Foundations of measurement of electrons, ions and species fluxes toward surfaces in low-temperature plasmas

Abstract

This foundation article discusses the diagnostics of electrons and ions in plasmas and fluxes of charged and neutral species toward plasma-facing surfaces by non-optical methods. The focus is laid on the fundamentals of the most common methods and their application to non-equilibrium low-pressure electropositive plasmas and ion beams. The key topics are Langmuir probes (LPs), Faraday cups (FCs) and energy analyzers, mass spectrometry, as well as novel calorimetric and force probes (FPs). For each method, the concepts are introduced, hints at best practice are given, and a survey of the recently published literature is included. Starting with the sheath–plasma boundary, the principles of plane and cylindrical LPs and the measurement of electron temperature and density are discussed. Beyond standard textbooks, double, triple and emissive probes are described and modulation techniques for obtaining the electron energy distribution function are presented. More technical issues related to probe compensation in radio-frequency plasmas, probe contamination, and the influence of a magnetic field are also included. The presentation is completed by modern radio-frequency techniques, such as impedance probes, multipole resonance probes, and self-excited electron resonance spectroscopy. FCs, retarding field analyzers (RFAs) as well as novel calorimetric and FPs are useful tools for the measurement of overall, not species resolved, ions and neutral species fluxes toward surfaces. RFAs provide overall ion energy distribution functions, whereas calorimetric and FPs can deliver information about fluxes of fast neutrals. The issues related to secondary electron emission, absolute signal calibration, and analysis of plasmas with complex chemistry are discussed. Mass spectrometry diagnostics is capable of mass and energy-resolved detection of ions and neutral species at the plasma-facing surfaces. Detection, identification and absolute density calibration of neutral stable and radical species are treated in detail and peculiarities of ion mass spectrometry, such as detection of negative ions, ion measurements in front of powered electrodes, or chromatic aberration and acceptance angle limitations in energy spectrum measurements are introduced.

Plasma modified and tailored defective electrocatalysts for water electrolysis and hydrogen fuel cells

“…In the fabrication of nanocatalysts, the plasma conditions are different from those in conventional ones because electrons move faster than ions and particles consequently creating a plasma sheath around the sample. 77 As a result, an equilibrium is created to control the dynamics of positive ions and electrons across the plasma sheath which can be regulated to produce the desirable modification effects. 54,56,78,79 For example, vertical transition metal dichalcogenide nanosheets and carbon nanotubes (CNT) can be prepared on different types of substrates to form electrocatalysts, supercapacitors, and batteries.…”

A Missing Puzzle in Dissociative Electron Attachment to Biomolecules: The Detection of Radicals

“…In these plasma studies, appearance mass spectrometry was used, which is based on the difference between the appearance potential for the ionization of radicals and that for the dissociative ionization of precursor molecules [40]. It has been used widely in the diagnostics of neutral species in plasmas [41]. This technique has been applied successfully in cross-sectional measurements of electron-impact neutral dissociation of gas-phase methane (CH4) into CH3 and CH2 radicals [33,34]; carbon tetrafluoride (CF4) into CF3, CF2, and CF [35]; silicon tetrafluoride (SiF4) into SiF3, SiF, and Si [36]; trifluoromethane (CHF3) into CF3, CF2, CF, CHF2, and CHF [37]; and sulfur hexafluoride (SF6) into SF3, SF2, and SF [38,39].…”

“…This technique has been applied successfully in cross-sectional measurements of electron-impact neutral dissociation of gas-phase methane (CH4) into CH3 and CH2 radicals [33,34]; carbon tetrafluoride (CF4) into CF3, CF2, and CF [35]; silicon tetrafluoride (SiF4) into SiF3, SiF, and Si [36]; trifluoromethane (CHF3) into CF3, CF2, CF, CHF2, and CHF [37]; and sulfur hexafluoride (SF6) into SF3, SF2, and SF [38,39]. To date, no studies of biomolecules in the gas phase have been performed, while the neutral products have been studied only for halogenated compounds [32][33][34][35][36][37][38][39][40] or small molecules, particularly hydrocarbons [41]. However, the neutrals formed from these compounds were not produced at energies corresponding to DEA, which occurs below 12 eV, but at much higher energies, above 14 eV and most often at approximately 70 eV, an electron energy region with the highest cross-section for ionization.…”

Engineered ion-bombardment as a tool in thin film deposition

“…There are several methods to characterize plasma properties. The most common methods are probe diagnostics such as the Langmuir probe and emissive probe, which enable the measurement of parameters such as plasma potential, floating potential, electron densities and electron energy distribution function [137]. Due to the abundance of ions of the sputtered species produced in HiPIMS discharges and their significant role in the growth of thin films (see Sec 3.2), it is in addition important to be able to characterize the nature of the ions arriving at a typical substrate position.…”