Small-diameter Nanotubes Research Articles

Simplified synthesis of double-wall carbon nanotubes

We demonstrate a simplified synthesis technique for double-wall carbon nanotubes that is an adaptation of chemical vapor deposition (CVD) techniques used previously for the production of single-wall nanotubes. Double-wall nanotubes (DWNTs) provide ideal geometries for numerous fundamental structural, electronic, thermal and vibrational studies, as well as providing a unique new platform for practical applications. The diameter distribution of DWNTs is broad, and it is possible that in previous studies using CVD-grown small-diameter nanotubes, presumed to be single-wall, there were significant numbers of DWNTs present.

Functionalization of carbon nanotubes by free radicals.

Free radicals generated by decomposition of benzoyl peroxide in the presence of alkyl iodides have been used to derivatize small-diameter single-wall carbon nanotubes (HiPco tubes). The degree of functionalization, estimated by thermal gravimetric analysis, is as high as 1 in approximately 5 carbons in the nanotube framework. The derivatized nanotubes exhibits remarkably improved solubility in organic solvents. The attached groups can be removed by heating in an atmosphere of argon. Derivatization was also accomplished by treating SWNTs with various sulfoxides employing Fenton's reagent. [reaction: see text]

On the elastic properties of carbon nanotube-based composites: modelling and characterization

The exceptional mechanical and physical properties observed for carbon nanotubes has stimulated the development of nanotube-based composite materials, but critical challenges exist before we can exploit these extraordinary nanoscale properties in a macroscopic composite. At the nanoscale, the structure of the carbon nanotube strongly influences the overall properties of the composite. The focus of this research is to develop a fundamental understanding of the structure/size influence of carbon nanotubes on the elastic properties of nanotube-based composites. Towards this end, the nanoscale structure and elastic properties of a model composite system of aligned multi-walled carbon nanotubes embedded in a polystyrene matrix were characterized, and a micromechanical approach for modelling of short fibre composites was modified to account for the structure of the nanotube reinforcement to predict the elastic modulus of the nanocomposite as a function of the constituent properties, reinforcement geometry and nanotube structure. The experimental characterization results are compared with numerical predictions and highlight the structure/size influence of the nanotube reinforcement on the properties of the nanocomposite. The nanocomposite elastic properties are particularly sensitive to the nanotube diameter, since larger diameter nanotubes show a lower effective modulus and occupy a greater volume fraction in the composite relative to smaller-diameter nanotubes.

Electrophoretic protein transport in gold nanotube membranes.

Gold nanotube membranes are ideal model systems for exploring how pore size affects the rate and selectivity of protein transport in synthetic membranes. This is because these membranes have cylindrical, monodisperse pores (the nanotubes) with diameters that can be varied at will from tens of nanometers down to less than 1 nm. We report here on the effects of nanotube inside diameter, solution pH, and applied transmembrane potential on the rate and selectivity of protein transport in PEG-thiol-treated gold nanotube membranes. The transport properties of four proteins of differing sizes and pI values--lysozyme, bovine serum albumin, carbonic anhydrase, and bovine hemoglobulin--were investigated. In general, membranes containing larger diameter nanotubes showed higher fluxes and lower selectivities than membranes with smaller diameter nanotubes. Transmembrane electrophoresis can be used to augment the diffusive transport selectivity. For example, for proteins that are oppositely charged, a combination of a large transmembrane potential and a large nanotube diameter can be used to optimize both selectivity and flux. In addition to transmembrane potential and nanotube diameter, solution pH value plays an important role in determining the transport selectivity. This is because pH determines the net charge on the protein molecule and this, in turn, determines the importance of the electrophoretic transport term.

Simple catalyst for the growth of small-diameter carbon nanotubes

We describe a simple technique to grow small (approximately 1–4 nm) diam carbon nanotubes both in and out of the plane of a silicon substrate using the decomposition of ethylene and a nickel–iron thin film catalyst. This procedure can be used to produce nanotubes for ultrasharp atomic force microscope probes as well as for nanotube-based electronic devices. The technique is compatible with the patterned growth of nanotubes and with standard device microfabrication processes.

