Spectroscopic evidence for the bulk polymerization of N-vinyl carbazole in the presence of single-walled carbon nanotubes

The bulk polymerization reaction of N-vinylcarbazole (VK) at 70 °C in the presence of single-walled carbon nanotubes (SWNTs) leads to a new composite, whose optical properties were studied by photoluminescence (PL), surface enhanced Raman scattering (SERS) and Fourier transform infrared (FTIR) spectroscopies. A dramatic reduction of the poly(N-vinylcarbazole) (PVK) PL efficiency and a change in the vibrational structure of the PL spectrum of this polymer were observed by adding SWNTs to the synthesis mixture. Steric hindrance effects were evidenced both in SERS spectra of the VK when it interacts mechanico-chemically with SWNTs and in FTIR spectra of the un-doped PVK/SWNTs' composites. Cyclic voltammetry was used to demonstrate the doping process of PVK in PVK/SWNTs' composite.     

A new method for the preparation of stable carbon nanotube organogels

Single-walled carbon nanotubes (SWNTs) functionalized by ferrocene-grafted poly(p-phenyleneethynylene)s can gelate common organic solvents such as chloroform to form a freestanding carbon nanotube organogel that cannot be redispersed in any organic solvents, indicating the robustness of 3D nanotube network. The drying of SWNT gel on silicon wafer in the air gives a highly convoluted film that is composed of numerous bumps and grooves at multiple length scales. In addition, we report a method for the preparation of an insoluble, homogeneous, electroactive SWNT film from a fresh SWNT solution, which may find applications in nanotube coatings and thin films that require both uniformity and solvent-resistance.     

Fabrication of pH‐ or temperature‐responsive single wall carbon nanotubes via a graft from photopolymerization

An ultraviolet light initiated “graft from” polymerization method to fabricate polymer‐functionalized single wall carbon nanotubes (SWNTs) with pendant pH‐ and temperature‐responsive polymer chains is utilized. The attached polymer chains, formed from methacrylic acid and poly(ethylene glycol) methyl ether methacrylate monomers, are well established for its pH‐responsive swelling/deswelling behavior. This special property was utilized here to control the aqueous dispersibility of the carbon nanotubes. Furthermore, poly(N‐isopropylacrylamide), a temperature‐responsive polymer, was utilized in the fabrication of SWNTs whose dispersibility was dependent on solution temperature. The morphology of the polymer‐functionalized carbon nanotubes was characterized by scanning electron microscopy (SEM) before and after functionalization. Environmental SEM was used to further characterize the morphology of the functionalized SWNTs. In addition, covalent bonding of the polymer to the carbon nanotube surface was established using Raman and Fourier transform infrared spectroscopic techniques. The physical and chemical properties of the functionalized nanotubes were further characterized by energy‐dispersive X‐ray spectroscopy, thermogravimetric analysis, and differential scanning calorimetry. © 2011 American Institute of Chemical Engineers AIChE J, 58: 2980–2986, 2012     

Fluorination of single-wall carbon nanotubes and subsequent derivatization reactions

This Account focuses on the most recent and systematic efforts in the area of functionalization chemistry of the single-wall carbon nanotubes (SWNTs) which utilizes direct fluorination for the preparation of "fluoronanonotubes" and their subsequent derivatization. The results obtained prove that the addition of fluorine drastically enhances the reactivity of the nanotube side walls. The use of this strategy as a versatile tool for preparation and manipulation of SWNTs with variable side-wall functionalities has been demonstrated. The functionalized SWNTs have shown an improved solubility in selected solvents and significantly altered electrical, mechanical, and optical properties. An overview of new synthetic methods for preparation and a discussion of characterization data for the functionalized SWNTs are provided.     

Selective interaction of single-walled carbon nanotubes with conducting dendrimer

We report on the transport properties of a system composed of single-wall carbon nanotubes (SWNTs) noncovalently linked to a new electrically conducting dendrimer poly(amidoamine) modified with a substituted naphthalenediimide (PAMAMC). SEM images show how the adsorption of the conducting dendrimer on SWNTs leads to the unroping of the bundles. The adsorption of PAMAMC molecules on SWNTs has been also investigated by electrical transport measurements. The electrical conductance of SWNTs drastically increases upon adsorption of conducting dendrimer. UV–Vis spectroscopy indicates that there was a modification in the electronic structure of the dendrimer as consequence of nanotube introduction while the appearance of new bands on the Raman spectra may suggest that metallic nanotubes are selectively functionalized.     

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.     

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.     

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.     

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.     

