Fraction Of Carbon Atoms Research Articles

Enhancing the chemoselective hydrogenation of nitroarenes: Designing a novel surface-strained carbon-based Pt nanocatalyst

Supported Pt nanoparticles (NPs) are highly active catalysts for heterogeneous catalytic hydrogenation reactions; however, controlling their selectivity remains the biggest challenge toward their applicability. Herein, we propose the formation of a highly selective and stable, surface-strained Pt-based nanocatalyst via a facile and scalable thermal reduction treatment. Spherical-aberration-corrected transmission electron microscopy (SACTEM) and various spectral analytic techniques reveal a strong metal-support interaction between the Pt NPs and carbon nanotubes (CNTs) support during the annealing process. Thereafter, a fraction of carbon atoms is etched from the carbon-coated Pt NPs, inducing a compressive strain on the surface of the Pt NPs. Notably, the chemoselectivity of the surface-strained Pt/CNTs-800H catalyst (where 800 represents the heat-treatment temperature; H represents a hydrogen atmosphere) is almost completely different compared to that of its pristine counterpart. This catalyst is used for the hydrogenation reactions of a styrene and nitrobenzene mixture as well as 4-nitrostyrene. Interestingly, similar findings were observed with 5 wt% Pt/C and Pt/rGO catalysts, confirming that this treatment could be generalized. Hence, it has great potential in the design and synthesis of carbon-based catalytic materials.

Thermodynamics of diamond formation from hydrocarbon mixtures in planets

Hydrocarbon mixtures are extremely abundant in the Universe, and diamond formation from them can play a crucial role in shaping the interior structure and evolution of planets. With first-principles accuracy, we first estimate the melting line of diamond, and then reveal the nature of chemical bonding in hydrocarbons at extreme conditions. We finally establish the pressure-temperature phase boundary where it is thermodynamically possible for diamond to form from hydrocarbon mixtures with different atomic fractions of carbon. Notably, here we show a depletion zone at pressures above 200 GPa and temperatures below 3000 K-3500 K where diamond formation is thermodynamically favorable regardless of the carbon atomic fraction, due to a phase separation mechanism. The cooler condition of the interior of Neptune compared to Uranus means that the former is much more likely to contain the depletion zone. Our findings can help explain the dichotomy of the two ice giants manifested by the low luminosity of Uranus, and lead to a better understanding of (exo-)planetary formation and evolution.

Efficiency of the top-down polycyclic aromatic hydrocarbon-to-fullerene conversion in ultraviolet irradiated environments

ABSTRACT Polycyclic aromatic hydrocarbons (PAHs) and fullerenes play a major role in the physics and chemistry of the interstellar medium (ISM). Based on a number of recent experimental and theoretical investigations we developed a model in which PAHs are subject to photo-dissociation (carbon and hydrogen loss) and hydrogenation. We take into account that dehydrogenated PAHs may fold into closed structures – fullerenes. Fullerenes, in their turn, can be also hydrogenated, becoming fulleranes, and photo-dissociated, losing carbon and hydrogen atoms. The carbon loss leads to shrinking of fullerene cages to smaller ones. We calculate the abundance of PAHs and fullerenes of different sizes and hydrogenation level depending on external conditions: the gas temperature, intensity of radiation field, number density of hydrogen atoms, carbon atoms, and electrons. We highlight the conditions, which are favourable for fullerene formation from PAHs, and we conclude that this mechanism works not only in H-poor environment but also at modest values of hydrogen density up to 104 cm−3. We found that fulleranes can be formed in the ISM, although the fraction of carbon atoms locked in them can be maximum around 10−9. We applied our model to two photo-dissociation regions, Orion Bar and NGC 7023. We compare our estimates of the fullerene abundance and synthetic band intensities in these objects with the observations and conclude that our model gives good results for the closest surroundings of ionizing stars. We also demonstrate that additional fullerene formation channels should operate along with ultraviolet (UV)-induced formation to explain abundance of fullerenes far from UV sources.

