Nucleation of single-walled carbon nanotubes

The nucleation pathway for single-wall carbon nanotubes on a metal surface is demonstrated by a series of total energy calculations using density functional theory. Incorporation of pentagons at an early stage of nucleation is energetically favorable as they reduce the number of dangling bonds and facilitate curvature of the structure and bonding to the metal. In the presence of the metal surface, nucleation of a closed cap or a capped single-wall carbon nanotube is overwhelmingly favored compared to any structure with dangling bonds or to a fullerene.     

F NMR and EPR study of fluorinated buckminsterfullerene C60F58

Solid state ¹⁹F NMR in the temperature range from 96 to 366 K and room temperature EPR studies of fluorinated buckminsterfullerene C₆₀F₅₈ have been carried out. The temperature dependence of the line width and the spin-lattice relaxation time show hindered molecular motion with the activation energy of ΔE_a=1.9 kcal/mol. Neither phase transition nor random rotation of C₆₀F₅₈ have been obtained. The spin-lattice relaxation rate is strongly affected by the presence of paramagnetic centers, namely, dangling C-C bonds yielding localized unpaired electrons. Such broken bonds are caused by C-C bond rupture in a cage-opened structure of hyperfluorinated species.     

EPR investigation of a-Si:H aerosol particles formed under silane thermal decomposition

The dangling bond defects were investigated in a-Si:H particles formed under silane thermal decomposition in flow reactor. EPR together with hydrogen evolution method were used. The experimental results allowed us to conclude that there are two kinds of dangling bond defects in a-Si:H aerosol particles. The defects of the first kind are localized on the surface of interconnected microvoids and microchannels (surface dangling bonds) and those of the second kind are embedded in amorphous silicon network (volume dangling bonds). The thermal equilibration of dangling bonds and temperature dependence of equilibrium dangling bond concentration were investigated. It was found that at temperatures > 400 K the dangling bond concentrationNApplied Magnetic Resonance _s reversibly depends on sample temperature. The volume dangling bond concentration increases with temperature increasing (the effective activation energy of dangling bond formationU > 0), and the surface dangling bond concentration decreases with temperature increasing (U < 0). It has been found that EPR line is considerably asymmetric for samples with high hydrogen content and for low hydrogen content the EPR line is weakly asymmetric. A conclusion was drawn that the asymmetry degree depends on amorphous silicon lattice distortions. This conclusion has been confirmed by EPR spectra simulations.     

Defects and impurities in nanodiamonds: EPR, NMR and TEM study

EPR, ¹³C NMR and TEM study of ultradisperse diamond (UDD) samples is reported. The compounds show a high concentration of paramagnetic centers (up to 10²⁰ spin/g), which are due to structural defects (dangling C-C bonds) on the diamond cluster surface. The anomalous reduction in the spin-lattice relaxation time of ¹³C (from several hours in natural diamond to ∼150 ms in UDD clusters) is attributed to the interaction between the unpaired electrons of the paramagnetic centers and nuclear spins. ¹³C NMR line-width reflects the fact that the structure of the UDD surface is distorted in comparison to the ‘bulk’ diamond structure.     

First-principles investigation of the intrinsic defects in Ti3SiC2

First-principles calculations have been carried out to investigate intrinsic defects including vacancies, interstitials, antisite defects, Frenkel and Schottky defects in the 312 MAX phase Ti₃SiC₂. The formation energies of defects are obtained according to the elemental chemical potentials which are determined by the phase stability conditions. The most stable self-interstitials are all found in the hexahedral position surrounded by two Ti(2) and three Si atoms. For the entire elemental chemical potential range considered, our results demonstrated that Si and C related defects, including vacancies, interstitials and Frenkel defects are the most dominant defects. Besides, the present calculations also reveal that the formation energies of C and Si Frenkel defects are much lower than those of all Schottky defects considered. In addition, the calculated profiles of densities of states for the defective Ti₃SiC₂ indicate that these defects should have great influence on its thermal and electrical properties.     

