Deposition Of Carbon Atoms Research Articles

Effect of catalyst preparation on the yield of carbon nanotube growth

Abstract Multi-wall carbon nanotubes (MWCNTs) were synthesized by catalytic chemical vapor deposition (CVD) on catalytic iron nanoparticles dispersed in a silica matrix, prepared by sol gel method. In this contribution, variation of gelation condition on catalyst structure and its influence on the yield of carbon nanotubes growth was studied. The precursor utilized were tetraethyl-orthosilicate and iron nitrate. The sols were dried at two different temperatures in air (25 or 80 °C) and then treated at 450 °C for 10 h. The xerogels were introduced into the chamber and reduced in a hydrogen/nitrogen (10%v/v) atmosphere at 600 °C. MWCNTs were formed by deposition of carbon atoms from decomposition of acetylene at 700 °C. The system gelled at RT shows a yield of 100% respect to initial catalyst mass whereas the yield of that gelled at 80 °C was lower than 10%. Different crystalline phases are observed for both catalysts in each step of the process. Moreover, TPR analysis shows that iron oxide can be efficiently reduced to metallic iron only in the system gelled at room temperature. Carbon nanotubes display a diameter of about 25–40 nm and several micron lengths. The growth mechanism of MWCNTs is base growth mode for both catalysts.

Artifacts in the sputtering of inorganics by C 60n+

In the present study, the basic issues in C 60 n+ sputtering are studied using silicon, gold and platinum samples. Sputtering yields are measured for energies in the range of 5–30 keV, by sputtering micrometre sized craters on the surface of flat clean samples and measuring their volumes using atomic force microscopy (AFM). Net deposition of carbon occurs for all three materials at 5 keV, and is not specific to silicon which forms a carbide. The threshold energy for net sputtering is dependent on the sputtering yield and the stopping power of the substrate. Away from the threshold, the sputtering yields agree well with Sigmund and Claussen's thermal spike model after allowance for the sputtering of the deposited carbon atoms. AFM images show the formation of unusual surface topography around the transition region between sputtering and deposition. Analysis of the bottom of a crater using imaging SIMS shows a significant enhancement of carbon clusters as well as various silicon–carbon groups, indicating the importance of carbon deposition and implantation in a gradual mixed layer formed from sputtering. The thickness of this interface layer is shown to be approximately 5 nm.

Comparative experimental analysis of the a-C:H deposition processes using CH4 and C2H2 as precursors

The plasma enhanced chemical vapor deposition of a-C:H films using methane and acetylene as precursors was studied. Noninvasive in situ techniques were used to analyze the plasma processes with respect to the self-bias voltage, the displacement currents to the grounded electrode, the neutral gas composition, the optical sheath thickness as well as current and energy of the ions hitting the powered electrode. The a-C:H films were characterized for their deposition rate, surface roughness, hardness, mass density, and hydrogen content. Ion mean free paths, suitable for low-pressure rf sheaths, have been quantified for both precursors. The film with the highest hardness of 25GPa was formed in the C2H2 discharge when the mean energy per deposited carbon atom was approximately 50eV. The hardness obtained with the CH4 discharge was lower at 17GPa and less sensitive to changes in the process parameters. It was found that the creation of hard (hardness >15GPa) a-C:H films from both precursors is possible if the mean energy per deposited carbon atom exceeds only ∼15eV. Further film characteristics such as surface roughness and hydrogen content show the interplay of ion flux and deposition from radicals to form the a-C:H structure and properties.

Atomic mechanism of twin formation in CVD diamond films

The macro-texture, grain boundary distribution and surface morphology in CVD free standing diamond films deposited with different methane concentrations were observed by X ray diffraction, electron backscatter diffraction and SEM. The atom stacking process of twins on {111} planes of diamond crystal were studied. It is shown that the twins form easily on {111} planes because of the 〈111〉 60° orientation relationship of first order twin and the stacking structure of {111} planes. At low methane concentration, carbon atoms tend to deposit on {111} planes with lower surface energy, which facilitates the formation of twins by means of rotating deposition of carbon atoms, while the high twinning frequency weakens the texture. With increasing methane concentration the {001} planes with lower growing activation energy become the main frontal growth fronts, and only those grains with 〈001〉 crystal direction which is parallel to the surface normal can continuously grow, so that the probability of twin nucleation decreases. In addition, second_order twins were found in diamond film, and their formation mechanism was analyzed.

