Plasma Exposure Time Research Articles

Plasma Modification of Poly Lactic Acid Solutions to Generate High Quality Electrospun PLA Nanofibers

Physical properties of pre-electrospinning polymer solutions play a key role in electrospinning as they strongly determine the morphology of the obtained electrospun nanofibers. In this work, an atmospheric-pressure argon plasma directly submerged in the liquid-phase was used to modify the physical properties of poly lactic acid (PLA) spinning solutions in an effort to improve their electrospinnability. The electrical characteristics of the plasma were investigated by two methods; V-I waveforms and Q-V Lissajous plots while the optical emission characteristics of the plasma were also determined using optical emission spectroscopy (OES). To perform a complete physical characterization of the plasma-modified polymer solutions, measurements of viscosity, surface tension, and electrical conductivity were performed for various PLA concentrations, plasma exposure times, gas flow rates, and applied voltages. Moreover, a fast intensified charge-couple device (ICCD) camera was used to image the bubble dynamics during the plasma treatments. In addition, morphological changes of PLA nanofibers generated from plasma-treated PLA solutions were observed by scanning electron microscopy (SEM). The performed plasma treatments were found to induce significant changes to the main physical properties of the PLA solutions, leading to an enhancement of electrospinnability and an improvement of PLA nanofiber formation.

Investigating Electron And Radical Interactions With Biomolecules And Cells Using A Droplet In Plasma Laboratory

Plasmas-liquid interactions are a valuable source of radical species for plasma medicine including cancer treatment and antimicrobial applications. However untangling the complex interplay between species and biology over very short timescales and distances is proving extremely challenging. Radiolysis induced damage to DNA is considered to be significantly affected by low energy secondary species such as OH radicals but also via direct interaction with low energy solvated electrons (LEE). Pt adducts and the presence of Au nanoparticles are also known to enhance the LEE effectiveness. However direct investigation of LEE and radical interactions with biomolecules e.g. DNA in a liquid environment at atmospheric pressure or in a living cell present very significant experimental challenges. LEE sources have been proposed such as 2D radioactive layers and UV-induced emission from metals. In this work we look at the potential of a living laboratory based on small liquid droplets containing biomolecules or cells passed through a plasma and exposed to a high flux of electrons and selectable radicals. We also consider the potential of using droplets to deliver plasma-activated media near instantaneously downstream. Transport of micron-sized liquid droplets through a low temperature non-equilibrium RF plasma [1] at atmospheric pressure has demonstrated a number of remarkable and unexpected effects. The microdroplet system allows for a controlled (air-free) gas ambient environment, a large surface area to volume ratio, very small reaction volume and low droplet temperature. In addition, flow induced convection in the liquid can be minimised. After a very short plasma exposure time, ~120°μ s, there is evidence that chemical reactions induced by the plasma and gas flux proceed at a rate that is significantly faster that observed in plasma – bulk liquid studies and many orders of magnitude faster than in standard bulk chemistry [2]. The high chemical reactivity is thought to depend not only on the picolitre droplet volume, as in microreaction chemistry, but also on the high level of surface charge due to net electron bombardment in the plasma and the high flux of low energy electrons which arrive at the charged droplet with almost zero net energy. We have observed very rapid electron reduction of metal (Au) salts to form Au nanoparticles at rates that are much greater than observed with gamma radiolysis or high energy electron beams. This points to the possible effectiveness of plasma-generated Ultra-Low Energy Electrons (ULEE) in reduction reactions that may prove valuable for electron – biomolecule studies. We have also observed H2O2 and OH in the liquid most likely due to generation of these species in the plasma phase, which contains only He and H2O vapour. We have used droplets as carriers for single bacteria cells, which are then exposed to plasma species (electrons, OH and H2O2) for a very short time (~0.1 ms) and the effects on cell viability and properties determined. It is clear that this system is complex but plasma-droplet system may yet offer unique access for plasma – liquid chemistry and biology studies. We will present plasma-DNA and cell studies in conjunction with liquid chemistry and electric field modelling.

Plasma Synthesis of Graphene from Mango Peel.

