Role of catalysts in the surface synthesis of single-walled carbon nanotubes

We demonstrate the role of catalysts in the surface growth of single-walled carbon nanotubes (SWNTs) by reviewing recent progress in the surface synthesis of SWNTs. Three effects of catalysts on surface synthesis are studied: type of catalyst, the relationship between the size of catalyst particles and carbon feeding rates, and interactions between catalysts and substrates. Understanding of the role of catalysts will contribute to our ability to control the synthesis of SWNTs on various substrates and facilitate the fabrication of nanotube-based devices.      

Single-walled carbon nanotube growth with non-iron-group “catalysts” by chemical vapor deposition

Various materials have been found to “catalyze” carbon nanotube growth in chemical vapor deposition (CVD) when they become nano-sized particles. These involve not only metals, such as Pd, Pt, Au, Ag, and Cu, but also semiconductors, such as Si, Ge, and SiC. Alumina and diamond nanoparticles also produce carbon nanotubes. These “catalysts”, which are better called “seeds”, can be categorized into two types: one type forms a eutectic liquid or highly-mobile alloy with carbon, and carbon atoms precipitate from the eutectic alloy; the other type remains as a solid phase and form a carbon surface layer during CVD growth. In this paper, we review recent studies of SWCNT growth with these non-iron-group materials and highlight the mechanisms involved.      

Self-organized growth of complex nanotube patterns on crystal surfaces

The organization of carbon nanotubes into well-defined straight or curved geometries and arrays on surfaces is a critical prerequisite for their integration into nanocircuits and a variety of functional nanosystems. We review the recent development of a new approach to carbon nanotube organization based on self-organized growth directed by well-defined crystal surfaces, or “nanotube epitaxy”. We identify three different modes of surface-directed growth, namely by atomic rows, atomic steps, and nanofacets. Particular emphasis is given here to the combinations of such surface-directed growth with external forces—like those exerted by an electric field or gas flow—for the creation of well-defined complex geometries, including crossbar architectures, serpentines, and coils.      

Inkjet printing of single-walled carbon nanotube/RuO<Subscript>2</Subscript> nanowire supercapacitors on cloth fabrics and flexible substrates

Single-walled carbon nanotube (SWNT) thin film electrodes have been printed on flexible substrates and cloth fabrics by using SWNT inks and an off-the-shelf inkjet printer, with features of controlled pattern geometry (0.4–6 cm²), location, controllable thickness (20–200 nm), and tunable electrical conductivity. The as-printed SWNT films were then sandwiched together with a piece of printable polymer electrolyte to form flexible and wearable supercapacitors, which displayed good capacitive behavior even after 1,000 charge/discharge cycles. Furthermore, a simple and efficient route to produce ruthenium oxide (RuO₂) nanowire/SWNT hybrid films has been developed, and it was found that the knee frequency of the hybrid thin film electrodes can reach 1,500 Hz, which is much higher than the knee frequency of the bare SWNT electrodes (˜158 Hz). In addition, with the integration of RuO₂ nanowires, the performance of the printed SWNT supercapacitor was significantly improved in terms of its specific capacitance of 138 F/g, power density of 96 kW/kg, and energy density of 18.8 Wh/kg. The results indicate the potential of printable energy storage devices and their significant promise for application in wearable energy storage devices.      

Nanopumping molecules via a carbon nanotube

We demonstrate the feasibility of using a carbon nanotube to nanopump molecules. Molecular dynamics simulations show that the transport and ejection of a C₂₀ molecule via a single-walled carbon nanotube (SWNT) can be achieved by a sustained mechanical actuation driven by two oscillating tips. The optimal condition for nanopumping is found when the tip oscillation frequency and magnitude correlate to form quasi steady-state mechanical wave propagation in the SWNT, so that the energy transfer process is optimal leading to maximal molecular translational motion and minimal rotational motion. Our finding provides a potentially useful mechanism for using an SWNT as a vehicle to deliver large drug molecules.      

