Carbon Dimer Research Articles

Squeezing carbon dimers into the smallest fullerenes: DFT prediction of new carbon clusters containing double hypercoordinate carbons

The viabilities of squeezing carbon dimers into the smallest fullerenes, C 20 and C 24, were investigated based on density functional theory computations. Each of the resulting endofullerenes, C 2@C 20 and C 2@C 24, contains double hypercoordinate carbons bonded with each other. These novel endofullerenes provide the first examples of hypercoordinate carbons achieved in fullerene derivatives and can be regarded as unprecedented bridged binuclear carbon sandwiches. Although the construction does not follow the eight electron counting rule, these structures are true energy minima on the potential energy surfaces because the remainder electrons on the polar rings are delocalized throughout the whole cage framework to obtain further stabilization. Interestingly, the 2( N+1) 2 rule is still valid for predicting the spherical aromaticities of these non-classical endofullerenes. These new carbon clusters would have less reactivity than fullerene C 20 and should be viable synthetic candidates. Extension of our constructing strategy would lead to the discovery of many new non-classical compounds and nanostructures.

Comparison of the characteristics of a plasma generated by a d.c. arc plasmatron and a high-voltage a.c. plasmatron with rail-shaped electrodes (gliding arc) as applied to the synthesis of carbon nanomaterials

Comparative results of investigating the parameters of plasmas generated by atmospheric-pressure electric-arc plasmatrons of two types operating in the regime of simultaneous partial oxidation of hydrocarbons and their pyrolysis, have been given. It has been established by spectral and thermophysical measurements that the component composition of the plasma and its thermal characteristics at exit from a d.c. plasmatron significantly differ from the parameters of a plasmatron of the second type — an a.c. plasmatron with rail-shaped electrodes of the gliding-arc type. The temperature nonequilibrium and the presence of the carbon dimers C2 in the plasma (in the absence of the monomer C) generated by the rail-shaped-electrode plasmatron point to the fact that realization of the synthesis of fullerene-containing particles and, probably, particles containing nanotubes is, in principle, possible in reactors based on it, just as in more energy-intensive reactors with low-voltage d.c. arc plasmatrons.

Charge-state dependence of energy loss of MeV dimers inGaAs(100)

Carbon and oxygen dimers with charge states $1+$ and $3+$ were implanted into $\mathrm{GaAs}$ along the [100] direction at an energy of $0.5\phantom{\rule{0.3em}{0ex}}\mathrm{MeV}∕\mathrm{atom}$. The defect depth profiles are extracted from Rutherford backscattering spectrometry and channeling. The depth profile of carbon is extracted from secondary ion mass spectrometry measurements. The defect density produced by dimer ions is larger than monomer ions. The depth profile of carbon in dimer implanted $\mathrm{GaAs}$ is deeper than that of monomer implanted $\mathrm{GaAs}$ showing negative molecular effect. The defect depth profile of oxygen dimer implanted $\mathrm{GaAs}$ is deeper for $3+$ than that for $1+$ charge state. This indicates that energy loss of ${\mathrm{O}}_{2}^{3+}$ is smaller than that of ${\mathrm{O}}_{2}^{+}$. It is attributed to charge asymmetry and a higher degree of alignment of ${\mathrm{O}}_{2}^{3+}$ along the [100] axis of $\mathrm{GaAs}$.

A Numerical Study of the Size and Rate Effects on the Mechanical Response of Single Crystal Diamond and UNCD Films

To better understand the mechanical response of ultrananocrystalline diamond (UNCD) and its grain boundary mechanism, a numerical study is performed of the specimen size and rate effects on the mechanical properties of single crystal diamond and UNCD films under uniaxial and shear loading paths, respectively. To compare with the UNCD films, single crystal diamond blocks of various sizes under tensile loading in the 100 hi direction and shear loading with the {100} 110 hi slip at different rates are first investigated via the molecular dynamics (MD) simulation. A combined kinetic Monte Carlo (KMC) and MD procedure is then developed for large-scale atomistic simulation of the mechanical response of UNCD films. In this numerical procedure, two single crystal diamond films, that are formed by the KMC method based on the mechanisms of UNCD growth from carbon dimers on the hydrogen-free (001) surface, are compressed along the [001] direction with two growth surfaces contacting each other at an elevated temperature in the MD simulation to create a polycrystalline UNCD film with certain grain boundary. The mechanical response of the resulting UNCD film is investigated by applying displacement-controlled shear loading in the MD simulation, and is compared with that of single crystal diamond. The preliminary results presented in this article provide a better understanding of the size and rate effects on the material properties of diamond and the role played by the grain boundary on influencing the mechanical response of UNCD films.