The excess of mango peels is considered manufacturing waste in the sugar and juice industry. There is an increasing interest in looking for alternative ways to employ this waste to address this overload. Here, we show the efficient use of mango peels as a noncost carbon source for the synthesis of graphene. We demonstrate for the first time the synthesis of graphene on Cu substrates from mango peels, a biomass rich in pectin. It is observed that plasma presence is essential for the growth of graphene from mango peels. At 15 and 30 min of plasma exposure, we observed the presence of multilayered graphene, at longer plasma exposure, i.e., 60 min, there is the formation of monolayer graphene, attributed to the etching of multiple layers formed at short times due to long plasma exposure time. When employing this technique, precautions must be taken due to the etching effect of plasma, such as reducing either the plasma exposure time or the plasma power. Finally, we present a graphene growth pathway under plasma environment on the basis of our experimental observations.

Covalent nitrogen doping in molecular beam epitaxy-grown and bulk WSe2

Covalent p-type doping of WSe2 thin films grown by molecular beam epitaxy and WSe2 exfoliated from bulk crystals is achieved via remote nitrogen plasma exposure. X-ray photoelectron and Raman spectroscopies indicate covalently bonded nitrogen in the WSe2 lattice as well as tunable nitrogen concentration with N2 plasma exposure time. Furthermore, nitrogen incorporation induces compressive strain on the WSe2 lattice after N2 plasma exposure. Finally, atomic force microscopy and scanning tunneling microscopy reveal that N2 plasma treatment needs to be carefully tuned to avoid any unwanted strain or surface damage.

Non-equilibrium atmospheric pressure plasma as innovative method to crosslink and enhance mucoadhesion of econazole-loaded gelatin films for buccal drug delivery

In this paper we developed an innovative, effective and rapid one-step approach to crosslink mucoadhesive gelatin films for buccal drug delivery. The method, which involves the application of non-equilibrium pressure plasma for 3 or 5 minutes/side, was compared with a classical approach based on the use of a chemical crosslinking agent, namely genipin. Econazole nitrate (ECN), an imidazole antifungal agent used for the treatment of skin infections and mucosal candidiasis, was selected as model drug. X-Ray Diffraction characterization performed on the drug-containing gelatin films revealed that ECN undergoes to a topotactic transformation into Econazole (EC) immediately after mixing with gelatin suggesting the occurrence of an acid-base reaction between drug and gelatin during film processing. Plasma treatment, as well as genipin crosslinking, did not provoke any further variation of EC structure. However, plasma exposure significantly improved films adhesiveness and allowed to reach mucoadhesive strength values more than double with respect to those obtained with genipin, ascribable to the presence of polar and hydrophilic groups on the plasma treated film’s surface. A residence time of at least 48 h was obtained by properly selecting the plasma exposure times. These results, together with the in-vitro data showing retention of antifungal efficacy against a strain of Candida albicans, demonstrated that plasma treatment was a valid and rapid alternative, easy to scale-up, to chemical crosslinking methods for the production of highly mucoadhesive gelatin-based films.

Influence of N2/H2 and N2 plasma on binary III-nitride films prepared by hollow-cathode plasma-assisted atomic layer deposition

The authors reported the hollow-cathode plasma-assisted atomic layer deposition of AlN, GaN, and InN films using N2-only and N2/H2 plasma. In this study, the authors analyzed the effect of plasma gas composition on the properties of deposited binary III-nitride thin films. Toward this goal, AlN, GaN, and InN films were deposited on Si (100) substrates using N2-only (50 sccm), as well as N2/H2 (50 + 50, 50 + 25 sccm) plasma to investigate the impact of H2 flow. Grazing-incidence x-ray diffraction (GIXRD) patterns of AlN and GaN thin films deposited with N2/H2 plasma remained almost unchanged when H2 flow decreased from 50 to 25 sccm. On the other hand, the use of N2 plasma without any H2 resulted in amorphous GaN thin films with significant carbon impurity within the bulk film. In the case of AlN, similar behavior was observed as the crystal structure is significantly altered to amorphouslike material. Thicknesses of AlN and GaN thin films increased tremendously when N2-only was used as the plasma gas. Furthermore, refractive index values of both AlN and GaN films decreased upon the use of N2-only plasma, which confirm the deterioration of the film quality. Structural weaknesses of GaN and AlN films deposited with N2-only plasma are due to presences of carbon impurities that are trapped inside the growing film. Interestingly, the authors did not observe similar results in InN films grown with N2/H2 plasma. For InN, GIXRD and spectroscopic ellipsometry results show that the phases of deposited films change from InN to In+InN as H2 content in the plasma gas is increased. On the other hand, InN films grown with N2-only plasma show improved structural properties. However, significantly higher N2 plasma exposure times are needed to minimize the residual carbon content in deposited InN layers.

