Rational engineering of thin nanocomposite layers, deposited in reactive plasmas, requires knowledge on the plasma behavior in order to produce multifunctional deposits with tailored properties ...(structural, optical, electrical, etc.) This work presents an experimental study of nanoparticles synthesized in the plasma gas-phase and their subsequent use as building-blocks to form layer-by-layer nanostructures. The experiment is performed in a plasma process that successfully combines plasma polymerization of an organosilicon molecular precursor (hexamethyldisiloxane, HMDSO) and sputtering of a metallic (silver) target. Pulsed injection of the precursor is found to promote cyclic nanoparticle formation in Ar/HMDSO reactive plasmas. The plasma electron temperature is found to vary in the range 1.6—2.2 eV as derived from time-resolved optical emission spectroscopy of the plasma energetic conditions. This diagnostic method is also shown to provide a reliable tool for online monitoring of the nanoparticle synthesis process. Two types of layer-by-layer structured nanocomposites can be obtained depending on the type of nanoparticles synthesized: (i) organosilicon nanoparticles of size less than 100 nm in all studied plasma conditions for a large quantity of injected HMDSO and (ii) raspberry-like nanoparticles of size less than 150 nm when the quantity of injected HMDSO is reduced. The organosilicon nanoparticle growth follows a polydimethylsiloxane (PDMS)-like oligomerization scheme in which the R 2 -Si(-O) 2 silicon bond tends towards the formation of polymeric structure in a R 3 -Si(-O) 1 silicon chemical environment, containing Si-(CH 2 )-Si type bridges that are involved in cross-linking. The elemental composition of the raspberry-like nanoparticles is similar to that of the organosilicon nanoparticles, supplemented by the Ag component. The decorating silver nanoparticles are ∼15 nm of size, round in shape and polycrystalline. There is no evidence for silver oxides in the nanostructures. The Si-O-Ag bridges, revealed by infrared spectroscopy, suggest the presence of junction sites between the metallic and the organosilicon parts of the raspberry-like nanoparticles. The silver nanoparticles are found to decorate the organosilicon nanoparticles to form the raspberry-like nanoparticles in the plasma gas-phase, before being deposited. This reveals a very interesting phenomenon of simultaneous growth of the silver- and organosilicon-parts in the plasma without mixing during the nucleation phase.
The ever increasing field of application of nanodielectrics in electrical insulations calls for description of the mechanisms underlying the performance of these systems and for identification of the ...signs exposing their aging under high electric fields. Such approach is of particular interest to electrically insulating polymers because their chemical defects are of deleterious nature for their electrical properties and can largely degrade their performance at high electric fields. Although these defects usually leave spectroscopic signatures in terms of characteristic luminescence peaks, it is nontrivial to assign, in an unambiguous way, the identified peaks to specific chemical groups or defects because of the low intensity of the signal with the main reason being that the insulating polymers are weakly emitting materials under electric field. In this work, we go beyond the conventional electroluminescence technique to record spectroscopic features of insulating polymers. By introducing a single plane of silver nanoparticles (AgNPs) at the near-surface of thin polypropylene films, the electroluminescent signal is strongly enhanced by surface plasmons processes. The presence of AgNPs leads not only to a much higher electroluminescence intensity but also to a strong decrease of the electric field threshold for detection of light emission and to a phase-stabilization of the recorded spectra, thus improving the assignment of the characteristic luminescence peaks. Besides, the performed analyses bring evidence on the capability of AgNPs to trap and eject charges, and on the possibility to adjust the energetics of charge carriers in electrically insulating polymers at the electrode/dielectric contact via AgNPs.
Laboratory experiments are essential in exploring the mechanisms involved in stardust formation. One key question is how a metal is incorporated into dust for an environment rich in elements involved ...in stardust formation (C, H, O, Si). To address experimentally this question we have used a radiofrequency cold plasma reactor in which cyclic organosilicon dust formation is observed. Metallic (silver) atoms were injected in the plasma during the dust nucleation phase to study their incorporation in the dust. The experiments show formation of silver nanoparticles (~15 nm) under conditions in which organosilicon dust of size 200 nm or less is grown. The presence of AgSiO bonds, revealed by infrared spectroscopy, suggests the presence of junctions between the metallic nanoparticles and the organosilicon dust. Even after annealing we could not conclude on the formation of silver silicates, emphasizing that most of silver is included in the metallic nanoparticles. The molecular analysis performed by laser mass spectrometry exhibits a complex chemistry leading to a variety of molecules including large hydrocarbons and organometallic species. In order to gain insights into the involved chemical molecular pathways, the reactivity of silver atoms/ions with acetylene was studied in a laser vaporization source. Key organometallic species, Ag
C
H
(n=1-3; m=0-2), were identified and their structures and energetic data computed using density functional theory. This allows us to propose that molecular Ag-C seeds promote the formation of Ag clusters but also catalyze hydrocarbon growth. Throughout the article, we show how the developed methodology can be used to characterize the incorporation of metal atoms both in the molecular and dust phases. The presence of silver species in the plasma was motivated by objectives finding their application in other research fields than astrochemistry. Still, the reported methodology is a demonstration laying down the ground for future studies on metals of astrophysical interest such as iron.
Plasma sampling mass spectrometry (PSMS) has been carried out to study the fragmentation kinetics of hexamethyldisiloxane (HMDSO) in a low‐pressure, axially asymmetric argon rf discharge designed for ...the growth of nanocomposite thin films through a hybrid PVD/PECVD method. Experiments have been conducted with a pulsed injection of HMDSO over a 5‐s period. Plasma conditions have been chosen to favor formation and disappearance of dust occurring in cycles of a few hundred seconds. The dissociation degree of HMDSO and the relative intensities of HMDSO‐related fragments are reported and analyzed regarding these two specific time‐scales. PSMS showed that formation of dust increases HMDSO dissociation. The same result can be deduced from the particle balance equation of HMDSO using the electron density and temperature obtained from optical emission spectroscopy as the only input parameters. For HMDSO, electron‐impact dissociation is the dominant loss pathway over diffusion and recombination on the reactor walls. Small C
xH
y compounds and H
2 are mostly generated from surface recombination mechanisms and lost by electron‐impact dissociation.
Plasma sampling mass spectrometry was used to study the fragmentation kinetics of hexamethyldisiloxane (HMDSO) in a low‐pressure argon rf discharge with pulsed precursor injection.
Plasma conditions have been chosen to favor formation and disappearance of dust occurring in cycles of a few hundred seconds.
Formation of dust increases HMDSO dissociation.
For HMDSO, electron‐impact dissociation is the dominant loss pathway over diffusion and recombination on the reactor walls.
Small CxH
y compounds and H
2 are mostly generated from surface recombination mechanisms and lost by electron‐impact dissociation.
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