A continuous model for the joining of two fullerenes

The successful design of many novel nano-electronic devices will require a thorough understanding of the geometric joining issues of certain nano-structures. In this paper, we adopt a continuous approach and we employ the calculus of variations to model the nanostructure obtained by the joining of two fullerenes. We model the fullerenes as spheres and we assume symmetric defects on both fullerenes so that the three-dimensional problem is axially symmetric and can therefore be reduced to a problem in two dimensions. We propose two models depending upon the curvature of the join profile which can be either positive or both positive and negative. However, there is at present no experimental or simulation data to verify the theoretical connecting structures predicted by this study.     

Continuum modelling of spherical and spheroidal carbon onions

Carbon nanostructures are of considerable interest owing to their unique mechanical and electronic properties. Experimentally, a wide variety of different shapes are obtained, including both spherical and spheroidal carbon onions. A spheroid is an ellipsoid with two major axes equal and the term onion refers to a multi-layered composite structure. Assuming structures of either concentric spherical or ellipsoidal fullerenes comprising n layers, this paper examines the interaction energy between adjacent shells for both spherical and spheroidal carbon onions. The Lennard-Jones potential together with the continuum approximation is employed to determine the equilibrium spacing between two adjacent shells. We also determine analytical formulae for the potential energy which may be expressed either in terms of hypergeometric or Legendre functions. We find that the equilibrium spacing between shells decreases for shells further out from the inner core owing to the decreasing curvature of the outer shells of a concentric structure.     

Mechanisms of the oxidation of defect-free surfaces of carbon nanostructures: the influence of surface curvature

This work is concerned with reaction paths in the interaction of carbon defect-free nanostructures with different surface curvatures (graphene, tubulenes, and fullerene C₆₀) with atomic and molecular oxygen. The interaction energies of atoms were calculated by the density functional theory method using the basis set of plane waves and the VASP package. The potential surface of reactions with molecular oxygen was studied by the nudged elastic band method. The energy parameters of the reaction (released energy and barrier) strongly depended on the curvature of carbon structure surfaces. The interaction of atomic oxygen in the ground state with the surface of carbon nanostructures is an exothermic reaction. The barrier to the reaction with molecular oxygen (0.5–2.5 eV) decreases as the curvature of nanostructure surfaces increases. The calculation results are in agreement with the experimental data and other ab initio calculations.     

Nature of the bound states of molecular hydrogen in carbon nanohorns

The effects of confining molecular hydrogen within carbon nanohorns are studied via high-resolution quasielastic and inelastic neutron spectroscopies. Both sets of data are remarkably different from those obtained in bulk samples in the liquid and crystalline states. At temperatures where bulk hydrogen is liquid, the spectra of the confined sample show an elastic component indicating a significant proportion of immobile molecules as well as distinctly narrower quasielastic line widths and a strong distortion of the line shape of the para-->ortho rotational transition. The results show that hydrogen interacts far more strongly with such carbonous structures than it does to carbon nanotubes, suggesting that nanohorns and related nanostructures may offer significantly better prospects as lightweight media for hydrogen storage applications.     

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.     

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     

Controllable irregular phenomenon induced by Ag atomic segregation on the melting of (AgPd)561 bimetallic clusters

Here focusing on the very new experimental finding on carbon nanomaterials for solid-state electron mediator applications in Z-scheme photocatalysis, we have investigated different graphene-based nanostructures chemisorbed by various types and amounts of species such as oxygen (O), nitrogen (N) and hydroxyl (OH) and their electronic structures using density functional theory. The work functions of different nanostructures have also been investigated by us to evaluate their potential applications in Z-scheme photocatalysis for water splitting. The N-, O?CN-, and N?CN-chemisorbed graphene-based nanostructures (32 carbon atoms supercell, corresponding to lattice parameter of about 1?nm) are found promising to be utilized as electron mediators between reduction level and oxidation level of water splitting. The O- or OH-chemisorbed nanostructures have potential to be used as electron conductors between H₂-evolving photocatalysts and the reduction level (H⁺/H₂). This systematic study is proposed to understand the properties of graphene-based carbon nanostructures in Z-scheme photocatalysis and guide experimentalists to develop better carbon-based nanomaterials for more efficient Z-scheme photocatalysis applications in the future.     

