Optical Emission Spectroscopy Measurements Research Articles

Helium metastable density determination in the COST reference source by absolutely calibrated optical emission spectroscopy

Helium metastable densities in the COST Reference Microplasma Jet are estimated for a variety of He/N2 admixtures and dissipated powers by applying a collisional-radiative model to absolutely calibrated optical emission spectroscopy measurements. This is accomplished by delineating the excitation mechanisms that result in the N2(C–B) and N2+(B–X) emission bands, the latter of which is strongly coupled to the presence of helium metastables. A number of other plasma parameters are established and discussed for each operating condition including the electron energy distribution function, reduced electric field, rate constants, and electron density. With these parameters, the reaction rates for the primary ionization pathways are also calculated, emphasizing the importance of helium metastables for discharge sustainment. Good agreement with the existing literature is found for most plasma parameters and for helium metastable densities, in particular. A clear [N2]−1 relationship between the nitrogen concentration and density of helium metastables is demonstrated, as has been identified in previous studies in analogous atmospheric pressure plasma jets. This validates the efficacy of this optical technique for determining helium metastable densities and establishes it as a viable, and in many cases, more accessible alternative to other means of quantifying helium metastables in low-temperature plasmas.

Quantitative analysis of optical emission spectroscopy for plasma process monitoring

In the field of plasma materials processing, various plasma parameters should be evaluated quantitatively and precisely to control the plasma process adequately, particularly with non-invasive methods, one of which is optical emission spectroscopy (OES) measurement. It has sufficient scientific feasibility to derive the electron density N e, electron temperature T e, and the electron energy distribution function (EEDF) even for various processing plasmas in a state of non-equilibrium. In this review, previous studies are reviewed to measure the N e, T e, and EEDF values of argon plasma with low-electron temperature (T e ≃ 1–10 eV) under not only low-pressure conditions but also atmospheric-pressure discharge using the OES measurement. First, to diagnose low-pressure discharge argon plasmas, we explain the basics and applications of the “collisional radiative model”, which models the population kinetics of the excited states in plasma at the elementary process level in non-equilibrium plasma. Methods for analyzing the plasma parameters are shown from the actual measurement results of emission spectra, including machine learning analysis of the excited-state populations. Next, the research results of the method to measure N e, T e, and EEDF are introduced for the measurement of atmospheric-pressure non-equilibrium plasmas using OES measurement of continuum emission, which also includes methods based on machine learning and data-scientific methods for the analysis of the OES data observed as bremsstrahlung of free electrons scattered against neutral molecules.

Ensuring the Quality of the Analytical Process in a Research Laboratory

This paper discusses approaches for verification of methods of measurements of chemical and physical characteristics of specific samples. The limitations of well-known approaches are discussed. Some examples of alternatives are given to demonstrate specific issues encountered in the research laboratory analyzing new materials or characterizing new properties of materials. Application of sequential procedure using lower quantities of samples and reagents is presented. A standard addition to solid samples is discussed. The approach of control charts for estimation of method uncertainty for determination of plant available phosphorus is presented. The method comparison is applied as an approach to verification of alkaline reactivity by inductively coupled plasma–optical emission spectroscopy (ICP-OES) measurement, as well as density of newly synthesized chalcogenide glass materials. The presented examples demonstrated that alternative approaches are needed in order to verify the methods applied due to the great variety of activities and corresponding tasks in a research laboratory.

Using a coupled optical and electrical monitoring method to follow the R-HiPIMS TiO2 deposition process drifts

In this work, coupled optical and electrical discharge measurements have been implemented to investigate the plasma state of a reactive HiPIMS TiO2 deposition process running at a fixed duty cycle of 2% and at a repetition rate of 1 kHz. Investigations focus on both the effect of the erosion target and substrate-holder temperature in an Ar/O2 gas mixture at fixed working pressure. First, as the racetrack shape evolves with the use of the target, the deposition rate is modified, in the same way as the emission intensity measured. Second, with the heater set at 400 °C in pure Ar, the coating appears thinner, and the optical emission spectroscopy measurement reveals the presence of oxygen atoms. Thus, the results from the coupled optical and electrical measurements are sensitive enough to track potential drift of the process.