Quantitative characterization of plasma treated PDMS microfluidic substrates by inverse gas chromatography

Abstract The effect of air plasma exposure time on the surface energies and acid-base characteristics of polydimethylsiloxane (PDMS) particles was studied. Polymerized PDMS powder was radio frequency induced air plasma irradiated for 2–10 s with the power of 500 W. The efficiency of the plasma treatments was investigated by a new generation inverse gas chromatograph, a surface energy analyzer. The dispersive component of surface energy was determined by the Dorris-Gray method describing the van der Waals interactions, while the specific component of surface energy expressed the surface ability for Lewis acid-base interactions. It was demonstrated that the air plasma treatment did not affect the dispersive and acidic parts of the surface energy and the change of surface hydrophilicity was caused by the raise of the electron-donor ability of the PDMS surface, characterized by van Oss-Good-Chaudhury’s base number. The optimal plasma treatment time was found to be 5 s. Analysis of the specific surface energy and acid-base characteristics with respect to exposure time showed that partial to complete hydrophobic recovery occurred within 38 h.

UV–vis spectroscopy study of plasma-activated water: Dependence of the chemical composition on plasma exposure time and treatment distance

Plasma-activated water (PAW) is receiving much attention in biomedical applications because of its reported potent bactericidal properties. Reactive oxygen and nitrogen species (RONS) that are generated in water upon plasma exposure are thought to be the key components in PAW that destroy bacterial and cancer cells. In addition to developing applications for PAW, it is also necessary to better understand the RONS chemistry in PAW in order to tailor PAW to achieve a specific biological response. With this in mind, we previously developed a UV–vis spectroscopy method using an automated curve fitting routine to quantify the changes in H2O2, NO2−, NO3− (the major long-lived RONS in PAW), and O2 concentrations. A major advantage of UV–vis is that it can take multiple measurements during plasma activation. We used the UV–vis procedure to accurately quantify the changes in the concentrations of these RONS and O2 in PAW. However, we have not yet provided an in-depth commentary of how we perform the curve fitting procedure or its implications. Therefore, in this study, we provide greater detail of how we use the curve fitting routine to derive the RONS and O2 concentrations in PAW. PAW was generated by treatment with a helium plasma jet. In addition, we employ UV–vis to study how the plasma jet exposure time and treatment distance affect the RONS chemistry and amount of O2 dissolved in PAW. We show that the plasma jet exposure time principally affects the total RONS concentration, but not the relative ratios of RONS, whereas the treatment distance affects both the total RONS concentration and the relative RONS concentrations.

Treatment of polyacrylonitrile co‐polymer membrane by low temperature radio‐frequency nitrogen plasma

Radio frequency plasma discharge of nitrogen gas was employed for modification of the surface of polyacrylonitrile membranes. The enhancement of hydrophilicity of the plasma modified membrane was evaluated by measuring the contact angle. The influence of various plasma operating conditions, namely, power and exposure time on improvement of permeability and hydrophilicity of the membrane was investigated. Hydrophilicity of the plasma‐treated membranes was more at higher exposure time and power. Approximately 67% increase in permeability of the modified membrane was obtained, and it was retained up to 2 months. Hydrophilicity of the membrane surface was due to attachment of functional groups derived from nitrogen and oxygen.

Effect of oxygen plasma treatment on tensile strength of date palm fibers and their interfacial adhesion with epoxy matrix

Abstract In recent years, natural fibers have received much attention from various industrial applications. As these fibers are lightweight, nonabrasive, low cost, ecofriendly and biodegradable, they can be sometimes considered as alternatives to synthetic fibers in lightweight composite structures. In this work, date palm fibers (DPFs) were treated by oxygen plasma at various plasma discharge power and exposure time. The effects of plasma treatment on tensile strength of DPF and interfacial adhesion between DPF and epoxy were determined by single fiber tensile test and microbond test, respectively. Scanning electron microscopy was used to investigate the surface morphologies of DPFs before and after the plasma treatment. The functional groups on the surface were studied by attenuated total reflectance-Fourier transform infrared spectroscope (ATR-FTIR). Decrease in hemicellulose and lignin content of DPF was indicated in ATR-FTIR spectra of the treated sample with plasma treatment. The results show that plasma treatment cleans the fiber surface and increases the surface roughness by etching effect. Moreover, fiber surface modification significantly improves tensile properties of DPFs and interfacial shear stress (IFSS) of fiber/matrix. However, the effects of plasma power and exposure time on tensile properties and IFSS values of DPFs are not found significant. Moreover, Weibull statistics show that plasma treatment could not decrease the variability in fiber strength due to the nature of fibers.

