Dynamic self-organization phenomena in complex ionized gas systems: new paradigms and technological aspects

Nonthermal plasmas have emerged as a viable synthesis technique for nanocrystal materials. Inherently solvent and ligand-free, nonthermal plasmas offer the ability to synthesize high purity nanocrystals of materials that require high synthesis temperatures. The nonequilibrium environment in nonthermal plasmas has a number of attractive attributes: energetic surface reactions selectively heat the nanoparticles to temperatures that can strongly exceed the gas temperature; charging of nanoparticles through plasma electrons reduces or eliminates nanoparticle agglomeration; and the large difference between the chemical potentials of the gaseous growth species and the species bound to the nanoparticle surfaces facilitates nanocrystal doping. This paper reviews the state of the art in nonthermal plasma synthesis of nanocrystals. It discusses the fundamentals of nanocrystal formation in plasmas, reviews practical implementations of plasma reactors, surveys the materials that have been produced with nonthermal pla...

Nonthermal Plasma Synthesis of Nanocrystals: Fundamental Principles, Materials, and Applications

The growth of particle size has been measured in a low-pressure argon-silane plasma using high-resolution transmission electronic microscopy. The results show that formation and growth of dust particles is an homogeneous process; the first generation size distribution is monodispersed; and the growth kinetics reveals a three-step process from molecular ions to large particles. Together with measurements of particle concentration obtained by laser light scattering, these measurements give a clear indication that the growth proceeds through three successive steps: (i) 'rapid' formation of crystalline clusters (as shown by dark-field high-resolution transmission electron microscopy) with concentrations of up to 1010 cm-3; (ii) formation of aggregates, of diameters up to 50 nm, by coagulation (during coagulation the particle concentration decreases dramatically); and (iii) growth of the particles with a constant concentration by surface deposition of SiHx radicals, whilst the numerical density remains constant. Laser-induced particle explosive evaporation has been performed using a XeCl (308 nm) laser. This experiment allowed detection of nanocrystallites and also the beginning of their coagulation and gave clear evidence of the temperature effect on particle formation.

https://iopscience.iop.org/article/10.1088/0963-0252/3/3/004/pdf

Particle nucleation and growth in a low-pressure argon-silane discharge

The experiments on solitons in plasmas performed during the period 1970 to 1982 are reviewed. Suggestions for future experiments are presented.

https://iopscience.iop.org/article/10.1088/0032-1028/25/9/001/pdf

Soliton experiments in plasmas

Semiconductor nanocrystals have attracted considerable interest for a wide range of applications including light-emitting devices and displays, photovoltaic cells, nanoelectronic circuit elements, thermoelectric energy generation and luminescent markers in biomedicine A particular advantage of semiconductor nanocrystals compared with bulk materials rests in their size-tunable optical, mechanical and thermal properties While nanocrystals of ionically bonded semiconductors can conveniently be synthesized with liquid phase chemistry, covalently bonded semiconductors require higher synthesis temperatures Over the past decade, nonthermal plasmas have emerged as capable synthetic approaches for the covalently bonded semiconductor nanocrystals Among the main advantages of nanocrystal synthesis in plasmas is the unipolar electrical charging of nanocrystals that helps avoid or reduce particle agglomeration and the selective heating of nanoparticles immersed in low-pressure plasmas This paper discusses the important fundamental mechanisms of nanocrystal formation in plasmas, reviews the range of synthesis approaches reported in the literature and discusses some of the potential applications of plasma-synthesized semiconductor nanocrystals

https://iopscience.iop.org/article/10.1088/0022-3727/42/11/113001/pdf

Nonthermal plasma synthesis of semiconductor nanocrystals

Stable negative ions containing up to sixteen silicon atoms have been measured by mass spectrometry in RF power-modulated silane plasmas for amorphous silicon deposition. These hydrogenated silicon cluster ions reach much higher masses than the positive ions, which have no more then six silicon atoms. This supports the view that negative ions are the precursors to particulate formation in silane plasmas. The time-dependent fluxes of positive and negative ions from the plasma are shown with a 5 mu s time resolution. Possible cluster reaction sequences are discussed and the effect of visible light on the negative ion signal is commented upon.