Reversible creation of nanostructures between identical or different species of materials

In this study, accurate nanostructures with various aspect ratios are created on several types of material. This work is highly applicable to the energy, optical, and nano-bio fields, for example. A silicon (Si) nano-mold is preserved using the method described, and target nanostructures are replicated reversibly and unlimitedly to or from various hard and soft materials. It is also verified that various materials can be applied to the substrates. The results confirm that the target nanostructures are successfully created in precise straight line structures and circle structures with various aspect ratios, including extremely high aspect ratios of 1:18. It is suggested that the optimal replicating and demolding process of nanostructures with high aspect ratios, which are the most problematic, could be controlled by means of the surface energy between the functional materials. Relevant numerical and analytical studies are also performed. It is possible to expand the applicability of the nanostructured mold by adopting various backing materials, including rounded substrates. The scope of the applications is extended further by transferring the nanostructures between different species of materials including metallic materials as well as identical species.     

A DFT study of carbon nanobuds

In the framework of the density functional theory (DFT) calculations, we present a first time investigation of the properties of four kinds of configurations of carbon nanobuds (CNBs) in which a perfect or defective C60 molecule attaches covalently on the surface of an armchair single-walled carbon nanotube (SWCNT). Chemical shielding (CS) parameters were calculated for the optimized structures. Our results indicate that carbon nanobuds have different values of formation energy, band gap energy, dipole moment, charge transfer and chemical-shielding isotropy (CSI), which result from the many covalent combinations of the fullerenes with the carbon nanotubes. These calculations were carried out using the Gaussian 09 software package.     

First-principles study of electronic and elastic properties of Stone–Wales defective zigzag carbon nanotubes

The interaction and coupling between the electrical, mechanical properties and formation energy for SW defective (10,0) carbon nanotube is studied in density functional theory. The investigated configurations include the axial and circumferential orientations for single defect as well as four distribution types for double ones. The more stable defective configurations, namely, SW-I configurations for single SW defective carbon nanotube and II–II-(2) and I–I ones for double SW defective tubes are related to high symmetry distribution of the defects. Moreover, we found that the σ^?–π^* hybridization induced by curvature effect causes the semiconductor to metal transition for double axial SW defects case. Young's modulus reduction of SW defective carbon nanotube with respect to defect-free one is less than 8%. The energy bands and Young's moduli of double SW defective tubes are mostly affected by the defect distribution and concentration but insensitive to the circumferential distance between the double defects.     

Nanotube nanoscience: A molecular-dynamics study

Carbon nanotubes, fullerenes, and other nanostructured carbon materials are now the most important material phases in the field of nanoscience and nanotechnology. We study the structural stabilities and the interconversion of carbon nanotubes and various other carbon nanostructured phases at elevated temperatures as well as under high pressure using the molecular dynamics method combined with a newly parametrized transferable tight-binding model. The model can deal with not only sp² and sp³ covalent bonds but also the interaction between sp² layers, which plays an important role in the structural and electronic properties of carbon nanostructured materials. It is found that, during a thermal transformation process of carbon nanotubes with C₆₀ fullerenes trapped inside into double-walled carbon nanotubes, the outer carbon-nanotube wall is chemically active and forms covalent bonds with inner carbon atoms, and that most vacancies on the initially imperfect outer tube wall are eventually filled with atoms migrated from inner fullerenes. It is also found that external pressure of about 20 GPa induces a variety of structural transformations in carbon nanostructures. On the other hand, pressure of 30 GPa or higher usually results in sp³-rich amorphous carbon materials. Finally, the rotational interlayer friction force in double-walled carbon nanotubes is studied for the system of (4,4)@(9,9), and the torque of the friction force per unit area acting on each nanotube of the system is found to be as small as . This small value indicates the importance of carbon nanostuctured materials not only for nanoelectronics but also for nanometer-scale machines in the future.     

Metastable Frenkel pair defect in graphite: source of Wigner energy?

The atomic processes associated with energy storage and release in irradiated graphite have long been subject to untested speculation. We examine structures and recombination routes for interstitial-vacancy (I-V) pairs in graphite. Interaction results in the formation of a new metastable defect (an intimate I-V pair) or a Stone-Wales defect. The intimate I-V pair, although 2.9 eV more stable than its isolated constituents, still has a formation energy of 10.8 eV. The barrier to recombination to perfect graphite is calculated to be 1.3 eV, consistent with the experimental first Wigner energy release peak at 1.38 eV. We expect similar defects to form in carbon nanostructures such as nanotubes, nested fullerenes, and onions under irradiation.     

