The Fate of a Cluster Colliding onto a Substrate Dissipation of Translational Kinetic Energy

The fate of clusters emitted onto a substrate falls into several categories including repulsion, soft landing, migration, and explosion, depending on the interaction between the cluster and the substrate. This dynamic behavior of the clusters controls thin-film formation processes from clusters such as cluster ion beam deposition and chemical vapor deposition. Here we describe the collision processes of both Al and Au clusters with a kinetic energy of 0.56 eV/atom onto an amorphous SiO₂ substrate studied by molecular dynamics simulation, focusing on the dissipation of translational kinetic energy during the collision process. The simulation elucidated that the activation of thermal vibrational energy of the substrate promoted the sticking of the colliding clusters on the substrate. This result suggests that the dissipation of the translational kinetic energy of the colliding cluster is one of the factors that determine the structure formed on a substrate from vapor phase.     

Glass transitions in quasi-two-dimensional suspensions of colloidal ellipsoids

We observed a two-step glass transition in monolayers of colloidal ellipsoids by video microscopy. The glass transition in the rotational degree of freedom was at a lower density than that in the translational degree of freedom. Between the two transitions, ellipsoids formed an orientational glass. Approaching the respective glass transitions, the rotational and translational fastest-moving particles in the supercooled liquid moved cooperatively and formed clusters with power-law size distributions. The mean cluster sizes diverge in power law as they approach the glass transitions. The clusters of translational and rotational fastest-moving ellipsoids formed mainly within pseudonematic domains and around the domain boundaries, respectively.     

Impact and spreading behavior of cluster atoms bombarding substrates

The purpose of this study is to investigate the behavior of copper cluster atoms bombarding a substrate using molecule dynamics based on tight-binding second moment approximation (TB-SMA) potential. The simulated results show that a crater on the substrate surface was created by the impact of the clusters. The variations of kinetic energy of cluster bombardments can be divided into three stages. At the initial impact level, the kinetic energies of the clusters and the substrate were constant. Then, the system went into a sluggish stage of energy variation, in which the kinetic energy of the clusters reduced. In the final stage, the kinetic energy of the system became stable. The high slip vector region around the crater had a disorder damage zone. The symmetry-like cross-slip occurred beneath the top layer of the substrate along the 〈1 1 0〉 orientations. The spreading index, temperature, and potential functions that affect the bombardments are also discussed.     

Deformation of slow liquid and solid clusters upon deposition: A molecular-dynamics study of Al cluster impact on an Al surface

Using molecular-dynamics simulation, we investigate the self-deposition of Al_n clusters (n < 4000) on an Al substrate at velocities below the velocity of sound. Both cold crystalline and hot liquid clusters are studied. We examine the cluster deformation after impact on the surface, which we quantify by its height and base radius. At a given cluster velocity, the shape of deposited crystalline clusters is rather independent of the cluster size; only at small cluster sizes, n ? 40, the clusters are less strongly deformed. With increasing cluster size, liquid clusters are more strongly deformed than crystalline clusters. Faster projectiles become more strongly flattened by the deposition process. Even clusters depositing with vanishing velocity show a finite deformation, which is considerable for smaller clusters. At large cluster speed, clusters penetrate deeper into the (1 0 0) surface than into the (1 1 1) surface and also deform more strongly.     

Cluster growth processes by direct simulation monte carlo method

Thin films obtained by cluster deposition have attracted strong attention both as a new manufacturing technique to realize high-density magnetic recording media and to create systems with unique magnetic properties. Because the film’s features are influenced by the cluster properties during the flight path, the relevant physical scale to be studied is as large as centimeters. In this paper, a new model of cluster growth processes based on a combination of the Direct Simulation Monte Carlo (DSMC) method and the cluster growth model is introduced to examine the effects of experimental conditions on cluster growth by an adiabatic expansion process. From the macroscopic viewpoint, we simulate the behavior of clusters and inert gas in the flight path under different experimental conditions. The internal energy of the cluster, which consists of rotational and vibrational energies, is limited by the binding energy which depends on the cluster size. These internal and binding energies are used as criteria of the cluster growth. The binding energy is estimated by surface and volume terms. Several types of size distribution of generated clusters under various conditions are obtained by the present model. The results of the present numerical simulations reveal that the size distribution is strongly related to the experimental conditions and can be controlled. Received: 23 January 2001 / Accepted: 3 May 2001 / Published online: 30 August 2001     

Comparison of surface damage under the dry and wet impact: Molecular dynamics simulation

Molecular dynamic simulation was applied in analyzing the difference of surface damage during the impact of a large silica cluster on a crystal silicon substrate in dry and wet condition, respectively. The simulation results show that the damage of silicon substrate under the dry impact is more severe than that under the wet impact. A reason is that the water film buffers the impact of the incident cluster at the init stage under the wet impact. The more important reason is that the water film might be solidified into a continuous thin film at the cluster deformation stage. So, the considerable great impact energy of the cluster will be absorbed by the water film and transformed into thermal dissipation of energy between the simulation ensemble and the water film.     