Atmospheric pressure plasma assisted calcination by the preparation of TiO2 fibers in submicron scale

Atmospheric pressure plasma assisted calcination by the preparation of TiO2 submicron fibers as a low-temperature alternative to the conventional thermal annealing was studied. A special type of dielectric barrier discharge was used for plasma treatment of hybrid titanium butoxide/polyvinylpyrrolidone (Ti(Bu)/PVP) fibers prepared by forcespinning to decompose and oxidize the base polymer and precursor. The obtained fibers were characterized by changes in chemical bonds on the surface using Fourier Transform Infrared Spectroscopy (FTIR), chemical composition by using Energy-Dispersive X-Ray Spectroscopy (EDX), X-ray Photoelectron Spectroscopy (XPS). The morphology of fibers was investigated by Scanning Electron Microscopy (SEM). A significant decrease of organic components was reached by short plasma exposure times less than 1 h. The obtained fibers exhibit a high surface porosity without degradation of the fibrous structure. The results obtained indicate that atmospheric pressure plasma assisted calcination can be a viable low-temperature, energy-and time-saving alternative or pre-treatment method for the conventional high-temperature thermal calcination.

Modification of W surfaces by exposure to hollow cathode plasmas

In this work, we assess the surface modifications induced on W samples following exposure to He and He/H2 radiofrequency plasmas in hollow cathode discharge configuration. Our study addresses issues that relate to the use of W in next-generation fusion reactors and, therefore, the investigation of W surface degradation following exposure and heating by plasmas to temperatures above 1000 °C is of practical importance. For these experiments, we used commercially available tungsten samples having areas of 30 × 15 mm and 0.1 mm thickness. The hollow cathode plasma was produced using a radiofrequency (RF) generator (13.56 MHz) between parallel plate electrodes. The W samples were mounted as one of the electrodes. The He and He/H2 plasma discharges had a combined effect of heating and bombardment of the W surfaces. The surface modifications were studied for discharge powers between 200 and 300 W, which resulted in the heating of the samples to temperatures between 950 and 1230 °C, respectively. The samples were weighed prior and after plasma exposure, and loss of mass was measured following plasma exposure times up to 90 min. The analysis of changes in surface morphology was carried out by optical microscopy, scanning electron microscopy and atomic force microscopy. Additionally, optical emission spectra of the respective plasmas were recorded from the region localized inside the hollow cathode gap. We discuss the influence of experimental parameters on the changes in surface morphology.

(Invited) Thermal Cyclic Atomic-Level Etching of Nitride Films: A Novel Way for Atomic-Scale Nanofabrication

