Synthesis of Nanocomposite Thin Films in an Atmospheric Pressure Plasma

Atmospheric pressure PECVD (AP-PECVD) combines the advantages of PECVD: low temperature process and low temperature, and AP: no pumps and in-line coating. A configuration like that of industrial corona treatment, useful for in-line coating of large surface, appears as the best one. It is based on a linear dielectric barrier discharge directly interacting with the substrate and typically working in a range of frequency of several tens of kHz. As example, in a Penning gas mixture like Ar with some tens of ppm of NH3, by adding SiH4, it leads to high quality SiNx:H thin films for photovoltaic cells.<br/>Another advantage of being at atmospheric pressure is that liquid or colloidal solution are easily injected in the plasma using an aerosol. To produce a nanocomposite, two different precursors should be used, one to synthesize the matrix and the other for the nanoparticles (NPs) that are embedded in the matrix. At first a colloidal suspension of pre-synthetized NPs in a solvent was tried. The study of the coatings made from an aerosol of TiO2 NPs in isopropanol showed that all the NPs contained in a droplet aggregate, leading to porous thin films. However, this study also showed that the matrix growth and the NPs transport onto the substrate can be controlled by the alternation of two plasma frequencies: a high one to polymerize the matrix and a low one to control the NPs transport. This was confirmed using SiO2 NPs in ethyl lactate as precursors.<br/>Then, to try to avoid NPs aggregation, the possibility of forming NPs directly in the plasma is explored. A great advantage of this solution is to be safe by design as NPs are not at all manipulated. Thus, an aerosol of a solution of metallic salt in a solvent able to form a polymer thin film is used. The first results made with a gold salt in isopropanol will be presented. They show that plasmonic layers are easily obtained. The frequency alternation is also useful to transport NPs on to the surface and have enough power to reduce the salt and induces the growth of the matrix. Gold NPs are formed in the plasma, their aggregation is avoided and plasmonic resonance peaks range from 540 nm to 750 nm depending on the process parameters. Thus the coupling of a dielectric barrier discharge in a configuration useful for in-line treatment and an aerosol of metallic salt in a polymerizable liquid appears as promising solution to synthesize dense nanocomposite thin film in safe by design and ecofriendly manner at atmospheric pressure.