Field emission characteristics of an individual carbon nanotube inside a field emission-scanning electron microscope

Field emission (FE) properties of individual single-walled carbon nanotubes (SWCNT) were investigated inside a field emission-scanning electron microscopy. The individual SWCNT turned on a voltage of 23 V defined to produce a current of 10 pA, and was saturated at around 43 V and 880 nA. The FE characteristic of individual SWCNT also followed a conventional Fowler–Nordheim (F–N) theory in which a single linear slope in the F–N plots is measured below their limit of current level corresponding to the saturation regime of emission current. Energy-dispersive X-ray spectroscopy analysis showed that carbon atoms were deposited on the anode surface by the local heating of SWCNT tip during the FE processes and indicated about atomic 83% of carbon atoms. The carbon atoms were newly found to be evaporated and deposited on the anode surface during the FE process such that it was assumed that the degradation of FE was caused by evaporation and deposition of carbon atoms during the FE process.

A Tight-Binding Molecular Dynamics Simulation Analysis of Carbon Nanotube Growth Process Parameters

This paper employs tight-binding molecular dynamics simulations to investigate the effects upon the final nanotube morphology of three principal carbon nanotube growth process parameters, namely the radii of the initial carbon nanotube patterns, the temperatures of the carbon nanotube patterns, and the energies of the incident carbon atoms. The simulation results provide an understanding of the roles played by these parameters in the fabrication of carbon nanotubes from an atomic perspective. It is shown that carbon nanotube patterns with small radii tend to form close-capped ends, while patterns with larger radii tend to promote lifted-up growth. Furthermore, the effect of pattern temperature is more obvious in the case of larger radii carbon nanotube patterns, in which disordered structures are evident at low pattern temperatures. Finally, it is shown that incident atoms with higher energies not only form unstable bonds with other carbon atoms, but may also destroy the covalent bonds of previously deposited carbon atoms. The present results provide a valuable insight into the promotion of close-capped nanotubes or lifted-up growth during the fabrication of carbon nanotubes.

Growth precursors for a-C:H film deposition in pulsed inductively coupled methane plasmas

Film deposition in pulsed inductively coupled plasmas is investigated using methane as precursor gas. The densities of neutrals as well as ion and radical fluxes are quantified using mass spectrometry. Plasma composition is uniquely determined by the mean dissipated energy per source gas molecule in the plasma, Emean. Film growth rates are measured by in situ real-time ellipsometry. The contributions of individual precursors to film growth are discussed quantitatively by comparing absolute net growth fluxes of reactive species with deposited carbon atoms. It is shown that chemisorption of CH3 or implantation of energetic carbonaceous ions cannot explain the observed growth rates. Instead, film deposition occurs either by incorporation of CH radicals at small values of Emean(<10eV) or by incorporation of unsaturated hydrocarbon radicals such as C2H3 or C2H at intermediate values of Emean(10<Emean<100eV). Film growth depends strongly on incorporation of carbonaceous ions only at larger values of Emean(>100eV).

Molecular dynamics simulation of the growth of thin films by deposition of carbon atoms andC60molecules on diamond and silicon substrates

