Microwave Surface Wave Research Articles

Plasma-deposited reactive species assisted synthesis of colloidal zinc-oxide nanostructures

A surface-wave microwave discharge is applied to deposit reactive oxygen and nitrogen species (RONS) into the liquid subsequently used as a medium for laser ablation of a Zn metallic target. It is shown that during laser ablation in plasma-treated liquids the H2O2 concentration decreases, while in deionized water (DIW) significant H2O2 is produced. Meanwhile, the pH—initially adjusted by applying reductive metals—increases in the acidic liquids and decreases in the alkaline ones. During months of storage the pH of colloids stabilize around pH 6, which insures the long-term stability of RONS. It is demonstrated that in DIW metallic Zn NPs are created, which gradually oxidize during storage, while in the plasma-treated liquids ZnO NPs are produced with the mean size of 18 nm. In the alkaline plasma-treated liquid the NPs form large aggregates, which slows the dissolution of NPs. In the acidic and neutral solutions besides NPs nanosheets are also formed, which during storage evolve into nanosheet networks as a result of the dissolution of NPs. The band gap of the colloidal ZnO is found to decrease with the formation of aggregates and nanosheet networks. The ZnO NPs ablated in plasma-treated liquids exhibit a high-intensity visible emission covering the green-to-red spectral region. The photoluminescence spectra is dominated by the orange-red emission—previously not detected in the case of laser-ablated ZnO NPs and attributed to the interstitial Zn and oxygen sites—and the yellow emission, which can be attributed to the OH groups on the surface. It is shown that during months of storage, due to the dissolution of NPs and formation of nanosheets, the intensity of the visible emission decreases and shifts to the blue-green spectral region.

Deposition of vertical carbon nanostructures by microwave plasma source on nickel and alumina

Vertical carbon nanostructures on metal and ceramic substrates are deposited successfully using planar microwave plasma source at frequency of 2.45 GHz. The PECVD (Plasma Enhanced Chemical Vapor Deposition) with microwave surface wave discharge is applied because it produces a dense plasma providing efficient decomposition of the methane and creation of a large number of reactive particles which results in lower substrate temperature for graphene deposition compared to CVD method. Optimization of the gas mixture of H2, Ar and CH4, and the gas pressure in the chamber results in a homogeneous graphene structures deposition on the substrates of Ni-foil, Ni-foam and alumina ceramics at substrate temperatures ∼600 °C. The plasma parameters of surface wave discharge such as gas temperature, electron temperature and density are obtained by measuring OH-band and Ar-lines using optical emission spectroscopy. The morphology of the vertical carbon structures is obtained using SEM analysis and the characteristics of the graphene layers were determined by Raman spectroscopy.

Broadband negative-index surface-waves on arrays of capped helices

A novel metasurface comprised of capped helices arranged as a hexagonal array that supports a broadband near-isotropic negative-index microwave surface-wave is designed, manufactured, and experimentally characterized. The surface-mode dispersion is studied both numerically and experimentally with the operational band more than double the bandwidth of structures previously reported in the literature. Hence, it is shown how one may provide a structured surface that supports just a negative-index mode over a wide operational band with no forward wave being simultaneously excited.

Characterization of radical-enhanced atomic layer deposition process based on microwave surface wave generated plasma

A plasma-assisted atomic layer deposition system employing a microwave surfatron plasma was developed and characterized by spatially resolved Langmuir probe diagnostics and optical emission spectroscopy. The deposition process was applied on TiO2 thin films prepared on Si wafers. The surfatron is equipped with a small ring electrode serving as a source of weak radio frequency plasma helping with fast and reliable ignition of the discharge in molecular gas. Results evaluated in the pure argon plasma proved that the plasma potential and the plasma density are homogeneous in the radial direction, while a rapid decrease was observed in the axial direction. Adding up to 30% of nitrogen into the gas mixture led to less homogeneous plasma parameters in the radial direction together with the increase of the electron effective temperature. Optical emission spectra revealed many Ar I lines of neutral atoms with only a few Ar II ions’ lines. The gradual addition of nitrogen causes a systematic decrease in the Ar I line intensity. We expect that excited nitrogen molecules are produced by the inelastic collisions with electrons and by the collisional quenching of metastable Ar(4s) states. On the other hand, oxygen atom and ion lines are detected when oxygen was mixed with argon. Deposited TiO2 thin films are characterized by the anatase phase when the substrate temperature is 250 °C. The anatase phase is observed even for the substrate temperature of 200 °C; however, the microwave power delivered into the surfatrons must be lower in comparison with the prior case.

