Plasma Chemistry and Plasma Processing (v.34, #3)

Plasma Processing and Polymers: A Personal Perspective by Michael R. Wertheimer (363-376).
Low-temperature plasma science and technology has made enormous strides during the past half-century. To a significant extent, the driver has been integrated circuit (IC) processing, because without plasma-based etching and deposition techniques, the progression of Moore’s law would have ceased its course long ago. Scientific and technological advances no less spectacular than those in the IC sector, and of comparable economic impact, have occurred in the area of plasmas and polymers, where a third process type, surface modification, can be added to those, etching—or ablation, and deposition, mentioned above. In this article, we start by listing a number of particular features associated with the exposure of polymers to low-temperature plasmas, for example the liberation of volatile molecular fragments from the surface due to bond scissions, the accompanying creation of free radicals, cross-linking and grafting reactions induced by this and by vacuum-ultraviolet photon irradiation from excited species in the plasma, etc. All these aspects are of far lesser impact, if any at all, when the same plasmas are in contact with inorganic surfaces. We then list some high-volume industrial processes and operational details drawn from diverse economic sectors. These are accompanied by detailed example-cases from our own laboratories, based on both low- and atmospheric-pressure low-temperature plasma processes.
Keywords: Plasma processing; Polymers; Personal perspective

Plasma Processing Based Synthesis of Functional Nanocarbons by Rikizo Hatakeyama; Toshiaki Kato; Yongfeng Li; Toshiro Kaneko (377-402).
Our recent research has shown that plasma processing techniques, which allow versatile control of both chemical and physical aspects, have considerable potential for the innovative synthesis and functionalization of three varieties of low-dimensional nanocarbons, which show great promise in the development of nanoscience and its applications. In the case of 0-D fullerenes, the mission is the high-yield production of atom (X) encapsulated fullerenes (X@C60). The formation of macro-quantities of charge-exploited Li@C60 and overwhelmingly-high purity spin-exploited N@C60 are realized for the first time by the control of alkali-fullerene and nitrogen double plasmas, respectively. In the case of 1-D carbon nanotubes the challenge is precise structure control, i.e., chirality control of single-walled carbon nanotubes (SWNTs). The extremely narrow-chirality distributed growth of SWNTs is realized with time-programmed and nonmagnetic-catalyzed plasma CVD. As for functionalization of SWNTs, the enhanced p-type C60@SWNTs created under the substrate-bias control in collisionless plasmas are found to be effective for harvesting solar energy in the infrared wavelength range and adapted to the use for multiple exciton generation in solar cells. Concerning 2-D graphene, our aim is to overcome two serious issues for electronics applications. One is the realization of the direct growth of graphene on an insulating (SiO2) substrate by adjusting the growth parameters using non-equilibrium diffusion plasma CVD. The other is the direct fabrication of field-effect transistor device of a narrow-width (≥20 nm) graphene nanoribbon using a new, simple, and scalable method based on rapid heating plasma CVD, which shows a clear transport gap and a high on/off ratio. Finally the prospects for the above-mentioned results are discussed together with ripple effects of the nanocarbon research on the progress of nanoscience and its applications.
Keywords: Plasma processing; Nanocarbons; Nanoscientific applications

A Model of Plasma-Biofilm and Plasma-Tissue Interactions at Ambient Pressure by C. Chen; D. X. Liu; Z. C. Liu; A. J. Yang; H. L. Chen; G. Shama; M. G. Kong (403-441).
This paper presents the development of a model framework for plasma-biofilm and plasma-tissue interactions that can link molecular simulation of plasma chemistry to functions at a cell population level or a tissue level. This is aided with a reactive penetration model for mass transfer of highly transient plasma species across the gas–liquid boundary and a panel of electrical and thermal thresholds considering pain sensation, protein denaturation and lethal electric currents. Application of this model reveals a number of previously little known findings, for example the penetration of plasma chemistry into highly hydrated biofilms is about 10–20 μm deep for low-power He–O2 plasma and this is closely correlated to the penetration of liquid-phase plasma chemistry dominated by O2 , H2O2, and HO2 or O2 , H2O2, and O3. Optimization by manipulating liquid-phase chemistry is expected to improve the penetration depth to 40–50 μm. For direct plasma treatment of skin tissues at radio frequencies, the key tolerance issue is thermal injuries even with a tissue temperature <50 °C and these can lead to induction of pain and protein denaturation at a small discharge density of 8–15 mA/cm2 over few tens of seconds. These and other results presented offer opportunities to improve plasma-biofilm and plasma-tissue interactions. The model framework reported may be further extended and can be used to non-biomedical applications of low-temperature plasmas.
Keywords: Plasma medicine; Low-temperature plasmas; Biofilm interaction; Living tissues; Biophysics model

