Neutral Carbon Atoms Research Articles

Study of CNT synthesis mechanism by in-situ spectroscopy

Optical emission spectra (OES) of the plasma during synthesis of carbon nanotubes (CNTs) were in-situ examined for the purpose of understanding the formation mechanism of CNTs. It is observed that simultaneous presence of abundant amount of neutral Fe-atoms and the neutral (or singly-ionized) carbon-atoms in the plasma, but without the induction of carbon clusters, is necessary for growing CNTs. The optimum chamber pressure required is 400 Torr in 100 W microwave powers, which corresponds to 5000 K plasma temperature. The CNTs are presumed to form via gas phase nucleation process and thus obtained CNTs are multi-wall, approximately 30–70 nm in diameter.

Distribution of the [Ci] Emission in the ρ Ophiuchi Dark Cloud

The 3P1-3P0 fine-structure line of the neutral carbon atom ([C I]) has been mapped over the 18 × 13 area of the L1688 cloud in the ρ Ophiuchi region with the Mount Fuji submillimeter-wave telescope. The 3P2-3P1 line of [C I] has also been observed toward two representative positions to evaluate the excitation temperature of the [C I] lines. The overall extent of the [C I] distribution generally resembles that of the 13CO distribution. The [C I] distribution has two major peaks; one (peak I) is at ρ Oph A, and the other (peak II) is toward the east side of the C18O core in the southern part of L1688. Peak II is located beyond the C18O core with respect to the exciting star HD 147889. The C0 column density is 5.0 × 1017 cm-2 toward peak II. The spatial distribution of the [C I] emission is compared with plane-parallel photodissociation region (PDR) models, which suggest that peak II is associated with a lower density PDR front, adjacent to the dense cloud cores observed in the C18O line emission. Alternatively, peak II is in the early stage of chemical evolution, where C0 has not been completely converted to CO. In this case, the difference in the [C I] and C18O distributions represents an evolutionary sequence. This is consistent with a picture of a shock-compressed formation of the dense cores in this region due to influences from the Sco OB2 association.

Ionization of carbon, nitrogen, and oxygen by electron impact

Total ionization cross sections of neutral carbon, nitrogen, and oxygen atoms by electron impact are presented. In our theory we have included the possibilities that (a) some target atoms used in an experiment were in metastable states close to the ground state, (b) excitation-autoionization of ${2s2p}^{m}$ exited states may be substantial, and (c) ions produced in experiments may be in excited, low-lying metastable states. The binary-encounter Bethe (BEB) model of Kim and Rudd [Phys. Rev. A 50, 3954 (1994)] is used to calculate the cross sections for direct ionization. Plane-wave Born cross sections scaled by the method developed by Kim [Phys. Rev. 64, 032713 (2001)] are used to determine the contributions from excitation-autoionization. A sum of the BEB cross sections for direct ionization weighted according to the statistical weights of the final ion states is used to modify the direct ionization cross sections. The combination of the BEB model and the scaled Born cross sections is in excellent agreement with available experimental data. The present method can easily be extended to heavier open-shell atoms.

Influence of the energy of sputtered carbon atoms on the constitution of diamond-like carbon thin films

Diamond-like carbon thin films have been deposited by unbalanced radio frequency magnetron sputtering of a graphite target in a pure argon discharge. The formation of a dense sp3-coordinated structure is usually described by the subplantation model in which the role of two critical factors, i.e. the energy of bombarding argon ions EAr+ and the flux ratio of ions to neutral carbon atoms, ΦAr+/ΦC, is well-known. The present study aimed, furthermore, at describing the influence of the energy of neutral carbon atoms EC on the constitution of resulting films. To this end, the ion energy dependence of the micro-density and the sp3 content of the deposited films were examined by means of EELS for two different working pressures (0.2 Pa and 2 Pa) at a constant flux ratio ΦAr+/ΦC=5. Due to enhanced collisional processes in the plasma at 2 Pa, the effects of an energy reduction of carbon atoms are revealed. The results can be explained again according to subplantation processes.