Reshaping elastic nanotubes via self-assembly of surface-adhesive nanoparticles

Elastic sheets with macroscopic dimensions are easy to deform by bending and stretching. Yet shaping nanometric sheets by mechanical manipulation is hard. Here we show that nanoparticle self-assembly could be used to this end. We demonstrate that spherical nanoparticles adhering to the outer surface of an elastic nanotube can self-assemble into linear structures: rings or helices on stretchable nanotubes, and axial strings on nanotubes with high rigidity to stretching. These self-assembled structures are inextricably linked to a variety of deformed nanotube profiles, which can be controlled by tuning the concentration of nanoparticles, the nanoparticle-nanotube diameter ratio and the elastic properties of the nanotube. Our results open the possibility of designing nanoparticle-laden tubular nanostructures with tailored shapes, for potential applications in materials science and nanomedicine.     

Multi-disclination configurations in pentagonal microcrystals and two-dimensional carbon structures

A mechanism of decrease in the elastic (latent) energy of a solid containing disclination defects by introducing multi-disclination configurations of opposite sign has been considered. The relation of the proposed model with relaxation modifications of microcrystals with pentagonal symmetry, as well as with the structure of two-dimensional carbon films, has been discussed. An approach to the prediction of new carbon structures inherently containing multi-disclination configurations with screening has been demonstrated.     

Electron spectroscopy of fluoridated fullerene films

The chemical and energy structures of highly fluoridated fullerene films have been investigated. Analysis of the complex structure of carbon 1s spectra showed the presence of C-F and C-F2 fragments as well as nonfluoridated carbon atoms with an overwhelming quantity of C-F bonds. The band gap in fluoridated-fullerene and its films was estimated to be 8.0 eV and energy loss on an interband transition at 11 eV was also observed. Comparison of the valence-band spectra of the experimental samples showed that the valence band of fluoridated fullerenes is divided into anionic and cationic parts (similarly to alkali-halide crystals) and suggests that fluoridated fullerenes possess the corresponding properties, making it possible to find new materials with wide practical applications. Fiz. Tverd. Tela (St. Petersburg) 40, 168–172 (January 1998)     

Interaction of Methane with Single-Walled Carbon Nanotubes： Role of Defects,Curvature and Nanotubes Type

We investigate the interaction of single-walled carbon nanotubes （SWCNTs） and methane molecule from the first principles. Adsorption energies are calculated, and methane affinities for the typical semiconducting and metallic nanotubes are compared. We also discuss role of the structural defects and nanotube curvature on the adsorption capability of the SWCNTs. We could observe larger adsorption energies for the metallic CNTs in comparison with the semiconducting CNTs. The obtained results for the zig zag nanotubes with various diameters reveal that the adsorption energy is higher for nanotubes with larger diameters. For defected tubes the adsorption energies are calculated for various configurations such as methane molecule approaching to the defect sites pentagon, hexagon, and heptagon in the tube surface. The results show that the introduce defects have an important contribution to the adsorption mechanism of the methane on SWNTs.     

Generation of surface plasmons in a conductor by moving charges

The surface polarization fields generated by a charge moving near a sphere or cylinder have been considered. This problem is related to the phenomena arising in an arc discharge in a gas near a solid surface during the formation of conducting cylindrical or spherical nanostructures, in particular, carbon nanotubes or fullerenes. The polarization fields, forces, energy losses, and other characteristics have been calculated.     

Controlled surface nanopatterning with buried dislocation arrays

Self-assembled configurations of nanostructures are expected to play an increasing role in devices design, as an alternative to conventional microelectronics. The key limitation is the lack of control on localisation, density and size uniformity of the structures. Here we show how to create a template to overcome these problems. A periodic nanometre scale patterning can be induced at a silicon surface by buried dislocation networks obtained by twist wafer bonding, using stress selective etching of the surface. These templates are morphologically characterised by scanning tunnelling microscopy and grazing incidence X-ray diffraction. Stress fields and elastic energy densities are calculated for the non-etched solid, and the selective etching mechanisms are discussed. Germanium growth experiments on such a Si patterned surface give a demonstration of the ordering efficiency. This study provides a general method to create a template, which organises nanostructures with controlled periodicity over the full size of a Si wafer.