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PTDC/NAN-MAT/30565/2017 D01-284/2019 (INFRAMAT) IBB BASE 2020-2023 UID/FIS/00068/2019. The ability to change the secondary electron emission properties of nitrogen-doped graphene (N-graphene) has been demonstrated. To this end, a novel microwave plasma-enabled scalable route for continuous and controllable fabrication of free-standing N-graphene sheets was developed. High-quality N-graphene with prescribed structural qualities was produced at a rate of 0.5 mg/min by tailoring the high energy density plasma environment. Up to 8% of nitrogen doping levels were achieved while keeping the oxygen content at residual amounts ( 1%). The synthesis is accomplished via a single step, at atmospheric conditions, using ethanol/methane and ammonia/methylamine as carbon and nitrogen precursors. The type and level of doping is affected by the position where the N-precursor is injected in the plasma environment and by the type of precursors used. Importantly, N atoms incorporated predominantly in pyridinic/pyrrolic functional groups alter the performance of the collective electronic oscillations, i.e. plasmons, of graphene. For the first time it has been demonstrated that the synergistic effect between the electronic structure changes and the reduction of graphene $-plasmons caused by N doping, along with the peculiar “crumpled” morphology, leads to sub-unitary (textless 1) secondary electron yields. N-graphene can be considered as a prospective low secondary electron emission and plasmonic material. publishersversion published

An experimental and theoretical investigation on microwave plasma-based synthesis of free-standing N-graphene, i.e., nitrogen-doped graphene, was further extended using ethanol and nitrogen gas as precursors. The in situ assembly of N-graphene is a single-step method, based on the introduction of N-containing precursor together with carbon precursor in the reactive microwave plasma environment at atmospheric pressure conditions. A previously developed theoretical model was updated to account for the new reactor geometry and the nitrogen precursor employed. The theoretical predictions of the model are in good agreement with all experimental data and assist in deeper understanding of the complicated physical and chemical process in microwave plasma. Optical Emission Spectroscopy was used to detect the emission of plasma-generated &lsquo, &lsquo, building units&rsquo, &rsquo, and to determine the gas temperature. The outlet gas was analyzed by Fourier-Transform Infrared Spectroscopy to detect the generated gaseous by-products. The synthesized N-graphene was characterized by Scanning Electron Microscopy, Raman, and X-ray photoelectron spectroscopies.

Hybrid graphene-based nanostructures are considered promising materials for energy storage applications. However, the synthesis of high-quality hybrid graphene nanostructures at high yields is challenging. In the present work we propose a novel, single-step microwave plasma-enabled approach to synthetize customizable hybrid graphene-based nanostructures at high-yield while preserving their quality. Hybrid N-graphene (nitrogen-doped graphene) metal-based nanostructures, for instance, can be produced at a rate of ∼ 19 mg/min. The high energy density region of a microwave plasma provides sufficient energy and “building particles” fluxes towards the low-energy density plasma afterglow for the processes of assembly and growth of N-graphene sheets. Simultaneously, a controlled jet of metal-oxide(-sulfide) microparticles is sprayed into the plasma afterglow region where they bind to N-graphene sheets. Methane/methylamine are used as carbon and nitrogen precursors, combined with micron-sized MnO2 and oxy-MnS particles to synthesize the hybrid structures. As a result, nano-sized (∼10–30 nm) MnOx particles decorated N-graphene (4.6 at. N%) and oxidized metal sulfide anchored N-graphene sheets (3.1 at. N%) are produced at atmospheric conditions. High structural quality and distribution of metal-based nanostructures on N-graphene sheets are revealed using transmission and scanning electron microscopes and other advanced spectroscopic techniques. Finally, an electrode for supercapacitor based on the N-graphene-metal-oxide(sulfide) hybrid nanostructures is developed with promising specific capacitances (∼273 F.g−1 at 0.5 A.g−1). The described chemically engineered process is one of the fastest approaches reported for designing the high-quality hybrid nanostructures produced at a high-yield, and as such, is expected to provide a high impact on the design of electrode materials for sustainable energy storage systems.

