Afterglow Plasma Research Articles

Therapeutic effect of cold atmospheric plasma and its combination with radiation as a novel approach on inhibiting cervical cancer cell growth (HeLa cells)

Cervical cancer is one of the important cancers in women. Research on novel treatment approach can reduce the mortality and burden. Although radiotherapy is a common treatment, its negative side effects have concerned physician. In our study, we studied impact of cold atmospheric pressure plasma on the Hela cancer cells, as an alternative treatment. The effect of three different types of such plasma; dielectric barrier discharge (DBD), plasma jet, and afterglow plasma, on the cancer cells were studied. Moreover, some effective operating parameters such as exposure time, applied voltage, composition of working gas in plasma treatment were investigated on the survival of the afterglow plasma. Finally, treatments by the afterglow plasma, gamma radiation (1 Gy), and combination of both were compared. Analysis showed that DBD and plasma jet (direct exposure) effectively killed the cancer cells, even by a minimum applied voltage. But a fraction of the cells survived after the exposure of indirect diffused afterglow plasma. In the case of this plasma, we realized that higher applied voltage and exposure time led to less cell viability. Fewer fractions of survival cells were detected in the case of argon afterglow plasma comparing to oxygen afterglow. Cold atmospheric plasma and its combination with radiation therapy showed a significant decrease in viability of the cells, comparing to the radiation alone. Our research showed that plasma and its combination with radiation therapy have superiority over radiation therapy.

Charge neutralisation of microparticles by pulsing a low-pressure shielded spatial plasma afterglow

In this paper, it is shown that microparticles can be effectively neutralised in the (spatial) plasma afterglow of an inductively coupled plasma. A key element in the reported experiments is the utilisation of a grounded mesh grid separating the plasma bulk and the ‘shielded’ plasma afterglow. Once particles—being injected in and charged by the inductively coupled plasma—had passed this mesh grid, the plasma was switched off while the particles continued to be transported under the influence of both flow and gravity. In the shielded spatial plasma afterglow region, the particle charge was deducted from their acceleration in an externally applied electric field. Our experiments demonstrate that all particles were neutralised independently of the applied electric field magnitude. The achieved neutralisation is of primary importance for the further development of plasma-assisted contamination control strategies as well as for a wide range of other applications, such as colourimetric sensing, differential mobility analysers, and medical applications.

Enhancement of hydrogen peroxide production from an atmospheric pressure argon plasma jet and implications to the antibacterial activity of plasma activated water

We explore how to configure an argon atmospheric-pressure plasma jet for enhancing its production of hydrogen peroxide (H2O2) in deionised water (DIW). The plasma jet consists of a quartz tube of 1.5 mm inner diameter and 3 mm outer diameter, with an upstream internal needle electrode (within the tube) and a downstream external cylindrical electrode (surrounding the tube). The plasma is operated by purging argon through the glass tube and applying a sinusoidal AC voltage to the internal needle electrode at 10 kV (peak–peak) with a frequency of 23.5 kHz. We study how the following operational parameters influence the production rate of H2O2 in water: tube length, inter-electrode separation distance, distance of the ground electrode from the tube orifice, distance between tube orifice and the DIW, argon flow rate and treatment time. By examining the electrical and optical properties of the plasma jet, we determine how the above operational parameters influence the major plasma processes that promote H2O2 generation through electron-induced dissociation reactions and UV photolysis within the plasma core and in the plasma afterglow; but with a caveat being that these processes are highly dependent on the water vapour content from the argon gas supply and ambient environment. We then demonstrate how the synergistic action between H2O2 and other plasma generated molecules at a plasma induced low pH in the DIW is highly effective at decontaminating common wound pathogens Gram-positive Staphylococus aureus and Gram-negative Pseudomonas aeruginosa. The information presented in this study is relevant in the design of medical plasma devices where production of plasma reactive species such as H2O2 at physiologically useful concentrations is needed to help realise the full clinical potential of the technology.

Incorporation-limiting mechanisms during nitrogenation of monolayer graphene films in nitrogen flowing afterglows.

