High-temperature electrical transport properties of buckypapers composed of doped single-walled carbon nanotubes

Data on temperature-dependent electrical resistance of buckypaper flakes are presented in this paper. The buckypapers are composed of ropes of aligned single-walled carbon nanotubes doped with HNO₃, which are treated as mixed systems with their properties being dependent on the treatment performed. The measurements cover rather wide temperature range from 300 up to 900 K. In case of untreated samples, curves with two well-defined activation energies are seen, which are discussed in terms of different DC conductivity mechanisms, with a great attention paid to the parallel metal-semiconductor system. In turn, in heat-treated samples the resistance is found nearly temperature-independent except for the significant peak centered at about 600-650 K. Observed characteristics are also fitted using the parallel model, although with a less accuracy suggesting influence of another conductivity mechanisms. At any rate, the resistance peak is possibly related to the metal/non-metal transition observed in disordered solids.     

Sorting single-walled carbon nanotubes by strain-based electrical burn-off

Armchair metallic single-walled carbon nanotubes (SWCNTs) retain their high symmetry under tensile stretching, and their electrical properties are the least sensitive to tensile strain. Other types of SWCNTs with lower symmetries, including quasi-metallic (or small band-gap semiconducting) and semiconducting SWCNTs, are more sensitive and can exhibit band-gap changes under tensile stretching. In this study, we demonstrate a simple and reliable method for selectively removing armchair metallic SWCNTs from suspended SWCNT arrays to tailor the strain property of these composite SWCNTs. Our method takes advantage of the band-gap changes of three SWCNT species with respect to strain. Proof of the effectiveness of selection is given by a comparative strain sensitivity study on the initial and treated SWCNT arrays.     

Flexible single-walled carbon nanotubes/polyaniline composite films and their enhanced thermoelectric properties

Flexible single-walled carbon nanotubes/polyaniline (SWNT/PANi) composite films with enhanced thermoelectric properties were prepared via a simple method. Furthermore, these paper-like composite films show good flexibility, which makes them possible to be widely applied in various flexible energy converter devices.     

Nonlocal material properties of single-walled carbon nanotubes

Natural frequencies of single-walled carbon nanotubes (SWCNTs) obtained using a model based on Eringen's nonlocal continuum mechanics and the Timoshenko beam theory are compared with those obtained by molecular dynamics simulations. The goal was to determine the values of the material constant, considered here as a nonlocal property, as a function of the length and the diameter of SWCNTs. The present approach has the advantage of eliminating the SWCNT thickness from the computations. A sensitivity analysis of natural frequencies to changes in the nonlocal material constant is also carried out and it shows that the influence of the nonlocal effects decreases with an increase in the SWCNT dimensions. The matching of natural frequencies shows that the nonlocal material constant varies with the natural frequency and the SWCNT length and diameter.     

Reinforcing randomly oriented transparent freestanding single-walled carbon nanotube films

We report an improvement of the mechanical properties of transparent randomly oriented freestanding single-walled carbon nanotube (SWCNT) films by deposition of polymers using a drop casting method and aluminum oxide utilizing an atomic layer deposition (ALD) technique. Due to the thickness increase, the polymer coating resulted in an increase in toughness, however, simultaneously decreasing the ultimate tensile strength. The 100 nm thick SWCNT films ALD-coated with Al₂O₃ layer revealed significant increase in the ultimate tensile strength from 46 ± 5 to 213 ± 17 and 80 ± 4 to 318 ± 16 MPa depending on the network density, preserving the high level of porosity of the structure.     

Functionalization of carbon nanotubes with amphiphilic molecules and their Langmuir-Blodgett films

Crown ether-modified full-length multi-walled carbon nanotubes (MWNTs) and single-walled carbon nanotubes (SWNTs) have been prepared by a direct heating method. Subsequently, the Langmuir-Blodgett (LB) method was used to prepare a monolayer of modified carbon nanotubes on an air-water interface. The corresponding Langmuir-Blodgett (LB) films appeared to be very stable and could be transferred onto a hydrophilic silicon substrate easily. Scanning electron microscopy (SEM) images revealed the morphology of the films, in which locally aligned structures could be observed.     