Surface wave reflection from a metasurface termination

The reflection coefficient of a microwave surface wave incident at the termination of a metasurface is explored. Two different surface types are examined. One is a square array of square metallic patches on a dielectric-coated metallic ground plane, the other a Sievenpiper ‘mushroom’ array. In the latter the surface wave fields are more confined within the structure. Comparison of the measured surface-wave reflection spectra is made with that obtained from analytic theory and numerical modelling. The reflection coefficient is shown to be dependent on both the momentum mismatch between the surface wave and the freely propagating modes as well as the different field distributions of the two modes.

Controlling: the composition of plasma-activated water by Cu ions

It is demonstrated that Cu ions can be used to control the ageing of plasma-activated liquids through the Fenton-type reaction, which acts as a competitor to the + H2O2 + H+ → products reaction, and thus moderates the recombination of ions. It is shown that the ageing dynamics is influenced by the type of Cu-ions source used, due to the different ion dissolution dynamics. Using CuNP-colloidal and copper acetate salt solutions, respectively, similar ageing dynamics can be achieved: the lifetime of can be increased to 104 min comparing to 102 min obtained in plasma-activated water, while the H2O2 concentration decreases to about half of its initial value. While the treated salt solutions contain the degradation products of the salt compounds, the CuNPs can be filtered out from the plasma-activated colloid (PAC). It is shown that filtration does not influence the concentration of active species, while the ageing of the filtrated PAC depends on the moment of filtration, which determines the concentration of dissolved Cu ions. Using copper foil, the dissolved Cu-ion concentration can be controlled by removing the foil during ageing, and in this way the lifetime of species and the H2O2 concentration can be adjusted. The effect has been validated in the case of two different plasma sources: a surface-wave microwave discharge and a kHz plasma jet, which produce PALs with different concentration ratio, which is the main governor of the ageing process.

Laser-assisted doping of graphene for transparent conducting electrodes

In this work, we study the role of laser irradiation in the control of the doping processing for directly grown graphene using microwave surface wave plasma chemical vapor deposition (MW-SWP-CVD) on the glass substrates. Raman spectrum and X-ray photoelectron spectroscopy (XPS) were used to investigate the evolution of the structure of doping graphene as a function of laser irradiation. Also, the effect of laser on the morphology for doping graphene using AuCl3 was observed by field emission scanning electron microscopy (FESEM), atomic force microscopy (AFM), and optical microscope techniques. The gold particles were collected on graphene network wrinkles, which were resulted from laser irradiation. These gold network wrinkles were further used to enhance the electrical properties of the graphene, resulting in a significant reduction in sheet resistance and an improvement of its stability after storage in ambient air. The presented approach paves the way to design promising transparent conductive electrodes for optoelectronic devices such as light-emitting diodes (LED), photodiodes and solar cells.

Direct Synthesis of Graphene on Silicon at Low Temperature for Schottky Junction Solar Cells

Graphene thin films synthesized directly at low temperature (550˚C) on silicon substrate by microwave (MW) surface wave plasma (SWP) chemical vapor deposition (CVD) using the cover on substrates for avoiding plasma emission ultraviolet ray’s effect during film deposition. Analytical methods such as Raman spectroscopy, Transmission electron microscopy (TEM) and Scanning electron microscopy (SEM), four-point probe method, and JASCO V-570 UV/VIS/NIR spectrophotometer were employed to characterize the properties of the graphene films. Here, we report that it is possible to grow graphene directly on the silicon substrate (without using catalyst) due to the high radical density of MW SWP CVD. Furthermore, we fabricated graphene/silicon Schottky junction solar cells with an efficiency of up to 6.39%. Compared to conventional silicon solar cells, the fabrication process is greatly simplified; just graphene is synthesized directly on n-type crystalline Si substrate at low temperate.

