Instability driven fragmentation of nanoscale fractal islands

Formation and evolution of fragmentation instabilities in fractal islands, obtained by deposition of silver clusters on graphite, are studied. The fragmentation dynamics and subsequent relaxation to the equilibrium shapes are controlled by the deposition conditions and cluster composition. Sharing common features with other materials' breakup phenomena, the fragmentation instability is governed by the length-to-width ratio of the fractal arms.     

Thermal and chemical nanofractal relaxation

We studied shape relaxation of nano-fractal islands, during annealing, after their growth from antimony cluster deposition on graphite surface. Annealing at 180^°C shows evidence of an increase of the fractal branch width with time followed by branch fragmentation, without changing the fractal dimension. The time evolution of the width of the arm suggests the surface self-diffusion mechanism as the main relaxation process. With Monte Carlo simulations, we confirmed the observed behavior. Comparison is done with our previous results on fragmentation of nano-fractal silver islands when impurity added to the incident cluster promotes rapid fragmentation by surface self-diffusion enhancement [1].     

沉积在硅油表面上的Ag原子分形凝聚体

研究了沉积在硅油表面上的Ag原子团簇,经过随机扩散和转动,最终形成大尺度分形凝聚体的凝聚过程．研究结果表明：Ag原子团簇在这种液体基底上的转动为随机转动,转动角位移的方均值<(Δθ)²>和测量时间间隔Δt满足广义爱因斯坦关系<(Δθ)²>=4D_θΔt．随机转动系数D_θ与凝聚体面积S满足指数关系D_θ∝S^{－γ_θ,其中指数γ_θ＝2.4±0. 关键词：}     

Processes involved in the formation of silver clusters on silicon surface

We analyze scanning electron microscopy measurements for structures formed in the deposition of solid silver clusters onto a silicon(100) substrate and consider theoretical models of cluster evolution onto a surface as a result of diffusion and formation of aggregates of merged clusters. Scanning electron microscopy (SEM) data are presented in addition to energy dispersive X-ray spectrometry (EDX) measurements of the these films. Solid silver clusters are produced by a DC magnetron sputtering source with a quadrupole filter for selection of cluster sizes (4.1 and 5.6 nm or 1900 and 5000 atoms per cluster in this experiment); the energy of cluster deposition is 0.7 eV/atom. Rapid thermal annealing of the grown films allows analysis of their behavior at high temperatures. The results exhibit formation of cluster aggregates via the diffusion of deposited solid clusters along the surface; an aggregate consists of up to hundreds of individual clusters. This process is essentially described by the diffusion-limited aggregation (DLA) model, and thus a grown porous film consists of cluster aggregates joined by bridges. Subsequent annealing of this film leads to its melting at temperatures lower than to the melting point of bulk silver. Analysis of evaporation of this film at higher temperatures gives a binding energy in bulk silver of ɛ₀= (2.74 ± 0.03) eV/atom. The text was submitted by the authors in English.     

Stability of a deposited liquid cluster

The fragmentation of silver liquid clusters deposited onto a silicon surface is observed after heating the clusters to 1073 K and subsequent annealing with the exposition time 3 min. This contradicts macroscopic models of a liquid drop deposited on a surface if we use critical parameters of bulk silver. Some versions are analyzed that are based on cluster properties and may explain the phenomenon of cluster fragmentation. An experiment is suggested for clarifying the nature of the cluster fragmentation phenomenon and cluster critical phenomena.     

Kinetic Simulation of Fractal Aggregation on Liquid Surfaces

A modified fractal growth model based on the deposition, diffusion, and aggregation (DDA) with cluster rotation is presented to simulate two-dimensional fractal aggregation on liquid surfaces. The mobility (including diffusion and rotation) of clusters is related to its mass, which is given by D_m=D₀ s^-γ_D and θ_m=θ₀s^-γ_θ, respectively. We concentrate on revealing the details of the influence of deposition flux F, cluster diffusion factor γ_D and cluster rotation factor γ_θ on the dynamics of fractal aggregation on liquid surfaces. It is shown that the morphologies of clusters and values of cluster density and fractal dimension depend dramatically on the deposition flux and migration factors of clusters.     

