Plasma Chemistry and Plasma Processing (v.38, #1)

A Comparison of Floating-Electrode DBD and kINPen Jet: Plasma Parameters to Achieve Similar Growth Reduction in Colon Cancer Cells Under Standardized Conditions by Sander Bekeschus; Abraham Lin; Alexander Fridman; Kristian Wende; Klaus-Dieter Weltmann; Vandana Miller (1-12).
A comparative study of two plasma sources (floating-electrode dielectric barrier discharge, DBD, Drexel University; atmospheric pressure argon plasma jet, kINPen, INP Greifswald) on cancer cell toxicity was performed. Cell culture protocols, cytotoxicity assays, and procedures for assessment of hydrogen peroxide (H2O2) were standardized between both labs. The inhibitory concentration 50 (IC50) and its corresponding H2O2 deposition was determined for both devices. For the DBD, IC50 and H2O2 generation were largely dependent on the total energy input but not pulsing frequency, treatment time, or total number of cells. DBD cytotoxicity could not be replicated by addition of H2O2 alone and was inhibited by larger amounts of liquid present during the treatment. Jet plasma toxicity depended on peroxide generation as well as total cell number and amount of liquid. Thus, the amount of liquid present during plasma treatment in vitro is key in attenuating short-lived species or other physical effects from plasmas. These in vitro results suggest a role of liquids in or on tissues during plasma treatment in a clinical setting. Additionally, we provide a platform for correlation between different plasma sources for a predefined cellular response.
Keywords: Atmospheric pressure argon plasma jet; Dielectric barrier discharge; Hydrogen peroxide; kINPen; Plasma medicine

Prospective Applications of Low Frequency Glow Discharge Plasmas on Enhanced Germination, Growth and Yield of Wheat by N. C. Roy; M. M. Hasan; M. R. Talukder; M. D. Hossain; A. N. Chowdhury (13-28).
Medium pressure (~ 10 torr) low frequency (3–5 kHz) glow discharge (LFGD) plasmas were applied to treat wheat (Triticum aestivum) seeds to investigate the effects on water absorption, seed germination rate, seedling growth and yield. The LFGD plasmas were produced with air and air/O 2. Optical emission spectroscopic diagnostic methods were revealed that the $${ ext{N}}_{2} left( {{ ext{C}}^{3}Pi _{ ext{u}} - { ext{B}}^{3}Pi _{ ext{g}} } ight)$$ N 2 C 3 Π u - B 3 Π g , $${ ext{N}}_{2}^{ + } left( {{ ext{B}}^{2}Sigma _{ ext{u}}^{ + } - { ext{X}}^{2}Sigma _{ ext{g}}^{ + } } ight)$$ N 2 + B 2 Σ u + - X 2 Σ g + and $${ ext{N}}_{2} left( {{ ext{B}}^{3}Pi _{ ext{g}} - { ext{A}}^{3}Sigma _{ ext{u}}^{ + } } ight)$$ N 2 B 3 Π g - A 3 Σ u + produced with air, and O species were produced along with nitrogen species with air/O 2 plasmas, respectively. The SEM images were revealed that the surface architectures and functionalities of the seeds were modified due to plasma treatments. Water absorption was found to increase with treatment time. 6 min treatment was provided 95–100% seed germination. The plants grown from treated seeds for 3 and 9 min duration by air/O 2 plasma were showed the highest growth activity and dry matter accumulation. Total chlorophyll contents of the leaves, longest spikes and number of spikes/spikelet were also increased. The wheat yield was increased ~ 20% over control by 6 min treatment with air/O 2 plasma. Overall results revealed that LFGD plasmas can significantly change seed surface architecture, water absorption, germination rate, seedling growth and yield of wheat.
Keywords: Glow discharge plasma; Reactive nitrogen and oxygen species; Seed germination; Growth; Yield

