Microplasma Devices Research Articles

High stability plasma illumination from micro discharges with nanodiamond decorated laser induced graphene electrodes

Superior lifetime stability for the microplasma device developed by decorating nanodiamonds (nDs) on laser induced graphene (LIG) is reported. Upon overwhelming the difficulty of poor stability in graphene, the nD-LIG displays exceptional lifetime stability of 1770s verified at an applied voltage of 340 V. But, the lifetime stability of LIG is only 718 s at the same applied voltage. Therefore, the nD-LIG with enhanced lifetime stability have pronounced prospective as cathodes in microplasma device applications.

A portable piezoelectric transformer microplasma optical emission spectrometry for the determination of mercury

Miniaturization of microplasma-based optical emission spectrometry (OES) has gained great attention due to the advantages of simplicity, compactness, and cost-effectiveness. However, the energy feed-in affects the generation, maintenance and excitation capability of discharge microplasma, which also limits the potential of microplasma for portable OES. Herein, we report that a piezoelectric transformer-driven microplasma (PTM) was first utilized as an excitation source of miniature OES for quantitative analysis of mercury (Hg). A piezoelectric transformer (PT) can directly convert mechanical energy into electrical energy and emanate plasma at the wire of the output distal end, which provides a path to construct microplasma devices without an extra high voltage power supply. A syringe needle acted as another ungrounded electrode, which was hung a proper distance below the PT output, to generate glow discharge. Hg was effectively converted into volatile species by hydride generation (HG) and transferred into PTM through the syringe needle by carrier gas (argon). Under the optimized experimental conditions, Hg was detected in the range of 10–1000 μg/L, with a limit of detection (LOD) of 2.8 μg/L by using the analytical line of Hg 253.65 nm. The accuracy and practicability of the proposed method were validated by analyzing the Certified Reference Materials (CRMs, dogfish liver and soil) and real water samples with satisfied results. In addition to no need of high voltage power supply and closed circuit, this HG-PTM-OES system features low power consumption, small size, light weight, sturdiness and high safety, which would be beneficial to fabricate a miniaturized and portable device for on-site environmental monitoring.

A microplasma device and its switching behavior triggered by modulated pulse signal

Since the physical and electrical properties of plasmas are similar to those of semiconductor devices, plasma devices have been proposed to be good candidates for switching controlled devices when applied in harsh environments. In the proposed paper, a microplasma device with a dielectric barrier structure constructed with three electrodes (two driven electrodes and one trigger electrode) is fabricated, and the electrical characteristics of the proposed device are investigated in 2 kPa of argon. From the experimental results, a stable conducting current is obtained through the two driven electrodes in the device due to gas discharge, since the hysteresis characteristic of discharge plasmas (discharge is still maintained when the driven voltage is below the breakdown voltage of the gas because of the existence of residual charge particles), the device can be switched from the OFF to ON state through pre-discharge by a pulse applied to the trigger electrode. While in the device ON state, this trigger voltage attracts channel charged particles to the surface of the dielectric layer, quenching the discharge plasma current and the device can be switched from ON to OFF state. The trigger pulse that makes the device switch successfully goes from single to continuous up to 80 kHz. The influence of pulse parameters on the switching process is also investigated, pulse amplitude and pulse width are found to be important to determine whether the device can switch ON or OFF, peak current after switching, and the response speed of switching ON current; however, these switching parameters are barely affected by the rise and fall time of the pulse. The results are significant for the application of microplasma switching devices.

Sensitive determination of Ag, Bi, Cd, Hg, Pb, Tl, and Zn by inductively coupled plasma optical emission spectrometry combined with the microplasma-assisted vapor generation