Compton Profiles of Variously Ionized Carbon Atoms with Configurations 1sm2sn2pq

Compton profiles have been computed for variously ionized carbon atoms with configurations 1sm2sn2pq, m = 1-2, n = 0-2, q = 0-2. The values of the Compton profiles from the present calculation can be used to interpret experimental cross sections of variously ionized carbon atoms colliding with other atoms. The calculation s have been performed in impulse approximation using numerical Hartree-Fock wave functions. Compton profiles of neutral carbon atoms, available in the literature, are in excellent agreement with the present calculation.

Observation of the C [CSC]i[/CSC] [TSUP]3[/TSUP][ITAL]P[/ITAL][TINF]2[/TINF]–[TSUP]3[/TSUP][ITAL]P[/ITAL][TINF]1[/TINF] Line toward the Orion Kleinmann-Low Region

The 3P2-3P1 fine-structure line of the neutral carbon atom (809 GHz) has been observed toward the Orion Kleinmann-Low (KL) region with the Mount Fuji submillimeter-wave telescope. The 6' × 6' area centered at Orion KL has been mapped with a grid spacing of 15. The intensity distribution of the 3P2-3P1 line is found to be similar to that of the 3P1-3P0 line; these lines are rather weak toward Orion KL, while they are both bright at Orion KL's northern and southern positions. The excitation temperature determined from the intensity ratio between the 3P2-3P1 and 3P1-3P0 lines ranges from 40 to 110 K. The excitation temperature is not enhanced toward Orion KL, whereas it tends to be high in the vicinity of θ1 Orionis C. These results indicate that the C I emission arises from a photodissociation surface illuminated by strong UV radiation from θ1 Ori C. The relative reduction in the C I intensities toward Orion KL is found to originate from a relatively low excitation temperature rather than from the depletion of the C I column density. The origin of the low-excitation temperature of C I toward Orion KL is discussed in terms of a radiative transfer effect.

The cosmic microwave background radiation temperature at a redshift of 2.34.

The existence of the cosmic microwave background radiation is a fundamental prediction of hot Big Bang cosmology, and its temperature should increase with increasing redshift. At the present time (redshift z = 0), the temperature has been determined with high precision to be T(CMBR)(0) = 2.726 +/- 0.010 K. In principle, the background temperature can be determined using measurements of the relative populations of atomic fine-structure levels, which are excited by the background radiation. But all previous measurements have achieved only upper limits, thus still formally permitting the radiation temperature to be constant with increasing redshift. Here we report the detection of absorption lines from the first and second fine-structure levels of neutral carbon atoms in an isolated cloud of gas at z = 2.3371. We also detected absorption due to several rotational transitions of molecular hydrogen, and fine-structure lines of singly ionized carbon. These constraints enable us to determine that the background radiation was indeed warmer in the past: we find that T(CMBR)(z = 2.3371) is between 6.0 and 14 K. This is in accord with the temperature of 9.1 K predicted by hot Big Bang cosmology.

The Mt. Fuji submillimeter-wave telescope

We have developed a 1.2 m submillimeter-wave telescope at the summit of Mt. Fuji to survey emission lines of the neutral carbon atom (CI) toward the Milky Way. A superconductor-insulator-superconductor mixer receiver on the Nasmyth focus is used to observe the 492 GHz band in SSB and the 345 GHz band in DSB simultaneously. The receiver noise temperature is 300 K in SSB and 200 K in DSB for 492 and 345 GHz, respectively. The intermediate frequency frequency is 1.8–2.5 GHz. An acousto-optical spectrometer which has the total bandwidth of 0.9 GHz and 1024 channel outputs has also been developed. The telescope was installed at the summit of Mt. Fuji (alt. 3725 m) in July 1998. It has been remotely operated via a satellite communication system from Tokyo or Nobeyama. Atmospheric opacity at Mt. Fuji was 0.4–1.0 at 492 GHz during 30% of the time and 0.07–0.5 at 345 GHz during 60% of the time from November 1998 to February 1999. The system noise temperature was 1000–3000 K in SSB at 492 GHz and 500–2000 K in DSB at 345 GHz. We observed the CI (3P1−3P0: 492 GHz) and CO (J=3−2: 345 GHz) emission lines from nearby molecular clouds with the beam size of 2.̓2 and 3.̓1, respectively. We describe the telescope system and report the performance obtained in the 1998 winter.

