Cluster Growth Process Research Articles

Formation of clusters composed ofC60 molecules via self-assembly in critical fluids

Fullerenes are promising candidates for intelligent, functional nanomaterials because oftheir unique mechanical, electronic and chemical properties. However, it is necessary toinvent some efficient but relatively simple methods of producing structures composed offullerenes for the development of nanomechatronic, nanoelectronic and biochemical devicesand sensors. In this paper, we show that various structures such as straight fibres,networks formed by fibres, wide sheets and helical structures, which are composed ofC60 molecules, arecreated by placing C60-crystals in critical ethane, carbon dioxide and xenon even thoughC60 moleculesdo not dissolve or disperse in the above fluids. It is supposed, judging by the intermolecular potentialsbetween C60 and C60,between C60 and ethane, and between ethane and ethane, thatC60-clusters grow with the assistance of solvent molecules, which are trapped betweenC60 molecules under critical conditions. This room-temperature self-assembly cluster growthprocess in critical fluids may open up a new methodology of forming structures builtup with fullerenes without the need for any ultra-fine processing technologies.

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.

Electron diffraction study of two-component clusters Ar–Kr: Features of the nucleation, growth mechanisms, and structural states

The structure and growth processes of clusters formed in adiabatically expanding supersonic jets of gaseous mixtures of rare gases are investigated for the first time. Ar–Kr mixtures containing 0.1, 0.5, 2.5, and 20% Kr atoms are studied at temperatures and pressures of the gaseous mixtures at the nozzle entrance T0=100 and 120K and P0=0.15–2.5atm. The experiments give a number of results that shed light on the picture of cluster formation in Ar–Kr gaseous mixtures. It is established that the mechanisms of cluster formation depend substantially on the krypton impurity concentration and the temperature of the gaseous mixture at the nozzle entrance. The evolution of the character of the cluster formation, from homogeneous to heterogeneous, is traced. The intensification of rare-gas cluster growth by an impurity is revealed for the first time, and the concentration dependence of the transition from a process of formation of solid solutions to the formation of clusters containing only impurity krypton is traced.

Sensitivity of Water Condensation in a Supersonic Plume to the Nucleation Rate

The direct simulation Monte Carlo (DSMC) method has recently been developed to simulate homogeneous water condensation in a free expansion rocket plume. A nucleation rate, based on the classical nucleation theory (CNT), was used in the DSMC simulation to predict initial nuclei in the condensation region. However, recent experimental research suggests that the CNT nucleation rate should be corrected and the magnitude of the correction is a function of the vapor temperature. Because the nucleation rate is one of the most important factors that impacts the accuracy of the numerical simulation results for a condensation plume, the impact is investigated of the corrected nucleation rate on cluster growth processes and flow macroparameters in an expanding flow using the DSMC method to model both the gas and condensate flow.

Nucleation and growth of defect clusters in CeO2 irradiated with electrons

Nucleation and growth process of defect clusters in cerium dioxide (CeO2) with fluorite-type crystal structure has been investigated in situ under electron irradiation by using high voltage transmission electron microscopy. Planar defect clusters were formed with electron irradiation ranging from 200 to 1000keV at temperatures below 450K. The defect clusters were determined to be faulted-interstitial type dislocation loops lying on {111} planes. The growth rate of dislocation loops was found to increase with decreasing electron energy. An analysis of the fluence dependence of the growth process of dislocation loops suggests an increase in the vacancy mobility with decreasing electron energy. The rate of the electronic excitation is discussed in terms of the radiation-induced diffusion of oxygen-ion vacancies.

Cluster growth and dissolution of fullerenes in non-polar solvents

The processes of fullerene cluster formation and growth in non-polar solvents are described basing on the numerical solutions of kinetic equations of nucleation theory. The liquid drop model and the limited growth model are analyzed. For supersaturated solutions it is shown, that the model of limited growth is an adequate model and can qualitatively describe the experimental data. The model of fullerene dissolution processes is proposed.

Multifractal and percolation study on thin film growth with a cell-automata model

A new cell-automata model describing the growth process of cluster by an infection probability is introduced and investigated with Monte Carlo method. The multifractal property and the percolation property are included in this model. Analysis on the surface morphology shows interesting multifractal behavior in great agreement with the experiment AFM images, indicating that this model can simulate a series of film dynamic growth. A novel form of percolation-related phase transition other than power law in traditional scaling theory is also showed by introducing a simple grand canonical ensemble partition function Z ( P ) .