A technique of vapor generation assisted by a microplasma was used for sample introduction into inductively coupled plasma optical emission spectrometry (ICP OES). Replacing a pneumatic nebulizer with a novel microplasma device improved the sensitivities of analytical signals for Ag, Bi, Cd, Pb, Tl, and Zn (by 2–13 times), as well as a concomitant reduction in their detection limits (DLs). Moreover, an outstanding improvement (30-fold) was achieved for Hg. The factors contributing to the boosted signal intensities were higher analyte fluxes and less water vapor produced by the microplasma system. The DLs of Ag, Bi, Cd, Hg, Pb, Tl, and Zn in microplasma-ICP OES were 0.4, 4, 0.06, 0.2, 2, 5, and 0.2 μg L−1, respectively, and the measurement precision was within the range of 0.7–2.4% (it was significantly improved as compared to that achievable with pneumatic nebulization). The proposed microplasma-assisted vapor generation eliminates the use of toxic reductants, e.g., sodium tetrahydridoborate, and it is characterized by higher resistance to matrix effects from transition metal ions (as compared to chemical vapor generation (CVG) and photochemical vapor generation (PVG)). To validate the trueness of the SAGD-ICP OES method, certified reference materials of lobster hepatopancreas (TORT-2), cormorant tissue (MODAS-4) as well as spiked tap water and seawater samples were analyzed to determine levels Cd and Hg. The standard additions method was used for calibration in both cases. Recoveries of the analytes in the case of the analysis of TORT-2 and MODAS-4 samples as well as the spiked tap water and seawater was within the range of 98–113%, which indicated that the developed sample introduction system can be successfully used for very sensitive determinations of selected hazardous elements in environmental samples.

Coupling of chemical vapor generation with atmospheric pressure glow discharge optical emission spectrometry generated in contact with flowing liquid electrodes for determination of Br in water samples

Miniaturized atmospheric pressure glow discharge (APGD) microplasma devices, operated in contact with a flowing liquid anode (FLA) or a flowing liquid cathode (FLC) and a He jet, were studied for the determination of Br by optical emission spectrometry (OES), after oxidizing the Br- ions to volatile Br2 in the reaction with KMnO4. The concentrations of all reagents needed for the successful operation of the proposed systems were found to be the most crucial parameters and their impact on the intensity of the Br analytical line was studied. Under the optimal conditions, the analytical figures of merit were assessed. The endurance to the matrix effects of both developed methods was also investigated and both of them were found to be strongly insusceptible to the presence of foreign ions. The detection limits (DLs) of Br and was established to be 0.05 and 0.2 mg L-1 for CVG-FLA- and CVG-FLC-APGD, respectively. The developed methods were successfully applied for the determination of relatively low concentrations of Br in selected water samples, spiked with this element, with very good precision (0.9–6.1% as RSD) and trueness (94–110% as recovery). In addition, no significant differences between the results obtained using the external standard calibration and the standard addition method were noted.

Nitrogen-Incorporated Boron-Doped Nanocrystalline Diamond Nanowires for Microplasma Illumination.

The origin of nitrogen-incorporated boron-doped nanocrystalline diamond (NB-NCD) nanowires as a function of substrate temperature (Ts) in H2/CH4/B2H6/N2 reactant gases is systematically addressed. Because of Ts, there is a drastic modification in the dimensional structure and microstructure and hence in the several properties of the NB-NCD films. The NB-NCD films grown at low Ts (400 °C) contain faceted diamond grains. The morphology changes to nanosized diamond grains for NB-NCD films grown at 550 °C (or 700 °C). Interestingly, the NB-NCD films grown at 850 °C possess one-dimensional nanowire-like morphological grains. These nanowire-like NB-NCD films possess the co-existence of the sp3-diamond phase and the sp2-graphitic phase, where diamond nanowires are surrounded by sp2-graphitic phases at grain boundaries. The optical emission spectroscopy studies stated that the CN, BH, and C2 species in the plasma are the main factors for the origin of nanowire-like conducting diamond grains and the materialization of graphitic phases at the grain boundaries. Moreover, conductive atomic force microscopy studies reveal that the NB-NCD films grown at 850 °C show a large number of emission sites from the grains and the grain boundaries. While boron doping improved the electrical conductivity of the NCD grains, the nitrogen incorporation eased the generation of graphitic phases at the grain boundaries that afford conducting channels for the electrons, thus achieving a high electrical conductivity for the NB-NCD films grown at 850 °C. The microplasma devices using these nanowire-like NB-NCD films as cathodes display superior plasma illumination properties with a threshold field of 3300 V/μm and plasma current density of 1.04 mA/cm2 with a supplied voltage of 520 V and a lifetime stability of 520 min. The outstanding plasma illumination characteristics of these conducting nanowire-like NB-NCD films make them appropriate as cathodes and pave the way for the utilization of these materials in various microplasma device applications.

