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International audience; In this article, a detailed description of the light extinction spectrometry diagnostics is given. It allows the direct in-situ measurement of the particle size distribution and absolute concentration of a dust cloud levitating in plasmas. Using a relatively simple and compact experimental setup , the dust cloud parameters can be recovered with a good accuracy making minimum assumptions on their physical properties. Special emphasises are given to the inversion of light extinction spectra and all the required particle shape, refractive index and the light extinction models. The parameter range and the limitations of the diagnostic are discussed. In addition, two examples of measurements in low-pressure gas discharges are presented: in a DC glow discharge in which nanopar-ticles are growing from the sputtering of a tungsten cathode, and in an Argon-Silane radio-frequency discharge.

Dusty or complex plasmas are partially ionized gas composed of neutral species, ions, electrons, and charged dust particles. In laboratory experiments, these particles can be either injected or grown directly in the plasma. Injected dust particles are usually micron-size particles. Due to their mass, they are confined near the bottom electrode where the electric force counterbalances gravity. The microgravity condition is necessary to study dust clouds of micrometer-size particles filling the whole plasma chamber. In the laboratory, dense clouds of submicron particles light enough to completely fill the gap between the electrodes can be obtained using reactive gases such as silane or using a target sputtered with ions from plasma. Dust-particle charge is a key parameter in complex plasma. It determines the interaction between a dust particle and electrons, ions, its neighboring dust particles, and electric field. However there are only a few papers devoted to dust charging, or decharging to be more specific, in the discharge afterglow. In this paper, we report an on-ground experiment on the residual charge measurement of dust particles after the decay of a dusty plasma. The experiment was performed in the PKE-Nefedov reactor where the dust particles were physically grown in the plasma. A temperature gradient was introduced in the chamber to create an upward thermophoretic force to balance gravity in order to maintain the dust particles in the volume of the chamber when the discharge is switched off. The residual charges were determined from an analysis of dust oscillations, which were excited by applying a sinusoidal bias to the bottom electrode. It was shown that dust particles do keep a residual charge of few electrons and that both positively and negatively charged dust particles coexist in the late plasma afterglow. In order to understand these results, measurements on electron density decay have been performed into other dusty discharges such as argon/methane dilution plasma and sputtering discharge.

International audience; Silane-based plasmas are widely used to deposit nanostructured silicon thin films or to synthesize silicon nanoparticles. Dust particle formation in Ar/SiH4 plasmas is a continuous phenomenon: as long as silane precursors are provided, new dust generations are formed. Successive generations can be monitored thanks to various electrical (Vdc/3H) and optical (OES, video imaging) diagnostics. Experiments presented in this paper have been performed in a capacitively-coupled radiofrequency discharge, at low pressure (12 Pa) in an Argon/Silane mixture (92:8).

International audience; The aim of this paper is to contribute to improve the knowledge concerning the growth steps of hydrocarbon dust particles in Ar-CH4 plasma. To achieve this aim, we monitored the plasma impedance modification, due to the particle formation, by measuring the time-evolution of the self bias voltage, the current third harmonic and the electron density. Emission Spectroscopy turns out to be a good tool for estimating the electron temperature. While, Transmission and Scanning Electron Microscopy are used to characterize the morphology and the size of collected powders.

International audience; When a three-dimensional dust cloud is present in a plasma, a dust-free region, called void, is usually observed in the plasma centre. Forces involved in the void existence are an inward electrostatic force and an outward ion drag one. Under certain conditions, this force balance can be disturbed and this region exhibits a self-excited unstable behaviour consisting in successive contractions and expansions of its size. This low frequency instability (few Hz), called "heartbeat", is here characterised by various diagnostics: electrical and optical measurements, high-speed video imaging. The results brought to the fore the main features of this instability. Correlations between physical processes in the plasma volume and the dust cloud motion are investigated through experimental results.

International audience; The dust residual charge distribution in the late afterglow of a dusty plasma has been measured for different operating pressures. It has been shown that the dust charge distribution can be approximated by gaussian function with mean value close to −3e, rms about 1e and a tail in the positive charge region. Numerical simulations of the evolution of the dust charge distribution showed that the shape of the charge distribution is very dependent on the transition from ambipolar to free diffusion during the plasma decay process.