Simulation of the Embryonic Stage of ZnS Formation from Aqueous Solution

We investigate the processes of cluster formation and growth of ZnS from aqueous solution using molecular dynamics simulation techniques. The influence of both temperature and concentration is studied. We show that, at lower temperatures, the crucial process is the transformation of an outer-sphere Zn/S complex to an inner-sphere ion pair. Further growth of the latter is fast to generate negatively charged planar clusters. These clusters interact to form more stable, closed structures, which are found to be the global minima configurations in vacuo. At higher temperatures, no outer-sphere ion pairs are formed, and the larger cluster configurations form much more quickly.

IR Spectroscopy of Microsolvated Aromatic Cluster Ions: Ionization-Induced Switch in Aromatic Molecule–Solvent Recognition

Abstract IR spectroscopy, mass spectrometry, and quantum chemical calculations are employed to characterize the intermolecular interaction of a variety of aromatic cations (A+) with several types of solvents. For this purpose, isolated ionic complexes of the type A+–L n , in which A+ is microsolvated by a controlled number (n) of ligands (L), are prepared in a supersonic plasma expansion, and their spectra are obtained by IR photodissociation (IRPD) spectroscopy in a tandem mass spectrometer. Two prototypes of aromatic ion–solvent recognition are considered: (i) microsolvation of acidic aromatic cations in a nonpolar hydrophobic solvent and (ii) microsolvation of bare aromatic hydrocarbon cations in a polar hydrophilic solvent. The analysis of the IRPD spectra of A+–L dimers provides detailed information about the intermolecular interaction between the aromatic ion and the neutral solvent, such as ion–ligand binding energies, the competition between different intermolecular binding motifs (H-bonds, π-bonds, charge–dipole bonds), and its dependence on chemical properties of both the A+ cation and the solvent type L. IRPD spectra of larger A+–L n clusters yield detailed insight into the cluster growth process, including the formation of structural isomers, the competition between ion–solvent and solvent–solvent interactions, and the degree of (non)cooperativity of the intermolecular interactions as a function of solvent type and degree of solvation. The systematic A+–L n cluster studies are shown to reveal valuable new information about fundamental chemical properties of the bare A+ cation, such as proton affinity, acidity, and reactivity. Because of the additional attraction arising from the excess charge, the interaction in the A+–L n cation clusters differs largely from that in the corresponding neutral A–L n clusters with respect to both the interaction strength and the most stable structure, implying in most cases an ionization-induced switch in the preferred aromatic molecule–solvent recognition motif. This process causes severe limitations for the spectroscopic characterization of ion–ligand complexes using popular photoionization techniques, due to the restrictions imposed by the Franck–Condon principle. The present study circumvents these limitations by employing an electron impact cluster ion source for A+–L n generation, which generates predominantly the most stable isomer of a given cluster ion independent of its geometry.

Cluster Relations in the Media Industry: Exploring the 'Distanced Neighbour' Paradox in Leipzig

Bathelt H. (2005) Cluster relations in the media industry: exploring the 'distanced neighbour' paradox in Leipzig, Regional Studies39, 105-127. This paper uses a multidimensional cluster concept that views processes of knowledge creation as being decisive in explaining how clusters are established, why they grow and how they reproduce themselves. It is suggested that clusters can only create new knowledge and continue to grow if the cluster firms have linkages with external markets and employ a mix of local and non-local transactions. Local interaction or 'buzz' and interaction through global or trans-local 'pipelines' create a dynamic process of knowledge creation which is the key to understanding a cluster's growth process. It is argued that the lack of a reflexive mechanism of local and trans-local interaction is the reason as to why the Leipzig media industry cluster has stagnated in recent years after a decade of substantial growth. In this cluster, we are confronted with what is referred to as the '...

Molecular dynamics study on self-assembly of C36 clusters

A tentative plan to prepare new clusters by self-assembly of C36 clusters in gas-phase is given in this paper. The growth process of the carbon clusters under a real He atmosphere is simulated by molecular dynamics method. We find that the ambient temperature is the key factor to affect the final structures of the n ew clusters: from 1000 to 2000 K, the new clusters keep a pod shape after self-assembly of C36 clusters; and above 2000 K, the structures of the final clusters change into a ball shape.