Non-thermal Plasma Induced Expression of Heat Shock Factor A4A and Improved Wheat (Triticum aestivum L.) Growth and Resistance Against Salt Stress by Alireza Iranbakhsh; Narges Oraghi Ardebili; Zahra Oraghi Ardebili; Mohammadreza Shafaati; Mahmood Ghoranneviss (29-44).
There is a huge interest in making and applying innovating functional devices based on basic sciences (like physics) to improve plant growth and resistance against various stress conditions. This research was carried out in order to investigate effects of cold plasma on expressions of heat shock factor A4A (HSFA4A), plant growth and post reactions to salt stress. Wheat seedlings were treated with plasma (0.84 W/cm2 surface power densities) at different exposure times. In both three and 6 h after plasma, inductions in expressions of HSFA4A were recorded in roots, compared to control. Six hours after treatments, plasma-induced the shoot expressions of HSFA4A in the treated seedlings, contrasted to 3 h. Plasma treatment caused not the only enhancement in shoot fresh and dry mass and total leaf area, but also alleviated adverse impacts of salinity. Destroying impacts of salinity on chlorophyll contents were mitigated by plasma. Peroxidase activity was decreased by 27% for salinity treatment alone over control, while it was increased by 15% for plasma and salinity-treated samples, compared to salinity control. The highest activities of phenylalanine ammonia lyase (PAL) were found in plasma treatment alone. PAL activity was found to be higher in plasma-pretreated seedlings counteracted to salt stress, relative to the salinity control. The plasma treatment may act as an effective elicitor to modify gene expression, thereby improving plant growth and resistance. Plasma technology should be considered as a new functional technology in plant sciences.
Keywords: Cold plasma; DBD; Heat shock factor; Elicitor; Nitric oxide; Signaling

A quantitative interpretation of the schlieren technique applied to a non-thermal atmospheric-pressure oxygen plasma jet driven at low-frequency (50 Hz) is reported. The jet was operated in the turbulent regime with a hole-diameter based Reynolds number of 13,800. The technique coupled to a simplified kinetic model of the jet effluent region allowed deriving the temporally-averaged values of the gas temperature of the jet by processing the gray-level contrast values of digital schlieren images. The penetration of the ambient air into the jet due to turbulent diffusion was taken into account. The calibration of the optical system was obtained by fitting the sensitivity parameter so that the oxygen fraction at the nozzle exit was unity. The radial profiles of the contrast in the discharge off case were quite symmetric on the whole outflow, but with the discharge on, relatively strong departures from the symmetry were evident in the near field. The time-averaged gas temperature of the jet was relatively high, with a maximum departure of about 55 K from the room temperature; as can be expected owing to the operating molecular gas. The uncertainty in the temperature measurements was within 6 K, primarily derived from errors associated to the Abel inversion procedure. The results showed an increase in the gas temperature of about 8 K close to the nozzle exit; thus suggesting that some fast-gas heating (with a heating rate ~0.3 K/μs) still occurs in the near field of the outflow.
Keywords: Non-equilibrium plasma jet; Gas temperature; Turbulent air mixing; Schlieren technique

Plasma conductivity as a probe for ambient air admixture in an atmospheric pressure plasma jet by F. J. J. Peeters; R. F. Rumphorst; M. C. M. van de Sanden (63-74).
By utilizing a fully floating double electrical probe system, the conductivity of a linear atmospheric pressure plasma jet, utilizing nitrogen as process gas, was measured. The floating probe makes it possible to measure currents in the nanoamp range, in an environment where capacitive coupling of the probes to the powered electrodes is on the order of several kilovolts. Using a chemical kinetic model, the production of reactive nitrogen oxide and hydrogen-containing species through admixture of ambient humid air is determined and compared to the measured gas conductivity. The chemical kinetic model predicts an enhanced diffusion coefficient for admixture of O2 and H2O from ambient air of 2.7 cm2 s−1, compared to a literature value of 0.21 cm2 s−1, which is attributed to rapid mixing between the plasma jets and the surrounding air. The dominant charge carriers contributing to the conductivity, aside from electrons, are NO+, NO2 and NO3 . Upon admixture of O2 and H2O, the dominant neutral products formed in the N2 plasma jet are O, NO and N2O, while O2(1Δg) singlet oxygen is the only dominant excited species.
Keywords: Plasma jet; Double probe; Chemical kinetic model

