Internally consistent verification of mean-field models for aggregation using large-scale molecular dynamics

The underlying atomistic mechanisms that govern vacancy aggregation in crystalline silicon are probed using a parametrically consistent, two-scale approach. The essential ingredient in this framework is a direct, quantitative comparison between the predictions of atomistic and continuum simulations for the transient size distribution of vacancy clusters. The former is carried out with parallel molecular dynamics simulation of a silicon system containing 215?000 atoms and 1000 vacancies. The continuum model is based on a sequence of coupled Master equations and is parametrized based on the same empirical potential used to perform the atomistic aggregation simulation. An excellent representation of the cluster size distribution can be obtained with consistent parameters only if the relevant physical mechanisms are captured correctly. The inclusion of vacancy cluster diffusion and a model to capture the dynamic nature of cluster morphology at high temperature are necessary to reproduce the results of the large-scale atomistic simulation. Finally, the continuum model is used to investigate cluster evolution for longer times, which are relevant for process simulation of defect-optimized silicon substrates for microelectronic device fabrication.     

Self-assembly of CuSO4 nanoparticles and bending multi-wall carbon nanotubes on few-layer graphene surfaces

When a colloidal suspension is allowed to wet a suitable substrate, various patterns emerge that can be varied from isolated island-like structures to fractal patterns. In this work we investigate the patterns arising from the interplay of colloidal copper sulfate suspensions containing carbon nanotubes with few-layer graphene substrates. The compositions of the thin film samples were investigated using X-ray photoelectron spectroscopy, surface topography and the nanostructure of the thin films were probed with atomic force microscope and transmission electron microscope respectively. The colloidal suspensions were characterized using contact angle and viscosity measurements. The colloidal suspensions when dip coated on few-layer graphene substrates exhibited fractal like morphology with the aggregation of copper sulfate crystallites to hexagonal platelets. This aggregation is explained invoking the depletion attraction theory. The various patterns observed experimentally were reproduced using a Monte Carlo simulation.     

In-situ characterization of metal clusters supported on a birefringent substrate using reflectance difference spectroscopy

Reflectance difference spectroscopy has been applied for the in-situ characterization of the growth of Ag cluster films on insulating birefringent Al₂O₃ (10[`1]0)(10\overline{1}0) substrates in the spectral range of 1.5–5 eV. Information on the individual cluster, cluster film morphology and growth are derived from the anisotropy of the in-plane plasmon resonances in comparison with scanning electron microscopy images. In particular, the evolution of the dipolar resonance has been attributed to two distinct stages of coarsening involving particle aggregation and ripening, and to the development of anisotropic particle shapes for higher Ag coverages. The effect of the formation of anisotropic electrostatic images in the birefringent substrate is used to explain the spectra even in the absence of structural anisotropies.     

气体团簇离子束装置的设计及其在表面平坦化、自组装纳米结构中的应用

根据超声膨胀原理,n(10-10^4)个气体原子可以绝热冷却后凝聚在一起形成团簇,经过离化后,形成带一个电荷量的团簇离子,比如Arn^+.当团簇离子与固体材料相互作用时,由于平均每个原子携带的能量(~eV)较低,仅作用于材料浅表面区域,因此,气体团簇离子束是材料表面改性的优良选择.本文介绍了一台由武汉大学加速器实验室自主研制的气体团簇离子束装置,包括整体构造、工作原理及实验应用.中性团簇束由金属锥形喷嘴(F=65-135μm,q=14°)形成,平均尺寸为3000 atoms/cluster,经离化后,其离子束流达到了50μA.Ar团簇离子因其反应活性较低,本文运用Ar团簇离子(平均尺寸为1000 atoms/cluster)进行了平坦化和自组装纳米结构的研究.单晶硅片经Ar团簇离子束处理后,均方根粗糙度由初始的1.92 nm降低到0.5 nm,同时观察到了束流的清洁效应.利用Ar团簇离子束的倾斜(30°-60°)轰击,在宽大平坦的单晶ZnO基片上形成了纳米波纹,而在ZnO纳米棒表面则形成了有序的纳米台阶,同时,利用二维功率谱密度函数分析了纳米结构在基片上的表面形貌和特征分布,并计算了纳米波纹的尺寸和数量.     

