Density Of Reactive Species Research Articles

Nanosecond-pulsed spark discharge plasma in liquid nitrogen: synthesis of polynitrogen from NaN3

Nanosecond-pulsed plasma ignited in liquid nitrogen is a unique tool for the synthesis of unconventional materials due to a combination of energetic properties of the discharge itself (high densities of reactive species, pressures and radiation) with the low temperature of the surrounding dense liquid. Here, we report on applications of such plasma for synthesis of a polymeric nitrogen compound, preliminarily identified as neutral or ionic N6, from sodium azide precursors.

Reactive species in cold atmospheric-pressure He+Air plasmas: The influence of humidity

A He+Air mixture is a common working gas for cold atmospheric-pressure plasmas which compromises the discharge stability of helium and the chemical reactivity of air. Water vapor is an important component in the gas mixture which can be either introduced as a precursor for reactive species or inevitably entrapped in the plasma system from the surroundings, and the presence of water can cause a profound change on the characteristics of the plasma and consequently its application efficacy. A global model of cold atmospheric-pressure He+Air plasmas is presented in this paper so as to study the influence of humidity. The densities of reactive species and power dissipation are studied as a function of water concentration spanning from 10 to 10 000 ppm. It is found that the dominant cation changes from NO+ to H3O+, and the dominant anion changes from NO2− to OH− with the increasing water concentration. HNO2 and O2(a) are the most abundant reactive nitrogen species and reactive oxygen species, respectively. When the water concentration is above 100 ppm, the electronegativity of the plasma increases dramatically, which has a significant effect on the way of power dissipation in the plasma. Also, the plasma chemistry under different air contents (100, 1000, and 10 000 ppm) is studied as a function of the water concentration. Among different cases of air concentration, the presence of water can enrich the density of most hydrogen-containing species (OH, H2O2, HO2, H3O+, and HNO3) except for HNO2, and it can also enrich the density of hydrogen-free species [O, O3, O2−, and O2(a)] at a low air concentration (100 ppm) while running it down at a high air concentration (1000 and 10 000 ppm).

Utilizing surface modification in plasma‐enhanced cyclic etching of tantalum nitride to surpass lithographic limits

AbstractPitch subdivision of tantalum nitride (TaN) lines is demonstrated across a 200 mm wafer using a cyclic quasi‐atomic layer etch process in an inductively coupled plasma reactor. Chlorine (Cl2) and hydrogen (H 2) chemistries are introduced sequentially to an argon plasma in discrete steps to etch the TaN film. The starting lithographic pattern with critical dimension (CD) of approximately 82 nm and pitch of 200 nm thus yields lines of approximately 40 nm CD and 100 nm pitch with minimal line edge roughness increase. We identify a synergistic effect between H 2‐exposed TaN and Cl 2 plasma as contributing to this result, as well as a potential link to surface oxidation. Optical emission spectroscopy analysis of the plasma discharge is used to characterize reactive species densities and explain the observed changes in profile.

Global model of an atmospheric-pressure capacitive discharge in helium with air impurities from 100 to 10 000 ppm

Helium is a common working gas for cold atmospheric plasmas (CAPs) and this is often mixed with other gases, such as oxygen and nitrogen, to increase its reactivity. Air is often found in these plasmas and it can be either introduced deliberately as a precursor or entrapped in systems that operate in open atmosphere. In either case, the presence of small traces of air can cause a profound change on the composition of the plasma and consequently its application efficacy. In this paper, a global model for He + Air CAPs is developed, in which 59 species and 866 volume reactions are incorporated, and a new boundary condition is used for the mass transport at the interface between the plasma and its surrounding air gas. The densities of reactive species and the power dissipation characteristics are obtained as a function of air concentrations spanning from 100 to 10 000 ppm. As the air concentration increases, the dominant cation changes from to NO+ and then to the dominant anion changes from to and then to the dominant ground state ROS changes from O to O3, and the dominant ground state reactive nitrogen species changes from NO to HNO2. O2(a) is the most abundant metastable species and its density is orders of magnitude larger than other metastable species for all air concentrations considered in the study. Ion Joule heating is found important due to the electronegative nature of the plasma, which leads to the fast decrease of electron density when the air concentration is larger than 1000 ppm. The generation and loss pathways of important biologically relevant reactive species such as O, O3, OH, H2O2, NO, HNO2, HNO3 are discussed and differences with the pathways observed in He + O2, He + H2O, Ar + Air and pure air plasmas are highlighted. Based on the simulation results, a simplified chemistry set with 47 species and 109 volume reactions is proposed. This simplified model greatly reduces the computational load while maintaining the accuracy of the simulation results within a factor of 2. The simplified chemistry model is computationally much less intensive, facilitating its integration into multidimensional fluid models for the study of the spatio-temporal evolution of He + Air CAPs.