Superhydrogenated Polycyclic Aromatic Hydrocarbon Molecules: Vibrational Spectra in the Infrared

Abstract Superhydrogenated polycyclic aromatic hydrocarbons (PAHs) may be present in H-rich and ultraviolet-poor benign regions. The addition of excess H atoms to PAHs converts the aromatic bonds into aliphatic bonds, the strongest of which falls near 3.4 μm. Therefore, superhydrogenated PAHs are often hypothesized to be a carrier of the 3.4 μm emission feature that typically accompanies the stronger 3.3 μm aromatic C–H stretching feature. To assess this hypothesis, we use density function theory to compute the infrared (IR) vibrational spectra of superhydrogenated PAHs and their ions of various sizes (ranging from benzene and naphthalene to perylene and coronene) and of various degrees of hydrogenation. For each molecule, we derive the intrinsic oscillator strengths of the 3.3 μm aromatic C–H stretch ( ) and the 3.4 μm aliphatic C–H stretch ( ). By comparing the computationally derived mean ratio of with the mean ratio of the observed intensities , we find that the degree of superhydrogenation—the fraction of carbon atoms attached with extra hydrogen atoms—is only ∼2.2% for neutral PAHs, which predominantly emit the 3.3 and 3.4 μm features. We also determine for each molecule the intrinsic band strengths of the 6.2 μm aromatic C–C stretch ( ) and the 6.85 μm aliphatic C–H deformation ( ). We derive the degree of superhydrogenation from the mean ratio of the observed intensities and for neutrals and for cations to be ≲3.1% for neutrals and ≲8.6% for cations. We conclude that astrophysical PAHs are primarily aromatic and are only marginally superhydrogenated.

Impact of Physicochemical Properties on Dose and Hepatotoxicity of Oral Drugs.

A database containing maximum daily doses of 1841 marketed oral drugs was used to examine the influence of physicochemical properties on dose and hepatotoxicity (drug induced liver injury, DILI). Drugs in the highest ∼20% dose range had significantly reduced mean lipophilicity and molecular weight, increased fractional surface area, increased % of acids, and decreased % of bases versus drugs in the lower ∼60% dose range. Drugs in the ∼20-40% dose range had intermediate mean properties, similar to the mean values for the full drug set. Drugs that are both large and highly lipophilic almost invariably do not have doses in the upper ∼20% range. The results show that oral druglike physicochemical properties are different according to these dose ranges, and this is consistent with maintenance of acceptable safety profiles as efficacious exposure increases. Verified DILI annotations from a compilation of >1000 approved drugs (Chen, M.; et al. Drug Discov. Today, 2016, 21, 648 ) were used. The drugs classified as "No DILI" ( n = 163) had significantly lower dose and lipophilicity, and higher Fsp3 (fraction of carbon atoms that are sp3 hybridized) versus the "Most DILI" ( n = 163) drugs. The percentages of acids were reduced and bases increased in the "No DILI" versus the "Most DILI" groups. Drugs classified as "Less DILI" or "Ambiguous DILI" had intermediate mean values of dose, lipophilicity, Fsp3, and % acids and bases. The impact of lipophilicity and Fsp3 on DILI increases in the upper 20% versus the lower 80% dose range, and a simple decision tree model predicted "No DILI" versus "Most DILI" outcomes with 82% accuracy. The model correctly classified 19 of 22 drugs (86%) that failed in development due to human hepatotoxicity. Because many oral drugs lacking DILI annotations are predicted to be "Most DILI", the model is best used preclinically in conjunction with experimental DILI mitigation.

