Optical extinction resonance of Au and Ag clusters formed by ion irradiation in SiO2 and Al2O3

Three methods based on ion-beam irradiation were used to fabricate Ag and Au colloids in silica and alumina. Their surface-plasmon resonance in the visible was characterised by transmittance measurements and interpreted on the basis of transmission electron microscope observations. Despite their bimodal size distribution, particles formed by ion-beam mixing of sandwich layers exhibit much narrower resonances than those obtained by ion implantation. This unusual effect of an inhomogeneity in cluster size is ascribed to the spatial organisation of these clusters. Irradiation of supersaturated solid solutions at much lower ion fluences produces colloids with more uniform size and spatial distributions, and equally strong resonances. Received: 17 March 2001 / Accepted: 31 July 2001 / Published online: 11 February 2002     

Nucleation of atmospheric aerosol particles

A significant fraction of the total number of particles present in the atmosphere is formed originally by nucleation from the gas phase. Binary nucleation of sulphuric acid and water, ternary nucleation of sulphuric acid, water and ammonia and ion-induced nucleation are thought to be the most important aerosol nucleation processes in the atmosphere. Within the last two decades, instrumentation to observe and characterize nucleation has improved greatly and numerous observations of nucleation have been made including quantification of the nucleation rate, characterization of the growth process and first chemical characterizations of the freshly formed particles. Nucleation has been observed at many different places in the atmosphere: in the boundary layer, in the free troposphere, in remote locations, in coastal areas, in boreal forests as well as urban areas and pollution plumes. In most cases gaseous sulphuric acid is assumed to be the key precursor gas. After nucleation, other supersaturated substances, especially low vapour pressure organics often take part in the subsequent aerosol growth. Iodine oxides seem to be responsible for nucleation observed in some coastal areas.Recent advances in modelling allow for a kinetic treatment of the nucleation process based on measured thermochemical data for the cluster formation. Considerable improvement over the classical nucleation treatment is expected from this approach.A detailed understanding of atmospheric aerosol nucleation processes is needed as the freshly formed particles directly influence the number concentration and size distribution of the atmospheric aerosol. The formation of clouds and precipitation is affected and influences on climate are anticipated. Anthropogenic emissions influence atmospheric aerosol nucleation processes considerably.Despite the comprehensive research efforts, substantial inconsistencies remain and conflicting results of laboratory studies, model studies as well as atmospheric observations persist. Several key questions about the predictability of atmospheric nucleation in general, about the substances, that take part in nucleation and subsequent growth and about the size and composition of the critical cluster, have not been resolved so far. To cite this article: J. Curtius, C. R. Physique 7 (2006).     

Evolution of cluster size distribution during nucleation with rapidly changing dynamic gas processes

A non-steady-state solution of the Zel’dovich equation for the cluster size distribution function has been obtained. The solution is applicable in the entire range of cluster sizes. The relaxation time of the quasi-steady-state distribution and the partial time lags of cluster formation were derived and analyzed. The dependence of these times on cluster size was investigated. Expressions for the quasi-steady-state flux and the induction times have been derived. The accuracy of these calculations is confirmed by comparison with numerical calculations by other authors.     

On the morphology of carbon nanotubes growing from catalyst particles: Formulation of the model

A model is constructed for the growth of nanotubes from metal catalyst particles supersaturated with carbon. An island of the graphene plane on the catalyst surface serves as a nucleus for the formation of nanotubes with different morphologies. The dependence of the type of nanotube nucleating from an island on the catalyst particle size and the minimum number of carbon-metal interaction parameters is determined. These parameters are calculated using the semiempirical quantum-chemical methods. The results of calculations in the framework of the proposed model are compared with the experimental data obtained for the simultaneous formation of nanotubes of several types.     

Theory of diffusive decomposition of supersaturated multicomponent systems

A closed system of equations, which describes the evolution of supersaturated multicomponent solutions, is obtained. A general solution of these equations is determined in the late stage of decomposition, which is determined by nonlinear diffusive interaction of macrodefects.The algebraic equations, which determine the coexistent phases, are obtained.The size of distribution function for particles is determined for each phase. The boundaries of the coexistent phases are defined.     

