Source Of Atomic Carbon Research Articles

Nanodiamonds from Fuel

In this paper, we present the results of the synthesis of nanodiamonds from carbon nanostructures and diamondoids in an arc discharge. The electrodes used in the experiments were a molybdenum cathode and a copper anode coated with carbon nanostructures such as carbon nanotubes and diamondoids. The formation of carbon nanostructures on the surface of a copper electrode was carried out in an electric arc sunk into fuel oil. At the next stage, these carbon nanostructures served as a source of atomic carbon and diamondoids from the surface of the arc discharge anode. The use of fuel oil as a feedstock for creating the electrode material probably led to the retention of diamondoids contained in fuel oil during two vaporizations. It was found that, in addition to carbon nanotubes, nanodiamonds also formed on the cathode surface.

An atomic carbon source for high temperature molecular beam epitaxy of graphene

We report the use of a novel atomic carbon source for the molecular beam epitaxy (MBE) of graphene layers on hBN flakes and on sapphire wafers at substrate growth temperatures of ~1400 °C. The source produces a flux of predominantly atomic carbon, which diffuses through the walls of a Joule-heated tantalum tube filled with graphite powder. We demonstrate deposition of carbon on sapphire with carbon deposition rates up to 12 nm/h. Atomic force microscopy measurements reveal the formation of hexagonal moiré patterns when graphene monolayers are grown on hBN flakes. The Raman spectra of the graphene layers grown on hBN and sapphire with the sublimation carbon source and the atomic carbon source are similar, whilst the nature of the carbon aggregates is different - graphitic with the sublimation carbon source and amorphous with the atomic carbon source. At MBE growth temperatures we observe etching of the sapphire wafer surface by the flux from the atomic carbon source, which we have not observed in the MBE growth of graphene with the sublimation carbon source.

Growth and electronic structure of graphene on semiconducting Ge(110)

The direct growth of graphene on semiconducting or insulating substrates might help to overcome main drawbacks of metal-based synthesis, like metal-atom contaminations of graphene, transfer issues, etc. Here we present the growth of graphene on n-doped semiconducting Ge(110) by using an atomic carbon source and the study of the structural and electronic properties of the obtained interface. We found that graphene interacts weakly with the underlying Ge(110) substrate that keeps graphene's electronic structure almost intact promoting this interface for future graphene-semiconductor applications. The effect of dopants in Ge on the electronic properties of graphene is also discussed.

ATOMIC CARBON IN THE UPPER ATMOSPHERE OF TITAN

The atomic carbon emission C I line feature at 1657 A (^(3)P^(0)_(J) -^(3)P_J ) in the upper atmosphere of Titan is first identified from the airglow spectra obtained by the Cassini Ultra-violet Imaging Spectrograph. A one-dimensional photochemical model of Titan is used to study the photochemistry of atomic carbon on Titan. Reaction between CH and atomic hydrogen is the major source of atomic carbon, and reactions with hydrocarbons (C_2H_2 and C_2H_4) are the most important loss processes. Resonance scattering of sunlight by atomic carbon is the dominant emission mechanism. The emission intensity calculations based on model results show good agreement with the observations.

Ballistic hole transport in a quantum wire

We report ballistic hole conduction in 2μm long GaAs quantum wires of nominal cross-section 15 nm by 15 nm. In each of eight test wires, we observe several quantized conduction steps of approximate height 0.77e2∕h. The wires were fabricated using the cleaved edge overgrowth molecular-beam epitaxy process, and the modulation-doped acceptors were incorporated using a carbon filament as the source of atomic carbon.

Production of carbon nanotubes by microwave plasma torch at atmospheric pressure

The key requirements of nanotube formation are an atomic carbon source and a source of nanometal particles. Carbon nanotubes (CNTs) have been synthesized by an argon/nitrogen microwave plasma torch using a mixture of acetylene and vapor-phase iron pentacarbonyl at the atmospheric pressure. The synthesized CNTs have been analyzed by scanning electron microscopy, field-emission transmission electron microscopy, and Raman spectroscopy, and are shown to be multiwalled and have a bamboo-shaped structure. The synthesized CNTs in some areas are well aligned. It is also found that the higher the content of nitrogen gas used, the higher the number of rough and wavy surfaces and the inner intersecting layers.