Surfactants: Perfect heteroepitaxy of Ge on Si(111)

The hetero growth of Ge on Si results in formation of 3D clusters with an uncontrolled defect structure. Introduction of a monolayer of a surfactant completely changes the growth mode to a 2D-layer growth (Frankvan der Merwe) with a continuous and smooth Ge film on Si(111). The surfactant is not incorporated but segregates and floates on the growing Ge film. The saturation of the dangling bonds of the semiconductor reduces the surface free energy and drives the strong segregation. The effect on the growth process is the selective change of activation energies which are important for diffusion and the mobility of the Ge. Up to a thickness of 8 MLs (MonoLayers) the misfit-related strain of the pseudomorphic Ge film is relaxed by formation of a micro rough surface. This allows a partial relaxation of the Ge towards its bulk lattice constant which would not be possible for a flat and continuous film. For thicker Ge films the misfit of 4.2% is relieved by a periodic dislocation network, which is confined to the Si-Ge interface. Ge-films thicker than 20 MLs are free of defects and completely relaxed to the Ge bulk lattice constant: a model system for perfect heteroepitaxial growth.     

Density functional calculations of dangling bonds for CN and SiN films

Hyperfine interaction constants (HFICs) of dangling bonds for CN and SiN films were calculated by density-functional theory. The averaged ¹⁴N isotropic HFICs for C dangling bonds are almost equal to those for Si dangling bonds. The anisotropic ¹⁴N HFICs calculated for C dangling bonds are larger than those for Si dangling bonds by a factor 2. The calculated results were compared with experimental results obtained by electron spin resonance and electron nuclear double resonance. It was indicated that the carbon dangling bonds are located such that they avoid N atoms in CN films.     

Metastability of amorphous silicon from silicon network rebonding

We propose a network rebonding model for light-induced metastability in amorphous silicon, involving bonding rearrangements of silicon and hydrogen atoms. Nonradiative recombination breaks weak silicon bonds and generates dangling bond-floating bond pairs, with very low activation energies. The transient floating bonds annihilate, generating local hydrogen motion. Charged defects are also found. Support for these processes is found with tight-binding molecular dynamics simulations. The model accounts for major experimental features of the Staebler-Wronski effect including electron-spin resonance data, the t(1/3) kinetics of defect formation, two types of metastable dangling bonds, and hysteretic annealing.     

Simulating the thermal stability and phase changes of small carbon clusters and fullerenes

The thermal stability, phases and phase changes of small carbon clusters and fullerenes are investigated by constant energy Molecular Dynamics simulations performed over a wide range of temperatures, i.e., from to above the melting point of graphitic carbon. The covalent bonds between the carbon atoms in the clusters are represented by the many-body Tersoff potential. The zero temperature structural characteristics of the clusters, i.e., the minimum energy structures as well as the isomer hierarchy can be rationalized in terms of the interplay between the strain energy (due to the surface curvature) and the number of dangling bonds in the cluster. Minimization of the strain energy opposes the formation of cage structures whereas minimization of the number of dangling bonds favors it. To obtain a reliable picture of the processes experienced by carbon clusters as a function of temperature, both thermal and dynamical characteristics of the clusters are carefully analyzed. We find that higher excitation temperatures are required for producing structural transformations in the minimum energy structures than in higher lying isomers. We have also been able to unambiguously identify some structural changes of the clusters occurring at temperatures well below the melting-like transition. On the other hand, the melting-like transition is interrupted before completion, i.e., the thermal decomposition of the clusters (evaporation or ejection of or units) occurs, from highly excited configurations, before the clusters have fully developed a liquid-like phase. Comparison with experiments on the thermal decomposition of and a discussion of the possible implications of our results on the growth mechanisms leading to the formation of different carbon structures are included. Received: 25 March 1998 / Received in final form: 30 October 1998     

Substitutional impurity in the graphene quantum dots

The process of formation of the localized defect states due to substitutional impurity in sp²-bonded graphene quantum dot is considered using a simple tight-binding-type calculation. We took into account the interaction of the quantum dot atoms surrounding the substitutional impurity from the second row of elements. To saturate the external dangling sp² orbitals of the carbon additionally 18 hydrogen atoms were introduced. The chemical formula of the quantum dot is H₁₈C₅₁X, where X is the symbol of substitutional atom. The position of the localized levels is determined relative to the host-atoms (C) ε_p energies. We focused on the effect of substitutional doping by the B, N and O on the eigenstate energies and on the total energy change of the graphene dots including for O the effect of lattice distorsion. We conclude that B, N, and O can form stable substitutional defects in graphene quantum dot.     

Electronic structure of substitutional defects and vacancies in GaSe

We have investigated the nature of defect states associated with substitutional impurities (Cd, In, Sn) and both Ga and Se vacancies in GaSe using ab initio electronic structure methods within density functional theory. These calculations were done using supercell model allowing for internal atomic relaxation. Binding energies (BEs) of defects obtained in this model are compared with effective mass approximation results. Significant central cell corrections are present for most of the defects. This is consistent with charge densities associated with the defect states that show clearly their strongly localized nature. Because of the difficulties associated with LDA/GGA in giving the correct band gap in semiconductors, we have only compared the acceptor BEs with available experiments. Our theoretical results agree well with the experiment for Cd_Ga and V_Ga. The fundamental role played by the Ga dimers in the formation of defect states is discussed.     

