Scaling of silicon nanoparticle growth in low temperature flowing plasmas (original) (raw)
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Plasma Chemistry and Plasma Processing, 2016
Control of the size and material properties of silicon nanoparticles plays a critical role in optimizing applications using those nanoparticles, such as photovoltaics and biomedical devices. While synthesis of silicon nanoparticles in low temperature plasmas has many attractive features, the basic mechanisms leading to formation of nanoparticles in these plasmas are poorly understood. A two-dimensional numerical model for synthesis of silicon nanoparticles (\5 nm in diameter) in radio frequency (RF) discharges was developed and used to investigate mechanisms for particle growth for Ar/He/SiH 4 gas mixtures. Algorithms for the kinetics of nanoparticle formation were self-consistently embedded into a plasma hydrodynamics simulation to account for nucleation, growth, charging, and transport of nanoparticles. We found that with RF excitation in narrow tubes at pressures of a few Torr, the electric field does not fully confine charged nanoparticles in the axial direction, which then results in a finite residence time of particles in the plasma. We found that because of the high neutral nanoparticle density, coagulation plays a significant role in
Growth dynamics of hydrogenated silicon nanoparticles under realistic conditions of a plasma reactor
Computational Materials Science, 2006
We present results of an extensive numerical study that was motivated by the experimental problem to understand under which conditions Si n H m nanoparticles deposited by plasma enhanced chemical vapor deposition (PECVD) take an amorphous or a crystalline structure. A crystalline structure of those particles is crucial, for example, for the electrical properties and lifetime of polymorphous solar cells. First, we use a fluid dynamics model to characterize the experimentally employed silane plasma. The resulting relative densities for all plasma radicals, their temperatures, and their collision interval times are then used as input data for detailed semiempirical quantum molecular dynamics simulations. As a result the growth dynamics of nanometric hydrogenated silicon Si n H m clusters is simulated starting out from the collision of individual SiH x radicals under the plasma conditions derived above. We demonstrate how the details of the plasma determine the amorphous or crystalline character of the forming nanoparticles. Finally, we show a preliminary absorption spectrum based on ab initio time-dependent DFT calculations for a crystalline Si 10 H 16 cluster to demonstrate the possibility to monitor the cluster growth in situ.
Nanoparticles in SiH4-Ar plasma: Modelling and comparison with experimental data
Journal of Applied Physics, 2011
Stabilization and growth of non-native nanocrystals at low and atmospheric pressures J. Chem. Phys. 136, 044703 (2012) Metal-insulator transition sustained by Cr-doping in V2O3 nanocrystals Appl. Phys. Lett. 100, 043103 Near-field optical imaging with a nanotip grown on fibered polymer microlens Appl. Phys. Lett. 100, 033107 (2012)
Nanomaterials
A three-dimensional numerical modelling of a time-dependent, turbulent thermal plasma jet was developed to synthetize silicon nanopowder. Computational fluid dynamics and particle models were employed via COMSOL Multiphysics®v. 5.4 (COMSOL AB, Stockholm, Sweden) to simulate fluid and particle motion in the plasma jet, as well as the heat dependency. Plasma flow and particle interactions were exemplified in terms of momentum, energy, and turbulence flow. The transport of nanoparticles through convection, diffusion, and thermophoresis were also considered. The trajectories and heat transfer of both plasma jet fields, and particles are represented. The swirling flow controls the plasma jet and highly affects the dispersion of the nanoparticles. We demonstrate a decrease in both particles’ velocity and temperature distribution at a higher carrier gas injection velocity. The increase in the particle size and number affects the momentum transfer, turbulence modulation, and energy of parti...
Improved size distribution control of silicon nanocrystals in a spatially confined remote plasma
Plasma Sources Science and Technology, 2015
This work demonstrates how to improve the size distribution of silicon nanocrystals (Si-NCs) synthesized in a remote plasma, in which the flow dynamics and the particular chemistry initially resulted in the formation of small (2-10 nm) and large (50-120 nm) Si-NCs. Plasma consists of two regions: an axially expanding central plasma beam and a background region around the expansion. Continuum fluid dynamics simulations demonstrate that a significant mass flow occurs from the central beam to the background region. This mass flow can be gradually reduced upon confinement of the central beam, preventing the mass transport to the background region. Transmission electron microscopy and Raman spectroscopy analyses demonstrate that the volume fraction of large Si-NCs decreases from ∼77% to below 45% in parallel with the decrease of mass flow to the background region upon confinement, which indicates that large Si-NCs are synthesized in the background and small Si-NCs are synthesized in the central beam. Spatially resolved ion flux analyses demonstrate that the ions are localized in the central beam despite the mass flow to the background, indicating that the formation of small Si-NCs is governed by ion-assisted growth while the formation of large Si-NCs is governed by radical-neutral-assisted growth in the absence of ions. According to these observations, a better uniformity in the size distribution of Si-NCs can be obtained by creating a more uniform plasma flow and controlling the density of plasma species in the plasma.