Spectral properties of single-walled carbon nanotubes encapsulating fullerenes

Single-walled carbon nanotubes (SWCNTs) with diameter ranged from 1.22 to 1.6 nm filled with C₆₀, C₇₀ and C₆₀H₂₈ molecules (peapods), as well as double-walled carbon nanotubes (DWCNTs) derived from peapods, were studied by HRTEM, UV-vis-NIR and Raman spectroscopy. Suspensions with accurate concentration were used for spectroscopic studies to enable quantitative comparison of different substances. Filling of the SWCNTs with C₇₀ molecules resulted in a reduced van der Waals interaction between the tubes in a bundle. The DWCNTs have lower intensity of the van Hove bands and weaker photoluminescence. Raman spectra at 633 and 1064 nm excitation wavelengths reveal that RBM frequencies of C₆₀ and C₇₀ peapods are equally downshifted compared to empty tubes. It was found that filling of the nanotubes with C₆₀ and C₇₀ caused spectral shifts of absorption bands: thin tubes display red shifts, while thick ones show blue shifts. DWCNTs and C₆₀H₂₈@SWCNTs do not show any shifts. All the results suggest that the filling of nanotubes with fullerenes alters the average diameter of the electron cloud around SWCNT framework; namely, it increases for thin SWCNTs, and decreases for thick ones. Our attempts to structurally assign thick nanotubes using reported extrapolations from data for thin tubes were unsuccessful.     

Prediction of elastic properties for single-walled carbon nanotubes

Based on a link between molecular and solid mechanics, an analytical method was developed for modeling the elastic properties of single-walled carbon nanotubes (SWNTs). A SWNT is regarded as a continuum-shell model which is composed of the discrete molecular structures linked by the carbon-to-carbon bonds. The elastic properties were investigated for the SWNTs as a function of the nanotube size in terms of the chiral vector integers (n,m). The theoretical prediction on elastic properties agreed reasonably with the existing experiment and theoretical results. The present formulas are able to serve as a good approximation of the elastic properties for SWNTs.     

Fundamental electronic properties and applications of single-walled carbon nanotubes

Recent scanning tunneling microscopy studies of the intrinsic electronic properties of single-walled carbon nanotubes (SWNTs) are overviewed in this Account. A brief theoretical treatment of the electronic properties of SWNTs is developed, and then the effects of finite curvature and broken symmetry on electronic properties, the unique one-dimensional energy dispersion in nanotubes, the interaction between local spins and carriers in metallic nanotubes systems, and the atomic structure and electronic properties of intramolecular junctions are described. The implications of these studies for understanding fundamental one-dimensional physics and future nanotube device applications are also discussed.     

Freestanding vertically oriented single-walled carbon nanotubes synthesized using microwave plasma-enhanced CVD

Freestanding single-walled carbon nanotubes (SWCNTs) have been synthesized in a vertical direction, perpendicular to the growth substrate, using applied DC substrate bias in a microwave plasma-enhanced chemical vapor deposition (PECVD) synthesis process. The degree of alignment and spatial density of SWCNTs demonstrate a strong dependence on the magnitude of applied bias, with increased alignment and decreased density with increased bias. The unique synthesis environment created by the application of a negative substrate bias in PECVD aligns SWCNTs along electric field lines and decreases SWCNT density due to bombardment by positively charged hydrogen ions. Multi-excitation wavelength Raman spectroscopy reveals shifts in dominant RBM peaks with the application of dc bias. Use of this technique to orient SWCNTs in the vertical direction may allow for three-dimensional SWCNT-based device architectures.     