Calculation of the structure of carbon clusters based on fullerene-like C<Subscript>24</Subscript> and C<Subscript>48</Subscript> molecules

Equilibrium structures obtained by linking with valence bonds the carbon carcasses of two fullerene-like molecules have been studied by molecular dynamics simulation. In free fullerene, carbon atoms form sp² hybridized bonds, but at places of links between fullerenes, sp³ hybridized bonds are formed, which determines the changes in the properties of such structures. In the literature, the topology of diamond-like phases is described, but equilibrium clusters based on fullerene-like molecules are underexplored. The right angles between the C–C bonds are energetically unfavorable, and the reduction in the energy of clusters in the process of relaxation is connected with the optimization of valence angles, which leads to a reduction in the symmetry of clusters and, in a number of cases, even to disruption of some valence bonds. It is shown that different fashions of linking two fullerenes result in the formation of clusters with different structures and energies. Different initial conditions can lead to different configurations of clusters with the same topology. Among the analyzed clusters, a structure with the minimum potential energy per atom was found. The results of this work contribute to the study of the real structure of carbon clusters.     

Nuclear magnetic resonance study of ultrananocrystalline diamonds

We report on a nuclear magnetic resonance (NMR) study of ultrananocrystalline diamond (UNCD) materials produced by detonation technique. Analysis of the ¹³C and ¹H NMR spectra, spin-spin and spin-lattice relaxation times in purified UNCD samples is presented. Our measurements show that UNCD particles consist of a diamond core that is partially covered by a sp ²-carbon fullerene-like shell. The uncovered part of outer diamond surface comprises a number of hydrocarbon groups that saturate the dangling bonds. Our findings are discussed along with recent calculations of the UNCD structure. Significant increase in the spin-lattice relaxation rate (in comparison with that of natural diamond), as well as stretched exponential character of the magnetization recovery, are attributed to the interaction of nuclear spins with paramagnetic centers which are likely fabrication-driven dangling bonds with unpaired electrons. We show that these centers are located mainly at the interface between the diamond core and shell.     

Type I FA (Rb, Cs) and II FA (Li, Na) tunable laser activities and donor-acceptor properties of O and O at KCl (001) surface: first principle calculations

Type I F_A (Rb⁺, Cs⁺) and II F_A (Li⁺, Na⁺) tunable laser activities, adsorptivity and donor-acceptor properties of O and O⁻ adsorbates at the flat surface of KCl crystal were investigated using an embedded cluster model and ab initio methods of molecular electronic structure calculations. Ion clusters were embedded in a simulated Coulomb field that closely approximates the Madelung field of the host surface, and the nearest neighbor ions to the defect site were allowed to relax to equilibrium. Based on the calculated Stokes shifted optical transition bands, F_A tunable laser activities were found to be inversely proportional to the size of the dopant cation (Li⁺, Na⁺, Rb⁺, Cs⁺) relative to the host cation (K⁺). This relation was explained in terms of the axial perturbation of the impurity cation. The probability of orientational bleaching attributed to the RES saddle point ion configuration along the 〈110〉 axis was found to be inversely proportional to the size of the dopant cation, with activation energy barriers of ca. 0.44-3.34 eV. Surface relaxation energies of type II F_A centers were more important than those of type I F_A centers. In terms of defect formation energies, the products of type II F_A center imperfection were more stable than those of type I F_A. The difference between F or F_A band energies and exciton bands depended almost exclusively on the size of the positive ion species. As far as the adsorptivity of O and O⁻ is concerned, the results confirm that surface imperfection enhances the adsorption energies by ca. 4.38-16.37 eV. O and O⁻ penetrate through the defect-containing surface. The energy gap between the adsorbate and the defect containing surface and the donor-acceptor property of adsorbate play the dominant role in the course of adsorbate substrate interactions and the results were explained in terms of electrostatic potential curves and Mulliken population analysis.     