TR-LII for sizing of carbon particles forming at room temperature

Time-resolved laser-induced incandescence (TR-LII) was applied for the determination of particle sizes during carbon-particle formation from supersaturated atomic carbon vapor that was generated by laser photolysis of carbon suboxide (C₃O₂) at room temperature. Thus, the solid carbon particles were formed under hydrogen-free conditions. The TR-LII technique was used for in situ size measurement of growing carbon particles and samples of final particles were analyzed by transmission electron microscopy (TEM). It was found that the particles grow to a final size of 4–12 nm within 0.02–1 ms. The properties of the obtained particles depend on the initial conditions in the reaction volume, i.e. concentration of carbon suboxide, pressure and type of gas diluter, photolysis wavelength, and laser pulse energy. The comparison of TR-LII and TEM particle sizing results yields information about the effective thermal energy accommodation coefficients for He, Ar, CO, and C₃O₂ molecules on carbon particles. PACS 61.46.Df; 07.60.-j; 78.70.-g     

Effect of ultrasound on sodium arsenate induction time and crystallization property during solution crystallization processes

The effect of ultrasound vibrations on the cooling crystallization of sodium arsenate in supersaturated solutions was investigated. In particular, the effects of ultrasound vibrations on induction time and crystal size distribution were studied using a laser-based apparatus with relative supersaturation ranging from 1.3 to 1.8. The results show that ultrasound vibrations have a significant effect on reducing induction time and crystal size distribution. The application of ultrasound vibrations to the system resulted in a small change in surface tension; however, the induction time and crystal size significantly decreased. The mean size of sodium arsenate crystals decreased from 398.87 ± 3.27 to 168.68 ± 2.07 μm, as the ultrasound power increases from 26 to 130 W. Ultrasound vibrations significantly reduced the induction time in a highly supersaturated solution compared to that in a low supersaturated solution.     

Analytical results for the cluster size distribution in controlled deposition processes

This paper proposes a controlled particle deposition model for cluster growth on the substrate surface and then presents exact results for the cluster (island) size distribution. In the system, at every time step a fixed number of particles are injected into the system and immediately deposited onto the substrate surface. It investigates the cluster size distribution by employing the generalized rate equation approach. The results exhibit that the evolution behaviour of the system depends crucially on the details of the adsorption rate kernel. The cluster size distribution can take the Poisson distribution or the conventional scaling form in some cases, while it is of a quite complex form in other cases.     

In Situ Liquid SAXS Studies on the Early Stage of Calcium Carbonate Formation

Calcium carbonate is a model system to investigate the mechanism of solid formation by precipitation from solutions, and it is often considered in the debated classical and nonclassical nucleation mechanism. Despite the great scientific relevance of calcium carbonate in different scientific areas, little is known about the early stage of its formation. Therefore, contactless devices are designed that are capable of providing informative investigations on the early stages of the precipitation pathway of calcium carbonate in supersaturated solutions using classical scattering methods such as wide‐angle X‐ray scattering (WAXS) and small‐angle X‐ray scattering (SAXS) techniques. In particular, SAXS is exploited for investigating the size of entities formed from supersaturated solutions before the critical conditions for amorphous calcium carbonate (ACC) nucleation are attained. The saturation level is controlled and kept constant by mixing four diluted solutions (i.e., NaOH, CaCl₂, NaHCO₃, H₂O) at constant T and pH. The scattering data are collected on a liquid jet generated about 75 s after the mixing point. The data are modeled using parametric statistical models providing insight about the size distribution of denser matter in the liquid jet. Theoretical implications on the early stage of solid formation pathway are inferred.     

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.     

Surface Nucleation Kinetics on Active Centers from Supersaturated Vapor

The formation of nuclei from a supersaturated vapor on a surface with active centers is treated within standard self-consistent classical model with exhaustion of active centers. Basic characteristics of nucleation process (total number of nuclei, nucleation rate, time lag and size distribution of nuclei) are determined by numerical solution of kinetic equations. It is shown that standard approach to nucleation on active centers based on Avrami model coincides with our approach in the values of time lag of nucleation process, but it differs in the total number of nuclei.     

