Microplasmas have attracted a tremendous amount of interest from the plasma community because of their small physical size, stable operation at atmospheric pressure, non-thermal characteristics, high electron densities and non-Maxwellian electron energy distributions. These properties make microplasmas suitable for a wide range of materials applications, including the synthesis of nanomaterials. Research has shown that vapour-phase precursors can be injected into a microplasma to homogeneously nucleate nanoparticles in the gas phase. Alternatively, microplasmas have been used to evaporate solid electrodes and form metal or metal-oxide nanostructures of various composition and morphology. Microplasmas have also been coupled with liquids to directly reduce aqueous metal salts and produce colloidal dispersions of nanoparticles. This topical review discusses the unique features of microplasmas that make them advantageous for nanomaterials synthesis, gives an overview of the diverse approaches previously reported in the literature and looks ahead to the potential for scale-up of current microplasma-based processes.

/pdf/microplasmas-for-nanomaterials-synthesis-22udkqo6sx.pdf

Microplasmas for nanomaterials synthesis

Electric probe methods for diagnostics of plasmas are reviewed with emphasis on the link between the appropriate probe theories and the instrumental design. The starting point is an elementary discussion of the working principles and a discussion of the physical quantities that can be measured by the probe method. This is followed by a systematic classification of the various regimes of probe operation and a summary of theories and methods for measurements of charged particle distributions. Application of a single probe and probe clusters for measurements of fluid observables is discussed. Probe clusters permit both instantaneous and time-averaged measurements without sweeping the probe voltage. Two classes of applications are presented as illustrations of the methods reviewed. These are measurements of cross sections and collision frequencies (plasma electron spectroscopy), and measurements of fluctuations and anomalous transport in magnetized plasma.

Electric probes for plasmas: The link between theory and instrument

We use a global (volume averaged) model to study the dissociation processes and the presence of negative ions and metastable species in a low pressure high density O2/Ar discharge in the pressure range 1?100?mTorr. The electron density and the fractional dissociation of the oxygen molecule increases with increased argon content in the discharge. We relate this increase in fractional dissociation to an increase in the reaction rate for electron impact dissociation of the oxygen molecule which is due to the increased electron temperature with increased argon content in the discharge. The electron temperature increases due to higher ionization potential of argon than for molecular and atomic oxygen. We find the contribution of dissociation by quenching of the argon metastable Arm by molecular oxygen (Penning dissociation) to the creation of atomic oxygen to be negligible. The negative oxygen ion O? is found to be the dominant negative ion in the discharge. Dissociative attachment of the oxygen molecule in the ground state and in particular the metastable oxygen molecule O2(a?1?g) are the dominating channels for creation of the negative oxygen ion O?.

/pdf/oxygen-discharges-diluted-with-argon-dissociation-processes-4qg2kdm010.pdf

Oxygen discharges diluted with argon: dissociation processes

The relations between the surface reaction probability β of an atom or a radical in a reactive gas discharge, its diffusive flux to the wall, spatial density profile and temporal density decay during the postdischarge, are examined. Then, the values of β for H, SiH3, and Si2H5 on a growing a-Si:H film, and CH3 and C2H5 on an a-C:H film are derived from the temporal decay of radical densities during the discharge afterglow by using time-resolved threshold ionization mass spectrometry. For SiH3 on a-Si:H, β=0.28±0.03 in excellent agreement with previous determinations using other experimental approaches, and for Si2H5, 0.1<β<0.3. For H on a-Si:H, 0.4<β<1 and mostly consists of surface recombination as H2, while the etching probability of Si as SiH4 is only e≈0.03 at 350 K in good agreement with other studies of H reaction kinetics on crystalline silicon. At high dilution of SiH4 in H2 the sticking probabilities of Si hydride radicals are affected by the flux of H atoms of hydrogen ions which enhances surfac...

Surface reaction probabilities and kinetics of H, SiH3, Si2H5, CH3, and C2H5 during deposition of a-Si:H and a-C:H from H2, SiH4, and CH4 discharges

We use a global (volume averaged) model to study the presence of negative ions and metastable species in low-pressure high-density oxygen discharges. We find the negative oxygen ion O- to be the dominant negative ion in the discharge, the density of the negative ion O2- to be small and the density of the negative ion O-3 to be negligible in the pressure range of interest, 1-100?mTorr. Dissociative attachment of the oxygen molecule in the ground-state O2(3?g-) and the metastable oxygen molecule O2(a1?g) are the dominating channels for the creation of the negative oxygen ion O-. At low pressure (<5?mTorr) recombination involving O- and O+ ions is the main loss channel for O- ions. At higher pressure, the detachment on O(3P) becomes the main loss channel for the O- ion. The creation of O-2 is mainly through dissociative attachment of ozone O3. Ozone is almost entirely created through detachment by the collision of O- with the metastable oxygen molecule O2(a1?g). The creation of O-2 is thus greatly influenced by this detachment process and neglecting the detachment has a significant influence on the density of O-2 ions. At low pressure (<10?mTorr) the O-2 ion is mainly lost through recombination while at higher pressure the charge transfer to form O2 is the dominating loss process.

https://iopscience.iop.org/article/10.1088/0022-3727/34/7/312/pdf

Electronegativity of low-pressure high-density oxygen discharges

Diffusion theory is often employed to calculate the effects of wall destruction on the local concentration of an active species immersed in a scattering gas. In many situations the spatial dependence of the concentration is given to a good approximation by the fundamental diffusion mode, and the local loss frequency can be calculated using the container’s fundamental mode diffusion length Λ. The additional assumption that the density of the active species may be taken to be zero at the container boundaries gives a value of Λ=Λ0 which depends only on the container dimensions, but use of Λ0 can be seriously in error if the diffusion mean free path λm is comparable to the dimensions, or if the particle reflection coefficient R becomes of significance. An improved boundary condition may be written simply in terms of the linear extrapolation length λ, whose inverse is the logarithmic gradient of the particle density at the boundary. The equation λ=2(1+R)λm/3(1−R) allows the representation of the full range of ...

P. J. Chantry

Papers

A simple formula for diffusion calculations involving wall reflection and low density