Carbon Ad-Dimer Defects in Carbon Nanotubes

The adsorption of carbon dimers on carbon nanotubes leads to a rich spectrum of structures and electronic structure modifications. Barriers for the formation of carbon dimer induced defects are calculated and found to be considerably lower than those for the Stone-Wales defect. The electronic states introduced by the ad-dimers depend on defect structure and tube type and size. Multiple carbon ad-dimers provide a route to structural engineering of patterned tubes that may be of interest for nanoelectronics.

Scandium iodides without and with carbon

There are two binary iodides of scandium, the rather trivial insulating ScI 3 and the scandium deficient Sc 0.9I 2, metallic above and insulating below about 100 K. At low temperatures, six out of eight excess electrons corresponding to Sc 0.89I 2 × 9 = Sc 8□ 1I 18 = (Sc 3+) 2(Sc 2+) 6(□ 1)(I −) 18 are localized in 3d states, and one regular metal site of a hypothetical CdI 2-like superstructure of ScI 2 is empty. At high temperatures, the electrons delocalize into a conduction band, in accord with the formulation (Sc 3+) 8(□ 1)(e −) 6(I −) 18. Excess electrons may also be added or consumed by a third partner. The first case is realized in the, again, scandium deficient ternary halides ASc x X 3 (A = Rb, Cs; X = Cl, Br, I). These have all, in principle, the CsNiCl 3-type of structure in which halide octahedra share common faces. Their centres are, in accord with, e.g., CsSc 0.75I 3 × 4 = Cs 4Sc 3□ 1I 12 = (Cs +) 4(Sc 3+) 3(□ 1)(e −) 1(I −) 12, occupied in three out of four cases. At sufficiently low temperatures, the one excess electron is localized at the middle scandium atom of a triad of scandium atoms. The scandium carbide iodides Sc 4C 2I 6 and Sc 6C 2I 11 are examples of reduced scandium iodides in which electrons are consumed by carbon dimers which occupy, as interstitials, the centres of scandium metal octahedra. In attempts to reproduce these in pure form, we have obtained Sc 24C 10I 30. It is built of nano-sized Sc 24C 10I 30 molecules which consist of a shell of 30 I − ions, a truncated and hollow T4 supertetrahedron. It envelopes a T3 supertetrahedron of 20 scandium atoms in which the tetrahedral interstices are occupied by carbon atoms. The T2 supertetrahedron C 10 itself incorporates a tetrahedron of scandium atoms. This inner scandium tetrahedron traps two electrons in a four-centre-two-electron molecular orbital. In total, Sc 24C 10I 30 may be viewed as (e −) 2(Sc 3+) 4(C 4−) 10(Sc 3+) 20(I −) 30.

Prediction of molecular-dynamics simulation results using feedforward neural networks: Reaction of a C2 dimer with an activated diamond (100) surface

A new approach involving neural networks combined with molecular dynamics has been used for the determination of reaction probabilities as a function of various input parameters for the reactions associated with the chemical-vapor deposition of carbon dimers on a diamond (100) surface. The data generated by the simulations have been used to train and test neural networks. The probabilities of chemisorption, scattering, and desorption as a function of input parameters, such as rotational energy, translational energy, and direction of the incident velocity vector of the carbon dimer, have been considered. The very good agreement obtained between the predictions of neural networks and those provided by molecular dynamics and the fact that, after training the network, the determination of the interpolated probabilities as a function of various input parameters involves only the evaluation of simple analytical expressions rather than computationally intensive algorithms show that neural networks are extremely powerful tools for interpolating the probabilities and rates of chemical reactions. We also find that a neural network fits the underlying trends in the data rather than the statistical variations present in the molecular-dynamics results. Consequently, neural networks can also provide a computationally convenient means of averaging the statistical variations inherent in molecular-dynamics calculations. In the present case the application of this method is found to reduce the statistical uncertainty in the molecular-dynamics results by about a factor of 3.5.

