Atomic Nitrogen Density Research Articles

Development and diagnostic study of the RF nitrogen atom source

With the development of thin film technology, particularly the increasing demand for research on group III nitride materials, the need for N ions has significantly increased. A wider and more efficient RF nitrogen atom source specifically designed for nitride film growth has been developed. To evaluate the nitrogen atom density from the RF nitrogen source, a global model of RF nitrogen gas discharge plasma was created. This model was calibrated by measuring the emission spectrum of RF nitrogen gas discharge, and the relationships between nitrogen atom density and actual working pressure, power, and discharge chamber size were determined. It was found that the nitrogen atom density increases with increasing power and initially increases with pressure before decreasing. Finally, the nitrogen atom density was determined, achieving an 18 % nitrogen atom dissociation rate and a nitrogen atom density of 5 × 1012/cm³ at an RF power of 400 W. This density is significantly higher than that of nitrogen ions, indicating that this is a nitrogen atom source with high purity.

Absolute atomic nitrogen density spatial mapping in three MHCD configurations

In this work, nanosecond two-photon absorption laser induced fluorescence (TALIF) is used to perform spatial mappings of the absolute density of nitrogen atoms generated in a micro-hollow cathode discharge (MHCD). The MHCD is operated in the normal regime, with a DC discharge current of 1.6 mA and the plasma is ignited in a 20% Ar/ 80% N2 gas mixture. A 1-inch diameter aluminum substrate, acting as a third electrode (second anode), is placed further away from the MHCD to emulate a deposition substrate. The spatial profile of the N atoms is measured in three MHCD configurations. First, we study a MHCD having the same pressure (50 mbar) on both sides of the anode/cathode electrodes and the N atoms simply diffuse in three dimensions from the MHCD. The recorded N atoms density profile in this case satisfies our expectations, i.e. the maximal density is found at the axis of the hole, close to the MHCD. However, when we introduce a pressure differential, thus creating a plasma jet, an unexpected N atoms distribution is measured with maximum densities away from the jet axis. This behavior cannot be simply explained by the TALIF measurements. Then, as a first simplified approach in this work, we turn our attention to the role of the gas flow pattern. Compressible gas flow simulations show a correlation between the jet width and the radial distribution of the N atoms at different axial distances from the gap. Finally, a DC positive voltage is applied to the third electrode (second anode), which ignites a micro cathode sustained discharge (MCSD). The presence of the pressure differential unveils two stable working regimes depending on the current repartition between the two anodes. The MCSD enables an homogenization of the density profile along the surface of the substrate, which is suitable for nitride deposition applications.

The effect of gas composition on the properties of silicon oxynitride thin film prepared by low-pressure inductively coupled Ar/N2 plasma

Silicon nitride (Si3N4) has been widely used in semiconductor devices and applications, including the passivation layer in organic light-emitting diode (OLED). However, it becomes more important to adjust the refractive index value of silicon nitride layers. For example, the refractive index difference between the passivation layer and emission layer is one of the problems degrading OLED performance. Recently, silicon oxynitride (SiOxNy) thin films possessing different refractive indexes have been fabricated by varying their nitrogen and oxygen content. In this work, we control the refractive index of SiOxNy thin film by changing the N2 and Ar flow ratio in 13.56 MHz radio-frequency inductively coupled plasma (RF-ICP). To study the effects of gas composition in the plasma on the properties of thin films, nitrogen atom (N) and nitrogen molecular ion (N2+) density are investigated using actinometry and simulation via the synthetic spectrum method, respectively. We observed that when the gas ratio defined as [N2/(N2+Ar)] increases from 0.1 to 0.9, the density of N2+ increases from 4.76×1015m − 3 to 1.71×1016m − 3 and the refractive index of the thin film increases from 1.54 to 1.57. The results indicate that N2+ plays the important role in changing the properties of thin film during the plasma process.

Evaluation of isocyanate functionalization ratio on multi-walled carbon nanotubes during N2/CO2 plasma treatment

We recently reported how effective and important carbon nanotubes with isocyanate groups after plasma treatment are to polyimide composite. This article is the following report showing the monitoring method for the isocyanate functionalization by plasma treatment on carbon nanotubes using optical emissions from nitrogen and oxygen atoms. We measured the functionalization ratio on the nanotubes with the fluorescent measurement and measured the number density in the gas phase with plasma actinometry. The number of the functionalized nanotubes became maximized when the plasma contained almost the same number density of nitrogen atoms as the number density of oxygen atoms in the gas phase. Our plasma utilized in this article reached this condition when the gas mixture of nitrogen and carbon dioxide had 90% of nitrogen content. These overall results indicated that the functionalization during plasma processing can be evaluated with the optical emission from the plasma.