Precise fabrication of nanomaterials: a nonlinear dynamics approach

Modeling of the precise fabrication in the self-assembling of particles is studied using the nonlinear Langevin equation system. The numerical simulation showed a marked ordering of the particles as a function of time after some induction period. The abnormally enlarged fluctuation was found around the start of the evident ordering. After the fluctuation, a sudden increase of the cluster size was observed. The results corresponded well to the dynamics due to the formation of the critical cluster. The shape of the critical cluster around the enlarged fluctuation was not compact and showed fractal-like structures. The fluctuation of the cluster size around the formation of the critical cluster was explained by the anomalous fluctuation theorem for the generalized Langevin equation. The characterization of the stochastic dynamics of the critical clusters rationalized the concept of dynamic templating for the fabrication technique of the self-assembling of nanoparticles, that is, the structural constraint on the particle assembly by externally adding the resonance frequencies that match with the localized nonlinear vibrational modes of the target structures originating from thermal (Brownian) activation.     

Check of the scaling procedure of track structures of ionizing radiation in nanometric volumes

In nanodosimetry, the track structure of ionizing radiation is characterized by the probability distribution of the number of ionizations produced by an ionizing particle in a target volume. By Monte-Carlo simulations of the track structure, this so called ionization cluster size distribution can be determined for nanometric cylindrical water targets used as substitute for the radiation sensitive biological target, namely a DNA-segment of about 20 base pairs length. Measurements of ionization cluster size distributions are carried out in an ion-counting nanodosimeter filled with an appropriate working gas at low pressure. A scaling procedure was proposed by Grosswendt (2006) to derive an operating pressure depending on the working gas for which the measured ionization cluster size distribution becomes equivalent to the ionization cluster size distribution within a nanometric target of liquid water. This scaling procedure was previously tested by means of Monte-Carlo simulations and is now tested experimentally.Ionization cluster size distributions produced by mono-energetic proton and alpha particle beams in the energy range between 0.1 MeV and 20 MeV were measured at the accelerator facilities of the PTB. The working gases used in this experiment were C₃H₈ and N₂. According to the NIST databases for stopping power and mean ionization energy (NIST, 2009), equivalent cluster size distributions for protons and alpha particles should be obtained for pressures of 0.25 mbar C₃H₈ and 1.2 mbar N₂. Measurements reveal the best agreement with pressures of 0.425 mbar C₃H₈ and 1.2 mbar N₂ for protons and 0.46 mbar C₃H₈ and 1.2 mbar N₂ for alpha particles.     

Kinetic description of the Coulomb explosion of a spherically symmetric cluster

The particle distribution function is calculated for the Coulomb explosion of a spherically symmetric charged cluster formed through the interaction of intense ultrashort laser pulses with a cluster gas. The particle density and mean velocity distributions as well as the energy spectra of the accelerated particles are obtained. These characteristics are analyzed in detail for a cold cluster plasma, where the kinetic effects determine the physics of multiple flows emerging after the turnover of the cluster particle velocity profile. We find the boundaries of the multiple-flow regions and study the characteristics of an exploding cluster as a function of its initial density profile. The energy spectra of the accelerated ions are obtained for a cluster plasma with a specified cluster size distribution.     

On aggregate structures in a rod-like haematite particle suspension by means of Brownian dynamics simulations

We have investigated aggregation phenomena in a suspension composed of rod-like haematite particles by means of Brownian dynamics simulations. The magnetic moment of the haematite particles lies normal to the particle axis direction and therefore the present Brownian dynamics method takes into account the spin rotational Brownian motion about the particle axis. We have investigated the influence of the magnetic particle–field and particle–particle interactions, the shear rate and the volumetric fraction of particles on the particle aggregation phenomena. Snapshots of aggregate structures are used for a qualitative discussion and the cluster size distribution, radial distribution function and the orientational correlation functions of the direction of particle axis and magnetic moment are the focus for a quantitative discussion. The significant formation of raft-like clusters is found to occur at a magnetic particle–particle interaction strength much larger than that required for a magnetic spherical particle suspension. This is because the rotational Brownian motion has a significant influence on the formation of clusters in a suspension of rod-like particles with a large aspect ratio. An applied magnetic field enhances the formation of raft-like clusters. A shear flow does not have a significant influence on the internal structure of the clusters, but influences the cluster size distribution of the raft-like clusters.     

