Afterglow Plasma Research Articles

Optical and spectroscopic study of a supersonic flowing helium plasma: energy transport in the afterglow

Flowing plasma jets are increasingly investigated and used for surface treatments, including biological matter, and as soft ionization sources for mass spectrometry. They have the characteristic capability to transport energy from the plasma excitation region to the flowing afterglow, and therefore to a distant application surface, in a controlled manner. The ability to transport and deposit energy into a specimen is related to the actual energy transport mechanism. In case of a flowing helium plasma, the energy in the flowing afterglow may be carried by metastable helium atoms and long-lived helium dimer ions. In this work a systematic investigation of the optical and spectroscopic characteristics of a supersonic flowing helium plasma in vacuum and its afterglow as function of the helium gas density is presented. The experimental data are compared with numerical modeling of the plasma excitation and helium dimer ion formation supported by a Computational Fluid Dynamic simulation of the helium jet. The results indicate that the plasma afterglow is effectively due to helium dimer ions recombination via a three-body reaction.

TiO2 powder modified by plasma afterglow: A correlation between active species, microstructure, and optical properties

TiO2 powder was modified by the afterglow of plasma of pure N2 or H2, and mixed N2 + H2 gas. The ion species in plasma afterglow were measured by an ion-energy analyzer. It is found that the main ion is N2+ in N2 afterglow and the yield is changed to NH4+ with a six times higher concentration, when 10% H2 was input. The modified TiO2 shows significant differences of microstructure and optical properties depending on the plasma gas used. This work enables one to find a correlation between active species in the plasma afterglow and TiO2 microstructure/optical properties.

Electron and negative ion dynamics in a pulsed 100 MHz capacitive discharge produced in an O2 and Ar/O2/C4F8 gas mixture

This paper presents an experimental study of electron and negative ion dynamics in a pulsed 100 MHz capacitive discharge. The plasma is produced in an O2 and Ar/O2/C4F8 gas mixture at 2 Pa gas pressure. Laser photo-detachment combined with a microwave resonance probe is used to diagnose the absolute densities of multiple negative ion species, F− and O−. In pure oxygen discharge, both the quasi-steady state electron and negative ion densities increase during the active glow with an increase in RF power level. In the afterglow plasma, the electron density decay rate is ∼10–15 μs, whereas the O− decays more slowly with a decay time constant in the range of 30–35 μs. At a low RF power, a peak in the O− density is observed in the early afterglow plasma. In the Ar/O2/C4F8 gas mixture, the quasi-steady state electron density in the active glow plasma increases with RF power, whereas the O− density decreases and F− density increases. In comparison to pure O2 discharge, in the Ar/O2/C4F8 gas mixture the electron density decay is faster. In the afterglow the F− density is ten times higher than the O− density. These results suggest a change in the dominant negative ion production mechanism within the discharge pulse from dissociative electron attachment of CxFy in the active on-phase to electron attachment to fluorine atoms in the afterglow plasma.

Cold High-Frequency Plasma Versus Afterglow Plasma in the Preservation of Mobile Cultural Heritage on Paper Substrate

This article presents a comparison between the results obtained at the application of a conservation treatment in cold high-frequency (HF) and afterglow plasma on historical papers dating in the 19th century. This article is focused on plasma biocide effects and on the impact of active species from the two types of plasma on the surface, highlighted by mechanical and physical-chemical analyses. The working gas was nitrogen and the treatment time was varied from 5 to 10 and 15 min. The result of physical-chemical analyses as SEM, pH determination, color, and tensile strength analyses, as well as microbiological investigations, converges to the conclusion that a treatment of 10 min achieves a complete decontamination, and the impact of active species is less intense in the afterglow plasma as compared to the direct discharge in cold HF plasma.

Understanding the ion acceleration mechanism in bipolar HiPIMS: the role of the double layer structure developed in the after-glow plasma

