Large-diameter Single-walled Carbon Nanotubes Research Articles

Narrow and intense resonances in the low-frequency region of surface-enhanced Raman spectra of single-wall carbon nanotubes

Previously unexplored low-frequency Raman modes of single-wall carbon nanotubes (SWNTs) are reported. Gold deposited onto a randomly-oriented top layer of vertically aligned carbon nanotube arrays was used to induce surface-enhanced Raman scattering (SERS) ``hot spots'' on large-diameter SWNTs with SERS intensities up to 1900 times greater than normal Raman intensities from undecorated arrays. The linewidths of the resonances (down to $0.3\text{ }{\text{cm}}^{\ensuremath{-}1}$) are ten times narrower than previously measured for individual SWNTs. Pairs of intense sharp resonances with identical excitation profiles were found and tentatively interpreted as the low-energy longitudinal optical and radial breathing modes of the same nanotube. SERS lines in the region of $14--30\text{ }{\text{cm}}^{\ensuremath{-}1}$ were tentatively assigned to the ring modes of SWNTs in agreement with existing theories.

Diameter-Dependent Solubility of Single-Walled Carbon Nanotubes

We study the solubility and dispersibility of as-produced and purified HiPco single-walled carbon nanotubes (SWNTs). Variation in specific operating conditions of the HiPco process are found to lead to significant differences in the respective SWNT solubilities in oleum and surfactant suspensions. The diameter distributions of SWNTs dispersed in surfactant solutions are batch-dependent, as evidenced by luminescence and Raman spectroscopies, but are identical for metallic and semiconducting SWNTs within a batch. We thus find that small diameter SWNTs disperse at higher concentration in aqueous surfactants and dissolve at higher concentration in oleum than do large-diameter SWNTs. These results highlight the importance of controlling SWNT synthesis methods in order to optimize processes dependent on solubility, including macroscopic processing such as fiber spinning, material reinforcement, and films production, as well as for fundamental research in type selective chemistry, optoelectronics, and nanophotonics.

Selection of single-walled carbon nanotubes according to both their diameter and chirality via nanotweezers

Diameter- and chirality-dependent interactions between aromatic molecule-based nanotweezers and single-walled carbon nanotubes (SWNTs) are revealed by density functional theory calculations. We found that the threshold diameter of selected SWNTs is determined by the end-to-end distance of the nanotweezer. Large-diameter SWNTs are preferred by a nanotweezer with an obtuse folding angle, whereas small-diameter SWNTs are favored by a nanotweezer with an acute folding angle. The adsorption can be further stabilized by the orientational alignment of the hexagonal rings of the nanotweezer and the SWNT sidewall. Therefore, by taking advantage of the supramolecular recognition ability of the aromatic molecule-based nanotweezer, SWNTs can be enriched with both controllable diameter and chirality. Open image in new window

Investigations of niobium carbide contact for carbon-nanotube-based devices

Single-walled carbon nanotube (SWCNT) field effect transistors (FETs)with Nb contacts have been fabricated and upon annealing in vacuum at700 °C for 1 h, niobiumcarbide (Nb2C) is formed at the Nb/SWCNT interface. TheNb2C/SWCNT contacts demonstrate a very small Schottky barrier height of ∼ 18 meV(decreased by > 80% relative to that of pristine Nb/SWCNT contact of ∼ 98 meV) to p-type transport. This is attributed to the higher work function ofNb2C (∼5.2 eV) than Nb (∼4.3 eV) and better bondingbetween Nb2C and SWCNTs.The performance of Nb2C-contacted SWCNT FETs is as follows: the p-channel ON current is as high as0.5 µA atVDS = 0.1 V, theION/IOFF ratio is up to ∼ 105 and the subthresholdslope is ∼ 550 mV/dec, which is as good as that of titanium carbide (TiC-) and Pd-contacted SWCNT FETs. Compared withTiC, Nb2C contacts yield more unipolar p-type SWCNT FETs, as a result of theNb2Cs higher work function.More importantly, Nb2C contacts can form near-ohmic contacts to both large-(≥1.6 nm) andsmall-diameter (∼1 nm) SWCNTs, while Pd can only form near-ohmic contacts for large-diameter SWCNTs. Moreover,the Nb2C contacts demonstrate good stability in air.

The reaction of lithium metal vapor with single walled carbon nanotubes of large diameters

AbstractRaman spectroscopy was used to evaluate the effect of lithium vapor on single walled carbon nanotubes (SWCNTs) having diameters from 1.2 to 1.6 nm. The Raman spectra were excited by five different laser lines. A strong doping of SWCNTs after the reaction with Li vapor was confirmed by the corresponding attenuation of the Raman signal. The relative intensities and frequencies of the radial breathing mode of the Li vapor treated SWCNT were recovered after subsequent reaction with water. Our data demonstrate that the reaction of Li with large diameter SWCNTs is reversible, which is different from previous results showing irreversible changes for SWCNT samples containing both large diameter and small diameter tubes. The subsequent in situ Raman spectroelectrochemical measurement of our sample confirmed that after water treatment of the Li etched sample there is no significant residual doping of the SWCNT sample.

