Single-walled Carbon Nanotubes Material Research Articles

Studies of electronic distribution in potassium-doped mats of single-walled carbon nanotubes, double-walled carbon nanotubes, and peapods

In this work, we have performed a systematic study of the electrical transport properties (both the resistivity and the thermoelectric power) on bundles of single-walled carbon nanotubes (SWCNTs), double-walled carbon nanotubes (DWCNTs), and peapods (derived from the same batch of initial SWNTs) during in situ doping with potassium (donor). The charge transfer effects in K-doping are similar in SWNTs and DWNTs, but drastically different in C60 at SWNTs. An impurity level associated with the chain of C60 in the band gap of the tube with the Fermi level pinned near the top of the valence band, leading to the p-type behavior for the peapod. Significant charge transfer is found to occur only in the case of very low coverage of these nanostructures.

Characterization of single-walled carbon nanotube mats and their performance as electromechanical actuators

Carbon nanotube mats (buckypapers) were prepared from three commercial grades of single-walled carbon nanotubes (SWCNTs) and by two processing variants (i.e. filtration of centrifuged and uncentrifuged dispersions). Material properties such as Young’s modulus, tensile strength, electrical conductivity, electrical capacitance, specific surface area and morphology were investigated and put in relation to the in-plane actuation performance in an aqueous electrolyte (1 M NaCl). A dynamic mechanical analyzer was adapted for actuation strain measurements on the samples under various tensile prestress levels. High SWCNT purity and dispersibility were found to be crucial for preparing dense and strong cohesive mats with good actuation performance.

Purification of Single-Walled Carbon Nanotubes by a Highly Efficient and Nondestructive Approach

Single-walled carbon-nanotubes (SWCNTs) in their as-grown state contain a large quantity of impurities such as catalytic particles and encapsulating graphitic shells. Previous purification approaches usually depended upon severe oxidation. But such oxidation in either a gas phase or a liquid phase induces structural destruction and a big loss of the SWCNT material. In this study, a new method is reported that involves transformation of catalytic iron particles to iron oxide at a low temperature of 400 °C, removal of graphitic shells by reaction with the encapsulated iron oxide at a high temperature of 800–900 °C, and final elimination of iron oxide particles by HCl. Experimental results, based on transmission electron microscopy, Raman spectroscopy, thermogravimetric analysis, and near-infrared spectroscopy, show that catalytic particles and encapsulating graphitic shells can be removed. More importantly, this highly efficient purification induces little damage to the graphitic structure of SWCNTs. These ...

Mechanism Analysis of Interrupted Growth of Single-Walled Carbon Nanotube Arrays

We investigated the growth mechanism of layered single-walled carbon nanotube (SWNT) mats by a cutting method. Transmission electron microscope observations revealed that new SWNTs grown below first grown SWNTs also have caps at their tips. Raman spectroscopy suggests that the SWNTs in each layer have the same chirality distribution. This growth method might be a way to prove a factor of chirality selection of SWNTs.

Switching Behavior in π-Conjugated Molecules Bridging Nonmetallic Electrodes: A Density Functional Theory Study

We report a first-principles study to investigate the switching behavior in a nitro-oligo(phenylene ethylene) molecule bridged between a silicon slab and a single wall carbon nanotube (SWCNT) mat. Our results suggest that the switching in the device may be driven by conformational changes in the molecule upon the application of an electric field. In addition, we find that the nature of the contact at the interface of the SWCNT mat plays an important role in the switching.

Sonochemical Optimization of the Conductivity of Single Wall Carbon Nanotube Networks

Networks of single-wall carbon nanotubes (SWCNTs) are covalently functionalized with oxygen-containing groups. In lower concentration, these functional groups act as stable dopands improving the conductivity of the SWCNT material. In higher concentration however, their role as defects with a certain scattering potential becomes dominant, decreasing the conductivity of the CNT material. These effects lead to a trade-off in the number of functional groups which has been investigated using time-dependent sonochemistry.

Integrated, Electrically Contacted NAD(P)+‐Dependent Enzyme–Carbon Nanotube Electrodes for Biosensors and Biofuel Cell Applications

