Microwave Plasma Process Research Articles

Microwave Plasma Synthesis of Materials—From Physics and Chemistry to Nanoparticles: A Materials Scientist’s Viewpoint

In this review, microwave plasma gas-phase synthesis of inorganic materials and material groups is discussed from the application-oriented perspective of a materials scientist: why and how microwave plasmas are applied for the synthesis of materials? First, key players in this research field will be identified, and a brief overview on publication history on this topic is given. The fundamental basics, necessary to understand the processes ongoing in particle synthesis—one of the main applications of microwave plasma processes—and the influence of the relevant experimental parameters on the resulting particles and their properties will be addressed. The benefit of using microwave plasma instead of conventional gas phase processes with respect to chemical reactivity and crystallite nucleation will be reviewed. The criteria, how to choose an appropriate precursor to synthesize a specific material with an intended application is discussed. A tabular overview on all type of materials synthesized in microwave plasmas and other plasma methods will be given, including relevant citations. Finally, property examples of three groups of nanomaterials synthesized with microwave plasma methods, bare Fe2O3 nanoparticles, different core/shell ceramic/organic shell nanoparticles, and Sn-based nanocomposites, will be described exemplarily, comprising perspectives of applications.

Microwave Plasma Process for SiCN:H Thin Films Synthesis with Composition Varying from SiC:H to SiN:H in H2/N2/Ar/Hexamethyldisilazane Gas Mixture

AbstractSiCxNy:H thin films are obtained with the microwave plasma assisted chemical vapour deposition (MPACVD) method in the gas mixture H2/Ar/Hexamethyldisilazane. When very few amounts of nitrogen are added to the gas mixture, the film composition changes drastically from SiCx:H like films to SiNx:H like films, according to X‐rays Photoelectron Spectroscopy and Fourier Transform Infra‐Red Spectroscopy (FTIR) analysis. The refractive index (n) and Tauc's optical gap (Eg) are modified over a wide range of values (1.75 ≤ n ≤ 2.15 and 3.5 eV ≤ Eg ≤ 5 eV) with nitrogen addition to the feed gas leading to thin films optical constants close to those of SiC or Si3N4. Therefore, for the films obtained without nitrogen, SiC nanoparticles with a size of about 20 nm embedded in an amorphous SiCN:H matrix are synthesized, leading to nanocomposite films.

Improvement of multilayer graphene synthesis on copper substrate by microwave plasma process using helium at low temperatures

A pretreatment method for copper foil substrates for graphene synthesis with microwave plasma has been developed using a helium and hydrogen gas mixture. Contaminants on the surface of the substrate, particularly copper oxides, were effectively removed by this method, which was confirmed by X-ray photoelectron spectroscopy (XPS). Graphene was grown by microwave-plasma-assisted chemical vapor deposition (MWCVD) subsequent to the pretreatment of the substrate in the same apparatus. Using the hydrogen and helium gas mixtures as the diluent gas for methane, the contamination in the deposited graphene by undesirable impurities from the ambient in the reaction apparatus, particularly silicon etched from the quartz window by microwave plasma, was successfully suppressed. The multilayer graphene films synthesized at low temperatures on the pretreated copper foil substrates exhibited higher crystallinity, as shown in transmission electron microscopy (TEM) images and Raman scattering spectra, than those on the untreated substrates.

A‐SiC:H Films by Remote Hydrogen Microwave Plasma CVD From Ethylsilane Precursors**

Amorphous, hydrogenated, silicon carbide (a‐SiC:H) films are deposited in the remote hydrogen microwave plasma (RP‐CVD) process using diethylsilane as a single‐source precursor. The effect of substrate temperature (TS) on the kinetics of RP‐CVD, chemical composition, structure, surface morphology, and properties (density, refractive index, and extinction coefficient) of the resulting a‐SiC:H films is investigated. The TS dependence of film growth rate implies that RP‐CVD is an adsorption‐controlled process. The increase of TS from 30 °C to 350 °C causes the elimination of organic moieties from the film and the formation of a SiC network structure. The relationships between the content of SiC bonds, represented by the relative integrated intensity of the SiC IR band, and the film properties are determined. The number of SiC bonds is found to be a key parameter in the control of the examined film properties. The films deposited at TS = 350 °C appear to be very dense materials exhibiting small surface roughness and high refractive index. The results of the present study are compared with those reported for a‐SiC:H films produced by RP‐CVD from a triethylsilane precursor.