Monolayer graphene films are exposed to the flowing afterglow of a low-pressure microwave nitrogen plasma, characterized by the absence of ion irradiation and significant populations of N atoms and N2(A) metastables. Hyperspectral Raman imaging of graphene domains reveals damage generation with a progressive rise of the D/G and D/2D band ratios following subsequent plasma treatments. Plasma-induced damage is mostly zero-dimensional and the graphene state remains in the pre-amorphous regime. Over the range of experimental conditions investigated, damage formation increases with the fluence of energy provided by heterogenous surface recombination of N atoms and deexcitation of N2(A) metastable species. In such conditions, X-ray photoelectron spectroscopy reveals that the nitrogen incorporation (either as pyridine, pyrrole, or quaternary moieties) does not simply increase with the fluence of plasma-generated N atoms but is also linked to the damage generation. Based on these findings, a surface reaction model for monolayer graphene nitrogenation is proposed. It is shown that the nitrogen incorporation is first limited by the plasma-induced formation of defect sites at low damage and then by the adsorption of nitrogen atoms at high damage.

Influence of a Helium–Nitrogen RF Plasma Jet on Onion Seed Germination

This paper presents an experiment using a radio frequency atmospheric pressure plasma jet to generate cold plasma for pre-sowing stimulation of Wolska onion seeds. Impact of the He + N2 afterglow plasma on germination was investigated. Eight groups of seeds characterized by different exposure times (2, 5, 10, 15, 60, 120, 240, and 480 s) and distance from the electrodes (20 mm and 50 mm) were used. Pre-sowing plasma stimulation of the seeds improved the germination capacity and germination energy for all tested groups, relative to control. The impact of radio frequency plasma on the onion seed germination parameters was statistically significant. The highest germination parameters were obtained for seeds stimulated for 240 s at a distance of 50 mm. No significant differences in physical and morphological properties of onion seeds were found.

Selective nitrogen doping of graphene due to preferential healing of plasma-generated defects near grain boundaries

Hyperspectral Raman IMAging (RIMA) is used to study spatially inhomogeneous polycrystalline monolayer graphene films grown by chemical vapor deposition. Based on principal component analysis clustering, distinct regions are differentiated and probed after subsequent exposures to the late afterglow of a microwave nitrogen plasma at a reduced pressure of 6 Torr (800 Pa). The 90 × 90 µm2 RIMA mapping shows differentiation between graphene domains (GDs), grain boundaries (GBs), as well as contaminants adsorbed over and under the graphene layer. Through an analysis of a few relevant band parameters, the mapping further provides a statistical assessment of damage, strain, and doping levels in plasma-treated graphene. It is found that GBs exhibit lower levels of damage and N-incorporation than GDs. The selectivity at GBs is ascribed to (i) a low migration barrier of C adatoms compared to N-adatoms and vacancies and (ii) an anisotropic transport of C adatoms along GBs, which enhances adatom-vacancy recombination at GBs. This preferential self-healing at GBs of plasma-induced damage ensures selective incorporation of N-dopants at plasma-generated defect sites within GDs. This surprising selectivity vanishes, however, as the graphene approaches an amorphous state.

Outside Front Cover Picture: Plasma Process. Polym. 12/2020

Outside Front Cover: Surface microdischarge has shown great potential in practical applications. In our work, the effects of macroscopic parameters on the temporal evolution of OH radicals are investigatedin both plasma layer and afterglow region. The results reveal that OH lifetime dependence on the power is more pronounced compared to the modulation time, and the density decay is mainly ascribed to the transport process and recombination reactions of OH with itself. Further details can be found in the article by Zhiwei Wang, Chunlei Feng, Liang Gao, and Hongbin Ding (e2000122).

Collisionless adiabatic afterglow

We study, by numerical and analytical means, the evolution of a collisionless plasma initiated between absorbing walls. The ensuing flow is described by rarefaction waves that travel inward from the boundaries, interact, and eventually vanish after crossing through, leading up to the asymptotic stage of the decay. Particle simulations indicate that the kinetic evolution strongly resembles one found in isentropic gas dynamics. Namely, a very gradual density profile forms in the expanding central region where the rarefaction waves interact, with an accompanying linear velocity profile. Asymptotically, the density falls off as 1/t. The density and the flux at the boundary show little variation over the period when rarefaction waves still exist. Plasma potential, on the other hand, drops quite rapidly (on the underlying ion-acoustic timescale) to less than initial electron temperature Te when over 70% of the particles still remain in the system. This is due to electron kinetics being governed by conservation of adiabatic invariant in a slowly varying potential well. Analytical model of the velocity distribution is presented to explain the simulations. The results have implications for afterglow plasmas used in material processing and also for ion-extraction devices. One property of potential interest is good uniformity of the decaying plasma that occurs after approximately one ion-acoustic time.