Shaping of the flame geometry by non-conventional cold plasma arrangements

Experimental examination of possibility to affect the shapes of flames under combustion of the liquified petroleum gas (LPG) were performed by several non-conventional cold atmospheric plasma arrangements. The lateral fused hollow cathode, the microwave surface wave plasma jet and the combination of these systems confirmed possibility of an efficient control of the flame shapes, increasing stability of flames and broadening of their front parts.

Propagation of Microwave Discharge Sustained by Surface Wave in Quartz Tube Filled with Low-Pressure Air

Propagation was studied of the ionization front of the gas discharge sustained by the microwave surface wave in a quartz tube filled with low-pressure air. In a wide pressure range, the characteristic propagation velocities of the discharge front were measured, as well as the parameters of the stationary plasma column. The experimental results obtained are in satisfactory agreement with the discharge propagation model based on the concept of nonlocal electron heating in the plasma resonance region. The electrodynamic characteristics of the discharge were studied and the electron density averaged over the cross section of the plasma column was measured. Based on the experimental measurements of the ionization and electron loss rates, the electric field distribution was estimated in the ionization front region.

Radiation Losses From a Microwave Surface-Wave Sustained Plasma Source (Surfatron)

Surfatron is an electromagnetic (EM) field applicator yielding surface-wave (SW) discharges (SWDs) within dielectric tubes in a broad range of operating conditions. In this article, the vector wave equation for the electric field is solved numerically in 2-D to determine the influence of plasma parameters, such as electron density, normalized electron–neutral collisions frequency, plasma column length and radius, as well as the length of the surfatron body on the field strength distribution, radiation patterns, and ratio of radiated power to power absorbed in plasma. This analysis allows for estimating the efficiency of plasma generation. The model takes into account the surfatron configuration, reproducing the typical SWD plasma column behavior, namely the fact that the plasma column is “pushed out” of the surfatron interstice as absorbed power increases. The surfatron is assumed to be placed in an air-filled sphere with the surface transparent to the wave. Calculations are performed for a field frequency of 2450 MHz and a fused silica tube, considering plasma parameters typical for both atmospheric- and lower-pressure discharge gases for achieving a parametric survey. It is found that for given plasma parameters, varying the plasma column length from a few up to 30 cm significantly affects the distribution of the electric field around the surfatron and the radiation patterns. It is further shown that the radiated to absorbed power ratio increases with increasing electron density and plasma radius, and decreasing electron–neutral collision frequency. This ratio can exceed 30% for a 30-cm plasma column with a plasma radius of 3 mm and parameters typical for atmospheric and reduced gas pressures. The surfatron length at which radiation losses are minimal is at half the vacuum wavelength.

Silicon Oxide Barrier Films Deposited on Polycarbonate Substrates in Pulsed Plasmas

For many applications of polycarbonate (PC) from packaging to micro-electronics improved barrier properties are necessary. In this contribution, silica thin films were deposited from hexamethyldisiloxane/oxygen (HMDSO/O2) on polycarbonate substrate in three step plasma processes by combining a microwave (MW) surface wave discharge of 2.45 GHz with an optional radio-frequency (RF) bias of 13.56 MHz. The influence of interlayer thickness, HMDSO flow and oxygen to HMDSO ratio on barrier performance for three step-coating processes was investigated. The morphology and surface properties of the coated surface of PC were studied by atomic force microscopy (AFM). The surface topography showed a silica particles distribution on the PC substrate with relatively smooth surface roughness. AFM-QNM provides more insight into the surface morphology and stiffness. The results identify the coating structure for PC film coated with and without bias. High barrier improvement of the deposited films on PC substrates was obtained after plasma silicon coating process with a barrier improvement factor up to 337. It was found that the deposition process is optimal for food packaging applications by using combined MW-RF PECVD technology.

