Concentration Of Nitrogen Atoms Research Articles

Nitrogen-Rich Mesoporous Carbons: Highly Efficient, Regenerable Metal-Free Catalysts for Low-Temperature Oxidation of H2S

We demonstrate that it is possible to transform traditional mesoporous carbons into a superior metal-free catalyst for low-temperature H2S removal via doping a high concentration of nitrogen atoms into the carbon. The nitrogen doping level is important for the activity of mesoporous carbons as a metal-free catalyst. Although carbons doped with only an intermediate amount of nitrogen (e.g., 4.3 wt %), show little aptitude for the catalytic oxidation of H2S when nitrogen doping reaches a certain high level (e.g., 8.5 wt %), the nitrogen-rich mesoporous carbons (NMC) exhibit high catalytic activity and selectivity toward H2S oxidation at low temperature. Further study suggests that the pyridinic nitrogen atoms are responsible for the catalytic activity in H2S oxidation. Owing to the metal-free nature of the NMC catalyst, it can be easily regenerated by CS2 scrubbing, and the product sulfur can be recovered. Our desulfurization results suggest that such metal-free carbons could, indeed, overcome the limitations of the conventional H2S catalysts and provide suitable, sustainable, and inexpensive solutions for technological development in H2S removal.

Low temperature rate constants for the N(4S) + CH(X2Πr) reaction. Implications for N2 formation cycles in dense interstellar clouds

Rate constants for the potentially important interstellar N((4)S) + CH(X(2)Πr) reaction have been measured in a continuous supersonic flow reactor over the range 56 K ≤T≤ 296 K using the relative rate technique employing both the N((4)S) + OH(X(2)Πi) and N((4)S) + CN(X(2)Σ(+)) reactions as references. Excess concentrations of atomic nitrogen were produced by the microwave discharge method upstream of the Laval nozzle and CH and OH radicals were created by the in situ pulsed laser photolysis of suitable precursor molecules. In parallel, quantum dynamics calculations of the title reaction have been performed based on accurate global potential energy surfaces for the 1(3)A' and 1(3)A'' states of HCN and HNC, brought about through a hierarchical construction scheme. Both adiabatic potential energy surfaces are barrierless, each one having two deep potential wells suggesting that this reaction is dominated by a complex-forming mechanism. The experimental and theoretical work are in excellent agreement, predicting a positive temperature dependence of the rate constant, in contrast to earlier experimental work at low temperature. The effects of the new low temperature rate constants on interstellar N2 formation are tested using a dense cloud model, yielding N2 abundances 10-20% lower than previously predicted.

Investigation into the adsorption of atomic nitrogen on an Al2O3 (0001) surface

Computer simulation of sapphire nitridation used to obtain nitride-based heterostructures (GaN) on an Al2O3 substrate has been performed. The adhesion of atomic nitrogen to the sapphire (0001) surface is investigated ab initio. The possibility of replacing surface-layer oxygen atoms with nitrogen atoms has been examined. The calculated results indicate that adsorbed nitrogen atoms occupy the most stable positions above surface oxygen atoms at different nitrogen concentrations. The changes in the total system energy after replacement of surface oxygen atoms with nitrogen atoms have been calculated. It turns out that oxygen replacement is energetically unfavorable for a single nitrogen adatom. However, this process becomes energetically favorable if the concentration of nitrogen atoms increases. This outcome, obtained for the first time, enables better understanding of the atomic-scale mechanism of sapphire nitridation.

ESR studies of nitrogen atoms stabilized in aggregates of krypton–nitrogen nanoclusters immersed in superfluid helium

Impurity–helium condensates (IHCs) containing nitrogen and krypton atoms immersed in superfluid 4He have been studied via CW electron spin resonance (ESR). The IHCs are gel-like aggregates of nanoclusters composed of impurity species. It was found that the addition of krypton atoms to the nitrogen–helium gas mixture used for preparation of IHCs increases the efficiency of stabilization of nitrogen atoms. We have achieved high average (5 · 1019 cm−3) and local (2 · 1021 cm−3) concentrations of nitrogen atoms in krypton–nitrogen–helium condensates. The analysis of ESR lines shows that in krypton-nitrogen nanoclusters three different sites exist for stabilization of nitrogen atoms. Nitrogen atoms are stabilized in the krypton core of nanoclusters, in the nitrogen molecular layer that covers the Kr core, and on the surface of the nanoclusters. High concentrations of nitrogen atoms achieved in IHCs provide an important step in the search for magnetic ordering effects at low temperatures.

