Low-pressure Plasma Reactor Research Articles

Direct Liquid Reactor-Injector of Nanoparticles: A Safer-by-Design Aerosol Injection for Nanocomposite Thin-Film Deposition Adapted to Various Plasma-Assisted Processes

The requirements of nanocomposite thin films, having non-aggregated nanoparticles homogeneously dispersed in the matrix, have been realized using a new method of Direct Liquid Reactor-Injector (DLRI) of nanoparticles. In this approach, unlike conventional aerosol-assisted plasma deposition, the nanoparticles are synthesized before their injection as an aerosol into plasma. In our experiments, we have used two different plasma reactors, namely an asymmetric low-pressure RF plasma reactor and a parallel plate dielectric barrier discharge at atmospheric pressure. Our results have shown that DLRI can be easily coupled with various plasma processes as this approach allows the deposition of high-quality multifunctional nanocomposite thin films, with embedded nanoparticles of less than 10 nm in diameter. Hence, DLRI coupled with plasma processes meets the specifications for the deposition of multifunctional coatings.

Surface-Active Plasma-Polymerized Nanoparticles for Multifunctional Diagnostic, Targeting, and Therapeutic Probes

Surface-functionalized polymeric nanoparticles have advanced the field of nanomedicine as promising constructs for targeted delivery of molecular cargo as well as diagnostics and therapeutics. Conventionally, the functionalization of polymeric nanoparticles incorporates tedious wet chemical methods that require complex, multistep protocols. Surface-active plasma-polymerized nanoparticles (PPNs) produced by a dry, low-pressure plasma process can be easily functionalized with multiple ligands in a simple step. However, plasma polymerization remains limited by the challenge of efficient collection of PPNs from low-pressure plasma reactors. Here, we demonstrate a simple method to overcome this limitation by delaying the inflow of the polymer-forming precursor gas, acetylene, into a nitrogen and argon plasma discharge. We provide evidence that this cutting-edge development in the plasma polymerization method drastically enhances the collection yield of nanoparticles by 2.5-fold, compared to the simultaneous inflow of the gases. COMSOL Multiphysics simulations support our experimental data and provide insights into the role of pressure gradients in regulating the forces controlling the collection of the particles. Surface characterization data revealed that changing the sequence of the precursor gas inflow had no significant effect on the physicochemical properties of the nanoparticles, as critically important for theranostic applications. A model, green fluorescent protein, was successfully conjugated to the surface of the PPNs via a reagent-free, one-step incubation process that immobilized the biomolecule while retaining its biological activity. Cytotoxicity of the particles was assessed by a lactate dehydrogenase (LDH) assay at concentrations of up to 5 × 105 nanoparticles per cell. Despite their high concentrations, the nanoparticles were remarkably well tolerated by the cells, demonstrating their superb potential for in vivo cellular uptake. This study advances previous research on plasma-polymerized nanoparticles, introducing a low-waste synthesis method that achieves higher yields. This sustainable technology has important implications for the production of multifunctional nanoparticles for drug delivery, tumor targeting, and medical imaging.

Enhanced Energetic Performance of Aluminum Nanoparticles by Plasma Deposition of Perfluorinated Nanofilms.

The performance of Al as nanoenergetic material in solid fuel propulsion or additive in liquid fuels is limited by the presence of the native oxide layer at the surface, which represents a significant weight fraction, does not contribute to heat release during oxidation, and acts as a diffusion barrier to Al oxidation. We develop an efficient technique in which the oxide layer is effectively turned into an energetic component via a reaction with fluorine that is coated in the form of a fluorocarbon nanofilm on the Al surface by plasma-enhanced chemical vapor deposition. Perfluorodecalin vapors are introduced in a low-pressure plasma reactor to produce nanofilms on the surface of Al nanoparticles, whose thickness is controlled with nanolevel precision as demonstrated by high-resolution transmission electron microscopy images. Coated particles show superior heat release, with a maximum enhancement of 50% at a thickness of 10 nm. This significant improvement is attributed to the chemical interaction between Al2O3 and F to form AlF3, which removes the oxide barrier via an exothermic reaction and contributes to the amount of heat released during thermal oxidation. The chemistry and mechanism of the enhancement effect of the plasma nanofilms are explained with the help of X-ray photoelectron spectroscopy, X-ray diffraction, high-angle annular dark-field scanning transmission electron microscopy-energy dispersive spectroscopy, thermogravimetric analysis, and differential scanning calorimetry.