Near Infrared Spectroscopic Identification of Alkyl Aromatic Esters and Phenyl Ketones

Bands characterizing the content of carbon atoms in alkyl (7177–7205 cm–1) and phenyl structural fragments (9175–9192 cm–1) in organic molecules were revealed by studying the near infrared spectra of such compounds. The optical density at the maxima of these absorption bands was shown to depend strongly on the fraction of carbon atoms in the corresponding fragments. The developed models proved to be adequate for determining the fraction of carbon atoms in alkyl aromatic esters and phenyl ketones. The feasibility of modeling the molecular structure of alkyl aromatic esters using regression models was demonstrated for the product of the condensation of oleic acid and benzyl alcohol.

Thermodynamic description of graphitizable carbons and the irreversible graphitization process

The present paper develops a thermodynamic model for the calculation of the Gibbs energy of graphitizable carbons near the graphitization temperature threshold of 2550K. The graphitization threshold is the minimal heat treatment temperature for the establishment of true graphitic ordering in a graphitizable carbon. The present model represents the structure of graphitizable carbons with a simplified cluster approach. In the model, two distinct clusters share the totality of the carbon atoms in the material: Oberlin's Local Molecular Orientation clusters (LMOs) and mesoscale grain boundary clusters (mGBs). The ability of a carbon material to graphitize is proportional to the size of the LMOs (larger LMOs = more graphitizable). Strongly graphitizing carbons have a large average size for their LMOs. Thus, for these carbons, the fraction of carbon atoms in mGBs can be neglected relative to the one in LMOs. In a LMO, two phases are present in pseudo-thermodynamic equilibrium: the intercrystalline matter phase (IMP) and the coke crystallite phase (CCP). If the average crystallite diameter (La) of the CCP is sufficiently large, the model assumes that the number of carbon atoms in the IMP relative to the CCP is negligible. The paper presents an additive scheme to calculate the Gibbs energy of carbon atoms in a graphitizable carbon based on the present cluster description. The current Gibbs energy model accounts for the contribution of intercrystalline matter removal by the irreversible densification of LMOs up to the graphitization temperature threshold of 2550K.

АТОМЫ УГЛЕРОДА В МЕЖДОУЗЛИЯХ КРИСТАЛЛИЧЕСКОЙ РЕШЁТКИ ЦЕМЕНТИТА: AB INITIO МОДЕЛИРОВАНИЕ

It was shown in previous works that iron sublattice in the structure of cementite Fe3C has four different types of interstitial sites that may potentially be occupied by carbon atoms, viz. ‘normal’ and ‘distorted’ prismatic (NP, DP) and ‘normal’ and ‘distorted’ octahedral (NO, DO) sites. Distances between their centres and the centres of z nearest-neighbour iron atoms are 1.99–2.04 A (NP sites, z = 6); 1.80–1.87 A (NO sites, z = 6); 1.62 A (DP sites, z = 4) and 1.25 A (DO sites, z = 2). It is usually believed that all carbon atoms are located in NP positions. In this work other possible variants are considered by ab initio calculations using FP-LAPW full-potential method as realized in WIEN2k program package and a cementite supercell containing 16 atoms (4 formula units). It is found that the structure containing all carbon atoms in NP positions has the lowest energy and volume. The structure with all carbon atoms in NO positions has the energy 0.267 eV/f.u. higher and the volume 9.20 % greater. If only one of the four carbon atoms in the supercell is shifted from NP to NO site the energy and volume increase by 0.233 eV/f.u. and 3.59 % correspondingly. The structure with one carbon atoms located in DP site and the other three in DO sites has the energy 0.452 eV and the volume 3.75 % higher than those of cementite with all carbon atoms in NP sites. The structures with all carbon atoms in DP sites and one or all carbon atoms in DO sites are mecha-nically unstable and transform spontaneously to conventional NP structure. Thermodynamic estima-tion based on these data shows that even at temperatures below 1000 K the fraction of carbon atoms that may move from NP sites to other ones may be as great as 20 %. Evaluation of the vacancy formation energy in cementite (supercell of 128 atoms containing 96 atoms of Fe and 32 atoms of C) resulted in the values of 0.50 eV for carbon vacancy and 1.34 (FeG) or 1.60 eV (FeS) for iron vacancy. In all these cases vacancy formation did not result in volume change of the system.

