We analyze theoretically the optical properties of ideal semiconductor crystallites so small that they show quantum confinement in all three dimensions [quantum dots (QD's)]. In the limit of a QD much smaller than the bulk exciton size, the linear spectrum will be a series of lines, and we consider the phonon broadening of these lines. The lowest interband transition will saturate like a two-level system, without exchange and Coulomb screening. Depending on the broadening, the absorption and the changes in absorption and refractive index resulting from saturation can become very large, and the local-field effects can become so strong as to give optical bistability without external feedback. The small QD limit is more readily achieved with narrow-band-gap semiconductors.

Theory of the linear and nonlinear optical properties of semiconductor microcrystallites

Macromolecular plasma-chemistry: an emerging field of polymer science

Surface reactions during growth and erosion of hydrocarbon films

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

To determine self-consistently the time evolution of particle size and their number density in situ multi-angle polarization-sensitive laser light scattering was used. Cross-polarization intensities (incident and scattered light intensities with opposite polarization) measured at 135 degrees and ex situ transmission electronic microscopy analysis demonstrate the existence of nonspherical agglomerates during the early phase of agglomeration. Later in the particle time development both techniques reveal spherical particles again. The presence of strong cross-polarization intensities is accompanied by low-frequency instabilities detected on the scattered light intensities and plasma emission. It is found that the particle radius and particle number density during the agglomeration phase can be well described by the Brownian free molecule coagulation model. Application of this neutral particle coagulation model is justified by calculation of the particle charge whereby it is shown that particles of a few tens of nanometer can be considered as neutral under our experimental conditions. The measured particle dispersion can be well described by a Brownian free molecule coagulation model including a log-normal particle size distribution. (C) 1996 American Institute of Physics.

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Particle agglomeration study in rf silane plasmas: In situ study by polarization-sensitive laser light scattering

Measurements of anions and cations are reported for hydrocarbon and silane radio frequency capacitive glow discharges. Series of anions were observed by quadrupole mass spectrometry using power‐modulated plasmas, and their structures are interpreted from the form of the mass spectra. Various experiments in silane plasmas show that anion confinement results in particles and conversely, anion detrapping can inhibit particle formation. In contrast, the polymerized neutral flux magnitudes, mass spectra and dynamics are independent of the powder formation. Powder is known to form readily in deposition plasmas containing electronegative free radicals, and the general role of anions in particle formation is discussed in the light of these experiments.

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Anionic clusters in dusty hydrocarbon and silane plasmas

In situ Fourier transform infrared absorption spectroscopy has been used to study the composition of particles formed and suspended in radio-frequency discharges of silane-oxygen-argon gas mixtures. The silane gas consumption was observed by infrared absorption. The stoichiometry of the produced particles depends on the silane flow rate and was compared with commercial colloidal silica. A small proportion of silane gas produces nanometric stoichiometric particles whereas a large proportion produces larger under-stoichiometric particles. Absorption spectroscopy was sufficiently sensitive to reveal particles too small to be visually observed by laser light scattering. Post-oxidation of hydrogenated silicon particles trapped in an argon plasma was obtained by adding oxygen. Mass spectrometry of negative and positive ions showed an extensive range of ionic clusters which may be at the origin of the observed particle formation. A model based on an iterative reaction sequence gives a good agreement with the measured positive ion mass spectrum.

/pdf/silicon-oxide-particle-formation-in-rf-plasmas-investigated-4i7yqsf35b.pdf

Silicon oxide particle formation in RF plasmas investigated by infrared absorption spectroscopy and mass spectrometry

Partial-depth modulation of the rf power in a capacitive discharge is used to investigate the relative importance of negative ions and neutral radicals for particle formation in low-power low-pressure silane plasmas. For less than 85% modulation depth, anions are trapped indefinitely in the plasma and particle formation ensues, whereas the polymerized neutral flux magnitudes and dynamics are independent of the modulation depth and the powder formation. These observations suggest that negative ions could be the particle precursors in plasma conditions where powder appears many seconds after plasma ignition. Microwave interferometry and mass spectrometry were combined to infer a rough estimate of anion density of similar to 7 x 10(9) cm(-3) which is approximately twice the free electron densit in these modulated plasmas.

https://iopscience.iop.org/article/10.1088/0963-0252/5/2/014/pdf

Partial-depth modulation study of anions and neutrals in low-pressure 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

C. Courteille

Papers

Particle agglomeration study in rf silane plasmas: In situ study by polarization-sensitive laser light scattering

Anionic clusters in dusty hydrocarbon and silane plasmas

Silicon oxide particle formation in RF plasmas investigated by infrared absorption spectroscopy and mass spectrometry

Partial-depth modulation study of anions and neutrals in low-pressure silane plasmas

From molecules to particles in Silane plasmas