Feasibility of two-dimensional die-level plasma process monitoring using spatially resolvable optical emission spectrometers

We demonstrated the feasibility of die-level process monitoring using spatially resolvable optical emission spectrometers (SROESs) in a low-pressure plasma reactor. The spatially resolved Ar emission line intensities and their intensity ratios were obtained from uniformly generated and intentionally perturbed plasma by inserting conductors of known sizes at known locations. The sheath and perturbed plasma distributions were identified from the Ar emission line intensity and their intensity ratio in the lateral direction to the SROES installment orientation. However, the resolution in the axial direction was insufficient to identify the sheath and perturbed the plasma distribution. We propose the superimposition of measured plasma emission intensities from two perpendicularly aligned SROESs for die-level process monitoring. The insufficient axial resolution can be compensated for by the lateral resolution of another SROES, and die-level process monitoring using SROES can thus be realized.

Control and data acquisition system to study dielectric breakdown in low-pressure DC plasma reactor with parallels electrodes

This work aimed to present the theory of dielectric breakdown of a glow discharge through the automation of gas injection (mass flow) systems with pressure control (pressure gauge) and voltage source with voltage control, in the reactor for the acquisition of rupture parameters for low-pressure glow discharges. Through the analysis of Paschen’s curves for the configuration with argon gases and atmospheric air, it was observed that the rupture potential for argon gas is lower than for atmospheric air, although the ionization potential of atmospheric air is lower about argon.

Control and data acquisition system to study dielectric breakdown in low-pressure DC plasma reactor with parallels electrodes

This work aimed to present the theory of dielectric breakdown of a glow discharge through the automation of gas injection (mass flow) systems with pressure control (pressure gauge) and voltage source with voltage control, in the reactor for the acquisition of rupture parameters for low-pressure glow discharges. Through the analysis of Paschen’s curves for the configuration with argon gases and atmospheric air, it was observed that the rupture potential for argon gas is lower than for atmospheric air, although the ionization potential of atmospheric air is lower about argon.

Facilitation of adhesion and spreading of endothelial cells on silicone oxide-coated dacron material by microwave-excited low-pressure plasma.