Acid-base properties of Baltic Sea dissolved organic matter

Abstract Calculations related to the marine CO2 system that are based on alkalinity data may be strongly biased if the contributions of organic compounds are ignored. In coastal seas, concentrations of dissolved organic matter (DOM) are frequently high and alkalinity from inorganic compounds is low. In this study, based on measurements of total alkalinity, total CO2, and pH, we determined the organic alkalinity, Aorg, in water from the central Baltic Sea. The maximum Aorg measured in the surface mixed layer during the spring bloom was > 50 μmol/kg-SW but the Aorg decreased with depth and approached zero below the permanent halocline. This behavior could be attributed to the decreased pH of deeper water layers. The data were used to calculate the bulk dissociation constant, KDOM, for marine DOM and the fraction f of dissolved organic carbon (DOC) that acts as a carrier for acid-base functional groups. The p KDOM (7.27) agreed well with the value (7.34) previously estimated in a preliminary study of organic alkalinity in the Baltic Sea. The fraction of carbon atoms carrying acid-base groups was 17% and was somewhat higher than previously reported (12%). Spike experiments performed using artificial seawater and three different humic/fulvic substances tested whether the acid-base properties of these substances explain the results of our field study. Specifically, Aorg was determined at different concentrations (DOC) of the added humic/fulvic substances. The relationship between Aorg and the DOC concentrations indicated that humic/fulvic substances are more acidic (p KDOM

Deposition and thermal conductivity of diamond-like carbon film on a silicon substrate

Diamond-Like Carbon (DLC) is thought to be a potential material in solving heat dissipation problems in light emitting diode module packages. It is of vital importance in evaluating the thermal conductivity of DLC film deposited on a silicon substrate. In this paper, the molecular dynamics method is used to simulate the formation of a DLC film by the deposition of carbon atoms on a isilicon substrate. Tersoff potential is adopted to reproduce the structures and densities of silicon, carbon, and SiC. A silicon substrate consisting of 544 atoms is located at the bottom of the simulation domain. The substrate is kept at a temperature of 600 K through a Noose-Hover thermostat. Carbon atoms are injected into the substrate individually every 0.5 ps at an energy of 1 eV. After a 7.5 ns deposition process, a 4 nm amorphous film containing 15000 carbon atoms is formed. Injected carbon atoms and substrate silicon atoms are intermixed at the bottom layer of the deposited film while the rest of the film contains only carbon atoms. The density of the film decreases slightly with the increase of the height of the deposited film and the average density is 2.8 g/cm3. Analysis of the coordination number shows that the sp3 fraction of carbon atoms in the film also decreases with the increase of the height of the deposited film, with a maximum value of 22%. It might be caused by the continuous impacts of the subsequently injected carbon atoms on the previously formed DLC film. The thermal conductivities of the DLC film in the planar and normal directions are calculated by the Green-Kubo method. The thermal conductivity of pure diamond film is also calculated for comparison. The results show that the planar thermal conductivity of the deposited DLC film is approximately half of that of the pure diamond film with the same size. It is higher than the normal thermal conductivity of the deposited film. The thermal conductivities of the DLC film in both planar and normal directions increase with the increase of film density and sp3 fraction in the DLC film. The results indicate that the local tetrahedral structure of sp3 carbon atoms contributes to the improvement of thermal conductivity in the DLC film.