Bismuth nanotubes: potential semiconducting nanomaterials

We have employed the first-principles molecular dynamics method tostudy the stabilities and electronic properties of bismuth nanotubes.There are some stable bismuth nanotubes. The strain energies ofBi(n, n) nanotubes followthe classical 1/D2 strainlaw. For small Bi(n, 0)nanotubes, the strain energies show a non-linear dependence on1/D2.Bismuth nanotubes are predicted to be semiconducting in both(n, n) and(n, 0)forms. For small-diameter bismuth nanotubes, the band structure varies stronglybecause of hybridizations. As the diameters are larger than 18 Å, the band gaps of bothBi(n, n) andBi(n, 0)nanotubes approach 0.63 eV, corresponding to the band gap of a bismuth sheet at theΓpoint. Thus bismuth nanotubes are seen as potential semiconductor nanomaterialsfor future nano-electronics applications.

Synthesis of nearly uniform single-walled carbon nanotubes using identical metal-containing molecular nanoclusters as catalysts.

Uniform and small-diameter single-walled carbon nanotubes (SWNTs) have been produced using identical molecular nanoclusters containing Fe and Mo atoms with a defined molecular formula and a specific structure as catalysts in a chemical vapor deposition method. The average diameter of the SWNTs produced in these experiments is 1.0 nm with a standard deviation for the diameter distribution of 17%. The diameters of SWNTs were obtained by atomic force microscopy and Raman spectroscopy.

Rapid imaging of nanotubes on insulating substrates

We demonstrate the use of field-emission scanning electron microscopy for rapid imaging of small-diameter carbon nanotubes on insulating SiO2 substrates. The image contrast stems from local potential differences between the nanotube and substrate and is insensitive to surface roughness and defects. This technique may also be used as a probe of the electrical connectivity of small structures without external leads.

Evolution of the molecular structure of metallic and semiconducting carbon nanotubes under laser irradiation

The temperature dependence of the morphology of single wall carbon nanotubes (SWNTs) has been studied by Raman spectroscopy at different temperatures by changing the incident laser power. It is shown that a high power laser irradiation treatment of as-grown samples anneals the SWNTs, improving their structural order and perhaps also removing adsorbed gases. As a result, a significant increase in the Raman cross-section of the nanotubes upon laser irradiation can be observed, for both metallic or semiconducting tubes. The investigation of the power level dependence of the Raman spectra also reveals that smaller diameter nanotubes are burned off first, increasing the mean diameter of the nanotubes in the sample.

Synthesis of SiC nanorods from sheets of single-walled carbon nanotubes

We report the unexpected synthesis of SiC nanorods during thermally annealing single-walled carbon nanotube sheets (SWNTs) at 1000 °C between two silicon wafers. The exterior layers of the carbon nanotube sheets were converted into a network of SiC nanorods, while the carbon nanotubes interior to the sheet remain unchanged. The nanotube sheets that underwent this reaction were comprised of small diameter nanotubes made by a high-pressure carbon monoxide process (HiPco), which contain iron catalyst. Using the same reaction conditions, SiC formation was not observed for sheets of purified, larger diameter nanotubes made by laser ablation, which contained a small amount of residual metal catalysts.

The formation of low-dimensional ionic crystallites in carbon nanotubes

Atomistic computer simulation models are used to interpret the results of recent high resolution transmission electron microscopy (HRTEM) experiments which have studied the filling of carbon nanotubes by liquid KI. In the HRTEM experiments, the liquid KI fills narrow width nanotubes to form low-dimensional crystallites which display specific distortions in comparison with the idealized bulk fragments. The atomistic origin of these distortions are discussed. Molecular dynamics simulations are then used to directly model the filling of these tubes and an understanding of the filling mechanisms is developed. The dependence of the filling structure on the pore radius and morphology of the carbon nanotube is discussed. Novel crystalline structures, often incorporating specific twisting, is observed for the smaller diameter nanotubes. The physical origins of the observed filling mechanisms are discussed.