ObjectivesAutologous transplants are still the means of choice for bypass surgery. In addition to good tolerability, there is a reduced thrombogenicity and fewer neointima hyperplasia compared to artificial materials. However, since viable transplants are limited, attempts are being made to improve existing artificial vascular prosthesis material. Next to the reduction of thrombogenicity, a rapid endothelialization of the vascular graft should reduce intimal hyperplasia and thus prevent stenoses. The effect of newly developed silicon oxide coatings on the growth of endothelial cells was therefore the goal of this work in a cell culture study.MethodsA woven, uncoated polyethylene terephthalate (PET) vessel prosthesis was used. The coating process was carried out in a low-pressure plasma reactor in a multi-step process. After preparation of the vacuum chamber hexamethyldisiloxane (HDMSO) with oxygen was evaporated using argon plasma. By this an approx. 1 nm thin adhesion promoter layer was separated from plasma and HMDSO. The silicone oxide barrier layer was applied to the PET vessel samples. The carbon content of the layer could be selectively altered by changing the HMDSO oxygen flow ratio, resulting in coatings of 100 nm, 500 nm, and 1,000 nm. In addition, two different oxygen-to-HMDSO ratios were used. To achieve a carbon coating as low as possible, the ratio was set to 200:1. A carbon-rich layer was obtained with the 1:1 setting. The various coatings were then examined for their surface texture by scanning electron microscopy (SEM) as well as by cell culture experiments for cell viability and growth using EA.hy 926 cells.ResultsSEM showed no changes in the surface morphology; however a layer thickness of 1,000 nm showed peeled off coating areas. Alamar blue assays showed a significantly higher metabolic activity (p=0.026) for the coating 500 nm, ratio 200:1 compared to untreated control samples and a significantly lower metabolic activity (p=0.037) of the coating 500 nm, ratio 1:1 compared to the coating 500 nm, ratio 200:1. This underlines the apparent tendency of the 1:1 coating to inhibit the metabolic activity of the cells, while the 200:1 coating increases the activity. Fluorescence microscopy after calcein acetoxymethyl ester (AM) staining showed no significant difference between the different coatings and the uncoated PET material. However, a tendency of the increased surface growth on the coating 500 nm, ratio 200:1, is shown. The coatings with the ratio 1:1 tend to be less densely covered.ConclusionsThe results of this work indicate a great potential in the silicon coating of vascular prosthesis material. The plasma coating can be carried out easy and gently. Cell culture experiments demonstrated a tendency towards better growth of the cells on the 200:1 ratio coating and a poorer growth on the carbon-rich coating 1:1 compared to the uncoated material. The coating with silicon oxide with a thickness of 500 nm and an oxygen-HMDSO ratio of 200:1, a particularly low-carbon layer, appears to be a coating, which should therefore be further investigated for its effects on thrombogenicity and intimal hyperplasia.

Comparison of Plasma Deposition of Carbon Nanomaterials Using Various Polymer Materials as a Carbon Atom Source.

Carbon nanowalls are promising materials for various electrochemical devices due to their chemical inertness, desirable electrical conductivity, and excellent surface-to-mass ratio. Standard techniques, often based on plasma-assisted deposition using gaseous precursors, enable the synthesis of top-quality carbon nanowalls, but require long deposition times which represents a serious obstacle for mass applications. Here, an alternative deposition technique is presented. The carbon nanowalls were synthesized on titanium substrates using various polymers as solid precursors. A solid precursor and the substrate were mounted into a low-pressure plasma reactor. Plasma was sustained by an inductively coupled radiofrequency discharge in the H-mode at the power of 500 W. Spontaneous growth of carbon nanomaterials was observed for a variety of polymer precursors. The best quality of carbon nanowalls was obtained using aliphatic polyolefins. The highest growth rate of a thin film of carbon nanowalls of about 200 nm/s was observed. The results were explained by different degradation mechanisms of polymers upon plasma treatment and the surface kinetics.

Surface Modification of Polyamides by Gaseous Plasma-Review and Scientific Challenges.

A review of the most significant scientific achievements in the field of surface modification of polyamides by non-equilibrium plasma treatments is presented. Most authors employed atmospheric pressure discharges and reported improved wettability. The super-hydrophilic surface finish was only achieved using a low-pressure plasma reactor and prolonged treatment time, enabling both the nanostructuring and functionalization with polar functional groups. The average increase of the oxygen concentration as probed by XPS was about 10 at%, while the changes in nitrogen concentrations were marginal in almost all cases. The final static water contact angle decreased with the increasing treatment time, and the oxygen concentration decreased with the increasing discharge power. The need for plasma characterization for the interpretation of experimental results is stressed.