Study of surface cleaning methods and pyrolysis temperatures on nanostructured carbon films using x-ray photoelectron spectroscopy

Nanostructured carbon (ns-C) films fabricated by stabilization and pyrolysis of diblock copolymers are of interest for a variety of electrical/electronic applications due to their chemical inertness, high-temperature insensitivity, very high surface area, and tunable electrical resistivity over a wide range [Kulkarni et al., Synth. Met. 159, 177 (2009)]. Because of their high porosity and associated high specific surface area, controlled surface cleaning studies are important for fabricating electronic devices from these films. In this study, quantification of surface composition and surface cleaning studies on ns-C films synthesized by carbonization of diblock copolymers of polyacrylonitrile-b-poly(n-butyl acrylate) at two different temperatures were carried out. X-ray photoelectron spectroscopy was used for elemental analysis and to determine the efficacy of various surface cleaning methods for ns-C films and to examine the polymer residues in the films. The in-situ surface cleaning methods included HF vapor treatment, vacuum annealing, and exposure to UV-ozone. Quantitative analysis of high-resolution XPS scans showed 11 at. % nitrogen was present in the films pyrolyzed at 600 °C, suggesting incomplete denitrogenation of the copolymer films. The nitrogen atomic concentration decreased significantly for films pyrolyzed at 900 °C confirming extensive denitrogenation at that temperature. Furthermore, quantitative analysis of nitrogen subpeaks indicated higher loss of nitrogen atoms residing at the edge of graphitic clusters relative to that of nitrogen atoms within the graphitic clusters, suggesting higher graphitization with increasing pyrolysis temperature. Of the surface cleaning methods investigated, in-situ annealing of the films at 300 °C for 40 min was found to be the most efficacious in removing adventitious carbon and oxygen impurities from the surface.

Effect of nitrogen plasma on the surface of indium oxide nanowires

The change in the atomic nitrogen concentration on a semiconducting nanowire’s surface and the consequent changes in the electrical characteristics of a nanowire transistor were investigated by exposing In2O3 nanowires to nitrogen (N2) plasma. After plasma was applied at N2 flow rates of 20, 40, and 70 sccm with a fixed source power of 50 W, the In2O3 nanowire transistor exhibited changes in the threshold voltage (Vth), subthreshold slope (SS), and on-current (Ion). In particular, after treatment at an N2 flow rate of 40 sccm, Vth shifted positively by ∼2.3 V, the SS improved by ∼0.24 V/dec, and Ion increased by ∼0.8 μA on average. The changes are attributed to the combination of nitrogen ions produced by the plasma with oxygen vacancies or indium interstitials on the nanowires. Optimization of the plasma treatment conditions is expected to yield desirable device characteristics by a simple, nondestructive process.

Surface modification of NiTi by plasma based ion implantation for application in harsh environments

The substitution of conventional components for NiTi in distinct devices such as actuators, valves, connectors, stents, orthodontic arc-wires, e.g., usually demands some kind of treatment to be performed on the surface of the alloy. A typical case is of biomaterials made of NiTi, in which the main drawback is the Ni out-diffusion, an issue that has been satisfactorily addressed by plasma based ion implantation (PBII). Even though PBII can tailor selective surface properties of diverse materials, usually, only thin modified layers are attained. When NiTi alloys are to be used in the harsh space environment, as is the case of devices designed to remotely release the solar panels and antenna arrays of satellites, e.g., superior mechanical and tribological properties are demanded. For this case the thickness of the modified layer must be larger than the one commonly achieved by conventional PBII. In this paper, new nitrogen PBII set up was used to treat samples of NiTi in moderate temperature of 450°C, with negative voltage pulses of 7kV/250Hz/20μs, in a process lasting 1h. A rich nitrogen atomic concentration of 85at.% was achieved on the near surface and nitrogen diffused at least for 11μm depth. Tribological properties as well as corrosion resistance were evaluated.

