Many-particle charged-particle plasma simulations using spatial meshes for the electromagnetic field solutions, particle-in-cell (PIC) merged with Monte Carlo collision (MCC) calculations, are coming into wide use for application to partially ionized gases. The author emphasizes the development of PIC computer experiments since the 1950s starting with one-dimensional (1-D) charged-sheet models, the addition of the mesh, and fast direct Poisson equation solvers for 2-D and 3-D. Details are provided for adding the collisions between the charged particles and neutral atoms. The result is many-particle simulations with many of the features met in low-temperature collision plasmas; for example, with applications to plasma-assisted materials processing, but also related to warmer plasmas at the edges of magnetized fusion plasmas. >

Particle-in-Cell Charged Particle Simulations, plus Monte Carlo Collisions with Neutral Atoms, PIC-MCC

Macroscopic organic aggregates, which are >500 µm and known as marine and lake snow, are important components in the turnover, decomposition and sinking flux of both organic and inorganic matter and elements in aquatic ecosystems. They are composed of various organic and inorganic materials depending largely on the given system and environmental conditions. The sys- tems include the pelagic limnetic, the neritic and oceanic marine region, as well as shallow turbid environments, e.g. rivers, the littoral zone of lakes, estuaries and tidally affected coastal areas with intense turbulence and a high load of suspended matter. Aggregate abundance and size vary greatly among these systems. Macroaggregates are heavily colonized by bacteria and other heterotrophic microbes and greatly enriched in organic and inorganic nutrients as compared to the surrounding water. During the last 15 yr, many studies have been carried out to examine various aspects of the formation of aggregates, their microbial colonization and decomposition, nutrient recycling and their significance for the sinking flux. They have been identified as hot-spots of the microbial decomposi- tion of organic matter. Further, microaggregates, which are <5 to 500 µm in size and stained by different dyes, such as transparent exopolymer particles (TEP) and Coomassie blue-stained particles, have been discovered and shown also to be important in the formation and decomposition of macroaggregates. In this review we give an overview of the present state of the microbial ecology of macro- and microaggregates, including the mentioned points but highlighting in particular the recent findings on the bacterial colonization of aggregates using molecular tools, their microbial decomposition and mineralization, and the significance of protozoans and metazoans for the colo- nization and decomposition of macroaggregates. Today it is evident that not only the aggregates but also their surroundings are sites and hot-spots of microbial processes, with the plume of solutes leak- ing out of the aggregates and greatly extending the volume of the intense decomposition processes. This microheterogeneity has important implications for the spatial and temporal dynamics of the organic-matter field in aquatic ecosystems and for our understanding of how heterotrophic organisms are involved in the decomposition of organic matter. The significance of aggregate-associated micro- bial processes as key processes and also for the overall decomposition and flux of organic matter- varies greatly among the various systems, and is greatly affected by the total amount of suspended particulate matter. A conclusion from the presented studies and results is that the significance of bac- teria for the formation and decomposition of aggregates appears to be much greater than previously estimated. For a better understanding of the functioning of aquatic ecosystems it is of great impor- tance to include aggregate-associated processes in ecosystem modeling approaches.

/pdf/microbial-ecology-of-organic-aggregates-in-aquatic-1kw4w22q3c.pdf

Microbial ecology of organic aggregates in aquatic ecosystems

Localized surface plasmon resonance (LSPR) in semiconductor nanocrystals (NCs) that results in resonant absorption, scattering, and near field enhancement around the NC can be tuned across a wide optical spectral range from visible to far-infrared by synthetically varying doping level, and post synthetically via chemical oxidation and reduction, photochemical control, and electrochemical control In this review, we will discuss the fundamental electromagnetic dynamics governing light matter interaction in plasmonic semiconductor NCs and the realization of various distinctive physical properties made possible by the advancement of colloidal synthesis routes to such NCs Here, we will illustrate how free carrier dielectric properties are induced in various semiconductor materials including metal oxides, metal chalcogenides, metal nitrides, silicon, and other materials We will highlight the applicability and limitations of the Drude model as applied to semiconductors considering the complex band structures 

Localized Surface Plasmon Resonance in Semiconductor Nanocrystals

A Coulomb crystal was successfully formed as a result of the growth of spherical and monodisperse carbon particles suspended in a methane plasma. The crystal structure was confirmed to be hexagonal from top- and side-view photographs. The particle growth was monitored by Mie-scattering ellipsometry and correlated with the formation process of the Coulomb crystal. The liquid-to-solid phase transition occurred when particle diameter grew to 1.3 µm, and when the Wigner-Seitz radius was about 90 µm.