Growth and physical properties of individual single-walled carbon nanotubes

We have developed a versatile catalyst-assisted chemical vapor deposition (CVD) technique for the synthesis of single-walled carbon nanotubes (SWNTs) from discrete nickel nanoparticles (average diameter of 4.7 ± 1.4 nm). Atomic force microscopy (AFM), transmission electron microscopy (TEM) and micro-Raman spectroscopy are used to characterize these as-grown nanotubes. Using a conventional set-up, we are able to produce isolated SWNTs with a narrow diameter distribution (1.5 ± 0.5 nm). The advantages of such a versatile CVD method for the study of physical properties at the single nanotube level are illustrated by means of two prospective studies on vibrational and electrostatic properties of SWNTs.     

Anisotropic electrical conduction of vertically-aligned single-walled carbon nanotube films

Anisotropic electrical conduction measurements have been carried out for thin films of vertically-aligned single-walled carbon nanotubes (VA-SWCNTs) grown by an alcohol catalytic CVD process. Combined with controlled synthesis and structure characterization by optical spectroscopy, the influence of the aligned structure on the electrical conduction has been identified. The out-of-plane conductivity of the films was measured to be about 0.56 S/mm, independently of the film thickness. On the other hand, the in-plane conductivity was found to be more than an order of magnitude smaller, which gives rise to highly anisotropic electrical conduction, reflecting the high degree of alignment in the VA-SWCNT films. The in-plane conductivity decreases with increasing film thickness, in contrast to the film of random SWCNT networks, which exhibit thickness-independent in-plane resistance. The thickness-dependent in-plane conductivity can be expounded by a growth model of vertically aligned SWCNT films in which a thin layer of nanotube networks form on top of films at the initial stage of the growth. Such electrical anisotropy of VA-SWCNT films can be useful in miniaturized sensing devices.     

Electrochemical properties of carbon nanotubes-hydrogenase conjugates Langmuir-Blodgett films

We report the preparation of Langmuir-Blodgett (LB) films composed of oxidized carbon nanotubes (CNTs) and hydrogenase (H₂ase) conjugates and their electrochemical properties. Both single-walled (SWNTs) and multi-walled CNTs (MWNTs) were used to form mixed monolayers with H₂ase on the Tris-HCl subphase surfaces. By using the LB method, the CNTs-H₂ase monolayers were transferred onto CaF₂ and indium tin oxide (ITO) electrode surfaces. The LB film modified electrodes showed a couple of waves centered at around −500 mV (versus Ag/AgCl), which corresponding to the redox reaction of [4Fe-4S]^2+/1+ clusters in the H₂ase. The current intensity was enhanced after co-assembly with CNTs. Because of the different diameters of CNTs, this current intensity was proportional to the scan rate (υ) for the electrodes modified with the LB films of pure H₂ase and SWNTs-H₂ase, but to the root of scan rate (υ^1/2) for those modified with the MWNTs-H₂ase LB film. The products of diffusion coefficient and concentration (D^1/2C) increased in the order of pure H₂ase, SWNTs-H₂ase, and MWNTs-H₂ase LB films.     

Piezoresistive,optical and electrical properties of diamond like carbon and carbon nitride films

In the present study diamond like carbon (DLC) and carbon nitride (a-CN_x:H) films were deposited by closed drift ion source from the acetylene and nitrogen gas mixture. The piezoresistive, electrical and optical properties of ion beam synthesized DLC films were investigated. Piezoresistive properties of the diamond like carbon and carbon nitride films were evaluated by four point bending test. The piezoresistors were fabricated on crystalline alumina substrates using Al-based interdigitated finger type electrodes. Effects of the nitrogen concentration on the piezoresistive gauge factor were investigated. The dependence of the resistance of the metal/a-CN_x:H/metal structures on temperature has been studied. Current–voltage (I–V) and capacitance–voltage characteristics were measured for a-CN_x:H/Si heterostructures. The main current transport mechanisms were analyzed. Optical parameters of the synthesized films such as optical bandgap and B parameter (slope of the linear part of the Tauc plot) were investigated to study possible correlation with the piezoresistive properties.     