/pdf/negative-hydrogenated-silicon-ion-clusters-as-particle-12gw8y3wh4.pdf

Negative hydrogenated silicon ion clusters as particle precursors in RF silane plasma deposition experiments

In situ Fourier transform infrared (FTIR) absorption spectroscopy has been used to determine the fractional depletion of silane in a radio-frequency (rf) glow discharge. The technique used a simple single-pass arrangement and was implemented in a large-area industrial reactor for deposition of amorphous silicon. Measurements were made on silane plasmas for a range of excitation frequencies. It was observed that, at constant plasma power, the fractional depletion increased from 35% at 13.56 MHz to 70% at 70 MHz. With a simple model based on the density continuity equations in the gas phase, it was shown that this increase is due to a higher dissociation rate and is largely responsible for the observed increase in the deposition rate with the frequency.

/pdf/degree-of-dissociation-measured-by-ftir-absorption-puqpg24bdl.pdf

Degree of dissociation measured by FTIR absorption spectroscopy applied to VHF silane plasmas

Particle formation has been investigated experimentally from the initial molecular precursors up to the final micron-sized particles in a low pressure silane rf capacitive discharge. Neutrals and ions were studied by quadrupole mass spectro- metry in power-modulated plasmas: Whole series of negative ions were observed, ranging from monosilicon anions through to nanometric clusters. Anion confinement results in particles and conversely, anion de-trapping can inhibit particle formation. Plasma polymerisation is considered in terms of neutral and ionic species. Laser light scattering measurements show that particles appear during a rapid coalescence phase and possible mechanisms are discussed.

/pdf/from-molecules-to-particles-in-silane-plasmas-5c6kka8py2.pdf

From molecules to particles in Silane plasmas

Plasma spraying is a well developed and widely used technology, successfully applied for ceramic and metal coatings in many fields of applications such as aeronautics, gas turbine, automotive or medical. The coatings obtained are usually intentionally porous and thick (more than 100 pm). Presently, thin films (<mu m) are deposited using various physical vapour deposition (PVD) or chemical vapour deposition (CVD) processes with low deposition rates. In this paper, we make use of the high enthalpy and high ionisation degree of the plasma jet of conventional plasma spraying guns operated at low pressure (mbar) to obtain dense coatings by CVD from gaseous and/or liquid precursors. The advantages of such thermal plasma CVD processes are the high deposition rates to obtain dense and thin layers, and the possibility of combining these thin films with thermally sprayed coatings using the same equipment.

Mechanisms of films and coatings formation from gaseous and liquid precursors with low pressure plasma spray equipment

Films were prepared under argon and xenon dilution with silane concentrations ranging from 3 to 100% using the very high frequency glow discharge (VHF-GD) technique. The H-content and the microstructure were determined by IR-spectroscopy. A surface profiler was used to measure the stress and thickness of the films. The surface roughness of the films was evaluated by the UV-light reflectance loss. The samples were further characterized by dark- and photoconductivity, by CPM and by PDS, both in annealed and light-soaked state. Down to silane concentrations of about 10–20% film properties change only little; however, for both dilution series the microstructure parameter shows a minimum and the internal stress a maximum near 20% silane concentration. At still higher rare gas dilution the film properties change drastically. Surprisingly the photoconductivity remains almost constant for all gas dilutions. It is shown how these changes in the film properties are linked with the light induced degradation.

Ch. Hollenstein

Papers

Negative hydrogenated silicon ion clusters as particle precursors in RF silane plasma deposition experiments

Degree of dissociation measured by FTIR absorption spectroscopy applied to VHF silane plasmas

From molecules to particles in Silane plasmas

Mechanisms of films and coatings formation from gaseous and liquid precursors with low pressure plasma spray equipment

Microstructure, Optoelectronic Properties and Saturated Defect Density of A-SL:H Prepared in VHF-Glow Discharge Using AR and XE Dilution