Absolute N-atom density measurement in an Ar/N2 micro-hollow cathode discharge jet by means of ns-two-photon absorption laser-induced fluorescence

In this work, nanosecond two-photon absorption laser-induced fluorescence (TALIF) is used to probe the absolute density of nitrogen atoms in a plasma generated using a micro-hollow cathode discharge (MHCD). The MHCD is operated in the normal regime, and the plasma is ignited in an Ar/N2 gas mixture. First, we study a MHCD configuration having the same pressure (50 mbar) on both sides of the electrodes. A good agreement is found between the density of N atoms measured using TALIF in this work and previous measurements using vacuum ultraviolet Fourier transform absorption spectroscopy. Then, we introduce a pressure differential between the two electrodes of the MHCD, creating a plasma jet. The influence of the discharge current, the percentage of N2 in the gas mixture, and pressures on both sides of the MHCD is studied. The current has a small impact on the N-atom density. Furthermore, an optimal N-atom density is found at around 95% of N2 in the discharge. Finally, we demonstrate that the pressure has a different impact depending on the side of the MHCD: the density of N atoms is much more sensitive to the change of the pressure in the low-pressure side when compared to the pressure change in the high-pressure side. This could be due to several competing phenomena: gas residence time in the cathodic region, recirculation, or recombination of the N atoms at the wall. This study contributes to the optimization of MHCD as an efficient N-atom source for material deposition applications.

Strategies for enhancing hole utilization on organic-POM hybrid materials and photocatalytic degradation of neonicotinoid insecticides

Ammonium phosphotungstate/pyromrllitic diimide (NH4PW/PMDI) composite photocatalysts were prepared using organic amines and ammonium respectively. The characterization results showed that although the chemical structure and chemical environment of each element were the same, the NH4PW/PMDI composite photocatalysts showed differences in photoelectric properties, which was caused by their different morphologies. The source of organic amines and ammonium may influence the distribution of NH4PW in the NH4PW/PMDI. The NH4PW on the surface can improve the light absorption ability, while the NH4PW on the inside can inhibit the recombination of electron and hole. And imidacloprid can be adsorbed at the surface of NH4PW through hydrogen bonding between nitrogen atoms and ammonium, which increases the electron density of nitrogen atoms that is conducive to the attack of photogenerated holes. The degradation rate constants of 15UNPWPI-325 for imidacloprid, thiamethoxam and acetamiprid were 2.8488, 1.4784, and 0.9372 h−1, respectively.

Sp-nitrogen and γ-ray modulating multiply γ-graphyne for anchoring Pt nanoparticles to boost oxygen reduction activity and stability

γ-Graphyne (GY) is a potential carbon support for anchoring platinum nanoparticles (Pt NPs) in oxygen reduction reaction (ORR). However, the high oxygen penetration barrier of primitive GY intercepts its application for ORR. Herein, based on the stronger negative charge density of sp-hybridized nitrogen (sp-N) atoms and the regulation of γ-ray irradiation on carbon nanostructure, a Pt/N50-GY catalyst with a multi-level pore structure, larger specific surface area (twice that of original GY) and more Pt NPs anchor sites was fabricated by sp-N doping and γ-ray irradiation, which reduced the oxygen penetration barrier of original GY, exposed more active sites, shortened ion migration path and accelerated mass transfer, thus improving the ORR performance. The ORR activity of Pt/N50-GY catalyst is superior to that of the commercial Pt/C and even more outstanding when compared with the mostly reported graphdiyne related and noble meal catalysts under lower catalyst loading. Additionally, the Pt/N50-GY catalyst showed superior stability to the commercial Pt/C. Our work breaks the bottleneck that GY can not be applied to ORR, and provides a breakthrough way for the application of GY in ORR.