Isolation of single-walled carbon nanotube enantiomers by density differentiation

Current methods of synthesizing single-walled carbon nanotubes (SWNTs) result in racemic mixtures that have impeded the study of left- and right-handed SWNTs. Here we present a method of isolating different SWNT enantiomers using density gradient ultracentrifugation. Enantiomer separation is enabled by the chiral surfactant sodium cholate, which discriminates between left- and right-handed SWNTs and thus induces subtle differences in their buoyant densities. This sorting strategy can be employed for simultaneous enrichment by handedness and roll-up vector of SWNTs having diameters ranging from 0.7 to 1.5 nm. In addition, circular dichroism of enantiomer refined samples enables identification of high-energy optical transitions in SWNTs. Electronic Supplementary Material  Supplementary material is available for this article at and is accessible for authorized users.     

Growth of serpentine carbon nanotubes on quartz substrates and their electrical properties

A simple method for high-yield, chemical vapor deposition (CVD) synthesis of serpentine carbon nanotubes, employing quartz substrates and a molecular cluster catalyst, is described. The growth mechanism is analyzed by controlled addition of nanoscale barriers, and by mechanical analysis of the curved sections. The serpentine structures are used to study the electrical transport properties of parallel arrays of identical nanotubes, which show three-terminal conductance that scales linearly with the number of nanotube segments. This article is published with open access at Springerlink.com     

Odako growth of dense arrays of single-walled carbon nanotubes attached to carbon surfaces

A novel process is demonstrated whereby dense arrays of single-walled carbon nanotubes (SWNT) are grown directly at the interface of a carbon material or carbon fiber. This growth process combines the concepts of SWNT tip growth and alumina-supported SWNT base growth to yield what we refer to as “odako” growth. In odako growth, an alumina flake detaches from the carbon surface and supports catalytic growth of dense SWNT arrays at the tip, leaving a direct interface between the carbon surface and the dense SWNT arrays. In addition to being a new and novel form of SWNT array growth, this technique provides a route toward future development of many important applications for dense aligned SWNT arrays. Electronic Supplementary Material  Supplementary material is available for this article at and is accessible for authorized users. This article is published with open access at Springerlink.com     

Effect of the laser heating of nanotube nuclei on the nanotube type population

Many potential applications of carbon nanotubes are expected to benefit from the availability of single-walled carbon nanotube materials enriched in metallic species, and specifically armchair nanotubes. The present work focuses on the modification of the pulsed laser vaporization (PLV) technique to selectively produce certain carbon nanotube structures. Nanotube nuclei were “warmed-up” with an additional laser pulse, timed to coincide approximately with the nucleation event. The effect of the second laser on the carbon vapor temperature was studied by emission spectroscopy. Nanotube type populations with and without warm-up were compared by means of absorption, photoluminescence, and Raman spectroscopy. It was found that the warm-up of nanotube nuclei with a laser pulse has a noticeable, albeit small, effect on the nanotube population. The intensity of spectral features associated with (9,7) nanotube and its large chiral angle neighbors increased, while small chiral angle nanotubes decreased, with exception of the (15,0) tube. This experiment demonstrates that nanotube population during PLV synthesis can be manipulated in a controlled fashion.      

Direct evidence for lip-lip interactions in multi-walled carbon nanotubes

The stability of open edged multi-walled carbon nanotubes has been investigated by using in situ high resolution transmission electron microscopy at elevated temperatures. Formation of inter-shell structures was experimentally observed for the first time and attributed to a robust interaction between adjacent concentric shells (so-called lip-lip interaction). The fl uctuating behavior of the inter-shell structures suggests a mechanism by which the carbon atoms can pass in or out through the inter-shell edges during carbon nanotube growth or shrinkage processes. This article is published with open access at Springerlink.com     

Design of a carbon nanotube/magnetic nanoparticle-based peroxidase-like nanocomplex and its application for highly efficient catalytic oxidation of phenols