An investigation on the effects of air on electron energy in atmospheric pressure helium plasma jets

In this work, the effects of air on electron energy in the atmospheric pressure helium plasma jet produced by a needle-plane discharge system have been investigated by means of the numerical simulation based on a two-dimensional fluid model, and the air concentration dependences of the reactive species densities have also been calculated. In addition, the synergistic effects of the applied voltage and air concentration on electron energy have been explored. The present work gives the following significant results. For a fixed applied voltage, the averaged electron energy is basically a constant at air concentrations below about 0.5%, but it evidently decreases above the concentration of 0.5%. Furthermore, the averaged densities of four main reactive species O, O(1D), O2(1Δg), and N2(A3Σu+) increase with the increasing air concentration, but the increase becomes slow at air concentrations above 0.5%. The air concentration dependences of the averaged electron energy under different voltage amplitudes are similar, and for a given air concentration, the averaged electron energy increases with the increase in the voltage amplitude. For the four reactive species, the effects of the air concentration on their averaged densities are similar for a given voltage amplitude. In addition, the averaged densities of the four reactive species increase with increasing voltage amplitude for a fixed air concentration. The present work suggests that a combination of high voltage amplitude and the characteristic air concentration, 0.5% in the present discharge system, allows an expected electron energy and also generates abundant reactive species.

Numerical study on the time evolutions of the electric field in helium plasma jets with positive and negative polarities

This paper presents 2D simulations of atmospheric pressure discharges in helium with N2 and O2 admixtures, propagating in a dielectric tube between a point electrode and a grounded metallic target. For both positive and negative polarities, the propagation of the first ionization front is shown to correspond to a peak of the absolute value of the axial electric field inside the tube, but also outside the tube. After the impact on the metallic target, a rebound front is shown to propagate from the target to the point electrode. This rebound front is 2–3 times faster than the first ionization front. Close to the high voltage point, this rebound front corresponds to a second peak of the absolute value of the axial electric field. Close to the target, as the first ionization and rebound fronts are close in time, only one peak is observed. The dynamics of the absolute value of the radial component of electric field outside the tube is shown to present an increase during the first ionization front propagation and a fast decrease corresponding to the propagation of the rebound front. These time evolutions of the electric field components are in agreement with experiments. Finally, we have shown that the density of metastable He* in 99% He—1% N2 and 99% He—1% O2 atmospheric pressure discharges are very close. Close to the grounded target, the peak density of reactive species is significantly increased due to the synergy between the first ionization and rebound fronts, as observed in experiments. Similar results are obtained for both voltage polarities, but the peak density of metastable He* close to the target is shown to be two times less in negative polarity than in positive polarity.

Plasma–surface interaction at atmospheric pressure: A case study of polystyrene etching and surface modification by Ar/O2 plasma jet

In this paper, the authors studied atmospheric pressure plasma–surface interactions using a well-characterized radio-frequency Ar/O2 plasma jet with polystyrene (PS) polymer films in controlled gas environments as a model system. A number of plasma processing parameters, such as the treatment distance, environmental gas composition, and substrate temperature, were investigated by evaluating both the changes in the thickness and the surface chemical composition of PS after treatment. The authors found that the polymer average etch rate decayed exponentially with the nozzle–surface distance, whereas the surface oxygen composition increased to a maximum and then decreased. Both the exponential decay constant and the oxidation maximum depended on the composition of the gaseous environment which introduced changes in the density of reactive species. The authors previously reported a linear relationship between measured average etch rates and estimated atomic O flux based on measured gas phase atomic O density. In this work, the authors provided additional insights into the kinetics of surface reaction processes. The authors measured the substrate temperature dependence of the PS etch rate and found that the apparent activation energy (Ea) of the PS etching reaction was in the range of 0.10–0.13 eV. Higher values were obtained with a greater nozzle-to-surface distance. This relatively low Ea value suggests that additional energetic plasma species might be involved in the etching reactions, which is also consistent with the different behavior of etching and surface oxidation modification reactions at the polymer surface as the treatment distance is varied.

