Dysprosium-Catalyzed Growth of Single-Walled Carbon Nanotube Arrays on Substrates

In this letter, we report that dysprosium is an effective catalyst for single-walled carbon nanotubes (SWNTs) growth via a chemical vapor deposition (CVD) process for the first time. Horizontally superlong well-oriented SWNT arrays on SiO₂/Si wafer can be fabricated by EtOH-CVD under suitable conditions. The structure and properties are characterized by scanning electron microscopy, transition electron microscopy, Raman spectroscopy and atomic force microscopy. The results show that the SWNTs from dysprosium have better structural uniformity and better conductivity with fewer defects. This rare earth metal provides not only an alternative catalyst for SWNTs growth, but also a possible method to generate high percentage of superlong semiconducting SWNT arrays for various applications of nanoelectronic device.     

Effect of chemisorption on the interfacial bonding characteristics of carbon nanotube-polymer composites

The influence of chemical functionalization on the interfacial bonding characteristics of single-walled nanotubes (SWNTs) reinforced polymer composites was investigated using molecular mechanics and molecular dynamics simulations. The simulations show that functionalization of nanotubes at low densities of functionalized carbon atoms drastically increases their interfacial bonding and shear stress between the nanotubes and the polymer matrix, where chemisorption to as little as 5.0% of the nanotube carbon atoms increases the shear stress by about 1000%. This indicates that increasing the load transfer between SWNTs and a polymer matrix in a composite via chemisorption may be an effective way and chemical attachment of nanotubes during processing may be in part responsible for the enhanced stress transfer observed in some systems of the nanotube-polymer composites. Furthermore, this suggests the possibility to use functionalized nanotubes to effectively reinforce other kinds of polymer-based materials as well.     

X-ray absorption near-edge structure and photoelectron spectroscopy of single-walled carbon nanotubes modified by a HBr solution

X-ray absorption near-edge structure (XANES) spectroscopy and photoelectron spectroscopy (PES) have been used to investigate single-walled carbon nanotubes (SWNTs) modified by immersion in a HBr solution at room temperature. After treatment XANES spectra of SWNTs show a new pronounced feature, which has been assigned to new bonds between the sidewall of the SWNTs and Br atoms. This investigation demonstrates the unique capabilities of the XANES spectroscopy as a tool to achieve structural and bonding information of carbon nanotubes induced by chemical processes.     

Hybrid peptide–carbon nanotube dispersions and hydrogels

Scanning probe microscopy (SPM) techniques based on nano-mechanical measurements (topography, adhesion, modulus) and electric force microscopy (EFM) have been used to examine mica surfaces modified with the ionic-complementary peptide EFK8 alone and with EFK8–single-walled carbon nanotube (SWNT) dispersions in water in order to gain a deeper understanding of the interaction between nanotubes and ionic-complementary peptides. Through the use of these techniques, it has been shown for the first time that peptide fibers can be distinguished from SWNTs and peptide-wrapped SWNTs. SPM images reveal features consistent with two types of helical structures: EFK8 fibers wrapped around each other during self-assembly and EFK8 fibers wrapped around SWNTs. In this second structure, EFK8 chains should be oriented with their hydrophobic sides oriented toward the SWNTs and their hydrophilic sides toward the water, thereby enabling the dispersion of the nanotubes in aqueous media. We have also demonstrated the formation of hybrid EFK8–SWNT hydrogels that have potentially superior physical and mechanical properties over those of other hydrogels and opens up new applications for this type of material. To the best of our knowledge, this is the first work reporting the formation of a composite hydrogel made of an ionic-complementary peptide and carbon nanotubes.     

Purification process for single-wall carbon nanotubes

Single-wall carbon nanotubes (SWNTs) have exceptional strength and stiffness and high thermal and electrical conductivity, making them excellent candidates for aerospace structural materials. However, one of the most fundamental challenges is purifying the SWNTs. The purpose of this study was to develop a simple purification process for SWNTs, along with an understanding of the purification process. In addition, uncomplicated analytical methods were sought to screen and compare various purification methods. In this study, we demonstrate an easy method of cleaning SWNTs and evaluating their purity. The cleaning method, which employed oxidative heat treatment followed by acid reflux, was straightforward, inexpensive, and fairly effective. The purification mechanism was determined to be, first, that much of the non-nanotube carbon and iron catalyst was oxidized and, second, that the acid washing removed the iron oxide, leaving relatively pure SWNTs. Also, it was shown that a combination of thermal gravimetric analysis and Raman spectroscopy, both of which take only a few minutes and require little sample preparation, are sufficient as qualitative screening tools to determine the relative purity of SWNTs. Other analytical techniques were used to verify the validity of the screening techniques.     