Plasma-Assisted Deposition of Magnesium-Containing Coatings on Porous Scaffolds for Bone Tissue Engineering

Magnesium plays a pivotal role in the formation, growth, and repair of bone tissue; therefore, magnesium-based materials can be considered promising candidates for bone tissue engineering. This study aims to functionalize the surfaces of three-dimensional (3D) porous poly-ε caprolactone (PCL) scaffolds with magnesium-containing coatings using cold plasma-assisted deposition processes. For this purpose, the radiofrequency (RF) sputtering of a magnesium oxide target was carried out in a low-pressure plasma reactor using argon, water vapor, hydrogen, or mixtures of argon with one of the latter two options as the feed. Plasma processes produced significant differences in the chemical composition and wettability of the treated PCL samples, which are tightly related to the gas feed composition, as shown by X-ray photoelectron spectroscopy (XPS) and water contact angle (WCA) analyses. Cytocompatibility assays performed with Saos-2 osteoblast cells showed that deposited magnesium-containing thin films favor cell proliferation and adhesion on 3D scaffold surfaces, as well as cell colonization inside them. These films appear to be very promising for bone tissue regeneration.

Superlocal chemical reaction equilibrium in low temperature plasma

AbstractLow temperature plasmas (LTP) are a unique class of open‐driven systems in which chemical reactions are unpredictable using established concepts. The terminal state of chemical reactions in LTP, termed the superlocal equilibrium state, is hypothesized to be defined by a proposed set of state variables. Using a LTP reactor wherein the state variables have been measured, it is shown that CO2 spontaneously splits and the effluent speciation is independent of the influent speciation if the state variables are held constant and the residence time is long. CO2 conversion at long residence times, which is expected to be nominally zero from equilibrium thermodynamics, can be as high as 70% in the LTP. The employed low pressure plasma reactor (P = 10 mbar) had a similar volume, productivity, and energy efficiency compared to an atmospheric pressure dielectric barrier discharge reactor, thanks to reaction rates that were three orders of magnitude faster.

Determination of nanoparticle collision cross section distribution functions in low pressure plasma synthesis reactors via ion mobility spectrometry

Low pressure (<20 Torr) non-thermal plasma flow tube reactors are commonly used for the synthesis of high purity nanoparticles (NPs) via nucleation and subsequent growth from vapor phase precursors. In spite of their utility, process monitoring (i.e. output NP size characterization) in such reactors is difficult, as there is a dearth of techniques available for online NP size distribution function measurement. In this study, we developed and applied an ion mobility spectrometry (IMS) system consisting of a low pressure differential mobility analyzer (LPDMA) and electrical detector to determine the collision cross section distribution functions of Si NPs synthesized in a radio frequency non-thermal SiH4–Ar plasma operated at ∼2 Torr (266 Pa). The collision cross section, roughly proportional to projected area, is a parameter quantifying the size and structure of nanometer scale species in the vapor phase. We introduce the collision cross section distribution function as a metric for potential use in online process monitoring in NP synthesis. Proper inversion of the collision cross section distribution function requires a priori knowledge of the LPDMA transfer function. We utilized a tandem differential mobility analyzer approach, coupled with a Twomey–Markwoski based scheme to determine LPDMA transfer functions. Subsequent application of these transfer functions in collision cross section distribution function determination showed that at the outlet of the non-thermal plasma flow tube reactor, both negatively and positively charged Si NPs persisted with nearly identical collision cross section distribution functions. NPs were found to have mode ‘mobility equivalent’ diameters near 10 nm, and via TEM analysis were found to persist at the reactor outlet as small aggregates composed of ∼5 nm diameter primary particles. Size distribution functions inferred from collision cross section measurements were compared to size distribution functions inferred from TEM images; excellent agreements were found between IMS and TEM for both mean mobility equivalent diameter and distribution function width. In total, this study shows that (1) IMS is a viable approach for process monitoring in non-thermal plasma NP flow tube synthesis systems, (2) although NPs are modestly aggregated at low-pressure plasma reactor outlets, the extent of aggregation is considerably less than observed in most atmospheric pressure and equilibrium NP synthesis systems (e.g. flames), and (3) after exiting the plasma reactor, the decharging of NPs from highly negative charge levels to a bipolar charge distribution likely drives aggregation on the plasma boundary.