Effects of the pulse width on the reactive species production and DNA damage in cancer cells exposed to atmospheric pressure microsecond-pulsed helium plasma jets

Plasma-liquid and plasma-cell interactions were investigated using an atmospheric pressure dc microsecond-pulsed helium plasma jet. We investigated the effects of the electrical parameters such as applied voltage and pulse width (determined by the pulse frequency and duty ratio) on the production of reactive species in the gas/liquid phases and on the DNA damage responses in the cancer cells. The densities of reactive species including OH radicals were estimated inside the plasma-treated liquids using a chemical probe method, and the nitrite concentration was detected by Griess assay. Importantly, the more concentration of OH resulted in the more DNA base oxidation and breaks in human lung cancer A549 cells. The data are very suggestive that there is strong correlation between the production of OH in the plasmas/liquids and the DNA damage.

A novel large-scale plasma source: two discharge modes and their correlation to the production of aqueous reactive species

In this paper, a novel large-scale plasma source is put forward, which can generate two modes of discharge, i.e. the surface dielectric barrier discharge and the plasma jet array, by just varying the helium gas flow rate. It is found that the discharge power changes a little with the increasing gas flow rate, but the densities of reactive species in the gas phase and in the treated water change a lot. In particular, the gaseous O3 has its density decreasing while the aqueous O3 has its concentration increasing with the increasing gas flow rate. In the plasma-treated water, the reactive nitrogen species such as nitrite and nitrate have their concentrations first increasing and then decreasing, while the reactive oxygen species such as H2O2, O3 and OH have their concentrations increasing monotonically, implying that the plasma source is well-adaptive for different application requirements.

Influence of reactive gas-phase species on the structure of an air/water interface

Highly-reactive gas-phase species, such as OH, O3 and NOx, present at the air/water interface, play a crucial role in natural environments. Unique hydrogen bond structures, not seen in the bulk, exist at the air/water interface. Here, by means of vibrational sum-frequency generation spectroscopy, we probe the interfacial water structure, and demonstrate how reactive gas-phase species interact with the water surface. We found that the reactive gas-phase species supplied to the water surface largely influenced the water surface structure. Furthermore, we suggest a higher density of reactive species at the water surface as compared to the bulk.

Pseudo actinometry for the measurement of reactive species density.

To compensate for the effect of plasma parameters in the signal of optical emission spectroscopy, we should normalize the emission intensity from the species against that of an inert gas (i.e., the actinometer). In many plasma processes in the semiconductor industry, plasma etching without using a neutral gas for the actinometer has become popular to achieve better critical dimension uniformity. We propose "pseudo actinometry" for normalization in the absence of an inert gas in the process plasma. Based on the theory of optical actinometry, we define a correction factor as the ratio of the emission intensity to the number density of the inert gas. As we reduced the density of the inert gas, we experimentally determined the correction factor by taking its convergence when the concentration of the inert gas was zero. As proof of concept, we applied pseudo actinometry to measure the density distribution of atomic chlorine in a photomask etching process without an inert gas. By comparing the distributions of chlorine radicals and the etch rate as measured by an ellipsometer, we calculated the correlation coefficient between the distributions. The correlation coefficient rapidly decreased to 0.60 when we used the correction factor determined at a flow rate for the inert gas of 20 standard cubic centimeters per minute at STP. By using pseudo actinometry, we successfully determined the distribution of chlorine radicals with a correlation coefficient of 0.98 in a plasma etching process without an inert gas.

Effect of background ionization on plasma ignition dynamics

The influence of background ionization on the ignition dynamics of the pulsed plasma plume is studied. The ignition delay time of each pulse is investigated by recording the voltage signal and the light emission signal. By changing the frequency, the relationship between the pulse-off time and the ignition delay time is revealed. This indicates that residual active species produced in the previous discharge play a role in the next one. With the decrease in the frequency, both time delay and ignition delay time increase. This is due to the decay of the reactive species densities in the pulse-off time. Lower concentrations of these species lead to a longer ignition delay time. The functions for calculating the ignition delay time are utilized to explain the effect of residual species. The independent data of each discharge also evidence the impact of the previous pulse. The exotic relationship between the ignition delay times of the first two pulses may be due to the electrode configuration used in this work. For a pin-to-plane electrode structure, the active species produced during the breakdown are accumulated around the anode (pin-point) where the discharge initiated for the asymmetrical electric field distribution.

