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.     

Physical properties of silver-coated silica clusters: A computer experiment

The effect on the physical properties of a silica cluster after silver coating has been studied by the molecular dynamics method. It has been established that silver atoms make a sphere-like cluster shape, increase mechanical stability and give a stable positive value of the surface tension. They increase the number of electrons able to interact with the electromagnetic field, and, hence, enhance the efficiency of thermal radiation transfer by clusters.     

Kinetics and energy states of nanoclusters in the initial stage of homogeneous condensation at high supersaturation degrees

The condensation of metal vapor in an inert gas is studied by the molecular dynamics method. Two condensation regimes are investigated: with maintenance of partial pressure of the metal vapor and with a fixed number of metal atoms in the system. The main focus is the study of the cluster energy distribution over the degrees of freedom and mechanisms of the establishment of thermal equilibrium. It is shown that the internal temperature of a cluster considerably exceeds the buffer gas temperature and the thermal balance is established for a time considerably exceeding the nucleation time. It is found that, when the metal vapor concentration exceeds 0.1 of the argon concentration, the growth of clusters with the highest possible internal energy occurs, the condensation rate being determined only by the rate of heat removal from clusters.     

Influence of burrowing the atoms on the density of the Fe and Co nanoclusters on the Cu(100) surface

The dependence of the density of the Co and Fe clusters on the Cu(100) surface on the substrate temperature and the deposition flux of the atoms has been studied by the molecular dynamics method and the kinetic Monte Carlo method. It has been shown that burrowing the impurity atoms into the first substrate layer can considerably increase the density of nanoclusters. The influence of burrowing the atoms on the size distribution of nanoclusters has been investigated.     

Structure and thermal properties of supported catalyst clusters for single-walled carbon nanotube growth

Structure and thermal properties of supported iron clusters were studied using molecular dynamics simulations. When supported clusters are in the liquid state, their surfaces have spherical curvature, whereas solid clusters form a layered crystalline structure. The cluster freezing (melting) point increases dramatically with increasing cluster-substrate interaction strength, and rapid diffusion of cluster surface atoms is observed below the freezing point.     

Inhomogeneity of local temperature in small clusters in microcanonical equilibrium

Overall homogeneity of temperature is a condition for thermal equilibrium, but, as is demonstrated by classical molecular dynamics simulations, the local temperatures of atoms in small, isolated crystalline clusters in microcanonical equilibrium are not uniform. The effective temperature determined from individual atomic velocity decreases with distance from the cluster center. It is argued that these effects are due to the conservation of angular and translational momentum. A general microcanonical expression is derived for the spatial dependence of the statistics of the kinetic energies of individual atoms; this fits the numerical observations well.     

Small rare gas clusters in XUV laser pulses

Semi-classical molecular dynamics simulations of small rare gas clusters in short laser pulses of 100 nm wavelength were performed. For comparison, the cluster response to 800 nm laser pulses was investigated as well. The inner ionization dynamics of multi-electron atoms inside the cluster was treated explicitly. The simulation results underpin the belief that at XUV (extreme ultraviolet) wavelengths collisions play an important role in the energy absorption. The generation of the surprisingly high charge states of Xe atoms inside clusters, as they were observed in a free-electron laser experiment at DESY, Hamburg, Germany by Wabnitz et al. is due to the reduced ionization potential of atoms inside charged clusters, the ionization ignition mechanism, and collisions. PACS 36.40.Gk; 36.40.Vz; 31.15.Gy     

Formation of argon cluster with proton seeding

ABSTRACT

We employ force-field molecular dynamics simulations to investigate the kinetics of nucleation to new liquid or solid phases in a dense gas of particles, seeded with ions. We use precise atomic pair interactions, with physically correct long-range behaviour, between argon atoms and protons. Time dependence of molecular cluster formation is analysed at different proton concentration, temperature and argon gas density. The modified phase transitions with proton seeding of the argon gas are identified and analysed. The seeding of the gas enhances the formation of nano-size atomic clusters and their aggregation. The strong attraction between protons and bath gas atoms stabilises large nano-clusters and the critical temperature for evaporation. An analytical model is proposed to describe the stability of argon-proton droplets and is compared with the molecular dynamics simulations.     

Nucleation of catalyst clusters during the growth of carbon nanotubes

The formation of cluster in the course of pyrolysis of ferrocene is studied experimentally and theoretically, and the parameters of the clusters are determined. The thermodynamics of homogeneous formation of catalyst clusters is developed. It is shown that the surface tension and sizes of the clusters substantially depend on the temperature and the properties of the substrate. Surface tension coefficients, as well as the characteristic sizes of clusters, are calculated. The resultant dependences and parameters make it possible to reconstruct the size distribution function for the clusters growing on a certain substrate by setting the temperature in the working zone. This permits the prediction of the cluster size depending on the synthesis conditions.     