Controlling N and C-atom densities in N2/H2 and N2/CH4 microwave afterglows for selective TiO2 surface nitriding

Radiative states of N2/<5% H2 and N2/<50/00 CH4 HF(microwave) flowing afterglows of microwave plasmas at reduced gas pressures (4–20 Torr) have been analyzed by emission spectroscopy. The N2 1st pos (580 nm), 2nd pos (316 nm), N2+ 1st neg (391.4 nm), NH (336 nm) and CN (386 nm) bands in the pink (early) and the late afterglows were measured and the NO titration was used to determine the concentrations of N-atoms, O-atoms in impurity, N2(X,v > 13) and N2(A) metastable molecules, N2+ ions. Introduction of small percentages of H2 (or CH4) into N2 lead to a size reduction of the early afterglow region which was followed by a late afterglow zone where the N + N recombination is the dominant process. In addition, it also gave an effect of reducing the densities of N atoms (as well as the N2 excited states) in the afterglow regions. At the same time, it introduced other active species such as H (or C) atoms, of which the densities strongly vary depending on the percentage of H2 (or CH4) in the mixture. Such changes in the densities of active species such as N and C atoms in the afterglow region had a strong influence on the incorporation characteristics of N atoms into the surface of TiO2 films introduced into the afterglow region. These results deliver an important meaning in devising process conditions especially for selective nitrogen or carbon doping into a skin-depth layers of oxide materials, which is desired in the case of preparing a few nm-thick functional films for photovoltaic and photocatalytic applications.

Depletion of atomic hydrogen in a high power helicon discharge

Depletion of the ground state atomic hydrogen density has been directly measured using two-photon laser-induced fluorescence in a high-density helicon plasma. The depletion is correlated with the plasma pressure becoming increasingly higher than the neutral gas fill pressure. Spatially resolved measurements show depletion of atomic hydrogen in the centre of the discharge chamber. Temporally resolved measurements display a replenishment of atomic hydrogen in the plasma afterglow at high plasma densities in comparison to the typical two-step decay at lower plasma densities.

Tarnished silver–copper surfaces reduction using remote helium plasma at atmospheric pressure studied by means of high-resolution synchrotron x-ray photoelectron microscopy

Previous investigations suggested that sulphide layers on pure silver surfaces were easily removed with a reducing plasma afterglow at atmospheric pressure, but were harder to remove on silver-copper alloys. In this study we investigated by synchrotron x-ray photoelectron spectroscopy the effect of plasma afterglow on tarnished pure silver, pure copper and sterling silver alloy. The results confirmed that plasma was able to reduce in depth all sulphides except Cu2S. The process was explained by standard reduction potentials and allowed to draw a detailed picture of the submicrometric evolution of the tarnished layers.

Inactivation of biofilms in endotracheal tube by cold atmospheric plasma treatment for control and prevention of ventilator‐associated pneumonia

AbstractVentilator‐associated pneumonia (VAP) is a type of hospital‐acquired infection with high mortality and morbidity rates that is seen in critically ill patients who are connected to mechanical ventilators in intensive care units. Conventional infection control measures may remain insufficient for control and prevention of VAP. In the present study, biofilms of most common pathogens, which are responsible for VAP, namely Pseudomonas aeruginosa, Acinetobacter baumannii, Staphylococcus aureus, and Candida albicans, were grown in the lumen of endotracheal tubes (ETs) under low shear to mimic the biofilm formation in clinical settings using the drip flow reactor system. Antibiofilm efficacies of two different plasma treatment methods were tested on the biofilms that were grown in ETs. In the first method, deionized water (DIW), phosphate‐buffered saline (PBS) solution, and N‐acetylcysteine (NAC) solution were treated with air dielectric barrier discharge (DBD) plasma and then nebulized on the biofilms. In the second method, air plasma afterglow (APA), which was generated using a DBD jet electrode, was flown through the lumen of ETs. Effects of two different plasma treatment methods on biofilms were evaluated using colony‐counting and XTT assays. The quantity of extracellular polymeric substance (EPS) was evaluated with the safranin assay. Scanning electron microscopy (SEM) was used to visualize the effect of various plasma treatment methods on biofilms. Cytotoxic effects of two different plasma treatment methods were tested on the human tracheal epithelial cell line. Our results suggest that the nebulized plasma‐treated NAC solution had a superior antibiofilm effect as compared with DIW and PBS. Two different plasma treatment methods used effectively inactivated biofilms and significantly reduced the quantity of EPS without a significant damage to the human tracheal epithelial cell line. Furthermore, SEM images showed the inactivation of microbial cells mainly through membrane damage.

