Multifrequency low temeprature plasma at atmospheric pressure and metallic salt for a onde step nanocomposite thin film process, safe by design: application to plasmonic and magnetic nanocomposite – PLASSEL

The explored solution had not been considered before. It involves jointly using an aerosol of metal salts dissolved in a polymerizable solvent and an atmospheric-pressure cold plasma (AP). The metal salts are soluble in water and most alcohols, offering a wide range of solvent choices for aerosol formation required for synthesis. The diversity and low cost of metal salts pave the way for numerous metals and metal oxides derived from clean, low-cost precursors. The plasma reduces the salt to form metallic particles and polymerizes the solvent to create the composite matrix.

The project focused on two thin-film systems:

Plasmonic gold (Au)/polymer. This system served as a model for the entire experimental and numerical study. From an application standpoint, gold was chosen for its plasmonic properties—the ability of gold nanoparticles to oscillate under the influence of photon interaction at specific energies. This oscillation absorbs energy at a defined wavelength, which lies in the visible spectrum for gold nanoparticles. This absorption can be fully modulated by controlling the size, shape, and dispersion state of the gold nanoparticles within a given matrix, making it a simple way to characterize the Au/polymer composite. Another advantage of gold is its chemical stability.

Magnetic nickel (Ni)/polymer. This system was tested at the end of the project to demonstrate the versatility of the process. Nickel was selected based on research showing that, under non-equilibrium conditions similar to those in plasma processes, metastable allotropic forms of Ni with enhanced magnetic properties—compared to its conventional face-centered cubic allotrope—can be produced.

The overall approach involved studying each step of the process:

The formation of the saline aerosol and its transformation during transport to the plasma,

The formation of particles and their deposition in the plasma, as well as the effect of various plasma parameters, particularly power and discharge regime, by comparing two carrier gases (Ar and N₂) and the frequency of the voltage generating the plasma. Frequencies included:

A relatively low frequency (800 Hz) to transport nanoparticles to the surface and allow ion contribution to deposition,

Intermediate frequencies (20 or 60 kHz) to enhance ion involvement,

Radio frequency (13.56 MHz), which traps all charges in the gas volume, generating a denser plasma.

The morphology, chemistry, and final properties of the synthesized thin films.

At each stage, modeling and experiments were combined.

The overall results enabled a clear understanding of how the process works by linking the nature of the precursors entering the plasma to the morphological, chemical, and final properties of the thin films.

The minimum amount of solvent required to achieve a stable aerosol is 2%, which is too high to obtain a homogeneous discharge at atmospheric pressure, regardless of the carrier gas or the frequency of the voltage generating the plasma. However, homogeneous deposits are achieved by sufficiently alternating between a very low-frequency voltage (800 Hz), which electrostatically transports gold nanoparticles to the substrate, and a higher-frequency voltage that provides enough power to polymerize the polymer.

The salt is reduced very efficiently and rapidly by the plasma. Each nanoparticle is formed from the gold atoms contained within an aerosol droplet. Depending on the solvent's polymerization rate, the nanoparticle is coated with a more or less thick polymer layer. Depending on the nature of the salt and the carrier gas, the nanoparticles either aggregate or remain dispersed. For example, the presence of NHₓ in the salt or gas promotes aggregation. Conditions that robustly prevent any aggregation and lead to arrays of core-shell gold/polymer nanoparticles have been defined.

Key factors to control (i) the quantity and density of gold, (ii) the growth rate of thin films, (iii) nanoparticle aggregation, and (iv) the formation of metal-polymer core-shell particles have been identified. Some limitations have also emerged: the discharge remains filamentary, and the reactivity of the salts prevents the use of nitrogen as a non-reactive carrier gas. It has been shown that the effect of the parameters is independent of the nature of the metal ions, which is very promising for the diversity of nanocomposites that could be synthesized. However, further study is needed to determine how to prevent nickel oxidation and whether metastable forms can be obtained.

The PLASSEL process robustly synthesizes plasmonic gold/polymer nanocomposites with controlled morphology.

The next step is the upscaling. Further work will also be needed to find solutions to prevent nickel oxidation, with the first approach being to reduce the amount of oxygen in the plasma.

Open questions remain, particularly regarding the mechanisms leading to the formation of nanoparticle arrays or aggregates, and understanding the significant variation in deposition rate depending on the discharge power in radio-frequency mode. This understanding is crucial because this discharge mode results in high deposition rates and gold concentrations.

PLASSEL aims to develop a low cost, one step, process able to prepare polymer/metal nanocomposite (NCs) thin films, with well-controlled properties on large surface substrates. This safe by design process at low environmental impact will be developed for gold plasmonic and nickel magnetic NCs. Its originality comes from the association of an aerosol of metallic salt dissolved in a solvent with a multifrequency Dielectric Barrier Discharge (DBD) producing a low temperature plasma at atmospheric pressure easily upgraded on large-scale surface. The plasma induces metallic nanoparticles (NPs) formation, their transport onto the substrate and the polymerization of the solvent to make the NCs matrix and/or NPs shell. The scientific approach combines numerical modeling and experiments. This project aims to understand the transformation of metallic salt into NPs, designs an optimal plasma source and optimizes NCs properties. The equilibrium properties of the plasma will be used to get original NP’s shapes and/or structures.
By continuously referring to the literature on plasmonic NCs, PLASSEL will develop, for the Au/polymer system, plausible models of nucleation and growth of NPs and will identify the key phenomena and associated experimental parameters, which will allow the morphological control of NCs. It will also develop specific tools for the in situ characterization of NPs to follow their formation along the whole production chain. This know-how will then be transposed to the case of magnetic NCs, with a special emphasis on the use of the off-equilibrium conditions of DBDs to promote the formation of allotropic varieties other than the thermodynamically stable one of Ni. These new metallic phases will allow PLASSEL to explore new magnetic properties, scarcely studied to date.
The fine control of the properties of CNs involves (i) determining the transformation processes of metal salts droplets into metal NPs in the presence of energetic plasma species (ii) controlling the size, shape, structure of NPs and their density in NCs. The understanding of all the process parameters and in particular the discharge regime on solvent polymerization and NPs growth in the plasma volume and on the substrate will allow controlling each of the mechanisms as independently as possible.
Finally, thanks to a pilot plasma reactor available in the consortium, the production of these NCs will be scaled up to determine how to make the process industrially recoverable in the long term.
The feasibility of this new NC manufacturing process has just been established by the coordinator's laboratory on the Au/polymer system. These pioneer experimental results are the origin of the present proposal.
The consortium brings together specialists of modeling and experimentation having large knowledge of the growth and transport of NPs in plasma (LSPM, LAPLACE, PROMES), discharges at atmospheric pressure and their associated processes (PROMES, LAPLACE) as well as specialists in the processing and characterization of NCs (PROMES, ITODYS) and their plasmonic (PROMES) and magnetic properties (ITODYS).
The versatility of PLASSEL process in terms of variety of materials prepared, structural and microstructural properties explored and its easy upscaling make it a unique process. It places our consortium as a clear leader in these fields. The significant expected advances in functional metallic nanomaterials should accentuate this scientific leadership in many domains as the synthesis of materials and their physical properties, the experimental in-situ analysis and the physic of plasma interaction with salt droplets or NPs. Results will pave the way for technology transfers.