Rapid Formation of Diamond-Like Nano-Carbons in a Gas Bubble Discharge in Liquid Ethanol by Zhiqiang Chen; Kevin Magniez; Marin Duchemin; Nikki Stanford; Arun T. Ambujakshan; Adam Taylor; Cynthia S. Wong; Yan Zhao; Xiujuan J. Dai (75-87).
This work demonstrates that diamond-like nano-carbons can be rapidly grown at atmospheric pressure and near ambient temperature in Ar gas bubble discharge in liquid ethanol. The method uses a discharge between point-to-plate electrodes immersed in ethanol, with plasma being generated inside Ar gas bubbles introduced through the needle electrode. The ethanol was dissociated at the liquid/gas interface into reactive species such as C2 and CH, which are the primary species responsible for diamond formation. A mixture of lonsdaleite nano-diamonds, amorphous carbon nano-spheres, and a graphitic carbon network, was formed. The rapid bubble movement distributes the reaction products almost immediately into the liquid phase, ensuring that nucleation of new material continues throughout the process. This simple, inexpensive and fast process avoids the elevated temperatures and extreme pressures of current methods.
Keywords: Diamond-like nano-carbons; Gas bubble discharge; Ethanol

Aqueous Reactive Oxygen Species Induced by He + O2 Plasmas: Chemistry Pathways and Dosage Control Approaches by Chen Chen; Dingxin Liu; Aijun Yang; Hai-Lan Chen; Michael G. Kong (89-105).
Plasma–liquid interaction has already been a hotspot in the research field of plasma medicine. Aqueous reactive oxygen species (ROS) generated in this process are widely accepted playing a crucial role in plasma biomedical effects. In this paper, chemistry pathways among various aqueous ROS induced by He + O2 plasmas are investigated by a numerical model. Simulation results show that these aqueous ROS can be classified into two groups according to their production ways: the group of species including O, 1O2 and e directly produced in plasma, and the other group of species including O2 , H2O2, O3, etc. produced by liquid reactions. A key reaction chain of e → O2  → HO2(→ HO2 ) → H2O2 is found to be important in the plasma-induced liquid chemistry. Furthermore, impacts of changes in plasma and solution conditions on aqueous ROS concentrations are studied as well. It is found that changes in plasma conditions (O2 ratio in the discharge gas/power density) can globally influence the concentrations of almost every aqueous ROS, while conditions changes of the treated liquid (pH/dissolved oxygen) only partially influence the concentrations of some specific species including O2 /HO2, O3 /HO3 and H2O2. The revelations of the liquid chemistry pathways and the dependence of ROS dosage on the treatment conditions offer a better understanding on the plasma–liquid interactions, as well as provide optimized dosage control approaches for biomedical applications.
Keywords: Cold atmospheric plasma; Plasma–liquid interaction; Aqueous reactive oxygen species; Chemistry pathway; Dosage control

Synthesis of CdO Ultradisperse Powders Using Atmospheric Pressure Glow Discharge in Contact With Solution and the Investigation of Intermediate Products by Dmitriy A. Shutov; Kristina V. Smirnova; Mikhail V. Gromov; Alexander N. Ivanov; Vladimir V. Rybkin (107-121).
A new plasma-solution method of the CdO ultradisperse powders synthesis was described. The atmospheric pressure direct current discharge was excited in the ambient air by applying a high direct voltage to two pointed titanium electrodes placed above liquid anode and liquid cathode in the H-shaped cell. The discharge current was 40 mA and the total input power was about 40 W. The action of the DC glow discharge on the cadmium nitrate water solution in the absence of additional reagents and without electrodes-solution contact was shown to result in the production of the solids in the liquid phase. The kinetics of particles formation was studied using turbidimetry and nephelometry methods. Powders’ chemical composition and morphology was obtained using X-ray diffraction spectroscopy (XRD), electron-dispersive X-ray spectroscopy (EDX), thermogravimetric analysis (TGA), differential-scanning calorimetry (DSC) and scanning electron microscopy (SEM). It was found that as-synthesized powders are not the pure cadmium hydroxide but the mixture of the cadmium nitrate, hydroxy nitrate and hydroxide. Some assumptions regarding the mechanisms and pathway of the chemical processes both under the plasma action on the solution and during the calcination of as-synthesized powders were discussed.
Keywords: Plasma; CdO synthesis; Liquid anode; Kinetics; Intermediate Products; Mechanisms