A density-functional study of the structural, electronic, magnetic, and vibrational properties of Ti8C12 metallocarbohedrynes

Calculations are presented for the structural, electronic, and vibrational properties of the different Ti8C12 metallocarbohedrynes. (Please note that we adopt the name "metallocarbohedrynes" instead of "metallocarbohedrenes" to denote the acetylenic nature of C2 units in this class of clusters demonstrated by several contributions in literature.) The density-functional theory (DFT) calculations are performed with the all-electron projector augmented-wave method and generalized gradient approximation for the exchange-correlation functional. We study the seven low-energy isomers of the Ti8C12 metallocarbohedrynes using spin-polarized DFT, where we find a correlation between the number of rotated carbon dimers and the cohesive energy of the structure. The electronic density of states (eDOS) show that C3nu, D*3d, and D3d isomers are spin polarized. The partial eDOS shows that, depending on the dimer orientation, carbon atoms and a subgroup of the metal atoms form a covalent framework while other metal atoms are bonded to this framework more ionically. This picture is further supported by the charge density of the different structures, where we see that the Ti atoms with higher charge density show less contribution to the covalent bonding of the Ti-C framework. The vibrational spectra of the different structures are calculated using the frozen-vibration method. Also, we calculate the vibrational spectra of the C3nu and C2nu structures using molecular-dynamics simulations at two different temperatures. The results of the simulations demonstrate the local stability of the structures beyond the harmonic limit explored by the frozen-vibration method.

Defects inSiO2as the possible origin of near interface traps in theSiC∕SiO2system: A systematic theoretical study

A systematic study of the level positions of intrinsic and carbon defects in ${\mathrm{SiO}}_{2}$ is presented, based on density functional calculations with a hybrid functional in an $\ensuremath{\alpha}$-quartz supercell. The results are analyzed from the point of view of the near interface traps (NIT), observed in both $\mathrm{SiC}∕{\mathrm{SiO}}_{2}$ and $\mathrm{Si}∕{\mathrm{SiO}}_{2}$ systems, and assumed to have their origins in the oxide. It is shown that the vacancies and the oxygen interstitial can be excluded as the origin of such NIT, while the silicon interstitial and carbon dimers give rise to gap levels in the energy range inferred from experiments. The properties of these defects are discussed in light of the knowledge about the $\mathrm{SiC}∕{\mathrm{SiO}}_{2}$ interface.

Determination and mapping of diameter and helicity for single-walled carbon nanotubes using nanobeam electron diffraction

The atomic structures of 124 single-walled carbon nanotubes, described by their diameter and helicity or equivalently by the two chiral indices $(u,v)$ that define the perimeter of each nanotube, have been determined unambiguously by nanobeam electron diffraction. A mapping of 70 possible nanotubes in the range of 1.20--1.65 nm in diameter with all possible helicities has been constructed experimentally for a carbon nanotube sample produced by arc discharge. Among the total 124 nanotubes characterized experimentally, 58 nanotubes of different structure have been identified. By examining the histogram of occurring helicities, we find that, while certain nanotubes were observed slightly more often than others, the overall feature showed a rather uniform distribution in occurrence. Basing on a nucleation-and-growth model, we suggest that the uniform distribution of helicity be originated from the weak dependence on helicity of the formation energy of carbon nanotubes, while the growth prefers slightly the structure with helicity 15\ifmmode^\circ\else\textdegree\fi{}--30\ifmmode^\circ\else\textdegree\fi{} for which the addition of carbon dimers would facilitate the growth of carbon nanotubes.

Application of variational reduced-density-matrix theory to the potential energy surfaces of the nitrogen and carbon dimers

The acceleration of the variational two-electron reduced-density-matrix (2-RDM) method, using a new first-order algorithm [D. A. Mazziotti, Phys. Rev. Lett. 93, 213001 (2004)], has shown its usefulness in the accurate description of potential energy surfaces in nontrivial basis sets. Here we apply the first-order 2-RDM method to the potential energy surfaces of the nitrogen and carbon dimers in polarized valence double-zeta basis sets for which benchmark full-configuration-interaction calculations exist. In a wave function formalism accurately stretching the triple bond of the nitrogen dimer requires at least six-particle excitations from the Hartree-Fock reference. Furthermore, cleaving the double bond of C2 should produce a "non-Morse"-like potential curve because the ground state near equilibrium (X 1sigma(g)+) has an avoided crossing with the second excited state (B' 1sigma(g)+) and a level crossing with the first excited state (B 1delta(g)). Because the 2-RDM method variationally optimizes the energy over correlated 2-RDMs on the two-electron space without parametrization of the many-electron wave function, it captures multireference correlations that are difficult to describe with approximate wave functions. The 2-RDM method yields for N2 a potential energy surface with features and spectroscopic constants that are more accurate than those from single-reference methods and similar in accuracy to multireference techniques, and it describes the non-Morse-like behavior of C2 which is not captured by single-reference methods.