Afterglow of atmospheric non-thermal plasma for disinfection of lentil seeds from Botrytis Grey Mould

Seed-borne fungal diseases of grains are a serious threat to grain crops' yield due to lack of either resistant crop varieties or dependence on fungicides. Therefore, there is a growing demand to develop sustainable technologies for crop protection. In the present study, an atmospheric microwave plasma was employed to eradicate Botrytis cinerea from lentil seeds. Argon, air and a combination of them were applied to create the plasma. There was a 41% reduction in the percentage of artificially contaminated seeds (with an initial contaminated seed percentage of 95.8%) after 100 s treatment of 2.5 g of lentil seeds with the afterglow of air plasma followed by 24-h holding time. A 32.3% reduction occurred when a 30% air/70% Argon was applied for 10 s and 60 min of holding time. The holding time of 24 h increased catalase activity from 0.8 to 1.1 mM H2O2 mg−1 min−1 that was an indicator of early plant immune system fortification. This also changed seeds' colour toward redness and yellowness. Conclusively, the afterglow of microwave plasma could be considered as a part of integrated disease management in lentil crops.

Time- and space-resolved optical Stark spectroscopy in the afterglow of laser-produced tin-droplet plasma.

The afterglow emission from Nd:YAG-laser-produced microdroplet-tin plasma is investigated, with a focus on analyzing Stark effect phenomena and the dynamical evolution of the plasma. Time- and space-resolved optical imaging spectroscopy is performed on 11 lines from Sni-iv ions, in the 315-425-nm wavelength range. Stark shift-to-width ratios serve as the basis for unambiguous experimental tests of atomic physics theory predictions. Experiment and theory, where available, are found to be in poor agreement, and are in disagreement regarding the sign of the ratio in several cases. Spectroscopic measurements of the Stark widths in tandem with Saha-Boltzmann fits to Sni and Snii lines, establish the evolution of the local temperature and density of the plasma afterglow, 20-40ns after the end of the 15-ns-long temporally box-shaped laser pulse. A clear cool-down from ∼2 to 1eV is observed of the plasma in this time window, having started at ∼30eV when emitting extreme-ultraviolet (EUV) light. An exponential reduction of the density of the plasma from ∼10^{18}-10^{17}e^{-}cm^{-3} is observed in this same time window. Our work is relevant for understanding the dynamics of the decaying, expanding plasma in state-of-the-art EUV nanolithography machines.

Synthesis of hard diamond-like carbon films by double-pulse high-power impulse magnetron sputtering

Hard diamond-like carbon (DLC) films were prepared by a double-pulse high-power impulse magnetron sputtering (HiPIMS) at an Ar gas pressure of 0.6 Pa and average power of 90 W. The objective was to increase the sp3 C content in the films, in comparison to single-pulse HiPIMS. The first HiPIMS plasma temporally evolved during a pulse-on time of the voltage source with a width of about 15 μs and the discharge current reached 50 A corresponding to 1.8 A/cm2. After the application of the first-pulse voltage for HiPIMS, a second-pulse voltage was applied to produce the second HiPIMS from the afterglow plasma. External setting parameters such as the time interval between the first pulse and second pulse and the target current in the second HiPIMS were varied. The fraction of sp3 C–C bonds was estimated from the ratio of the peak intensity related to sp3 C–C bonds to the total peak intensity of C1s core spectrum in X-ray photoelectron spectroscopy. The fraction of sp3 C–C bonds in the DLC film prepared by double-pulse HiPIMS reached 52% with a hardness of 36.5 GPa.