Nanomorphological characteristics of the single-crystal Si(100) surface subjected to microwave plasma processing at weak adsorption

The influence of the conditions and composition of the highly ionized plasma of an electron cyclotron resonance low-pressure microwave gas discharge on the nanomorphology of the single-crystal Si(100) surface is studied. Model mechanisms of the processes controlling the main nanomorphological parameters of silicon crystals subjected to low-energy microwave plasma processing in chemically active and inactive gaseous media under the conditions of weak adsorption are considered.

A Practical Electrode for Microwave Plasma Processes

In-liquid plasma chemical vapor deposition (CVD) method is useful for high speed production of high quality diamond films. The method is simple and secure comparing usual CVD methods. In this paper, new electrodes for in-liquid plasma CVD by receiving microwaves are developed and introduced. This paper examines the fabrication of electrodes that enable the generation of plasma by effectively receiving microwaves. The fabricated electrodes are used to generate plasma in a waste liquid and the performance of the electrode is evaluated by the speed at which gases are created by decomposing the liquid. By using experiments in decomposing n-dodecane using in-liquid plasma and through detailed investigation into the length of the electrode used, it is confirmed that the half wavelength is optimal for the generation of plasma. The fabri-cated electrodes are used to test the formation of diamond film by plasma CVD in the microwave oven. Diamond film is successfully created inside microwave oven by using the vapor from a mixture of methanol and ethanol.

Achieving enhanced DSSC performance by microwave plasma incorporation of carbon into TiO2 photoelectrodes

The photoactivity of carbon-incorporated titanium dioxide (TiO2) has been widely reported. This study involves a novel approach to the incorporation of carbon into TiO2 through the use of microwave plasma processing. The process involved thermally treating printed TiO2 nanoparticle coatings in a microwave-induced argon-oxygen plasma containing low concentrations of methane. The resulting deposited carbon layer was characterized using XRD, XPS, Raman, UV–vis, ellipsometry, and optical profilometry. It was found that the methane gas was dissociated in the microwave plasma into its carbon species, which were then deposited as a nm-thick layer onto the TiO2 coatings, most likely in the form of graphite. The photovoltaic performances of both the TiO2 and the carbon-incorporated TiO2 were assessed through J-V and IPCE measurements of the N719-sensitized solar cells using the titania as their photoanodes. Up to a 72% improvement in the maximum power density (Pd-max) was observed for the carbon-incorporated TiO2 samples as compared to the TiO2, onto which no carbon was added. This improvement was found to be mainly associated with an increase in the short-circuit current density (Jsc), but independent from the open-circuit voltage (Voc), the filter factor (FF), and the level of dye adsorption. Possible contributory factors to the improved performance of the carbon-incorporated TiO2 were the enhanced electron conductivity and electron lifetime, both of which were elucidated through electrochemical impedance spectroscopy (EIS). When the surface layer was examined using XPS, the optimal carbon content on the TiO2 coating surface was found to be 8.4%, beyond which there was a reduction in the DSSC efficiency.

Purification of metallurgical grade silicon by a microwave-assisted plasma process

A new microwave plasma process is developed to refine and purify metallurgical grade silicon (MG-Si) effectively. Inductively coupled plasma-atomic emission spectrometry analysis (ICP-AES) indicates that the concentrations of impurities in silicon decrease significantly in the process, particularly for phosphorus, whose average removal rate is close to 100% after microwave plasma treatment of only 5min. The underlying mechanisms of the ultra-high removal rate of impurity atoms are discussed in detail in this paper. The photoresponse switching behavior of n+-Si wafers that are made of as-purified silicon provides further evidence for the unique advantage arising from the use of microwave plasma in the purification of MG-Si.

Surface modification of silica nanopowders in microwave plasma

In this work, surface modification of nanopowders in microwave plasma process was demonstrated. In order to obtain silica nanopowder with polymeric coating, a two-step microwave process was designed. 3-Aminopropyltrimethoxysilane was used as the surface modifier. Transmission electron microscopy and electron energy loss spectroscopy have been used for the direct microstructural imaging and chemical analysis of surface modified-silica nanopowders. The silica nanopowders were uniformly coated with a thick layer of amino-silane. Surface modification in microwave plasma is a simple, direct and easy to control process without using any solvents. The surface modified-silica nanopowders can be incorporated directly into polymer matrix for the preparation of hybrid nanocomposite materials.

마이크로웨이브 플라즈마를 이용한 탄화공정 및 PAN fiber의 강도 향상에 관한 연구

A study to replace a high temperature thermal carbonization process with microwave plasma process is carried for PAN fiber as a starting material. Near atmospheric pressure microwave plasma (1 Torr~45 Torr) was used to control to get the fiber temperature up to 1000 o C. Even argon is an inert gas, its plasma state include high internal energy particles; ion (15.76 eV) and metastable (11.52 eV). They are very effective to lower the necessary thermal temperature for carbonization of PAN fiber and the resultant thermal budget. The carbonization process was confirmed by both EDS (energy dispersive spectroscopy) of plasma treated fibers and OES (optical emission spectroscopy) during processing step as a real time monitoring tool. The same trend of decreasing oxygen content was observed in both diagnostic methods.