2003
Self-organization and dynamic processes of nano/micron-sized solid particles grown in low-temperature chemically active plasmas as well as the associated physico-chemical processes are reviewed. Three specific reactive plasma chemistries, namely, of silane (SiH 4 ), acetylene (C 2 H 2 ), and octafluorocyclobutane (c-C 4 F 8 ) RF plasma discharges for plasma enhanced chemical vapor deposition of amorphous hydrogenated silicon, hydrogenated and fluorinated carbon films, are considered. It is shown that the particle growth mechanisms and specific self-organization processes in the complex reactive plasma systems are related to the chemical organization and size of the nanoparticles. Correlation between the nanoparticle origin and self-organization in the ionized gas phase and improved thin film properties is reported. Self-organization and dynamic phenomena in relevant reactive plasma environments are studied for equivalent model systems comprising inert buffer gas and mono-dispersed organic particulate powders. Growth kinetics and dynamic properties of the plasma-assembled nanoparticles can be critical for the process quality in microelectronics as well as a number of other industrial applications including production of fine metal or ceramic powders, nanoparticleunit thin film deposition, nanostructuring of substrates, nucleating agents in polymer and plastics synthesis, drug delivery systems, inorganic additives for sunscreens and UV-absorbers, and several others. Several unique properties of the chemically active plasma-nanoparticle systems are discussed as well.
A self-consistent model for the production and growth of nanoparticles in low-temperature plasmas
Russian Journal of Physical Chemistry B
A theoretical global model is presented for describing the kinetics of generation and growth of clusters and nanoparticles in low-pressure plasmas, where important processes for clusters and grains are collisions with monomers, electrons, and ions and particle coagulation and loss because of diffusion and gas flow drag. Simple equations are given for calculations of monomer density, particle-size distribution function, critical cluster size, the rate of particle production and particle density and mean size, and plasma characteristics (the densities and average energies of "cold" and "hot" electrons and the density of positively charged ions). The model is self-consistent; that is, the above-mentioned properties of clusters, nanoparticles, electrons, and ions are calculated jointly from coupled equations as functions of a small number of radio frequency (RF) discharge parameters (discharge geometry; absorbed electric power; voltage across the RF sheath; gas pressure; composition; and flow rate). Comparisons are made with the experimental data on SiH 4-Ar mixtures.
Nanoparticle formation using a plasma expansion process
Plasma Chemistry and Plasma Processing, 1995
We describe a process in which nanosize particles with a narrow size distribution are generated by expanding a thermal plasma carrying vapor-phase precursors through a nozzle. The plasma temperature and velocity profiles are characterized by enthalpy probe measurements, by calorimetric energy balances, and by a model of the nozzle flow. Aerosol samples are extracted from the flow downstream of the nozzle by means of a capillary probe interfaced to a two-stage ejection diluter. The diluted aerosol is directed to a scanning electrical mobility spectrometer (SEMS) which provides on-line size distributions down to particle diameters of 4 nm. We have generated silicon, carbon, and silicon carbide particles with number mean diameters of about 10 nm or less, and we have obtained some correlations between the product and the operating conditions. Inspection of the size distributions obtained in the e.~:periments, together with the modeling results, suggests that under our conditions silicon carbide formation is initiatect by nucleation of extremely small silicon particles from supersaturated silicon vapor, followed by chemical reactions at the particle surfaces involving carbon-containing species from the gas phase.
Plasma Sources Science and Technology, 2019
The coagulation enhancement factor due to electrostatic (Coulomb and polarization-induced) interaction between silicon nanoparticles was numerically computed for different nanoparticle sizes and charges in typical low-temperature argon-silane plasma conditions. We used a rigorous formulation, based on a multipole moment coefficients, to describe the complete electrostatic interaction between dielectric particles. The resulting interaction potential is non-singular at the contact point, which allows to adapt the orbital-motion limited theory to calculate the enhancement factor. It is shown that, due to induced polarization, coagulation is enhanced in neutral-charged particles encounters up to several orders of magnitude. Moreover, the short-range force between like-charged nanoparticles can become attractive as a direct consequence of the dielectric nature of the nanoparticles. The multipolar coefficient potential is compared to an approximate analytic form which can be readily used to simplify the calculations. The results presented here provide a better understanding of the electrostatic interaction in coagulation and can be used in dust growth simulations in low-temperature plasmas where coagulation is a significant process.