Stainless Steel Activation for Efficient Alkaline Oxygen Evolution in Advanced Electrolyzers.

Designing robust and cost-effective electrocatalysts for efficient alkaline oxygen evolution reaction (OER) is of great significance in the field of water electrolysis. In this study, an electrochemical strategy to activate stainless steel (SS) electrodes for efficient OER is introduced. Bycycling the SS electrode within a potential window that encompasses the Fe(II)↔Fe(III) process, its OER activity can be enhanced to a great extent compared to using a potential window that excludes this redox reaction, decreasing the overpotential at current density of 100mA cm-2 by 40mV. Electrochemical characterization, Inductively Coupled Plasma - Optical Emission Spectroscopy, and operando Raman measurements demonstrate that the Fe leaching at the SS surface can be accelerated through a Fe → γ-Fe2O3 → Fe3O4 or FeO → Fe2+ (aq.) conversion process, leading to the sustained exposure of Cr and Ni species. While Cr leaching occurs during its oxidation process, Ni species display higher resistance to leaching and gradually accumulate on the SS surface in the form of OER-active Fe-incorporated NiOOH species. Furthermore, a potential-pulse strategy is also introduced to regenerate the OER-activity of 316-type SS for stable OER, both in the three-electrode configuration (without performance decay after 300h at 350mA cm-2) and in an alkaline water electrolyzer (≈30mV cell voltage increase after accelerated stress test-AST). The AST-stabilized cell can still reach 1000 and 4000mA cm-2 at cell voltages of 1.69 and 2.1V, which makes it competitive with state-of-the-art electrolyzers based on ion-exchange membrane using Ir-based anodes.

Evaluation of Surface Discharge of Polluted Insulator Based on Optical Emission Spectroscopy Measurements

Evaluation of Surface Discharge of Polluted Insulator Based on Optical Emission Spectroscopy Measurements

Frequency-dependent electron power absorption mode transitions in capacitively coupled argon-oxygen plasmas

Phase Resolved Optical Emission Spectroscopy (PROES) measurements combined with 1d3v Particle-in-Cell/Monte Carlo Collisions simulations are performed to investigate the excitation dynamics in low-pressure capacitively coupled plasmas (CCPs) in argon-oxygen mixtures. The system used for this study is a geometrically symmetric CCP reactor operated in a 70% Ar-30% O2 mixture at 120 Pa, applying a peak-to-peak voltage of 350 V, with a wide range of driving RF frequencies (2 MHz ⩽f⩽ 15 MHz). The measured and calculated spatio-temporal distributions of the electron impact excitation rates from the Ar ground state to the Ar 2p1 level show good qualitative agreement. The distributions show significant frequency dependence, which is generally considered to be predictive of transitions in the dominant discharge operating mode. Three frequency ranges can be distinguished, showing distinctly different excitation characteristics: (i) in the low frequency range ( f⩽ 3 MHz), excitation is strong at the sheaths and weak in the bulk region; (ii) at intermediate frequencies (3.5 MHz ⩽f⩽ 5 MHz), the excitation rate in the bulk region is enhanced and shows striation formation; (iii) above 6 MHz, excitation in the bulk gradually decreases with increasing frequency. Boltzmann term analysis was performed to quantify the frequency-dependent contributions of the Ohmic and ambipolar terms to the electron power absorption.

Spatial distribution diagnosis of electron temperature and density of argon inductively coupled plasma by tomographic optical emission spectroscopic measurement and collisional-radiative model

There are few reported cases in which the spatial distribution of spectral emission coefficients of plasmas from tomographic optical emission spectroscopy measurements is analyzed based on a collisional-radiative model to diagnose the spatial distribution electron temperature of Te and density Ne. This study aimed at in situ diagnosis of process plasma. The spectral radiance of 18 lines-of-sight was measured simultaneously in argon inductively coupled plasma. The spatial distribution of the excited level number density distribution was calculated from the spatial distribution of spectral emission coefficients obtained from spectral tomography calculations. The three-dimensional distribution of Te and Ne was analyzed using a collisional-radiative model from the obtained spatial distribution of the excited levels number density. The effects of power and pressure on the dependence of the spatial distribution of Te and Ne were discussed. Furthermore, data processing methods for spectral tomographic measurements with coarse wavelength resolution were also discussed.

