(Invited) Negative Ions, Nanoparticles and Their Transport to Substrates: Towards Better Control of Plasma-Based Fabrication of Surfaces with Tunable Wetting Properties

Abstract

The removal of pollutants from water, such as oil residues, requires the production of materials with controlled surface properties, especially in terms of their specific surface area and wetting properties [1]. Environmentally friendly processes based on plasma techniques enable the production of surfaces with a wide variety of different properties in terms of their surface topography and chemical composition. An important component that often forms in low-temperature plasmas operated with organic monomers are nanoparticles. Although often considered an undesirable side effect in many applications, nanoparticles can also be used to create nanocomposites or to decorate surfaces to control their wetting properties and their surface roughness [2]. In order to harness the nanoparticles from a plasma it is therefore an essential task to control their formation and ultimately their transport to the reactor walls or to potential substrates. This contribution will focus on the formation of nanoparticles in low temperature, low pressure plasmas operated with different organic monomers as a precursor gas. Although the exact mechanism responsible for the creation of nanoparticles clearly depends on the nature of the process, it is believed that negative ions play a crucial role in the initial stages of the nucleation [3]. In contrast to neutral radicals or positive ions, which are rapidly lost due to diffusion to the reactor walls the residence time of negative ions in the plasma bulk is usually much larger. Negative ions can therefore more easily form larger molecules, which can serve as the basis for the formation of primary clusters in the nanometer or subnanometer range and in this way trigger the agglomeration processes that lead to the formation of larger particles. Since negative ions can leave the plasma during the plasma-off phase of a pulsed discharge, pulsing the plasma is a suitable means to control the accumulation of negative ions and thus the formation of nanoclusters. The experimental results show a complex interplay between pulse frequency or duty cycle and nanoparticle formation. Even the smallest changes in plasma off-time can lead either to a discharge regime that favors nanoparticle formation or to a discharge regime that prevents nanoparticle formation. The minimum plasma off time required to prevent the build- up of particles in a pulsed discharge depends strongly on parameters like power, pressure, gas mixture ratio and the plasma on time. Although the formation of nanoparticles can be very precisely controlled by an appropriate choice of pulse frequency and duty cycle, their transport and controlled deposition on substrates remains a difficult task. In this context, the use of new structured "filtering" electrodes [4] is discussed. The results show that these electrodes allow the deposition of materials with very different surface topographies. Depending on the plasma conditions and the DC voltages applied to parts of the electrodes, the resulting deposits can range from highly localized polymer structures to dense layers of nanoparticles or polymer films with Turing-like patterns. Literature [1] M. Padaki, R. Surya Murali , M.S. Abdullah, N. Misdan , A. Moslehyani , M.A. Kassim , N. Hilal , A.F. Ismail , Desalination, 357,P. 197 (2015)[2] Berndt, J., Acid, H., Kovacevic, E., Boufendi, L., Journal of Applied Physics, 113(6), p.063302. (2013)[3] Berndt, J., Kovacevic, E., Boufendi,L. Journal of Applied Physics, 106, 063309 (2009)[4] Sciacqua, D., Pattyn, C., Jagodar, A., von Wahl, E., Lecas, T., Strunskus, T., Kovacevic, E. and Berndt, J., 2020.. Scientific Reports, 10(1), pp.1-8. Acknowledgement Authors acknowledge the project PEGASUS (Plasma Enabled and Graphene Allowed Synthesis of Unique nano- Structures), funded by the European Union's Horizon research and innovation programme under grant agreement No 766894 and the EU Graphene Flagship FLAG-ERA III JTC 2021 project VEGA (PR-11938). TS, CB ,and JB want to thank HZB for the allocation of synchrotron radiation beamtime at Bessy II. This project has received funding from the European Union’s Horizon 2020 research and innovation programme under grant agreement No 730872 (Nr. 18207084-ST and 18207393- ST). TV, CB and JB acknowledge also support obtained via ARD MATEX Region Centre and Erasmus + Program with WHS Recklinghausen and Prof dr Franziska Traeger.