Regulation of the near-IR spectral properties of individually dissolved single-walled carbon nanotubes in aqueous solutions of dsDNA

Two different single-walled carbon nanotubes (SWNTs), the so-called HiPco and CoMoCAT, have been individually dissolved in aqueous solutions of double-stranded DNA (dsDNA). Atomic force microscopy (AFM) revealed the fine structures of the dsDNA-wrapped SWNTs. The near-IR absorption and photoluminescence (PL) spectra of aqueous solutions of dsDNA-wrapped SWNTs were recorded and, in pure water, we observed only a single two-dimensional PL spot from (6,5) SWNTs for both HiPco and CoMoCAT. In sharp contrast, when Tris-EDTA (TE) buffer was used in place of pure water, the PL-mapping images of the solutions showed chirality indices of (6,5), (7,5), (7,6), (8,4), (9,4), and (10,2) for HiPco-SWNTs, and (6,5) and (7,5) for CoMoCAT-SWNTs. The first semiconducting bands in the near-IR absorption spectra of solutions of dsDNA-wrapped SWNTs are different. To explain the observed differences in the near-IR absorption and PL behavior we conducted several experiments and found that the near-IR optical properties of the SWNTs can be modulated by changing the pH of the solutions. The pH break-points for near-IR absorption bleaching and PL quenching are different and the phenomena are explained by differences in the numbers of holes generated on the SWNTs. These findings are important from both fundamental and applied viewpoints.     

Metal-enhanced fluorescence of carbon nanotubes

The photoluminescence (PL) quantum yield of single-walled carbon nanotubes (SWNTs) is relatively low, with various quenching effects by metallic species reported in the literature. Here, we report the first case of metal enhanced fluorescence (MEF) of surfactant-coated carbon nanotubes on nanostructured gold substrates. The photoluminescence quantum yield of SWNTs is observed to be enhanced more than 10-fold. The dependence of fluorescence enhancement on metal-nanotube distance and on the surface plasmon resonance (SPR) of the gold substrate for various SWNT chiralities is measured to reveal the mechanism of enhancement. Surfactant-coated SWNTs in direct contact with metal exhibit strong MEF without quenching, suggesting a small quenching distance for SWNTs on the order of the van der Waals distance, beyond which the intrinsically fast nonradiative decay rate in nanotubes is little enhanced by metal. The metal enhanced fluorescence of SWNTs is attributed to radiative lifetime shortening through resonance coupling of SWNT emission to the reradiating dipolar plasmonic modes in the metal.     

Photoluminescence and population analysis of single-walled carbon nanotubes produced by CVD and pulsed-laser vaporization methods

Band gap photoluminescence (PL) behaviors of single-walled carbon nanotubes (SWNTs) grown by the methods of chemical vapor deposition and pulsed-laser vaporization are investigated over the wide diameter range (≈0.8–1.4 nm). The peak intensity of the PL signals strongly depends on chirality and the ‘(2n + m) family type’ of SWNTs. Based on the PL results, a population analysis of these SWNTs is conducted by combining the calculated PL yields for each (n, m) tube. The results are directly compared with the histograms of diameter distributions estimated by the transmission electron microscope (TEM) observations to check the validity of the analysis.     

Length-dependent optical effects in single walled carbon nanotubes

Recently, Heller et al. reported length-dependent effects on the relative photoluminescence (PL) quantum yield of single walled carbon nanotubes (SWNTs) [Heller et al J. Am. Chem. Soc. 2004, 126, 14567-14573]. We propose a simple model involving thermal diffusion of excitons along the nanotube axis and quenching at the ends, to explain the observed trend in their data. By fitting to our model, we extract a diffusion coefficient of 6 cm(2)/s for excitons in SWNTs. Assuming a mono exponential decay of exciton PL, we also predict that effective length-dependent PL lifetimes for these excitons lie in the range of 1-27 ps. Experimental observations are shown to be consistent with stochastic rather than wavepacket-like exciton migration, which is in agreement with ultrafast excitonic dephasing. Edge effects seem to limit the use of short SWNTs in imaging and optical sensing applications.     