Is It Possible to Synthesize a Low-Valent Transition Metal Complex with a Neutral Carbon Atom as Terminal Ligand? A Theoretical Study of (CO)4FeC

Quantum chemical calculations at the NL-DFT (B3LYP) and CCSD(T) levels of theory have been carried out for the carbon complex (CO)4FeC. The bonding situation was analyzed with the NBO partitioning scheme and with the topological analysis of the electron density distribution. The results have been compared with the bonding situations in (CO)4FeCH2, I(CO)3FeCH, and Fe(CO)5. The trigonal-bipyramidal complex (CO)4FeC with an axial Fe−C bond is a minimum on the singlet potential energy surface, while the equatorial form is a transition state. The Fe−C bond has a large dissociation energy, De = 84.1 kcal/mol at B3LYP/II and De = 94.5 kcal/mol at CCSD(T)/II. The carbon ligand is a strong π-acceptor and an even stronger σ-donor. The analysis of the electronic structure suggests that (CO)4FeC should behave like a carbon nucleophile. Geometry optimization of the donor−acceptor complex (CO)4FeC−BCl3 yielded a strongly bonded compound, which has a calculated C−B bond energy of De = 25.6 kcal/mol at B3LYP/II. Lewis acid stabilized carbon complexes such as (CO)4FeC−BCl3 might become isolated under appropriate conditions.

Studies of the ablation plume arising in 193 nm laser irradiation of graphite in vacuum

Abstract Pulsed ArF ( λ =193 nm) laser ablation of graphite in vacuum has been investigated using wavelength, spatially and temporally resolved measurements of the plume emission. Electronically excited neutral carbon atoms, along with singly and doubly charged atomic carbon ions are identified amongst the ablated material. Plume expansion velocities are estimated from time gated CCD imaging of specific emissions, by measuring the time dependence of selected emissions through localised regions of space, and via complementary Langmuir probe measurements of the charged material within the plume. The relative merits and limitations of each measurement type are considered. Neutral C atoms are observed to expand with a velocity distribution which is symmetrical about the surface normal. In contrast, the emission from C + ions expanding in vacuum is found to maximise in the solid angle between the laser propagation axis and the surface normal. Such behaviour is rationalised in terms of excitation, and subsequent multiphoton ionisation, of C atoms in the leading edge of the expanding plasma ball whilst the 193 nm laser pulse is incident.

Large-Scale Mapping Observations of the C [CSC][CLC]i[/CLC][/CSC] [TSUP]3[/TSUP][ITAL]P[/ITAL][TINF]1[/TINF]–[TSUP]3[/TSUP][ITAL]P[/ITAL][TINF]0[/TINF] Line toward Heiles Cloud 2 in the Taurus Dark Cloud

A distribution of the neutral carbon atom (C i) in Heiles cloud 2 (HCL2) has been investigated with the Mount Fuji submillimeter-wave telescope. A region of 1.2 deg 2 covering a whole region of HCL2 has been mapped with the 3 P1‐ 3 P0 fine-structure line (492 GHz) of C i. The global extent of the C i emission is similar to that of 13 CO, extending from southeast to northwest. However, the C i intensity is found to be rather weak in dense cores traced by the line of C 18 O. On the other hand, strong C i emission is observed in a south part of J= 1‐0 HCL2 in which the C 18 O intensity is fairly weak. The C i/CO abundance ratio is greater than 0.8 for the C i peak, whereas it is 0.1 for the dense cores such as the cyanopolyyne peak. The C i‐rich cloud found in the south part may be in the early evolutionary stage of dense core formation where C i has not yet been converted completely into CO. This result implies that formation of dense cores is taking place from north to south in HCL2. Subject headings: ISM: atoms — ISM: clouds — ISM: evolution — ISM: individual (Heiles’s cloud 2)

Optical Emission Spectra during Carbon Nanotube Production by Arc Discharge in H2, CH4 or He Gas

Optical emission spectra during carbon nanotube production by DC arc discharge in H2, CH4 or He gas were investigated. Emission spectra for C(I) (neutral carbon) atoms and C2 dimers were commonly observed in the three gases. The existence of H atoms and CH molecules was verified by in situ observation during the evaporation in H2 or CH4 gas, because strong H and CH spectra were identified. It was concluded that H atoms were conducive for production of high-quality multiwalled carbon nanotubes by means of forming hydrocarbon species, but were not conducive for production of C60 and C70.