The physical vapor deposition techniques are nowadays widely used at industrial scale to produce thin films and surface coatings. Despite their proven utilities and benefits, these techniques still need to be improved in order to build other exigent and innovative coating systems, able to satisfy the market demand and to meet modern society’s needs. In this context, the bipolar high power impulse magnetron sputtering (BP-HiPIMS) technology is gaining ground and popularity due to its extraordinary ability to control the energy of the incoming ion flux to the growing film, enabling an energy-enhanced deposition process thanks to the existence of a dynamic double layer (DL) structure which develops in the after-glow plasma. In this work, the HiPIMS discharge was operated in bipolar mode, in which the negative pulses are followed by positive pulses whose amplitude, duration and delay can be independently controlled. The temporal and spatial evolutions of the plasma potential in BP-HiPIMS discharge were investigated using an emissive probe. Energy-resolved mass spectrometry was used to study the influence of the pulsing configuration (positive/negative pulse duration) on the energy distribution and flux of the ionic species bombarding the substrate. It was found that the ion energy distributions and ion flux were mainly determined by the specific time-and-space evolution of the plasma potential throughout the cycle of the voltage pulses applied to the target and the spatial distribution of the ions at the onset of the positive pulse. The ion propagation dynamics is strongly related to the potential drop across the DL structure whose characteristic features are mainly influenced by the amplitude and duration of the positive pulse. Short negative HiPIMS pulses followed by short and highly positive reverse pulses are favorable for an efficient acceleration mechanism of the metal ions in the potential drop of the DL.

Equipments for Treatment in High Frequency Plasma Discharge

This article presents five alternative plasma equipments patented by the author, designed for specific applications like activation, cleaning, decontamination, or monomer polymerization on surfaces. Two equipments perform HF cold plasma treatment of highly adherent powdery materials (pigments, plastics, drugs, and so on), that are used to modify the physical-chemical characteristics of surfaces. Another HF plasma equipment is employed for the treatment of nonwoven stratified materials used as filters for gaseous media. A specially designed afterglow plasma equipment is used for decontamination and cleaning of highly fragile flat or 3-D cultural heritage items. An equipment for producing HF torch plasma is also described.

Drift-Alfvén fluctuations and transport in multiple interacting magnetized electron temperature filaments

The results of a basic electron heat transport experiment using multiple localized heat sources in close proximity and embedded in a large magnetized plasma are presented. The set-up consists of three biased probe-mounted crystal cathodes, arranged in a triangular spatial pattern, that inject low energy electrons along a strong magnetic field into a pre-existing, cold afterglow plasma, forming electron temperature filaments. When the three sources are activated and placed within a few collisionless electron skin depths of each other, a non-azimuthally symmetric wave pattern emerges due to interference of the drift-Alfvén modes that form on each filament’s temperature gradient. Enhanced cross-field transport from chaotic ( $\boldsymbol{E}\times \boldsymbol{B}$ , where $\boldsymbol{E}$ is the electric field and $\boldsymbol{B}$ the magnetic field) mixing rapidly relaxes the gradients in the inner triangular region of the filaments and leads to growth of a global nonlinear drift-Alfvén mode that is driven by the thermal gradient in the outer region of the triangle. Azimuthal flow shear arising from the emissive cathode sources modifies the linear eigenmode stability and convective pattern. A steady-current model with emissive sheath boundary predicts the plasma potential and shear flow contribution from the sources.

Modifications produced on a large magnetized plasma column by a floating end-plate that is partially emissive: Experiment and theory

An experiment is performed on a large plasma device operated by the Basic Plasma Science Facility at the University of California, Los Angeles, in which an electrically floating structure is placed near the end of the 18-m magnetized plasma column. The structure consists of a flat carbon plate that acts as a mask for a smaller, ring-shaped LaB6 emissive surface whose temperature can be externally controlled. This configuration has been previously used to study electron heat transport and pressure-driven avalanches [B. Van Compernolle and G. J. Morales, Phys. Plasmas 24, 112302 (2017)] by biasing the LaB6 ring-cathode with respect to a distant anode in a cold afterglow plasma. In contrast, the present study is performed during the active portion of the steady-state discharge in which the nominal plasma parameters are determined by the injection of an electron beam from a BaO cathode at the opposite end. It is found that, even without an applied bias on the LaB6 cathode, the self-consistent potential and current profiles are modified near the end plate as the LaB6 temperature is increased, resulting in density increases on the field lines in contact with the ring-cathode. In the absence of enhanced ionization, at the largest cathode temperatures, the ambient density can be doubled. A theoretical model is presented that provides a quantitative explanation for the experimental observations.