Ammonia-free epitaxy of single-crystal InN using a plasma-integrated gas-injection module

Group III nitride semiconductors are used for optical, high-frequency electronic, and power devices. In particular, indium nitride (InN) and In-rich indium gallium nitride (InGaN) are attractive materials for a wide range of carbon-neutral and post-5 G (5th generation mobile communication system) applications. However, the growth of high-quality bulk single crystals of both InN and In-rich InGaN cannot be ideally achieved owing to a mismatch between the low evaporation temperature of indium and the high temperature required to provide an adequate supply of reactive nitrogen species via the thermal decomposition of ammonia gas. In this study, we obtained a very-high-quality bulk InN material via epitaxial growth on a GaN template with the lowest dislocation density (∼3 × 109 cm–2) reported to date, and a narrow room-temperature full-width-at-half-maximum (∼0.1 eV) of the photoluminescence peak at 0.687 eV. These were achieved using a newly developed method obtained by combining microstrip-line microwave plasma irradiation with metal organic chemical vapor deposition. In this method, trimethylindium gas and pure nitrogen plasma (for producing a high density of nitrogen atoms) were supplied to a template independently, simultaneously, and continuously. We believe that the proposed procedure can serve as a standard guideline for non-toxic, low-energy-consumption, and low-cost manufacturing of numerous nitride-compound semiconductors.

Time evolution of atomic nitrogen density in pure-nitrogen-pulsed barrier discharge at sub-atmospheric pressure

Ground state atomic nitrogen N(2p34S) was analyzed using two-photon absorption laser-induced fluorescence (TALIF) in sub-atmospheric pressure nitrogen pulsed barrier discharge under needle-to-hemisphere electrode configuration. By reducing the pressure from 90 to 30 kPa, the discharge form transitioned from multiple filaments to a single column, improving the reacting region uniformity. The TALIF measurement revealed that the amount of atomic nitrogen near the needle anode increased over tens of microseconds after the discharge, and this N-production during afterglow was enhanced by reducing the pressure. Reducing the pressure from 90 to 30 kPa extended the half-life period of atomic nitrogen near the anode by 350 μs, while maintaining the peak amount of atomic nitrogen. The lifetime extension with the same amount of atomic nitrogen helped improving the chemical activity near the anode. The origin of the N-production during afterglow was not identified as a single factor, but its time constant indicated the contribution of N(2P) quenched by the ground state atomic nitrogen, along with the quenching of N(2D), which was previously considered as a major source of afterglow production of the ground state atomic nitrogen. Under 30 kPa, higher discharge energy resulted in faster and larger amount of atomic nitrogen production during afterglow, which indicates the involvement of highly excited particles including metastable atomic nitrogen. In contrast, the decay rate of atomic nitrogen did not depend on the discharge energy. This suggests that the increasing discharge energy broadens the N-productive region while maintaining the local N density.

Numerical study of nitrogen bulk dissociation in DC magnetron discharge in Ar+N2 mixture

The effect of the composition of the gas feed mixture on the dissociation rate constant of molecular nitrogen is examined by numerical solving the Boltzmann equation written in a two-term approximation for conditions corresponding to nitride aluminum films deposition. By solving of macroscopic balance equations in volume occupied with plasmas of stationary low-pressure glow discharge in a nitrogen-argon mixture the atomic nitrogen density was determined. It’s shown that there is a maximum of atomic nitrogen density at certain stoichiometry of gas feed mixture.

Effect of plasma-species to functionalize isocyanate-groups on multiwalled carbon nanotubes

This article reports our recent investigation of the relationship between the density of the active species in nitrogen-oxygen plasma and the degree of isocyanate (R-NCO) functionalization onto multiwalled carbon nanotubes. We first monitored the plasma active species with optical emission spectroscopy focusing on atomic nitrogen and oxygen species, which are the major components to form NCO groups. The optical emission intensity allowed us to find the density of atomic nitrogen and oxygen in plasma through the plasma optical actinometry method. The actinometry result showed that the density of atomic nitrogen and oxygen becomes as close as the nitrogen content in plasma increased to 90%. We then investigated the functionalization ratio with the fluorescent method and Raman spectroscopy to monitor the degree of NCO functionalization. The results suggested that NCO groups on carbon nanotubes should increase as the content ratio of atomic nitrogen and oxygen becomes equal in the plasma.