Ejector-based sampling from low-pressure aerosol reactors

Online measurements of nanoparticles are necessary when rapid information about the particle size and mass distribution is needed. Currently, the application of online measurement techniques with commonly used instruments such as SMPS, CPMA and ELPI+ is not possible at low-pressure conditions. In this work, a commercial vacuum ejector is used as a simple tool to transfer nanoparticles from a low-pressure region to atmospheric pressure. The vacuum ejector is investigated for different process pressures between 120 and 170 mbar to measure size-selected aerosols in the range from 10 to 100 nm. It was found that the sampling with the vacuum ejector does not change the particle size. The gas and particle dilution factors as well as the particle losses are determined, so that quantitative measurements of the aerosol size distribution can be obtained. Additionally, the applicability of the vacuum ejector is tested during particle synthesis in a low-pressure microwave plasma reactor with a combination of online instrumentation. The direct transfer of the aerosol to atmospheric pressure allows real-time measurements. The primary particle size, mass mobility exponent and effective density are calculated exemplary based on parallel online ELPI+, SMPS and CPMA measurements and are compared to offline TEM analysis.

Composition, Mixing State and Water Affinity of Meteoric Smoke Analogue Nanoparticles Produced in a Non-Thermal Microwave Plasma Source

Abstract The article reports on the composition, mixing state and water affinity of iron silicate particles which were produced in a non-thermal low-pressure microwave plasma reactor. The particles are intended to be used as meteoric smoke particle analogues. We used the organometallic precursors ferrocene (Fe(C5H5)2) and tetraethyl orthosilicate (TEOS, Si(OC2H5)4) in various mixing ratios to produce nanoparticles with radii between 1 nm and 4 nm. The nanoparticles were deposited on sample grids and their stoichiometric composition was analyzed in an electron microscope using energy dispersive X-ray spectroscopy (EDS). We show that the pure silicon oxide and iron oxide particles consist of SiO2 and Fe2O3, respectively. For Fe:(Fe+Si) ratios between 0.2 and 0.8 our reactor produces (in contrast to other particle sources) mixed iron silicates with a stoichiometric composition according to FexSi(1−x)O3 (0≤x≤1). This indicates that the particles are formed by polymerization of FeO3 and SiO3 and that rearrangement to the more stable silicates ferrosilite (FeSiO3) and fayalite (Fe2SiO4) does not occur at these conditions. To investigate the internal mixing state of the particles, the H2O surface desorption energy of the particles was measured. We found that the nanoparticles are internally mixed and that differential coating resulting in a core-shell structure does not occur.

Organic chemistry in a CO2 rich early Earth atmosphere

The emergence of life on the Earth has required a prior organic chemistry leading to the formation of prebiotic molecules. The origin and the evolution of the organic matter on the early Earth is not yet firmly understood. Several hypothesis, possibly complementary, are considered. They can be divided in two categories: endogenous and exogenous sources. In this work we investigate the contribution of a specific endogenous source: the organic chemistry occurring in the ionosphere of the early Earth where the significant VUV contribution of the young Sun involved an efficient formation of reactive species. We address the issue whether this chemistry can lead to the formation of complex organic compounds with CO2 as only source of carbon in an early atmosphere made of N2, CO2 and H2, by mimicking experimentally this type of chemistry using a low pressure plasma reactor. By analyzing the gaseous phase composition, we strictly identified the formation of H2O, NH3, N2O and C2N2. The formation of a solid organic phase is also observed, confirming the possibility to trigger organic chemistry in the upper atmosphere of the early Earth. The identification of Nitrogen-bearing chemical functions in the solid highlights the possibility for an efficient ionospheric chemistry to provide prebiotic material on the early Earth.

Multiscale Modeling of Low Pressure Plasma Etching Processes: Linking the Operating Parameters of the Plasma Reactor with Surface Roughness Evolution

Α multiscale modeling framework, linking the operating parameters of a low pressure plasma reactor with the surface roughness being formed on the etched substrate, is developed. It consists of a) a reactor scale model, which calculates densities, energies, and fields in the reactor, b) a Monte Carlo (MC) particle tracing model, which calculates the ion energy and angular distributions on the substrate, and c) a MC surface model, which calculates the evolution of the surface morphology during etching. The case study is etching of polymeric substrates with Ar plasma in the GEC reactor. Based on a generic surface model, the effects of the operating conditions (pressure, power) on the roughness are investigated. The potential for non‐uniform and anisotropic roughness along the wafer radial direction is demonstrated.

