Two-dimensional Nucleation Process Research Articles

Crossover from BKT-Rough to KPZ-Rough Surfaces for Crystal Growth/Recession

We found a crossover from a Berezinskii-Kosterlitz-Thouless (BKT, logarithmic-rough surface to a Kardar-Parisi-Zhang (KPZ, algebraic)-rough surface for growing/recessing vicinal crystal surfaces in the non-equilibrium steady state using the Monte-Carlo method. We also found that the crossover point from a BKT-rough surface to a KPZ-rough surface is different from the kinetic roughening point for the (001) surface. Multilevel islands and negative islands (island-shaped holes) on the terrace formed by the two-dimensional nucleation process are found to block surface fluctuations, which contributes to making a BKT-rough surface.

Study of the Nucleation and Growth Mechanisms of Copper Electrodeposition on Bare and Nitrogen-Doped Reduced Graphene Oxide Modified SnO2:F/glass Substrates

In this article, the influence of a nitrogen-doped electrochemically reduced graphene oxide layer on the nucleation and growth mechanisms of copper electrodeposition was studied. Thus, copper electrodeposition from an acidic solution was evaluated using two different substrates: fluorine-doped tin oxide (FTO), and fluorine-doped tin oxide covered with a nitrogen-doped reduced graphene oxide layer (FTO/N-ERGO). In both cases, chronoamperometric curves were obtained, which were analyzed and deconvoluted using pre-established models and equations, where the different contribution and nucleation parameters were determined. Field emission scanning electron microscopy (FESEM) images were acquired in order to observe the morphology and verify the nuclei density for each case considered in this study. In the case of copper electrodeposition onto FTO, an instantaneous three-dimensional nucleation was observed, together with a proton reduction reaction. When FTO/N-ERGO was used as a substrate, a new instantaneous two-dimensional nucleation process was observed in addition to the processes previously described. Furthermore, the increased density of active sites and the changes in copper morphology are directly related with the N-ERGO layer, which also increased the kinetic constant for the proton reduction reaction and the nucleation rate per active site during the copper electrodeposition process.

Well-Ordered "Ripple-Shaped" Microstructures of Mn Thin Films on GaAs Substrates

This study was aimed to investigate the thickness dependent morphological changes of Mn films deposited on GaAs substrates by thermal evaporation technique. Ni films were deposited under same conditions to perform comparative study of the morphological changes with respect to the Mn films. The scanning electron microscopy and atomic force microscopy studies revealed ripple-shaped structure of Mn film with good periodicity, while Ni film only exhibited small granules deposited throughout the surface. The influence of the thickness of the Mn film in producing the ripple structure was clearly observed. In addition, the annealing time was considered as the major parameter to control the ordering of the ripple structure. X-ray diffraction pattern indicated the formation of different phases of Ga-Mn and Mn-As due to diffusion of atoms during annealing. A model for the creation of stress-driven microstructure is proposed which indicates that Mn thin films grow on GaAs substrates in three stages: in the primary stage, the growth occurs via two-dimensional nucleation process; as the thickness increases, the stress is released by the film via creation of additional surface roughness which produce ripples; and finally an island-like growth occurs because of the non-uniform distribution of stress along the surface of the film.

A turbulent Eulerian multi-fluid model for homogeneous nucleation of water vapour in transonic flow

A model for homogeneous nucleation in rapid expansion that is applicable to moist gas and wet steam nucleating flow is presented. The governing equations of the two-phase flow are set up using the Eulerian multi-fluid model. Droplet formation is described by the Classical Nucleation Theory, and the turbulence of the two-phase flow is closed by a high-Reynolds k−ε−kp model. In this model, the droplet turbulence and the influences of gas compressibility, as well as droplet generation and influence on gas phase turbulence are all considered. CFD is used to solve the equations governing the gas phase, droplets and turbulence. The model is validated with the experimental data, and good agreement has been obtained. Moreover, this model is applied to simulate the two-dimensional nucleation process of moist gas and wet steam in the symmetrical converging–diverging nozzles with rectangular section, and some new results are obtained.

Abnormal grain growth of WC with small amount of cobalt

Abnormal grain growth (AGG) often appears in the synthesis of cemented tungsten carbide (WC), especially where ultra-fine powder is used. In this study, it was observed that AGG was strongly affected by the amount of Co. The obvious AGG was found to occur when the amount of Co was between 0.2 and 0.9 wt%. Careful examination of the etched samples found crack-like defects in abnormal grains, which were indications of grain boundaries in these large grains. Therefore, it is suggested that the considerable AGG of WC at lower Co concentrations be attributed to a grain boundary re-entrant edge (GBRE)-assisted two-dimensional nucleation process. Grain coalescence during sintering was the source of the grain boundaries. Due to the selective wetting of (0001) WC planes with the Co liquid, those grain boundaries were parallel to the (0001) planes resulting in platelet-like large grains with high aspect ratio. The amount of Co would affect the number of the wetted (0001) planes and the probability of grain boundary formation, and hence affect the AGG. The correspondence of the calculated weight percentage of Co required surrounding all the planes of WC grains and the experimental upper limit of the range supported this mechanism.