Monte Carlo Simulation of Sculptured Thin Films Growth of SiO2 on Si for Applications

A fully three-dimensional Monte Carlo model for simulation of sculptured thin-film growth is presented. After explaining the model, the simulation results are compared with the corresponding experiments, and encouraging consistency is proven. The morphology of sculptured thin films is then compared on periodical patterned and bare substrates. It is shown that there are more uniform structures and hence possible better optical properties by fabricating on patterned substrates. Finally, with the aid of computer simulation, we examine the sell-shadowing effect and our theoretical analysis of simulated morphology data deals with the accuracy of this model.     

Correlated nucleation model for simulating nanocluster pattern formation on Si(111)7 × 7 surface

We study the aggregation mechanisms of metal nanoclusters on the Si(111)7 × 7 reconstructed surface using a correlated nucleation model, in which the nucleation and growth behavior of a cluster (irreversible or partially reversible growth) depend on the local environment of the cluster. The kinetic Monte Carlo simulation of the model shows that with increasing temperature, the correlated nucleation effect causes a transition of growth behavior from asymmetric adatom aggregation between faulted and unfaulted half cells with a strong preference of occupation of faulted half cells, to compact cluster aggregation with a low occupation preference at high temperatures. As a result the preference as a function of the temperature exhibits a nonmonotonous behavior, with a maximum located at the temperature at which the transition of growth behavior has been observed. Both the simulated cluster morphologies and the quantitative analysis of the cluster distribution are in good agreement with the results observed from relevant growth experiments.     

含杂质无格点基底表面分枝状凝聚体的计算机模拟

根据含杂质熔融玻璃表面金原子凝聚的实验规律，在原子团簇具有随机的线扩散步长和刚性 转动角度的特征条件下，建立了含杂质无格点基底表面的各向异性团簇-团簇凝聚模型，对 团簇的无规扩散、刚性转动以及凝聚全过程进行了计算机模拟，系统地研究了基底表面无规 分布的杂质区域对分枝状凝聚体诸多特性的影响，所得结果与实验事实相符合. 关键词： 薄膜生长 Monte Carlo模拟 分形     

具有排斥相互作用的原子团簇非平衡系统研究

在无格点基底表面建立了存在排斥相互作用的原子团簇凝聚及扩散模型，对沉积在均匀带电硅油基底表面的银原子凝聚过程进行了研究.结果表明：当沉积原子凝聚成稳定的原子团簇后，由于带同种电荷，团簇之间存在库仑排斥作用，团簇数密度随时间呈指数形式衰减，衰减时间常数为Oa_a；两团簇相互离散的相对平均速率V与它们之间的相对距离L在 统计意义上成正比，即V=HL.停止沉积后初期,H≈Oa_a，然后随着扩散时间的增 加，H逐渐趋于零；随着液体基底黏滞系数增加，摩擦力增大，H逐渐减小，摩 关键词： 排斥相互作用 团簇 扩散     

Analysis of gas-phase condensation of nickel nanoparticles

Gas-phase condensation of 8000 nickel atoms is examined by molecular dynamics simulation with a tight-binding potential. A detailed study of the evolution of the system cooled at a constant rate from 1000 K to 77 K is presented. The results are used to identify four distinct stages of the evolution from a hot atomic gas to a few synthesized particles. An analysis of possible nanoparticle formation mechanisms suggests that cluster-cluster aggregation is the dominant one. The simulation shows that there two stages of cluster formation are of primary importance with regard to aggregation. At the first stage, spherical liquid clusters nucleate with uniform size distribution. The second stage is characterized by a distinct transition from uniform to bimodal size distribution due to aggregation of relatively large clusters. The particles obtained by gas-phase synthesis are analyzed by the CNA method [25]. It is found that most nanoparticles produced in the simulation have either icosahedral or mixed FCC/HCP structure.     