Influence of a magnetic field on deposition of diamond-like carbon films

A magnetic field was incorporated into pulsed laser deposition of diamond-like carbon films. Due to the collision of neutral carbon atoms with energized electrons in the plume, more carbon ions were generated. As a result, the hardness of the diamond-like carbon films was increased in comparison with the hardness of those deposited without a magnetic field. In order to confirm this mechanism, the plume current was measured. As expected, the plume current was increased considerably (about two times larger than that without the magnetic field). It is known that the collision probability is proportional to the magnitude of the magnetic field. So we can expect that a strong magnetic field should be more beneficial to the deposition of harder diamond-like carbon films. Our experimental results were in agreement with the prediction. Much harder diamond-like carbon films have been obtained under a strong magnetic field.

Low-temperature sputter deposition and characterisation of carbon nitride films

Bombarding a carbon target with low-energy nitrogen ions causes the release of neutral carbon atoms (physical sputtering) and volatile carbon nitride compounds (chemical sputtering) with relative yields dependent on the energy of the nitrogen beam. The chemically sputtered species are volatile and can be condensed on the substrate by reducing its temperature. Carbon nitride films have been deposited at varying nitrogen beam energies and substrate temperatures in a dual ion beam deposition chamber. Films were grown both with and without the presence of an additional assisting nitrogen beam. Reduction of the substrate temperature in conjunction with low sputter beam voltages (<200 V) caused the nitrogen concentration to attain a maximum value of 44%, the optical band gap to increase to 2.2 eV, the sheet conductivity to decrease to less than 10 −9 (Ω cm) −1 and the density to be reduced to 1.6 g cm −3. The chemical structure was investigated by Fourier transform infra-red, X-ray photoelectron and Auger electron spectroscopies. The increasing transparency is accompanied by structural changes indicating a transition from a predominantly sp 2-bonded amorphous sp 2/ sp 3 C–N network to a more linear polymer-like structure consisting predominantly of doubly and triply bonded C and N atoms. No evidence for the formation of a β-C 3N 4 phase was found.

Mechanisms of intrinsic stress generation in amorphous carbon thin films prepared by magnetron sputtering

Abstract Amorphous carbon (a-C) films have been deposited on biased silicon substrates at room temperature by conventional and unbalanced magnetron sputtering from a graphite target in pure argon discharges. The flux and energy of positive ions striking the surface of growing films were determined as functions of the substrate bias voltage by grid probe measurements. The flux of neutral carbon atoms condensed on Si substrates was deduced from Rutherford backscattering spectroscopy measurements. The mass density of a-C films, the argon concentration and intrinsic stress in these films were investigated as functions of the substrate bias voltage, i.e. the energy of positive ions impinging on the film surface. The similarity between the variations of the mass density and argon concentration as the substrate bias voltage was increased suggests that the increase in mass density was likely caused by the argon incorporation in the films. The intrinsic stress level was essentially dependent on the energy of positive ions striking the film surface. The experimental values of the intrinsic stress were compared with the values predicted by models of intrinsic stress generation in thin films reported in the literature. The good agreement between experimental and theoretical values of the intrinsic stress in a-C films sputter-deposited using conventional and unbalanced magnetron modes contributes to assess the validity of the model proposed by Davis.