Electron spectroscopy of objects for nanoelectronics

An electron-spectroscopic analysis is made of layered nanostructures and clusters at the surface and in the bulk of a solid. A new method of forming metal/insulator/semiconductor (superconductor) nanostructures is proposed based on ion-stimulated metal segregation effects at the surface of low-temperature gallium arsenide and a 123 high-temperature superconductor. The geometric parameters and electronic structure of these nano-objects are studied. It is shown that their electronic properties can be controllably varied in situ by acting on the surface. The dimensional transformation of the electronic properties of metal clusters is studied for clusters in the insulator SiO₂, in the superconductor LTMBE-GaAs, and on silicon and graphite surfaces. The nature of this transformation is clarified. A diagnostics for cluster ensembles is developed by which one can determine the parameters needed to describe singleelectron transport: the average number of atoms per cluster, the average distance between clusters and isolated atoms, and the chemical state of the atoms. Ensembles of silver clusters with specified parameters are obtained on a silicon surface. It is shown that these ensembles are potentially useful for developing single-electron devices. Zh. Tekh. Fiz. 69, 85–89 (September 1999)     

Ag-Cu双金属团簇中Ag原子偏析行为的 分子动力学研究

采用了恒温分子动力学方法系统模拟研究了不同尺寸不同组分的Ag-Cu双金属团簇的退火过程。分析低温退火结构可得团簇中Ag原子的偏析行为：在Ag原子所占比率较少时,Ag原子全部占据在团簇表面;随着Ag原子数的增多,直到Ag和Cu的比率接近时,绝大多数Ag原子仍占据在团簇表面;这与实验观测Ag-Cu混合团簇中Ag原子的偏析行为完全一致。通过细致研究Ag-Cu双金属团簇中Ag原子的偏析行为与团簇尺寸、组分和温度的关系发现：当Ag原子所占比重明显少于Cu原子时,在各不同尺寸下,Ag原子偏析温度点均高于熔点,即在熔点以上一定温度范围内仍会出现Ag原子的偏析现象;当Ag原子所占比重明显多于Cu原子时,在各不同尺寸下,体系偏析温度点均低于团簇熔点;对于较大尺寸团簇,在Ag原子所占比重接近于Cu原子时,体系偏析温度点与团簇熔点相同。     

The effect of thermal treatment on the organization of copper and nickel nanoclusters synthesized from the gas phase

The condensation of 85000 Cu or Ni atoms from the high-temperature gas phase has been simulated by molecular dynamics with the tight binding potential. The efect of the subsequent thermal treatment on the shape and structure of synthesized particles was studied by simulating their gradual heating in a range of 100–1200 K. Some tendencies are revealed that are characteristic of the influence of heat treatment on the nanoparticles synthesized from the gas phase. It is concluded that short-term heating leads to significant ordering of the internal structure in 70% of agglomerated nanoparticles with the predominant formation of spherical shapes. In order to explain this result, the main mechanisms of cluster formation from the gas phase have been analyzed and it is found that the agglomeration temperature plays the main role in the formation of clusters with unified shape and structure. This opens the fundamental possibility of obtaining Cu and Ni nanoclusters with preset size, shape, and structure and, hence, predictable physical properties.     

Molecular dynamics analysis of the substrate temperature effect on thermomechanical characteristics of vapor deposited nanofilms

Modern engineering applications are in need for technologies of nanostructures and nanofilms with controllable properties. The detection of these structures requires methods of atomic research, among which are molecular dynamics techniques, Monte-Carlo simulation, and ab initio calculation. The most efficient method to deal with systems of about several thousands of atoms is molecular dynamics simulation. We used this method to analyze the formation of nanolayers on a Cu substrate in vapor deposition of Cu atoms. It is shown that the film deposited on the substrate surface replicates the crystalline structure of the substrate. It is found that at low deposition temperatures, the deposited layer reveals a great quantity of vacancies and vacancy clusters (nanopores). It is demonstrated that increasing the substrate temperature in metal vapor deposition ensures a more perfect lattice in the nanocoating, and the cohesive energy of atoms in the nanolayer thus approximates experimental values. It is also found that the increase in substrate temperature in the process causes Young’s modulus and elastic limit to tend to the values of a perfect crystal.     