CO2+ dissociative recombination: A source of thermal and nonthermal C on Mars

CO2+ dissociative recombination has been assumed in the past to proceed overwhelmingly by the channel that produces CO + O. Although the channel that leads to the products C + O2 is energetically possible, the significant rearrangement of bonds that is required has led to the belief that this channel contributes minimally. Seiersen et al. [2003] have recently measured the branching ratio for the latter channel, and they have reported a value of ∼9% of the total. We have constructed both low and high solar activity models of the Martian thermosphere, and we have tested the effect of including the C + O2 channel on the production of both thermal and escaping C atoms in the Martian atmosphere. We find that dissociative recombination of CO2+ is by far the dominant source of atomic carbon in both models, and its inclusion leads to larger densities of ambient C. The contribution of the source to the escape flux of C, however, is found to be small, both because the altitude profile of the production rate falls off rapidly near the exobase and because the O2 molecule is probably produced with considerable internal energy. A calculation of the statistical energy partitioning into vibrational and rotational energy of the product O2 molecule and the translational energy of the products of the reaction indicates that the C atom is produced with translational energy exceeding the escape energy only 3.7% of the time. The models predict that photodissociation of CO is the most important source of escaping C, as have other recent investigations. The computed escape fluxes for the minor sources, however, differ considerably from those of previous models. We find that the second most important source of escaping C is electron impact dissociation of CO, followed by dissociative recombination of CO+. The total predicted escape fluxes are comparable to those of previous models at high solar activity and are a factor of 2 larger at low solar activity.

Consequences of cometary aurora on the carbon chemistry at comet p/Halley

Various experimental data acquired during the visit of Halley's comet in 1986 have shown that the amount of carbon produced due to photodissociation of parent carbon bearing species is not ample enough to explain the observations. This requires the presence of an additional source of atomic carbon. One of the possible source could be auroral-type activities resulting from the precipitation of high-energy “auroral electrons” of solar wind origin, the evidence of which have been inferred from many observations at comet Halley. We have developed a coupled chemistry-transport model to study the role of auroral and photoelectron impact as well as of chemistry on the modelling of carbon in the inner coma (≤10 4 km) of comet Halley. Our study suggest that electron impact dissociation of CO is the major source of carbon production in the inner coma, not the recombination of CO + as suggested by earlier workers, while transport is the main loss process.

Ion composition and dynamics at comet Halley

The ion mass spectrometer aboard the Giotto spacecraft measured the composition and velocity distributions of cometary ions at distances of ∼7.5×l06 to ∼1,300 km from the nucleus of comet Halley. Well outside the bow shock, pick-up cometary H+ ions were found in a diffuse shell-like distribution. Heavier ions (C+, H2O+-group, CO+ and S+) with similar distributions have been identified at ≤3×l05 km. Solar-wind He2+ was found throughout the coma to as close as ∼5,000 km; He+ produced by charge exchange was seen inside ∼2×105 km. Deeper within the coma the main cometary hot-ion species identified were H+, H2+, C+, O+, OH+, H2O+, H3O+, CO+ and S+. A pile-up of heavy cometary ions was found at ∼104 km from the nucleus. Giotto crossed the contact surface at ∼4,600 km, based on changes in ion flow velocity and temperature. Inside, ion temperatures as low as ∼340 K and outflow velocities of ∼1 km s−1 were found. Outside the contact surface ion densities vary as r−2, with a transition to an r−l dependence approximately at the contact surface. A large C+ abundance throughout the coma indicates an unexpected direct source of atomic carbon. The nitrogen abundance, on the other hand, is relatively low.

Vibrational and rotational excitation of CO in comets Nonequilibrium calculations

Ultraviolet observations of atomic carbon in cometary comae show that carbon-bearing compounds must comprise a significant fraction (approximately equal to or greater than 10 percent) of the volatiles. One likely source is CO. This compound is an important constituent of at least some cometary nuclei, even though it may not be the ultimate source of atomic carbon in all comets. Feldman (1983) has suggested that the relative abundance of CO may be one of the few fundamental characteristics which distinguish one comet from another. The present investigation is, therefore, concerned with the vibrational and rotational excitation of the CO molecule in cometary comae. Two previously neglected factors are taken into account in the study. The fractional populations of CO as solutions to time-dependent differential equations are derived, and a nonisothermal and cold kinetic temperature profile for the inner coma is considered.