The impact of C atoms and ${\mathrm{C}}_{60}$ molecules with ideal diamond and silicon (100) substrates and the subsequent growth of carbon films have been investigated by molecular dynamics simulations. The interatomic many-body potential proposed by Tersoff has been used. The structural and vibrational properties of the as-grown and annealed films are studied as a function of the deposition energy (in the range $1--150\phantom{\rule{0.3em}{0ex}}\mathrm{eV}$ for C atoms and $1--1000\phantom{\rule{0.3em}{0ex}}\mathrm{eV}$ for ${\mathrm{C}}_{60}$ molecules) and are compared with experimental results. Analysis of films grown from ${\mathrm{C}}_{60}$ molecules reveals a behavior with deposition energy similar to that experimentally observed. For low deposition energies (below $100\phantom{\rule{0.3em}{0ex}}\mathrm{eV}$) fullerene cages preserve their identity, constructing low-density structures with large intermolecular holes and practically no interface with the substrate. For higher deposition energies the molecules are broken into pieces, giving as a result high-density amorphous carbon films. Although the penetration depth of molecular fragments into the substrate increases with deposition energy, the resulting interface is considerably thinner than in the case of using individual atoms as projectiles. This is in agreement with experimental evidence of a poor adherence of films obtained by accelerating $\mathrm{C}_{60}{}^{+}$ ions on silicon substrates.

Isomerization of n-butane: Conversion as a function of sulfate loading on sulfated zirconia catalysts

Sulfur loading on a sulfated zirconia catalyst was studied as a function of H 2 SO 4 concentration. The resulting catalytic activity was tested using the isomerization of n -butane as a reaction probe at 200°C. Optimum catalytic activity was observed for a catalyst prepared using 0.5 N H 2 SO 4 . The amount of carbon required to completely deactivate the catalyst was determined using a TGA/FTIR technique. For all of the catalysts synthesized in this study it was found that two active sulfate sites were poisoned by a single deposited carbon atom. Using this information, an active site composed of two coupled sulfate sites, perhaps a surface pyrosulfate site, is suggested. Using a TGA/FTIR technique it was found that the number of catalytically active sulfate sites never exceeded 14% of the total sulfate loading. These results were also supported by calorimetric studies.

Process of Energetic Carbon Atom Deposition on Si (001)Substrate by Molecular Dynamics Simulation

The process of energetic C atom deposition on Si (001)-(2×1) is studiedby the molecular dynamics method using the semi-empirical many-bond Tersoffpotential. It is found that the incident energy of the carbon atom has animportant effect on the collision process and its diffusion process on thesubstrate. Most of the incident energy of the carbon atom is transferred to thesubstrate atoms within the initial two vibration periods of substrate atomsand its value increases with the incident energy. The spreading distance andpenetration depth of the incident atom increasing with the incident energy arealso identified. The simulated results imply that an important effect ofenergy of incident carbon on the film growth at low substrate temperatureprovides activation energy for silicon carbide formation through thevibration enhancement of local substrate atoms. In addition, suppressingcarbon atom inhomogeneous collection and dispensing with the silicondiffusion process may be effectively promoted by the spreading andpenetration of the energetic carbon atom in the silicon substrate.

Ion-assisted pulsed magnetron sputtering deposition of ta-C films

The process of ion-assisted deposition of ta-C films by pulsed magnetron sputtering of a graphite target has been investigated. Probe measurements of the magnetron discharge plasma have been performed and its space- and time-dependent characteristics have been obtained as functions of the sputtering parameters and the bias voltage applied to the substrate. It has been shown that the density of the pulsed magnetron discharge plasma approaches values typical of pulsed laser or vacuum arc cathode sputtering of graphite (10 17−10 18 m −3). Raman scattering was used to examine the ta-C films produced at both low and high pulsed bias voltages applied to the substrate ( U sub<1 kV, τ=160 s and 1 kV< U sub<6 kV, τ=40 μs, respectively). It has been shown that in the first case the maximum content of diamond-like carbon in the coating (50–60%) is achieved at a lower energy per deposited carbon atom ( E C=100–300 eV) than in the second one (40–50% at E C=1100–1200 eV). Despite the lower diamond-like phase content in the coatings produced at a high bias voltage, they show better adhesion to the substrate due to the ion mixing between the film and the substrate and between the film layers. The use of shorter bias pulses is also important to prevent breakdowns of the deposited dielectric ta-C film.