The role of carbon nanotube structure in purification and hydrogen adsorption

Structural properties of carbon nanotubes were studied by using samples from various manufacturers synthesized by different processes. A two-stage purification method was applied to all samples. Relationships between synthesis techniques and carbon nanotube structure are discussed. The role of carbon nanotube structural features such as degree of crystallinity, tube diameter, tube wall structure, and bundling behavior in purification and hydrogen adsorption were investigated by a combination of transmission electron microscopy and magnetic resonance techniques. It is suggested that MWNTs with low crystallinity and SWNTs with large diameters and open tube ends yielded the highest hydrogen uptake capacities. Both MWNTs and SWNTs show low hydrogen storage capacities (less than 1 wt%) at hydrogen pressures up to 1480 kPa.     

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.     

Synthesis and Characterization of Novel Hybrids of Tris[3-(trimethoxysilyl)propyl] Isocyanurate (TTPI) Capped Palladium Nanoparticles and Single-Walled Carbon Nanotubes

The conjugation of nanoparticles to carbon nanotubes (CNTs) involves various steps including premodification of the nanotubes, which is known to be a very tedious process and sometimes leads to a mixture of products. In this regard, a direct route to generate such conjugates is a worthwhile endeavor. In this paper, we report a novel, mild, one-pot, approach to a controlled and direct coordination of Pd nanoparticles (Pd NPs) onto the surface of single walled carbon nanotubes (SWNTs), without any pre-modification of the SWNTs surface. We also present detailed characterization of the SWNT-Pd NP hybrid systems using High Resolution Transmission Electron Microscopy (HRTEM), Energy Dispersive X-ray Spectroscopy (EDS), Mid-Infrared Spectroscopy (Mid-IR) and UV-Visible Spectroscopy (UV–vis) along with the stability studies of the nanoconjugates.     

Covalent functionalization of single-walled carbon nanotubes by aniline electrochemical polymerization

Electrochemical polymerization of aniline in an HCl solution on a single-walled carbon nanotubes (SWNTs) film has been studied by Raman and FTIR spectroscopy. It is shown that this method leads to a covalent functionalization of SWNTs with polyaniline (PANI). A careful study in Raman scattering shows that the increase in the intensity of the band at 178 cm⁻¹ associated with radial breathing modes of SWNTs bundles suggests an additional nanotubes roping with PANI as a binding agent. A post chemical treatment with the NH₄OH solution of polymer-functionalized SWNTs involves an internal redox reaction between PANI and carbon nanotubes. As a result, the polymer chain undergoes a transition from the semi-oxidized state into a reduced one.     

New hybrid system: Poly(ethylene glycol) hydrogel with covalently bonded pegylated nanotubes

Hydrogels containing carbon nanotubes (CNTs) are expected to be promising conjugates because they might show a synergic combination of properties from both materials. Most of the hybrid materials containing CNTs only entrap them physically, and the covalent attachment has not been properly addressed yet. In this study, single‐walled carbon nanotubes (SWNTs) were successfully incorporated into a poly(ethylene glycol) (PEG) hydrogel by covalent bonds to form a hybrid material. For this purpose, SWNTs were functionalized with poly(ethylene glycol) methacrylate (PEGMA) to obtain water‐soluble pegylated SWNTs (SWNT–PEGMA). These functionalized SWNTs were covalently bonded through their PEG moieties to a PEG hydrogel. The hybrid network was obtained from the crosslinking reaction of poly(ethylene glycol) diacrylate prepolymer and the SWNT–PEGMA by dual photo‐UV and thermal initiations. The mechanical and swelling properties of the new hybrid material were studied. In addition, the material and lixiviates were analyzed to elucidate any kind of SWNT release and to evaluate a possible in vitro cytotoxic effect. © 2010 Wiley Periodicals, Inc. J Appl Polym Sci, 2011     

