Buckling of functionalized single-walled nanotubes under axial compression

Buckling characteristics of functionalized single-walled carbon nanotubes under axial compression are investigated by molecular mechanics simulation. The influences of the content, the distribution density and the location of the sp³-hybridized carbon atoms as well as the chirality on the critical buckling strains of functionalized single-walled carbon nanotubes are carefully studied. The results indicate that the chirality and the distribution density have dominant effect on the critical buckling strains. The critical buckling strains of present armchair (5, 5) and zigzag (10, 0) carbon nanotube are degraded by about 43% and 70%, respectively, due to the dense distribution of the sp³-hybridized carbon atoms. The reduction amplitude of the critical strain increases with increasing the tubule radius of an armchair or zigzag single-wall carbon nanotube. The dramatic reduction of the critical strain could cause a great loss of reinforcing role of carbon nanotubes in composites.     

Structure changes of single-wall carbon nanotubes and single-wall carbon nanohorns caused by heat treatment

Raman spectra and transmission electron microscope images showed that diameter enlargement of HiPco, a kind of single-wall carbon nanotube, accompanied by tube-wall corrugation was caused by heat treatment (HT) at 1000 to 1700 °C. Further enlargement accompanied by straightening of the tube walls and incorporation of carbon fragments within the tubes became obvious after HT at 1800 to 1900 °C. The transformation of some single-wall carbon nanotubes into multi-wall nanotubes was observed after HT at 2000 °C, and most single-wall tubes were transformed into multi-wall ones by HT at 2400 °C. What influence the Fe contained in the HiPco tubes had on these structure changes was unclear; similar changes were observed in single-wall carbon nanohorns that did not contain any metal. This indicates that thermally induced changes in the structure of single-wall carbon nanotubes can occur without a metal catalyst. Heat treatment increased the integrity of the nanotube-papers, and this increase may have been due to tube-tube interconnections created by HT.     

X-ray diffraction of multiwalled carbon nanotube under high pressure: Structural durability on static compression

The structural durability of multiwalled carbon nanotubes under hydrostatic and non-hydrostatic compression was examined by in situ X-ray powder diffraction at room temperature. No interlayer interaction such as sp³ hybridization that could lead to hexagonal diamond in graphite was observed under compression up to 52 GPa, even though the nanotubes were similar in compressibility to graphite. This result could be attributed to the nested structure, which makes the interlayer stacking of carbon atoms take on an irregular arrangement. Despite the history of non-hydrostatic compression, electron microscopic observation revealed that the structure remained nested tubular. This reversibility suggests the nanotubes have strong durability on non-hydrostatic compression under extreme pressures.     

Conversion of carbon nanotubes to diamond by spark plasma sintering

The diamond phase has been converted directly from carbon nanotubes by spark plasma sintering (SPS), at 1500 °C under 80 MPa pressure, without any catalyst being involved. Well-crystallized diamond crystals, with particle sizes ranging from 300 nm to 10 μm were obtained. After sintering at 1200 °C, the tips of the carbon nanotubes were found to be open and the conversion from carbon nanotubes to diamond started. The mechanism for carbon nanotube to diamond conversion in SPS may be described as that from carbon nanotubes to an intermediate phase of carbon nano-onion, and then to diamond. It is believed that the plasmas generated by the low-voltage, vacuum spark, via a pulsed DC in the SPS process, played a critical role in the low pressure diamond formation. This SPS process provides an alternative approach to diamond synthesis.     

Influence of the gas pressure on single-wall carbon nanotube formation

Experiments and modeling are performed to predict the effect of gas pressure on species distributions and nanotube growth rate under specific conditions of synthesis of single-wall carbon nanotubes (SWCNTs) by arc discharge. Numerical results are compared with experiments in order to find a consistent correlation between the nanotube growth and the pressure. We use argon and helium as buffer gases with a total pressure varied between 0.1 and 1 bar. We experimentally observe that both the anode erosion rate and the Brunauer-Emmett-Teller (BET) surface area of the as-produced nanotube soot material are very sensitive to the total gas pressure in the reactor.     

Coalescence of single-walled carbon nanotubes and formation of multi-walled carbon nanotubes under high-temperature treatments

Electric arc-discharge single-wall carbon nanotubes are annealed between 1600 and 2800 °C under argon flow. Their stability and evolution are studied by coupling TEM, X-ray diffraction and Raman spectroscopy. The first modifications appear at 1800 °C with a significant decrease of the crystalline order. It is due to SWNTs coalescence leading to smaller bundles but with an increase of the tube diameters from 2 to 4 nm. From 2200 °C, SWNTs progressively disappear to the benefit of MWNTs having at first two to three carbon layers then reaching 7 nm external diameter. The possible mechanisms responsible for the SWNTs coalescence and instability and their transformation in MWNTs are discussed.     

