Direct Current Microplasma Research Articles

Exploring the Photoluminescence Characteristics of Nitrogen-Doped Carbon Dots

Carbon dots, or C-Dots, are graphitic particles below 10 nm that exhibit room temperature photoluminescence (PL). The specific PL mechanism of C-Dots varies with synthesis paths and remains unresolved. This study utilizes direct current microplasmas for reliable and reproducible synthesis, exploring precursor outcomes and the discharge current's impact on surface chemistry and optical properties. The nitrogen-doped C-Dots (N-CQDs) synthesized display crystalline structures with a graphitic core, nitrogen doping, and a functionalized surface. Luminescence, originating from both core and surface states, can be finely controlled by adjusting synthesis conditions. N-CQDs, with a high photoluminescence quantum yield, hold promise for next-gen solar cell applications. The N-CQDs exhibit intriguing quantum-confined optical properties dependent on the level of nitrogen incorporation. Notably, the band energy structure of the N-CQDs is clarified, and they are integrated into a photovoltaic device as the photoactive layer, achieving an extraordinary open-circuit voltage of 1.8 V corroborating well with energy gat of CQDs and PL properties [1, 2]. We specifically highlight the use of nitrogen-doped C-Dots as an active absorbing layer in solar cells and discuss how doping can engineer their electrical, optical, and chemical properties. Nitrogen doping mainly occurs in the graphitic core as a substituted type of doping (63–67 atomic%), with enough to create emissive states without impacting the core structure. The optical and chemical properties remain stable even with re-dispersion, suggesting easy applicability for cellular imaging or optoelectronics [3]. Highly stable nitrogen-doped graphite quantum dots, prepared by atmospheric pressure microplasma, without post-synthesis treatments, yields stable, water-dispersible, and non-toxic quantum dots with a high PL quantum yield of approximately 70%.

Correlations between energy flux and thin film modifications in an atmospheric pressure direct current microplasma

A custom-built atmospheric pressure direct current microplasma discharge was designed utilizing Au coated Si3N4 transmission electron microscopy (TEM) grids as electrodes to enable a direct imaging of the plasma treated anode and cathode surfaces. Significant differences between the anode and the cathode, as well as between Ar or He operation of the microplasma, respectively, were observed already by examination of the electrode surfaces by light microscopy. Scanning electron microscopy as well as TEM imaging revealed details such as grain growth and changes in surface morphology. The energy fluxes from plasma towards the anode and cathode were determined and correlated with the surface modifications. As expected from structure zone diagrams higher energy fluxes result in higher degree of crystallinity even at atmospheric pressure. Furthermore, ions and their respective energy contributions were identified to play a major role for the surface modification, despite their low kinetic energy due to the large number of collisions. Depending on the working gas, Ar or He, the identified energetic contributions of the total ion energy flux change and, subsequently, result in visible differences in the plasma treated Au films.

Modification of Nitrogen Doped Carbon Quantum Dots Emissive States Using Plasma-Induced Non-Equilibrium Electrochemistry

Highly luminescent nanomaterials with ambient stability, environmental friendliness, low-cost and abundance are in demand for various applications in biological sensing, imaging and optoelectronics [1-6]. Many of the photoluminescent nanomaterials are semiconductors and usually contain toxic elements, heavy and expensive metals which have limited their applications. Carbon nanoparticles are a promising alternative to semiconductor nanocrystals as next generation green nanomaterials due to excellent biocompatibility, low cytotoxicity and solution processability which results in ease of production and incorporation in devices [7]. In the current work, nitrogen doped carbon quantum dots (N-CQDs) are synthesized by plasma-induced non-equilibrium electrochemistry utilizing a direct-current atmospheric pressure microplasma. The direct current microplasmas used in this work serve as a reliable and highly reproducible synthesis method for tuning the optical properties of carbon-based quantum dots. The outcome of precursors and discharge current affecting the surface chemistry and optical properties are studied using various characterization techniques, which have contributed to determine the mechanisms leading to QD formation from the plasma-induced reactions at the interface. The synthesized N-CQDs are crystalline with graphitic core doped with nitrogen and functionalized surface. The luminescence which originates from the combined effect of core and surface states can be finely controlled by changing synthesis conditions. These N-CQDs have potential to be used as an active material in next-generation solar cells or even as down-converters for high energy photons to be absorbed by a lower bandgap transporter [8]. The particularly high photoluminescence quantum yield (33% to 68%) can be exploited for optoelectronic applications.