Ag-Co clusters deposition on Ag(100): an atomic scale study

The slowing down of Co₁₀Ag₁₉₁ and Co₂₈₅Ag₃₀₁ nanoclusters on a Ag (100) surface is studied at the atomic scale by means of classical Molecular Dynamics simulations. The slowing down energy, 0.25 to 1.5 eV/atom, is characteristic of low energy cluster beam deposition and aerosol focused beam techniques. The two clusters differentiate by their size, stoechiometry and structure. While Co forms one or several groups just beneath the cluster surface in Co₁₀Ag₁₉₁, Co₂₈₅Ag₃₀₁ displays a core-shell structure where Ag forms one complete monolayer around the Co core. As a consequence of the impact, the smallest cluster undergoes deep reorganization and becomes fully epitaxial with the substrate. The larger one only undergoes partial accommodation and partially retains the memory of its initial morphology. For both, after impact, the Co forms one group covered by Ag. The substrate damage is significant and depends on the slowing down energy. It results in a Ag step surrounding the cluster which may be more than one atomic layers high and isolated add-atoms or small monolayer islands apart from the step. The latter originate from the cluster and the former from the substrate. Further details in the consequences of the impact are given, concerning the cluster penetration, its deformation and lattice distortions, with emphasis on the cluster size and stoechiometry.Received: 11 June 2004, Published online: 31 August 2004PACS: 35.40.-c - 61.46. + w Nanoscale materials: clusters, nanoparticles, nanotubes, and nanocrystals - 07.05.Tp Computer modeling and simulation     

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.     

Cluster-solid interactions: A molecular dynamics investigation

Abstract

The interaction of small energetic clusters of metal atoms with surfaces has been investigated by molecular dynamics computer simulations. A wide variety of cluster-solid combinations have been studied so that the effects of the energy, size, and angle of incidence of the cluster, and the relative elastic properties of the cluster and substrate could be elucidated. ‘Soft landings’ were also investigated. From these studies, a mechanism map for cluster-solid interactions is developed. The results have particular importance for cluster beam deposition of thin films. The simulations are fully dynamical and 3-dimensional; they employ embedded-atom method potentials, which have been modified for interactions at close separations. A scheme for reducing the CPU time that is required for these and other simulations of radiation effects is described.     

Hyperthermal cluster-surface scattering

Using molecular-dynamics simulation, we study the processes occurring after impact of clusters on a rigid wall. Comparing the impact of model clusters consisting of 13 atoms, or of 13 diatomic molecules with varied bond strength, the systematics in the results of the collision process are investigated. Four regimes of impact-induced cluster fragmentation are identified: intact reflection, shattering into large fragments, complete fragmentation, and molecule dissociation. The effect of the number of degrees of freedom activated in the collision on the translational and internal energies of the reflected fragments is discussed in detail. As a rule, with increasing number of degrees of freedom which can be activated in the collision, the translational energy sinks. On the other hand, for weak intramolecular bonding, intramolecular vibrations are easily excited at small impact energies, reducing the resulting translational energy. The presence of even a very weak attractive well epsilon_w at the surface has a major influence on the sticking behavior of the clusters — and hence also on the absolute reflected energies — even at impact energies E₀ ≫ epsilon_w.     

纳米粒子与单晶硅表面碰撞的反弹机理研究

应用分子动力学方法模拟了纳米粒子与单晶硅(001)表面碰撞、反弹飞离的现象，分析了粒子的反弹行为、基体弹性形变和塑性形变的原子构型特征，以及碰撞过程的能量转化.碰撞后单晶硅表面形成半球形的小坑，小坑周围的基体原子呈非晶态.碰撞过程中与颗粒相邻的基体原子立即非晶化，在非晶层外面基体以可恢复的(111)［110］滑移结构存储弹性形变能.在射入过程，基体发生压缩弹性形变；颗粒反弹时基体势能振荡下降,交替形成压缩形变构型和拉伸形变构型.射入过程中存贮的压缩弹性形变能的释放为颗粒提供了反弹、飞离的能量. 关键词： 碰撞 纳米粒子 单晶硅表面 分子动力学模拟     

Dynamics of cluster deposition on Ar surface

Using a combined quantum mechanical/classical method, we study the dynamics of deposition of small Na clusters on Ar(001) surface. We work out basic mechanisms by systematic variation of substrate activity, impact energy, cluster orientations, cluster sizes, and charges. The soft Ar material is found to serve as an extremely efficient shock absorber which provides cluster capture in a broad range of impact energies. Reflection is only observed in combination with destruction of the substrate. The kinetic energy of the impinging cluster is rapidly transfered at first impact. The distribution of the collision energy over the substrate proceeds very fast with velocity of sound. The full thermalization of ionic and atomic energies goes at a much slower pace with times of several ps. Charged clusters are found to have a much stronger interface interaction and thus get in significantly closer contact with the surface.     