C123 レーザー照射による炭素クラスター生成過程に関する分子動力学的研究(OS-11 ナノスケール伝熱II)

The emission process of high temperature, high density carbon atoms into cold buffer gas is simulated by using the molecular dynamics method in order to investigate the formation process of carbon clusters under laser irradiation. The slag-like carbon material are initially decomposed from C_n (n>20) mostly into very small clusters C_n (n=2〜4) atoms, and the re-growth growth process of these small cluster into middle sized clusters of C_n (n=6〜10) follows afterwards. In the case of high pressure buffer gas condition, the second process is enhanced due to the trapping effect.

A new 19-metal-atom cluster [(Me2PhP)10Au12Ag7(NO3)9] with a nearly staggered–staggered M5 ring configuration

New mixed metal clusters with M19 metal frameworks have been synthesized by NaBH4 reduction of Au(NO3)(PMe2Ph) together with AgNO3 in ethanol. Single crystal X-ray diffraction has revealed Au12Ag7 and Au17Ag2 metal skeletons for these clusters, which are best described in terms of bicapped pentagonal antiprismatic cages with a staggered-staggered M(5) ring configuration. These clusters connect the missing link between M13 icosahedral and M25 biicosahedral clusters providing a view of the cluster growth process. A TEM image of this cluster has been observed, which has clearly demonstrated single-sized nano-particles of less than 1.0 nm.

On the Titanium Oxide Neutral Cluster Distribution in the Gas Phase: Detection through 118 nm Single-Photon and 193 nm Multiphoton Ionization

Titanium oxide clusters are generated in a supersonic expansion by laser ablation of the metal and reaction with oxygen (0.1-6%) in He expansion gas. Mass spectra of the titanium oxide clusters are observed by photoionization with lasers of three different wavelengths: 118, 193, and 355 nm. Only the 118 nm (10.5 eV) light can ionize Ti(m)O(n) neutral clusters without fragmentation. Both the 193 nm (6.4 eV) and 355 nm (3.5 eV) multiphoton ionization cause fragmentation of the neutral clusters during the ionization process and, thus, can complicate the determination of the stable neutral Ti(m)O(n) gas-phase species. Employing 118 nm single-photon ionization and line-width data, the Ti(m)O(2m) and Ti(m)O(2m+1) series are found to be the most stable neutral cluster species for high oxygen content in the expansion gas. Fragmentation during the multiphoton ionization process for 193 nm light yields the cluster ions Ti(m)O(2m-1,-2)+. These ions are formed by the loss of one or two oxygen atoms from Ti(m)O(2m,2m+1) neutral species. The dominant cluster growth process is suggested to be through the addition of TiO2 species. For low oxygen content (<2%) in the expansion gas, oxygen-deficient clusters of the form Ti(m)O(2m-1,-2) are also observed. These latter series are not fragmented by the 193 nm ionization process.

Formation of Pd dimers at regular and defect sites of the MgO(100) surface: cluster model calculations

The formation of Pd dimers on the surface of MgO has been studied by means of density functional theory (DFT) cluster model calculations. The following surface sites have been considered: regular five-coordinated anions at the (1 0 0) terraces, monoatomic steps, OH groups, and neutral vacancies (F centers). We discuss the energy balance of forming a dimer at a given site with respect to two isolated Pd atoms, one adsorbed at the defect and one on a regular terrace site. We found that all the defects considered lead to an energy gain when the dimer is formed, suggesting that they can be involved in nucleation and growth processes of metal clusters on the MgO surface. The dimerization energy is moderate for steps (≈0.8 eV), large for OH groups (≈1.3 eV) and rather small (<0.5 eV) for F centers.

Growth and structure of Ge clusters produced by Ge?C co-evaporation

Complete mixture films of carbon and germanium produced by the co-evaporation in vacuum were directly heated in a transmission electron microscope. It was noticed that the diffusion and growth of Ge dots in the mixture film take place at approximately 500°C. On the other hand, Ge dots coated with amorphous carbon were produced by simultaneous evaporation of carbon and Ge in Ar gas at 10kPa. In addition to Ge clusters of 3–10nm in size, multiple-twin Ge clusters with the size of 10nm have been produced in carbon soot. These Ge clusters showed a diamond structure. The growth process of Ge clusters in amorphous carbon soot has been discussed using the result in the mixture film experiment and temperature distribution in the particle formation process.