A multi-phase alternating current (AC) arc has been applied to glass melting technology. The large volume discharge produced by a stable multi-phase AC arc is preferable to melt the granulated glass materials. The discharge behavior and the high-temperature region of the plasma can be controlled by the electrode configurations. In this study, the spatial characteristics of the arc discharge were examined by image analysis of high-speed camera. Results show arc existence area is related with electrode configuration. This study provides the useful information of efficient particle treatment in the preferred electrode configuration. However, the electrode erosion is one of the most considerable issues to be solved. The combination of high-speed video camera and band-pass filters was introduced to measure the electrode temperature to investigate the erosion mechanism of the multi-phase AC arc. The dynamic behavior of the electrode vapors in the arc was investigated by using the same high-speed camera system. Results show the tungsten electrode mainly evaporates at the anodic period during AC cycle.
Keywords: Thermal plasma; Multi-phase AC arc; Electrode fluctuation; Electrode temperature; Electrode erosion

Measurement of Reactive Hydroxyl Radical Species Inside the Biosolutions During Non-thermal Atmospheric Pressure Plasma Jet Bombardment onto the Solution by Yong Hee Kim; Young June Hong; Ku Youn Baik; Gi Chung Kwon; Jin Joo Choi; Guang Sup Cho; Han Sup Uhm; Do Young Kim; Eun Ha Choi (457-472).
Non-thermal atmospheric pressure plasma jet could generate various kinds of radicals on biosolution surfaces as well as inside the biosolutions. The electron temperature and ion density for this non-thermal plasma jet have been measured to be about 0.8~1.0 eV and 1 × 1013 cm−3 in this experiment, respectively, by atmospheric pressure collisional radiative model and ion collector current. In this context, the hydroxyl OH radical density inside the biosolutions has been also investigated experimentally by ultraviolet absorption spectroscopy when the Ar non-thermal plasma jet has been bombarded onto them. It is found that the emission and absorption profiles for the other reactive oxygen species such as NO (226 nm) and O2* (245 nm) are shown to be very small inside the biosolution in comparison with those for the OH radical species. It is found that the densities of OH radical species inside the biosolutions are higher than those on the surface in this experiment. The densities of the OH radical species inside the deionized water, Dulbecco’s modified eagle medium, and phosphate buffered saline are measured to be about 2.1 × 1016, 1.1 × 1016, and 1.0 × 1016 cm−3, respectively, at 2 mm downstream from the surface under optimized Ar gas flow of 200 sccm. It is also found that the critical hydroxyl OH radical density for the lung cancer H460 cells to experience an apoptosis is observed to be around 0.3 × 1016 cm−3 under 1 min plasma exposure in this experiment.
Keywords: Non-thermal atmospheric pressure plasma jet; Atmospheric pressure collisional radiative model; Hydroxyl OH radical density inside the biosolution; Ultraviolet absorption spectroscopy

Low pressure plasma technologies have been widely and successfully utilized for the production of a large variety of organic–inorganic nanocomposite (NC) thin films consisting of metal or metal oxide nanoparticles embedded in a polymer matrix. Recently, the deposition of this class of coatings has been also accomplished by atmospheric pressure cold plasmas using aerosol-assisted processes in which a dispersion containing preformed inorganic nanoparticles and the liquid precursor of the polymeric component is atomized and injected in aerosol form in the atmospheric plasma. This short review is aimed at presenting this approach which is expected to enlarge the range of structures and properties of organic–inorganic NC coatings deposited by cold plasma technologies.
Keywords: Organic–inorganic nanocomposite; Thin film; Atmospheric pressure cold plasmas; Aerosol; Nanoparticle

A one-dimensional numerical model and simulation results are presented for a capacitively-coupled radio frequency parallel-plate argon–silane dusty plasma. The model includes self-consistently coupled numerical modules, including a plasma fluid model, a sectional aerosol model, and a simple chemistry model to predict rates of particle nucleation and surface growth. Operating conditions considered include 13.56 MHz frequency, 100 mTorr pressure, a 4-cm electrode gap, gas flow through the top electrode with a 30:1 ratio of argon to silane, and applied radio frequency voltage amplitude of either 100 or 250 V. In the higher voltage case two lobes of relatively large particles are formed by ion drag, while fresh nucleation occurs in the void between these lobes. It is shown that the reason that fresh nucleation occurs in the void involves an interplay among several coupled phenomena, including nanoparticle transport, the plasma potential profile, and trapping of silicon hydride anions that drive nucleation in this system.
Keywords: Dusty plasmas; Silane; Nanoparticles