Direct current microplasma formation around microstructure arrays

We demonstrate the formation and transition behaviors of a microplasma around microstructure arrays at different gas pressures via two-dimensional particle-in-cell/Monte Carlo collision simulations. It is found that the microdischarge occurs outside the cathode microcavities at the lowest pressure and starts penetrating the microcavities with a curved sheath edge as the pressure increases. At higher pressure, coupled periodic microhollow cathode discharges (MHCDs) are formed inside the microcavities. Further increasing the gas pressure results in the disappearance of the MHCDs, and the dominant discharge shifts outside of the microcavity, locating above the protrusion tips. The effect of the space charge shielding on the discharge and the conditions for MHCD formation are discussed. The macroscopic discharge parameter scalings with the gas pressure and the electron kinetics are also examined. The results are helpful for deeply understanding the microplasma formation with nonplanar electrodes, which inform the scaling, design, and optimization of microplasma array devices across a wide range of pressure regimes in practical applications.

Computational analysis of gas breakdown modes in direct current micro-plasmas at elevated pressures

Direct current micro-plasmas in the non-homogeneous electric field are analyzed over a wide pressure range using the self-consistent two-dimensional axisymmetric fluid model. We observe that the breakdown voltage is not the unique function of Pd, where P is the gas pressure and d is the interelectrode spacing, but also depends on the aspect ratio d/r, where r is the anode radius. This result agrees with the data reported in the literature. For fixed d, we find two modes of ionization wave propagation on the right branch of the breakdown curve: an axial streamer mode that is obtained at low pressures and a hollow streamer mode obtained at high pressures. By varying the ballast resistance connected to the anode, we analyze the steady-state parameters of the micro-discharge for the cathode–anode gap of 200 μm. We obtain normal and sub-normal glow modes of the micro-discharge operation. The instability of the latter mode is analyzed.

Plasma Induced Non-Equilibrium Electrochemistry for Synthesis of Nitrogen Doped Carbon Quantum Dots Applied to Third-Generation Solar Cells

Highly luminescent nanomaterials with ambient stability, environmental friendliness, low-cost and abundance are in demand for various applications in biological sensing, imaging and optoelectronics [1-6]. Many of the photoluminescent nanomaterials are semiconductors and usually contain toxic elements, heavy and expensive metals which have limited their applications. Carbon nanoparticles are a promising alternative to semiconductor nanocrystals as next generation green nanomaterials due to excellent biocompatibility, low cytotoxicity and solution processability which results in ease of production and incorporation in devices [7]. In the current work, nitrogen doped carbon quantum dots (N-CQDs) are synthesized by one-step atmospheric pressure microplasma process. The direct current microplasmas used in this work serve as a reliable and highly reproducible synthesis method for tuning the optical properties of carbon-based quantum dots in colloid form which result as being highly stable and environmentally-friendly. The outcome of precursors and discharge current affecting the particle morphology and optical properties are studied using various characterization techniques, which have contributed to determine the mechanisms leading to QD formation from the plasma-induced reactions at the interface. The synthesized N-CQDs are crystalline with graphitic core doped with nitrogen and functionalized surface. The particle size and luminescence can be finely controlled to give either excitation dependent or fixed wavelength emission in the visible region. The synthesis conditions can be easily controlled by changing the precursors or the discharge current of the plasma to give rise to required particle size and absorbance in the ultraviolet-visible region. These N-CQDs have potential to be used as an active material in next-generation solar cells or even as down-converters for high energy photons to be absorbed by a lower bandgap transporter [8]. The possible synthesis mechanisms have been analysed and potential chemical pathways leading to the formation of the QDs are described. The particularly high photoluminescence quantum yield (33% to 68%) can be exploited for applications.