A pacs-type metal-organic framework based on [Cd3(OH)] clusters for effective C2H2/CO2 separation and fluorescent detection of TNP in water

Metal-organic frameworks (MOFs) have become a versatile platform for gas capture/separation and luminescent sensing of toxic pollutants, and the combination of both of these properties depends on the robust framework and pre-designed pore functionality which is critical to achieve the targeted applications. In this work, a novel Cd(II)-MOF {(NH2Me2)[Cd3 (μ3-OH) (tpt) (TZB)3](H2O)2 (1,4-dioxane)5}n (1, H2TZB ​= ​4-(1H-tetrazol-5-yl)benzoic acid, tpt ​= ​2,4,6-tri (pyridin-4-yl)-1,3,5-triazine) comprising of a {Cd3(OH)}5+ cluster-based unit (SBU) has been solvothermally formed from nitrogen-rich and highly fluorescent organic ligands. The porous structure 1 is composed of cages and segments which are affixed by high density of free nitrogen atoms. The activated Cd(II)-MOF (1a) exhibits a relative high C2H2 sorption capacity than that of CO2 which endows it with a C2H2/CO2 sorption selectivity of 7.26 ​at room temperature. Furthermore, complex 1 could behave as a luminescent sensor for highly discriminative detection of explosive 2,4,6-trinitrophenol (TNP) via remarkable fluorescence quenching (Ksv ​= ​6.16✕104 ​M−1).

Atomic nitrogen density measurements by actinometry method in the toroidal device NAGDIS-T

Nitrogen (N) atomic density is measured by an actinometry method in the toroidal shaped plasma device NAGDIS-T, where the plasma produced in a DC arc discharge is cooled as spiraling down inside the vacuum vessel. The plasma was characterized with an electrostatic probe, and we applied the actinometry method to obtain the N density using the deduced electron density and temperature. The actinometry method used the Ar line emissions at 750.4 or 811.5 nm and N emission at 746.8 nm. The measured N densities from the actinometry was compared with that from the vacuum ultraviolet absorption spectroscopy.

Measurements of nitrogen atom density in a microwave‐excited plasma jet produced under moderate pressures

Using a microwave‐excited plasma source based on a microstrip line, we have generated a nitrogen plasma jet at a moderate pressure in the range from 1 to 10 kPa. The densities of nitrogen (N) atoms produced by the plasma jet were measured with a vacuum ultraviolet absorption spectroscopy. The results show that the plasma jet is able to provide a high density of N atoms at least 4.5 × 1014 cm−3 at 1.5 kPa. The N atom densities vary widely with the change in gas flow rate and substrate placement. We expect that this plasma source will provide a high performance as an advanced N radical source in various applications. © 2020 Institute of Electrical Engineers of Japan. Published by Wiley Periodicals LLC.

TALIF measurements of atomic nitrogen in the afterglow of a nanosecond capillary discharge

The atomic nitrogen (N) density in a nanosecond pulse capillary discharge is measured using two-photon laser induced fluorescence. The capillary discharge is favored for its unique combination of both large reduced fields (E/N) and high specific deposited energies. Under such conditions, we find that a pure nitrogen (N2) capillary discharge at a pressure of 27 mbar and initial temperature of about 300 K, produces a peak N-atom density of 1.29 × 1017 cm−3, corresponding to an extremely high dissociation degree of about 10%. The time evolution of the N-atom density is tracked from a few hundred ns after discharge initiation, up to several ms when the concentration of N-atoms falls below the detection limit. The temporal evolution curve exhibits a trapezoidal-like shape, characterized by an initial rise in the N-atom density up to a few μs, followed by a relatively flat and constant profile until about 1 ms, and finally terminating with a drop to near detection limits at about 10 ms. The high electron densities (≈1015 cm−3) and efficient production of electronically excited states associated with this type of discharge is found to have a profound effect on the consequent kinetics. A process of stepwise dissociation through electron impact of the ( ) excited states is examined and proposed as a possible explanation for the unusually high energy efficiency of N-atom production. The present study shows that the capillary discharge is an extremely effective source of N-atoms, and forms the impetus for continued study of discharges with both high levels of specific deposited energies (≥1 eV/molecule) and reduced electric fields (E/N ≥ 150 Td).

Low temperature plasma-assisted nitriding of austenitic steel in a ferromagnetic enhanced inductive discharge

Plasma-assisted nitriding of austenitic stainless steel AISI 321 has been performed with the use of a ferromagnetic enhanced inductive discharge as the source of nitrogen ions and radicals, in the temperature range of 300–400°C, gas pressure of 50 mTorr and ion densities of about (2.5–7.7)·1010 cm−3. A significant increase in the nitriding rate was achieved, in comparison with a low temperature glow-discharge nitriding process. A possible mechanism of the nitriding rate increase is a high atomic nitrogen density achieved in the ferromagnetic enhanced inductively coupled plasma.

