Maxime Mikikian

Atomic structure of the nanocrystalline Si particles appearing in nanostructured Si thin films produced in low-temperature radiofrequency plasmas

Nanostructured Si thin films, also referred as polymorphous, were grown by plasma-enhanced chemical vapor deposition. The term “polymorphous” is used to define silicon material that consists of a two-phase mixture of amorphous and ordered Si. The plasma conditions were set to obtain Si thin films from the simultaneous deposition of radical and ordered nanoparticles. Here, a careful analysis by electron transmission microscopy and electron diffraction is reported with the aim to clarify the specific atomic structure of the nanocrystalline particles embedded in the films. Whatever the plasma conditions, the electron diffraction images always revealed the existence of a well-defined crystalline structure different from the diamondlike structure of Si. The formation of nanocrystallinelike films at low temperature is discussed. A Si face-cubic-centered structure is demonstrated here in nanocrystalline particles produced in low-pressure silane plasma at room temperature.

Sheath modification in the presence of dust particles

Negatively charged dust particles are expected to modify the local sheath potential where they are in equilibrium. In the conditions of a hot cathode discharge, sheath profiles are deduced from the measurement of ion drift velocities, with dust particles in suspension and without. In the unperturbed potential profile, the surface potential of an isolated dust particle, its charge, and its potential energy can be estimated as a function of the position in the sheath. In the presence of dust particles, an average increase of the ion drift velocity is measured showing a modification of the local sheath profile. This experimental result suggests that the dust particle charge due to the plasma particle fluxes in the sheath, modifies in turn the local plasma particle distributions.

Experimental investigations of void dynamics in a dusty discharge

The first electrical and spectroscopic characterizations of an instability, usually called the “heartbeat” instability, occurring in a laboratory dusty plasma are reported. The heartbeat instability consists of successive contractions and expansions of the central dust free region observed in a dense cloud of dust particles. This cloud is formed in a radio-frequency plasma by sputtering polymer material deposited on the electrodes. The evolution of the discharge current reveals the relatively complex shape of the instability and allows one to measure its evolution as a function of gas pressure and radio-frequency power.

Single-crystal silicon nanoparticles: An instability to check their synthesis

An instability occuring in electrical signals of the discharge is used as a mark to detect the end of the single-crystal silicon nanoparticle formation in Ar∕SiH4 rf plasmas. Scanning electron microscopy and atomic force microscopy studies of depositions show that the exact beginning of the coalescence phase corresponds to the onset of the instability. At the end of the instability, no single-crystal nanoparticles are remaining in the gas phase. These results based on a nonperturbative method allow to control depositions of single-crystal silicon nanoparticles of a well-defined size distribution with the highest density available during dust particle growth.

Self-excited instability occurring during the nanoparticle formation in an Ar-SiH4 low pressure radio frequency plasma

An experimental investigation of an instability occurring during dust nanoparticle formation is presented in this paper. The present study has been performed in radio frequency low pressure plasma in an argon-silane mixture. The formation and growth of nanoparticles is followed, thanks to the analysis of the amplitude of the third harmonics (40.68MHz) of the discharge current and the self-bias voltage (Vdc). In some cases, at the end of the accumulation phase of the nanocrystallites an instability occurs. It seems to be an attachment induced ionization instability as observed in electronegative plasmas. A detailed study of the influence of different operating conditions (injected power, gas temperature, and silane flow rate) on this instability behavior and frequencies is presented. The paper concludes by examining a very particular case of the instability.

Electron diffraction and high-resolution transmission microscopy studies of nanostructured Si thin films deposited by radiofrequency dusty plasmas

Abstract Nanostructured Si thin films, also referred to as polymorphous Si, were grown by chemical vapor deposition using modulated radiofrequency (RF) plasmas of SiH4, highly diluted in Ar. The plasma conditions were adapted to obtain a new type of Si thin films, between the amorphous and the microcrystalline ones, by allowing the formation of nanocrystalline Si particles in the plasma gas phase and their incorporation into the growing film. These films consist of Si-ordered domains (1–5 nm) embedded in an amorphous matrix, as seen by high-resolution transmission electron microscopy. Films with different crystallite size and density were obtained depending on the plasma modulation and discharge conditions. Selected area electron diffraction has highlighted that such Si-ordered domains are crystals with face-centered-cubic structure, clearly different from the Si diamond-like structure.

Dust particles in low-pressure plasmas: Formation and induced phenomena*

Formation of dust particles is a common mechanism in low-pressure plasmas. These big particles (in comparison with other plasma species) are sometimes the desired final products of the process, but they may also constitute a severe drawback in certain contexts. In either situation, it is necessary to understand growth mechanisms well, in order to control or avoid dust particle formation. One of the problems that has to be overcome is that dust particle growth is usually a continuous mechanism: once started, it can enter into a cyclic regime where new generations of dust particles are succeeding one after the other. This cyclic phenomenon often induces a side effect consisting of instabilities of a few tens of Hz. This paper discusses the main characteristics of dust successive generations, and particularly the importance of dust-free spaces (void) involved in this process. Finally, some aspects related to deposition when several generations coexist will be presented.

Dust charge distribution in complex plasma afterglow

Dust charge distributions have been measured for the first time in a complex plasma. Experiments were performed late in a discharge afterglow. Residual charges of few hundred particles have been determined. It was found that the mean residual charge is negative but the tail of the distribution extends into the positive-charge region. The experiments were complemented by numerical simulations taking into account discreteness of the charging process and dependence of plasma parameters on the transition from ambipolar to free diffusion. It is shown that the existence of positively charged particles can be explained by the broadening of the dust charge distribution in the late complex plasma afterglow.

Carousel Instability in a Capacitively Coupled RF Dusty Plasma

Rotating plasma spheroids are observed in a capacitively coupled radio-frequency discharge containing grown dust particles. These plasma spheroids are regions of slightly enhanced emission that rotate along the circumference of electrodes. This effect is not observed systematically, and when it occurs, the number and the speed of the spheroids can vary with the discharge parameters. These spheroids have been evidenced by using high-speed imaging.

PdPt catalyst synthesized using a gas aggregation source and magnetron sputtering for fuel cell electrodes

, +33 (0)2 3849 4352 KEYWORDS. gas aggregation source, magnetron sputtering, platinum, nanoclusters, catalyst. Abstract. PdPt catalysts with different morphologies and atomic ratios have been synthesized on native SiO2/Si and on proton exchange membrane. The combination of the gas-aggregation source and of the magnetron sputtering techniques allows the formation of quasi core-shell Pd0.97Pt0.03@Pt nanoclusters. Transmission electron microscopy and grazing incidence wide angle X-ray scattering measurements on Pd-rich core reveal a mean diameter of 4 nm and a fcc structure. The Pt shell around the half of the Pd-rich core is formed by magnetron sputtering which leads to the increase of nanocluster diameter (up to 10 nm) and of the overall Pt content (up to 85%). The membranes coated by PdPt core catalyst and PdPt@Pt catalyst (resulting in the formation of catalyst coated membrane) are incorporated into fuel cells and their electrical characteristics are measured. The association of the two deposition techniques resulting in the formation of quasi core-shell PdPt@Pt nanoclusters improves the startup step of the fuel cell.