Interactions of Carbon Atoms and Dimer Vacancies on the Si(001) Surface

Abstract We investigate the interactions between substitutional carbon atoms on the defect free, (2×1) reconstructed Si(001) surface, and bring evidence that the interaction energy differs significantly from the inverse-cube distance dependence that is predicted by the theory of force dipoles on an elastic half-space. Based on Tersoff potentials, we also calculate the interactions between carbon atoms and dimer vacancies. The calculations indicate that dimer vacancies (DVs) are strongly stabilized by fourth-layer C atoms placed directly underneath them. By use of simple model Monte Carlo simulations, we show that the computed interactions between carbon atoms and DVs lead to self-assembled vacancy lines, in qualitative agreement with recent experimental results.

Theoretical Analysis of Diamond Mechanosynthesis. Part I. Stability of C<SUB>2</SUB> Mediated Growth of Nanocrystalline Diamond C(110) Surface

A theoretical investigation of the chemical vapor deposition (CVD) growth on clean diamond C(110) surfaces from carbon dimer precursors shows that isolated deposited C2 dimers appear stable at room temperature, but a second carbon dimer subsequently chemisorbed in close vicinity to the first can adopt one of 19 local energy minima, five of which require barriers � 0.5 eV to reach the global minimum and, thus, constitute stabilized defects. Three chemisorbed C2 dimers can adopt one of 35 local minimum energy structures, 10 of which are stable defects located in deep potential energy wells. The number of potential defects increases with the number of deposited carbon dimers, which suggests an isolated rather than clustered growth mechanism in CVD on bare diamond C(110). These results also provide information regarding outcomes of the misplacement of a carbon dimer and establish constraints on the required dimerplacement positional precision that would be needed to avoid the formation of stable defects during diamond surface growth.

Theoretical Analysis of Diamond Mechanosynthesis. Part II. C<SUB>2</SUB> Mediated Growth of Diamond C(110) Surface via Si/Ge-Triadamantane Dimer Placement Tools

This paper presents a computational and theoretical investigation of the vacuum mechanosynthesis of di- amond on the clean C(110) surface from carbon dimer (C2) precursors positionally constrained throughout the reaction pathway by silicon- or germanium-doped triadamantane derivatives mounted on a scanning probe tip. Interactions between the dimer placement tools and the bare diamond C(110) surface are inves- tigated using Density Functional Theory (DFT) with generalized gradient approximation (GGA) by con- structing the reaction path potential energy profiles and analyzing ab initio molecular dynamics simulations. Similar methods are applied to study the energetics and kinetics of recharging the tool with acetylene. Molecular mechanics simulations on extended tool tips are carried out to elucidate the positional uncer- tainty of the carbon dimer due to thermal fluctuations, and the possibility of intermolecular dimerization and dehydrogenation of the dimer placement tools is explored.

Carbon dimer in silicon cage: A class of highly stable silicon carbide clusters

A class of silicon carbide cage clusters with two carbon atoms inside the silicon cage and with high stabilities are presented. The theoretical formalism used is Hartree-Fock theory followed by second-order many-body perturbation theory to account for correlation effects, and geometry optimizations at the second-order perturbation theory level are performed without any symmetry constraints. The smallest ``cage'' is found to be a silicon cube with the carbon dimer inside the cube. Based on the simultaneous criteria of high binding energy, high vertical ionization potential, high [highest occupied--lowest unoccupied molecular orbital (HOMO-LUMO)] gap, and a low vertical electron affinity, ${\mathrm{Si}}_{14}{\mathrm{C}}_{2},$ with a close fullerenelike structure, is predicted to be a particularly stable cluster both at the all-electron and at the pseudopotential level of calculations. The $\mathrm{C}---\mathrm{C}$ bond lengths and the HOMO-LUMO gaps of the clusters are both found to oscillate with cluster size.