Time-resolved ion energy distribution functions in the afterglow of an EUV-induced plasma

Since the introduction of extreme ultraviolet (EUV) lithography (EUVL), the inevitable presence of EUV-induced plasmas inside the lithography tools impacts the operation of EUV optical components. EUV-induced plasmas are created everywhere in the optical path due to the ionizing interaction between the high energy (92 eV) EUV photons and the tools' background gas, which typically is hydrogen gas at a pressure of 1–10 Pa. From a physical point of view, the main impact of the plasma is due to the presence of ions that imping the plasma-facing surfaces. Experimental research into the fluence and energy distribution functions (IEDFs) of ions from EUV-induced plasmas has been limited to time-averaged measurements. In this Letter, we present time-resolved measurements of IEDFs for H+, H2+, and H3+ ions from an EUV-induced plasma in pure hydrogen gas. To this end, an electrostatic quadrupole plasma (EQP) analyzer has been used. The measurements pinpointed momentary fluxes up to three orders of magnitude higher than earlier reported average ion fluxes. In addition, the mean ion energy was unexpectedly found to remain elevated up to 50 μs after the gas had been irradiated with EUV photons. Also, it was shown that the EQP detects H2+ ions on time scales much larger than expected. The presented results are valuable not only for the understanding of elementary processes regarding EUV-induced plasmas interacting with surfaces but also for simulating and predicting the impact of EUV-induced plasma on the lifetime and stability of optical components in EUVL.

Magnetic field enhanced cold plasma sterilization

Cold plasma sterilization offers an efficient way to sterilize medical components and instruments. This paper reports using a magnetized plasma to realize low-temperature sterilization. A radio frequency dielectric barrier discharge is created in a quartz tube using a mixture of argon and oxygen gas. A uniform amount of Escherichia coli is applied onto glass slides and exposed to the plasma afterglow at different pressures with and without a magnetic field. Optical emission spectroscopy is used to identify the plasma species present. The magnetic field significantly promotes the intensity of the plasma and the sterilization efficiency. A process gas pressure of 100 mTorr presents the most effective treatment with a sterilization time less than one minute and sample temperature below 32 °C.

Immobilization of Chitosan Onto Polypropylene Foil via Air/Solution Atmospheric Pressure Plasma Afterglow Treatment

The combination of an atmospheric pressure plasma afterglow operated in air with wet grafting was utilized for the immobilization of chitosan on the surface of polypropylene foil. The plasma treatment results in a local modification of polymer surface along the sample axis with a modified zone of width 2 cm. Moreover, plasma treatment initiates etching and melting of the polymer on micro-level. Processing in combination with chitosan solution in-line prevents thermal effects and results in the formation of island-like chitosan structures. X-ray photoelectron spectroscopy (XPS) and Fourier transform infrared spectroscopy (FT-IR) reveal the presence of chemical groups typical for chitosan structure.

Application of Molecularly Imprinted Polymers (MIP) and Magnetic Molecularly Imprinted Polymers (mag-MIP) to Selective Analysis of Quercetin in Flowing Atmospheric-Pressure Afterglow Mass Spectrometry (FAPA-MS) and in Electrospray Ionization Mass Spectrometry (ESI-MS).

Molecularly imprinted polymer (MIP) and magnetic molecularly imprinted polymer (mag-MIP) for solid extraction and pre-concentration of quercetin have been successfully prepared by thermal polymerization method using quercetin (Q) as a template, acrylamide (AA) as a functional monomer, and ethylene glycol dimethacrylate (EGDMA) as a cross-linking agent. The MIP and mag-MIP were successfully applied in analysis of quercetin by mass spectrometry (MS) methods. To perform ambient plasma ionization experiments, a setup consisting of the heated crucible, a flowing atmospheric-pressure afterglow (FAPA) plasma ion source, and amaZon SL ion trap (Bruker, Bremen, Germany) was used. The heated crucible with programmable temperature allowed desorption of the analytes from MIPs structure which resulted in their direct introduction into the ion stream. The results of Q-MIP/Q-mag-MIP and FAPA-MS measurements were compared with those of the analysis of quercetin by the ESI-MS method without extractions and pre-concentration of analytes on polymers. Limits of detection (LOD) for quercetin solutions in both positive and negative ESI-MS were established at 10−8 M and 10−7 M, respectively. The linearity (R2 = 0.9999) of the proposed analytical procedure for quercetin determination in positive ions was provided in the range between 10−4 M and 10−7 M. Moreover, the same parameters were established for FAPA-MS in positive ions, reaching LOD at 0.005 mg/gMIP and the linearity of the method in the range of 0.015–0.075 mg/gMIP with the correlation coefficient value R2 = 0.9850.