Dynamic Scaling of Ramified Clusters Formed on Liquid Surfaces

A comprehensive simulation model -- deposition, diffusion, rotation, reaction and aggregation model is presented to simulate the formation processes of ramified clusters on liquid surfaces, where clusters can disuse and rotate easily. The mobility （including diffusion and rotation） of clusters is related to its mass, which is given by Dm = Dos^-γD and θm = θos^-γθ, respectively. The influence of the reaction probability on the kinetics and structure formation is included in the simulation model. We concentrate on revealing dynamic scaling during ramified cluster formation. For this purpose, the time evolution of the cluster density and the weight-average cluster size as well as the cluster-size distribution scaling function at different time are determined for various conditions. The dependence of the cluster density on the deposition flux and time-dependence of fractal dimension are also investigated. The obtained results are helpful in understanding the formation of clusters or thin film growth on liquid surfaces.     

Mechanisms of small clusters production by short and ultra-short laser ablation

The mechanisms involved into the formation of clusters by pulsed laser ablation are studied both numerically and experimentally. To facilitate the model validation by comparison with experimental results, the time and length scales of the simulation are considerably increased. This increase is achieved by using a combination of molecular dynamics (MD) and the direct simulation Monte Carlo (DSMC) methods. The combined MD-DSMC model is then used to compare the relative contribution of the two channels of the cluster production by laser ablation: (i) direct cluster ejection upon the laser-material interaction, and (ii) collisional sticking and aggregation in the ablated gas flow. Calculation results demonstrate that both of these mechanisms play a role. The initial cluster ejection provides cluster precursors thus eliminating the three-body collision bottleneck in the cluster growth process. The presence of clusters thus facilitates the following collisional condensation and evaporation processes. The rates of these processes become considerable, leading to the modification of not only the plume cluster composition, but also the dynamics of the plume expansion. Calculation results explain several recent experimental findings.     

Scanning tunneling microscopy investigation of CoO/Fe(001) and Fe/CoO/Fe(001) layered structures

We report on the nanometer scale morphology of CoO thin films grown on top of Fe(001) substrates from the early stages of interface formation (few atomic layers), and on the surface topography of Fe/CoO/Fe(001) layered structures. The growth of the CoO films is dominated by formation of islands up to about 5 nominal atomic layers, then it proceeds in the layer-plus-island regime. The surface topography of thin Fe films grown on top of the CoO/Fe systems is strongly influenced by the morphology of the latter. Moreover, we observe a strong relationship between the growth mode and the chemical interactions at the CoO/Fe interface, since thick layers of iron oxides develop only below the CoO islands, as an effect of the proximity between Fe and Co atoms. We finally discuss possible implications of our observations on the magnetic properties of these layered magnetic structures.     

Effects of the System Temperature on Dust Cluster Formation in Plasma

Effects of the system temperature on dust aggregation in plasmas are investigated using two‐dimensional molecular dynamics simulations. It is shown that as the system temperature increases, the boundary of the clusters becomes gradually irregular (i.e., deviating from sphere‐like), and the cluster system gradually changes from solid to liquid and finally to gas state. The mean square displacement, mean nearest‐neighbor distance in the clusters, cluster size and coupling parameter of the system are obtained and the properties of the system structure and dynamics are investigated. The time τ needed for reaching equilibrium for different temperatures is obtained. It is shown that τ firstly decreases and then increases with the temperature, indicating that there is an optimum temperature allowing a dust aggregation to reach an equilibrium state most rapidly. The simulation results agree qualitatively with the experimental observations. (© 2015 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

