Fundamentals and Applications of Plasma Nonequilibrium at Interfaces

Plasma-generated high nonequilibrium fluxes of energetic ions, metastables, reactive radicals or electrons reaching the surfaces are widely used in the research and industry. Still, some of the microscopic interaction processes including ion induced secondary electron emission or electron reflection on realistic surfaces are not fully understood. Additionally, the <i>in situ</i> monitoring and control of for example ion energy-flux distribution function is needed to provide precise control of material processing needed for applications such as processing of 2D materials or manufacturing of components for quantum computing. This contribution will discuss the application of energy-resolved ion mass spectrometry (ERIMS) in two experiments used to analyze plasma-surface interaction and perform controlled material processing.<br/><br/>First, a technique that combines ERIMS measurements and 1d3v particle-in-cell /Monte Carlo collision simulations to determine effective ion induced secondary electron emission (SEE) yield and the effective elastic electron reflection probability parameters simultaneously and independently in a symmetric rf capacitively coupled plasma in argon [1]. It is possible, because SEE and electron reflection processes at the surface influence plasma and sheath properties and with it also the ion flux-energy distribution function (IEDF) measured at the electrode. In particular, the energy width and mean energy of the bimodal peak feature in the IEDF can provide the effective values of both parameters for any material on the electrode surface. The method was applied to stainless steel and aluminum oxide surfaces resulting in a good agreement with literature values.<br/>Second, a new reactor with inverted electrode geometry was assembled which is combining ICP plasma with CCP bias applied to reactor wall to control the plasma density and ion energy separately at the small grounded electrode during the treatment [2]. The small size of the grounded electrode results in the largest voltage drop and largest ion energy there, which can be analyzed by ERIMS during the plasma treatment including the analysis of the sputtered or desorbed material on the ion composition. The CCP bias allow further control of the ion energy. We have observed previously that low-pressure plasma treatment improves electrochemical performance of transition metal oxides used at air electrode of zinc-air batteries [3]. We use this material here as a test substrate for the study of the effect of the ion energy on their performance in hydrogen and nitrogen discharges. Additionally, the nitrogen plasma has also been used to treat graphene and results relating ion IEDF and structural changes of graphene will be presented. The graphene has been chosen here as a model material for the 2D materials. Experiments with transition metal dichalcogenides and their monolayers will follow. All examples in this contribution are part of the larger research program at Kiel University focusing on the experimental and theoretical study of the plasma nonequilibrium at interfaces.<br/><br/>[1] C. Schulze, Z. Donkó, J. Benedikt, <i>A computationally assisted technique to measure material-specific surface coefficients in capacitively coupled plasmas based on characteristics of the ion flux-energy distribution function</i>, Plasma Sources Sci. Technol. 31 (2022) 105017<br/>[2] C. Schulze, H. Li, L. Mohn, M. Müller, J. Benedikt, <i>Chamber with Inverted Electrode Geometry for Measuring and Control of Ion Flux-Energy Distribution Functions</i>, Plasma 2022, 5(3), 295<br/>[3] H. Li, S. Askari, J. Wang, N. Wolff, M. Behrens, L. Kienle, J. Benedikt, <i>Nitrogen-Doped NiCo₂O₄ Nanowires on Carbon Paper as a Self-Supported Air Cathode for Rechargeable Zn-Air Batteries</i>, accepted for publication in Journal of Hydrogen Energy; DOI:10.1016/j.ijhydene.2023.03.146