Optimizing the density of nitrogen implantation for generating high-density NV center ensembles for quantum sensing

We prepared four NV center ensembles with different nitrogen atom doses at the same diamond by ion implantation. The densities of NV centers in the samples were analyzed using their fluorescence intensities and PL spectra, which increases with the implantation dose and reaches a maximal value at a dose of 1014 cm−2, corresponding to the nitrogen density of 3.1 × 1019 cm−3. Ramsey measurement showed that no significant difference was found in the dephasing time between the four NV− center ensembles. The results suggest that the quantum sensor utilizing NV− ensemble centers can achieve its best sensitivity when the density of nitrogen atoms in diamond is about 3.1 × 1019 cm−3.

Experimental characterization of a ns-pulsed micro-hollow cathode discharge (MHCD) array in a N2/Ar mixture

An array of seven micro-hollow cathode discharges (MHCDs) operating in a N2/Ar mixture and ignited by a nanosecond pulsed high voltage is experimentally characterized in this work. Compared to other non-equilibrium discharges, high electron densities can be obtained with this type of microdischarge, and, thus, a high dissociation degree of the molecules is expected to be achieved. This is particularly suited for nitride deposition, which is the targeted application of this work, given that the atomic nitrogen density is a key parameter for this deposition process. The electrical diagnostics reveal that the energy deposited in the plasma during one voltage pulse increases with the Ar fraction in the gas mixture and the voltage pulse repetition frequency. A small effect of the repetition frequency is observed on the voltage and current characteristics of the discharge, while a significant increase in the amplitude of these two electrical parameters is observed with increasing Ar admixture. Consequently, a higher electron density is expected with increasing Ar fraction in the gas mixture and repetition frequency. Time-integrated optical emission spectroscopy is carried out to identify the excited emissive species in the plasma and to determine the time-averaged vibrational and rotational temperatures. For a discharge operating in pure N2, at a repetition frequency of 40 kHz, the time-averaged rotational temperature is found to be 390 K ± 40 K and the time-averaged vibrational temperature is 3580 K ± 200 K, revealing the non-equilibrium nature of the discharge. Both temperatures increase with the Ar fraction in the N2/Ar mixture, while they are almost independent of the repetition frequency. Finally, the different phases of the spatio-temporal evolution of the discharge are mapped using space-time resolved ICCD images. The spatial evolution of the discharge between two consecutive voltage pulses can be decomposed in four phases, corresponding to the different stages of the temporal evolution of the discharge voltage.

Validation cases for recombining nitrogen and air plasmas

We report on Raman scattering and optical emission spectroscopy (OES) measurements in recombining atmospheric pressure plasmas of air and nitrogen. An inductively coupled plasma torch is used to create an equilibrium plasma, which is then forced to rapidly recombine by flowing through a water-cooled tube. For all conditions, temperature measurements are performed using OES and Raman scattering at the exit of tubes of varying lengths. The density of atomic nitrogen is also determined. Evidence of strong chemical nonequilibrium is found in a number of cases. For these cases, we observe that the rotational temperatures measured with OES differ from those measured with Raman scattering, and that the atomic nitrogen density is elevated with respect to equilibrium. A power balance analysis confirms that a large fraction of gas enthalpy is stored in the non-recombined nitrogen atoms. For cases where the plasma remains in equilibrium, we perform numerical simulations using the Eilmer3 computational fluid dynamics (CFD) code. Eilmer3 does not predict the observed drop in gas temperature measured using Raman scattering and OES. Prior efforts by the CFD community have also failed to correctly predict this temperature drop. The results presented in this paper are therefore intended as validation test cases for CFD simulations.

Highly efficient iodine capture by task-specific polyethylenimine impregnated hypercrosslinked polymers

Abatract The primary challenge in environmental remediation is keeping the balance between performance and economy for practical application. The usage of hypercrosslinked polymers as iodine capture adsorbents overcomes the shortcomings of commonly used porous organic polymers adsorbents including the usage of expensive catalysts or monomers and complicated synthesis procedures. This issue is the first attempt to prepare polyethylenimine impregnated hypercrosslinked polymers with a high density of nitrogen atoms designed for iodine capture. The resulting polymers exhibited ultrahigh iodine uptake capacities up to 607 wt% and the capture process fitted the pseudo-second-order model well. Porous characteristics, electron-rich aromatic networks, and abundance nitrogen atoms in polyethylenimine contributed to the high capture capacity. Results indicated that the interactions between iodine and adsorbents were charge transfer and the form of captured iodine was polyiodide. Furthermore, the polymers could be recycled with 93.4% iodine uptake capacity maintained after four cycles. The work presented a new benchmark for using hypercrosslinked polymers as economical materials for iodine capture.