Synthesis,characterization, and electrical properties of nitrogen-doped single-walled carbon nanotubes with different nitrogen content

Nitrogen-doped single walled carbon nanotubes (SWCNTs) have been synthesized via the thermal decomposition of ferrocene using different ratios of acetonitrile/ethanol feedstock mixtures during the chemical vapor deposition process. The experiments were performed at 950 °C and 2 bar. The concentration of acetonitrile in the mixtures was varied from 0% to 100%. High resolution transmission electron microscopy and Raman spectroscopical measurements revealed the formation of SWCNTs for all mixtures. X-ray photoelectron spectroscopical analysis show nitrogen doping levels of up to 2 at.%. The doping levels increase as the acetonitrile concentration increases. The nitrogen incorporation is predominantly in the pyridine form. Electrical conductivity measurements show the dependence of conductivity as a function of nitrogen incorporation in the SWCNTs.     

Polymeric nanocomposite films from functionalized vs suspended single-walled carbon nanotubes

The reported work was to demonstrate that the defect-derived photoluminescence in functionalized single-walled carbon nanotubes could be exploited in probing the dispersion of these nanotubes in polymeric nanocomposites because the luminescence emissions are sensitive to the degree of nanotube bundling and surface modification. The polyimide-SWNT nanocomposite thin films obtained from nanotubes with and without functionalization were compared. The spectroscopic results suggest that despite a similar visual appearance in the two kinds of films, the nanotube dispersion must be significantly better in the film with functionalized nanotubes, as reflected by the strong photoluminescence. In fact, the nanotubes embedded in polymer matrix that can be readily characterized by Raman spectroscopy are non-luminescent, while those that are difficult for Raman are strongly luminescent. Therefore, Raman and photoluminescence serve as complementary tools in the investigation of nanocomposites concerning the nanotube dispersion-related properties.     

Ferrocene-filled single-walled carbon nanotubes

Ferrocene molecules are successfully introduced into the inner hollow space of Single-walled carbon nanotubes (SWNTs) to get ferrocene-filled SWNTs (Fc@SWNTs). This nanohybrid material was carefully characterized by high resolution microscopy, FTIR spectrum, and Cyclic voltammetry (CV). This new material may not only act as air stable n-type field-effect transistors based on nanotubes, but it may also be employed as building blocks for various devices based on the redox activity of ferrocene. What’s more, upon high temperature annealing, the encapsulated ferrocene molecules will decompose and change into interior tubes, forming double-walled carbon nanotubes (DWNTs). This provides convincing evidence that ferrocene molecules are inserted into the hollow cavities SWNTs. This result also presented a controllable way to synthesize DWNTs.     

Structure and electrical properties of the oriented polyaniline films

The structure of oriented polyaniline (PANI) films were characterized by elemental analysis, FTIR, XPS, SEM, and X-ray diffraction, and their electrical properties were measured as a function of the protonation state, elongation ratio, temperature, and applied pressure. A maximum conductivity at room temperature for oriented PANI films can be achieved up to 500 s/cm with conductivity anisotropy as high as 20 : 1. The temperature dependence of conductivity for both unoriented and oriented films at 77–300 K and applied pressure of 0–11.4 kbar is consistent with the 3-D variable-range hopping model; however, the hopping barrier of oriented films is one order magnitude lower than that of unoriented films. The mechanism of enhanced conductivity for oriented PANI films is discussed. © 1994 John Wiley & Sons, Inc.     

Chemistry of single-walled carbon nanotubes

In this Account we highlight the experimental evidence in favor of our view that carbon nanotubes should be considered as a new macromolecular form of carbon with unique properties and with great potential for practical applications. We show that carbon nanotubes may take on properties that are normally associated with molecular species, such as solubility in organic solvents, solution-based chemical transformations, chromatography, and spectroscopy. It is already clear that the nascent field of nanotube chemistry will rival that of the fullerenes.