Colloidal Analysis of Particles Extracted from Microalloyed Steels

Different colloidal particle characterization methods are examined for their suitability to determine the particle size distribution of particles extracted from steels. Microalloyed steels are dissolved to extract niobium and titanium carbonitride particles that are important for the mechanical properties of these steels. Such particles have sizes ranging from several nanometers to hundreds of nanometers depending on the precipitation stage during the thermomechanically controlled rolling process. The size distribution of the particles is analyzed by dynamic light scattering (DLS), analytical ultracentrifugation (AUC), and hollow fiber flow field-flow fractionation (HF5) and compared to data obtained for reference particles as well as data from electron microscopy, the standard sizing technique used in metallurgy today. AUC and HF5 provide high-quality size distributions, average over large particle numbers that enables statistical analysis, and yield useful insights for alloy design; however, DLS fails due to a lack of resolution. Important aspects in the conversion and comparison of size distributions obtained for broadly distributed particle systems with different measurement principles and the role of surfactants used in sample preparation are discussed.     

Order-disorder transitions in atactic polystyrene films

The results of the electron microscopy investigation of the surface topology of the films obtained from solutions of linear atactic polystyrene in chloroform are presented. It has been shown that the distribution of density fluctuations in the films can be described using the model of a fractal percolation cluster of macromolecular coils. A decrease in the local packing density of particles upon going from θ-coils to blobs is associated with mutual penetration of the coils. An increase in density fluctuations and a decrease in the relative area and fractal surface of the cluster of the particles are associated with a decrease in the short-range order caused by the formation of the percolation cluster, which reflects portions of the chains not involved in the blobs.     

Interatomic potential effects on dynamical heterogeneities in liquid SiO2

Dynamical heterogeneities (DH) in low density liquid SiO₂ have been investigated by molecular dynamics (MD) method. Simulations were done in the basic cube under periodic boundary conditions containing 3000 particles with the pair interatomic potentials, which have a weak electrostatic interaction and a Morse type short range interaction (PMSI). We have evaluated the non-Gaussian parameter for the self part of the van Hove correlation function and we found a clear evidence of the existence of DH in low density liquid SiO₂. Moreover, the atomic displacement distribution (ADD) in a model has been obtained and it deviates from a Gaussian form. The results have been compared with those obtained in another liquid SiO₂ system with the Born-Mayer interatomic potentials (BMP) in order to observe the interatomic potential effects on the DH in the system and indeed, the effects are strong. Calculations showed that particles of extremely low or fast mobility have a tendency to form a cluster and mean cluster size of most mobile and immobile particles in PMSI models increases with decreasing temperature. In contrast, no systematic changes have been obtained for the most mobile and immobile particles in BMP models. Calculations show that there is no relation between local particle environment and particle mobility in the system.     

Characterization of TiO2 Smoke Prepared Using Gas‐Phase Hydrolysis of TiCl4

The formation of submicron TiO₂ smoke (a gas‐phase suspension) from titanium tetrachloride in a low‐pressure hydrolysis reaction in a simple reactor configuration has been studied. Particle size distribution, particle morphology and degree of crystallinity have been characterized as a function of reaction conditions. Highly crystalline anatase TiO₂ particles with narrow size distribution and smaller particle size were formed at high reactor temperature, while larger, amorphous particles were found at lower reactor temperatures. These results are consistent with literature studies. At 817 °C, small (450 nm), spherical, unagglomerated particles could be produced. A gas‐phase dispersion of these particles is intended for use as seeds in subsequent kinetic studies of titanium dioxide formation reactions involving a rapid compression methodology.