The charge of micro-particles in a low pressure spatial plasma afterglow

In this letter, we present charge measurements of micro-particles in the spatial afterglow (remote plasma) of an inductively coupled low pressure radiofrequency plasma. The particle afterglow charge of , being deducted from their acceleration in an externally applied electric field, is about three orders of magnitude lower compared to the typical charge expected in the bulk of such plasmas. This difference is explained by a relatively simplistic analytical model applying orbital motion limited theory in the afterglow region. From an application perspective, our results enable further understanding and development of in situ plasma-based particle contamination control for ultra-clean low pressure environments.

Tunable morphologies of ultrathin ZnO nanostructures synthesized by a plasma afterglow-assisted oxidation process and their photocatalytic properties

The fast growth of ultrathin zinc oxide nanostructures is reported in this study, by using a flowing microwave plasma oxidation method: zinc films coated by a thin copper buffer layer are oxidized in low-pressure afterglow. This work highlights the versatility of plasma afterglow treatments and demonstrates that the morphology of ZnO nanostructures (nanowires, nanoribbons and nanocombs) can easily be controlled with good reproducibility by adjusting experimental parameters such as the working pressure during the oxidation process. The rapid growth of ultrathin nanostructures (∼1 min) is due to the afterglow area which is a very oxidizing medium since it contains high concentrations of oxygen atoms and excited oxygen molecules. The surface morphology and structural and optical properties of the as-synthesized nanostructures are studied by means of SEM, HRTEM and UV–visible absorption, respectively. Ultrathin ZnO nanowires and nanoribbons with high aspect ratios exhibit excellent photocatalytic activity for the degradation of a model organic dye: methyl orange (MO), by comparison with pure ZnO thin films without nanostructures and thick ZnO nanowires synthesized by thermal oxidation. These results are supported by photocurrent measurements. The enhancement in photocatalytic performance could be attributed to higher surface area and higher transfer efficiency of photoinduced charge carriers to the surface of ultrathin nanowires and nanoribbons. The reusability of ZnO nanowire samples is also investigated by recording the photoactivity after several cycles of photodegradation.

Strong Sensitized Ultraviolet Luminescence from He–C2F4–NO Flowing Plasma Afterglow: A Route to Short-Wavelength Gas-Flow Lasers?

Unexpected strong ultraviolet emission has been observed from the merger of two low-pressure supersonic gas-flows previously leaving two radiofrequency (RF) discharges, one containing Ar, He, O2 and traces of NO, the other He and plasma-chemically produced C2F4. Analysis of the spectral emission data over a range of 190 to 710 nm shows that the main energy carriers in this radiofrequency powered flowing discharge afterglow are He, C2F4 (or CF2) and NO. Plasma-chemical excitation and energy transfer amongst the main energy carriers result in a spectrum dominated by a vibronic structure, covering the spectral range from 190 to 300 nm, which is assigned to the spontaneous emission from excited nitric oxide. The strongest line located at 236.2 nm is due to the NO(A2Σ+, 0–1) transition. The distance between the point of observation and the nearest RF discharge together with the residence time of gases in this region excludes any direct excitation of NO by the RF discharge itself. The observed phenomenon has potential as a plasma-chemical gas-flow laser emitting at a fundamental frequency in the mid-ultraviolet spectral region.

The role of hydrogen in the nitriding of anatase TiO2 films in the N2-H2 microwave afterglows

Nitriding in the afterglows can be beneficial in the surface-selective nitridation of oxide materials for the control of surface defects or active sites in electronic devices or catalysts. Here we introduce our results on the nitriding of anatase TiO2 films in the afterglows of N2-H2 microwave plasmas at the substrate temperatures up to 300 °C. We employed optical emission spectroscopy (OES) and X-ray photoemission spectroscopy (XPS) to determine active species in the afterglows and the N species incorporated into the film, respectively. Adding a few percent of H2 into the N2 gas flow generates active species such as NH and H atoms in the afterglow region in addition to the N2-derived active species such as N atoms and excited N2 species. The XPS results revealed that such changes in the type and densities of active species in the afterglows can significantly enhance the surface nitriding performance. The origin for the enhanced surface nitriding is attributed to the efficient removal of surface oxygen atoms by reactive H atoms via the formation of H2O to result in an efficient surface uptake of nitrogen. Our results show that the N2-H2 afterglows can be effectively used for a selective surface nitriding of oxide films such as TiO2 at the relatively low temperature range.

High-Sensitivity Elemental Mass Spectrometry of Fluorine by Ionization in Plasma Afterglow.