Distribution patterns and controllable transport of water inside and outside charged single-walled carbon nanotubes

The density distribution patterns of water inside and outside neutral and charged single-walled carbon nanotubes (SWNTs) soaked in water have been studied using molecular dynamics simulations based on TIP3P potential and Lennard-Jones parameters of CHARMM force field, in conjunction with ab initio calculations to provide the electron density distributions of the systems. Water molecules show different electropism near positively and negatively charged SWNTs. Different density distribution patterns of water, depending on the diameter and chirality of the SWNTs, are observed inside and outside the tube wall. These special distribution patterns formed can be explained in terms of the van der Waals and electrostatic interactions between the water molecules and the carbon atoms on the hexagonal network of carbon nanotubes. The electric field produced by the highly charged SWNTs leads to high filling speed of water molecules, while it prevents them from flowing out of the nanotube. Water molecules enter the neutral SWNTs slowly and can flow out of the nanotube in a fluctuating manner. It indicates that by adjusting the electric charge on the SWNTs, one can control the adsorption and transport behavior of polar molecules in SWNTs to be used as stable storage medium with template effect or transport channels. The transport rate can be tailored by changing the charge on the SWNTs.     

Optical Properties of Single-Walled Carbon Nanotubes Separated in a Density Gradient; Length, Bundling, and Aromatic Stacking Effects

Single-walled carbon nanotubes (SWNTs) are promising materials for in vitro and in vivo biological applications due to their high surface area and inherent near infrared photoluminescence and Raman scattering properties. Here, we use density gradient centrifugation to separate SWNTs by length and degree of bundling. Following separation, we observe a peak in photoluminescence quantum yield (PL QY) and Raman scattering intensity where SWNT length is maximized and bundling is minimized. Individualized SWNTs are found to exhibit high PL QY and high resonance-enhanced Raman scattering intensity. Fractions containing long, individual SWNTs exhibit the highest PL QY and Raman scattering intensities, compared to fractions containing single, short SWNTs or SWNT bundles. Intensity gains of approximately ~1.7 and 4-fold, respectively, are obtained compared with the starting material. Spectroscopic analysis reveals that SWNT fractions at higher displacement contain increasing proportions of SWNT bundles, which causes reduced optical transition energies and broadening of absorption features in the UV-Vis-NIR spectra, and reduced PL QY and Raman scattering intensity. Finally, we adsorb small aromatic species on "bright," individualized SWNT sidewalls and compare the resulting absorption, PL and Raman scattering effects to that of SWNT bundles. We observe similar effects in both cases, suggesting aromatic stacking affects the optical properties of SWNTs in an analogous way to SWNT bundles, likely due to electronic structure perturbations, charge transfer, and dielectric screening effects, resulting in reduction of the excitonic optical transition energies and exciton lifetimes.     

Thermal Stability of Oxidized Single‐Walled Carbon Nanotubes: Competitive Elimination and Decomposition Reaction Depending on the Degree of Functionalization

The thermal stability of oxidized single‐walled carbon nanotubes (SWNTs) with various degrees of oxidation was investigated. The oxidized SWNTs exhibited lower absorption and radial breathing mode (RBM) peaks and a higher intensity ratio of the D band to the G band (D/G) in their absorption and Raman spectra than those of the pristine SWNTs. After the thermal treatment, the D/G ratio of the oxidized SWNTs almost recovered its original intensity, regardless of the degree of oxidation. The absorption, photoluminescence (PL), and RBM peaks could not recover their original intensities when the oxidation degree was high. The results indicate that the elimination and decomposition reactions proceeded competitively depending on the degree of oxidation. In addition, a new PL peak was observed in the near‐infrared region, and the PL peak intensity increased with the subsequent thermal treatment. The theoretical calculations provided an insight into the possible pathways for the decomposition of oxidized SWNTs, showing that the O₂ elimination and CO/CO₂ evolution proceed competitively during thermal treatment.     

Azafullerene encapsulated single-walled carbon nanotubes with n-type electrical transport property

Electrical transport properties of C60 and C59N encapsulated single-walled carbon nanotubes (SWNTs) are investigated by fabricating them as the channels of field effect transistor (FET) devices at room temperature. Their measurements indicate that C60@SWNTs exhibit the enhanced p-type characteristics compared with the case of pristine SWNTs, whereas C59N@SWNTs show the n-type behavior. The novel transport properties of peapods can be explained by the charge-transfer effect, which can modify the electronic structure of SWNTs.     