Iron oxide/graphene oxide nanocomposite synthesis using atmospheric cold plasma.

Herein, we demonstrate the use of an atmospheric pressure plasma with a Dielectric Barrier Discharge (DBD) for the synthesis of FeOx nanoparticles with a simultaneous formation of graphene oxide domains at low substrate temperature. For that, the interaction of the plasma to control good decomposition of the Fe precursor is essential and this is demonstrated by FTIR analyses. Thanks to a fine tuning of the plasma conditions, a homogeneous spatial distribution around 5 nm nanoparticles (NPs) was obtained, whereas without plasma, in the same configuration of the process, a heterogeneity regarding size and shape for the NPs was obtained. The Raman spectrum of the plasma deposit confirmed the presence of graphene oxide as the characteristic G and D bands were observed with I(D)/I(G) = 0.92. Thanks to optical emission spectroscopy (OES) measurements, it is proposed that the carbon deposition on FeOx nanoparticles is produced on the near plasma post discharge. XPS studies showed that the main contribution of iron was in Fe2+ form, corresponding to the FeO phase. No metallic Fe or carbide were detected. As there are many studies reporting the synergetic effect of FeOx NPs and graphene oxide, we believe that this new one-step simultaneous synthesis method may be of high interest for applications requiring direct deposition on temperature labile substrates such as polymers.

Characterization and Microstructure of Recycled Eroded Particles from Die-Sink Electro Discharge Machining of H11 Alloy for Applicability in Additive Manufacturing

Different waste streams from electro discharge machining (EDM) were investigated for an upcycled usage in processes for additive manufacturing (AM). These erosion sludges accumulate in filter cartridges and at the bottom of machining basins. The enclosed particles were extracted, sieved and investigated via laser diffraction, dynamic image analysis, scanning electron microscopy, optical emission spectroscopy, elemental analysis and flowability measurements. Additionally, thermal, crystallographic and metallographic investigations as well as X-ray micro-computed tomography (µ-CT) were utilized for the characterization of particle and material properties. In general, eroded powders fulfill the requirements for AM regarding particle size and shape very well, which is confirmed in morphological investigations and powder flow characteristics showing similar properties as the H11 AM reference material. The chemical composition of the powders is equal to the machined H11 alloy, except for the high carbon content. Carbon is entrapped in the iron lattice originating from pyrolysis of the present dielectric fluid and the graphite electrode during rapid solidification, which leads to a transition from martensite to cementite structures. This change is observed in the microstructure of powders, in which acicular primary cementite and austenite are present. After remelting with slow heating and cooling rates the microstructure changed to ledeburite II with retained austenite and martensitic phases. The pore size and shape distributions obtained by µ-CT measurements showed a pore formation in the compact sample. These results provide a fundament of major properties as well as handling and recycling suggestions for eroded particles enclosed in waste sludges.Graphical

Three-Dimensional Surface Treatment of MoS2 Using BCl3 Plasma-Derived Radicals.

The realization of next-generation gate-all-around field-effect transistors (FETs) using two-dimensional transition metal dichalcogenide (TMDC) semiconductors necessitates the exploration of a three-dimensional (3D) and damage-free surface treatment method to achieve uniform atomic layer-deposition (ALD) of a high-k dielectric film on the inert surface of a TMDC channel. This study developed a BCl3 plasma-derived radical treatment for MoS2 to functionalize MoS2 surfaces for the subsequent ALD of an ultrathin Al2O3 film. Microstructural verification demonstrated a complete coverage of an approximately 2 nm-thick Al2O3 film on a planar MoS2 surface, and the applicability of the technique to 3D structures was confirmed using a suspended MoS2 channel floating from the substrate. Density functional theory calculations supported by optical emission spectroscopy and X-ray photoelectron spectroscopy measurements revealed that BCl radicals, predominantly generated by the BCl3 plasma, adsorbed on MoS2 and facilitated the uniform nucleation of ultrathin ALD-Al2O3 films. Raman and photoluminescence measurements of monolayer MoS2 and electrical measurements of a bottom-gated FET confirmed negligible damage caused by the BCl3 plasma-derived radical treatment. Finally, the successful operation of a top-gated FET with an ultrathin ALD-Al2O3 (∼5 nm) gate dielectric film was demonstrated, indicating the effectiveness of the pretreatment.