Dust cloud dynamics and behaviour are studied during dust particle successive generations in a silane based plasma. Silicon particle growth is known to exhibit a periodic behaviour, depending on experimental conditions. By having a look to the inner structure of the periodic signal (and especially to the electrical signal increase leading to the second dust particle generation), a complex unstable behaviour can be evidenced. This behavior is characterized by at least two distinct phases: a high-ordered one and a less-ordered one. Thanks to correlation between electrical measurements, laser light scattering, optical emission spectroscopy and video imaging, we show that these different phases are closely linked to the steps of dust particle formation. These diagnostics prove the presence of a void region (strongly speculated in our plasma until now). Spatially resolved depositions performed during dust successive generations show that new dust particles appear in the void region. This fact leads to an important spatial inhomogeneity in silicon particle size and density deposited on the substrate. We especially show that in some cases, the deposited dust density drastically increases between a sample deposited in the plasma center and one deposited at the plasma edge. Those phenomena of instabilities are known to be linked to dust formation in silane-based plasma [1]. They are also observed in dust cloud containing a void [2-4]. These two aspects could be linked, and further investigations are underway. [1] M. Cavarroc, M.C. Jouanny, K. Radouane, M. Mikikian, L. Boufendi, J. Appl. Phys. 99, 064301 (2006) [2] G. Praburam, J. Goree, Phys. Plasmas 3(4), 1212 (1996) [3] D. Samsonov, J. Goree, Phys. Rev. E 59, 1047 (1999) [4] M. Mikikian, L. Boufendi, Phys. Plasmas 11, 3733 (2004)

International audience; This paper deals with the study of the growth kinetics of hydrocarbon dust particles in low pressure N2-CH4 gas mixture radio frequency discharges. The partial pressure of CH4 is about 2% of the gas mixture total pressure (0.9 mbar), and the power varies from 20 to 40 W. The initiation and growth of hydrocarbon particles are followed thanks to the measurement of the self bias voltage, the amplitude of the third harmonic of the discharge current, the electron density and the electron temperature. The microwave resonant cavity technique is used to measure the time evolution of the electron density, while the morphology and the size of the collected powders are studied by the means of Transmission and Scanning Electron Microscopy.

Dust particle growth in silane-based plasmas has been widely studied at room temperature and effects of higher gas temperatures are now well-attested [1,2]. However, effects of very low gas temperatures (below 0°C) on the dust formation are still little-known. In the present study, we grow silicon nanocrystals [3] from room temperature down to -40°C. We first show that low gas temperature increases dust formation kinetics. These results are consistent with the ones previously obtained at higher gas temperature, underlining an exponential behavior of the formation kinetics versus gas temperature. Moreover, as the plasma behaves like a supersaturated medium, silicon nanoparticle radius is expected to evolve as 1/T(K). At room temperature, single-crystal nanoparticles have a typical size of 1.5 nm in radius. First results at low gas temperature are very promising: spherical nanoparticles up to 40 nm in radius have been obtained for T= -40°C. These first insights into low gas temperature effects tend to confirm the hypothesis of a fast increasing radius for gas temperature below 0°C. Presently, TEM analyses are underway to determine a possible influence of the gas temperature on the nanoparticle crystalline phase and/or density in the plasma. This new method for silicon nanocrystal tailoring allows obtaining silicon nanoparticles in a size range of 1 to 40 nm with a quite monodispersed size distribution and a well-known density. This new technique can be of great interest for industrial applications requiring nanoparticles of well-defined size (photovoltaic applications, nanotechnologies...). [1] A. Fridman, L. Boufendi, T. Hbid, B. Potapkin, and A. Bouchoule, J.Appl.Phys. 79, 1303 (1995) [2] A. Bouchoule, A. Plain, L. Boufendi, J. Ph.Blondeau and C. Laure, ,J.Appl.Phys. 70, 1991 (1991) [3] M. Cavarroc, M. Mikikian, G. Perrier, L. Boufendi, Appl. Phys. Lett. 89, 013107 (2006)

International audience; The synthesis of single-crystal silicon nanocrystals we present is performed in the gas phase of a radio frequency (rf) discharge. The plasma is produced in a discharge box, enclosed in a vacuum vessel, containing an Ar/SiH4 mixture (92:8). The typical pressure is 0.12 mbar while typical rf injected power is 10 W. Gas temperature can be decreased to -40 °C. Dust particle formation and growth is monitored thanks to an electrical diagnostics based on the time evolution of the discharge current third harmonic amplitude (3H). Size, density and crystallinity of dust particles are determined thanks to microscopy (SEM, TEM, AFM).

International audience; Complex (dusty) plasmas are partially ionized gases composed of neutral species, ions, electrons and charged dust particles. The experiment was performed in the PKE-Nefedov reactor where dust particles were physically grown by sputtering in discharge chamber. It was found coexistence of positively and negatively charged dust particles as well as non-charged ones for more than one minute after the discharge was switched off. The residual charges for 200 nm radius particles have been measured for two different pressures. It was revealed that dusts kept residual charges only when the discharge was abruptly switched off. Indeed when the discharge power is decreased slowly until the plasma disappeared, there was no residual charge on dust particles.