Characterization of Orthorhombic α‐MoO 3 Microplates Produced by a Microwave Plasma Process

Orthorhombic α‐MoO3 microplates were produced from (NH4)6Mo7O24·4H2O solid powder by a 900 W microwave plasma for 40, 50, and 60 min. Phase, morphologies, and vibration modes were characterized by X‐ray diffraction (XRD), selected area electron diffraction (SAED), scanning electron microscopy (SEM), and Raman and Fourier transform infrared (FTIR) spectroscopy. Sixty min processing resulted in the best crystallization of the α‐MoO3 phase, with photoluminescence (PL) in a wavelength range of 430–440 nm.

Characterization of orthorhombic α-MoO3 microplates produced by a microwave plasma process

Orthorhombic α-MoO3 microplates were produced from (NH4)6Mo7O24ċ4H2O solid powder by a 900 W microwave plasma for 40, 50, and 60min. Phase, morphologies, and vibration modes were characterized by X...

Structural and optical properties of nanoparticulate Y2O3:Eu2O3 made by microwave plasma synthesis

Nanoparticulate Y2O3:Eu2O3 with a small, uniform particle size and a well-defined composition was synthesized using a low temperature microwave plasma process. The structural evolution and the luminescence properties were studied in different states of annealing and Eu2O3 addition using X-ray diffraction, transmission electron microscopy, and UV-photoluminescence spectroscopy. As synthesized, the samples were amorphous and showed only weak luminescence. Subsequent annealing steps from 500°C to 800°C lead to the formation and growth of cubic Y2O3:Eu2O3 nanocrystals (5–20 nm) and a concomitant strong increase of the luminescence yield already at small grain sizes in the range of 10 nm. No self-quenching effects were observed up to 11 mol% Eu2O 3.

Microwave Plasma Process Optimization to Produce Nano Titania through Design of Experiments

The numerous unique advantages afforded by microwave plasma synthesis have led to its extensive utilization in synthesis of a wide variety of highly pure nanopowders. The present research reports synthesis of commercially important titania nanopowder from low cost titanium tetrachloride using this process. The parametric impact of the process parameters on the conversion efficiency and percentage anatase in titania nanopowder has been studied using statistically designed experiments and analysis. The outcome of this research indicated that low levels of plasma forming gas flow rate (PFR), additional gas flow rate (AFR), and precursor feed rate (FR), along with high level of carrier gas flow rate (CFR) would result in the most efficient conversion process with suitable percentage of anatase for photocatalytic applications.

Microwave Plasma Process에 의한 N-Hexane으로부터 다이아몬드 박막제작 및 특성

In this paper, the best conditions for the deposition of the high quality diamond thin-film from N-hexane as a carbon source in the microwave plasma process was carried out. Major parameters are the deposition time, flow rates of oxygen and hexane. The deposition time for the steady state thin-film was required more than 4[h], and the suitable flow rates of hexane and oxygen for the high-quality thin-film are 0.4[sccm] and 0.1~0.2[sccm], respectively. In addition, amorphous carbons such as DLC and graphite were grown by increasing the flow rate of hexane, and it decreased by increasing the flow rate of oxygen. Specifically, the growth rate is about 1.5[] under no addition of oxygen and it decreased about 60[%] as ca. 1.0[] with oxygen.

Growth of uniform ZnO nanoparticles by a microwave plasma process

ZnO nanoparticle coatings with a controlled size distribution have been grown on quartz substrates by a novel microwave plasma assisted spray (MPAS) technique. This study presents the analysis of structure, photoluminescence, and magnetic properties of particle coatings with two distinctly different mean particle sizes (400 nm and 200 nm). X-ray diffraction patterns show a typical wurtzite structure without any impurity phases for the nanoparticle coatings. SEM and TEM investigations have shown the grown nanoparticles to be spherical and well separated with a narrow size distribution. Nanoparticles are polycrystalline with smaller grain sizes associated with the smaller particle sizes. Photoluminescence (PL) spectra reveal the presence of oxygen vacancy related defects in the 400 nm nanoparticles, which become less pronounced in the 200 nm nanoparticles. The 400 nm nanoparticles are found to exhibit room-temperature ferromagnetism with a clear hysteretic behavior, while the 200 nm nanoparticles are diamagnetic even down to 10 K. These results suggest the oxygen vacancies were the cause for defect-induced ferromagnetism in the 400 nm nanoparticles.