Integrated, electrically contacted beta-nicotinamide adenine dinucleotide- (NAD(+)) or beta-nicotinamide adenine dinucleotide phosphate- (NADP(+)) dependent enzyme electrodes were prepared on single-walled carbon nanotube (SWCNT) supports. The SWCNTs were functionalized with Nile Blue (1), and the cofactors NADP(+) and NAD(+) were linked to 1 through a phenyl boronic acid ligand. The affinity complexes of glucose dehydrogenase (GDH) with the NADP(+) cofactor or alcohol dehydrogenase (AlcDH) with the NAD(+) cofactor were crosslinked with glutaric dialdehyde and the biomolecule-functionalized SWCNT materials were deposited on glassy carbon electrodes. The integrated enzyme electrodes revealed bioelectrocatalytic activities, and they acted as amperometric electrodes for the analysis of glucose or ethanol. The bioelectrocatalytic response of the systems originated from the biocatalyzed oxidation of the respective substrates by the enzyme with the concomitant generation of NAD(P)H cofactors. The electrocatalytically mediated oxidation of NAD(P)H by 1 led to amperometric responses in the system. Similarly, an electrically contacted bilirubin oxidase (BOD)-SWCNT electrode was prepared by the deposition of BOD onto the SWCNTs and the subsequent crosslinking of the BOD units using glutaric dialdehyde. The BOD-SWCNT electrode revealed bioelectrocatalytic functions for the reduction of O(2) to H(2)O. The different electrically contacted SWCNT-based enzyme electrodes were used to construct biofuel cell elements. The electrically contacted GDH-SWCNT electrode was used as the anode for the oxidation of the glucose fuel in conjunction with the BOD-SWCNT electrode in the presence of O(2), which acted as an oxidizer in the system. The power output of the cell was 23 muW cm(-2). Similarly, the AlcDH-SWCNT electrode was used as the anode for the oxidation of ethanol, which was acting as the fuel, with the BOD-SWCNT electrode as the cathode for the reduction of O(2). The power output of the system was 48 microW cm(-2).

Influence of impurities on the x-ray photoelectron spectroscopy and Raman spectra of single-wall carbon nanotubes

The effect of impurities on the properties of single-wall carbon nanotubes (SWNTs) was investigated with multiple analytical techniques. Charge transfer is believed to occur between the impurities and the SWNTs as observed by combining the Raman scattering and x-ray photoelectron measurements. The impurity condition (type and level) was found to strongly affect the electronic and vibrational properties of the SWNT. The metal catalysts in the impurity usually behave as electron donors, which can downshift the graphitic (G) band as well as the radial breathing mode frequencies. The low temperature air oxidation of as-prepared SWNT material usually upshifts the radial breathing mode Raman peaks to higher frequencies.

Atomic size-limited intercalation into single wall carbon nanotubes

Intercalation of single wall carbon nanotubes (SWNTs) provides an important tool tomodify their electronic band structure. Using multiple excitation wavelengthRaman spectroscopy, we demonstrate that intercalation into SWNT interiors can belimited by intercalant size resulting in an unusual material comprising SWNTs withvarying charge density. In the particular case of iodine intercalation, larger SWNTswith iodine-filled interiors were found to carry significantly higher charge densityas compared to smaller empty ones. This difference was used to separate theintercalated SWNT material into fractions with homogeneous charge density.

FTIR and Thermogravimetric Analysis of Biotin-Functionalized Single-Walled Carbon Nanotubes

We report the covalent functionalization of single-walled carbon nanotubes (SWNTs) with biotin. The synthesized functionalized SWNT materials were fully characterized by FTIR and thermogravimetric analysis. These characterization techniques provided valuable information concerning the nature of the obtained SWNT materials as well as the efficiency of the employed synthetic route towards SWNT covalent functionalization with biotin.

Layer-by-layer deposition of ultra-thin films of carbon nanotubes

This paper reports about the characterisation of single-walled carbon nanotubes (SWNT) ultra-thin films fabricated through an automated layer-by-layer process. This consists of dipping the substrate alternately into a polycation namely, poly(diallyldimethylammonium) (PDDA), and an SWNT suspension. The morphology and roughness of samples containing varying numbers of PDDA/SWNT layers were studied, showing that the multi-layers are made of uniform nanotube networks with an average surface roughness between 2.3 and 7.6 nm. The average thickness of a single PDDA/SWNT layer is around 5 nm. The Raman spectra, using the 633 nm excitation line, show a significant change in the shape of radiant breathing mode (RBM) features and a blue-shift of the G-mode for PDDA/SWNT networks compared to the pristine SWNT material. This proves that PDDA is acting as an acceptor. The electrical conductance of the PDDA/SWNT networks is order of magnitude higher than that of equivalent networks fabricated without PDDA. Moreover, the temperature dependence of conductance differs from that measured in ultra-thin SWNT networks; the typical thermally activated electron transport mechanism changes towards low temperatures so that, close to zero temperature, a final value of conductance is observed. These results show that the use of a polyelectrolyte as adsorption promoter improves the quality of the film in terms of uniformity and density of nanotubes in the network.