Microstructure and growth mechanism of epitaxial SrRuO3 thin films on (001) LaAlO3 substrates

SrRuO 3 thin films deposited on (001) LaAlO3 substrates by 90° off-axis sputtering at 600 °C were studied by atomic force microscopy (AFM) and transmission electron microscopy (TEM). Both AFM and cross-section TEM investigations show that the films have a rough surface. Plan-view TEM studies demonstrate that the films are composed of all three different types of orientation domains (twins). These domain structures and surface morphology are different from the SrRuO3 film deposited on the (001) SrTiO3 substrate which has an atomically flat surface and is composed of only the [110]-type domains. The reason for these differences was ascribed as the effect of lattice mismatch across the film/substrate interface. It is proposed that a SrRuO3 thin film grows on a (001) SrTiO3 substrate through a two-dimensional nucleation process, while a film on LaAlO3 grows with three steps: the coherent growth of a few monolayers at the initial stage through a two-dimensional nucleation process; the formation of misfit dislocations when the film reaches a critical thickness; and an island-like growth thereafter due to the nonuniform distribution of stress along the film surface.

Investigation of the nucleation and growth of SiC nanostructures on Si

Using in situ reflection high energy electron diffraction (RHEED), ex situ atomic force microscopy (AFM) and transmission electron microscopy (TEM) the early stages of SiC growth on Si during the carbonisation were investigated in a solid source molecular beam epitaxy equipment. Different mechanisms of SiC precipitate growth by SSMBE were found. The SiC growth during carbonisation of Si(111) at 600°C is controlled by diffusion and at higher temperatures by a two-dimensional nucleation process, which is mononuclear at 660°C and polynuclear above 750°C. At temperatures greater than 750°C and 850°C three-dimensional nucleation occurs at (111) and (100) surfaces, respectively.

Lateral Growth of Extended-Chain Single Crystals of Vinylidene Fluoride and Trifluoroethylene Copolymers

Lateral growth of extended chain single crystals (ECSCs) of vinylidene fluoride and trifluoroethylene copolymers, P(VDF/TrFE), within the mobile hexagonal phase at high temperature and at atmospheric and high pressures was investigated directly by polarized optical microscopy. The VDF molar contents (x) studied were 0.75 and 0.78, which were less than x at the triple point (x=0.82) at atmospheric pressure. The lateral growth was steady growth. The plot of logarithmic lateral growth rate (log V) against the inverse of the degree of supercooling (ΔT-1) gave a straight line, i.e., V=V0 exp (-C/ΔT), where V0 and C are constants, and does not show any breaking. These results confirmed that the lateral growth is mainly controlled by a two-dimensional nucleation process and shows only one “regime”. The number of ECSCs increased linearly with time, which indicates homogeneous nucleation.

Structural characteristics of diamond films prepared by thermal chemical vapor deposition processes

Abstract Diamonds were synthesized by thermal chemical vapor deposition on molybdenum and silicon substrates. Using CH 4 H 2 as the precursor gas mixture, distinct growth steps on (111) faces were observed for diamonds prepared at 950°C on molybdenum substrates. No growth spirals were found on the (111) planes, indicating an apparent layerwise growth induced by a two-dimensional nucleation process rather than a screw dislocation mechanism. As the temperature of the molybdenum substrate was increased to 1060°C, the (100) planes of diamond were dominant in the film's surface morphology with no distinct growth steps observed. The termination of the outermost surface of the diamond films by CH 2 groups was suggested by Fourier transform IR analysis.

Non-equilibrium surface segregation of silicon in Fe-6.3at%Si(110)

The non-equilibrium segregation of silicon from the bulk to the (110) surface of a Fe-6.3at%Si single crystal was monitored in the temperature range 484 to 536°C by means of AES. Two distinct processes were observed. Initially a diffusion process controlling the initial surface enrichment of Si up to a temporary equilibrium value of about 15% was observed. This value corresponds to a 7 × 1 LEED pattern observed in similar crystals. Subsequently a two-dimensional nucleation process starts allowing more atoms onto the surface. Activation energies for both mechanisms were determined and are respectively 170 ± 20 and 480 ± 60 kJ mol .

Stochastic character of two-dimensional nucleation in the case of electrocrystallization of silver

The randomness of the two-dimensional nucleation process is experimentally demonstrated with the example of layer-by-layer electrolytic growth of a screw-dislocation free (100) face of a silver single crystal.

Microtopographical studies of first order prism faces of synthetic quartz

Surface structures of first order prism faces of synthetic quartz, grown on different types of seed crystals, are studied using optical techniques. Oriented hexagonal growth pyramids, which are so commonly found on such faces, are illustrated. Oriented, pentagonal elevations, observed on hexagonal growth pyramids, are described. Origin of growth islands is explained. Rows of growth hillocks on opposite faces are illustrated and are attributed to growth along the edges of the seed plate. It is proposed that the first order prism faces of synthetic quartz grow mainly by two-dimensional nucleation process due to spreading and piling up of hexagonal growth layers.

The growth mechanism of apophyllite crystals studied by surface microstructures

It is proposed that apophyllite crystals grow mainly by two-dimensional nucleation process. Main growth takes place on the ‘a’ (100) faces by spreading and piling up of hexagonal growth ayers. ‘p’ (111) faces develop by independent growth in the form of triangular growth sheets. Growth rate on ‘p’ faces is greater than that of ‘a’ faces. Occasionally ‘a’ faces seem to grow by piling up of elliptical growth layers. Observations of hexagonal and elliptical growth pyramids on ‘a’ faces, oval shaped, hexagonal and rhombus shaped natural etch pits on ‘a’ faces, triangular natural etch pits on ‘p’ faces etc. reported, here, endorse the proposed mechanism of growth of apophyllite crystals.