Surface tensions and stress tensors of liquid and solid clusters by molecular dynamics

Surface tension and pressure (stress) tensors of Lennard-Jones clusters, in the size range 200 ~ 2700 atoms/cluster, formed from evaporating liquid droplets were calculated in a Molecular Dynamics simulation. Icosahedral clusters have a much larger surface tension than decahedral, fcc, and hcp ones, meanwhile asymmetric icosahedral clusters have a lower surface tension. Fcc and hcp clusters have a very small surface tension. Decahedral clusters have a surface tension closer to that of fcc and hcp ones than to that of icosahedral ones, though both icosahedral and decahedral structures have five fold symmetry axis. Binary component clusters have a higher surface tension than single component ones.     

Structural transitions in small nickel clusters

The processes of a thermal impact on Ni nanoclusters with a radius of up to 0.8 nm have been studied by means of molecular dynamics with the use of a tight-binding potential. The simulation indicates that the structural transition from the initial fcc phase to the icosahedral modification occurs under the influence of temperature. The transition temperature is shifted towards the cluster melting temperature with an increase in the cluster size. A similar behavior has been observed for copper and gold nanoparticles. A conclusion has been drawn that 200–250 atoms is presumably the limiting size of a metallic cluster, below which the initial fcc modification cannot be kept under realistic industrial conditions. The adequacy of the results is checked in the computer experiments with Lennard-Jones nanoparticles. The results for the Lennard-Jones and metallic nanoparticles have been shown to agree with each other.     

An investigation of the internal temperature dependence of Pd-Pt cluster beam deposition: A molecular dynamics study

We investigated the internal temperature dependence of the Pd_1−aPt_a cluster beam deposition in the present study via the molecular dynamics simulations of soft-landing. By analysis of the velocity distribution and diffusion coefficient of the bimetallic cluster, Pd atoms with better mobility improved the diffusibility of Pt atoms. The radial composition distribution showed that a Pt-core/Pd-shell structure of the cluster formed at high internal temperatures through migrations of the Pd atoms from inner to surface shells. In the soft-landing process, the diffusing and epitaxial behaviors of the deposited clusters mainly depended on the internal temperature because the incident energy of the cluster was very small. By depositing clusters at high internal temperatures, we obtained a thin film of good epitaxial growth as the energetic cluster impact. Furthermore, nonepitaxial configurations such as scattered nonepitaxial atoms, misoriented particles, and grain boundaries of (1 1 1) planes were produced in the growth of the cluster-assembled film. As the size of the incident cluster increased, the internal temperature of the cluster needed for better interfacial diffusion and contact epitaxy on the substrate also rose.     

Atomic mechanism of glass formation in supercooled monatomic liquids

Atomic mechanism of glass formation in supercooled monatomic liquids is monitored via analyzing the spatial arrangement of solid-like atoms. The supercooled states are obtained by cooling from the melt using molecular dynamics (MD) simulation. Solid-like atoms, detected via Lindemann-like freezing criterion, are found throughout the liquid. Their number increases with decreasing temperature and they form clusters. In the deeply supercooled region, all solid-like atoms form a single percolation cluster which spans throughout the system. The number of atoms in this cluster increases steeply with further cooling. Glass formation in supercooled liquids occurs when a single percolation cluster of solid-like atoms involves the majority of atoms in the system to form a relatively rigid glassy solid. By analyzing the temperature dependence of static and dynamic properties, we identify three characteristic temperatures of glass formation in supercooled liquids including the Vogel–Fulcher temperature.     

A gas cluster ion beam accelerator

An assembled CO₂ gas cluster ion beam system was assessed using a retarding field analyzer and a time-of-flight mass spectrometer. The CO₂ gas was expanded to form gas clusters at the input pressure of 1–5 bar through a quartz Laval nozzle. At 4 bar, it is confirmed that the clusters consisted of about 500 molecules. Also the dependence of the mean cluster size distribution on source temperature was examined. At the low fluence of ion beam, an isolated gas cluster ion impact on solid surfaces was investigated. CO₂ gas cluster ions were irradiated at the acceleration voltage of 40–60 kV on highly oriented pyrolytic graphite. Si with native oxide layers, and Cu film deposited on Si wafer. After very short exposure of cluster ions, induced hillocks with about 0.8–1 nm in height and 20 nm in width were outgrown from the impacted surfaces. After prolonged irradiation on Si and Cu/Si, humping was more developed and consequently the surface morphology seemed to be saturated because of gradual filling the gap between the hillocks.     

Blue-shifted plasmon resonance of individual size-selected gold nanoparticles

The future implementation of integrated photonic devices requires the creation of nanostructures with well defined morphological and optical properties. To this end, we deposited size-selected gold nanoparticles produced by a gas phase aggregation cluster source on transparent substrates at room temperature with controlled impact energy. Interferometric optical detection measurements using a supercontinuum laser source demonstrated a blue-shifted plasmon resonance at the single particle level. The blue shift was observed to be more pronounced for small single clusters down to 3 nm in size.