Correlation of sp2 carbon bonds content in magnetron-sputtered amorphous carbon films to their electrochemical H2O2 production for water decontamination applications

The radio frequency (RF)-magnetron sputtering technique was used to deposit amorphous carbon (a-C) films onto different substrates at different deposition substrate temperatures (Td) ranging from room temperature (RT) to 750°C. The carbon bonding states of the a-C films were systematically investigated by means of Raman spectroscopy and X-ray photoelectron spectroscopy (XPS) as a function of Td. The Raman analyses clearly show that an increase of Td leads to significant microstructural changes of the initially mixed sp2/sp3 carbon bonds, for the films deposited at RT into a more graphitic like sp2 bonding at higher Td. This increase of [sp2]-bonded fraction of carbon atoms to the detriment of the sp3 bonded ones has been quantitatively confirmed by the XPS carbon C 1s core level spectra analyses. Indeed, the sp3-bonds content of the a-C films is found to diminish from 44% at RT to a minimum value of 16% around Td=450°C and then stabilizes around 26% at Td=750°C. On the other hand, the hydrogen peroxide (H2O2) production capacity of these a-C films was systematically evaluated, and found to reach its maximum of 11.2μg/L/cm2 at Td=450°C, at which the highest C sp2-bonds content was also observed. We were thus able to point up, for the first time, a fairly linear correlation between the produced amount of H2O2 of the a-C films and their relative [sp2] bonding content, over an [sp2] relative content range as wide as (0–84%). These results clearly demonstrate the relevance of a-C films with a pronounced graphitic like character for H2O2 production. Such a-C cathodes with an enhanced H2O2 production capacity are highly sought for in electrochemical decontamination of water from persistent organic pollutants.

Pore characteristics of carbide-derived carbons obtained from carbides with different carbon volume fractions

Porous carbide-derived carbons (CDCs) are synthesized by chlorination of carbides with different carbon volume fractions in the temperature range from 400 to 1000°C. It is found that the volume fraction of carbon atoms in carbide precursor has much influence on pore structure of the produced CDC. It is prone to form well ordered graphite to chlorinate carbides with high carbon volume fraction (e.g. VC and TiC). Interlayer spacing of the produced graphite is about 0.34nm. Its thickness increases with chlorination temperature, which leads to a monotonous increase of pore size in the produced CDC. As for the CDCs synthesized from carbides with low carbon volume fraction (e.g. NbC), it is mainly composed of single or bi-layer graphene. The pore size distribution of these NbC–CDCs is insensitive to chlorination temperature. NbC–CDC synthesized at moderate temperature (600 or 800°C) contains a large amount of micropores (∼0.8nm) and small amount of mesopores. Since almost all of these micropores are end-opened, the produced NbC–CDC has high specific surface area (>2000m2g−1).

THE CARRIERS OF THE INTERSTELLAR UNIDENTIFIED INFRARED EMISSION FEATURES: CONSTRAINTS FROM THE INTERSTELLAR C-H STRETCHING FEATURES AT 3.2-3.5 μm

The unidentified infrared emission (UIE) features at 3.3, 6.2, 7.7, 8.6, and 11.3 micrometer, commonly attributed to polycyclic aromatic hydrocarbon (PAH) molecules, have been recently ascribed to mixed aromatic/aliphatic organic nanoparticles. More recently, an upper limit of <9% on the aliphatic fraction (i.e., the fraction of carbon atoms in aliphatic form) of the UIE carriers based on the observed intensities of the 3.4 and 3.3 micrometer emission features by attributing them to aliphatic and aromatic C-H stretching modes, respectively, and assuming A_34./A_3.3~0.68 derived from a small set of aliphatic and aromatic compounds, where A_3.4 and A_3.3 are respectively the band strengths of the 3.4 micrometer aliphatic and 3.3 micrometer aromatic C-H bonds. To improve the estimate of the aliphatic fraction of the UIE carriers, here we analyze 35 UIE sources which exhibit both the 3.3 and 3.4 micrometer C-H features and determine I_3.4/I_3.3, the ratio of the power emitted from the 3.4 micrometer feature to that from the 3.3 micrometer feature. We derive the median ratio to be <I_3.4/I_3.3> ~ 0.12. We employ density functional theory and second-order perturbation theory to compute A_3.4/A_3.3 for a range of methyl-substituted PAHs. The resulting A_3.4/A_3.3 ratio well exceeds 1.4, with an average ratio of <A_3.4/A_3.3> ~1.76. By attributing the 3.4 micrometer feature exclusively to aliphatic C-H stretch (i.e., neglecting anharmonicity and superhydrogenation), we derive the fraction of C atoms in aliphatic form to be ~2%. We therefore conclude that the UIE emitters are predominantly aromatic.