Stable Field Electron Emission and Plasma Illumination from Boron and Nitrogen Co‐Doped Edge‐Rich Diamond‐Enhanced Carbon Nanowalls

AbstractSuperior field electron emission (FEE) characteristics are achieved in edge‐rich diamond‐enhanced carbon nanowalls (D‐ECNWs) grown in a single‐step chemical vapor deposition process co‐doped with boron and nitrogen. The structure consists of sharp, highly conductive graphene edges supplied by a solid, diamond‐rich bottom. The Raman and transmission electron microscopy studies reveal a hybrid nature of sp3‐diamond and sp2‐graphene in these nanowalls. The ab‐initio calculations were carried out to support the experimental observations of diamond‐graphene hybrid structure. Finally, this hybrid D‐ECNWs is employed as a cathode in an FEE device resulting in a low turn‐on field of 3.1 V µm−1, a large field enhancement factor, a high FEE Je of 2.6 mA cm−2, and long lifetime stability of 438 min. Such an enhancement in the FEE originates from the unique materials combination, resulting in good electron transport from the graphene phases and efficient FEE of electrons from the sharp edges on the nanowalls. The prospective application of these materials is displayed by employing these hybrids as cathodes in a microplasma device ensuing a low threshold voltage of 160 V and high plasma stability of 140 min, which confirms the role of these hybrid structured nanowalls in the enhancement of electron emission.

Combined atomic and molecular (CAM) ionization with the liquid sampling-atmospheric pressure glow discharge microplasma.

In a world where information-rich methods of analysis are often sought over those with superior figures of merit, there is a constant search for ionization methods which can be applied across diverse analytical systems. The liquid sampling-atmospheric pressure glow discharge (LS-APGD) is a microplasma device which has the inherent capabilities to operate as a combined atomic and molecular (CAM) ionization source. The plasma is sustained by placement of a high voltage (~500 V, dc) onto an electrolytic solution through which the analyte is generally delivered to the discharge. Judicious choice of the solvent provides a means of obtaining atomic/elemental and/or molecular mass spectra. Presented here are the diverse modes of sample introduction and mass spectrometer platforms to which the LS-APGD has been interfaced. Likewise, representative spectra and figures of merit are presented towards elemental and isotope ratio measurements, as well as application to small organic molecules, organometallic complexes, and intact proteins. It is believed that the diversity of analytical applications and ready implementation across the entirety of mass spectrometry platforms portends a level of versatility not realized with other ionization sources.

High frequency impedance characteristics of a tunable microplasma device

Computational studies on high frequency impedance characteristics of a microplasma device are reported. While microplasma is ignited using a primary excitation signal, frequency response of plasma impedance is determined by a secondary high frequency probe signal with significantly lower voltage amplitude such that it does not influence the plasma parameters. The computational model utilizing the drift–diffusion approximation is first validated by comparing with experimental data for microplasmas ignited at pressures ranging from 1 to 5 Torr. In spite of quantitative discrepancies, good overall agreement is obtained between the measured frequency response of impedance of the discharge. Comparisons are also presented for various plasma parameters including mean electron number density, sheath thickness, mean electron temperature, and collision frequency that were inferred from the impedance measurements. The computational model is then used to perform simulations of near-atmospheric pressure microplasmas with the probe signal frequency ranging from 3 to 20 GHz. The simulations demonstrate the presence of a resonance frequency at which the impedance vanishes. More importantly, it is shown that this resonant frequency can be tuned effectively by suitably modifying the operating parameters (gap size, pressure, and excitation voltage). The simulated impedance characteristics are used to determine the effective plasma inductance and capacitance using a non-linear fitting approach, thereby showing the dependence of these electrical parameters on the plasma operating conditions.

Transition characteristics and electron kinetics in microhollow cathode discharges

We demonstrate the transition characteristics and electron kinetics of microdischarges in a microgap with a cathode having microstructures using two-dimensional particle-in-cell/Monte Carlo collision (2d3v) simulations. It is found that the microdischarge is inside the hollow cavity at lower pressures, forming microhollow cathode discharges (MHCDs), while the dominant discharge moves outside the cavity at higher pressures, locating above the cathode rectangular protrusion tip. The spatial distributions of the microdischarge parameters (e.g., electric potential and electron density) are presented by capturing the transition characteristics. The electron kinetics of the microdischarges are examined based on the moment analysis of the electron Boltzmann equation. The collisional and non-collisional components of the electron power absorption are decomposed, which were seldomly investigated for microdischarges previously. The results provide fundamental insights into MHCD formation with a structured electrode at varying pressure conditions, which could be beneficial for the design and fabrication of microplasma devices in practical applications.