Feasible production of hydrogen from methanol reforming through single stage DC microplasma reactor

Plasma-assisted methanol reforming is an effective technology to produce hydrogen for various clean energy applications. In this study, hydrogen was produced from methanol reforming in a unique single stage microplasma reactor. Microplasma was produced between the capillary stainless steel tube electrodes by using high voltage direct current (DC) power supply. Blend of methanol and water was supplied to the microplasma reactor in a controlled flow rate using nitrogen as carrier gas. The effects of applied input power to the discharge and methanol feed rate on the performance of the plasma methanol decomposition were investigated. The experimental results showed that increasing the applied input power expressively increased the methanol conversion and hydrogen energy yield. In contrast, the increased feed rate significantly decreased the methanol conversion efficiency though it enriched the hydrogen energy yield. Under selective conditions, hydrogen energy yield of 24.14 g kW[Formula: see text] h[Formula: see text] was achieved with the conversion efficiency of 71% and 50% selectivity for H2, which is comparatively better than many of plasma-assisted methanol reforming processes. This investigation reveals that methanol reforming through a single stage microplasma reactor has the ability to produce hydrogen efficiently without coke formation at room-temperature and atmospheric pressure.

Benchmark continuum and kinetic simulations of argon microplasmas in the direct current and microwave regimes

This study presents benchmark comparisons between continuum and kinetic simulations of argon microplasmas operating in the direct current and microwave regimes. Kinetic simulations using the particle-in-cell with Monte Carlo collisions (PIC-MCC) method and continuum simulations using the full-momentum equation for both ions and electrons are performed at various operating conditions in order to study the influence of product of pressure and gap size, pd (for a given gap size), influence of gap size (for a given value of pd) and operating frequency. It is shown that using the electron energy distribution function (EEDF) predicted by zero-dimensional Boltzmann solvers (such as BOLSIG+) in continuum simulations of direct current microplasmas leads to a significant under-prediction of plasma number densities with continuum simulations based on the Maxwellian EEDF performing better particularly for higher values of pd. The discrepancy between kinetic and continuum simulations is attributed to the presence of hot electrons created as a result of secondary emission and subsequent acceleration in the sheath. On the other hand, simulations performed for argon microwave microplasmas operating at 0.5 GHz, 0.8 GHz, 2 GHz and 4 GHz demonstrated that continuum simulations performed using the rate constants from BOLSIG+ showed excellent agreement with kinetic simulations for the plasma density profiles in spite of over-predicting the voltage/power required to achieve a given plasma density.

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.

Review of electric discharge microplasmas generated in highly fluctuating fluids: Characteristics and application to nanomaterials synthesis

Plasma-based fabrication of novel nanomaterials and nanostructures is indispensible for the development of next-generation electronic devices and for green energy applications. In particular, controlling the interactions between plasmas and materials interfaces, and the plasma fluctuations, is crucial for further development of plasma-based processes and bottom-up growth of nanomaterials. Electric discharge microplasmas generated in supercritical fluids represent a special class of high-pressure plasmas, where fluctuations on the molecular scale influence the discharge properties and the possible bottom-up growth of nanomaterials. This review discusses an anomaly observed for direct current microplasmas generated near the critical point, a local decrease in the breakdown voltage. This anomalous behavior is suggested to be caused by the concomitant decrease of the ionization potential due to the formation of clusters near the critical point, and the formation of extended electron mean free paths caused by the high-density fluctuation near the critical point. It is also shown that in the case of dielectric barrier microdischarges generated close to the critical point, the high-density fluctuation of the supercritical fluid persists. The final part of the review discusses the application of discharges generated in supercritical fluids to synthesis of nanomaterials, in particular, molecular diamond—so-called diamondoids—by microplasmas generated inside conventional batch-type and continuous flow microreactors.

Carbon Structures Grown by Direct Current Microplasma: Diamonds, Single-Wall Nanotubes, and Graphene

Plasma assisted CVD is now an established technique for the growth of a variety of dielectrics and semiconductors. The versatility of an in-house developed direct-current (dc) microplasma deposition system is demonstrated here for the growth of a wide range of carbon-based materials. Diamond, nanodiamond, nanocrystalline graphite, single-wall carbon nanotubes, and few-layer graphene have been deposited using the same dc microplasma deposition system using 0.5% CH4/H2 gas feed, but changing only the substrate temperature (in the range 500–1150 °C) and the total pressure (0.3–200 Torr). The different structures have been characterized by scanning electron microscopy and micro-Raman spectroscopy. The experimental data have been interpreted from a thermodynamic point of view by applying a nonequilibrium nondissipative model. Nonequilibrium phase diagrams are presented and compared to the experimental data to provide a wide-ranging interpretation scenario.