Field-enhancement properties of nanotubes in a field emission setup

The polarization phenomenon, involved in the mechanisms of emission from carbon nanotubes, is investigated by means of a self-consistent resolution of Poisson's equation. We show that the field enhancement. responsible for the emission, varies in a logarithmic way with the nanotube length. This leads, for most of the nanotubes investigated, to a rapid saturation of the amplification of the field which does not allow for the recovery of experimental values for microscopic lengths, However, this saturation is less important with (n,0) nanotubes and values of the amplification factor around 2000 can be obtained with small diameter nanotubes of this kind. The case of nanotube films is also investigated, and the dependence of the amplification factor with the nanotube density is pointed out. Finally, the screening effect of the outer shells on the inner ones is investigated in the case of multiwall nanotubes.

Superconductivity in 4 angstrom single-walled carbon nanotubes.

Investigation of the magnetic and transport properties of single-walled small-diameter carbon nanotubes embedded in a zeolite matrix revealed that at temperatures below 20 kelvin, 4 angstrom tubes exhibit superconducting behavior manifest as an anisotropic Meissner effect, with a superconducting gap and fluctuation supercurrent. The measured superconducting characteristics display smooth temperature variations owing to one-dimensional fluctuations, with a mean-field superconducting transition temperature of 15 kelvin. Statistical mechanic calculations based on the Ginzburg-Landau free-energy functional yield predictions that are in excellent agreement with the experiments.

Functionalization of carbon nanotubes by electrochemical reduction of aryl diazonium salts: a bucky paper electrode.

Small-diameter (ca. 0.7 nm) single-wall carbon nanotubes are predicted to display enhanced reactivity relative to larger-diameter nanotubes due to increased curvature strain. The derivatization of these small-diameter nanotubes via electrochemical reduction of a variety of aryl diazonium salts is described. The estimated degree of functionalization is as high as one out of every 20 carbons in the nanotubes bearing a functionalized moiety. The functionalizing moieties can be removed by heating in an argon atmosphere. Nanotubes derivatized with a 4-tert-butylbenzene moiety were found to possess significantly improved solubility in organic solvents. Functionalization of the nanotubes with a molecular system that has exhibited switching and memory behavior is shown. This represents the marriage of wire-like nanotubes with molecular electronic devices.

Current-carrying capacity of carbon nanotubes

The current carrying capacity of ballistic electrons in carbon nanotubes that are coupled to ideal contacts is analyzed. At small applied voltages, where electrons are injected only into crossing subbands, the differential conductance is $4e^2/h$. At applied voltages larger than $\Delta E_{NC}/2e$ ($\Delta E_{NC}$ is the energy level spacing of first non crossing subbands), electrons are injected into non crossing subbands. The contribution of these electrons to current is determined by the competing processes of Bragg reflection and Zener type inter subband tunneling. In small diameter nanotubes, Bragg reflection dominates, and the maximum differential conductance is comparable to $4e^2/h$. Inter subband Zener tunneling can be non negligible as the nanotube diameter increases because $\Delta E_{NC}$ is inversely proportional to the diameter. As a result, with increasing nanotube diameter, the differential conductance becomes larger than $4e^2/h$, though not comparable to the large number of subbands into which electrons are injected from the contacts. These results may be relevant to recent experiments in large diameter multi-wall nanotubes that observed conductances larger than $4e^2/h$.

Band structure and CK α emission of ultrathin nanotubes

An attempt is made to calculate the energy bands and spectra of the characteristic CKα emission of small-diameter carbon nanotubes. The calculated spectra for the nanotubes are compared with similar spectra for graphite monolayers used as a test object and with known experimental results for nanotubes. It is concluded that the x-ray emission spectra can be used to identify thin carbon nanotubes. A classification of solid-phase carbon is proposed which takes into account the position of carbon nanotubes in the family of allotropic carbon forms. The type of hybridization of the electrons in the carbon atom is used as the criterion for classification.

Dependence of Elastic Properties on Morphology in Single-Wall Carbon Nanotubes

Small-diameter nanotubes are predicted to exhibit quantum size effects involving an interdependence between the diameter of the nanotubes and their electronic and magnetic properties. These authors ask whether this dependence on diameter might be extended to the mechanical properties of single-wall carbon nanotubes (SWNTs). Results were obtained by inducing a compressive deformation in SWNTs embedded in a polymer matrix by cooling the specimen to 81 K and monitoring the deformation via the Raman D* band frequency shift of the embedded nanotubes.