Kinetic Spectra of Polyatomic Clusters Formed via the Ion Sputtering of Metals

A theory is developed for the ion sputtering of metals resulting in the formation of excited neutral and charged clusters with their subsequent fragmentation to a stable state. The effect fragmentation has on the kinetic spectra and charge state of clusters is considered. The results are presented in the form of convenient formulas. Calculations for the kinetic spectra of polyatomic clusters of niobium and silver are given as examples.     

Softlanding and STM imaging of Ag<Subscript> 561</Subscript> clusters on a C<Subscript> 60</Subscript> monolayer

The low energy deposition of silver cluster cations with 561 (±5) atoms on a cold fullerene covered gold surface has been studied both by scanning tunneling microscopy and molecular dynamics simulation. The special properties of the C₆₀/Au(111) surface result in a noticeable fixation of the clusters without a significant change of the cluster shape. Upon heating to room temperature we observe a flattening or shrinking of the cluster samples due to thermal activation. Similar changes were observed also for mass selected Ag clusters with other sizes. For comparison we also studied Ag islands of similar size, grown by low temperature deposition of Ag atoms and subsequent annealing. A completely different behavior is observed with much broader size distributions and a qualitatively different response to annealing.     

Orientational averaging of integrated cross sections in the discrete dipole method

Using the discrete dipole method, exact and approximate analytical solutions for orientation-averaged cross sections for extinction, absorption, and scattering of light are obtained. The analytical solutions can be applied to the calculation of integrated cross sections of fractal clusters formed by primary particles with different optical properties (soot in air and aqueous suspensions of aggregates of polystyrene, gold, and silver nanoparticles). It is shown that two models of aggregates that differ only in trajectories (ballistic or Brownian) of primary particles and intermediate clusters and in average fractal dimensions give close values of averaged extinction cross sections.     

Scaling behavior of threshold epidemics

We numerically simulate the dynamics of atomic clusters aggregation deposited on a surface interacting with the growing island. We make use of the well-known DLA model but replace the underlying diffusion equation by the Smoluchowski equation which results in a drifted DLA model and anisotropic jump probabilities. The shape of the structures resulting from their aggregation-limited random walk is affected by the presence of a Laplacian potential due to, for instance, the surface stress field. We characterize the morphologies we obtain by their Hausdorff fractal dimension as well as the so-called external fractal dimension. We compare our results to previously published experimental results for antimony and silver clusters deposited onto graphite surface.     

Observation of fractal nanoclusters on the pulsed laser deposition of gold

The results of studying the fractal structure of Au clusters arising in pulsed laser deposition under conditions of strong deviations from thermodynamic equilibrium are presented. A mechanism is suggested in which the formation of fractal nanoclusters is considered as a result of the decay of a strongly correlated initial state in a non-equilibrium system of adatoms. The geometrical shape of the fractal structures observed experimentally can be described within the given mechanism, and their dimensionality can be estimated.     

Chemically induced morphology change in cluster-based nanostructures

Preformed clusters carrying surfactant are used as primary blocks for the building of nano structures. Self assembly of silver atom based clusters, soft landed on a HOPG surface, generates a large variety of new architectures depending on the nature and on the concentration of the impurities. Fractal shapes fragmented into multiple compact like islands, and chain like structures might be formed. A strong local enhancement of the silver atom mobility at the surface of islands is responsible for those morphology changes.     

Growth of Ag nanoclusters on a 1D template

Bi nanolines, which are 1.5 nm wide and can be over 500 nm long, have been used as a 1D template for the formation of arrays of small clusters of silver. Distinct nucleation and growth phases are observed. At room temperature, silver atoms preferentially adsorb onto the Bi nanoline templates to form arrays of spheroidal nanoclusters 0.55 nm in height, up to a saturation density of 6.7 clusters per 100 nm of nanoline. Further deposition of Ag results in the growth of the clusters on the nanolines up to 1.4 nm, accompanied by the formation of 0.55 nm clusters on the background. At 400 K, however, deposition of Ag results in the formation of clusters up to 4 nm in height, with the majority formed on the nanoline template.     