High-Purity Diamagnetic Single-Wall Carbon Nanotube Buckypaper

We describe a novel purification process for single-wall carbon nanotube (SWNT) materials that removes non-nanotube carbon and reduces ferromagnetic impurities to levels at which native SWNT magnetic properties predominate. Ferromagnetism is reduced from 1.04 to less than 0.013 emu/g by magnetic gradient filtration. This procedure creates samples of sufficient quality for spectroscopies such as nuclear magnetic resonance (NMR). The overall cleanliness and purity of the material is confirmed through NIR absorption spectroscopy, X-ray diffraction, C60 filling experiments with yields exceeding 90%, and high-resolution 13C NMR.

Raman characterisation of single-walled carbon nanotubes produced by the catalytic pyrolysis of methane

Bundles of single-wall carbon nanotubes (SWCNT) were synthesised using a chemical vapour deposition technique. This basic process was optimised over a wide range of process parameters. For the optimal results, ethane was decomposed at 950 °C over a catalyst material consisting of 2% by wt Fe chemically deposited on an MgO support. The samples were characterised using scanning electron microscopy and transmission electron microscopy to show the presence of nanotubes and also to measure their diameters and the size of amorphous carbon deposits. Raman scattering was also used to probe the electronic properties and hence derive the distribution of diameters of the SWCNT. Samples were measured in both the radial breathing mode and tangential mode ranges using three different laser lines. For comparison purposes, similar data have been reported for a standard commercial SWCNT material (HiPCO). From the data, we can conclude that the tubes in our sample are significantly different to those in the HiPCO sample. In particular, we conclude that samples produced by our method contain a much narrower distribution of tube diameters than does the commercial sample.

Integration of single-walled carbon nanotubes into polymer films by thermo-compression

We developed a simple and direct thermo-compression method for integrating single-walled carbon nanotube (SWCNT) mats of adjustable thicknesses, transparency and conductivity into polymer films. Produced SWCNT/polyethylene composite films have exhibited good optical transparency and conductivity as well as high mechanical flexibility. It was found that the electrical conductivity of the SWCNT mats could be significantly improved by ethanol densification. SWCNT/polyethylene thin films demonstrated excellent cold electron field emission properties.

Carbon Nanotubes Contain Residual Metal Catalyst Nanoparticles even after Washing with Nitric Acid at Elevated Temperature Because These Metal Nanoparticles Are Sheathed by Several Graphene Sheets

It is demonstrated that multiwalled (MWCNT) and single-walled (SWCNT) carbon nanotube materials contain residual metal impurities (Fe, Ni, Co, Mo) even after prolonged periods of "washing" with concentrated nitric acid at temperature of 80 degrees C. Transmission electron microscopy (TEM) and high-resolution TEM (HR-TEM) reveals that this is because such metal impurities are intercalated in the nanotube channel (in the case of MWCNT) or in the "bamboo" segment of the nanotube (in the case of "bamboo"-like MWCNT), or they create graphene sheet protected metal core/shell nanoparticles (in the case of SWCNT). TEM/energy-dispersive X-ray spectroscopy (TEM/EDS) elucidate that residual metal impurities presented in "washed" carbon nanotube materials are in some cases in the form of metal alloys or that there can be several different pure metal nanoparticles presented in one CNT material. It is shown by thermogravimetric analysis that "washing" with concentrated nitric acid removes up to 88% (w/w) of metal catalyst nanoparticles from as-received carbon nanotubes and that such removal has in some cases a significant effect on the electrochemical reduction of hydrogen peroxide.

Quantitative Control over Electrodeposition of Silica Films onto Single-Walled Carbon Nanotube Surfaces

Control over the thickness of a silica coating on single-walled carbon nanotubes (SWNTs) is highly desirable for applications in optics and in biomedicine. Moreover, a silica coating on SWNTs would also aid in avoiding tube−tube contact and bundle formation as well as tube oxidation, a scenario conducive to the use of appropriately functionalized carbon nanotubes as individualized gate dielectric materials in field effect transistors. In this work, we have developed two feasible and reliable means with which to coat SWNTs with various reproducible thicknesses of silica using an electrochemical sol−gel process. In one procedure, a SWNT mat was used as a working electrode for the direct deposition of silica. In the second, nanotubes were dispersed in solution and silica was deposited onto these solubilized nanotubes in the presence of a platinum working electrode. Applying a negative potential results in the condensation of a silica film onto the SWNT surface. The thickness of the silica coating can be cont...