Synthesis of nitrogen-doped diamond films using vibrational excitation of ammonia molecules in laser-assisted combustion flames

Nitrogen-doped diamond was synthesized in open air using laser-assisted combustion flame method. A wavelength-tunable CO2 laser was used to resonantly excite the vibration modes of ammonia molecules, which were added into the diamond forming combustion flame. The wavelength of the CO2 laser was tuned to match frequencies of the NH wagging mode of the ammonia molecules. High efficiency energy coupling was achieved at laser wavelengths of 9.219, 10.35, and 10.719 μm, which are related to a rotational–vibrational transition (1084.63 cm−1), and splitting of the NH wagging mode (υ2+, 932.51 cm−1 and υ2−, 968.32 cm−1). Vibrational excitations of the ammonia molecules under these wavelengths actively intervenes the reaction courses, which steers the chemical reaction in the combustion flame and eventually promotes nitrogen concentration in the deposited diamond films. Concentration of the doped nitrogen atoms reaches up to 1.5 × 1020 atoms/cm3 in the diamond films deposited with a laser wavelength of 9.219 μm. Optical emission spectroscopy and mass spectrometry were used to study the evolution of chemical reactions with and without laser excitations.

Effect of nitrogen concentration to the structural, chemical and electrical properties of tantalum zirconium nitride films

Tantalum zirconium nitride films with different nitrogen concentrations were deposited by reactive co-sputtering. Systematical material characterization was done to study the effect of nitrogen concentration to the ternary nitride films’ composition, microstructure, chemical and electrical properties. XRD and TEM showed that the microstructure of the films was mainly modulated by the Ta/Zr atomic ratio and nitrogen concentration, and amorphous structure formed in the nitrogen deficient films. XPS showed the chemical state of tantalum and zirconium atoms shifted systematically from metallic to ionic bonding state with the increasing of nitrogen. The stoichiometric ternary TaZrN films showed better conductivity than the binary TaN or ZrN constituents, because each tantalum atom contribute one excess d-orbital valence electron when the zirconium atom was substituted.

Structural and electronic properties of single-wall carbon nanotubes with various nitrogen content

The band structure of a carbon-nitrogen nanotube of “zigzag” type (8, 0) with nitrogen atom concentrations of 6.25%, 12.5%, and 25% and “armchair” (5, 5) carbon-nitrogen nanotube with nitrogen atom concentrations of 10% and 20% is calculated within the density functional theory approach. The calculations are carried out taking into account optimizations of both the longitudinal and transverse nanotube structural parameters. The structure is distorted significantly both lengthwise and across with increasing nitrogen concentrations. In particular, their cross-sectional profile ceases to be a circle. Dependence of the energies of interband electronic transitions on the concentration of nitrogen can be observed in the form of a peak shift in the optical absorption spectrum.

Ab initio simulations of doped single-walled carbon nanotube sensors

The interactions between oxygen and nitrogen atoms with single-walled carbon nanotubes were investigated for nanotubes with two different geometrical configurations using first-principle calculations within the framework of the density functional theory. We introduced a new type of toxic gas sensor that can detect the presence of H 2, Cl 2, CO, and NO molecules. We also demonstrated that the sensitivity of this device can be controlled by the concentration of the dopants on the surface of the nanotube. In addition, the transport properties of the doped nanotube were studied for different concentrations of oxygen or nitrogen atoms that were randomly distributed on the surface of the single-walled carbon nanotube. We observed that small amounts of dopants can modify the electronic and transport properties of the nanotube and can lend metallic properties to the nanotube. Band-gap narrowing occurs when the nanotube is doped with either oxygen or nitrogen atoms.