Destruction Kinetics of 2,4 Dichlorophenol Aqueous Solutions in an Atmospheric Pressure Dielectric Barrier Discharge in Oxygen by Andreiy A. Gushchin; Vladimir I. Grinevich; Valeriy Ya. Shulyk; Elena Yu. Kvitkova; Vladimir V. Rybkin (123-134).
The processes of degradation of 2,4-dichlorophenol (2,4-DCP) aqueous solutions under the action of atmospheric pressure of DBD in oxygen were studied. The degradation of 2,4-DCP proceeds efficiently, the degree of decomposition reaching 100%. The degradation kinetics of 2,4-DCP obeys a formal first-order kinetic law on concentration of 2,4-DCP. The effective rate constants depend weakly on the experimental conditions and are equal to ~ 2 s−1. Based on experimental data, the energy efficiency of 2,4-DCP decomposition was determined to be in the range of 0.039–0.173 molecules per 100 eV depending on the experimental conditions. The composition of the products was studied by gas chromatography, chromatography-mass spectrometry, UV/visible spectroscopy, fluorescent methods and some chemical methods. The main decomposition products present in the solution were found to be carboxylic acids, aldehydes and chloride ions, whereas carbon dioxide and molecular chlorine appear in the gas. The results obtained are compared with similar data from other advanced oxidation processes (AOP’s) methods.
Keywords: Oxygen DBD; Kinetics; 2,4 Dichlorophenol; Decomposition products

Plasma-Chemical Conversion of Hydrogen Sulfide in the Atmosphere of Methane with Addition of CO2 and O2 by Andrey N. Ochered’ko; Sergey V. Kudryashov; Andrey Yu. Ryabov (135-146).
Plasma-chemical conversion of hydrogen sulfide in the atmosphere of methane with addition of CO2 and O2 in the nonequilibrium plasma of barrier discharge is studied. The degree of hydrogen sulfide removal reaches 97 vol%. The degree of methane transformation does not exceed 14 vol%. Gaseous reaction products contain hydrogen, carbon oxides, and C2–C4 hydrocarbons. The energy consumption for the removal of hydrogen sulfide ranges from 84 to 182 eV molecule−1. The process is accompanied by the formation of deposits on the surface of reactor electrodes. The composition of deposits is studied. Organic linear and cyclic polysulfides, as well as sulfones of various structures are identified in soluble components of deposits. Based on the experimental data and the results of theoretical estimates, a radical chain reaction mechanism is proposed. It is shown that the formation of polysulfide compounds with terminal alkyl and oxygen-containing groups is provided by the reactions between atomic oxygen, SH, and alkyl radical which were formed in the initial stages of processes in the non-equilibrium plasma of barrier discharge.
Keywords: Methane; H2S; CO2 ; O2 ; Polysulfides; Barrier discharge

Modelling of the Plasma–Sheath Boundary Region in Wall-Stabilized Arc Plasmas: Unipolar Discharge Properties by B. Mancinelli; L. Prevosto; J. C. Chamorro; F. O. Minotti; H. Kelly (147-176).
A two-dimensional model of the non-equilibrium unipolar discharge occurring in the plasma–sheath boundary region of a transferred-arc was developed. This model was used to study the current transfer to the nozzle (1 mm diameter) of a 30 A arc cutting torch operated with oxygen. The energy balance and chemistry processes in the discharge were described by using a kinetic block of 45 elementary reactions and processes with the participation of 13 species including electronically excited particles. The nonlocal transport of electrons was accounted for into the fluid model. The dependence of the ion mobility with the electric field was also considered. Basic discharge properties were described. It has been found that a large part (~ 80%) of the total electric power (1700 mW) delivered in the bulk of the sheath region is spent in heating the positive ions and further dissipated through collisions with the neutral particles. The results also showed that the electron energy loss in inelastic collisions represents only ~ 25% of the electron power and that about 63% of the power spent on gas heating is produced by the ion–molecule reaction, the electron–ion and ion–ion recombination reactions, and by the electron attachment. The rest of the power converted into heat is contributed by dissociation by electron-impact, dissociative ionization and quenching of O(1D). Some fast gas heating channels which are expected to play a key role in the double-arcing phenomena in oxygen gas were also identified.
Keywords: Plasma–sheath; Wall-stabilized arc; Unipolar discharge

