(Invited) Particle Formation Mechanisms in Low Temperature Plasmas
Authors: Berndt Johannes , Pattyn Cedric , Kovacevic Eva 
Published: February 2020

Low temperature plasmas, in particular when they are operated in molecular (organic) gases, are rather complex systems. Their distinct non equilibrium nature, characterized by low neutral gas temperatures (close to room temperature) and relatively high electron temperatures (easily reaching values up to several eV) make them a breeding place for a great variety of different species and depending on the precursor gas and the process conditions even a source for nanoparticles. The formation of nanoparticles has been observed in various discharge types operated in different kinds of gases or gas mixture as e.g.in silane, in fluorocarbons such as CF4 or C2F6 or in hydrocarbons as e.g. acetylene. The formation of nanoparticles in a plasma is either an unwanted side effect, occurring for example during the plasma based deposition of thin films (or the etching of microstructures) or it is the deliberate result of a process used e.g. for the production of nanoparticle deposits. In either way the understanding of the underlying mechanisms which are eventually responsible for the nanoparticle formation in low temperature plasmas is indispensable for the control of the whole process. However, the plasma based formation of nanoparticles is a rather complex procedure, which involves different plasma species, various timescales and several physical and chemical processes. The process starts with the dissociation of the original parent molecule due to electron impact reactions. These reactions are responsible for the initial formation of molecular fragments which can induce subsequent polymerization reactions leading by and by to the formation of larger and larger molecules and eventually to the formation of nanometer sized clusters. These initial protoparticles can further grow due to coagulation and/or accretion processes till they reach a size of some 10 nanometer. Once the particles have reached this size they become rapidly negatively charged and their further growth is mainly governed by the collection of positive ions and neutral radicals. In contrast to CVD processes which are mainly driven by neutral molecules and radicals plasma based processes involve also charged species, namely electrons and different kinds of ions. Depending on the precursor gas used for the given process this ionic component can include positive as well as negative ions.
This contribution will focus on the nanoparticle formation in two exemplary systems: discharges operated in argon/acetylene and argon/aniline mixtures. While acetylene plasma are frequently used for the production of DLC films, the use of aniline discharges has been intensively studied for the (conformal) deposition of (ultra) thin polyaniline films or the production of polyaniline nanoparticle deposits. In particular the latter material class possess interesting potential applications in the field of biosensors and for the production of supercapacitors.
Based on various in-situ plasma diagnostics (plasma ion mass spectroscopy, in-situ multi-pass FTIR spectroscopy, microvawe interferometry, laser light scattering and optical emission spectroscopy) and different discharge designs the particle formation in both systems is studied aiming to understand the underlying formation mechanisms in order to either promote or prevent the generation of nanoparticles.
Special emphasis is paid to the role of the negative ion component. In non-pulsed discharges (low energy) negative ions are trapped in the positive plasma potential inside the plasma bulk and while not contributing to the flux of species reaching the wall of the plasma chamber they can be a dominant species inside the plasma volume. They may play therefore an important role in the initial formation of protoparticles i.e. in the early nucleation phase.

Acknowledgments

The authors would like to acknowledge the support obtained by the French National Research Agency via the project PlasmaBond. This work has received also funding from the European Union's Horizon 2020 research and innovation programme under grant agreement No 730872 (CALYPSO plus for the work at the HZB BESSY II synchrotron, Berlin) and under grant agreement No766894 FETOPEN project PEGASUS.