Cold Atmospheric Plasma: A Powerful Tool for Modern Medicine.

Cold atmospheric plasma use in clinical studies is mainly limited to the treatment of chronic wounds, but its application in a wide range of medical fields is now the goal of many analyses. It is therefore likely that its application spectrum will be expanded in the future. Cold atmospheric plasma has been shown to reduce microbial load without any known significant negative effects on healthy tissues, and this should enhance its possible application to any microbial infection site. It has also been shown to have anti-tumour effects. In addition, it acts proliferatively on stem cells and other cultivated cells, and the highly increased nitric oxide levels have a very important effect on this proliferation. Cold atmospheric plasma use may also have a beneficial effect on immunotherapy in cancer patients. Finally, it is possible that the use of plasma devices will not remain limited to surface structures, because current endeavours to develop sufficiently miniature microplasma devices could very likely lead to its application in subcutaneous and internal structures. This study summarises the available literature on cold plasma action mechanisms and analyses of its current in vivo and in vitro use, primarily in the fields of regenerative and dental medicine and oncology.

Modular constructed metal-grid arrays—an alternative to silicon-based microplasma devices for catalytic applications

Here, we present a modular constructed metal-grid micro cavity plasma array as a flexible, robust, and simple alternative to micro-structured devices based on silicon. They show great potential for applications requiring large-area treatment, catalytic conversion or decomposition of volatile organic compounds. The metal-grid array is an easily assembled layered structure consisting of a metal grid, a dielectric foil and a magnet. The grid contains between hundreds and thousands of uniformly arranged cavities with a diameter of 150 μm. The whole system is kept together by magnetic force. This also allows disassembling and exchange of the components independently. Typically, the arrays are operated close to atmospheric pressure with an alternating voltage of up to 1.4 kV peak-to-peak in the kHz range. For a first comparison with silicon-based configurations, the metal-grid array is examined from two different perspectives using phase-resolved imaging. The individual cavities show the same asymmetric discharge behaviour as in the silicon-based arrays. In addition, the expansion width of the discharge from the cavities could be measured. The same interaction between the cavities with the propagation of an ionization wave with velocities in the km/s range is observed as for the silicon-based devices. Thus, with respect to the most basic discharge properties, both configurations show the same behaviour, although they are different in structure and composition.

Direct EPR detection of atomic nitrogen in an atmospheric nitrogen plasma jet.

In this study, an atmospheric nitrogen plasma jet generated by a custom-built micro-plasma device was analyzed at room temperature by continuous wave and pulse EPR spectroscopy in real time. Transiently formed nitrogen atoms were detected without the necessity to use spin-traps or other reagents for their stabilization. In contrast to results from optical emission spectroscopy, only signals from the 4S ground state of 14N and 15N could be detected. EPR data analysis revealed an isotropic g value of 1.9971 and isotropic hyperfine coupling constants of a(14N) = (10.47 ± 0.02) MHz and a(15N) = (14.69 ± 0.02) MHz. Moreover, lifetime and relaxation data could be determined; both are discussed in terms of spectral widths and actual concentrations of the transiently formed nitrogen species within the plasma jet. The data show that the lifetimes of atomic nitrogen and charged particles such as N+ must be different, and for the latter below the observation time window of EPR spectroscopy. We demonstrate that the real-time (pulsed) EPR technique is a fast and reliable alternative to detect atomic nitrogen in atmospheric pressure plasma jets. The method may be used for a continuous monitoring of the quality of plasma jets.

Origin of Conductive Nanocrystalline Diamond Nanoneedles for Optoelectronic Applications.