Growth and structure of Ge clusters produced by Ge–C co-evaporation

Complete mixture films of carbon and germanium produced by the co-evaporation in vacuum were directly heated in a transmission electron microscope. It was noticed that the diffusion and growth of Ge dots in the mixture film take place at approximately 500°C. On the other hand, Ge dots coated with amorphous carbon were produced by simultaneous evaporation of carbon and Ge in Ar gas at 10 kPa . In addition to Ge clusters of 3– 10 nm in size, multiple-twin Ge clusters with the size of 10 nm have been produced in carbon soot. These Ge clusters showed a diamond structure. The growth process of Ge clusters in amorphous carbon soot has been discussed using the result in the mixture film experiment and temperature distribution in the particle formation process.

Two-photon fluorescence-guided laser tweezers for study of cluster growth and gelation process

Laser tweezers trapping technology has been used to monitor the bulk solution viscosity during the sol-gel gelation process at different depths from an interface. The gelation rate is the same in depth ranges 2–20 μm from the bounding surface. Simultaneously with the laser tweezers study, a microviscosity kinetic measurement of the sol-gel process was performed by fluorescent anisotropy and quantum yield methods. The large differences found between the bulk and microviscosities obtained in the experiment reflect the intrinsic differences in solution environment sensed by the laser tweezers on the macro level and by other optical techniques on the microlevels.

Molecular Dynamics Simulations of Cluster Fission and Fusion Processes

Results of molecular dynamics simulations of fission reactions Na102+ → Na7+ + Na3+ and Na182+ → 2Na9+ are presented. The dependence of the fission barriers on the isomer structure of the parent cluster is analyzed. It is demonstrated that the energy necessary for removing homothetic groups of atoms from the parent cluster is largely independent of the isomer form of the parent cluster. The importance of rearrangement of the cluster structure during the fission process is elucidated. This rearrangement may include transition to another isomer state of the parent cluster before actual separation of the daughter fragments begins and/or forming a “neck” between the separating fragments. A novel algorithm for modeling the cluster growth process is described. This approach is based on dynamic search for the most stable cluster isomers and allows one to find the optimized cluster geometries, as well as their essential formation mechanisms. Cluster growth paths for Lennard–Jones clusters have been investigated for cluster sizes of up to 150 atoms. All known global minima structures of the Lennard–Jones clusters are found.

Kinetics of phase transformation on a Bethe lattice.

A kinetic Ising model is applied to the description of phase transformations on a Bethe lattice. A closed set of kinetic equations for a model with the coordination number q=3 is obtained using a procedure developed in a previous paper. For T close to Tc(T>Tc), where Tc is the phase transition temperature, and zero external field (absence of supersaturation), the rate of phase transformation (RPT) for small deviations from equilibrium is independent of time and tends to zero as (T-Tc). At T=Tc, the RPT depends on time and for large times behaves as t(-1). For T<Tc, we examine the transformation from the initial state with almost all spins "down" to the state with almost all spin "up" after the external field jumped from Bi<0 to Bf>0. The role of different mechanisms responsible for growth (decay), splitting (coagulation), and creation (annihilation) of clusters are examined separately. In all cases there is a critical value Bc of the external field, such that the phase transformation takes place only for Bf>Bc. This result is also obtained from a more simple consideration involving spherical-like clusters on a Bethe lattice. The characteristic time tR at which the polarization becomes larger than zero diverges as (Bf-Bc)(-b) for Bf-->Bc with b=0.47. The RPT has a rapid growth near tR and remains constant for t>tR. The average cluster size (number of spins in a cluster) exhibits a rapid unrestricted growth at a time td approximately tR which indicates the creation of infinite clusters. The only exception to the latter behavior occurs when the kinetics is dominated by cluster growth and decay processes. In this case, the average cluster size remains finite during the transformation process. In contrast to the classical theory, the present approach does not separate the processes of creation of clusters of critical size (nucleation) and of their growth, both being accounted for by the kinetic equations employed.