Surfactant-free nanofluids containing double- and single-walled carbon nanotubes functionalized by a wet-mechanochemical reaction

Single-walled carbon nanotubes (SWNTs) and double-walled carbon nanotubes (DWNTs) have been functionalized through the wet-mechanochemical reaction method. Results from the infrared spectrum and zeta potential measurements show that the hydroxyl groups have been introduced onto the treated SWNT and DWNT surfaces. Transmission electron microscope observations revealed that the SWNTs and DWNTs were cut short after being milled. SWNTs and DWNTs with optimized aspect ratio can be obtained by adjusting the ball milling parameters. Thermal conductivity enhancement of water-based nanofluids containing treated carbon nanotubes (CNTs) shows augmentation with the increase of temperature mainly due to the effects of an ordering liquid layer forming around the chemical surfaces of CNTs. Moreover, the thicker interfacial layer of water molecules on the surfaces of CNTs with smaller diameter, such as SWNTs, is in favor of greater thermal conductivity enhancement compared with the thinner one on the surfaces of DWNTs or MWNTs with larger diameter.     

Insights in the plasma-assisted growth of carbon nanotubes through atomic scale simulations: effect of electric field

Carbon nanotubes (CNTs) are nowadays routinely grown in a thermal CVD setup. State-of-the-art plasma-enhanced CVD (PECVD) growth, however, offers advantages over thermal CVD. A lower growth temperature and the growth of aligned freestanding single-walled CNTs (SWNTs) makes the technique very attractive. The atomic scale growth mechanisms of PECVD CNT growth, however, remain currently entirely unexplored. In this contribution, we employed molecular dynamics simulations to focus on the effect of applying an electric field on the SWNT growth process, as one of the effects coming into play in PECVD. Using sufficiently strong fields results in (a) alignment of the growing SWNTs, (b) a better ordering of the carbon network, and (c) a higher growth rate relative to thermal growth rate. We suggest that these effects are due to the small charge transfer occurring in the Ni/C system. These simulations constitute the first study of PECVD growth of SWNTs on the atomic level.     

Identification of the structures of superlong oriented single-walled carbon nanotube arrays by electrodeposition of metal and Raman spectroscopy

In this Communication, we have demonstrated a facile and effective approach to identify the structure of the superlong well-aligned single-walled carbon nanotubes (SWNTs) by the combination of electrodeposition of metal (Ag) with Raman spectroscopy. The suitable density and the visibility of the Ag-deposited long oriented nanotubes make it possible to acquire Raman spectra from isolated individual nanotubes very easily. The results reveal that the well-oriented SWNT arrays on SiO2/Si wafer fabricated by EtOH chemical vapor deposition using Fe/Mo nanoparticles as catalyst exhibit a low percentage of metallic SWNTs (5%). Among other SWNTs about 62.3% are semiconducting SWNTs, and a small amount of nanotubes are quasimetallic. About 32% are a so-called quasi-insulator, which is caused inevitably by the defects during growth. Furthermore, the structural uniformity of the long SWNTs can be also evaluated by the deposition of Ag along the length and Raman spectroscopy. This method also provides an approach to deposit other metals on long SWNTs, which could have various potential applications such as for use as sensors, etc. More importantly, this facile method can be applied to long SWNT arrays fabricated from other different catalytic systems so that the relationship between the growth conditions and the structures of SWNTs are expected to be ruled out.     

Growth of high-density parallel arrays of long single-walled carbon nanotubes on quartz substrates

Herein we report a CVD approach to prepare high-density and perfectly aligned arrays of long SWNTs on stable temperature (ST)-cut quartz substrates using copper as catalyst and ethanol as carbon source. Compared with earlier reports, we have demonstrated that the aligned nanotube arrays can be grown on ST quartz substrate without the need of thermal annealing. The density can reach >50 nanotubes per micron and the length can be a few millimeters. Additionally, we have obtained direct proof on the "tip-growth" mechanism for the aligned nanotubes and important evidence that explained the termination of the growth.     