New carbon nanotube–conducting polymer composite electrodes for drug delivery applications

A novel drug delivery system (DDS) based on a carbon nanotube (CNT)–poly(3,4‐ethylenedioxythiophene) (PEDOT) composite was constructed via a layering method. Single‐walled CNTs (SWNTs) were immobilized on a gold electrode using a layer‐by‐layer technique. In particular, cysteamine (Cys) was firstly bonded to the gold surface through the strong S? Au association and SWNTs were subsequently linked onto the Cys layer through condensation reaction of ? NH₂ and carboxyl groups by 1‐ethyl‐3‐(3‐dimethylaminopropyl)carbodiimide/N‐hydroxysuccinimide coupling. X‐ray photoelectron spectroscopy and Raman spectroscopy demonstrate that this is a facile route for immobilizing CNTs on gold electrodes. Finally PEDOT was electropolymerized on the SWNT‐functionalized electrode to make a SWNT–PEDOT composite, and the modified electrode was applied as a DDS. Dexamethasone, as a model drug, was incorporated into PEDOT in the electropolymerization. Investigations of the electrochemical properties of SWNT–PEDOT demonstrate that SWNTs greatly improve the conductivity and increase the charge capacity of PEDOT. The composite exhibits a petal‐like surface structure, 20–30 nm thick and 100–200 nm wide. Compared to a DDS based on pure PEDOT synthesized under the same conditions, SWNT–PEDOT has the merits of higher drug release rate and larger release amount. The average mass release for every five voltammetry cycles increases from 1.4126 to 1.8864 mg cm^?2. Copyright © 2011 Society of Chemical Industry     

Raman spectroscopy and nitrogen vapour adsorption for the study of structural changes during purification of single-wall carbon nanotubes

Raman spectroscopy and nitrogen adsorption measurements were combined to study the surface features of semi-conducting and metallic single-wall nanotubes (SWNTs). The nanotubes were treated chemically and with heat under moderate conditions that more than doubled the mesopore volume of the tested samples, which consistently led to a significant rise in the total surface area of up to 1550 m²/g. The large increase in the number of micropores of less than 1 nm in diameter was associated with the loosening of nanotube bundles as well as the creation of structural flaws on the surface of individual SWNTs due to chemical treatment. Micropores in the 1.0-1.8 nm range were associated with the holes created on the surface of individual tubes. Heating at 1000 °C was shown to restore nanotube diameter to their initial pre-chemical treatment levels with the change in the chirality of SWNTs and diminish the porosity by closing small holes. It was assumed that the intermediate frequency range (500-1100 cm⁻¹) was associated with the degree of imperfection of HiPco SWNTs crystalline structures, and therefore provided information about the degree of tube surface damage due to the presence of functional groups. A hypothesis explaining the transformation of SWNT porous structure during heat treatment is proposed.     

Microwave-assisted poly(glycidyl methacrylate)-functionalized multiwall carbon nanotubes with a ‘tendrillar’ nanofibrous polyaniline wrapping and their interaction at bio-interface

Multiwall carbon nanotubes (MWCNTs) were activated by microwave irradiation and covalently functionalized with poly(glycidyl methacrylate) (PGMA) through free radical polymerization using ‘fishing process’ when the propagating polymer radicals were attached onto the graphitic surface of the nanotube. The PGMA-functionalized MWCNTs were then used as a precursor to non-covalently wrap polyaniline (PAni) nanofiber onto them. The functionalized nanotubes exhibited stable dispersion up to 180 days in tetrahydrofuran, dimethyl formamide and dimethyl sulfoxide. Fourier transform infrared analyses indicated the attachment of the epoxide and benzenoid–quinoid functional moieties onto the nanotube surface. The PGMA coating on the nanotube and surrounding PAni nanofiber on the MWCNT scaffold were confirmed by transmission electron microscopy. The Raman spectroscopy confirmed the phonon-assisted modification of the nanotube. The differential action of the pristine and functionalized MWCNTs against an opportunistic bacterium (Escherichia coli ) and its plasmid deoxyribonucleic acid was also investigated. Pristine nanotubes exhibited bacterial inhibitory action and no condensation with the pET-32α(+) plasmid. On the other hand, the anti-bacterial PAni nanofiber and functionalized nanotubes showed complex formation with the bacterial plasmid.     

Phenomenological characterization of fabrication of aligned pristine‐SWNT and COOH‐SWNT nanocomposites via dielectrophoresis under AC electric field