Selective Deposition and Alignment of Single-Walled Carbon Nanotubes Assisted by Dielectrophoresis: From Thin Films to Individual Nanotubes

Dielectrophoresis has been used in the controlled deposition of single-walled carbon nanotubes (SWNTs) with the focus on the alignment of nanotube thin films and their applications in the last decade. In this paper, we extend the research from the selective deposition of SWNT thin films to the alignment of small nanotube bundles and individual nanotubes. Electrodes with “teeth”-like patterns are fabricated to study the influence of the electrode width on the deposition and alignment of SWNTs. The entire fabrication process is compatible with optical lithography-based techniques. Therefore, the fabrication cost is low, and the resulting devices are inexpensive. A series of SWNT solutions is prepared with concentrations ranging from 0.0125 to 0.2 mg/ml. The alignment of SWNT thin films, small bundles, and individual nanotubes is achieved under the optimized experimental conditions. The electrical properties of these samples are characterized; the linear current–voltage plots prove that the aligned SWNTs are mainly metallic nanotubes. The microscopy inspection of the samples demonstrates that the alignment of small nanotube bundles and individual nanotubes can only be achieved using narrow electrodes and low-concentration solutions. Our investigation shows that it is possible to deposit a controlled amount of SWNTs in desirable locations using dielectrophoresis.     

Photophysics of individual single-walled carbon nanotubes

Single-walled carbon nanotubes (SWNTs) are cylindrical graphitic molecules that have remained at the forefront of nanomaterials research since 1991, largely due to their exceptional and unusual mechanical, electrical, and optical properties. The motivation for understanding how nanotubes interact with light (i.e., SWNT photophysics) is both fundamental and applied. Individual nanotubes may someday be used as superior near-infrared fluorophores, biological tags and sensors, and components for ultrahigh-speed optical communications systems. Establishing an understanding of basic nanotube photophysics is intrinsically significant and should enable the rapid development of such innovations. Unlike conventional molecules, carbon nanotubes are synthesized as heterogeneous samples, composed of molecules with different diameters, chiralities, and lengths. Because a nanotube can be either metallic or semiconducting depending on its particular molecular structure, SWNT samples are also mixtures of conductors and semiconductors. Early progress in understanding the optical characteristics of SWNTs was limited because nanotubes aggregate when synthesized, causing a mixing of the energy states of different nanotube structures. Recently, significant improvements in sample preparation have made it possible to isolate individual nanotubes, enabling many advances in characterizing their optical properties. In this Account, single-molecule confocal microscopy and spectroscopy were implemented to study the fluorescence from individual nanotubes. Single-molecule measurements naturally circumvent the difficulties associated with SWNT sample inhomogeneities. Intrinsic SWNT photoluminescence has a simple narrow Lorentzian line shape and a polarization dependence, as expected for a one-dimensional system. Although the local environment heavily influences the optical transition wavelength and intensity, single nanotubes are exceptionally photostable. In fact, they have the unique characteristic that their single molecule fluorescence intensity remains constant over time; SWNTs do not "blink" or photobleach under ambient conditions. In addition, transient absorption spectroscopy was used to examine the relaxation dynamics of photoexcited nanotubes and to elucidate the nature of the SWNT excited state. For metallic SWNTs, very fast initial recovery times (300-500 fs) corresponded to excited-state relaxation. For semiconducting SWNTs, an additional slower decay component was observed (50-100 ps) that corresponded to electron-hole recombination. As the excitation intensity was increased, multiple electron-hole pairs were generated in the SWNT; however, these e-h pairs annihilated each other completely in under 3 ps. Studying the dynamics of this annihilation process revealed the lifetimes for one, two, and three e-h pairs, which further confirmed that the photoexcitation of SWNTs produces not free electrons but rather one-dimensional bound electron-hole pairs (i.e., excitons). In summary, nanotube photophysics is a rapidly developing area of nanomaterials research. Individual SWNTs exhibit robust and unexpectedly unwavering single-molecule fluorescence in the near-infrared, show fast relaxation dynamics, and generate excitons as their optical excited states. These fundamental discoveries should enable the development of novel devices based on the impressive photophysical properties of carbon nanotubes, especially in areas like biological imaging. Many facets of nanotube photophysics still need to be better understood, but SWNTs have already proven to be an excellent starting material for future nanophotonics applications.     