Free-standing N-graphene was synthesized using a microwave plasma-based method at atmospheric pressure conditions through a single step and in a controllable manner. Using ethanol and ammonia as precursors, N-graphene with low relative amount of bonded oxygen and low level of saturated sp3 carbon bonds was produced. Adjusting the injection position of the nitrogen precursor in the plasma medium leads to selectivity in terms of doping level, nitrogen configuration and production yield. A previously developed theoretical model, based on plasma thermodynamics and chemical kinetics, was further updated to account for the presence of nitrogen precursor. The important role of HCN attachment to the graphene sheets as the main process of N-graphene formation is elucidated. The model predictions were validated by experimental results. Optical Emission Spectroscopy was used to detect the emission of plasma generated “building units” and to determine the gas temperature. The plasma outlet gas was analyzed by Fourier-Transform Infrared Spectroscopy to detect the generated gaseous by-products. The synthesized N-graphene was characterized by Scanning Electron Microscopy, Raman and X-ray photoelectron spectroscopies.

Vacuum ultraviolet (VUV) radiation emitted by microwave driven argon plasmas has been investigated at low-pressure conditions (0.36 mbar). A classical surface-wave sustained discharge at 2.45 GHz has been used as plasma source. VUV radiation has been detected by emission spectroscopy in the 30–125 nm spectral range. The spectrum exhibits atomic and ionic argon emissions with the most intense spectral lines corresponding to the atomic resonance lines, at 104.8 nm and 106.7 nm, and to the ion lines, at 92.0 nm and 93.2 nm. Emissions at lower wavelengths were also detected, including lines with no information concerning level transitions in the well-known NIST database (e.g., the atomic line at 89.4 nm). The dependence of the lines' intensity on the microwave power delivered to the launcher was investigated. The electron density was estimated to be around 1012 cm−3 using the Stark broadening of the hydrogen Hβ line at 486.1 nm. The main population and loss mechanisms considered in the model for the excited a...

The vacuum ultraviolet (VUV) radiation emitted by hydrogen surface-wave-driven plasmas operating at microwave frequency (2.45 GHz) and low-pressure conditions (0.1–2 mbar) was investigated, in particular the influence of microwave power and gas pressure on the intensity of the emissions. The strong emission of Lyman H2 and Werner H2 molecular bands in the 80–125 nm spectral range was detected, while the most intense atomic emissions observed correspond to Lyman-α and Lyman-β lines at 121.6 nm and 102.6 nm respectively. An increase of the atomic lines and molecular bands intensities with increasing microwave power at pressure 0.1 mbar was observed. At 2 mbar the VUV spectra are entirely dominated by molecular bands. Theoretical predictions, as obtained from a collisional-radiative model, were validated by the experimental results.

The extreme ultraviolet radiation emitted by helium microwave-driven (2.45 GHz) plasmas operating at low-pressure conditions was investigated. Novel data regarding emitted spectral lines of excited helium atoms and ions in the 20–33 nm wavelength range and their intensity behavior with variation of discharge operational conditions are presented. The intensity of all the spectral emissions was found to strongly increase with the microwave power delivered to the plasma. Furthermore, the intensity of the ionic spectral emissions decreases by nearly one order of magnitude as the pressure was raised from 0.2 to 0.5 mbar.

Generation of energetic hydrogen atoms, with energy in the range 4-8 eV, was detected throughout the volume of a surface wave generated (500 MHz) plasma column in H2 at pressure p = 0.01 mbar. The Hβ, Hγ, Hδ, and H, line profiles were found to be bi-Gaussian towards the plasma column end. The kinetic temperatures corresponding to the Doppler broadening of the Hβ, Hγ, Hδ, lines are higher than the rotational temperature of the hydrogen molecular Fulcher-α band and the wall temperature. At pressure p = 0.2 mbar, the kinetic temperature of excited H (n = 4-7) atoms, as determined from the fitting of the spectral lines with a single-Gaussian profile, increases with upper level principal quantum number. The experimental results have been analyzed in the framework of a global self-consistent kinetic model describing this surface wave sustained plasma column.