Improved electrical conductivity of a non-covalently dispersed graphene–carbon nanotube film by chemical p-type doping

Using a high-pressure air spray we developed a method to deposit electrically-conducting thin films consisting of non-covalently dispersed graphene and carbon nanotubes. The graphene–carbon nanotube film was immersed in a nitric acid and followed by exposure to fuming nitric acid. The acid treatment induced an increased concentration of atomic nitrogen on the graphene basal plane and carbon nanotube sidewall. This result indicates chemical p-type doping of the graphene oxide–carbon nanotube film. After the two acid treatments, the spray coated graphene oxide–carbon nanotube films on a glass substrate exhibit a low sheet resistance of 171 Ω/sq, and a high transmittance of 84% at a wavelength of 550 nm.

Room-temperature photoluminescence from nitrogenated carbon nanotips grown by plasma-enhanced hot filament chemical vapor deposition

Nitrogenated carbon nanotips with a low atomic concentration of nitrogen have been synthesized by using a custom-designed plasma-enhanced hot-filament plasma chemical vapor deposition system. The properties (including morphology, structure, composition, photoluminescence, etc.) of the synthesized nitrogenated carbon nanotips are investigated using advanced characterization tools. The room-temperature photoluminescence measurements show that the nitrogenated carbon nanotips can generate two distinct broad emissions located at ∼405 and ∼507 nm, respectively. Through the detailed analysis, it is shown that these two emission bands are attributed to the transition between the lone pair valence and σ* bands, which are related to the sp3 and sp2 C–N bonds, respectively. These results are highly relevant to advanced applications of nitrogenated carbon nanotips in light emitting optoelectronic devices.

Density functional study of super cell N-doped (10,0) zigzag single-walled carbon nanotubes as CO sensor

N-doped SWCNT with different concentration of doped nitrogen atoms were investigated through density functional theory (DFT) calculations for detecting CO molecule. The CO molecule was adsorbed to different sites of the modified nanotubes and their geometric structures and electronic properties were investigated after full optimization. A significant change can be observed in adsorption energies and electronic properties of N-doped SWCNT after CO adsorption. By increasing the number of nitrogen atoms in each unit cell, these properties change more obviously. So these modified nanotubes can be used as CO sensors.

(Invited) Band-Edge Effective Work Functions by Controlling HfO2/TiN Interfacial Composition for Gate-Last CMOS

Effective work function (EWF) changes of TiN/HfO2 annealed at low temperatures in different ambient environments are correlated to the atomic concentration of oxygen, nitrogen, and aluminum at the metal/dielectric interface. Low EWFs (4.0 eV) are obtained by allowing aluminum to migrate to the TiN/HfO2 interface during a forming gas anneal. High EWFs (5.1 eV) are achieved with anneals that incorporate oxygen throughout the TiN with [O] = 2.8x1021 cm-3 near the TiN/HfO2 interface. First-principles calculations indicate the exchange of O and N atoms near the TiN/HfO2 interface cause the formation of dipoles that increase the EWF.

Thermal properties of porous copolymers of BM-DVB and their carbonization products

Thermogravimetry and differential scanning calorimetry were used for characterization of the thermal properties of new 4,4′-bismaleimidodiphenylmethane (BM) and divinylbenzene (DVB) porous copolymers and their carbonization products. Bead-shaped porous copolymers BM-DVB with the following monomer ratios 1:4, 1:1, 4:1 were synthesized using suspension copolymerization under the same conditions. Differences in the monomer ratio caused a different degree of cross-linking of the starting polymers. Before carbonization, the BM-DVB copolymers were pretreated using two methods. In one method, the starting material was stabilized in hot air (product was labeled PO-C800). In the other method, the copolymer was soaked in H 3PO 4 (product was named P800). Then, materials obtained by both methods were carbonized at 800 °C in an argon atmosphere. To characterize the heat resistance of the BM-DVB copolymers and their carbonized derivatives, their thermostabilities were evaluated. The data suggest the existence of a relationship between the composition and thermal stability of the copolymers and their carbonized derivatives. The most thermally resistant copolymer was that obtained with a 4:1 molar ratio of BM to DVB. Its thermal stability is caused by the high concentration of nitrogen atoms in the polymeric structure. 1:4 BM-DVB copolymer with a high degree of cross-linking was the least thermally stable, which might be caused by its microporous nature and small fraction of nitrogen. The derived carbons have very similar thermal properties, and an insignificant influence of the nature of the polymer precursor was observed. More important factors affecting thermal stability were the porosity and surface chemistry, which were created in the thermal pretreatment processes.