Tuning the composition of plasma-activated water by a surface-wave microwave discharge and a kHz plasma jet

An atmospheric pressure surface-wave microwave discharge and a kHz plasma jet are used to activate purified water. It is shown, that by varying the treatment distance and the initial Ar/N2/O2 mixture composition of the surface-wave microwave discharge the concentration ratio of NO3− and H2O2 radicals created in the plasma activated water (PAW) can be varied over three orders of magnitude, which can be preserved during months of storage at room temperature. At the same time, with the 5 min treatment of the 32 ml water the absolute radical concentrations are varied in the range of 0.5–85 mg l−1 for H2O2, 20–180 mg l−1 for NO3− and 0.5–14 mg l−1 for NO2−. In the case of the N2 kHz plasma jet this concentration ratio can be tuned within one order of magnitude by varying the treatment distance. By treating different volumes very similar concentration ratios are obtained, which evolve differently during storage, as the ageing dynamics is determined by the absolute concentration of radicals. In general, the radical most affected by ageing is NO2−, whose recombination is found to be determined by the H2O2 radical. In order to control the H2O2 concentration and thus the NO2− radicals recombination, the application of a Fenton type reaction is suggested, which is implied by inserting a copper surface into PAW during or after plasma treatment.

Surface Amino Group Regulation and Structural Engineering of Graphitic Carbon Nitride with Enhanced Photocatalytic Activity by Ultrafast Ammonia Plasma Immersion Modification.

Surface amino group regulation and structural engineering of graphitic carbon nitride (g-CN) for better catalytic activity have increasingly become a focus of academia and industry. In this work, the ammonia plasma produced by a microwave surface wave plasma generator was developed as a facile source to achieve fast, controllable surface modification, and structural engineering of g-CN by ultrafast plasma treatment in minutes, thus enhancing photocatalytic performance of g-CN. The morphology, surface hydrophilicity, optical absorption properties, and states of C-N bonds were investigated to determine the effect of plasma immersion modification on the g-CN catalyst. The structure and photoelectric features of the plasma-modified samples were characterized by X-ray diffractometry, Fourier transform infrared spectroscopy, X-ray photoelectron spectroscopy, and electrochemical impedance spectroscopy. The results indicate that the ammonia plasma-treated g-CN-NH3 exhibits an ultrathin nanosheet structure, enriched amino groups, and an ideal molecular structure, a narrower band gap (2.35 eV), extended light-harvesting edges (560 nm), and enhanced electron transport ability. The remarkably enhanced photocatalytic activity demonstrated in the photoreduction and detoxification of hexavalent chromium (Cr(VI)) can be ascribed to the optimization of the structural and photoelectric properties induced by the unique ammonia plasma treatment. The effective and ultrafast approach developed in this work is promising in the surface amino group regulation and structural engineering of various functional materials.

Functionalized Carbon Nanowalls as Pro-Angiogenic Scaffolds for Endothelial Cell Activation.

Substrates that can be utilized to assist in expression of the inherent characteristics of cells under in vitro conditions are necessary to mimic the in vivo scenario to the maximum extent possible. We demonstrate the application of functionalized carbon nanowalls (CNW) in facilitating human endothelial cell attachment and proliferation. The CNW films were grown on silicon substrates by surface-wave microwave plasma-enhanced chemical vapor deposition (SWMWPECVD) technique. These CNW were then functionalized using nitrogen (N2) gas plasma to form functionalized N2 doped CNW (CNW-N2). Characterization of CNWs revealed a uniform petal-like morphology with the individual CNW width measured to be 1-5 nm and the film thickness in the range of 10-12 μm. The N2 functionalized CNWs proved to be highly efficient in providing cell-anchorage to the endothelial cells. Profiles of major cytoskeletal proteins revealed a higher degree of expression in the functionalized CNWs depicting the maintenance of structural integrity of cells. Interestingly, the CNW-N2 substrate was found to promote pro-angiogenic factors in the cells, which is observed here for the first time, that could pave way for the utilization of this substrate for detailed studies of angiogenesis processes in vitro and further biomedical applications.

A two-step modelling approach for plasma reactors – experimental validation for CO2 dissociation in surface wave microwave plasma

Modelling approach that comprises a 2D self-consistent plasma model for discharge characterization in the first step and a 0D global plasma model for performance analysis in the second step.