Because the structure of semiconductor devices has changed to miniaturized three-dimensional (3D) structures, etching technologies that enable 3D atomic-scale control have received increasing interest. Therefore, atomic level/layer etching (ALEt), which is a counterpart of atomic layer deposition (ALD), has attracted much attention due to its potential in atomic-scale 3D nanofabrication. ALEt typically involves a cyclic repetition of self-limiting formation and desorption of surface modified layers. Thus, geometric loading effects are mitigated when using ALEt because of the self-limiting behavior. The authors have developed a novel isotropic ALEt process for SiN using cyclic repetitions of hydrofluorocarbon-based plasma exposure and thermal annealing [1]. The first step of the etching cycle for SiN is exposing the sample surface at room temperature to hydrofluorocarbon-based radicals that form ammonium hexafluorosilicate ((NH4)2SiF6) on SiN. The second step is thermal annealing above 100°C to sublimate the ammonium hexafluorosilicate. SiN film with an atomically thin layer can be etched away by repeating the cycle of forming and by removing the ammonium hexafluorosilicate. This cyclic etching shows self-limiting behavior because the formation of the ammonium hexafluorosilicate saturates as the plasma exposure time increases. This technology is applicable to other nitride films besides SiN. The authors have demonstrated selective isotropic ALEt of TiN [2], which is widely used as a gate electrode and barrier metal in semiconductor devices. In this paper, we present our results with the isotropic ALEt process for SiN and TiN. This method features a variety of promising characteristics, such as high selectivity with respect to SiO2and poly Si, atomic-level control of the etching amount, and isotropic etched profiles. The authors developed a novel etching apparatus as an ALEt tool for 300-mm wafers. This apparatus utilizes an infrared (IR) lamp for thermal annealing to achieve rapid temperature cycles. Particular attention is devoted to the surface reaction mechanism in the novel isotropic ALEt process for SiN. The results of in situ X-ray photoelectron spectroscopy (XPS) and thermal desorption spectroscopy (TDS) are described; they reveal the formation and decomposition of ammonium hexafluorosilicate on SiN during the cycle of plasma exposure and annealing. The potential industrial applications of these two processes are also described. [1] K. Shinoda et al., Appl. Phys. Express 9, 106201 (2016). [2] K. Shinoda et al., AVS Symposium and Exhibition, PS+TF-WeM10 (2016).

Transmission of oxygen radicals through free-standing single-layer and multilayer silicon-nitride and silicon-dioxide films

Free radicals from processing plasmas are known to cause damage to dielectric films used in semiconductor devices. Many radicals are highly reactive and can readily interact with the material exposed to the plasma. This can modify the chemical structure of the material causing deterioration of electrical and mechanical properties of the films. This work detects the transmission of oxygen radicals through single- and double-layer silicon-nitride and silicon-dioxide freestanding films. The films were exposed to oxygen plasma. A fluorophore dye was used to detect the oxygen radicals traversing through the films. By measuring the fluorescence of the dye before and after multiple timed-plasma exposures, the transmission properties of oxygen radicals through the material were found. The results indicate that the absorption length of oxygen radicals increases with increasing plasma exposure times for Si3N4 films because the oxygen plasma oxidizes the top layer of the film and forms a less dense silicon oxynitride layer. For SiO2 films, the absorption length was found to decrease as a function of plasma exposure time because of oxidation of the SiO2 surface which leads to the formation of a denser oxide layer on the surface of the sample.

(Invited) Thermal Cyclic Atomic-Level Etching of Nitride Films: A Novel Way for Atomic-Scale Nanofabrication

A highly selective, rapid thermal-cyclic atomic-level etching (ALE) process for SiNx film has been developed. The first step of this process is exposing SiNx to hydrofluorocarbon plasma to form an (NH4)2SiF6 layer on the SiNx surface. The second step is rapid thermal annealing with an infrared (IR) irradiation to decompose and sublimate the (NH4)2SiF6 layer. Etching of SiNx was observed after the (NH4)2SiF6 layer was removed by thermal annealing. Cyclic etching tests were carried out by repeated plasma exposure and IR irradiation. It was found that the cyclic process is self-limiting because etching depth depends only on the cycle number and not on the plasma exposure time. A high selectivity over SiO2 and poly Si was confirmed. This paper reviews the novel isotropic ALE for nitride films focusing on the surface reaction mechanism investigated by x-ray photoelectron spectroscopy (XPS) and thermal desorption spectroscopy (TDS). Isotropic ALE of TiN was also demonstrated using the same approach.

TiO2 coatings obtained by reactive sputtering at room temperature: Physical properties as a function of the sputtering pressure and film thickness

TiO2 layers with crystalline anatase structure have been attained on unheated glass substrates, by reactive sputtering from a metallic target, using the sputtering pressure to change the energy of the particles arriving to the film surface. Such energy and the time of plasma exposure also influence the morphology and other physical properties of the sputtered films. The deposition time has been ranged to obtain TiO2 coatings with different thicknesses from 0.24 to 1.08μm and at various total pressures from 0.2 to 1.0Pa. Overall characterization of the samples has been performed by X-ray diffraction, atomic force microscopy and spectrophotometry. The effects of the sputtering pressure and the film thickness on the crystalline structure (interplanar distance and mean crystallite size), the morphology (roughness and surface area) as well as the optical properties (refractive index, extinction coefficient) have been established. A direct relation between optical and morphological characteristics, and the calculated film porosity has been proven. Such porosity is well controlled by the deposition parameters; these allow tuning the optical and morphological properties of the sputtered TiO2 coatings as required for different applications.