Use of the hydrocarbon plasma of a low-pressure arc discharge for deposition of highly adhesive a-C:H films

It has been investigated whether a source of a hydrocarbon plasma generated by a non-self-sustained low-pressure arc discharge is suitable for production of highly adhesive a-C:H films. The distinguishing feature of this plasma source is the possibility for varying the degree of plasma ionization and the degree of destruction of hydrocarbon gases, making possible to realize both pure plasma-immersion ion implantation (PIII) and plasma-immersion ion deposition (PIID) in a unified vacuum cycle. The plasma parameters were measured with probes as functions of discharge current. Based on these measurements, the parameters of plasma generator have been determined for the PIII and PIID operation. The parameters of the pulsed bias voltage applied to the substrate in the process of the ion implantation and growth of an a-C:H film have also been preliminary chosen. For PIID it has been demonstrated that the improvements in quality of the a-C:H coating and in its adherence to the substrate are attained when an energy per carbon atom built in the coating is lying in the range 200–500 eV. The growth rates of a-C:H films in this case are approximately 200–300 nm/h. It was shown that the most favorable for achieving high energies per deposited carbon atom and for reducing of residual intrinsic stress are short (∼60 μs) high-voltage (>1 kV) substrate bias pulses.

Surface reactions of trimethylgallium on MOVPE-grown GaN(0001)

The reaction of trimethylgallium (TMGa) on MOVPE-grown GaN(0001) was studied using temperature-programmed desorption (TPD), high-resolution electron energy loss spectroscopy (HREELS), Auger electron spectroscopy (AES), and Rutherford backscattering spectrometry (RBS). Dissociation of adsorbed TMGa to form monomethylgallium (MMGa) and/or dimethylgallium (DMGa) and surface methyl groups occurred at temperatures above 250 K. Chemisorbed MMGa/DMGa either desorbed between 250 and 450 K or underwent further dissociation forming adsorbed methyl groups and Ga atoms. For low coverages, surface methyl groups desorbed near 600 K producing gaseous methyl radicals. At higher coverages, a fraction of the methyl groups underwent dehydrogenation, resulting in the production of methane at 590 K and the deposition of carbon atoms.

Automatic Production of Fullerenes by A JxB ARC Jet Discharge Preventing Carbon Vapor from Depositing on the Cathode

ABSTRACT Deposition of carbon atoms on a cathode in a DC arc discharge to produce fullerenes is reduced by JxB force of a magnetic field in an arc flame. By this modification, more effective synthesis of fullerenes and easier control of discharge conditions are achieved.

Hard diamond-like carbon films prepared by various physical vapour deposition techniques

Amorphous diamond-like carbon (DLC) films were prepared both by dual-ion-beam sputtering (DIBS) and by ion-beam-assisted d.c. magnetron sputtering (IBAM) of a graphite target under an argon atmosphere. The growing film was bombarded with 50–300 eV inert gas ions. Homogeneous, adherent films up to 0.8 μm thick were deposited on glass, steel and silicon substrates at a highest deposition rate of 5 nm min −1 observed with IBAM. This technique, as compared with DIBS, allowed the ion energy per deposited carbon atom to vary in a larger range and achieved a reduced stress level. The film properties were carefully examined and related to the process parameters in order to determine the growth conditions favouring DLC films with respect to poor quality carbon ones. The hardness reached 40 000 MPa and the morphology was very compact. The inert gas concentration rose from 1 to 2.5 at.% with film bombardment. The low hydrogen content (0.5–1 at.%) measured for DIBS films was increased to 10 at.% with IBAM. The density was strongly dependent on the ion bombardment parameters (ion energy, nature and flux). It ranged from 1.6 to 2.7 g cm −3 for the DIBS films and from 1.4 to 2.3 g cm −3 for the IBAM layers. Ball-on-disc tests performed in a laboratory environment pointed out a low friction coefficient varying from 0.08 to 0.1. Furthermore, the ion bombardment raised the optical indices.