Carbon nanotunnels form from single-walled carbon nanotubes interacting with a diamond (100)-(2 × 1) surface

A quantum chemical study of the interaction of (5,5), (7,7), (9,9) and (8,0) single-walled carbon nanotubes with a clean (100)-(2 × 1) diamond surface is reported. Stable structures with covalent bonds at the interface were found for carbon nanotubes oriented parallel or perpendicular to the dimer rows on the reconstructed (100) surface. The binding energy of the most stable (5,5) nanotube-diamond structure is 1.7 eV/Å, and is attributed to strong covalent bonds formed between the carbon nanotube and the diamond surface. The structure of the nanotube is distorted by adsorption on the surface such that it adopts a tunnel-like geometry. Two other nanotunnel geometries were found for the (5,5) nanotube, with binding energies of 1.39 and 1.09 eV/Å. In the most stable (5,5) nanotube-diamond structure the interaction between the nanotube and the diamond surface produces a 0.6 eV band gap near the Fermi level, but the metallic character of the nanotube is maintained in the two other, less strongly bound nanotunnel structures. No charge transfer occurs between the diamond surface and the nanotunnels in any of the three orientations. Binding energies decrease with increases in tube diameter, to the extent that one of the three nanotunnel structures is not formed by (9,9) carbon nanotubes.     

Raman spectroscopy and nitrogen vapour adsorption for the study of structural changes during purification of single-wall carbon nanotubes

Raman spectroscopy and nitrogen adsorption measurements were combined to study the surface features of semi-conducting and metallic single-wall nanotubes (SWNTs). The nanotubes were treated chemically and with heat under moderate conditions that more than doubled the mesopore volume of the tested samples, which consistently led to a significant rise in the total surface area of up to 1550 m²/g. The large increase in the number of micropores of less than 1 nm in diameter was associated with the loosening of nanotube bundles as well as the creation of structural flaws on the surface of individual SWNTs due to chemical treatment. Micropores in the 1.0-1.8 nm range were associated with the holes created on the surface of individual tubes. Heating at 1000 °C was shown to restore nanotube diameter to their initial pre-chemical treatment levels with the change in the chirality of SWNTs and diminish the porosity by closing small holes. It was assumed that the intermediate frequency range (500-1100 cm⁻¹) was associated with the degree of imperfection of HiPco SWNTs crystalline structures, and therefore provided information about the degree of tube surface damage due to the presence of functional groups. A hypothesis explaining the transformation of SWNT porous structure during heat treatment is proposed.     

Preparation and characterization of boron doped diamond electrodes on diamond anvil for in situ electrical measurements under high pressure

A layer of boron doped diamond (BDD) film was deposited selectively on a diamond anvil and employed as electrodes for measuring the electrical resistivity of matter under high pressure. Both heavily doped and lightly doped electrodes were characterized by Raman spectroscopy and scanning electron microscopy. Though the BDD film electrodes contain sp² carbon, it is still suitable for in situ high pressure electrical measurements. The dependability of diamond film electrodes was tested at high pressure up to 36 GPa, by measuring the electric resistance of C60 fullerene powder, and no damage of the electrodes was observed.     

Thermo-physical and impact properties of epoxy nanocomposites reinforced by single-wall carbon nanotubes

The thermo-physical properties and the impact strength of diglycidyl ether of bisphenol F (DGEBF) epoxy nanocomposites reinforced with fluorinated single-wall carbon nanotubes (FSWCNT) are reported. A sonication technique was used to disperse FSWCNT in the glassy epoxy network resulting in nanocomposites having large improvement in modulus with extremely small amount of FSWCNT. The glass transition temperature decreased approximately 30 °C with an addition of 0.2 wt% (0.14 vol%) FSWCNT, without adjusting the amount of the anhydride curing agent. This was because of non-stoichiometry of the epoxy matrix that was caused by the fluorine on the single-wall carbon nanotubes. The correct amount of the anhydride curing agent needed to achieve stoichiometry was experimentally examined by dynamic mechanical analysis (DMA). The storage modulus of the epoxy at room temperature (which is below the glass transition temperature of the nanocomposites) increased up to 0.63 GPa with the addition of only 0.30 wt% (0.21 vol%) of FSWCNT, representing an up to 20% improvement compared with the neat epoxy. The Izod impact strength slightly decreased when the amount of FSWCNT was increased to 0.3 wt%. The excellent improvement in the storage modulus was achieved without sacrificing impact strength.     