Prediction of Normal Boiling Points of Hydrocarbons Using Simple Molecular Properties

Four hundred and seventy-six hydrocarbons (CnHm )w ere utilized to fit their normal boiling point temperatures (NBPT) as a function of molecular weight and carbon atomic fraction. The proposed model is of the following form: NBPT=a ∗ (Cfrac) ∧ b ∗ (MW) ∧ c ,w here a, b ,a ndc are the non-linear regressed parameters for the given model; Cfrac is the carbon atomic fraction in a molecule, which is equal to n/(n+m) for a hydrocarbon compound; and MW is the molecular weight, which is calculated as 12 ∗ n+ 1 ∗ m. The model was found to predict NBPT with an adequate accuracy, manifested via the associ- ated percent relative error (PRE) of the curve-fitted NBPT. Out of the examined 476 hydrocarbons, methane, ethylene, and acetylene were found to have PRE values higher than 10%. If the confidence interval is further reduced to PRE value less than 5%, then 43 compounds will be excluded, and then NBPT for the other 433 compounds could be well predicted by the proposed model. Although the proposed model does not differentiate among isomers having the same molecular weight and chemical formula, nevertheless, the difference in NBPT among isomers is not really significant to be picked up by a simple, straight- forward model. A more rigorous model will work hard to offset such small differences in NBPT among isomers, nevertheless, at the expense of model simplicity.

THE CARRIERS OF THE INTERSTELLAR UNIDENTIFIED INFRARED EMISSION FEATURES: AROMATIC OR ALIPHATIC?

The unidentified infrared emission (UIE) features at 3.3, 6.2, 7.7, 8.6, and 11.3 micrometer, commonly attributed to polycyclic aromatic hydrocarbon (PAH) molecules, have been recently ascribed to coal- or kerogen-like organic nanoparticles with a mixed aromatic-aliphatic structure. However, we show in this Letter that this hypothesis is inconsistent with observations. We estimate the aliphatic fraction of the UIE carriers based on the observed intensities of the 3.4 and 6.85 micrometer emission features by attributing them exclusively to aliphatic C-H stretch and aliphatic C-H deformation vibrational modes, respectively. We derive the fraction of carbon atoms in aliphatic form to be <15%. We conclude that the UIE emitters are predominantly aromatic with aliphatic material at most a minor part of the UIE carriers. The PAH model is consistent with astronomical observations and PAHs dominate the strong UIE bands.

The physicochemical characteristics and hydrocarbon composition of crude oils of the Timan–Pechora petroliferous basin

The total hydrocarbon composition and average structural-group characteristics of typical Paleozoic crude oils of the Timan–Pechora petroliferous basin are described. The hydrocarbon (HC) types of crudes are compared, which were conventionally determined from the composition of their gasoline fractions, by mass-spectrometric analysis of the HC composition of crudes, and by structural-group analysis of crudes, based on radiospectrometric data. The analyses have shown the presence of 30 structural types of HCs with up to 43 carbon atoms. The whole series of members up to C43 are specific only for HCs with no more than three rings in the molecule. It has been established that the carbonate strata in the basin under investigation, independently of the depth of their occurrence and the age of the host deposits, generate and accumulate heavy high-resin high-sulfur oils rich in alicyclic structures and assigned by HC composition to the naphthene–methane or even naphthene type. Terrigenous reservoirs here abound in crudes of methane–naphthene type. The contents of sulfur and resinous substances and the fraction of carbon atoms in alicyclic HC structures decrease as the depth of occurrence of the host deposits grows, thus reflecting the known gradual process of “methanization” of petroleum composition.