Temperature and pressure dependence of molecular adsorption on single wall carbon nanotubes and the existence of an “adsorption/desorption pressure gap”

The interaction of acetone with single wall carbon nanotubes (SWCNTs) was studied by temperature programmed desorption with mass spectrometry (TPD-MS), after reflux, sonication, or exposure to 7.6Torr of acetone vapors at room temperature. Acetone molecules adsorb strongly on SWCNTs desorbing at ∼400–900K, corresponding to desorption energies of ∼100–225kJ/mol, as intact molecules. Exchange of intact adsorbed molecules with gas phase species was observed in successive dosing of hydrogenated and deuterated acetone molecules. The desorption energies reported here are in stark contrast to the desorption energies (∼75kJ/mol) reported earlier for SWCNTs interacting with acetone under high vacuum at cryogenic temperatures. This result suggests activated adsorption/desorption, and is also observed for adsorption of ethanol, methane, n-butane and 1,3-butadiene on SWCNTs and on carbon black. Quantum chemical calculations suggest that adsorption in interstitial channels of bundles formed of large-diameter SWCNTs is possible and can account for high desorption barriers, a result of strong dispersion interactions between neighboring SWCNTs.

Spectral mixing formulations for van der Waals-London dispersion interactions between multicomponent carbon nanotubes.

Recognition of spatially varying optical properties is a necessity when studying the van der Waals-London dispersion (vdW-Ld) interactions of carbon nanotubes (CNTs) that have surfactant coatings, tubes within tubes, andor substantial core sizes. The ideal way to address these radially dependent optical properties would be to have an analytical add-a-layer solution in cylindrical coordinates similar to the one readily available for the plane-plane geometry. However, such a formulation does not exist nor does it appear trivial to be obtained exactly. The best and most pragmatic alternative for end-users is to take the optical spectra of the many components and to use a spectral mixing formulation so as to create effective solid-cylinder spectra for use in the far-limit regime. The near-limit regime at "contact" is dominated by the optical properties of the outermost layer, and thus no spectral mixing is required. Specifically we use a combination of a parallel capacitor in the axial direction and the Bruggeman effective medium in the radial direction. We then analyze the impact of using this mixing formulation upon the effective vdW-Ld spectra and the resulting Hamaker coefficients for small and large diameter single walled CNTs (SWCNTs) in both the near- and far-limit regions. We also test the spectra of a [16,0,s+7,0,s] multiwalled CNT (MWCNT) with an effective MWCNT spectrum created by mixing its [16,0,s] and [7,0,s] SWCNT components to demonstrate nonlinear coupling effects that exist between neighboring layers. Although this paper is primarily on nanotubes, the strategies, implementation, and analysis presented are applicable and likely necessary to any system where one needs to resolve spatially varying optical properties in a particular Lifshitz formulation.

Hydrogen adsorption on single-walled carbon nanotubes studied by core-level photoelectron spectroscopy and Raman spectroscopy

We have investigated the adsorption of atomic hydrogen on vertically aligned carbon nanotube (CNT) films using in situ synchrotron-radiation-based core-level (CL) photoelectron spectroscopy and Raman spectroscopy. From C 1s CL spectra, we identified a CL peak component due to C–H bonds of carbon atoms in single-walled carbon nanotubes (SWCNTs). We also found the suppression of π-plasmon excitation, indicating that the hydrogen adsorption deforms the bonding structure. Raman spectra of the SWCNT film indicated that the radial-breathing-mode intensities of SWCNTs decreased due to the adsorption-induced bonding-structure deformation. Moreover, the decrease for small-diameter SWCNTs was more severe than that for large-diameter SWCNTs. Our results strongly suggest that the hydrogen adsorption, which induces the structure deformation from sp 2 to sp 3-like bonding, depends on the diameter of SWCNTs.

단일벽 탄소나노튜브의 플러렌 캡슐화 및 라만분광 분석

Single walled carbon nanotubes (SWCNTs) can be modified to produce a new concept of hybrid materials by introducing atoms or molecules inside their cylindrical empty space. Such an endohedral doping of the SWCNTs is expected to decisively alter their electronic transport and mechanical properties, In this study, we purified SWCNTs by three-step purification processes and formed the peapod structure by introducing <TEX>$C_{60}$</TEX> fullerenes inside the SWCNTs. <TEX>$C_{60}$</TEX> molecules were observed to be regularly arranged by transmission electron microscopy. In Raman spectra, the radial breathing mode (RBM) rather than the other modes was significantly affected by the endohedral injection of <TEX>$C_{60}$</TEX>. The RBM intensity was more greatly reduced in the large-diameter SWCNTs than the small-diameter ones, Raman spectroscopy is expected to be a key technique for analyzing <TEX>$C_{60}$</TEX>-encapsulated SWCNTs.