Vacuum ultraviolet emission from Ar-H2 wave driven microwave (2.45 GHz) plasmas operating at low pressures (0.1–1 mbar) has been investigated. The emitted spectra show the presence of the Ar resonance lines at 104.8 and 106.7 nm and of the Lyman-α,β atomic lines at 121.6 nm and 102.6 nm, respectively. The increase of the hydrogen amount in the mixture results in an abrupt increase of the Werner and Lyman molecular bands intensity. The Lyman-β intensity shows little changes in the range of 5%–30% of hydrogen in the mixture while the Lyman-α intensity tends to decrease as the percentage of hydrogen increases.

The overall performance of a surface wave driven air-water plasma source operating at atmospheric pressure and 2.45 GHz has been analyzed. A 1D model previously developed has been improved in order to describe in detail the creation and loss processes of active species of interest. This model provides a complete characterization of the axial structure of the source, including the discharge and the afterglow zones. The main electron creation channel was found to be the associative ionization process N + O → NO+ + e. The NO(X) relative density in the afterglow plasma jet ranges from 1.2% to 1.6% depending on power and water percentage, according to the model predictions and the measurements. Other types of species such as NO2 and nitrous acid HNO2 have also been detected by mass and Fourier Transform Infrared spectroscopy. The relative population density of O(3P) ground state atoms increases from 8% to 10% in the discharge zone when the input microwave power increases from 200 to 400 W and the water percentage from 1% to 10%. Furthermore, high densities of O2(a1Δg) singlet delta oxygen molecules and OH radicals (1% and 5%, respectively) can be achieved in the discharge zone. In the late afterglow the O2(a1Δg) density is about 0.1% of the total density. This plasma source has a flexible operation and potential for channeling the energy in ways that maximize the density of active species of interest.

Doppler broadened Hγ emission was detected in high frequency (350 and 500 MHz) hydrogen surface wave sustained discharges, revealing the presence of fast excited H atoms with kinetic energies in the range 4–9 eV. Spatially resolved measurements of the Doppler-broadened emission indicate that these fast atoms are predominantly formed near the wall, which suggests that their generation may result from acceleration of H+ ions in the radial dc space charge field followed by recombination at the wall and the return of the neutral atom to the gas phase.

An experimental and theoretical investigation of the axial structure of a surface wave (2.45 GHz) driven atmospheric plasma source in air with a small admixture (1%) of water vapor has been performed. Measurements of the gas temperature and of the intensities of the O(777.4 nm), O(844.6 nm), and O(630 nm) atomic lines and the NO(γ) molecular band versus input power and axial position were carried out. Amplitude and phase sensitive measurements have also been performed to derive the surface wave dispersion characteristics. The experimental results are analyzed in terms of a one-dimensional theoretical model based on a self-consistent treatment of particle kinetics, gas dynamics, and wave electrodynamics. The predicted gas temperature and emission line intensities variations with power and axial position are shown to compare well with experiment. “Hot” excited O atoms (with kinetic energy ∼2 eV) have been detected.

“Super-hot” (kinetic energy ∼4–8 eV) and “hot” (kinetic energy ∼0.3 eV) H atoms were detected in a surface wave (500 MHz) generated H2 plasma column, at pressure p=0.01 mbar, from the analysis of the Hβ, Hγ, Hδ, and He emission line profiles. These profiles were found to evolve from single Gaussian to bi-Gaussian toward the plasma column end. Population inversion between the levels 5→4 and 6→4 was detected. At pressure p=0.2 mbar, super-hot atoms were not detected and the temperature of the hot atoms was found to increase with the upper level principal quantum number.