Molecular dynamics simulation of shear-induced graphitization of amorphous carbon films

The shear-induced graphitization of amorphous carbon (a-C) films in sliding contact with a diamond counterface is investigated by molecular dynamics (MD) simulations. The gradual formation of a graphene-like sp² dominant layer on the a-C film surface is observed after steady-state sliding has been achieved, which provides direct evidence for the experimental observations of friction induced graphitization of a-C film. After the graphitized layer is formed, the relative sliding occurs between the graphitized atomic layers. During the shearing process, the biaxial stress in the graphitized layer experiences a transition from highly compressive (42 GPa) to tensile (−3 GPa). It is the relaxation of the local biaxial stress that leads to the sp³-to-sp² structural transformation.     

Growth of tetrahedral amorphous carbon film: Tight-binding molecular dynamics study

Molecular dynamics simulation using tight-binding potential has been performed to examine the growth and performance of tetrahedral amorphous carbon during ion deposition. The sp³ hybrid atom content, density, and compressive stress of the tetrahedral amorphous carbon film depend on the growing conditions such as substrate temperature, ion energy, ion dose, and annealing temperature. The critical temperature for sp³ transition to sp² decreases with ion energy (40, 80, and 120 eV). At low temperatures (<300 K) and low ion energies, the sp³ fraction increases up to 82%. At the annealing temperature less than 1200 K or with a few ions (<20) implanted into the film, its sp³ content and density have only slight changes while the compressive stress has a large reduction with the annealing temperature and the number of implanted ions. This large reduction in the compressive stress is due to a structural relaxation.     

Characteristics of HPHT diamond grown at sub-lithosphere conditions (10-20 GPa)

We have conducted high pressure-high temperature (HPHT) diamond synthesis experiments at the conditions of growth of superdeep diamonds (10-20 GPa), equivalent to the transition zone, using MgCO₃ carbonate (oxidising) and FeNi (reducing) solvent catalysts. High rates of graphite-diamond transformation were observed in these short duration experiments (20 min). Transformation rates were higher using the metallic catalyst than in the carbonate system. High degrees of carbon supersaturation at conditions significantly above the graphite-diamond stability line, led to a high nucleation density. This resulted in the growth of aggregated masses of diamond outlined by polygonised diamond networks, resembling carbonado. Where individual crystals are visible, grown diamonds are octahedral in the lower pressure experiments (≤ 10 GPa in MgCO₃ and ≤ 15 GPa in FeNi) and, cubo-octahedral at higher pressure. All grown diamonds show a high degree of twinning. The diamonds lack planar deformation features such as laminations or slip planes, which are commonly associated with natural superdeep diamonds.     

Gels of carbon nanotubes and a nonionic surfactant prepared by mechanical grinding

Mixtures of carbon nanotubes (CNTs), including multi-wall CNTs and single-wall CNTs, and polyoxyethylene(12) tridecyl ether (POETE), a nonionic surfactant and a fluid at room temperature, became gels after mechanical grinding. The heavily entangled multi-wall or single-wall CNTs debundled during the grinding and dispersed with fewer bundles in POETE. The mechanism for the gel formation was studied by the dynamic mechanical measurements and scanning electron microscopy. The results suggest that the formation of the CNT/POETE gel is the result of the physical-crosslinking CNT networks, mediated by the van der Waals interaction between CNTs and the nonionic surfactant. The gels were stable from room temperature up to 200 °C and did not shrivel even in vacuum. The CNT/POETE gels were electrically conductive and could be processed into conductive CNT films by coating the CNT/POETE gels on a substrate by a doctor blade and subsequent heating. POETE was removed during the heating, while the heating did not degrade the CNTs. The CNT films had a conductivity of about 13 S cm⁻¹ and had good adhesion to the substrate.     

Room temperature dc electrical conductivity studies of electron-beam irradiated carbon nanotubes

The influence of electron-beam (E-beam) irradiation on the electrical (electronic) properties of single- (SW) and multi-walled (MW) carbon nanotube grown by microwave chemical vapor deposition is investigated. These films were subjected to a constant energy of 50 keV (50 A/cm²) from a scanning electron microscope gun for 2.5, 5.5, 8.0, and 15 h continuously — such conditions resemble increased temperature and/or pressure regime, enabling a degree of structural fluidity. To assess the structural modifications and electrical properties, the films were analyzed before and after irradiation. The experiments show that with increased exposure to ≥ 8–9 h, occasionally found individual bundles of single-wall nanotubes tend to collapse or pinch, graphitize/amorphize, and oxidize within the area of the electron-beam focus. Dramatic improvement in the I–V properties for single-walled (from semiconducting to quasi-metallic) and relatively small but systematic behavior for multi-walled with increasing exposure is discussed in terms of the critical role of controlled introduction of defects. The contact resistance decreases by orders of magnitude when exposed to electron beam and for all of the measurements the values ranged between 80 Ω and 10 kΩ at room temperature. These results also indicated that multi-walled nanotubes tend to reach a state of saturation degradation assessed by four-probe conductivity measurements. It is suggestive that there may be local gradual re-organization, i.e. sp^2 + δ, sp³ C ⇔ sp² C. More importantly, they provided a contrasting comparison between metallic/semiconducting (single/double-wall) and invariably metallic (multi-wall) carbon nanotubes.     