无格点基底表面的杂质分布对分枝状凝聚体的影响

根据含杂质熔融玻璃表面金原子凝聚的实验规律，在原子团簇具有随机的线扩散步长和刚性转动角的特征条件下，建立了含杂质无格点基底表面上改进的杂质限制团簇-团簇(IRCCA)凝聚模型.对团簇的扩散、刚性转动以及凝聚全过程进行了计算机模拟，系统地研究了杂质区域分布情况对分枝状凝聚体诸多特性的影响.结果表明规则分布的杂质对凝聚体生长的影响比随机分布的杂质大，导致杂质规则分布的基底表面上的分枝状凝聚体的数密度更大，分枝状凝聚体的回旋半径，凝聚体平均大小及分形维数更小. 关键词： 薄膜生长 Monte Carlo模拟 分形 杂质     

Fractal Aggregation Under Rotation

By means of the Monte Carlo simulation, a fractal growth model is introduced to describe diffusion-limited aggregation (DLA) under rotation. Patterns which are different from the classical DLA model are observed and the fractal dimension of such clusters is calculated. It is found that the pattern of the clusters and their fractal dimension depend strongly on the rotation velocity of the diffusing particle. Our results indicate the transition from fractal to non-fractal behavior of growing cluster with increasing rotation velocity, i.e. for small enough angular velocity ω; thefractal dimension decreases with increasing ω;, but then, with increasing rotation velocity, the fractal dimension increases and the cluster becomes compact and tends to non-fractal.     

Generation of intense and cold beam of Pt-Ag bi-element cluster ions having single-composition

An intense beam of bi-element Pt-Ag cluster ions with a single atomic-composition has been gained toward development of new-functional materials of the clusters fixed on a solid surface. Mass production of the bi-element cluster ions has been achieved by operating dual magnetron-sputtering devices independently in a gas aggregation cell, and the ions having a single composition are filtered out by passing through a quadrupole mass filter. The kinetic energies of the cluster ions have been reduced by collision with cold helium in order for low-energy cluster-impact deposition of the clusters on the surface. The cooling process was examined further by means of molecular-dynamics simulation.     

Control of island morphology by dynamic coalescence of soft-landed clusters

The deposition of preformed clusters on surfaces has been established as a new way for growing nano-suctures on surfaces. It has been shown that supported island morphology relies on the dynamics of clusters, during the growth, giving rise to shapes from compact to ramified types. This paper identifies and discusses, in the case of antimony cluster deposits, several processes responsible for the non-equilibrium island shapes: limited kinetic cluster aggregation, size dependent coalescence, “wetting-like behavior” of antimony clusters on antimony islands. Using successive predetermined cluster sizes during the deposition process to synthesize polymorphic structure involves the interplay of those mechanisms. Received 1st December 2000     

Different W cluster deposition regimes in pulsed laser ablation observed by in situ scanning tunneling microscopy

We report on how different cluster deposition regimes can be obtained and observed by in situ scanning tunneling microscopy by exploiting deposition parameters in a pulsed laser deposition process. Tungsten clusters were produced by nanosecond pulsed laser ablation in Ar atmosphere at different pressures and deposited on Au(1 1 1) and HOPG surfaces. Deposition regimes including cluster deposition-diffusion-aggregation, cluster melting and coalescence and cluster implantation were observed, depending on background gas pressure and target-to-substrate distance which influence the kinetic energy of the ablated species. These parameters can thus be easily employed for surface modification by cluster bombardment, deposition of supported clusters and growth of films with different morphologies. The variation in cluster mobility on different substrates and its influence on aggregation and growth mechanisms has also been investigated.     

Effects of substrate defects on the carbon cluster formation in graphene growth on Ni(111) surface

The growth of graphene by chemical vapor deposition on transition metal has shown promise in this regard. The main hurdle for further improvement is the lack of complete understanding of the atomistic processes involved in the early growth stages, which is conceivable because there are too many factors affecting the growth process. Using first-principles calculations, we investigate the effect of substrate defects on the graphene nucleation on the Ni(111) surface. Our calculations reveal that the defects on substrates can induce the carbon aggregation, and the corresponding structures are completely different from that on the perfect Ni surface. We also compare the critical cluster sizes for the transition from one-dimensional carbon chains to two-dimensional graphene flakes in the growth sequence. Our investigations on the effects of substrate defects would be extremely useful for the future experimental synthesis of high-quality graphene.