Intrinsic defects in GaAs and InGaAs through hybrid functional calculations

Vacancies, antisites, and dangling bonds in GaAs and In_0.5Ga_0.5As are studied through hybrid density functionals. The As antisite is found to have a low formation energy in As-rich conditions and defect levels at mid-gap in correspondence of experimental defect densities at the GaAs/oxide and InGaAs/oxide interfaces. In n-type GaAs, the Ga vacancy also shows defect levels in agreement with measured defect densities and competitive formation energies. For both GaAs and InGaAs, the As dangling bonds are located near the valence band maximum. The Ga dangling bond in GaAs is found just below the conduction band-edge in correspondence of experimentally observed states, whereas its defect level is resonant with the conduction band in InGaAs.     

Trends in stability, elastic and electronic properties of cubic Rh, Ir, Pd and Pt carbides depending on carbon content: MC versus M4C from first-principles calculations

First principles FLAPW-GGA calculations have been performed to understand the peculiarities of stability, elastic, electronic properties and chemical bonding for cubic carbides of four noble metals M=Rh, Pd, Ir and Pt depending on carbon stoichiometry: MC versus M₄C. Our main findings are as follows: (i) in contrast to mono-carbides MC with positive formation energies E_form>0, carbon-deficient sub-carbides M₄C are stable (E_form<0), thus carbon stoichiometry is one of the major factors determining successful synthesis of these materials, and (ii) as distinct from the majority of other 3d-5d metals (including Pd and Pt examined here), an unusual effect of Rh and Ir “metallization” and the increasing of ductility for these metals owing to the introduction of carbon has been established.     

Molecular junctions by joining single-walled carbon nanotubes

Crossing single-walled carbon nanotubes can be joined by electron beam welding to form molecular junctions. Stable junctions of various geometries are created in situ in a transmission electron microscope. Electron beam exposure at high temperatures induces structural defects which promote the joining of tubes via cross-linking of dangling bonds. The observations are supported by molecular dynamics simulations which show that the creation of vacancies and interstitials induces the formation of junctions involving seven- or eight-membered carbon rings at the surface between the tubes.     

Monte Carlo simulation of growth of porous SiOx by vapor deposition

A random network model containing defects has been developed and applied to the deposition of amorphous SiOx films on a flat substrate. A new Monte Carlo procedure enables dangling bonds to migrate and annihilate. The degree of porosity in the films is found to increase with oxygen content. As the oxygen content increases a larger fraction of pore surfaces is covered with oxygen, and the density of dangling bonds on pore surfaces decreases. Oxygen plays the role of a surfactant, lowering the energies of pore surfaces and enhancing the porosity of amorphous SiO2 compared to amorphous Si.     

Simultaneous interaction of methyl radicals and atomic hydrogen with amorphous hydrogenated carbon films, as investigated with optical in situ diagnostics

Methyl radicals (CH₃) and atomic hydrogen (H) are dominant radicals in low-temperature plasmas from methane. The surface reactions of these radicals are believed to be key steps leading to deposition of amorphous hydrogenated carbon (a-C:H) films or polycrystalline diamond in these discharges. The underlying growth mechanism is studied, by exposing an a-C:H film to quantified radical beams of H and CH₃. The deposition or etching rate is monitored via ellipsometry and the variation of the stoichiometry is monitored via isotope labeling and infrared spectroscopy. It was shown recently that, at 320 K, methyl radicals have a sticking coefficient of 10^-4 on a-C:H films, which rises to 10^-2 if an additional flux of atomic hydrogen is present. This represents a synergistic growth mechanism between H and CH₃. From the interpretation of the infrared data, a reaction scheme for this type of film growth is developed: atomic hydrogen creates dangling bonds by abstraction of bonded hydrogen within a surface layer corresponding to the range of H in a-C:H films. These dangling bonds serve at the physical surface as adsorption sites for incoming methyl radicals and beneath the surface as radicalic centers for polymerization reactions leading to carbon–carbon bonds and to the formation of a dense a-C:H film. Received: 18 July 2000 / Accepted: 12 December 2000 / Published online: 3 April 2001     

Metastable states in Si:H

The results of ab initio LDF calculations applied to large clusters of Si atoms containing H in various positions are described. We find the bond centred (BC) defect to be most stable for neutral and positively charged H. H placed at an antibonding site is also stable with an energy 0.1 eV higher than the BC defect. The stability of H₂ and H^*₂ is also discussed. New results are reported for the conversion of BC defects into Si dangling bonds. It is found that H attached to vacancy-like defects is bi-stable: for Si-H-Si lengths less than ≈3.8 Å, the BC defect is stable, whereas for longer separations, the Si-H ··· Si dangling bond is stable. A discussion of the relevance of this to the Staebler-Wronski effect is given.