱发的均质形核过程. 结果表明,噪声强度在一定范围内对系统平衡热力学参量、形核势垒、临界晶核尺寸影响甚微; 而对形核孕育时间影响较大,形核孕育时间随噪声强度的增大呈指数减小趋势. 进一步分析表明,噪声对形核孕育时间的影响主要是由于噪声的引入改变了原子动力学系数.     

硅中位错增殖的实验观察

用化学侵蚀法研究了在机械应力和热应力作用下硅中位错的增殖和非均匀成核。结果表明,在使位错增殖和成核作用上,热应力同机械应力是等效的。硅中小角晶界中的位错,原生孤立位错都能成为位错源;晶体内部的缺陷及表面蚀斑处的应力集中能够引起位错成核;硅中螺型位错能够通过交叉滑移机制发生增殖。对新生位错环空间分布的研究表明,Frank-Read机制可能是位错增殖的主要形式。位错能否发生增殖,主要决定于位错源所受分切应力的数值、晶体温度、位错本身的结构特点以及钉扎情况等。     

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.     

Computer simulation of water vapor nucleation on charged nanoparticles

The Monte Carlo method is applied to the study of the formation of condensed-phase nuclei from water vapor on electrically charged silver iodide nanocrystals. This study is a continuation of the investigations carried out earlier in [1] with electrically neutral nucleation centers. Nanoparticles with a size of up to 4 nm and flat nanoparticles with a size of up to 10 nm are investigated. The free energy, entropy, and the work of formation of nuclei with a size of up to 6729 molecules are calculated at the atomic level by the bicanonical statistical ensemble (BSE) method at a temperature of 260 K. Thermodynamic stability of nuclei is investigated depending on the size, shape, and charge of nanocrystal nucleation centers, as well as depending on the presence of crystal defects and the degree of spatial localization of charge on the surface of nanoparticles. The excess charge has a crucial effect on the work of formation of a nucleus only in the case of strong spatial localization of the latter near a point crystal defect; however, this effect is restricted to a relatively small size of the nuclei and therefore cannot substantially enhance the ice-forming activity of nanoparticles. A nucleus that grows on the surface of a nanoparticle evolves through three stages that differ in molecule retention mechanism and thermodynamic stability. The charge of a nanoparticle has a small effect on these factors. The leading factor that determines the ice-forming activity of ion nanocrystals is their intrinsic electric field due to the nonuniform distribution of charge within a unit cell of the crystal lattice.     

生长于微结构内点状结构的成核热力学

研究了汽相沉积过程中的衬底表面凹结构的成核热力学,指出可以通过衬底表面微结构设计来控制成核过程以实现点状结构的生长．计算了凹结构内的临界核形成能,表明与平面相比成核优先在凹结构发生．考虑凹结构的“等效势阱”效应,发现凹结构内稳定原子团的生长速率远远大于平面处稳定原子团的生长速率． 关键词：