A significant improvement of both yield and purity during SWCNT synthesis via the electric arc process

This paper deals with the optimisation of the single walled carbon nanotube (SWCNT) synthesis by the electric arc technique using so-called heterogeneous anodes filled with Ni and Y catalysts along with either graphite (large-grain or small-grain) or diamond powders. The various carbon nanophases produced were analyzed using high-resolution transmission electron microscopy. Plasma physical properties were determined by emission spectroscopy and were correlated to the variation in the carbon products formed. Using large-grain (100 μm) graphite powder corresponded to standard conditions since able to generate impurity-rich SWCNT material resembling that usually described in literature. However, replacing the large-grain graphite powder by small-grain graphite powder (∼1 μm) resulted in a dramatic increase in both the yield and purity of the SWCNTs obtained. On the other hand, a similar result was obtained by using diamond powder (grain size ∼1 μm) instead of the small-grain graphite powder. The results are explained via the erosion modes of the anodes with respect to the apparent density of the powder mixtures filling their cavities. Maintaining a steady plasma composition and a CI/NiI concentration ratio higher than 10 8 are identified as two conditions required for optimising SWCNT synthesis.

Functional Molecules from Single Wall Carbon Nanotubes. Photoinduced Solubility of Short Single Wall Carbon Nanotube Residues by Covalent Anchoring of 2,4,6-Triarylpyrylium Units

Raw, micrometric HiPCO single wall carbon nanotube (SWNT) material was submitted to harsh acid oxidative treatment with a 3:1 H2SO4/HNO3 mixture to give short residues of SWNT (s-SWNT, <200 nm length measured by TEM). s-SWNT was functionalized through the tip carboxylic groups by peptide bonds using 3-mercatopropanamine linkers that subsequently were reacted with 2,6-diphenyl-4-(4-vinylbiphenyl)pyrylium using azobis(isobutyronitrile) as a radical initiator. After purification by dialysis, the resulting s-SWNT having covalently linked through an ethylthiopropylamide tether the strong electron-transfer pyrylium photosensitizer (Py-sSWNT) was characterized by solution 1H NMR spectroscopy (observation of specific signals due to the heterocyclic protons). Emission spectroscopy shows that the fluorescence of 2,6-diphenyl-4-(4-dodecylthiobiphenyl)pyrylium (Py-SC12) tetrafluoroborate (a model compound to the tethered pyrylium moiety in Py-sSWNT) (lambdaem 533 nm) is quenched by s-SWNT and vice versa that the emission of s-SWNT (lambdaem 330 nm) is quenched by Py-SC12. Depending on the excitation wavelength, Py-sSWNT exhibits dual emission corresponding to each of the two moieties, but with much less intensity than each of the model components independently. Laser flash photolysis of model Py-SC12 allows detection of the triplet (lambdaT-T 750 nm, tau 11.7 micros) and the much longer-lived pyrylium centered radical (lambdamax 525 nm, tau 147 mus). The latter species arises from photoinduced electron transfer from the sulfur atom, as the donor, to the pyrylium heterocycle in its electronic excited-state, as the electron acceptor. Laser flash photolysis (355 nm) of Py-sSWNT also allows detection of the pyrylium centered radical together with a broad absorption spanning from 200 to 500 nm and peaking at 280 nm. The latter band is absent in the laser flash photolysis of the model s-SWNT and was attributed to the electron hole localized on the nanotube moiety of Py-SWNT. The most remarkable effect of the steady-state irradiation is a 1 order of magnitude increase in the solubility of Py-sSWNT. According to TEM images this photoinduced solubility can be attributed to the debundling of the nanotubes due to photoinduced charge separation through the nanotube walls. In addition to exemplify how molecular compounds with photoresponsive properties can be derived from SWNT materials, the observation of photoinduced solubility can serve to develop SWNT layers suitable for photolithography patterning.

Observation of localized charge transport in isolated microscopic mats of single-wall carbon nanotubes

We have formulated a composite consisting of a low dielectric permittivity, low loss granular organic crystal, and 0.1% single-walled carbon nanotubes (SWNTs). The composite morphology was electrically isolated “mats” of SWNTs distributed on the surface of individual crystals having a typical particle size of 20μm. The real part of the composite permittivity (relative) was not significantly different from that of the matrix material (∼2.0), while the dielectric loss increased from 0.0035 to 0.3 at 2.45GHz. We used Maxwell-Garnett mixing theory, which accounts for interfacial polarization, to calculate complex permittivity and conductivity of the SWNT mats. The results indicated weak electrical contact between SWNTs within the mats, implying that localized electron transport was the primary dielectric loss mechanism.

The Role of Carboxylated Carbonaceous Fragments in the Functionalization and Spectroscopy of a Single‐Walled Carbon‐Nanotube Material

The majority of carboxylic acid groups formed after treatment of a single-walled carbon nanotube (SWCNT) material with nitric acid are present on carboxylated carbonaceous fragments (CCFs) (see figure). These CCFs are removed from the SWCNTs by treatment with NaOH solution and identified as carriers of the COOH functionality, implying that previous reports claiming side-wall functionalization of the SWCNTs with COOH groups should be reconsidered.