Structural deformation, melting point and lattice parameter studies of size selected silver clusters

Silver clusters have been produced by magnetron sputtering in a gas aggregation nanocluster source. Clusters are size selected using a quadrupole mass filter (3–8 nm) or by varying the aggregation tube length (9–20 nm) of the nanocluster source. Mass selected clusters are deposited on a Si(100) substrate at different bias voltages and are characterized by atomic force microscopy. We observe a significant flattening of clusters on the surface due to the increase of impact energy as a result of increasing substrate bias voltage. The behavior of lattice parameters for size selected clusters are investigated by X-ray diffraction. All measured lattice constants exhibit a tensile strain; it is found that the lattice constant slightly increases with increasing cluster size up to a size of 12 nm and then decreases. The melting temperature of deposited clusters is found to be size-dependent and significantly lower than for bulk material, in agreement with theoretical considerations.     

Sound transmission across lightweight all-metallic sandwich panels with corrugated cores

The transmission of sound through all-metallic sandwich panels with corrugated cores is investigated using the space-harmonic method. The sandwich panel is modeled as two parallel panels connected by uniformly distributed translational springs and rotational springs, with the mass of the core sheets taken as lumped mass. Based on the periodicity of the panel structure, a unit cell model is developed to provide the effective translational and rotational stiffness of the core. To check the validity of the model, it is used first to study the sound insulation properties of double-panel structures with air cavity, and the analytical predictions agree well with existing experimental data. The model is then employed to quantify the influence of sound incidence angle and the inclination angle between facesheet and core sheet on sound transmission loss （STL） across sandwich panels with corrugated cores. The results show that the inclination angle has a significant effect on STL and it is possible to avoid STL dips by altering the inclination angle. Moreover, it is found that sandwich panels with corrugated cores are more suitable for the insulation of sound waves having small incidence angles.     

Formation and characterization of high-density Fe cluster-assembled films with soft magnetic behaviors

High-density, magnetically soft Fe cluster-assembled films were obtained at room temperature by an energetic cluster deposition. Size-monodispersed Fe clusters with the mean cluster size d = 9, 13 and 16 nm were produced using a plasma-gas-condensation technique. Ionized clusters in cluster beam were accelerated electrically and deposited onto the substrate together with neutral clusters from the same cluster source. The morphology, microstructure and magnetic properties of the cluster-assembled films have been studied by an atomic force microscopy, scanning electron microscopy, transmission electron microscopy, and superconducting quantum interference device magnetometer. By increasing the impact energy of the ionized clusters up to 0.6 eV/atom, the Fe cluster-assembled film has a packing fraction of 0.86±0.03, and reveals a soft magnetic behavior. In addition, it is found that oxidization of the cluster-assembled films is remarkably suppressed with the increase in the density of the films.     

Cluster–surface interaction studied by time-resolved two-photon photoemission

n ⁺ clusters (n=2-9)deposited onto highly oriented pyrolytic graphite (HOPG) substrates at liquid nitrogen temperatures. The deposition was carried out with variable kinetic energies of the clusters. Clusters deposited with high kinetic energy (up to 60 eV/cluster) become fragmented upon impact. For low deposition energies (1–4 eV/cluster) the size dependence of the photoelectron spectra reveals a pronounced odd/even effect, which is well known for gas phase silver clusters. This indicates that the soft deposited clusters retain their size and identity on the sample. The phase of the odd/even effect suggests that transient negatively charged cluster ions serve as an intermediate step in the two-photon photoemission process. The lifetime of the anions rises with cluster size. This is attributed to an increasing electronic density of states for larger clusters. Received: 26 October 1998 / Revised version: 16 December 1998     

Rotation/translation interplay in the recoil statistics of cluster evaporation

A new statistical model has been developed in the framework of Phase Space Theory to describe the evaporation process of non-rotating clusters. The novelty of the theoretical approach resides in its ability to easily separate the total kinetic energy released in an evaporation process into the rotational and translational contributions. This new model has been tested by comparing its predictions with the results of Molecular Dynamics (MD) simulations for the unimolecular evaporation of two types of van der Waals clusters: the aniline-(argon)_n molecular clusters and the neat argon Ar_n clusters. Received 16 March 2000     

Ion Beam "Photography": Decoupling Nucleation and Growth of Metal Clusters in Glass

We demonstrate that room temperature MeV ion irradiation of a glass containing copper oxide initiates nucleation of pure Cu clusters via the inelastic "electronic" component of the ion energy loss, when the latter is above a threshold value. The clusters grow under subsequent thermal annealing, following Lifshitz-Slyozov-Wagner kinetics. The decoupling of nucleation and growth is analogous to that occurring in the photographic process. It allows total control over the cluster density, average size, and size distribution.