Expanding microwave plasma process for thin molybdenum films nitriding: Nitrogen diffusion and structure investigations

Transition metal nitrides exhibit very interesting properties for mechanical and catalytic applications. Thin nitride layers are expected to prevent metal films from oxidizing under working conditions. Molybdenum thin films of about 200nm thick and deposited on Si (100) wafers are processed in pure N2, (Ar–35%N2) and (Ar–25%N2–30%H2) expanding microwave plasma at 673K. Secondary neutral mass spectrometry (SNMS) and Raman spectroscopy are both used to make correlations between the composition and the structure of the as-formed compounds. The low nitrogen diffusion up to a depth of about 40nm is correlated to the formation of well crystallized MoO2 of monoclinic structure acting as a barrier of diffusion for nitrogen, in molybdenum films exposed to pure N2 and (Ar–35%N2) plasma. The large nitrogen diffusion into the film exposed to ternary (Ar–25%N2–30%H2) plasma is correlated to the reduction of MoO2 oxides by hydrogen species such as atomic hydrogen, NHx<3… contained in ternary plasma. The formation of Mo–N phases with defects could take place in molybdenum films processed at 673K compared to the results obtained at 873K.

Nanoparticles in polymer-matrix composites

Regarding the development of nanoparticles for polymer matrix composites the particle/agglomerate size and particle/agglomerate distribution in the composites, respectively, is often crucial. This is exemplarily shown for, e.g. optical applications with measurements of refractive index and transmittance. Classical blending techniques, where nanoparticles are dispersed in polymers or resins, are compared to a combination of a special gas-phase synthesis method with subsequent in-situ deposition of nanoparticles in high-boiling liquids. The particles/agglomerates were characterized regarding particle size and particle size distribution using transmission electron microscopy and dynamic light scattering. Additionally, important material properties like mechanical properties, relevant for application, or like viscosity, relevant for processing, are determined. It is shown, that with in-situ dispersed nanoparticles synthesized in a microwave plasma process composites with finely dispersed particles/agglomerates are attainable.

Effect of Low‐pressure Microwave Plasma Discharges on the Surface of Vulcanized Rubbers Before Bonding with a Silicone Sheet

AbstractIn this work, the surface of vulcanized rubbers was treated by a low‐pressure microwave plasma process to enhance the adhesion with a silicone adhesive. The plasma reactor was fed with nitrogen, oxygen or synthetic air. Scanning electron microscopy and atomic force microscopy observations associated with X‐ray photoelectron spectroscopy, laser desorption/ionization time‐of‐flight mass spectrometry, secondary ion mass spectroscopy and surface energy measurements showed how the plasma gas treatment can affect the morphology and the chemistry of the rubber surface. The adhesion performances of the treated samples were evaluated using 90°‐peel tests. Correlations between the plasma surface modifications and the adhesion properties are reported. magnified image

Characterization of core/shell nanoparticle thin films for gas analytical applications

AbstractGas analytical microsystems such as the Karlsruhe micro nose (KAMINA) can be improved toward higher sensitivity when using SnO2 nanoparticle thin films instead of well‐established sputtered layers. For achieving long‐term stability, such nanoparticles must be prevented from growing and agglomerating. A very promising approach is to coat SnO2 nanoparticles with an open‐pored SiO2 shell, simply acting as a spacer, while preserving electrical contact between adjacent core particles. Such nanoparticulate thin films can easily be realized by the gas‐phase Karlsruhe microwave plasma process (KMPP). This provides an all‐in‐one approach to synthesize particles with diameters less than 10 nm, to coat them in situ with a protective ultrathin SiO2 shell in a downstream step and to finally deposit the core/shell particles solvent‐free onto prefabricated micro devices.The present study focuses on the surface analytical characterization of 200 nm thin films consisting of SnO2 core/SiO2 shell nanoparticles with various shell designs. For this purpose, the SiO2 shell thickness was systematically increased while keeping the SnO2 core size constant. X‐ray photoelectron spectroscopy (XPS) provides information concerning chemical binding states and shell thickness in a nondestructive manner. Angle‐resolved XPS together with transmission electron microscopy (TEM) validate the achieved core/shell structure. In the case of the desired ultrathin SiO2 shells, low‐energy ion scattering (LEIS) is solely the suitable means to prove that Sn and Si are the constituents of the outermost monolayer of the spherical particles and it demonstrates the attainability of open‐pored coatings. Copyright © 2010 John Wiley &amp; Sons, Ltd.