The study of the emission of “mixed gas” plasmas reveals many surprising results, especially when hydrogen is one of the gases. Anomalous, even extreme, hydrogen line broadening was found in a number of mixed discharge plasmas excited via direct current dc or radio frequency rf electric fields. 1–5 The Balmer line spectra emitted by these discharges have typical multimode behavior, with widely broadened “wings” “fast” hydrogen and a sharp top “slow” hydrogen. The results have usually been explained in terms of Doppler shift and broadening due to the acceleration of charges such as H + ,H 2 , and H3 ions in the high dc electric fields present in the sheath regions of these discharges. The acceleration of hydrogen ions in these dc fields is followed by neutralization and generation of fast excited H atoms. This is the origin of the “wings” in the spectra. Strikingly excessive Balmer- line broadening has been observed in He– H210% microwave discharge, 6 but this has not yet been confirmed by other research groups. On the contrary, measurements of H line profiles emitted by microwave discharges under similar conditions have not revealed excessive broadening. 7–10 Nevertheless, selective hydrogen line broadening has been detected in microwave discharges and their afterglows when there is no significant broadening of noble gas lines or hydrogen molecular lines. 8,9,11 A possible explanation for such selective heating of H atoms may be connected with the main creation processes of excited H atoms, namely, ion conversion and electron impact dissociation. Furthermore, hyperthermal hydrogen atoms have surprisingly been detected at atmospheric pressure Ar– H2 microplasma jets, where the Hn =3 temperatures were found to range from 12,000 to 19,600 K. 12 It is now clear that hydrogen line broadening causes controversy so that more experimental observations are currently needed in order to try to elucidate the mechanisms and processes behind this phenomenon in different types of discharges. The aim of this experimental work is to address some of these problems. This article presents spectroscopic measurements in He– H2 and Ar– H2 low-pressure plasmas generated by a surface wave of frequency / 2 = 2.45 GHz. Results on the line shape and the emission intensities of excited hydrogen and helium atoms, and the emission intensities of the Q-branch of the Fulcher- band d 3 uv =1 →a 3 g v =1 are presented and discussed. Different temperatures are determined from the measured hydrogen and helium emission line shapes and the rotational distribution of hydrogen molecular lines. Furthermore, the population distribution of excited H atoms is determined from measurements of the relative intensities of transitions within the Balmer series.

The axial structure of air and nitrogen–argon plasma torches driven by surface waves at atmospheric pressure is studied by means of optical emission spectroscopy and radiophysics. The rotational temperature of the Q1-branch of the OH (A2+Σ+, v' = 0 → X 2Π, v'' = 0) rotational band in the range 306–315 nm has been determined as a measure of the gas temperature. This temperature is around 3500 K along the main part of the air plasma torch and sharply drops to about 1000 K in the post-discharge zone. In the N2–Ar mixture, the Doppler temperatures (6000–8000 K) of the Hα and Hβ lines are higher than the OH rotational temperature (~4000 K). 'Hot' excited O(3p5P) oxygen atoms with temperature of approximately 20000 K have been detected in the air plasma torch.

An experimental and theoretical study on microwave (2.45 GHz) plasma enabled assembly of carbon nanostructures, such as multilayer graphene sheets and nanoparticles, was performed. The carbon nanostructures were fabricated at different Ar–CH4 gas mixture composition and flows at atmospheric pressure conditions. The synthesis method is based on decomposition of the carbon-containing precursor (CH4) in the “hot” microwave plasma environment into carbon atoms and molecules, which are further converted into solid carbon nuclei in the “colder” plasma zones. By tailoring of the plasma environment, a controlled synthesis of graphene sheets and diamond-like nanoparticles was achieved. Selective synthesis of graphene flakes was achieved at a microwave power of 1 kW, Ar and methane flow rates of 600 sccm and 2 sccm respectively, while the predominant synthesis of diamond-like nanoparticles was obtained at the same power, but with higher flow rates, i.e. 1000 and 7.5 sccm, respectively. Optical emission spectroscopy was applied to detect the plasma emission related to carbon species from the ‘hot’ plasma zone and to determine the main plasma parameters. Raman spectroscopy and scanning electron microscopy have been applied to characterize the synthesized nanostructures. A previously developed theoretical model was further updated and employed to understand the mechanism of CH4 decomposition and formation of the main building units, i.e. C and C2, of the carbon nanostructures. An insight into the physical chemistry of carbon nanostructure formation in a high energy density microwave plasma environment is presented.