Fluorine elemental analysis using inductively coupled plasma mass spectrometry (ICPMS) is challenging because of low F ionization efficiency in the plasma and severe isobaric interferences. Notably, there is an increasing demand for ppb level fluorine measurements due to the rising importance of fluorinated compounds in pharmaceutical, environmental, and food analyses. Here, we report a new elemental ionization method where fluorinated analytes are introduced into an ICP to produce NaF followed by Na2F+ formation in the atmospheric-pressure plasma afterglow. The new method offers over 2 orders of magnitude improved sensitivities (180-500 cps/ppb versus 1.6-3.2 cps/ppb) for F detection. This approach also yields compound-independent F response for quantitation without compound-specific standards. Detection limits of ∼50 ppb F are attained using a single-quadrupole instrument without discrimination against isobaric interferences. Similar LODs are achievable only by isobaric interference reduction in ICPMS/MS. Importantly, the new approach offers facile interfacing to molecular MS instruments where LODs can be further improved via MS/MS and high-resolution MS techniques. The tolerance to matrix is demonstrated by quantitation of fluoride in infant formula, yielding recoveries of 86%-98% with repeatabilities of 3.5-6.3 RSD%.

Afterglow dielectric-barrier discharge air plasma (ADDAP) for the inactivation of Escherichia coli O157:H7

The afterglow of an atmospheric pressure plasma can conveniently be used for large scale decontamination operations. In the present study, an afterglow dielectric-barrier discharge air plasma (ADDAP) was used to inactivate Escherichia coli O157:H7, which was used as a model microorganism for studying the inactivation effect of the plasma. ADDAP was generated at different input currents in the range of 0.4–0.8 A. At all tested plasma exposure times, temperatures inside sample treatment chamber remained less than 30 °C. Regarding chemical composition of ADDAP in the treatment chamber, NOx species (NO and NO2) were predominantly observed. The levels of the NOx species increased as the current intensity increases and the maximum NO and NO2 concentrations noted were 6 and 4 ppm, respectively, but that of CO was less than 1 ppm level at 0.8 A. Upon treatment using ADDAP generated at 0.4–0.8 A for 180 min, glass-bound E. coli O157:H7 counts were reduced in the range of 1.24–2.71 log CFU/mL. The inactivation patterns exhibited better fit to the Weibull tail model. The comparison of delta values indicated that superior inactivation effects were observed as current intensity increases.

Behavior of Historical Printing Inks on Paper in High-Frequency Cold Plasma Discharges

This paper reports on the behavior of red and black ink from a historical printed book, subjected to a plasma discharge treatment, applied for the decontamination and cleaning of naturally aged paper. As some preliminary experiments using high-frequency (HF) plasma showed a slight fading of red ink, the study was focused on the comparison of HF cold plasma and afterglow plasma treatments in order to get conclusive information on the effect of plasma active species on the red ink. Energy Dispersive X-ray Scanning Electron Microscopy SEM-EDX and Attenuated Total Reflection Fourier-Transform Infrared Spectroscopy FTIR-ATR analyses were used for the identification of inks and binder, while the impact of plasma on inks was assessed by SEM analysis and color determinations. The experiments have been performed on an especially designed modular equipment, allowing the samples to be subjected alternately to HF cold plasma or afterglow plasma treatment. The study proved that direct HF plasma could result in a more important deterioration at treatment duration exceeding 5 min, while in afterglow plasma, this discoloration is not observable. The color fading is imputable to the corrosion of pigment crystallites produced by direct plasma discharge.

Low-damage nitrogen incorporation in graphene films by nitrogen plasma treatment: Effect of airborne contaminants

Graphene films grown on copper by chemical vapor deposition were exposed to the late afterglow of a reduced-pressure N2 plasma sustained by microwave electromagnetic fields. X-ray photoelectron and Raman spectroscopies reveal extremely high incorporation of plasma-generated N atoms into the graphene film (N/C = 29%) while maintaining an unprecedentedly low-damage generation (D:G = 0.35–0.45) compared to the literature (0.5–2.5). The incorporation dynamics between graphene on copper and graphene on copper oxide are also compared and discussed. After transfer on SiO2/Si substrate, the N/C content decrease to only 6%. This reveals that a large part of the N atoms are weakly bonded to the graphene surface. Most of the nitrogen incorporation seems linked to the functionalization of weakly bonded hydrocarbons initially adsorbed from air exposure or carbon-nitrogen structures arising from plasma-surface interactions.