We report a novel nanotechnology-based approach for the highly efficient catalytic oxidation of phenols and their removal from wastewater. We use a nanocomplex made of multi-walled carbon nanotubes (MWNTs) and magnetic nanoparticles (MNPs). This nanocomplex retains the magnetic properties of individual MNPs and can be effectively separated under an external magnetic field. More importantly, the formation of the nanocomplex enhances the intrinsic peroxidase-like activity of the MNPs that can catalyze the reduction of hydrogen peroxide (H₂O₂). Significantly, in the presence of H₂O₂, this nanocomplex catalyzes the oxidation of phenols with high efficiency, generating insoluble polyaromatic products that can be readily separated from water.      

Imaging electronic structure of carbon nanotubes by voltage-contrast scanning electron microscopy

We introduce voltage-contrast scanning electron microscopy (VC-SEM) for visual characterization of the electronic properties of single-walled carbon nanotubes. VC-SEM involves tuning the electronic band structure and imaging the potential profi le along the length of the nanotube. The resultant secondary electron contrast allows to distinguish between metallic and semiconducting carbon nanotubes and to follow the switching of semiconducting nanotube devices, as confi rmed by in situ electrical transport measurements. We demonstrate that high-density arrays of individual nanotube devices can be rapidly and simultaneously characterized. A leakage current model in combination with fi nite element simulations of the device electrostatics is presented in order to explain the observed contrast evolution of the nanotube and surface electrodes. This work serves to fill a void in electronic characterization of molecular device architectures. Electronic Supplementary Material  Supplementary material is available for this article at and is accessible for authorized users. This article is published with open access at Springerlink.com     

Theory and practice of “Striping” for improved ON/OFF Ratio in carbon nanonet thin film transistors

A new technique to reduce the influence of metallic carbon nanotubes (CNTs)—relevant for large-scale integrated circuits based on CNT-nanonet transistors—is proposed and verified. Historically, electrical and chemical filtering of the metallic CNTs have been used to improve the ON/OFF ratio of CNT-nanonet transistors; however, the corresponding degradation in ON-current has made these techniques somewhat unsatisfactory. Here, we abandon the classical approaches in favor of a new approach based on relocation of asymmetric percolation threshold of CNT-nanonet transistors by a technique called “striping”; this allows fabrication of transistors with ON/OFF ratio >1000 and ON-current degradation no more than a factor of 2. We offer first principle numerical models, experimental confirmation, and renormalization arguments to provide a broad theoretical and experimental foundation of the proposed method.      

Bi2S3 nanotubes: Facile synthesis and growth mechanism

Synthesis of tubular nanomaterials has become a prolific area of investigation due to their wide range of applications. A facile solution-based method has been designed to fabricate uniform Bi₂S₃ nanotubes with average size of 20 nm × 160 nm using only bismuth nitrate (Bi(NO₃)₃·5H₂O) and sulfur powder (S) as the reactants and octadecylamine (ODA) as the solvent. Powder X-ray diffraction (XRD), transmission electron microscopy (TEM), high-resolution TEM (HRTEM), and energy dispersive spectroscopy (EDX) experiments were employed to characterize the resulting Bi₂S₃ nanotubes and the classic rolling mechanism was applied to explain their formation process.      

Functionalization of single- and multi-walled carbon nanotubes with cationic amphiphiles for plasmid DNA complexation and transfection

The possibility of delivering DNA efficiently to cells represents a crucial issue for the treatment of both genetic and acquired diseases. However, even although the efficiency of non-viral transfection systems has improved in the last decade, none have yet proven to be sufficiently effective in vivo. We report herein our results on the functionalization of single-walled carbon nanotubes (SWNT) and multi-walled carbon nanotubes (MWNT) by two cationic amphiphiles (lipid RPR120535 and pyrenyl polyamine), their use for the complexation of plasmid DNA, and their efficiency in transfecting cells in vitro. The experiments have shown that the efficiency of transfection is higher when using SWNT instead of MWNT, and that transfection efficiency is similar or slightly higher when using nanoplexes (SWNT/lipid RPR120535/DNA) instead of lipoplexes (lipid RPR120535/DNA) and several orders of magnitude higher than that of naked DNA. This study therefore shows both that the transfection is better when using SWNTs and that it is dependent on the nature of the amphiphilic molecules adsorbed on the nanotubes.      

