Self-assembly of three-dimensional metal islands: nonstrained versus strained islands

A theoretical model for the Volmer-Weber growth of three-dimensional metal islands is proposed, with a dipolar island edge-edge interaction. The existence of such an island edge effect makes the island shape dependent on island size. Furthermore, it induces a stable island size against coarsening, leading to self-assembled islands of uniform size. The dependence of the stable island size on total film coverage is shown to be different for nonstrained versus strained islands, in the regime of strong island-island interaction.     

Equilibrium morphologies of epitaxially strained islands

An analytical expression of the free energy consisting of the strain energy, surface energy and interfacial energy for the coherent island/substrate system, as well as the evolving relations of aspect ratio against volume of the island, and misfit of the system, which provides a broad perspective on island behavior, is obtained, and used to study the equilibrium shapes of the systems. A two-dimensional model assuming linear elastic behavior is used to analyze an isolated island with elastic properties similar to those of the substrate. The results show that in order to minimize the total free energy, a coherent island will adopt a particular shape and height-to-width aspect ratio that are a function of only the island volume. The effect of a misfit dislocation on the equilibrium shape of an island is in passing examined. These can serve as a basis for interpretation of experiments.     

Stable unidimensional arrays of coherent strained islands

We investigate the equilibrium properties of arrays of coherent strained islands in heteroepitaxial thin films of bidimensional materials. The model we use takes into account only three essential ingredients: surface energies, elastic energies of the film and of the substrate and interaction energies between islands via the substrate. Using numerical simulations for a simple Lennard-Jones solid, we can assess the validity of the analytical expressions used to describe each of these contributions. A simple analytical expression is obtained for the total energy of the system. Minimizing this energy, we show that arrays of coherent islands can exist as stable configurations. Even in this simple approach, the quantitative results turn out to be very sensitive to some details of the surface energy.     

Kinetic evolution and equilibrium morphology of strained islands

Self-assembled SiGe islands grown on Si(001) leave behind characteristic "footprints" that reveal that small islands shrink, losing material to nearby larger islands. The critical size, dividing shrinking from growing islands, corresponds to the pyramid-to-dome shape transition, consistent with "anomalous coarsening" While shrinking, {105}-faceted pyramids transform into truncated pyramids and ultimately into unfaceted mounds. The similarity to behavior during island growth indicates that island shape and facet formation are thermodynamically determined.     

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.     

Positioning of strained islands by interaction with surface nanogrooves

When strained Stranski-Krastanow islands are used as "self-assembled quantum dots," a key goal is to control the island position. Here we show that nanoscale grooves can control the nucleation of epitaxial Ge islands on Si(001), and can drive lateral motion of existing islands onto the grooves, even when the grooves are very narrow and shallow compared to the islands. A position centered on the groove minimizes energy. We use as prototype grooves the trenches which form naturally around islands. During coarsening, the shrinking islands move laterally to sit directly astride that trench. In subsequent growth, we demonstrate that islands nucleate on the "empty trenches" which remain on the surface after complete dissolution of the original islands.     

Stability of an hexagonal array of coherently strained conical islands against Ostwald ripening

We present a linear stability analysis of an hexagonal array of coherently strained islands against the exchange of material between islands. Surprisingly, ultra-dense arrays are found to be metastable due to the short-range stabilizing effect of elastic interactions which overcome the destabilizing contribution of surface energy. Mean-field simulations are used to investigate the ripening kinetics of local volume defects in arrays which are found to equilibrate through island volume oscillations. Simulations of global perturbations directly verify the existence of the metastable regime and confirm the nature of the most unstable mode as derived from the stability analysis for sub-critical island coverage.     

Ensembles of aluminum islands on the surface of strained poly(ethylene terephthalate) films

This paper reports on a study of the size distribution of aluminum islands forming an aluminum-coated poly(ethylene terephthalate) film subjected to tension. The islands are shown to form four statistical ensembles, with the distribution in each of them being thermodynamically optimized and coinciding with the canonic distribution of thermodynamic probability.     