Two-Dimensional Geometry Control of Graphene Nanoflakes Produced by Thermal Plasma for Catalyst Applications by J.-L. Meunier; N.-Y. Mendoza-Gonzalez; R. Pristavita; D. Binny; D. Berk (505-521).
The control of nanoparticle synthesis using thermal plasmas is difficult and often leads to problems of chemical and structural purity, and poor process robustness in terms of consistency of product from run to run. Good reactor design allowed to overcome these issues and to develop a new material based on graphene with a flake-like structure (labeled graphene nanoflakes, GNF) supporting nitrogen for catalytic applications, for example as platinum replacement in fuel cells. These structures showed not only to be active, but also stable in polymer electrolyte fuel cell operation. Characterization of these structures, in situ fuel cell studies and modeling analysis all indicate that achievement of stability relates on the crystalline two-dimensional graphene structure. This paper first reviews the basic aspects behind the structural objectives, describes the synthesis process design leading to this crystalline structure, and provides a two-dimensional analysis on the graphitic growth based on fundamental theory and CFD calculations. These calculations indicate that an independent control of the graphene structure thickness (number of atomic planes) and sheet lengths is possible in a thermal plasma reactor.
Keywords: Graphene nano-flakes; Thermal plasma; Particle nucleation; PEM fuel cell catalyst; 2D graphene structure control

This paper describes effects of design parameters for a three-dimensionally integrated micro-solution plasma (3D IMSP) reactor, which generates a large amount of microplasma in gas bubbles flowing with a liquid medium through a porous dielectric material. Electric fields in gas bubbles are calculated by solving Maxwell’s equations under the electro-quasi-static approximation. The calculated electric fields in the bubbles can be high enough for igniting electrical discharge in the bubbles even if the bubbles are surrounded by electrically conductive liquid. We show importance of higher permittivity of a dielectric-reactor tube and a higher voltage-rise rate for obtaining higher electric fields in the bubbles. Using a proto type 3D IMSP reactor, we demonstrate synthesis of gold nanoparticles and decomposition of methylene blue molecules in aqueous solution.
Keywords: Plasma; Liquid; Bubble; Methylene blue; Gold nanoparticles

Self-Assembly in Silane/Hydrogen Plasmas: from Silicon Atoms to Aromatic Silicon Nanocrystals by Nancy C. Forero-Martinez; Ha-Linh Thi Le; Holger Vach (535-543).
We theoretically predict that low pressure silane/hydrogen plasmas may present the ideal venue for the fabrication of a new nanostructured silicon material with strong aromatic properties. Precisely controlling the ratio of atomic and molecular hydrogen concentrations during the growth of hydrogenated silicon clusters in a pulsed plasma enhanced chemical vapor deposition reactor will allow us to “steer” the growth toward over-coordinated silicon nanocrystals that reveal aromatic properties as strong as those of benzene. In addition, those pure silicon nanocrystals demonstrate optical, electrical, and mechanical characteristics that could so far only be obtained by adding toxic or expensive elements (as PbS, PbSe, CdS, CdSe, or Au) in the cluster structure. Due to their light absorption not only in the ultraviolet, but also in the visible and the infrared spectrum, they might, for instance, play a major role for photovoltaic devices or in the hyperthermal treatment of cancer.
Keywords: PECVD; Plasma-aided fabrication; Silicon cluster nucleation; Silicon nanocrystals; Electron-deficiency; Silicon aromaticity; New silicon materials; Photovoltaics

Plasma polymerization has evolved in an important technology for generation of thin films that have found numerous advanced applications. The orthodox view to plasma polymers is as continuous, homogeneous and pinhole-free coatings. However, there is an emerging trend towards creating more advanced films though engineering them at the nanoscale. This paper presents a summary of our work and published studies, from other groups, which demonstrate the potential of plasma polymerization to generate advanced nanostructured interfaces. The focus is on applications in the area of biomaterials. Strategies for generation of antibacterial coatings through inclusion of metallic nanoparticles in plasma polymer films are described. Drug delivery platforms developed via templating and incorporation of drug particles are outlined. A record of recent progress in fabrication of cell guidance surfaces facilitated by nanoengineering of plasma polymer film is also included. The paper concludes with the author’s view to the future outlook of the niche area of nanoengineered plasma polymer films.
Keywords: Plasma polymerization; Thin films; Surface modification; Biomaterials; Nanostructures; Antibacterial surfaces; Drug release