Effect of single-walled carbon nanotubes on morphology and mechanical properties of NBR/PVC blends

Dynamically vulcanized thermoplastic elastomer based on Nitrile butadiene-rubber (NBR)/PVC with functionalized single-walled carbon nanotubes (f-SWNTs) and non-functionalized single-walled carbon nanotubes (SWNTs) were prepared using a brabender internal mixer. Effects of two types of SWNTs (functionalized and non-functionalized) on morphology and mechanical properties of NBR/PVC blends were studied. Results showed that the mechanical properties of NBR/PVC/SWNTs nanocomposites improved with the increasing of SWNTs content and in particular with the increase of f-SWNTs content. Moreover, the enhancement of mechanical properties of NBR/PVC blends reinforced with functionalized SWNT was higher than that of NBR/PVC blends with non-functionalized SWNT. Dispersion of SWNTs and morphology of NBR/PVC/SWNT nanocomposites were determined by scanning electron microscopy and transmission electron microscopy (TEM) techniques. TEM images illustrated that f-SWNTs were dispersed uniformly in NBR/PVC matrix while non-functionalized SWNTs showed much aggregation. Dynamic mechanical thermal analysis of NBR/PVC/SWNTs nanocomposites was also studied. The outcomes indicated that in the case of f-SWNTs, the intensity of tan ?? peak was lower than that in the case of non-functionalized SWNTs. Meanwhile, the intensity of tan ?? peak reduced when the content of f-SWNTs was increased.     

Chemical modification of single-walled carbon nanotubes with peroxytrifluoroacetic acid

A new and simple method for chemical modification of single-walled carbon nanotubes (SWNTs) is presented. Purified SWNTs ropes prepared by CVD growth were reacted with peroxytrifluoroacetic acid (PTFAA) under ultrasonication. Samples before and after treatment were characterized using Raman, FTIR, UV/Vis/NIR, XPS, and AFM. Data from these experiments conclusively showed that, in addition to oxygen-based functional groups, trifluoroacetic groups were covalently attached to the SWNTs. Moreover, these modified SWNTs were shortened into ca. 300 nm in length in the same step of functionalization, resulting in exfoliation of nanotube ropes to yield small bundles and individual nanotubes. The resultant SWNTs were easily dispersed in polar solvents such as dimethylformamide, water and ethanol. The PTFAA treatment described herein should be useful to tailor SWNTs’ chemical and physical properties and to broaden their chemical processibility and reactivity.     

Materials science of carbon nanotubes: fabrication,integration, and properties of macroscopic structures of carbon nanotubes

In this Account, we summarize some of our recent studies on the materials properties of the carbon nanotubes (CNTs). The focus is on single-wall carbon nanotubes (SWNTs). We describe experiments on synthesis of SWNTs with controlled molecular structures and assembly of functional macroscopic structures. In addition, we present results on the electron field emission properties of macroscopic CNT cathodes.     

Voltammetric quantum charging capacitance behaviour of functionalised carbon nanotubes in solution

The electronic properties of functionalised single and multi wall carbon nanotubes (SWNTs and MWNTs, respectively) were investigated in depth by conventional electrochemical techniques in solution. Significant differences were observed between these two classes of carbon nanotubes. In fact, despite functionalisation strongly modified the electronic properties of carbon nanotubes, the enrichment of the density of states of MWNTs with respect to SWNTs, due to larger tube diameters, is still appreciated. In addition, the redox behaviour of the MWNTs shows a series of discrete single electron-transfer events (a coulomb staircase), which are undetected in the case of SWNTs. This quantised capacitance charging phenomena is analogous to that observed for other nanosystems which is attributed to the discrete charging of conducting core, coated with a dielectric film in an electrolyte solution.     