Molecular Dynamics Simulations of Thin Film Diamond Growth

ABSTRACTThe growth of synthetic diamond thin films is studied by molecular dynamics computer simulations of low energy carbon atom deposition onto a low temperature diamond (111) surface. A previously reported Stillinger Weber potential, reparameterised for sp3 bonding in carbon using Hartree-Fock calculations for small carbon clusters, is used to model the interatomic interactions. The penetration of 1-100eV neutral carbon atoms into a (111) surface of carbon at 100K and the resultant surface atom rearrangements and induced film stress are studied. For intermediate energies (20-60eV) the incident atoms penetrate beneath the exposed (111) surface and increase the lateral compressive stress in the diamond film. It is suggested that diamond films grow from below the exposed surface in a region of locally high stress and tetrahedral coordination. The predicted radial distribution functions agree reasonably with electron diffraction studies of vacuum arc deposited amorphous diamond.

Carbon buildup by ion-induced polymerization under 100–400 keV H, He and Li bombardment

The ion-induced carbon buildup has been investigated on nickel surfaces in hydrocarbon atmospheres in the 10 −6 mbar range. Under bombardment with H, He, and Li ions of 100–400 keV energy, the number of deposited carbon atoms was measured by means of the nuclear reaction 12C(d, p) 13C. From the comparison of methane, benzene, roughing pump oil and squalane, the amount of deposited material was found to increase strongly with the molecular weight of the admitted gas. The deposition rate varies nonlinearly as function of the dose rate of the incident beam (between 1.5 × 10 13 and 1 × 10 15 cm −2 s −1). This is in agreement with a simple model that describes the deposition as being controlled by ion-induced polymerization in an adsorbed layer. According to this model, cross-sections of polymerization have been deduced as function of ion energy.

Characterization and performance of platinum-iridium reforming catalysts

Unsupported and ..gamma..-alumina-supported catalysts containing 0:1 to 64:1 iridium-platinum ratios were prepared and characterized by X-ray diffraction, Auger spectroscopy, and activity and selectivity tests for hexane and heptane reforming at 650/sup 0/K and 1 bar. Only monophasic solid solutions were observed. The surface concentration of iridium was low even at high bulk concentrations. The main effect of the iridium was suppression of the formation of carbonaceous deposits, possible by migration of deposited carbon atoms from platinum to iridium centers and their hydrogenation to methane. Under the mild reforming conditions used, no significant change in selectivity was observed with increasing iridium content, but at the higher pressures of industrial reforming when greater amounts of carbon deposit, the presence or iridium causes the known improvement of isomerization activity. Tests with acidic ..gamma..-alumina and with iridium/platinum on nonacidic ..cap alpha..-alumina confirmed the bifunctional character of the reforming catalysis, in which the alkane is dehydrogenated on a metal site, isomerized on an acidic site, and finally hydrogenated on a metal site.

ニッケル－炭素繊維間の高温挙動

Structural changes of carbon fibers coated with nickel were observed during holding at high temperatures.The inner thin layer of the coated nickel entered into the carbon fiber in the form of a ring at the earlier stages, and moved and aggregated to the center of the fiber. The moving ring in the fiber was found to be nickel metal by X-ray diffraction. The part of the carbon fiber passed by the nickel had a flakey appearance; it should be graphite.This may be due to diffusion of the carbon atoms in the nickel from the less graphitized (crystallized) part of the carbon to more graphitized part, and deposited carbon atoms crystallize in graphite.Graphitized carbon fiber is considerably weaker than the original fiber.

An electron spin resonance study of coke deposits on a silica-alumina catalyst

The coke deposited on a silica-alumina catalyst from the cracking of n-hexane, cyclohexane, n-hexene-1, benzene, thiophene, and pyridine was studied by electron spin resonance. Thiophene exhibited a different linear increase in spin concentration with carbon content and produced double the number of spins per deposited carbon atom as compared with the hydrocarbons. The number of carbon atoms was about 10 3 times the number of unpaired spins. In the absence of air the unpaired spins saturated easily, while the admission of air resulted in the adsorption of ogygen which rendered the spin system unsaturated. In addition, oxygen decreased the spin concentration by a factor of 3. These oxygen effects were reversible. The results of this study correlated well with the coke structure model proposed by Haldeman and Botty.