https://iopscience.iop.org/article/10.1143/JJAP.33.L804/pdf

Observation of Coulomb-Crystal Formation from Carbon Particles Grown in a Methane Plasma

Experimental determination of the charge on dust particles forming Coulomb lattices

The generation and behavior of particles in a low‐pressure silane‐argon discharge have been analyzed under continuous and pulsed radio‐frequency (rf) excitation conditions. In the continuous rf excitation regime, the influence of parameters such as gas temperature and silane partial pressure are determined. By using rf pulsed excitation, it is shown that gas‐flow effects play a predominant role for particle dynamics when the excitation is stopped. Radio‐frequency regimes with short and adjustable rf off sequences are used to study both the inhibition of particle formation and the elimination of particles from the dusty plasmas. The electrical properties of the discharge are shown to be sensitive to the presence of the particles. Simple models for particle trapping in the plasma edge and for particle dynamics when the discharge is turned off are presented.

Particle generation and behavior in a silane‐argon low‐pressure discharge under continuous or pulsed radio‐frequency excitation

The time evolution of the size of particles produced in a silane‐argon low‐pressure radio‐frequency discharge has been determined in the nanometer to the micrometer range using transmission electron microscopy Highly ‘‘monodisperse’’ distributions are observed and their temporal evolution is followed These studies are combined with laser light scattering measurements to obtain particle densities, which are of the order of 107 cm−3 For short plasma durations (Δt<5 s), a fast (10 nm/s) linear increase in diameter is measured

Measurements of particle size kinetics from nanometer to micrometer scale in a low‐pressure argon‐silane radio‐frequency discharge

Self-consistent fluid and particle-in-cell Monte Carlo (PIC-MC) models in conjunction with experimental measurements have been used to study the electrical characteristics of rf discharges in pure argon and in argon contaminated with dust particles (at 0.11 torr argon, 13.56 MHz). In the experiments, the dust particles were first created in a silane-argon rf discharge and subsequently trapped in an argon discharge in order to stop the growth process. The concentration and radius of the dust particles were measured experimentally and found equal to ${10}^{8}$ ${\mathrm{cm}}^{\mathrm{\ensuremath{-}}3}$ and 0.05 \ensuremath{\mu}m, respectively. The PIC-MC discharge model has been used to check the validity of the fluid model in pure argon. The discharge in dusty argon has been described with a fluid model modified to account for the presence of dust particles. This model uses as input data the number of negative charges carried by the dust particles and the frequency of electron loss to the particles deduced from a two-dimensional particle-in-cell Monte Carlo simulation of a dusty dc plasma. The results show good agreement between experiments and models within the whole range of rf voltages considered and for the pure and dusty argon plasmas. The presence of dust particles in the plasma modifies considerably the discharge impedance, which becomes much more resistive. The electron number density in the plasma is much smaller than the positive ion density and than the number density of negative charges carried by the dust particles. It appears that the phase shift between current and voltage is very sensitive to the presence of powders. This property could be useful for the detection of powder formation in a processing plasma.

Numerical and experimental diagnostics of rf discharges in pure and dusty argon

High densities of submicron particles have been created in an Ar/SiH4 parallel plate radio‐frequency (rf) discharge. The particles were collected and measured by electron microscopy and the mean particle diameter was found to be 230±60 nm. Laser scattering from the dense clouds of such particles showed that the concentration was 1×108 cm−3. A laser Doppler anemometer was used to measure the particle velocity distribution and hence the mean particle mass. This is consistent with the specific density of the hydrogenated amorphous silicon. The mean velocities of particles were measured at two different gas flows when the discharge was extinguished, so that the particles are neutral and do not interact, and the particles move with the gas velocity. However, during the discharge the particles have almost no mean axial velocity, even though the gas flow is as large as before. This is due to the strong interparticle interactions that keep the particle cloud, as a whole, stationary. The charge on the particles is estimated, leading to a value of the Coulomb coupling parameter of Γ=10. This large value suggests that the particle cloud can be viewed as a Coulomb liquid.

C. Laure

Papers

Particle generation and behavior in a silane‐argon low‐pressure discharge under continuous or pulsed radio‐frequency excitation

Measurements of particle size kinetics from nanometer to micrometer scale in a low‐pressure argon‐silane radio‐frequency discharge

Numerical and experimental diagnostics of rf discharges in pure and dusty argon

Particle-particle interactions in dusty plasmas