Improved Surface Modification of Polymer Films by Energy-Compressed Dielectric Barrier Discharge With Discharge-Time-Regulated Power Source

An energy-compressed dielectric barrier discharge (DBD) in the atmospheric-pressure air is established to improve the surface modification of the polymer films. The effective discharge time of per repetitive cycle in the DBD system is adjusted by employing an active regulated power source, which can compress the energy transferred to the DBD load in the time domain. The effect of the energy compression on the surface modification of the polyethylene films is studied at different energy densities and frequencies by using the contact angle measurement, the atomic force microscope, and the X-ray photoelectron spectroscope. The results show that, at the given energy density and frequency, the surface wettability is improved by increasing the energy compression degree due to the higher surface roughness and more oxygen-containing polar functional groups. Furthermore, less energy density is needed to achieve the same level of surface wettability by increasing the energy compression degree. The reasons for the high performance of the energy-compressed DBD are the increase of the reactive species density and the enhancement of the physical and chemical interactions. As a result, the energy-compressed DBD is a superior solution for the polymer surface modification.

A New Surface Discharge Source: Plasma Characteristics and Delivery of Reactive Species

A new surface dielectric barrier discharge source containing a matrix of holes in the dielectric slice is put forward in this paper, for which the plasma characteristics and the delivery of reactive species to the downstream sample are studied for three experimental conditions, i.e., EC1 with a helium gas flow of 1 SLM through the holes, EC2 with an artificial air flow of 1 SLM, and EC3 without any gas flow. All the other conditions are kept the same. For the same discharge power of 2 W, it is found that the surface plasma in EC1 is comparatively more homogeneous, the discharge voltage is lower by ~12%, and the densities of reactive species are much higher, since the emission spectral lines of N2(C), N2+(B), He(3s3S), and O(3p5P) are stronger by at least sixfold. A petri dish of deionized water is put 5-mm downstream the surface discharge, in which the concentrations of nitrite, nitrate, H2O2, and NO are measured after the plasma treatment of 3 min. Compared with EC1, EC2, and EC3, it is found that the gas flow can increase the concentration of H2O2 by ~1.4-fold, but when helium is participated, a further increase of sixfold is achieved. Similarly, further increase is also found for the other three reactive species, which may be ascribed to the higher densities of gaseous reactive oxygen and nitrogen species in the helium-participated plasma and their larger diffusion coefficients in helium compared with that in air.

Development of Al2O3 electrospun fibers prepared by conventional sintering method or plasma assisted surface calcination

In this paper, the electrospinning method was used for preparation of α-Al2O3 microfibers from PAN/Al(NO3)3 precursor solution. The precursor fibers were thermally treated by conventional method in furnace or low-temperature plasma induced surface sintering method in ambient air. The four different temperatures of PAN/Al(NO3)3 precursors were chosen for formation of α-Al2O3 phase by conventional sintering way according to the transition features observed in the TG/DSC analysis. In comparison, the low-temperature plasma treatment at atmospheric pressure was used as an alternative sintering method at the exposure times of 5, 10 and 30min. FTIR analysis was used for evaluation of residual polymer after plasma induced calcination and for studying the mechanism of polymer degradation. The polycrystalline alumina fibers arranged with the nanoparticles was created continuously throughout the whole volume of the sample. On the other side the low temperature approach, high density of reactive species and high power density of plasma generated at atmospheric pressure by used plasma source allowed rapid removal of polymer in preference from the surface of fibers leading to the formation of composite ceramic/polymer fibers. This plasma induced sintering of PAN/Al(NO3)3 can have obvious importance in industrial applications where the ceramic character of surface with higher toughness of the fibers are required.

Graphitization of Chlorohydrocarbons in Laser‐Induced Plasma Filaments

SUMMARYThe decomposition of aqueous chlorohydrocarbons regarded as environmental pollutants has been widely studied. However, it is usually difficult to separate liquid and/or volatile products originated from chlorohydrocarbons from reactant solution. We succeeded to precipitate hydrophobic carbon nanoparticles and their agglomerates from aqueous dichloromethane by femtosecond laser (0.8 μm, 40 fs) irradiation. In the case of water/dichloromethane bilayer solution, the precipitates were produced in abundance when the water layer was exposed to femtosecond laser pulses. The time evolution of pH and transmittance revealed that the precipitation of carbon particles was preceded by dechlorination. Focusing intense femtosecond laser pulses onto water creates a high density of reactive species in a well‐confined volume; that is, plasma filament. The graphitization hence solidification by using laser pulses could be a useful way to remove chlorinated compounds from their aqueous solution.