Small angle neutron scattering studies on the interaction of cationic surfactants with bovine serum albumin

The structure of the protein-surfactant complex of bovine serum albumin (BSA) and cationic surfactants has been studied by small angle neutron scattering. At low concentrations, the CTAB monomers are observed to bind to the protein leading to an increase in its size. On the other hand at high concentrations, surfactant molecules aggregate along the unfolded polypeptide chain of the protein resulting in the formation of a fractal structure representing a necklace model of micelle-like clusters randomly distributed along the polypeptide chain. The fractal dimension as well as the size and number of micelles attached to the complex have been determined.      

Fragmentation of plasma resonances in open-shell silver cluster ions

Summary We study the fragmentation of the dipole collective states in silver cluster ions by looking at the coupling with surface oscillations and deformations of a dropletlike structure. This coupling is found to be crucial in order to reproduce the experimental photoabsorption data in open-shell clusters.     

Interaction of gold and silver cluster cations with CH<Subscript>3</Subscript>Br: thermal and photoinduced reaction pathways

The reactions of free, size selected Au _n ⁺ (n = 1–3, 5, 7) and Ag₃ ⁺ clusters with CH₃Br as well as the photodissociation of the resulting complexes at 266 nm were studied in a radio frequency ion trap under multiple collision conditions. CH₃Br was found to interact more strongly with the gold clusters than with the silver clusters. All investigated metal clusters exhibited characteristic size dependent adsorbate coverages. Furthermore, the successive loss of methyl radicals was identified as a major thermal reaction channel of the adsorbed CH₃Br molecules. Photodissociation experiments were performed with the product complexes of the trimer clusters and revealed the strongly preferred light-induced fragmentation of Au₃Br₃ ⁺ and Ag₃Br₃ ⁺, respectively, over any other thermal reaction products. However, whereas in the case of the gold cluster complexes the bare Au₃ ⁺ was exclusively re-formed through laser irradiation, considerable photoinduced metal cluster fragmentation occurred in the case of Ag₃Br₃ ⁺.     

银原子在液体基底上的凝聚特性

研究了沉积在液体基底表面上的银原子的扩散和凝聚特性．实验结果表明：银原子在液体表面先形成准圆形的团族，然后通过无规扩散而逐渐凝聚成具有分枝状的凝聚体．准圆形团簇的平均半径以及凝聚速率均随沉积速率的变化而变化．还提出了一个凝聚速率概率模型，计算结果与凝聚速率的实验曲线相符合． 关键词：     

Effect of chromium interlayer deposition on periodic silver nanoparticle array structure fabricated by nanosphere lithography

Effect of chromium interlayer deposition on 2-dimensional, periodic silver nanoparticle array structure was systematically investigated. The silver nanoparticle array was fabricated by nanosphere lithography with assembled polystyrene nanospheres being as a deposition mask. The chromium interlayer was deposited by thermal evaporation either on the nanosphere mask or directly on the silicon substrate. The structures of the achieved silver nanoparticle arrays were characterized by scanning electron microscope and were compared with that of silver nanoparticle array without the interlayer. With analysis of the anomalies among the structures the critical role of the interlayer in the periodic nanoparticle array fabrication was revealed.     

SIMULATION OF MULTIPLE FRACTAL AND COMPACT GROWTH OF ULTRA-THIN FILMS ON HEXAGONAL SUBSTRATE

The multiple cluster growth of ultra-thin films with different deposition rate and different substrate temperature has been studied by kinetic Monte-Carlo simulation. With increasing diffusion rate along cluster edges (corresponding to an increasing substrate temperature), pattern structures change smoothly from fractal islands, compact islands with random shapes, to regular islands, and the average branch width of clusters increases continuously up to some constant value in the compact island limit. The formation of the multiple fractal and compact clusters can be described quantitatively by multifractal. The results of multifractal analysis show that with pattern change from fractal to compact islands, the Hausdorff dimension D₀, the information dimension D₁, and the correlation dimension D₂ decrease, while the width and height of the multifractal spectra increase.     

Ordering in an ensemble of nanometer-size Au clusters on a NaCl(100) surface at high deposition rates

The results of experimental studies on pulsed laser deposition of the Au atoms on the NaCl(100) surface are reported. Under the conditions far from thermodynamic equilibrium, the spatial ordering was observed in an ensemble of gold clusters at the initial deposition stage. A mechanism based on the idea of fractal structure decomposition caused by the development of unstable concentration modes in the condensed substance is proposed.