Inactivation of Bread Molds on Agar by Atmospheric Pressure Plasma Jet

The aim of this study was to evaluate the effect of argon plasma jet, generated at atmospheric pressure, on Aspergillus sp. The molds were isolated from commercial white bread and inoculated on potato dextrose agar (PDA) containing 10% tartaric acid. The atmospheric pressure plasma jet (APPJ) was generated at a high frequency (43 kHz) power of 10, 20, or 30W and applied to the inoculated molds on PDA for 5, 10, or 20 minutes. The PDA plates were incubated for 3 and 6 days and the fungal inactivation was investigated directly under light microscope and indirectly by comparing the fungal dry weights. The results indicated that higher power supply and longer plasma exposure time led to more reduction in the fungal dry weight. The APPJ appears to be effective in destroying Aspergillus sp., yet the optimal plasma treatment condition remains to be optimized.

Smelting of Industrial Solid Waste for Recovery of Aluminum: Effect of Charge Material

The present paper deals with the effect of charge material on the recovery of aluminum from the industrial solid waste. For the efficient recovery of aluminum metal having a lower vaporization temperature, smelting was carried out in the presence of metal (iron) solvent bath inside a graphite crucible. Nitrogen plasma arc was generated by passing the current as well as the nitrogen gas through a hollow graphite electrode. Charge containing bottom ash or a mixture of iron ore slime and bottom ash along with a reductant of different reactivity was used either in the form of powder or pellets. Liquid iron bath below the plasma arc zone was utilized as a solvent for the absorption of reduced metals after reduction of the oxides present in the wastes. The quick absorption of aluminum in the liquid iron bath subsequent to the reduction of oxides under an inert atmosphere minimized the vaporization loss of aluminum. The effects of carbon content and its reactivity, form of charge material, charge-layer thicknesses for different charge compositions on the recovery of aluminum were studied for 30 min of plasma exposure times in all cases. Maximum recovery of aluminum was found for the charge in powder form having a mixture of bottom ash and iron ore slime along with the increased carbon content as well as charge-layer thickness, respectively. The recovery of aluminum was found to be up to 40.80% for the powder charge containing 50% bottom ash and 50% iron ore slime with the char in double stoichiometry carbon.

Nanostructuring of iron thin films by high flux low energy helium plasma

High flux, low energy He plasma exposure is proven to nanostructure iron thin films over their entire thickness to a highly open structure with large surface area. From a large set of plasma exposure parameters, the ion flux, the surface temperature, and the plasma exposure time are found to be the most relevant parameters to process mechanically stable, nanostructured Fe thin films on brittle glass substrates. The nanostructure stays stable during oxidation. Different surface morphologies are found, depending on the location where the plasma plume interacts with the thin film. This method paves the way to a new direction in top down nanostructuring of thin films, which can be adopted for many functional materials in diverse applications that require a high ratio of active to projected surface area.

Non-thermal plasma modified growth and differentiation process of Capsicum annuum PP805 Godiva in in vitro conditions

With the aim of evaluating the possible impacts of cold plasma on the structure and growth pattern of Capsicum annuum, the current study was carried out. The seeds were exposed to an argon-derived plasma (0.84 W cm−2 surface power densities) for 0, 1 or 2 minutes. Plasma-treated seeds were grown in the Murashige and Skoog (MS) medium or MS medium supplemented with BA and IAA. The presence of purple stems was recorded in plasma-treated plants grown in the medium supplemented with hormones. The recorded morphological differences were dependent on the exposure time of plasma treatments and/or the presence of hormones in the culture media. Plasma treatment of 1 minute had an improving effect on the shoot and root lengths as well as total leaf area, whereas plasma treatment of 2 minutes had an adverse effect. In contrast to the 1 minute treatment, plasma treatment of 2 minutes significantly impaired growth and hence reduced the total biomass. Alterations in stem diameter and differences in tissue patterns (especially in the vascular system) occurred, and were mainly dependent on the plasma exposure time and/or the presence of hormones. This is a first report on the effects of cold plasma on plant growth in in vitro conditions.