Influence of Wire Initial Composition on Anode Microstructure and on Metal Transfer Mode in GMAW: Noteworthy Role of Alkali Elements by F. Valensi; N. Pellerin; S. Pellerin; Q. Castillon; K. Dzierzega; F. Briand; J.-P. Planckaert (177-205).
Metal Active Gas (MAG) welding in presence of Argon and CO2 mixture as shielding gas is a largely developed process allowing the transfer of the liquid metal from the consumable wire anode to the workpiece according to various modes (short-arc, globular, spray-arc). The CO2 presence in the shielding gas leads to the formation of an oxide layer, or gangue, wrapping the droplet, limiting the access to the spray-mode transfer, taking into account the low conductivity and the high viscosity of this layer. Several electrodes of various compositions have been tested thanks to Flux Cored Arc Welding, to limit the gangue formation or its negative contribution, based on Ti, La, Zr and alkali metals addition or reduction in silicon content. The results are interpreted considering the metal transfer mode for a given current intensity (330 and 410 A), with various CO2 concentrations in the shielding gas. Finally, the role of the gangue, compared to the other factors governing the droplet detachment, is discussed. A decrease in silicon content limits significantly the gangue formation and gives access to spray arc transfer up to 30 vol.% of CO2 at 330 A. Titanium addition leads to the same results. The tests in presence of zirconium proved the conductivity improvement of the gangue. The addition of alkali allows to stabilize the spray arc up to the noteworthy value of 60 vol.% of CO2 at 330 A, supporting the hypothesis of a strong influence of viscosity on droplets detachment in the process.
Keywords: Arc plasma; GMAW; Anode microstructure; Metal transfer; Alkali elements

Laser induced breakdown spectroscopy (LIBS) coupled with a laser ablation time of flight mass spectrometer (LA-TOF-MS) has been developed for discrimination/analysis of the precious gold alloys cartage. Five gold alloys of Karats 18K, 19K, 20K, 22K and 24K having certified composition of gold as 75, 79, 85, 93 and 99.99% were tested and their precise elemental compositions were determined using the laser produced plasma technique. The plasma was generated by focusing beam of a Nd:YAG laser on the target in air and its time integrated emission spectra were registered in the range 250–870 nm. The calibration free LIBS technique (CF-LIBS) was used for the quantitative determination of the constituent elements present in different Karats of gold. Elemental compositions of these gold alloys were also determined using a Laser Ablation time of flight mass spectrometer (LA-TOF-MS). The LIBS limit of detection was calculated from the calibration curves for copper, silver and gold. Results of CF-LIBS and LA-TOF-MS are in excellent agreement with the certified values. It is demonstrated that LIBS coupled with LA-TOF-MS is an efficient technique that can be used to analyze any precious alloys in a fraction of a second.
Keywords: LIBS; Karats of gold; Elemental composition; Plasma parameters; LA-TOF-mass spectrometry

Removal of Tin from Extreme Ultraviolet Collector Optics by In-Situ Hydrogen Plasma Etching by Daniel T. Elg; Gianluca A. Panici; Sumeng Liu; Gregory Girolami; Shailendra N. Srivastava; David N. Ruzic (223-245).
The original version of this article unfortunately contained a mistake in the “Deposition of Stannane (DOS) Chamber” section.Extreme ultraviolet (EUV) lithography produces 13.5 nm light by irradiating a droplet of molten Sn with a laser, creating a dense, hot laser-produced plasma and ionizing the Sn to the + 8 through + 12 states. An unwanted by-product is deposition of Sn debris on the collector optic, which focuses the EUV light emitting from the plasma. Consequently, collector reflectivity is degraded. Reflectivity restoration can be accomplished by means of Sn etching by hydrogen radicals, which can be produced by an H2 plasma and etch the Sn as SnH4. It has previously been shown that plasma cleaning can successfully create radicals and restore EUV reflectivity but that the Sn removal rate is not necessarily limited by the radical density. Additionally, while Sn etching by hydrogen radicals has been shown by multiple investigators, quantification of the mechanisms behind Sn removal has never been undertaken. This paper explores the processes behind Sn removal. Experiments and modeling show that, within the parameter space explored, the limiting factor in Sn etching is not radical flux or SnH4 decomposition, but ion energy flux. Thus the removal is akin to reactive ion etching.
Keywords: EUV; Sn; Etching; Reactive ion etching; Collector; Cleaning; In-situ ; Plasma; Hydrogen; SnH4 ; Decomposition