Effect of polymer chain length on the solubility of polystyrene grafted single‐walled carbon nanotubes in tetrahydrofuran

BACKGROUND: While carbon nanotubes are highly interesting materials for a variety of applications, their inherent insolubility limits widespread applications and solution‐phase processing. It is known that chemical functionalization can overcome this insolubility problem, and covalent grafting of polymers to the nanotube surface has been shown to be effective. In this study, the effect of polymer molecular weight on the solubility of polymer–nanotube conjugates was investigated. RESULTS: A series of nitroxide‐capped polystyrene polymers ranging in molecular weight from 2900 to 105 000 g mol^?1 were grafted to single‐walled carbon nanotubes (SWNTs). The resulting polystyrene–SWNT conjugates exhibited different degrees of solubility in tetrahydrofuran. Subsequent thermogravimetric and UV‐visible spectroscopy analyses indicated that carbon nanotube solubility reached a maximum when a polymer sample with a weight‐average molecular weight of 10 000 g mol^?1 was used. Higher and lower molecular weights resulted in reduced solubilities. CONCLUSION: Polymer chains of intermediate length maximize SWNT solubility, while lengths that are too low or too high seem to diminish the ability of the polymer–SWNT conjugates to remain in solution. Copyright © 2008 Society of Chemical Industry     

Effect of hydrocarbons precursors on the formation of carbon nanotubes in chemical vapor deposition

High-temperature decomposition of hydrocarbons may lead to the formation of carbon deposits. However in our present studies, we found that the morphology of carbon deposits over MgO supported Fe catalyst during chemical vapor deposition (CVD) process was closely related to the thermodynamic properties and chemical structures of hydrocarbon precursors. Six kinds of hydrocarbons (methane, hexane, cyclohexane, benzene, naphthalene and anthracene) were used as carbon precursors in this study. Methane which has a pretty simple composition and is more chemically stable was favorable for the formation of high-purity single walled carbon nanotubes (SWNTs). For high-molecular weight hydrocarbons, it was found that the chemical structures rather than thermodynamic properties of carbon precursors would play an important role in nanotube formation. Specifically, the CVD processes of aromatic molecules such as benzene, naphthalene and anthracene inclined to the growth of SWNTs. While the cases of aliphatic and cyclic hydrocarbon molecules seemed a little more complicated. Based on different pyrolytic behaviors of carbon precursors and formation mechanism of SWNTs and multi-walled carbon nanotubes (MWNTs), a possible explanation of the difference in CVD products was also proposed.     

Stabilisation of SWNTs by alkyl-sulfate chitosan derivatives of different molecular weight: towards the preparation of hybrids with anticoagulant properties

We have previously demonstrated that chitosan derivative N-octyl-O-sulfate chitosan (NOSC), which presents important pharmacological properties, can suspend single walled carbon nanotubes (SWNTs) up to 20 times more effectively than other chitosan derivatives in an aqueous environment. In an attempt to further investigate the impact of different molecular weights of chitosan to the solubilization and anticoagulant properties of these hybrids an array of NOSC derivatives varying their molecular weight (low, medium and high respectively) was synthesised and characterised by means of FT-IR spectroscopy, NMR spectroscopy and thermal gravimetric analysis (TGA). Microwave and nitric acid purified SWNTs, characterised by FT-IR spectroscopy, transmission electron microscopy (TEM) and Raman spectroscopy, were colloidally stabilised by these polymers and their anticoagulant activity was assessed. The results revealed that the low molecular weight NOSC coated SWNTs exhibit the highest activity when 0.5 mg mL(-1) NOSC solutions are used, activity which is similar to that of the free polymer. Preliminary studies by exposure of these hybrids to Brine Shrimp (Artemia) cysts revealed no effect on the viability of sub-adult Artemia. Our findings suggest the possibility of tailoring these nanomaterials to bear the required properties for application as biocompatible building blocks for nanodevices including biosensors and biomaterials.     

Synthesis and characterization of carbon nanotube/metal nanoparticle composites well dispersed in organic media

Single-walled carbon nanotubes (SWNTs) were decorated with Au, Pt and Rh nanoparticles coated with oleylamine. This modification improves substantially the colloidal dispersion of SWNTs in chloroform and other organic solvents affording stable organosols for realistic applications. The metal nanoparticles are well dispersed on the external surfaces of carbon nanotubes and among else could be used as catalyst in homogeneous or heterogeneous reactions.