An effective analytical system based on a pulsed direct current microplasma source for ultra-trace mercury determination using gold amalgamation cold vapor atomic emission spectrometry

A novel analysis system based on a low power atmospheric pressure pulsed direct current (Pdc) microplasma is described for the determination of ultra-trace mercury in natural water by cold vapor generation atomic emission spectrometry (CV-AES). The plasma was generated with a miniaturized home-built high-voltage Pdc power supply which decreased the volume and weight of the whole experiment setup. The CV-Pdc-AES system is based on the preconcentration of mercury vapor on a gold filament trapping micro-column prior to detection that provides fast, reproducible absorption and desorption of mercury. The micro-column is produced by winding 30μm diameter 100m long gold filament to a small ball and then insert it into a quartz tube of 6mm i.d, 8mm o.d. Under the optimized experimental conditions, the new system provides high sensitivity (detection limit: 0.08pgmL−1) and good reproducibility (RSD 3.0%, [Hg]=20pgmL−1, n=11). The calibration curve is linear at levels near the detection limit up to at least 200pgmL−1 and the accuracy is on the order of 1–4%. The proposed method was applied to 5 real water samples for mercury ultra-trace analysis. The advantages and features of the newly developed system include high sensitivity, simple structure, low cost, and compact volume with field portable potential.

Cathode fall model and current-voltage characteristics of field emission driven direct current microplasmas

The post-breakdown characteristics of field emission driven microplasma are studied theoretically and numerically. A cathode fall model assuming a linearly varying electric field is used to obtain equations governing the operation of steady state field emission driven microplasmas. The results obtained from the model by solving these equations are compared with particle-in-cell with Monte Carlo collisions simulation results for parameters including the plasma potential, cathode fall thickness, ion number density in the cathode fall, and current density vs voltage curves. The model shows good overall agreement with the simulations but results in slightly overpredicted values for the plasma potential and the cathode fall thickness attributed to the assumed electric field profile. The current density vs voltage curves obtained show an arc region characterized by negative slope as well as an abnormal glow discharge characterized by a positive slope in gaps as small as 10 μm operating at atmospheric pressure. The model also retrieves the traditional macroscale current vs voltage theory in the absence of field emission.

Improved Optoelectronic Properties of Silicon Nanocrystals/Polymer Nanocomposites by Microplasma-Induced Liquid Chemistry

We have demonstrated that three-dimensional (3D) surface engineering of silicon nanocrystals (SiNCs) by direct current microplasma processing in water with poly(3,4-ethylenedioxythiophene) doped by poly(styrenesulfonate) (PEDOT:PSS) can lead to nanocomposites with enhanced optoelectronic performance. Specifically, we have successfully shown improved photoluminescence properties of SiNCs inside water-based solution. The results also confirm that SiNCs become stable in water with potential application impact for biorelated applications. We have also shown that the microplasma processing in the presence of the polymer helps prevent the fast oxidation process over a longer period of time in comparison to the unprocessed sample. Furthermore, the assessment of transport properties confirmed the improvement of exciton dissociation after microplasma surface engineering; this can have direct implications for higher performance optoelectronic devices including solar cells.

Built-In Charges and Photoluminescence Stability of 3D Surface-Engineered Silicon Nanocrystals by a Nanosecond Laser and a Direct Current Microplasma

NEDO; JSPS Invitation Fellowship; JSPS Bridge Fellowship; University of Ulster Strategic Research Fund; University of Ulster Vice-Chancellor Studentship; Ad-Futura Fellowship

Analysis of cathode geometry to minimize cathode erosion in direct current microplasma jet