Effects of DC bias voltages on the RF-excited plasma–tissue interaction

We present in this study how DC bias voltage impacts on the fluxes of reactive species on the skin tissue by means of a plasma–tissue interaction model. The DC bias voltage inputs less than 2% of the total discharge power, and hence it has little influence on the whole plasma characteritics including the volume-averaged densities of reactive species and the heating effect. However, it pushes the plasma bulk towards the skin surface, which significantly changes the local plasma characteristics in the vicinity of the skin surface, and in consequence remarkably enhances the flux densities of reactive species on the skin tissue. With the consideration of plasma dosage and heat damage on the skin tissue, DC bias voltage is a better approach compared with the common approach of increasing the plasma power. Since the DC voltage is easy to apply on the human body, it is a promising approach for use in clincial applications.

Is it possible to deduce the ground state OH density from relative optical emission intensities of the OH(A2Σ+-X2Πi) transition in atmospheric pressure non-equilibrium plasmas?—An analysis of self-absorption

The measurement of absolute densities of reactive species and radicals such as OH is of growing interest for many plasma applications. In this paper, we extend the use of a self-absorption model for atomic emission spectroscopy to molecular emission spectroscopy. The proposed analysis of self-absorbed molecular emission spectra is a simple and inexpensive method to determine OH(X) densities and rotational temperatures compared to laser induced fluorescence. We compare the recorded absolute OH density in a non-equilibrium diffuse atmospheric-pressure RF glow discharge by this method with broadband UV absorption considering a number of rotational lines with J′ ⩽ 6.5, the detection limit of the line integrated OH(X) density with this method is of the order of 2 × 1019 m−2. The accuracy of the density is sensitive to the rotational temperature of the OH(A) state and the non-equilibrium rotational population distribution.

Effects of the electrical parameters and gas flow rate on the generation of reactive species in liquids exposed to atmospheric pressure plasma jets

In this work, an atmospheric pressure plasma jet was fabricated and studied for plasma–liquid interactions. The plasma jet consists of a quartz-covered pin electrode and outer quartz tube with a tapered nozzle. Using the current–voltage (I-V) and optical emission characteristics of the plasma jet, the plasma density and the speed of the plume were investigated. The optical emission spectra clearly indicated the excited NO, O, OH, N2, and N2+ in the plasma plumes. Then the plasma jets were applied to the deionized water. We investigated the effects of the operating parameters such as applied voltage, pulse frequency, and gas flow rate on the generation of reactive species in the gas and liquid phases. The densities of reactive species including OH radicals were obtained at the plasma–liquid surface and inside the plasma-treated liquids using ultraviolet absorption spectroscopy and chemical probe method. The nitrite concentration was detected by Griess assay. The data are very suggestive that there is a strong correlation among the production of reactive oxygen and nitrogen species (RONS) in the plasmas and liquids.

Measurement of OH, O, and NO densities and their correlations with mouse melanoma cell death rate treated by a nanosecond pulsed streamer discharge

Mouse melanoma cells in a culture medium are treated using a nanosecond pulsed streamer discharge plasma and the correlations between the rate of cell death and the densities of reactive species (OH, O, and NO) in the plasma are measured. The plasma is irradiated onto the culture medium surface with a vertical gas flow of an O2/N2 mixture from a glass tube at various gas flow rates and O2 concentrations. The densities of the reactive species are measured very close to the culture medium surface, where the reactive species interact with the culture medium, using laser-induced fluorescence. In the case of the N2 discharge (O2 = 0%), an increase in gas flow rate decreases OH density because it lowers the water vapor concentration by diluting the vapor, which is required for OH production. The increase in gas flow rate also leads to a decreased cell death rate. In the case of the O2/N2 discharge, on the other hand, an increase in O2 concentration at a fixed flow rate does not affect the rate of cell death, although it considerably changes the O and NO densities. These findings indicate that some reactive species derived from water vapor such as OH are responsible for the melanoma cell death, whereas those from O2, such as O and NO, are less likely responsible. They also indicate the importance of water evaporation from the culture medium surface in cell treatment.