Effects of Pressure and Electrode Length on the Abatement of N2O and CF4 in a Low-Pressure Plasma Reactor

The emission of greenhouse gases, such as N2O and fluorinated gases, has been increasingly regulated in the semiconductor industry. Pressure effects on the abatement of N2O and CF4 were investigated in a low-pressure plasma reactor by using Fourier transform infrared (FTIR) spectroscopy. The destruction and removal efficiency (DRE) of N2O and CF4 was significantly lowered below 0.2 Torr. When the pressure was increased, the DRE of CF4 with H2O as the reactant gas increased continuously, but that with O2 or without any reactant gas first increased and then decreased. A larger electrode length yielded a higher DRE of N2O and CF4, especially at lower pressures. To understand this phenomenon, the electrical waveforms for the discharge in N2O were analyzed in conjunction with its optical emission profiles, and the rotational temperatures for different electrode lengths were compared using the N2+ ion band (λ = 391.4 nm). They provided insights into the mechanism involved in terms of plasma property and gas residence time.

Numerical investigation of the film uniformity during the surface coating of charged nanoparticles in a low pressure plasma reactor

The uniformity of film deposition on charged nanoparticles, trapped near the sheath of a capacitively coupled plasma reactor, is studied by numerical simulation of the multi-fluid plasma equations, surface deposition processes, and nanoparticle heating effects. It is found that the anisotropy in the ion flux onto the powered electrode may be hold responsible for the film nonuniformity. The nonuniformity increases with increasing of the particle radius, although small particles lose sphericity faster than the large particles. Because of the electron temperature dependence of the deposition rate and the incident ion flux, higher electron temperatures lead to more nonuniform film deposition. However, the uniformity is improved and the sphericity is restored by the increase in the background gas pressure and/or temperature.

Abatement characteristics of N2O in low-pressure plasma reactor

The abatement characteristics of N2O were investigated in a plasma reactor positioned in front of a vacuum pump. Fourier transform infrared (FTIR) spectroscopy was used for evaluating the destruction and removal efficiency (DRE) and the identification of byproducts. The concentration of NOx (NO and NO2) was quantified by using an NOx analyzer. The DRE of N2O was enhanced by increasing the power or decreasing the N2O flow rate. A higher pressure yields a higher DRE of N2O and a lower concentration of NOx in the destroyed N2O. For understanding this phenomenon, the discharge characteristics were analyzed by using optical emission spectroscopy (OES). The spatial distribution of emission spectra from the discharge in a mixture of N2O and Ar gases was measured by varying the pressure. The mechanisms underlying the pressure effect on the N2O DRE and NOx concentration are discussed in terms of the electron density and the concentration of O radicals.

The effects of plasma inhomogeneity on the nanoparticle coating in a low pressure plasma reactor

A self-consistent model is used to study the surface coating of a collection of charged nanoparticles trapped in the sheath region of a low pressure plasma reactor. The model consists of multi-fluid plasma sheath module, including nanoparticle dynamics, as well as the surface deposition and particle heating modules. The simulation results show that the mean particle radius increases with time and the nanoparticle size distribution is broadened. The mean radius is a linear function of time, while the variance exhibits a quadratic dependence. The broadening in size distribution is attributed to the spatial inhomogeneity of the deposition rate which in turn depends on the plasma inhomogeneity. The spatial inhomogeneity of the ions has strong impact on the broadening of the size distribution, as the ions contribute both in the nanoparticle charging and in direct film deposition. The distribution width also increases with increasing of the pressure, gas temperature, and the ambient temperature gradient.