(Invited) Non-Thermal Plasmas for the Production of Sustainable Functional Materials
Authors: Alvarez Barragan Alejandro , Mangolini Lorenzo 
Published: February 2020

This talk will highlight the potential of non-thermal plasmas as a viable synthetic approach for materials that are difficult to produce using other techniques. Materials such as titanium and zirconium nitride are impossible to produce in nanoparticle form using liquid phase chemistry techniques, and are not compatible with approaches such as flame and spray pyrolysis because they are not oxide-based ceramics. At the same time, they are quickly attracting interest because they are predicted to show plasmonic response in the visible, making them great candidates for the replacement of silver and gold-based plasmonics. Our recent investigation in this material system confirms that low-temperature, non-thermal plasma processes are compatible with the synthesis of titanium and zirconium nitride nanocrystals. Particles are produced using metal chlorides and ammonia as precursors. The plasma reactor is designed in a continuous flow configuration, where precursors are continuously fed into the plasma volume and nanoparticles are continuously collected downstream of the plasma by filtering. This technique allows producing TiN and ZrN nanocrystals with average size below 10 nm, with narrow size distribution (all particles are within 5 nm and 15 nm) and with perfect stoichiometry. In partial agreement with the theoretical predictions, such particles show plasmon resonance in the visible and in the near-infrared part of the spectrum. Yet the plasmon energy is lower than theoretically predicted, a behavior that has not been observed before and that we attribute to the presence of a native oxide shell. This observation is justified by simple experiments in which the particles are annealed in air at moderate temperature (<300°C), leading to the progressive growth of an oxide shell, as confirmed by elemental analysis and by XPS. The plasmon peak energy continuously red shifts during the annealing process, suggesting that a strategy for limiting oxide growth needs to be developed to enable the use of these nanomaterials for refractory plasmonic applications. In addition to discussing the properties of these novel plasma-produce nanomaterials, this talk will summarize our most recent findings with respect of their nucleation and growth. A combination of in-situ FTIR and optical emission spectroscopy measurements suggests that the nucleation process is initiated via atomic hydrogen-induced dissociation of the chlorine-based metal precursor, in this case titanium tetrachloride.
This talk will also cover our most recent studies on the interaction between nanoparticles and plasmas. Despite operating near room temperature, and despite the short residence time (tens to hundreds of milliseconds) these systems are capable of producing nanocrystals of high melting point materials. Moreover, there is mounting evidence that such systems are compatible with the production of a broad range of high melting point materials. Therefore it is necessary to achieve a detailed understanding of the phenomena occurring in such reactors to fully take advantage of their capabilities as a nanomaterial synthetic approach. Using the silane-based chemistry as a testbed, we have performed careful in-situ FTIR characterization of the surface of silicon nanoparticles immersed in a non-thermal plasma. The hydrogen surface coverage decreases as the input power density increases. We attribute this to plasma-induced heating of nanoparticles to temperatures sufficiently high to lead to hydrogen desorption. This observation confirms the prediction of several theoretical studies that there is a non-equilibrium between nanoparticle temperature in a dusty plasma and the background gas temperature.