Shape transitions in strained Cu islands on Ni(100): kinetics versus energetics

We examine the ramified islands observed in submonolayer Cu/Ni(100) growth. Our results indicate that the strain-energy contribution to the dependence of island energy on shape is surprisingly weak. In contrast, our accelerated dynamics simulations indicate that unexpected concerted popout processes occurring at step edges may be responsible. Kinetic Monte Carlo (KMC) simulations which include these processes produce island shapes which are very similar to those observed in experiment. These results suggest that the shape transition is of kinetic origin but is strongly mediated by strain.     

Flow characteristics around a square cylinder with changing chamfer dimensions

Abstract  

It is known that for a square cylinder subjected to uniform flow, the drag force changes with the angle of attack. To clarify the flow characteristics around a square cylinder with corner cutoffs, we measured the drag coefficient and the Strouhal number for changing chamfer dimensions. We analyzed the flow around a square cylinder with corner cutoffs by applying the RNG k–ε turbulent model, and investigated the surface flow pattern using visualization by means of the oil film and mist flow method. From these results, we obtained the surface flow patterns by the oil film method and numerical analysis. The numerical results agreed well with the experimental values. The drag coefficient of the square cylinder with corner cutoffs decreased suddenly at an angle of attack of about α = 0°– 10° when compared with the drag coefficient for a square cylinder. The minimum value of the drag coefficient for the square cylinder with corner cutoffs decreased by about 30% compared with that for the square cylinder. The drag coefficient of the square cylinder with 10% corner cutoffs was found to be smallest, since the wake area of this square cylinder was smaller compared with that of the other square cylinder.     

Lattice—BGK simulation of a two—dimensional channel flow around a square cylinder

The confined flow around a square cylinder mounted inside a two-dimensional channel (blockage ratio $\be=1/8$) was investigated in detail by a newly developed incompressible nonuniform lattice-BGK model. It is found that the vortex shedding behind the cylinder induces periodicity in the flow field, and the periodicity of the flow will lose for $Re>$300. A detailed analysis for a range of Reynolds numbers between 1 and 500 was presented. Quantitative comparisons with other methods show that the model gives accurate results for complex flows.     

Flow structure investigation around two square cross-section beams under interference conditions

The dependence of the flow patterns around two prism bodies with square cross sections on the distance between them has been studied. The specific structures arising around two bluff bodies have been obtained and divided into four main types. The thermoanemometer probing in aerodynamic wake of the same two bodies has been performed and resulted in the presented dependence of Strouhal numbers on the relative distance between the investigated prisms.     

Formation of cobalt bilayer islands on Cu(100) surface

Physics of the Solid State - The formation and evolution of Co islands on Cu(100) surface are studied using molecular dynamics simulation and kinetic Monte Carlo method. Two regimes of cluster...     

环形开关电极的自动优化设计方法

 在脉冲功率技术中,开关电极的形状优化设计非常重要。利用电荷模拟法对常用的环形电极的电场分布进行了模拟计算,用旋转椭圆近似法优化电极轮廓外形,用微粒群（PSO）算法对轮廓外形的几何参数进行优化选择。结果表明：旋转椭圆近似法结合PSO算法非常适用于环形开关电极自动优化设计,优化所得形状和参数能满足电极之间电场分布的要求。     

环形开关电极的自动优化设计方法

在脉冲功率技术中,开关电极的形状优化设计非常重要。利用电荷模拟法对常用的环形电极的电场分布进行了模拟计算,用旋转椭圆近似法优化电极轮廓外形,用微粒群（PSO）算法对轮廓外形的几何参数进行优化选择。结果表明：旋转椭圆近似法结合PSO算法非常适用于环形开关电极自动优化设计,优化所得形状和参数能满足电极之间电场分布的要求。