Directed assembly of nanoscale building blocks such as single-walled carbon nanotubes (SWNTs) into desired architectures is a major hurdle for a broad range of basic research and technological applications (e.g., electronic devices and sensors). Here we demonstrate a parallel assembly process that allows one to simultaneously position, shape, and link SWNTs with sub-100-nm resolution. Our method is based on the observation that SWNTs are strongly attracted to COOH-terminated self-assembled monolayers (COOH-SAMs) and that SWNTs with lengths greater than the dimensions of a COOH-SAM feature will align along the boundary between the COOH-SAM feature and a passivating CH3-terminated SAM. By using nanopatterned affinity templates of 16-mercaptohexadecanonic acid, passivated with 1-octadecanethiol, we have formed SWNT dot, ring, arc, letter, and even more sophisticated structured thin films and continuous ropes. Experiment and theory (Monte Carlo simulations) suggest that the COOH-SAMs localize the solvent carrying the nanotubes on the SAM features, and that van der Waals interactions between the tubes and the COOH-rich feature drive the assembly process. A mathematical relationship describing the geometrically weighted interactions between SWNTs and the two different SAMs required to overcome solvent-SWNT interactions and effect assembly is provided.

Near-infrared magneto-optical spectroscopy of single-walled carbon nanotubes reveals two absorption peaks with an equal strength at high magnetic fields (>55 T). We show that the peak separation is determined by the Aharonov-Bohm phase due to the tube-threading magnetic flux, which breaks the time-reversal symmetry and lifts the valley degeneracy. This field-induced symmetry breaking thus overcomes the Coulomb-induced intervalley mixing which is predicted to make the lowest exciton state optically inactive (or dark).

We present the first quantitative assessment of the maximum amount of nanotubes that can exist in the isotropic phase () of single-walled carbon nanotubes (SWNTs) in Brønsted-Lowry acids. We employ a centrifugation technique in conjunction with UV-vis-nIR spectroscopy to quantify , which is also the critical concentration of the isotropic-nematic transition of SWNTs in strong acids. Centrifugation of biphasic dispersions of SWNTs, that is, acid dispersions consisting of an isotropic phase in equilibrium with an ordered nematic liquid crystalline phase, results in a clear phase separation, where the isotropic phase is supernatant. Dilution of the isotropic phase with a known amount of acid followed by UV-vis-nIR absorbance measurements yields , that is, the maximum concentration of SWNTs that can exist in the isotropic phase in a given acid for a given SWNTs' length distribution. At low SWNT concentration (below 200 ppm) in superacids, light absorbance in the range from 400 to 1400 nm scales linearly with concentration. This Beer's law behavior yields calibration curves for measuring SWNTs' concentration in acids. We find that the critical concentration of the isotropic-nematic transition increases with acid strength in accordance with the previously proposed sidewall protonation mechanism for dispersing SWNTs in acids.

Single-walled carbon nanotubes functionalized by phenyl groups can be sulfortated in oleum (H2SO4, 20% free SO3) to give materials that exhibit high solubility in water as determined by centrifugation in conjunction with UV-vis-near-infrared measurements. Cryo-transmission electron microscopy images show that extensive debundling occurs during the sulfonation step.

A two-step process is utilized for cutting single-walled carbon nanotubes (SWNTs). The first step requires the breakage of carbon-carbon bonds in the lattice while the second step is aimed at etching at these damage sites to create short, cut nanotubes. To achieve monodisperse lengths from any cutting strategy requires control of both steps. Room-temperature piranha and ammonium persulfate solutions have shown the ability to exploit the damage sites and etch SWNTs in a controlled manner. Despite the aggressive nature of these oxidizing solutions, the etch rate for SWNTs is relatively slow and almost no new sidewall damage is introduced. Carbon-carbon bond breakage can be introduced through fluorination to ∼C(2)F, and subsequent etching using piranha solutions has been shown to be very effective in cutting nanotubes. The final average length of the nanotubes is approximately 100 nm with carbon yields as high as 70-80%.