Nitrogen and boron doping effects on the electrical conductivity of graphene and nanotube

The effects of nitrogen and boron doping on the density of states and the temperature dependence of the electrical conductivity of graphene plane and (10,0) single-walled carbon nanotube are studied. We apply the Green’s function technique and coherent potential approximation within the tight-binding Hamiltonian model. We find that when dopants are introduced, van-Hove singularities in the density of states are broadened and Fermi level is shifted. The shift of the Fermi-energy depends on concentration of nitrogen and boron atoms doping. The electrical conductivity changes with the dopant concentration in the two temperature regions. In the low temperature region, the electrical conductivity increases when dopant concentration increases while the electrical conductivity decreases with increasing concentration of nitrogen and boron atoms in the high temperature region.

Effects of hydrogen addition in nitrogen atmospheric pressure plasma on its optical and electrical properties and silicon-based deposits compositions

Si-based coatings were deposited with a cold plasma jet (Plasmabrush® PB1 from Reinhausen Plasma) at atmospheric pressure with nitrogen as main carrier gas and hexamethyldisilazane (HMDSN) as precursor. Effects of hydrogen addition on the plasma characteristics and the coatings compositions have been evidenced with optical emission spectroscopy (OES), power measurements and XPS in-depth analyses. The intensity evolution of the nitrogen line (at 315.9 nm) with the applied voltage has a sigmoid shape for the pure nitrogen plasma but a quite linear one with hydrogen addition (up to 3%). Based on OES spectra, the presence of the NH specie in the nitrogen-hydrogen plasmas has been evidenced (around 336.0 nm) but not in the pure nitrogen plasmas. Although the plasma power is similar for both gases, the nitrogen atomic concentrations in the films as evidenced by XPS were lower with the nitrogen-hydrogen plasmas than with the pure nitrogen plasmas indicating a chemical effect of the presence of hydrogen in the plasma.

Disturbances in the F1 ionospheric layer in April 2005

Manifestations of ionospheric disturbances at heights from 120 km to the height h m of the F2-layer maximum are analyzed. The geomagnetic disturbances on April 5, 12, and 13, 2005, are considered. These disturbances are characterized by a pronounced synchronous decrease in the electron concentration over the entire indicated height range. Using the method proposed by the authors, the noontime changes in the relative content of atomic and molecular oxygen at a height of 120 km are estimated. There are a distinct decrease in the ratio of atomic oxygen and molecular nitrogen concentrations and an increase in the ratio of oxygen molecule and atom concentrations.

Removal of Ar+ beam‐induced damaged layers from polyimide surfaces with argon gas cluster ion beams

AbstractAn Ar Gas Cluster Ion Beam (GCIB) has been shown to remove previous Ar+ ion beam‐induced surface damage to a bulk polyimide (PI) film. After removal of the damaged layer with a GCIB sputter source, XPS measurements show minor changes to the carbon, nitrogen and oxygen atomic concentrations relative to the original elemental bulk concentrations. The GCIB sputter depth profiles showed that there is a linear relationship between the Ar+ ion beam voltage within the range from 0.5 to 4.0 keV and the dose of argon cluster ions required to remove the damaged layer. The rate of recovery of the original PI atomic composition as a function of GCIB sputtering is similar for carbon, nitrogen and oxygen, indicating that there was no preferential sputtering for these elements. The XPS chemical state analysis of the N 1s spectra after GCIB sputtering revealed a 17% damage ratio of altered nitrogen chemical state species. Further optimization of the GCIB sputtering conditions should lead to lower nitrogen damage ratios with the elemental concentrations closer to those of bulk PI. Copyright © 2010 John Wiley & Sons, Ltd.