Magnetic Field Resonance and Pressure Effects on Epitaxial Thin Film Deposition and In Situ Plasma Diagnostics by C. R. Yang; C. H. Yeh; L. C. Hu; T. C. Wei; C. C. Lee; J. Y. Chang; T. T. Li (247-259).
This study demonstrated the use of quadrupole mass spectrometry and optical emission spectrometry in diagnosing the plasma in the electron cyclotron resonance chemical vapor deposition (ECRCVD) process. The effects of adjusting the main magnetic coil current and process pressure on chemical composition of the plasma and the characteristics of the epitaxial thin film in the ECRCVD system were investigated. When the main magnetic coil current increased, the deposition rate of thin film increased, with no major effect on thin film crystallization. However, when the process pressure was higher, both the deposition rate and crystallization of epitaxial thin film increased.
Keywords: ECRCVD; Thin film; PECVD; QMS; OES

An one-dimensional fluid model is used to study the physico-chemical properties of a 90%Ar–10%Cl2 plasma produced in dielectric barrier discharges under different driving voltages. The spatio-temporal characteristics of the discharge are obtained by applying a sinusoidal voltage with different amplitudes and frequencies from 350 to 1200 V and 10 to 19 kHz, respectively. As the voltage amplitude increases the plasma electronegativity and plasma radiations are increased and their radiations move toward the cathode vicinity. With decreasing frequency, the plasma radiations move toward the anode vicinity and the plasma electronegativity and its radiations are enhanced. At low voltage amplitudes, plasma most radiates VUV photons while UV radiations take place at higher voltage amplitudes.
Keywords: Low-pressure plasma; Ar/Cl2 plasma; Dielectric barrier discharge

The Radial Distribution of Ions and Electrons in RF Inductively Coupled H2/T2B Plasmas by Xing Ai; Guo Chen; Ling Zhang; Lei Liu; Xiao-Shan He; Zhi-Bing He; Kai Du (281-292).
A glow discharge polymer (GDP) was fabricated using trans-2-butene (T2B) and hydrogen (H2) via a plasma-enhanced chemical vapor deposition (PECVD) system. The uniformity of the GDP films was significantly affected by the radial distribution of the H2/T2B plasma parameters. The plasma properties while discharging by a multi-carbon gas source of mixed H2/T2B were investigated during the GDP deposition process. The main positive ions and ion energy distributions in inductively coupled H2/T2B plasmas were analyzed by energy-resolved mass spectrometer (MS), and the electron density and the effective electron temperature were mainly analyzed using a Langmuir probe. The MS results show that the main positive ions in the plasmas are $${ ext{C}}_{ 2} { ext{H}}_{ 4}^{ + }$$ C 2 H 4 + , $${ ext{C}}_{ 2} { ext{H}}_{ 6}^{ + }$$ C 2 H 6 + , $${ ext{C}}_{ 3} { ext{H}}_{ 3}^{ + }$$ C 3 H 3 + , $${ ext{C}}_{ 3} { ext{H}}_{ 6}^{ + }$$ C 3 H 6 + , $${ ext{C}}_{ 3} { ext{H}}_{ 8}^{ + }$$ C 3 H 8 + , $${ ext{C}}_{ 4} { ext{H}}_{ 5}^{ + }$$ C 4 H 5 + , $${ ext{C}}_{ 4} { ext{H}}_{ 1 0}^{ + }$$ C 4 H 10 + , $${ ext{C}}_{ 5} { ext{H}}_{ 5}^{ + }$$ C 5 H 5 + , and $${ ext{C}}_{ 5} { ext{H}}_{ 7}^{ + }$$ C 5 H 7 + with mass-to-charge ratios (m/e) of 28, 30, 39, 42, 44, 53, 58, 65, and 67, respectively. For a normalized ion intensity, the relative intensities of saturated CH ions increase with increasing radial distance, while the unsaturated CH ions decrease with increasing radial distance. The ion energy distribution of $${ ext{C}}_{ 2} { ext{H}}_{ 6}^{ + }$$ C 2 H 6 + (m/e = 30) presents a bimodal structure. Additionally, both the electron density and the effective electron temperature decrease with increasing radial distance.
Keywords: Inductively coupled plasma; Radial distribution; Langmuir probe; Mass spectrometer