Manipulating optical properties of single-walled nanotubes (SWNTs) is necessary for the development of nanoscale optical devices and probes for biomedical research. In life sciences it will make possible the direct observation of SWNTs inside living cells using optical microscopes. In the nanotechnology field it will enable the development of nanosensors with fluorescent reporting. However, the direct fluorescent labeling of SWNTs is obstructed by their strong light quenching qualities. Besides, chemical functionalization of SWNTs needed for the covalent attachment of fluorescent dyes could change favorable properties of nanotubes. Here we report that optical properties of SWNTs can be manipulated without their covalent modification by wrapping them with fluorescently labeled polymer poly(vinylpyrrolidone) (PVP-1300). Fluorescent PVP-1300 forms a monomolecular approximately 2.5 nm thick layer coiling around individual SWNTs and nanotube bundles. PVP casing is fluorescent although it is only several nanometers thick. This makes individual SWNTs observable by a fluorescent microscope. The spare polymer strands left over after wrapping around the relatively shorter nanotubes form junctions between SWNTs tying them together into new configurations, primarily Y- and psi-type junctions. The ability to use a single fluorescent polymer strand to fasten nanotubes together can be useful in assembly of nanotube-made devices. In PVP-covered SWNTs multiple fluorophores are attached to each single nanotube making them unique composite fluorophores attractive as parts of biological fluorescent probes and in the development of the new materials in photonics and nanotechnology.

We review our recent optical experiments on single-walled carbon nanotubes in high magnetic fields. The data revealed magnetic-field-induced optical anisotropy as well as broadening, splittings, and shifts of interband absorption and photoluminescence peaks. Quantitative comparison with theoretical predictions based on the Aharonov–Bohm effect is presented.

Liquid anhydrous sulfuric acid forms a partly ordered structure in the presence of single-walled carbon nanotubes (SWNTs). X-ray scattering from aligned fibers immersed in acid shows the formation of molecular shells wrapped around SWNTs. Differential scanning calorimetry of SWNT-acid suspensions exhibits concentration-dependent supercooling/melting behavior, confirming that the partly ordered molecules are a new phase. We propose that charge transfer between nanotube pi electrons and highly oxidizing superacid is responsible for the unique partly ordered structure.

We demonstrate the continued growth of single-walled carbon nanotubes (SWNTs) from ordered arrays of open-ended SWNTs in a way analogous to epitaxy. Nanometer-sized metal catalysts were docked to the SWNT open ends and subsequently activated to restart growth. SWNTs thus grown inherit the diameters and chirality from the seeded SWNTs, as indicated by the closely matched frequencies of Raman radial breathing modes before and after the growth.

Through ultrafast pump-probe spectroscopy with intense pump pulses and a wide continuum probe, we show that interband exciton peaks in single-walled carbon nanotubes (SWNTs) are extremely stable under high laser excitations. Estimates of the initial densities of excitons from the excitation conditions, combined with recent theoretical calculations of exciton Bohr radii for SWNTs, suggest that their positions do not change at all even near the Mott density. In addition, we found that the presence of lowest-subband excitons broadens all absorption peaks, including those in the second-subband range, which provides a consistent explanation for the complex spectral dependence of pump-probe signals reported for SWNTs.

A two-phase liquid-liquid extraction process is presented which is capable of extracting water-soluble single-walled carbon nanotubes into an organic phase. The extraction utilizes electrostatic interactions between a common phase transfer agent and the sidewall functional groups on the nanotubes. Large length-dependent van der Waals forces for nanotubes allow the ability to control the length of nanotubes extracted into the organic phase as demonstrated by atomic force microscopy.

Single-walled carbon nanotubes (SWNTs) uniquely combine characteristics of polymers (e.g., they are molecules composed of repeat units) as well as nanoscale colloids (e.g., they are extremely stiff); because of these characteristics, they may qualify as the ideal building blocks for the long-sought Staudinger's “continuous crystal.” To this end, neat SWNT fibers have been produced by wet-spinning concentrated dispersions of 6−10 wt % SWNTs in 102% H2SO4. A unique protocol that combined UV−ozone dry chemical etching and cryoultramicrotoming was developed to directly visualize the internal voids, alignment, and composition of these SWNT neat fibers. A substructure of large aligned SWNT ropes, 200−500 nm in diameter, was revealed throughout the fibers. This substructure may be linked to the SWNT spaghetti, threadlike liquid-crystalline domains observed in dispersions of SWNTs in H2SO4.

Using the High Pressure carbon monoxide (HiPco) reactor we conducted an experiment on the effects of changing the catalyst concentration. With each catalyst concentration tested the resulting raw HiPco material was characterized for average SWNT lengths, SWNT diameters, residual iron particle size, and large fullerene content. We were able to determine trends in each of these characteristics as the catalyst concentration was changed. As the catalyst concentration was decreased SWNT lengths increased, SWNT diameters increased, the residual iron particle size increased, and the large fullerene content decreased. From these trends we have developed a Competitive Growth model for nucleation and growth of SWNTs via the HiPco process.