Two-Temperature Chemical-Nonequilibrium Modelling of a High-Velocity Argon Plasma Flow in a Low-Power Arcjet Thruster by Hai-Xing Wang; Wei-Ping Sun; Su-Rong Sun; A. B. Murphy; Yiguang Ju (559-577).
A numerical simulation has been performed of a high-velocity argon plasma arc flow in a low power arcjet including a finite-rate chemical kinetic model. Electrons, ions, molecular ions ( $$ { ext{Ar}}_{2}^{ + } $$ Ar 2 + ), neutral atoms including the ground and excited argon atoms (Ar*) are treated as separate species in the plasma mixture. The chemical reactions considered are excitation, de-excitation, ionization and recombination processes, in which reactions involving excited argon atoms (Ar*) and molecular ions ( $$ { ext{Ar}}_{2}^{ + } $$ Ar 2 + ) are taken into account. The relative importance of different production and loss processes in determining the densities of excited argon atoms and ions is calculated inside the constrictor and expansion portion of the nozzle. The roles of the excited argon atoms and molecular ions are investigated. It is found that excited argon atoms play an important role in the ionization of argon atoms in the core of plasma arc, while the molecular ions have a significant effect on the recombination process at the arc fringes inside the constrictor and in the arc attachment zone of the anode.
Keywords: Two-temperature; Chemical-nonequilibrium; Plasma flow

Fundamentals and Environmental Applications of Non-thermal Plasmas: Multi-pollutants Emission Control from Coal-Fired Flue Gas by Shuran Li; Yifan Huang; Feifei Wang; Ji Liu; Fada Feng; Xinjun Shen; Qinzhen Zheng; Zhen Liu; Lihong Wang; Keping Yan (579-603).
Simultaneous control of multi-pollutants emission from coal-fired flue gas is essential for environmental quality. Non-thermal plasma (NTP) technologies have made numerous successful applications of combined removal for sulfur dioxide (SO2), nitrous oxide, particulate matter and mercury. Nitric oxide, elemental mercury can be oxidized in the gas phase, while fine particle gets agglomeration in the same NTP device. SO2 gets absorbed after heterogeneous oxidation by discharge. All the gaseous pollutants and aerosols generated will be further oxidized and captured in wet NTP device. Applications of NTP and typical configurations are also summarized.
Keywords: Multi-pollutant emission; Non-thermal plasma; Coal-fired flue gas

The influence of the addition of O2 on the OH production in a He + 0.1 % H2O discharge is investigated using laser induced fluorescence. The plasma properties $$(T_{ m g},;n_{ m e})$$ ( T g , n e ) are reported and used to explain the observed time and spatially resolved OH density, which is absolutely calibrated using Rayleigh scattering. Compared to the case when only H2O is added, an increase in the measured OH density is observed in the far afterglow. A zero-dimensional chemical kinetic model is constructed, which allows to determine the reactions responsible for the OH production in the far afterglow. When O2 is admixed, the key reaction $$hbox{O} + hbox{OH} longrightarrow hbox{O}_{2} + hbox{H}$$ O + OH ⟶ O 2 + H causes quenching of OH and production of increased densities of H, HO2 and H2O2, which subsequently leads to additional OH production in the late afterglow.
Keywords: Nanosecond pulsed discharges; Radical chemistry; Atmospheric pressure plasma; Laser induced fluorescence