Modeling frequency- and temperature-invariant dissipative behaviors of randomly entangled carbon nanotube networks under cyclic loading

Recent experiments have shown that entangled networks of carbon nanotubes exhibit temperature- and frequency-invariant dissipative behaviors under cyclic loading. We have performed coarse-grained molecular dynamics simulations which show that these intriguing phenomena can be attributed to the unstable attachments/detachments between individual carbon nanotubes induced by van der Waals interactions. We show that this behavior can be described by a triboelastic constitutive model. This study highlights the promise of carbon nanomaterials for energy absorption and dissipation under extreme conditions.      

Separation of metallic and semiconducting single-walled carbon nanotubes through fluorous chemistry

Separation of metallic from semiconducting single-walled carbon nanotubes has been a major challenge for some time and some previous efforts have resulted in partial success. We have accomplished the separation effectively by employing fluorous chemistry wherein the diazonium salt of 4-heptadecafluorooc tylaniline selectively reacts with the metallic nanotubes present in the mixture of nanotubes. The resulting fluoroderivative was extracted in perfluorohexane leaving the semiconducting nanotubes in the aqueous layer. The products have been characterized by both Raman and electronic absorption spectroscopy. The method avoids the cumbersome centrifugation step required by some other procedures. This article is published with open access at Springerlink.com     

Quantum transport properties of chemically functionalized long semiconducting carbon nanotubes

We present a first-principles study of the electronic transport properties of micrometer long semiconducting carbon nanotubes randomly covered with carbene functional groups. Whereas prior studies suggested that metallic tubes are hardly affected by such addends, we show here that the conductance of semiconducting tubes with standard diameter is, on the contrary, severely damaged. The configurational-averaged conductance as a function of tube diameter, with a coverage of up to one hundred molecules, is extracted. Our results indicate that the search for a conductance-preserving covalent functionalization route remains a challenging issue.      

Synthesis of single-walled carbon nanotubes by induction thermal plasma

The production of high quality single-walled carbon nanotubes (SWCNTs) on a bulk scale has been an issue of considerable interest. Recently, it has been demonstrated that high quality SWCNTs can be continuously synthesized on large scale by using induction thermal plasma technology. In this process, the high energy density of the thermal plasma is employed to generate dense vapor-phase precursors for the synthesis of SWCNTs. With the current reactor system, a carbon soot product which contains approximately 40 wt% of SWCNTs can be continuously synthesized at the high production rate of ∼100 g/h. In this article, our recent research efforts to achieve major advances in this technology are presented. Firstly, the processing parameters involved are examined systematically in order to evaluate their individual influences on the SWCNT synthesis. Based on these results, the appropriate operating conditions of the induction thermal plasma process for an effective synthesis of SWCNTs are discussed. A characterization study has also been performed on the SWCNTs produced under the optimum processing conditions. Finally, a mathematical model of the process currently under development is described. The model will help us to better understand the synthesis of SWCNTs in the induction plasma process.      

Synthesis and Ex situ doping of ZnTe and ZnSe nanostructures with extreme aspect ratios

We report synthesis windows for growth of millimeter-long ZnTe nanoribbons and ZnSe nanowires using vapor transport. By tuning the local conditions at the growth substrate, high aspect ratio nanostructures can be synthesized. A Cu-ion immersion doping method was applied, producing strongly p-type conduction in ZnTe and ionic conduction in ZnSe. These extreme aspect ratio wide-bandgap semiconductors have great potential for high density nanostructured optoelectronic circuits.