Electron impact fine-structure cross-section calculations of Cu and their applications in the diagnostics of laser-produced Cu-plasma through the collisional radiative model

We present the calculations of electron impact excitation cross-sections for the transitions from the ground state of Cu 3d104sS1/22 to the excited fine structure energy levels of the configurations 3d94s2,3d94s4p,3d104l,(l=p,d,f),3d105l,(l=s,p,d),3d10nl,(n=6,7;l=s,p) using the fully relativistic distorted wave theory. In addition, cross-sections for the transitions from the metastable states 3d94s2D5/2,3/22 to the upper excited levels are also presented. For calculating these cross-sections, we have obtained the atomic wave functions of copper using the multi-configuration Dirac–Fock approximation theory. To ensure the accuracy of these wave functions, we compared our calculated oscillator strengths of some dipole-allowed transitions of Cu with the available earlier reported values. We have utilized all the calculated electron impact fine-structure level excitation cross-sections from the ground as well as the metastable states of Cu to develop a fine structure-resolved collisional-radiative (CR) model for the diagnostics of laser-produced Cu plasma at atmospheric pressure. The model has been applied for diagnostics by coupling it with the spatially resolved optical emission spectroscopy (OES) measurements of Singh and Sharma [Phys. Plasmas 23 122104 (2016)] in the presence and absence of a static magnetic field. For diagnostics, the intensities of three OES observed strong atomic emission lines of Cu viz, 510.5, 515.3, and 521.8 nm have been utilized in the absence and presence of a magnetic field at different axial lengths (z = 0.5–6.5 mm) from the Cu target. Also, these intensities are corrected using the internal reference self-absorption method to overcome the effect of radiation trapping. The normalized intensities of the considered lines obtained from the CR model have been compared with their corresponding OES-measured values to extract the plasma parameters, i.e., electron temperature and electron density. Further, the obtained electron temperatures from the CR model are compared with the corresponding values reported in the experimental paper of Singh and Sharma [Phys. Plasmas 23 122104 (2016)] obtained using the Boltzmann plot method.

Radial dependence diagnosis of inductively coupled Ar plasma based upon optical emission spectroscopic measurement with spectral reflectance compensation

This study proposed a simple method to evaluate the spectral reflectance of the inner wall of a vacuum chamber. A method for calculating spectral emission coefficients by taking the spectral reflectance of the chamber inner wall into account was proposed. Furthermore, plasma diagnosis based on optical emission spectroscopic (OES) measurement was performed so as to obtain radial dependence of electron temperature Te and density Ne of a radio frequency inductively coupled Argon (Ar) plasma by applying a collisional-radiative model to radially resolved emission spectra of the Ar plasma assuming axial symmetry. In addition, Langmuir probe measurement and electromagnetic simulation were performed and compared with the OES-based plasma diagnosis results. The spectral radiance compensation improved the diagnostic result by 0.6% and 3.1% for Te and Ne, respectively.

Breakdown and quasi-DC phase of a nanosecond discharge: Comparison of optical emission spectroscopy measurements with numerical simulations

A nanosecond atmospheric pressure plasma jet operated in pure nitrogen is studied by spatially and temporally resolved optical emission spectroscopy complementing the companion paper (Kuhfeld et al 2023 Plasma Sources Sci. Technol. 32 084001), where the discharge is investigated by means of Particle-in-Cell/Monte Carlo collisions (PIC/MCC) simulations and fluid models. Two temporal phases of the evolution of the discharge are identified: a fast breakdown and a quasi-DC phase. It is shown that during the breakdown phase several ionization waves develop, while after the breakdown the discharge has a structure similar to DC glow discharges, in agreement with the modeling predictions. The results of the measurements of the spatial-temporal dynamics of the light emission are compared with the distribution of densities of the and states reconstructed from the PIC/MCC simulations. A good agreement is demonstrated.