Plasma Bromination of Graphene for Covalent Bonding of Organic Molecules by Jörg F. Friedrich; Gundula Hidde; Andreas Lippitz; Wolfgang E. S. Unger (621-645).
Plasma-chemical bromination applied to graphitic materials, in particular to highly ordered pyrolytic graphite is reviewed. The resulting bromination efficiency of the plasma-chemical treatment was subject of systematic process optimization. The plasma of elemental bromine vapour produced bromine concentrations on graphene surfaces of more than 30 % Br/C using either inductively or capacitively coupled low-pressure radio-frequency plasmas. Plasma brominated graphite surfaces have been studied by Near Edge X-ray Absorption Fine Structure, X-ray Photoelectron Spectroscopy, Atomic Force Microscopy and Scanning Electron Microscopy. The introduction of bromine into graphene-like graphite layers and its binding situation were investigated. To study the physical effect of the plasma bromination process, Kr plasma was also used because of its chemical inertness but similar atomic mass. Covering the samples with a Faraday cage or with a LiF window should help to differentiate between physical, chemical and radiation effects of the plasma. Bromination was assigned to radical or electrophilic addition of bromine onto fully substituted aromatic double bonds (sp2 C) with exothermal reaction enthalpy. Low bromination shows a strong decay of aromatic double bonds, higher bromination percentage let disappear all aromatic rings. The formed C–Br bonds were well suited for efficient grafting of organic molecules by post-plasma wet-chemical nucleophilic substitution. This grafting onto the graphene surface was demonstrated using aminosilane and different diamines. The bromination of double bonds changes the hybridization of carbon atoms from plane sp2 to tetrahedral sp3 hybridization. Thus, the plane topography of graphene is destroyed and the conductivity is lost.
Keywords: Graphene; Bromination; Grafting of diamines; Plasma

The enhancement by ion–neutral collisions of the momentum delivered to ions for specified power is compared for acceleration under electric and magnetic pressures. The enhancement in both cases is shown to be proportional to the square-root of the number of collision mean-free-paths when neutral-gas density is low and to the number of collision mean-free-paths when the neutral-gas density is high. The distributions along the acceleration channel of the ion density and velocity and of the electric potential are calculated at the space-charge limit for acceleration by electric pressure and at the diamagnetic-current limit for acceleration by magnetic field pressure, for both collisionless and collisional ions. Optimal magnetic field profile is found for acceleration under magnetic field pressure. Accelerating ions colliding with neutrals under magnetic field pressure can provide a high thrust at a low accelerating voltage.
Keywords: Electric propulsion; Thrust over power; Ion-neutral collisions; Ionic wind; Hall thruster

Chemical derivatization analysis of polyethylene surfaces plasma-treated in the afterglow regions of dielectric barrier discharges in mixtures of nitrogen and hydrogen was studied, using nucleophilic instead of electrophilic reagents which have commonly been used in studies of polymer surfaces exposed to discharges in nitrogen or nitrogen-containing gases. Vapors of strongly nucleophilic 2-mercaptoethanol and 4-(trifluoromethyl)-phenylhydrazine (TFMPH), respectively, were used for derivatization. XPS spectroscopy was subsequently applied in order to quantify the amount of sulfur and fluorine, respectively, introduced to the surface due to the presence of electrophilic moieties generated by the plasma treatment. Using FTIR-ATR spectroscopy following TFMPH derivatization, a quantitative determination of the hydrazone groups formed was possible, based on a comparison with spectra of a low-molecular weight model hydrazone. The results of these investigations confirm conclusions from earlier studies showing that the formation of electrophilic groups such as imines on polymers treated in afterglows of nitrogen–hydrogen DBDs must not be disregarded.
Keywords: Plasma treatment; Polymers; Chemical derivatization; FTIR ATR; XPS

Effect of Nanoparticles on Discharge Plasma and First Steps of Their Formation by I. V. Schweigert; A. L. Alexandrov; D. A. Ariskin (671-702).
We present particle-in-cell dust models for kinetic simulation of interaction between dust and plasma in a capacitive radio frequency discharge and afterglow. We study the properties of a capacitive 13.56 MHz discharge in pure argon and in a mixture of $$hbox {Ar/C}_{2}hbox {H}_{2}$$ Ar/C 2 H 2 with nanosize particles. For the first time it is found that at initial stage of growth the mobile nanoparticles are accumulated near the sheath–plasma boundaries. The transition between different modes of discharge operation is found to be associated with the growing nanoparticles. In discharge afterglow, it is shown that the electrons released from the dust surface contribute to the anomalous growth of the electron density. A hybrid model was developed to combine the kinetic description for electron motion and the fluid approach for negative and positive ion transports and plasmochemical processes. The growth of heavy hydrocarbons is studied in a capacitive 13.56 MHz discharge in a mixture of $$Ar/C_{2}H_{2}$$ A r / C 2 H 2 taking into account the plasmochemistry. The densities of negatively and positively charged heavy hydrocarbons are sufficiently large to be precursors for the formation of nanoparticles in the discharge volume. We study also the influence of the fraction of acetylene in the mixture of $$Ar/C_{2}H_{2}$$ A r / C 2 H 2 on the negative ion density.
Keywords: Nanoparticles; Discharge plasma; Heavy hydrocarbon radicals; Plasma chemistry; Kinetic simulation