Dielectrophoresis under the application of AC electric fields is one of the primary fabrication techniques (DEPFT) for obtaining aligned carbon nanotube (CNT)–polymer nanocomposites, and is used here to generate data sets from which DEPFT fabrication models in terms of CNT dispersion and orientation distribution can be developed. While the general understanding of how CNTs form aligned filaments under the influence of dielectrophoretic forces and moments is well established, detailed multi‐CNT‐filament formation predictions of microstructure evolution from a random dispersion into a more ordered structure remain intractable. As such, effort here is focused towards the development of phenomenological fabrication models for controlling local CNT dispersion and orientation as a function of applied electric field magnitude, frequency, and exposure time. In this study, 0.03 wt% single‐wall nanotubes (SWNTs) and acid treated functionalized SWNTs (COOH‐SWNTs) were dispersed in a photopolymerizable monomer blend (urethane dimethacrylate (UDMA) and 1,6‐hexanediol dimethacrylate (HDDMA)). Ultrasonication techniques were used to obtain the two different acrylate solutions i.e., 0.03% SWNTs/ UDMA/ HDDMA(9/1) solution and a 0.03% COOH‐SWNTs/UDMA/HDDMA(9/1) solution, consisting of randomly oriented, well dispersed SWNTs. Pristine SWNTs and acid treated SWNTs solutions were then subjected to controlled AC electric fields in order to explore the formation of aligned SWNT‐filaments. To assess key morphological features of the as‐produced SWNT‐acrylate and SWNT‐COOH‐acrylate nanocomposite samples, such as SWNT distribution and filament thicknesses, transmission optical microscopy has been used to observe the SWNT alignment and filament formation obtained by digitally mapping individual overlapping images. The acquisition of a large field of view with high magnification allows statistically meaningful distribution functions for morphological features to be constructed. Measurements of the as‐produced nanocomposite electrical properties in the SWNT alignment direction and transverse to it were used as a macroscale measure to confirm alignment and contiguity of the SWNT‐filament structure, with polarized Raman spectroscopy used to assess the degree of SWNT alignment at the local microscale level. It is observed that a combination of exposure time to AC electric field, and its frequency, is the key driver of filament thickness and spacing and that in general, the COOH‐SWNTs align to a greater extent than the pristine SWNTs, though they do not form filaments that are as thick and contiguous for the exposure times studied. POLYM. COMPOS., 36:1266–1279, 2015. © 2014 Society of Plastics Engineers     

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.     

Photoluminescence intensity of single-wall carbon nanotubes

The photoluminescence (PL) intensity of a single-wall carbon nanotube (SWNT) is calculated for each (n, m) by multiplying the photon-absorption, relaxation and photon-emission matrix elements. The intensity depends on chirality and “type I vs type II” for smaller diameter semiconducting SWNTs (less than 1 nm). By comparing the calculated results with the experimental PL intensity of SWNTs prepared by chemical vapor deposition at different temperatures, we find that the abundance of (n, m) nanotubes with smaller diameters should exhibit a strong chirality dependence, which may be related to the stability of their caps.     

PET-SWNT Nanocomposite Fibers through Melt Spinning

Poly(ethylene terephthalate)-single-walled carbon nanotube (PET-SWNT) nanocomposite fibers have been prepared through melt spinning and drawing. While acting as effective nucleating agents for PET melt crystallization, SWNTs also provide significant reinforcement to PET fibers. For example, the tenacity and initial modulus of the composite fiber with 1 wt% SWNTs were, respectively, 1.8 times and 3.2 times higher as compared to those of the pristine PET fiber prepared under identical conditions. When the draw ratio was increased, tenacity and modulus of the fibers increased, indicating that drawing induced orientation of the polymer molecules and SWNTs. Thermal stability of the fibers was not significantly affected by the presence of SWNTs at low concentrations.     

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.     

Fabrication of carbon nanotube field effect transistors by AC dielectrophoresis method

Single wall carbon nanotubes (SWNTs) suspended in isopropyl alcohol have been placed between two electrodes by AC dielectrophoresis method. The number of SWNTs bridging the two electrodes is controlled by SWNT concentration of the suspension and deposition time. Through selectively burning off the metallic SWNTs by current induced oxidation, the back-gate carbon nanotube field effect transistors (CNTFETs) with a channel current on-off ratio of up to 7 × 10⁵ have been successfully fabricated. The success rate of the CNTFETs in 20 samples is 60%. These results suggest that AC dielectrophoresis placement method is an efficient technique to fabricate CNTFETs with some flexibilities of controlling CNT reconnection, length and orientation.     

A method for creating reliable and low-resistance contacts between carbon nanotubes and microelectrodes

An ultrasonic bonding technique has been developed for bonding single wall carbon nanotubes (SWNTs) onto metal microelectrodes. The bonding was formed by pressing SWNTs against the electrodes with a vibrating press at an ultrasonic frequency. With this technology, low-resistance contacts are achieved between both metallic and semiconducting SWNTs and electrodes. After bonding, the effective Schottky barrier height between semiconducting SWNT and Ti electrode is as low as ∼6.6 meV in the ON-state and the barrier width is ∼0.9 nm at V_g = 0. The performance of carbon nanotube field-effect transistors (FETs) fabricated by this ultrasonic bonding technique is also significantly improved, with a transconductance as high as 3.4 μS for solid-state back-gate individual nanotube FETs.     

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.