Non-invasive optical characterization and estimation of Zn porosity in gas tungsten arc welding of Fe–Al joints using CR model and OES measurements

In this study, we employed a non-invasive approach based on the collisional radiative (CR) model and optical emission spectroscopy (OES) measurements for the characterization of gas tungsten arc welding (GTAW) discharge and quantification of Zn-induced porosity during the GTAW process of Fe–Al joints. The OES measurements were recorded as a function of weld current, welding speed, and input waveform. The OES measurements revealed significant line emissions from Zn-I in 460–640 nm and Ar-I in 680–800 nm wavelength ranges in all experimental settings. The OES coupled CR model approach for Zn-I line emission enabled the simultaneous determination of both essential discharge parameters i.e. electron temperature and electron density. Further, these predictions were used to estimate the Zn-induced porosity using OES-actinometry on Zn-I emission lines using Ar as actinometer gas. The OES-actinometry results were in good agreement with porosity data derived from an independent approach, i.e. x-ray radiography images. The current study shows that OES-based techniques can provide an efficient route for real-time monitoring of weld quality and estimate porosity during the GTAW process of dissimilar metal joints.

A novel N-heterocyclic carbene-palladium modified metal–organic framework (Sm-MOF-Pd) for Suzuki-Miyaura cross-coupling reaction

A novel 3D Sm-MOF, namely {n[Sm(L)2·2(H2O)]·(Cl)n, H2L+Cl- = 1,3-bis(4-carboxybenzyl)-imidazolium chloride)}, was prepared under hydrothermal conditions and structurally characterized by single-crystal X-ray diffraction. Then the N-heterocyclic carbene-Pd(II) functionalized metal–organic framework (Sm-MOF-Pd) was yielded by introducing Pd active sites into the Sm-MOF by the post-synthesis modification (PSM) method. This new material was characterized by powder X-ray diffraction (PXRD), thermogravimetric analyses (TGA), scanning electron microscopy (SEM), transmission electron (TEM), energy-dispersive X-ray spectroscopy (EDS), inductively coupled plasma optical emission spectroscopy (ICP-OES) and X-ray photoelectron spectroscopy (XPS) measurements, showing that Pd(II) ions were uniformly dispersed in the Sm-MOF after PSM. The Sm-MOF-Pd reached a catalytic yield of 99 % in Suzuki-Miyaura cross-coupling reactions under optimal reaction conditions. After four cycles, it still maintained high catalytic activity.

Electron impact cross-sections and rate coefficients of Si+2 ions relevant to collisional radiative modeling of silicon plasma

We report on the fine structure resolved electron impact excitation (EIE) cross-sections of Si+2 from its ground state 3s 2 (J= 0) to the 41 excited fine structure levels of the configurations 3s3p, 3p 2, 3s3d, 3s4s, 3s4p, 3s5s, 3s4d, 3s4f, 3s5p, 3s5d and 3s5f using relativistic distorted wave theory. The excitation cross-sections from the excited metastable levels (3P0,3P2) of the configuration 3s3p to higher excited levels as well as for some other dominant transitions relevant to plasma modeling are also obtained. In addition, the ionization cross-sections are evaluated from the ground and metastable levels to higher ionized state Si+3 (2S1/2). The calculated cross-sections are utilized to obtain the rate coefficients corresponding to EIE and ionization processes affecting the intensity of prominent Si+2 emission lines 379.61, 380.65, 456.78, and 457.48 nm recorded through optical emission spectroscopic measurements by Wang et al (2020 Phys. Plasmas 27 063513) on laser produced silicon plasma. Further, the rate coefficients corresponding to radiative, and three body recombination are also presented. The reported cross-sections and rate coefficients will be useful for developing rigorous collisional radiative model for the diagnostics of silicon plasma.