Low-temperature formation of β-silicon carbide

Silicon carbide thin films were deposited by unbalanced radio frequency (RF) (13.56 MHz) magnetron sputtering from a silicon carbide target in a pure argon discharge. Deposition parameters were 80 W RF target power, 1.6 × 10 −3 mbar argon pressure and 6 cm target substrate distance. As determined with energy and mass analysis, the flux of the film-forming particles ( Φ Si + Φ C ≈ 0.6 × 10 15 cm −2 s −1 ) consists of mainly neutral silicon and carbon atoms with typical energies of a few electronvolts. The flux of the plating particles ( Φ Ar + ≈ 1.5 × 10 15 cm −2 s −1 ) is composed of argon ions with a well-defined energy of 24 eV. The low film growth rate, in combination with a high flux ratio of plating argon ions to film-forming particles Φ Ar +/( Φ Si + Φ C ) of about 2.5, shifts the temperature of the phase transition from amorphous silicon carbide to nanocrystalline silicon carbide from normally 1000 °C in the case of plasma-enhanced chemical vapour deposition (PECVD) or chemical vapour deposition (CVD) deposition techniques down to 420 °C. The films were characterized by measurements of the mechanical properties, e.g. hardness and stress, as well as X-ray diffraction (XRD) and Auger electron spectroscopy (AES). Due to the crystallization at 420 °C the stress of the films is reduced from 6.3 GPa (at 60 °C and for the amorphous phase) to approximately 0.8 GPa. The hardness is between 53 and 37 GPa. At higher plating energies (> 85 eV) the ion-plating-induced densification is diminished by preferential sputtering of silicon and consequently stoichiometry and hardness are negatively affected.

The First Observation of Sulfur in Anomalous Cosmic Rays by the [ITAL]Geotail[/ITAL] and the [ITAL]Wind[/ITAL] Spacecrafts

The Geotail high-energy particle instruments have observed cosmic-ray particles in the energy range from 3 MeV n-1 to 150 MeV n-1 at 1 AU during the period 1992 September-1995 August. A remarkable enhancement of anomalous cosmic-ray (ACR) N, O, Ne, and C is observed during the period. A measurable enhancement of the sulfur flux below about 20 MeV n-1 was observed. This is the first evidence showing the existence of sulfur in the anomalous component. The flux increase of anomalous sulfur, with a first ionization potential (FIP) of 10.4 eV, is smaller than that of ACR carbon with an FIP of 11.3 eV and much smaller than those of high-FIP elements, which suggests that the fractions of neutral carbon and sulfur atoms are significantly low in the very local interstellar medium.

Improvement of mechanical properties of amorphous carbon films deposited on polycarbonate plastics

Amorphous carbon films have been deposited on polycarbonate surfaces using laser ablation technique. The polycarbonate substrates were treated with oxygen plasma prior to film deposition. Mechanical properties of the films were examined as a function of plasma pretreatment time. Results showed that the mechanical behavior of the carbon film improved with pretreatment time. A one-hour pretreated sample exhibited no cracking, good adhesion, low stress and a hardness of 5 GPa which is 20 times higher than that of bare polycarbonate. Spectroscopic ellipsometry was used to characterize the film and interface properties in order to understand the effect of plasma on polycarbonate surface. Computer modeling of the ellipsometry data revealed that the improvement of mechanical behavior of the films is due to the modifications of the polycarbonate surfaces after the oxygen plasma treatment. Further correlation studies of surface modification by oxygen plasma was carried out by atomic force microscopy and stress measurement. Optical emission spectra from the plasma plume during laser ablation showed that the neutral and ionic carbon atoms were the dominant species. We believe their presence plays a major role in controlling the properties of our diamond-like carbon films.

The charge effects in the low-energy ion depositing processes

The charge effects in the low-energy carbon ion depositing process on a graphite surface were discussed by using an ab initio molecular orbital calculation with a UHF method and a 3-21G basis set. A positively charged carbon ion is strongly bound on a graphite surface by a covalent bond. A neutral carbon atom is adsorbed by a van der Waals interaction. A negatively charged carbon ion is not bound to a graphite surface and is repulsed. These charge effects are independent of the approach points of the incident carbon ion on a graphite surface.

Composition and energy spectra of anomalous cosmic rays observed by the GEOTAIL satellite

The composition and energy spectra of anomalous cosmic-rays (ACRs) in the energies 6 – 200 MeV/n have been measured during the period from September 1992 to August 1995 using the High Energy Particle instrument onboard the GEOTAIL satellite orbiting at 1 AU. A remarkable enhancement of ACR N, O, Ne and Ar is observed during the period. A flux enhancement of anomalous argon at 1 AU is confirmed by the GEOTAIL satellite. The first possible evidence for the emergence of a measurable sulfur component in ACR below about 20 MeV/n is found. The flux increase of anomalous sulfur with 10.4 eV FIP is smaller than that of ACR carbon with 11.3 eV FIP, and much smaller than those of high FIP elements, which suggests that the fraction of neutral carbon and sulfur atoms is significantly lower in the very local interstellar medium.