Microplasma jets are now widely used for deposition, etching, and materials processing. The present study focuses on the investigation of the influence of cathode geometry on deposition quality, for microplasma jet deposition systems in low vacuum. The interest here is understanding the influence of hydrogen on sputtering and/or evaporation of the electrodes. Samples obtained with two cathode geometries with tapered and rectangular cross-sections have been investigated experimentally by scanning electron microscopy and energy dispersion X-ray spectroscopy. Samples obtained with a tapered-geometry cathode present heavy contamination, demonstrating cathode erosion, while samples obtained with a rectangular-cross-section cathode are free from contamination. These experimental characteristics were explained by modelling results showing a larger radial component of the electric field at the cathode inner wall of the tapered cathode. As a result, ion acceleration is larger, explaining the observed cathode erosion in this case. Results from the present investigation also show that the ratio of radial to axial field components is larger for the rectangular geometry case, thus, qualitatively explaining the presence of micro-hollow cathode discharge over a wide range of currents observed in this case. In the light of the above findings, the rectangular cathode geometry is considered to be more effective to achieve cleaner deposition.

Low temperature, atmospheric pressure, direct current microplasma jet operated in air, nitrogen and oxygen

Micro-plasma jets in atmospheric pressure molecular gases (nitrogen, oxygen, air) were generated by blowing these gases through direct current microhollow cathode discharges (MHCDs). The tapered discharge channel, drilled through two 100 to 200 μm thick molybdenum electrodes separated by a 200 μm thick alumina layer, is 150 to 450 μm in diameter in the cathode and has an opening of 100 to 300 μm in diameter in the anode. Sustaining voltages are 400 to 600 V, the maximum current is 25 mA. The gas temperature of the microplasma inside the microhollow cathode varies between ~2000 K and ~1000 K depending on current, gas, and flow rate. Outside the discharge channel the temperature in the jet can be reduced by manipulating the discharge current and the gas flow to achieve values close to room temperature. This cold microplasma jet can be used for surface treatment of heat sensitive substances, and for sterilization of contaminated areas.

Characteristic responses of an atmospheric pressure DC micro-plasma detector for gas chromatography to organic functional groups

A gas chromatographic detector using direct current micro-plasma discharged a distance of less than 50 μm under helium at atmospheric pressure is described. The micro-plasma device was assembled on glass substrates; the volume of the plasma region was only 0.58 nL. Very low power consumption (15 mW) and only carrier helium flow were needed to maintain micro-plasma stability. Organic compounds with various functional groups were chromatographically separated and fed into the micro-plasma. Fluctuations in plasma current were detected as the sample passed through the micro-plasma. All non-halogen organics showed positive peaks, while simple alkyl halides showed negative peaks. Compounds with both aromatic and halogen groups exhibited hybrid peak shapes, consisting of both positive and negative peaks. The detection limits (3 σ) ranged from 24.5 pg for m-xylene to 756 pg for 1,2-dichloroethane, depending on differences in ionization and electron-capture efficiency.

Electron and Ion Kinetics in a DC Microplasma at Atmospheric Pressure

The results of a particle-in-cell Monte Carlo collision (PIC-MCC) simulation of a direct current (DC) helium microplasma that operates at atmospheric pressure are presented. Electron and ion kinetic information that is not available from previous fluid studies is reported. Despite the high collisionality at atmospheric pressure, electrons are found to be in nonequilibrium. Similar to large-scale low-pressure dc discharges, the electron energy probability function (EEPF) in the bulk plasma presents three temperatures near the cathode, and it evolves into a bi-Maxwellian distribution as electrons approach the anode. The bi-Maxwellian character of the EEPF in the elastic energy region is not accounted for in fluid models, and as a result, PIC-MCC simulations predict a lower electron temperature than fluid models. The mean energy of ions that are impinging on the cathode is found to be significantly lower than in low-pressure discharges due to the large collisionality of the sheaths.

Microplasma synthesis of metal nanoparticles for gas-phase studies of catalyzed carbon nanotube growth

Catalytic properties of metal nanoparticles toward gas-phase carbon nanotube (CNT) growth are presented. Narrow dispersions of iron (Fe) and nickel (Ni) nanoparticles are prepared in a direct current microplasma reactor and subsequently introduced with acetylene (C2H2) and hydrogen (H2) into a heated flow furnace to catalyze CNT growth. Aerosol size classification and high-resolution transmission electron microscopy show that CNT growth occurs on Ni particles at lower temperatures than that for similarly produced Fe nanoparticles. Activation energies of 117 and 73kJ∕mol are found for Fe and Ni catalyst particles, respectively, suggesting that CNT growth occurs by carbon surface diffusion.