Sp hybridization in free carbon nanoparticles—presence and stability observed by near edge X-ray absorption fine structure spectroscopy

The presence and stability of sp hybridized atoms in free carbon nanoparticles was investigated by NEXAFS spectroscopy. The experiments show that a predominant fraction of carbon atoms is found in linear sp-chains and that conversion into sp(2) structures proceeds already at low temperature and in the gas phase.

Structural and mechanical properties of diamond-like carbon films deposited by an anode layer source

An anode layer source is a special ion gun, which can be fed with carbon precursors like acetylene to deposit hard and highly defect-free hydrogenated diamond-like carbon films at room temperature. The present study focuses on the influence of the process parameters — discharge voltage, process pressure and acetylene flow — on structure and mechanical properties of the deposited films. Raman spectra show that an increased discharge voltage yields decreased structural disorder, i.e. a lower C–C sp 3 hybridised fraction of carbon atoms in the films. By an elevation of the discharge voltage from 1 to 3 kV the full width at half maximum of the G-band decreases from 194 ± 0.2 cm − 1 to 183 ± 0.7 cm − 1 . Films deposited at the lowest discharge voltage show in accordance to the spectroscopic data the highest nanohardness (36 ± 1 GPa), stress (− 2.34 ± 0.2 GPa) and reduced elastic modulus (180 ± 4 GPa).

A thermodynamic model for carbon trapping in lattice defects

A thermodynamic model in the framework of the CALPHAD method is proposed to describe the distribution of interstitial solute (carbon) atoms in bcc-iron during tempering of martensite. In the model, the crystal defects are introduced as a separate sublattice, which is a highly favored lattice position for carbon compared to undisturbed interstitial sites or precipitate sites in cementite. With the model, which is convenient for multicomponent, multiphase systems, the fraction of carbon atoms trapped by the defects during tempering can be calculated. It is demonstrated that, when martensite is heavily dislocated, the precipitation of cementite on tempering is greatly suppressed because the high dislocation density provides a sufficient number of stressed sites which can trap most of the carbon atoms. The calculations for an alloy steel show that, unlike cementite, the defect density and carbon trapping phenomenon influence the precipitation of stable carbides only marginally.

Effect of surface reactions of low-energy carbon ions on the secondary electron emission of TiN:O thin films

Low-energy secondary electron emission coatings are required for antimultipactor applications in several important technologies and their study and development is also a matter of scientific interest. For this purpose, titanium nitride was deposited on Si(100) substrates by reactive sputtering and the influence of low-energy carbon ion bombardment on the secondary electron emission yield of TiN:O coatings was studied. The composition and chemical bonds formed in the films after carbon ion implantation were analyzed by x-ray photoelectron spectroscopy (XPS). XPS revealed the formation of both carbidic and graphitic bonds. N was displaced while Ti-C bonds were formed up to a limiting value of 0.103 for the carbidic carbon atomic fraction, beyond which a graphitic surface layer was deposited. The secondary electron emission yields for TiN:O were measured before and after low-energy CHn+ ion bombardment and air exposure, and after heating. Initially, the carbon ion implantation reduced the secondary electron emission yield. Then an increase in secondary electron emission was obtained when the excess graphitic carbon was deposited on the sample. On the other hand, subsequent thermal treatment at 700 °C of the carburized samples produced a further reduction of the secondary electron emission yield. The maximum yields are about 53% lower for thermal annealed films than for similarly treated previously measured as-deposited layers. A narrowing of XPS peak line shapes is observed as a consequence of the annealing away of structural and chemical defects in the near-surface region. In addition, secondary electron emission (SEE) yield curves were used in a simulation of multipactor discharge. Both experimental multipactor threshold tests and computer multipactor simulations indicate that SEE yield values for low primary-electron energies are the most influencing parameters on multipactor threshold.