Efficiency of C 60 incorporation in and release from single-wall carbon nanotubes depending on their diameters

We show that the efficiency of incorporating C 60 in single-wall carbon nanotubes (SWCNTs) and that of the incorporated C 60’s release from the SWCNTs depend on the SWCNT diameter. Through transmission electron microscopy, we found that the C 60 incorporation efficiency reached its maximum at diameters of 1–2 nm, while the efficiency of C 60 release from SWCNTs in toluene was maximized at 3–5 nm. The difficulty of C 60 release from SWCNTs with diameters of 5–6 nm might reflect either the effective packing of C 60 inside SWCNTs or a flattened SWCNT structure. We occasionally observed C 60 molecules arranged in a line along the sidewall inside SWCNTs with large diameters/width (>7 nm), indicating that large diameter SWCNTs were sometimes flattened.

Diameter dependence of second-order Raman features of graphene tubes

The second-order Raman band ( ω G ′ ) for single wall carbon nanotubes (SWNTs) in the range ∼2600–2700 cm −1 depends on the excitation energy and the diameter. However, the diameter dependence is not well established and understood. Here we propose that the two different physical dependencies are separable and can be taken into account by a phenomenological here derived equation. Using this methodology we have been able to find consistency between data for double-WNTs and large diameter SWNTs. Further, the determined diameter dependence of G′ correctly predicts the frequency for an infinitely large tube diameter, which is equivalent to a planar graphene sheet.

Pyrolytic synthesis of long strands of large diameter single-walled carbon nanotubes at atmospheric pressure in the absence of sulphur and hydrogen

We describe the synthesis of cm-long strands consisting of single-walled carbon nanotube ropes. The method involves the thermolysis of ferrocene (FeCp 2)–alcohol solutions under an Ar atmosphere at 800–950 °C. The tubes within strands could exhibit large diameters (2–3.5 nm OD) in high yields by either increasing the ferrocene concentration in the alcohol solution or by increasing the pyrolysis temperature. We noted that the nanotube material with the highest degree of crystallinity was produced at 950 °C, and as the ferrocene concentration in the alcohol solution increases (e.g., 1.2 wt%), the tubes tend to be metallic. This method appears to be simple, safer and more efficient than others reported in the literature because it does not require vacuum, sulphur agents, relatively high temperatures or large amounts of H 2.

Production of Large-Diameter Single-Wall Carbon Nanotubes by Adding Fe to a NiCo Catalyst in Laser Ablation

We have found that the simple introduction of Fe into a conventional NiCo catalyst enables the production of single-wall carbon nanotubes (SWNTs) with controlled diameter distributions by laser ablation. The diameter of as-grown SWNTs can be made larger by increasing the atomic percentage of Fe in the target without changing any other synthesis parameters. We think that Fe helps increase the aggregation rate of metal in laser ablation and enlarges the crystal lattice constant of metal catalysts because of a change in the crystal structure from Ni to Ni3Fe, NiFe, and NiFe2, which then results in the enlargement of metal particles and the growth of larger-diameter SWNTs.

Direct growth of macroscopic fibers composed of large diameter SWNTs by CVD

Macroscopic nanotube fibers, composed of aligned large diameter single-walled carbon nanotubes (SWNTs), were prepared by a simple CVD method. These fibers, which constitute the nanotube ropes of loose structure like hemp cords, can be divided into the nanotube threads (mesoscopically) and subdivided into nanotube bundles (microscopically). The samples show good alignment at different scales. The fibers, several micrometers in diameter, have a relatively stable structure and only dissociated partially under ultrasonic dispersion. The obtained SWNTs have good crystalline structure and good purity. The influence of the CVD conditions on the fiber morphology was also investigated.

Enrichment of Large-Diameter Single-Walled Carbon Nanotubes by Oxidative Acid Treatment

Herein we reported a simple way to enrich the large-diameter single-walled carbon nanotubes (SWNTs) by using a mixed concentrated H2SO4/HNO3 treatment. The proportion of the SWNTs with diameter larger than 2 nm measured from transmission electron microscopy was only ca. 3% in the original sample prepared by chemical vapor deposition method. It is found that this proportion becomes remarkably increased with treatment time and reached ca. 20% after 96 h of acid treatment.

Electrical properties and devices of large-diameter single-walled carbon nanotubes

Individual large-diameter (∼3 to 5 nm) semiconducting single-walled carbon nanotubes (SWNTs) are found to exhibit ambipolar field-effect transistor (FET) behavior, with easily accessible n- and p-conduction channels by simple electrostatic gates. The effects of temperature and ultraviolet radiation on their electrical properties are elucidated, shedding light into the intrinsic behavior of SWNTs in this relatively large-diameter regime. The ambipolar SWNT-FETs can be readily used as building blocks for functional nanoelectronic devices such as voltage inverters that operate under ambient conditions.