Co-Evaluation of Reflection Loss and Surface Wave Attenuation for Magnetic Absorbing Material

Magnetic absorbing material (MAM) sheet was fabricated by calendaring carbonyl iron flakes into acrylonitrile-butadiene rubber. The complex permittivity and permeability of the sample were measured within 0.3–18 GHz. The calculated reflection loss (RL) and surface wave attenuation constants of the MAM are shown to be effectively manipulated by the layer thickness and electromagnetic (EM) parameters, respectively. It is found that the thickness of MAM should be suitable so as to realize good microwave wave absorption and surface wave attenuation performances in wide frequency range. In view of this goal, there also exist consistency requirements in EM parameters of MAM, i.e., a proper permittivity and high permeability. Furthermore, the measured RL and monostatic radar cross section reduction of a slab coated with the fabricated MAM sheet are in good agreement with the theoretical calculation. The present results provide a pathway to the development of MAM with high-efficiency absorption on both incidence EM waves and surface waves in the gigahertz range.

Microwave Plasma Torch Generated in Argon for Small Berries Surface Treatment

Demand for food quality and extended freshness without the use of harmful chemicals has become a major topic over the last decade. New technologies are using UV light, strong electric field, ozone and other reactive agents to decontaminate food surfaces. The low-power non-equilibrium (cold) atmospheric pressure operating plasmas effectively combines all the qualities mentioned above and thus, due to their synergetic influence, promising results in fruit surface decontamination can be obtained. The present paper focuses on the applicability of the recently developed microwave surface wave sustained plasma torch for the treatment of selected small fruit. Optical emission spectroscopy is used for the determination of plasma active particles (radicals, UV light) and plasma parameters during the fruit treatment. The infrared camera images confirm low and fully applicable heating of the treated surface that ensures no fruit quality changes. The detailed study shows that the efficiency of the microbial decontamination of selected fruits naturally contaminated by microorganisms is strongly dependent on the fruit surface shape. The decontamination of the rough strawberry surface seems inefficient using the current configuration, but for smooth berries promising results were obtained. Finally, antioxidant activity measurements demonstrate no changes due to plasma treatment. The results confirm that the MW surface wave sustained discharge is applicable to fruit surface decontamination.

Enhanced Microwave Absorption and Surface Wave Attenuation Properties of Co0.5Ni0.5Fe₂O₄ Fibers/Reduced Graphene Oxide Composites.

Co0.5Ni0.5Fe2O4 fibers with a diameter of about 270 nm and a length of about 10 μm were synthesized by a microemulsion-mediated solvothermal method with subsequent heat treatment. The Co0.5Ni0.5Fe2O4 fibers/reduced graphene oxide (RGO) composite was prepared by a facile in-situ chemical reduction method. The crystalline structures and morphologies were investigated based on X-ray diffraction patterns and scanning electron microscopy. Magnetization measurements were carried out using a vibrating sample magnetometer at room temperature. Co0.5Ni0.5Fe2O4 fibers/RGO composites achieve both a wider and stronger absorption and an adjustable surface wave attenuation compared with Co0.5Ni0.5Fe2O4 fibers, indicating the potential for application as advanced microwave absorbers.

Modelling of Argon Cold Atmospheric Plasmas for Biomedical Applications

Plasmas for biomedical applications are one of the newest fields of plasma utilization. Especially high is the interest toward plasma usage in medicine. Promising results are achieved in blood coagulation, wound healing, treatment of some forms of cancer, diabetic complications, etc. However, the investigations of the biomedical applications from biological and medical viewpoint are much more advanced than the studies on the dynamics of the plasma. In this work we aim to address some specific challenges in the field of plasma modelling, arising from biomedical applications – what are the plasma reactive species’ and electrical fields’ spatial distributions as well as their production mechanisms; what are the fluxes and energies of the various components of the plasma delivers to the treated surfaces; what is the gas flow pattern? The focus is on two devices, namely the capacitive coupled plasma jet and the microwave surface wave sustained discharge. The devices are representatives of the so called cold atmospheric plasmas (CAPs). These are discharges characterized by low gas temperature – less than 40°C at the point of application – and non-equilibrium chemistry.