Microstructural evolution of nanocrystalline diamond (NCD) nanoneedles owing to the addition of methane and nitrogen in the reactant gases is systematically addressed. It has been determined that varying the concentration of CH4 in the CH4/H2/N2 plasma is significant to tailor the morphology and microstructure of NCD films. While NCD films grown with 1% CH4 in a CH4/H2/N2 (3%) plasma contain large diamond grains, the microstructure changed considerably for NCD films grown using 5% (or 10%) CH4, ensuing in nanosized diamond grains. For 15% CH4-grown NCD films, a well-defined nanoneedle structure evolves. These NCD nanoneedle films contain sp3 phase diamond, sheathed with sp2-bonded graphitic phases, achieving a low resistivity of 90 Ω cm and enhanced field electron emission (FEE) properties, namely, a low turn-on field of 4.3 V/μm with a high FEE current density of 3.3 mA/cm2 (at an applied field of 8.6 V/μm) and a significant field enhancement factor of 3865. Furthermore, a microplasma device utilizing NCD nanoneedle films as cathodes can trigger a gas breakdown at a low threshold field of 3600 V/cm attaining a high plasma illumination current density of 1.14 mA/cm2 at an applied voltage of 500 V, and a high plasma lifetime stability of 881 min is evidenced. The optical emission spectroscopy studies suggest that the C2, CN, and CH species in the growing plasma are the major causes for the observed microstructural evolution in the NCD films. However, the increase in substrate temperature to ∼780 °C due to the incorporation of 15% CH4 in the CH4/H2/N2 plasma is the key driver resulting in the origin of nanoneedles in NCD films. The outstanding optoelectronic characteristics of these nanoneedle films make them suitable as cathodes in high-brightness display panels.

Self-organized multi-layered graphene-boron-doped diamond hybrid nanowalls for high-performance electron emission devices.

Carbon nanomaterials such as nanotubes, nanoflakes/nanowalls, and graphene have been used as electron sources due to their superior field electron emission (FEE) characteristics. However, these materials show poor stability and short lifetimes, which prevent their use in practical device applications. The aim of this study was to find an innovative nanomaterial possessing both high robustness and reliable FEE behavior. Herein, a hybrid structure of self-organized multi-layered graphene (MLG)-boron doped diamond (BDD) nanowall materials with superior FEE characteristics was successfully synthesized using a microwave plasma enhanced chemical vapor deposition process. Transmission electron microscopy reveals that the as-prepared carbon clusters have a uniform, dense, and sharp nanowall morphology with sp3 diamond cores encased by an sp2 MLG shell. Detailed nanoscale investigations conducted using peak force-controlled tunneling atomic force microscopy show that each of the core-shell structured carbon cluster fields emits electrons equally well. The MLG-BDD nanowall materials show a low turn-on field of 2.4 V μm-1, a high emission current density of 4.2 mA cm-2 at an applied field of 4.0 V μm-1, a large field enhancement factor of 4500, and prominently high lifetime stability (lasting for 700 min), which demonstrate the superiority of these materials over other hybrid nanostructured materials. The potential of these MLG-BDD hybrid nanowall materials in practical device applications was further illustrated by the plasma illumination behavior of a microplasma device with these materials as the cathode, where a low threshold voltage of 330 V (low threshold field of 330 V mm-1) and long plasma stability of 358 min were demonstrated. The fabrication of these hybrid nanowalls is straight forward and thereby opens up a pathway for the advancement of next-generation cathode materials for high brightness electron emission and microplasma-based display devices.

Early stages of diamond growth on substrates with different carbon diffusivity

In this work, nanocrystalline diamond was deposited with a direct-current micro-plasma device on substrates with different carbon diffusivity. No substrate pre-treatment was performed and the same deposition conditions were adopted for all substrates, with the intention to investigate the first stages of growth. Samples were grown with increasing deposition time. Scanning electron microscopy and visible Raman spectroscopy were used to derive morphological and structural information. The growth dynamics was found to be the same on all substrates, with the deposition of a graphite layer prior to diamond growth. This layer was extensively characterized and a link between the structure and morphology of this layer and the subsequent features of the diamond grown on it was found. Moreover, diamond tens of micrometers thick was deposited in few hours, opening the possibility of producing diamond samples of practical interest. Particularly, the growth of nanocrystalline diamond on iron without substrate pre-treatment or interlayer deposition is reported, showing the feasibility of depositing diamond even on ferrous materials.