Plasma parameter measurement on a RIT-10 using empirical correlations between non-invasive optical emission spectroscopy and Langmuir diagnostics

Characterising and understanding the plasma properties of a rf-coupled electric propulsion device is crucial during testing, qualification and development. Therefore, the optimization of existing diagnostic systems as well as the development of new ones is an important area of electric propulsion research. Here, we present an approach to non-invasively determine the plasma parameters of an operating radio-frequency ion-thruster. For this purpose, a correlation between non-invasive optical emission (OE) spectroscopy and intrusive Langmuir probe diagnostics measurements is established for a reference system. Both types of measurements are performed simultaneously for a wide range of operation points yielding a large reference data set. Based on a principal component analysis (PCA), a correlation between plasma parameters and corresponding OE spectra at different operational points is established. This correlation can then be applied to OE spectra of the plasma of an operating thruster to obtain non-invasively the corresponding plasma parameters, i.e., without having to employ intrusive Langmuir probes. This approach for evaluating optical spectroscopic data in terms of plasma parameters has no need for a theoretical microscopic modeling of the plasma. This makes this approach very versatile and easily transferable to cases where other propellants are used, since no knowledge of excitation cross sections or transition matrix elements and other microscopic parameters of the species of the plasma is required. Such an approach enables continuous monitoring of a thruster’s behavior during the qualification process.

Partial EEDF analysis and electron diagnostics of atmospheric-pressure argon and argon–helium DBD plasma

Partial arbitrary electron energy distribution function (EEDF) results for cold atmospheric-pressure plasma are reported. The EEDF is obtained using the visible bremsstrahlung inversion (VBI) method. This machine learning method requires only optical emission spectroscopy (OES) measurement and a momentum transfer cross section to determine a partial EEDF. Numerical EEDF of a pure-argon dielectric barrier discharge dataset with changing peak-to-peak voltage and a helium–argon discharge with changing mixture ratio are reported. Resemblance between the numerical EEDF and a two-temperature Maxwell distribution is observed and a simplified three-point numerical EEDF is obtained. The electron temperature and relative electron number density for the bulk and high-energy electron populations are measured. The bulk electron temperature was consistently 0.3 eV. For pure argon, the high-energy electron temperature decreased exponentially from 3 to 2.2 eV with increasing peak-to-peak voltage from 3.6 to 6.3 kV. For the helium–argon dataset, the high-energy electron temperature decreased linearly from 4.2 to 2.2 eV with increasing argon fraction 25%–100%. From an OES measurement, the arbitrary EEDF can be observed by utilization of the VBI method. Based on this numerical EEDF, appropriate assumptions can be applied to simplify the quantification of electron diagnostics.

N-doping FeNi@C(Nx) core-shell nanoparticles synthesized by arc plasma as a highly efficient bifunctional electrocatalyst for all-solid zinc-air batteries

Applications of zinc-air batteries (ZABs) are generally hampered by limited efficiency and poor stability of oxygen reduction reaction (ORR) and oxygen evolution reaction (OER) at the air cathode. The key to solving the problem is the highly efficient bifunctional electrocatalyst. Herein, the bifunctional electrocatalyst of nitrogen-doping carbon-coated FeNi (FeNi@C(Nx)) core-shell nanostructures was fabricated by one-step physical vapor evaporation technology of DC arc plasma and subsequent -nitrogen treatment. The energy state in the synthesis of FeNi@C precursor was monitored by a real-time optical emission spectroscopy (OES) measurement. The obtained FeNi@C(N15) nanoparticles was proved to be an excellent electrocatalyst for both ORR and OER processes, the OER overpotential is 326 mV which is better than that of RuO2 (405 mV) and the ORR catalytic reaction is close to the typical 4e− process. It is indicated that all-solid Zn-air battery assembled with this FeNi@C(N15) electrocatalyst provides a high open circuit voltage of 1.394 V, and a high peak power density of 59.7 mW cm−2. The synergistic effect between the bimetallic FeNi core and the N-doped carbon shell is responsible for the bifunctional activities, and such a core-shell nanostructure can prevent the catalyst from corrosion in working. This study provides a new approach for the design and preparation of transition metal alloys nanostructures encapsulated into the carbon-shell with quantitative nitrogen-doping, such a core-shell structured nanocapsule has excellent potential as the electrochemical bifunctional catalyst in some renewable energy devices.