Simultaneous determination of nitrophenol isomers at the single-wall carbon nanotube compound conducting polymer film modified electrode

One of the challenging areas of electrochemistry and electroanalytical chemistry is the simultaneous determination of isomers at the same electrode. Con- ventional electrode only possesses a single function of electron transfer; therefore, it is difficult…     

Probing the Selectivity of Azomethine Imine Cycloaddition to Single‐Walled Carbon Nanotubes by Resonance Raman Spectroscopy

Selective covalent surface modification of single‐walled carbon nanotubes (SWNTs) is of great importance to various carbon nanotube‐based applications as it might offer an alternative method for enriching metallic and semiconducting nanotubes. Herein, we report on the surface modification of SWNTs through 1,3‐dipolar cycloaddition of 3‐phenyl‐phthalazinium‐1‐olate, which is a stable and reactive azomethine imine. For this reaction, microwave heating was found to be more efficient than conventional and solvent‐free heating. The sensitivity of cycloaddition to the molecular structure of SWNTs was probed using resonance Raman spectroscopy with three different laser excitations. Based on the obtained results, azomethine imine addition to the surface of nanotubes is selective for metallic and large‐diameter semiconducting SWNTs. Thermogravimetric analysis coupled with mass spectrometry showed that fragments released at high temperatures corresponded to the phenylphthalazine group, thus confirming the covalent surface functionalization. Modified SWNTs were further characterized by X‐ray photoelectron and UV/Vis‐NIR spectroscopies.     

Coronene Encapsulation in Single‐Walled Carbon Nanotubes: Stacked Columns,Peapods, and Nanoribbons

Encapsulation of coronene inside single‐walled carbon nanotubes (SWNTs) was studied under various conditions. Under high vacuum, two main types of molecular encapsulation were observed by using transmission electron microscopy: coronene dimers and molecular stacking columns perpendicular or tilted (45–60°) with regard to the axis of the SWNTs. A relatively small number of short nanoribbons or polymerized coronene molecular chains were observed. However, experiments performed under an argon atmosphere (0.17 MPa) revealed reactions between the coronene molecules and the formation of hydrogen‐terminated graphene nanoribbons. It was also observed that the morphology of the encapsulated products depend on the diameter of the SWNTs. The experimental results are explained by using density functional theory calculations through the energies of the coronene molecules inside the SWNTs, which depend on the orientation of the molecules and the diameter of the tubes.     

Ordered DNA wrapping switches on luminescence in single-walled nanotube dispersions

An extensive study of the time dependence of DNA wrapping in single-walled nanotube (SWNT) dispersions has been carried out, revealing a number of unusual phenomena. SWNTs were dispersed in water with salmon testes DNA and monitored over a three-month period. Between 20 and 50 days after the sample was first prepared, the SWNT photoluminescence (PL) intensity was observed to increase by a factor of 50. This increase was accompanied by a considerable sharpening of the van Hove absorption peaks. High-resolution transmission electron microscopy (HRTEM) images showed the progressive formation of a coating of DNA on the walls of the nanotubes over the three-month period. HRTEM and circular dichroism spectroscopy studies showed that the improvement in both the NIR PL intensity and the van Hove absorption peaks coincided with the completion of a monolayer coating of DNA on the SWNT walls. HRTEM images clearly showed the DNA wrapping helically around the SWNTs in a surprisingly ordered fashion. We suggest that the initial quenching of NIR photoluminescence and broadening of absorption peaks is related to the presence of protonated surface oxides on the nanotubes. The presence of an ordered DNA coating on the nanotube walls mediates both deprotonation and removal of the surface oxides. An extensive DNA coating is required to substantially restore the photoluminescence, and thus, the luminescence switch-on and subsequent saturation indicate the completion of the DNA-wrapping process. The temperature dependence of the PL switch-on, and thus of the wrapping process, was investigated by measuring as functions of temperature both the time before PL switch-on and the time required for the PL intensity to saturate. This allowed the calculation of the activation energies for both the process preceding PL switch-on and the process limiting the rise of PL intensity, which were found to be 31 and 41 kJ mol (-1), respectively. The associated entropies of activation were -263 and -225 J mol (-1) K (-1), respectively. These negative activation entropies suggest that the rate-limiting step is characterized by a change in the system from a less-ordered to a more-ordered state, consistent with the formation of an ordered DNA coating.     