The charge transfer induced lithiation of single-wall carbon nanotubes (SWNTs) was investigated by in situ monitoring by Raman spectroscopy as lithium was added incrementally to a dispersion of SWNTs in liquid ammonia. Charge transfer from liquid ammonia solvated lithium to the SWNTs led to intercalation of lithium into the SWNT ropes, as well as to the semi-covalent lithiation of the SWNTs. Raman spectra of the SWNTs recorded as lithium was added showed a ∼30 wavenumber downshift of the G band (1594cm−1) with the concomitant appearance of a new peak at 1350cm−1 that was assigned as the signature of the lithiated SWNTs. Addition of 1-iodododecane to the lithiated SWNTs resulted in the covalent attachment of dodecyl groups. The intercalation of lithium throughout the SWNT ropes led to complete dodecylation of all individual SWNTs.

A controlled and scalable multistep purification method has been developed to remove iron impurity and nonnanotube carbon materials from raw single-walled carbon nanotubes (SWNTs) produced in the HiPco (high-pressure CO) process. In this study, iron nanoparticles, coated by carbon, are exposed and oxidized by multiple step oxidation at increasing temperatures. To avoid catalytic oxidation by iron oxide of carbon nanotubes, the exposed and oxidized iron oxide is deactivated by reaction with C(2)H(2)F(4) or SF(6). The iron fluorides are removed by a Soxhlet extraction with a 6 M HCl solution. The purity and quality of each sample were determined by thermogravimetric analysis (TGA), Raman spectrometry, ultraviolet-visible-near-IR (UV-vis-near-IR) spectrometry, fluorescence spectrometry, and transmission electron microscope (TEM) spectroscopy. The purity and yield of SWNTs are improved due to reduced catalytic activity of the iron oxide. Greater iron oxide removal also resulted from oxidation at higher temperatures.

Single walled carbon nanotubes (SWNTs) have been coated with fluorine-doped silica by liquid phase deposition (LPD) using a silica–H2SiF6 solution and a surfactant stabilized solution of SWNTs. The coating of individual SWNTs versus small ropes is controlled by the choice of surfactant. Since the LPD reaction is performed close to the isoelectric point of the silica, some of the SiO2–SWNTs are fused together but the SWNTs remain individual in these composite structures. The SiO2–SWNTs have been characterized by SEM, TEM, Raman and IR spectroscopy, and XPS. Raman fluoresence is maintained even with coatings >50 nm. Using the relative intensity of the Raman G peak and the 8,3 SWNT fluorescence as a convenient measure of bundling, it may be shown that any decrease of fluoresence during growth is not due to changes in ionic strength or pH, as a consequence of addition of the LPD solution or the presence of HF as a side product in the deposition. The Raman D and G modes show no change in intensity, while the Fano line increases, both suggesting that no sidewall functionalization or proton adsorption of the SWNTs occurs during coating. The UV-visible-near infrared spectra shows a red shift in the first Van Hove transitions of the coated SWNTs inferring that the SWNTs in SiO2–SWNTs are in a more polarizable and inhomogeneous environment than that of surfactant solutions. Mats of SiO2–SWNT may be deposited onto silicon and gold substrates and through lithography may be patterned by etching off selected areas of the silica coating.

Liquid anhydrous sulfuric acid forms molecular "shells" wrapped around single-walled carbon nanotubes (SWNTs). Temperature-dependent X-ray scattering from aligned acid-swollen fibers shows that crystallization of the bulklike acid surrounding the structured shells is templated by the aligned SWNTs, while the structured shells remain partly ordered, at least for temperatures from 100 to 500 K. The (2 0 0) or ( 0 2) planes of the templated H2SO4 crystallites are parallel to the nanotube axes. This provides solid evidence for the direct protonation of SWNT since the molecules are terminated by hydrogen bonds.

The oxidation reaction of piranha solutions with purified HiPco carbon nanotubes was measured as a function of temperature. At high temperatures, piranha is capable of attacking existing damage sites, generating vacancies in the graphene sidewall, and consuming the oxidized vacancies to yield short, cut nanotubes. Increased reaction time results in increasingly shorter nanotubes. However, significant sidewall damage occurs as well as selective etching of the smaller diameter nanotubes. On the other hand, room-temperature piranha treatments show the capability of cutting existing damage sites with minimal carbon loss, slow etch rates, and little sidewall damage. Combined with a method of introducing controlled amounts of damage sites, these room-temperature piranha solutions have the potential to yield an efficient means of creating short, cut nanotubes.