Molecular Dynamics Simulation of Hydrocarbon Reflection and Dissociation Coefficients from Fusion-Relevant Carbon Surfaces

Reflection coefficients for carbon and hydrocarbon atoms/molecules on carbon-based surfaces are critically needed for plasma-surface interaction analysis in fusion devices, as carbon will continue to be used in next step devices like ITER. These have been calculated at different energies and angles with a molecular dynamics code using the Brenner hydrocarbon potential. Hydrogen saturated graphite was prepared by bombarding a graphite lattice with hydrogen, until a saturation at ~0.42 H:C. Carbon at 45° has a reflection coefficient (R) of 0.64 ± 0.01 at thermal energy, decreasing to 0.19 ± 0.01 at 10 eV. Carbon dimers (Rthermal = 0.51, R>1 eV ~ 0.10) tend to stick more readily than carbon trimers (Rthermal = 0.63, R10 eV = 0.16). Hydrocarbons reflect as molecules at thermal energies and break up at higher energies. The total reflection via these fragments decreases with energy, the number of unpaired electrons, and changing hybridization from sp3 to sp2 to sp. The results compare reasonably well with binary collision modeling for higher energies and experimental sticking data at thermal energies. A second surface, representing a “soft” redeposited carbon layer formed by the deposition of hydrocarbons onto a graphite surface, is also analyzed. In general, reflection is lower from the “soft” surface by 0.1–0.2. This reflection data can and has been incorporated in erosion/redeposition codes to allow improved modeling of chemically eroded carbon transport in fusion devices.

Desorption of the C2- Anion from the Au−C-Deposited Y2O3-Stabilized ZrO2 Surface

Carbon dimer anions (C2-) and atomic oxygen radical anions (O-) desorbed from an Au−C-deposited Y2O3-stabilized ZrO2 (YSZ) surface have been observed. Both the emitted anions and electrons were found to be strongly dependent on the surface temperature and applied extraction field. The Arrhenius plots for the anions and electrons exhibit double-linear behavior, resulting in lower apparent activation energies in the high-temperature region (>913 K). It was found that the apparent activation energies can be reduced by the extraction field. Measurements of the absolute emission current density show similar temperature and field features as derived by TOF (time-of-flight) spectroscopic investigations. The observed anions of O- are attributed to the detachment of the transient anions of O2-(s) (i.e., O2-(s) → O-(s) + e-(s)), which migrate from the YSZ bulk onto the emission surface. The C2-(s) anions are attributed to the surface anion-exchange reactions between O-(s) and surface carbon atoms. An emission mecha...

Theoretical study of ionization potentials and dissociation energies of Cnq+ fullerenes (n=50–60, q=0, 1 and 2)

We have evaluated electronic energies of neutral, singly charged and doubly charged fullerenes with sizes n=50–60 using density functional (DFT) theory. For each value of the cluster charge, we have considered around 40 possible structures. We have found that, except for C522+, the most stable isomer always has the minimum possible number of C2 units between adjacent pentagons. We have evaluated adiabatic dissociation energies corresponding to the various dissociation channels leading to the emission of carbon dimers with different charges. Our findings for dissociation leading to C2 emission are in reasonable agreement with the latest experimental values. As a byproduct of our calculations, we have also evaluated the first and second adiabatic ionization potentials. Both dissociation energies and ionization potential are useful data to interpret fragmentation of fullerenes by impact of energetic photons, electrons and ions.

Nanodimensional carbon materials formed in a gas discharge plasma

We have experimentally studied the formation of nanodimensional carbon materials from a methane-hydrogen gas mixture activated by a dc discharge. The range of discharge voltages and currents ensuring stable deposition of carbon films was determined. Data on the carbon-containing components of the activated gas phase were obtained by in situ optical emission spectroscopy of the gas discharge plasma. It is shown that the formation of nanodiamond and nanographite particles, as well as carbon nanotubes, in the deposited films is correlated with the presence of C2 carbon dimers in the gas phase. A mechanism of the noncatalytic formation of carbon nanotubes from platelike graphite nanoparticles is proposed.

Sputtering yield measurements during low energy xenon plasma bombardment

The sputtering yields of molybdenum, titanium, beryllium, and carbon have been measured during xenon ion bombardment from a plasma in the energy range between 10 and 200 eV. The erosion rates of Mo, Be, and C are measured both spectroscopically in the plasma and using the standard weight loss technique. Spectroscopic measurements of Ti sputtering yields, where no atomic physics data is available, are normalized to the weight loss measurements. The erosion rates of the metals decrease with the reduced mass of the metal–xenon combination and decrease with the increasing metal’s binding energy, as expected. The erosion results for bombardment of graphite indicate that the sputtering rate of carbon, as atoms, from the surface is insufficient to explain the total carbon weight loss measured. The multiple mechanisms for carbon erosion during plasma bombardment are discussed and the sputter rates of carbon atoms and carbon dimers are presented.