Rational chemical strategies for carbon nanotube functionalization

Whereas the chemistry of fullerenes is well-established, the chemistry of single-walled carbon nanotubes (SWNTs) is a relatively unexplored field of research. Investigations into the bonding of moieties onto SWNTs are important because they provide fundamental structural insight into how nanoscale interactions occur. Hence, understanding SWNT chemistry becomes critical to rational, predictive manipulation of their properties. Among the strategies discussed include molecular metal complexation with SWNTs to control site-selective chemistry in these systems. In particular, work has been performed with Vaska's and Wilkinson's complexes to create functionalized adducts. Functionalization should offer a relatively simple means of tube solubilization and bundle exfoliation, and also allows for tubes to be utilized as recoverable catalyst supports. Solubilization of oxidized SWNTs has also been achieved through derivatization by using a functionalized organic crown ether. The resultant adduct yielded concentrations of dissolved nanotubes on the order of 1 g L(-1) in water and at elevated concentrations in a range of organic solvents, traditionally poor for SWNT manipulation. To further demonstrate chemical processability of SWNTs, we have subjected them to ozonolysis, followed by treatment with various independent reagents, to rationally generate a higher proportion of oxygenated functional groups on the nanotube surface. This protocol has been found to purify nanotubes. More importantly, the reaction sequence has been found to ozonize the sidewalls of these nanotubes. Finally, SWNTs have also been chemically modified with quantum dots and oxide nanocrystals. A composite heterostructure consisting of nanotubes joined to nanocrystals offers a unique opportunity to obtain desired physical, electronic, and chemical properties by adjusting synthetic conditions to tailor the size and structure of the individual sub-components, with implications for self-assembly.     

Structural transformation of fluorinated carbon nanotubes induced by in situ electron-beam irradiation

We have investigated the structural transformation of fluorinated singlewalled nanotubes (SWNTs) induced by electron-beam irradiation during the transmission electron microscope observations. Heavily fluorinated SWNT bundles were systematically transformed into multiwall-like nanotubes by releasing fluorine atoms during electron-beam irradiation and even broken into two pieces of the capped graphitic structures. Such structural transformations at relatively low kinetic energy (< or = 300 keV) could be explained by the local strains induced by fluorination, where C-C bonds that were fluorine-attached became 1.53 A, a single bond similar to that of a diamond, from our density functional calculations. We propose a possible concerted pathway for the structural transformation of fluorinated SWNTs induced by electron-beam irradiation based on the experimental observations.     

Pinning of single-walled carbon nanotubes by selective electrodeposition

A maskless method for the fabrication of electrical or mechanical contacts to the single-walled carbon nanotubes (SWNTs) by selective electrodeposition is reported. Both semiconducting SWNTs and metallic SWNTs can be pinned on prepatterned electrodes by the locally deposited metal, leaving the section of SWNTs between the electrodes clean. The distribution of deposited metal on the SWNTs is mainly determined by the covering power of the plating bath and the plating potential. This research provides a parallel method for the large-scale integration of SWNTs into electronic, optoelectronic, and sensing systems.     

Micelle-encapsulated carbon nanotubes: a route to nanotube composites

We report a general approach toward dispersing single-walled carbon nanotubes (SWNTs) in solvents and polymer materials, by encapsulating SWNTs within cross-linked micelles. Micelles made from polystyrene-block-poly(acrylic acid) (PS-b-PAA), an amphiphilic block copolymer, are first assembled around SWNTs by gradually adding H2O to a suspension of nanotubes in dimethylformamide. The hydrophilic, outer shells of these micelles are then chemically cross-linked with a difunctional linker molecule. Pure encapsulated SWNTs (e-SWNTs) can then be separated from empty cross-linked micelles by consecutive cycles of centrifugation and redispersion. Atomic force and transmission electron microscopies of the resulting nanostructures demonstrate that individual nanotubes (rather than bundles) have been completely encased in polymer shells whose thickness is slightly larger than that of empty micelles. e-SWNTs encapsulated in PS-b-PAA can be permanently redispersed in H2O, in organic solvents, and in both hydrophobic and hydrophilic polymer matrices with minimal sonication. Micelle encapsulation could improve the compositing of SWNTs in a wide variety of polymer materials for structural, electronic, and thermal applications.