First-principles prediction of the transition from graphdiyne to a superlattice of carbon nanotubes and graphene nanoribbons

Graphdiyne is a recently-synthesized carbon allotrope with a framework of sp- and sp²-hybridized carbon atoms. From first-principles calculations, we propose a possible transition of graphdiyne to a novel carbon allotrope (h-carbon) whose structure is a superlattice of carbon nanotubes and graphene nanoribbons. The energy barrier of this endothermic transition was estimated to be 4.30 kcal/mol at zero pressure, which is much lower than that of the graphite–diamond transition at high pressure. First-principles calculations on the phonon spectrum and the elastic constants of the h-carbon revealed that it is kinetically and mechanically stable. This unique framework of sp²- and sp³-hybridized carbon atoms is energetically neutral versus diamond. The hardness of the h-carbon (35.52 GPa) is 1/3 that of diamond and very close to β-SiC crystal. Accurate electronic structure calculations based on the Heyd, Scuseria, and Ernzerhof approach and GW approximation indicate that the h-carbon is a semiconducting material with a band gap of 2.20–2.56 eV.     

Characterization of single-walled carbon nanotubes containing defects from their local vibrational densities of states

The vibrational properties of defects in single-wall carbon nanotubes are investigated with an empirical dynamical model based on force constants fitted to graphite [Al-Jishi R, Venkataraman L, Dresselhaus MS, Dresselhaus G. Phonon modes in carbon nanotubules. Chem Phys Lett 1993;209:77-82]. The defected structures are: an isotopic substitution impurity in a chiral tube, a Stone-Wales defect in a series of (10, m) (m = 4, … , 10) nanotubes and in armchair tubes, and an intramolecular junction between (12, 0) and (9, 0) nanotubes. The study emphasizes differences in local vibrational densities of states (lVDOS) between perfect and imperfect nanotubes. lVDOS are computed with the recursion method. This is a first step towards a more complete approach for characterizing defects in carbon nanotube structures based on the recognition of localized vibrational fingerprint.     

Pressure dependence of Raman modes in double wall carbon nanotubes filled with 1D Tellurium

The preparation of highly anisotropic one-dimensional (1D) structures confined into carbon nanotubes (CNTs) in general is a key objective in nanoscience. In this work, capillary effect was used to fill double wall carbon nanotubes (DWCNTs) with trigonal Tellurium. The samples are characterized by high resolution transmission electronic microscopy and Raman spectroscopy. In order to investigate their structural stability and unravel the differences induced by intershell interactions, unpolarized Raman spectra of radial and tangential modes of DWCNTs filled with 1D nanocrystalline Te excited with 514 nm were studied at room temperature and high pressure. Up to 11 GPa we found a pressure coefficient of 3.7 cm⁻¹ GPa⁻¹ for the internal tube and 7 cm⁻¹ GPa⁻¹ for the external tube. In addition, the tangential band of the external and internal tubes broaden and decrease in amplitude. All findings lead to the conclusion that the outer tube acts as a protection shield for the inner tube (at least up 11 GPa). No pressure-induced structural phase transition was observed in the studied range.     

Chemical modification of carbon nanotubes with organic hydrazines

A reaction of single-wall carbon nanotubes with an organic hydrazine proceeds in an aqueous surfactant solution. Raman spectrum of the product shows the typical disorder band, indicating the occurrence of sidewall functionalization of nanotubes. Elemental analyses of the products suggest that C-N bonds are formed on the nanotube surface. The functionalized nanotubes are soluble in organic solvents up to 100 mg/L. The attached groups can be removed by heating.     

Mechanical properties of individual electrospun polymer-nanotube composite nanofibers

Singlewalled carbon nanotube/polyvinylalcohol composite nanofibers were electro-spun onto a silicon surface pre-patterned with trenches. These nanofibers were prepared with different loadings of SWCNTs and had radii between 20 and 40 nm. Individual fiber sections were pinned across the trenches and laterally loaded by an AFM tip to yield mechanical response curves. A simple model was exploited to extract the tensile mechanical properties from the lateral force-displacement data. Depending on the fiber composition, the tensile modulus was found to be between 3 and 85 GPa. In addition we have prepared fibers with tensile strength of up to 2.6 GPa. Such optimised fibers break at strains of ∼4% and exhibit toughness of up to 27 MJ/m³.