Microplasma Field Effect Transistors

Micro plasma devices (MPD) with power gains are of interest in applications involving operations in the presence of ionizing radiations, in propulsion, in control, amplification of high power electromagnetic waves, and in metamaterials for energy management. Here, we review and discuss MPDs with an emphasis on new architectures that have evolved during the past seven years. Devices with programmable impact ionization rates and programmable boundaries are developed to control the plasma ignition voltage and current to achieve power gain. Plasma devices with 1–10 μm gaps are shown to operate in the sub-Paschen regime in atmospheric pressures where ion-assisted field emission results in a breakdown voltage that linearly depends on the gap distance in contrast to the exponential dependence dictated by the Paschen curve. Small gap devices offer higher operation frequencies at low operation voltages with applications in metamaterial skins for energy management and in harsh environment inside nuclear reactors and in space. In addition to analog plasma devices, logic gates, digital circuits, and distributed amplifiers are also discussed.

Nanocrystalline diamond produced by direct current micro-plasma: Investigation of growth dynamics

In this work, the growth dynamics of nanocrystalline diamond, deposited on molybdenum and silicon substrates with a direct-current micro-plasma device, is experimentally investigated. No substrate pre-treatment was performed and comparable deposition conditions were adopted for both substrates, addressing both morphological and structural properties for various deposition times. An extensive characterization of the resulting different stages of growth was achieved, exploiting scanning electron microscopy, X-ray diffraction and Raman spectroscopy at two wavelenghts (both visible and UV). The deposition of graphite upon formation of a carbide layer prior to diamond nucleation has been found to be the triggering step towards diamond growth on both substrates, but with a different growth kinetics. Besides showing the potential of the adopted deposition method and characterization strategy, our results also show the possibility of producing nanocrystalline diamond with reduced deposition times and without any substrate pre-treatment.

Microplasma device architectures with various diamond nanostructures

Diamond nanostructures (DNSs) were fabricated from three different morphological diamonds, microcrystalline diamond (MCD), nanocrystalline diamond (NCD), and ultrananocrystalline diamond (UNCD) films, using a reactive ion etching method. The plasma illumination (PI) behavior of microplasma devices using the DNSs and the diamond films as cathode were investigated. The Paschen curve approach revealed that the secondary electron emission coefficient (γ value) of diamond materials is similar irrespective of the microstructure (MCD, NCD, and UNCD) and geometry of the materials (DNSs and diamond films). The diamond materials show markedly larger γ-coefficient than conventional metallic cathode materials such as Mo that resulted in markedly better PI behavior for the corresponding microplasma devices. Moreover, the PI behavior, i.e. the voltage dependence of plasma current density (Jpl−V), plasma density (ne−V), and the robustness of the devices, varied markedly with the microstructure and geometry of the cathode materials that was closely correlated to the electron field emission (EFE) properties of the cathode materials. The UNCD nanopillars, possessing good EFE properties, resulted in superior PI behavior, whereas the MCD diamond films with insufficient EFE properties led to inferior PI behavior. Consequently, enhancement of plasma characteristics is the collective effects of EFE behavior and secondary electron emission characteristics of diamond-based cathode materials.

Plasma cell treatment device Plasma-on-Chip: Monitoring plasma-generated reactive species in microwells

We have developed a plasma cell treatment device called Plasma-on-Chip that enables the real-time monitoring of a single cell culture during plasma treatment. The device consists of three parts: 1) microwells for cell culture, 2) a microplasma device for generating reactive oxygen and nitrogen species (RONS) for use in cell treatment, and 3) through-holes (microchannels) that connect each microwell with the microplasma region for RONS delivery. Here, we analysed the delivery of the RONS to the liquid culture medium stored in the microwells. We developed a simple experimental set-up using a microdevice and applied in situ ultraviolet absorption spectroscopy with high sensitivity for detecting RONS in liquid. The plasma-generated RONS were delivered into the liquid culture medium via the through-holes fabricated into the microdevice. The RONS concentrations were on the order of 10–100 μM depending on the size of the through-holes. In contrast, we found that the amount of dissolved oxygen was almost constant. To investigate the process of RONS generation, we numerically analysed the gas flow in the through-holes. We suggest that the circulating gas flow in the through-holes promotes the interaction between the plasma (ionised gas) and the liquid, resulting in enhanced RONS concentrations.