Addendum: Mapping electron dynamics in highly transient EUV photon-induced plasmas: a novel diagnostic approach using multi-mode microwave cavity resonance spectroscopy (2018 J. Phys: D. Appl. Phys. 52 034004)

A new approach for an in-line beam monitor for ionizing radiation was introduced in a recent publication (Beckers J et al 2018 J. Phys. D: Appl. Phys. 52 034004). Due to the recent detection and investigation of an additional third decay regime of the afterglow of an extreme ultraviolet photon-induced plasma described in a later article (Platier B et al 2020 Appl. Phys. Lett. 116 103703) there is an additional reason for a minimum number of photons for this approach to work. Near or below this threshold, we explain that the response time of the diagnostic method is a limiting factor. Further, a second limit for the number of photons within a pulse is formalized related to the trapping of highly energetic free electrons.

Magnetic field-enhanced beam monitor for ionizing radiation.

For the microwave cavity resonance spectroscopy based non-destructive beam monitor for ionizing radiation, an addition-which adapts the approach to conditions where only little ionization takes place due to, e.g., small ionization cross sections, low gas pressures, and low photon fluxes-is presented and demonstrated. In this experiment, a magnetic field with a strength of 57 ± 1 mT was used to extend the lifetime of the afterglow of an extreme ultraviolet-induced plasma by a factor of ∼5. Magnetic trapping is expected to be most successful in preventing the decay of ephemeral free electrons created by low-energy photons. Good agreement has been found between the experimental results and the decay rates calculated based on the ambipolar and classical collision diffusion models.

Charge control of micro-particles in a shielded plasma afterglow

In this work, charge control of micro-particles from ∼−40 to +10 elementary charges is presented. This is achieved at 0.9 mbar argon in the spatial afterglow of an inductively coupled plasma by solely changing the strength of an externally applied electric field. Crucial in the presented experiments is the use of a grounded mesh grid in the cross section of the setup, separating the active plasma region from the ‘shielded’spatial afterglow. While in the regions above the mesh grid all particles reached a constant negative equilibrium charge, the actual control achieved in the shielded spatial afterglow can most probably be explained by variations of the local ion density. The achieved charge control not only opens up possibilities to study nano-scale surface charging physics on micro-meter length scales, it also contributes to the further development of plasma-based contamination control for ultra-clean low-pressure systems.

Measurement of the densities of plasma and ambient gas particles using a short direct current discharge

In this work, a short direct current (dc) discharge that lacks a positive column for the measurement of the densities of plasma and ambient gas particles is demonstrated and studied. It is shown that a sensor based on the discharge can provide the absolute measurement of the densities of plasma and ambient gas particles at low-pressures (between 0.3 and 7 Torr). This sensor may work for afterglow plasma and active dc discharge. It also has a wide dynamic range of measurements and, depending on the studied species, its resolution can be in the range of a fraction of 1 ppb to a fraction of 1 ppm. The sensor is able to survive and function in harsh environments, including at cryogenic and high temperatures, in the presence of toxic substances, and at elevated ionizing radiation and vibration levels.

The Optical Properties of Individual Silver Nanoparticles in Ag/Ag2O Composites Synthesized by Oxygen Plasma Treatment of Silver Thin Films

In this work, we present a more complete description of the optical properties of the silver nanoparticles formed in silver oxide thin films. Ag/Ag2O composites were synthesized by treating silver thin films manufactured by thermal evaporation method with oxygen plasma afterglow. The results showed that exposing silver thin films to oxygen plasma afterglow leads to a monocrystalline structure of silver oxide. Consequently, the relationship between the crystal size and the characteristics of the plasmon optical absorption peaks was studied. A slight degradation of plasmon peaks of the individual silver nanoparticles was observed. We suggested that this degradation might be due to the mutual interaction between the individual silver nanoparticles located near the Ag2O grain shell and the larger silver nanoparticles in neighboring grains. We found that the degree of degradation is related to the Ag2O grain size. On the other hand, the origin of each fluorescence peak in the Raman spectra of the prepared films was determined.