Dependence of Optical Transition Energies on Structure for Single-Walled Carbon Nanotubes in Aqueous Suspension: An Empirical Kataura Plot

Spectrofluorimetric data for identified single-walled carbon nanotubes in aqueous SDS suspension have been accurately fit to empirical expressions. These are used to obtain the first model-independent prediction of first and second van Hove optical transitions as a function of structure for a wide range of semiconducting nanotubes. To allow for convenient use in support of spectral studies, the results are presented in equation, graphical, and tabular forms. These empirical findings differ significantly from Kataura plots computed using a simple tight-binding model. It is suggested that the empirically based results should be used in preference to conventional model-based predictions in spectroscopic nanotube research

Photoexcited Aromatic Reactants Give Multicolor Carbon Nanotube Fluorescence from Quantum Defects

Covalent functionalization of single-wall carbon nanotubes (SWCNTs) can be valuable for modifying their electronic properties and creating fluorescent quantum defects. We report here a previously unreported category of such reactions involving interactions of photoexcited aromatic compounds with SWCNT sidewalls. When aqueous suspensions of SWCNTs are exposed to organic aromatic compounds and then irradiated by UV light, fluorescent defects are formed in the nanotubes at rates that depend on the aromatic ring substituents. In reactions with aniline or iodoaniline, strong spectral sidebands appear within 1 min. Total SWCNT photoluminescence can be enhanced by a factor as large as ∼5. Notably, emission spectra of reacted SWCNTs depend on the presence or absence of dissolved oxygen during the reaction. For (6,5) SWCNTs, treatment when oxygen is present gives an additional emission band red-shifted by 160 meV from the pristine position, whereas treatment without oxygen leads to two additional emission bands red-shifted by 140 and 270 meV. Variance spectroscopy shows the presence of individual “multicolor” nanotubes with three distinct emission bands (pristine plus two shifted). The facile generation of dual fluorescent quantum defects in SWCNTs provides emission closer to standard telecom wavelengths, advancing the prospects for applications as single-photon sources in quantum information processing

Enantiomers of Single-Wall Carbon Nanotubes Show Distinct Coating Displacement Kinetics

It is known that specific oligomers of single-stranded DNA (ssDNA) can show remarkable selectivity when coating different structural species of single-wall carbon nanotubes (SWCNTs). We report that (ATT)₄ ssDNA coatings strongly distinguish between the two optical isomers of (7,5) SWCNTs. This causes resolvable shifts in their fluorescence spectra and differences of 2 orders of magnitude in the room temperature rates of coating displacement, as monitored through changes in nanotube fluorescence wavelength and intensity on exposure to sodium deoxycholate. During coating displacement, the enantiomer with high affinity for the ssDNA oligomer is deduced to form an intermediate hybrid that is not observed for the low affinity enantiomer. These results reveal that enantiomeric differences in SWCNTs complexed with ssDNA are more diverse and dramatic than previously recognized

Kinetics of Single-Wall Carbon Nanotube Coating Displacement by Single-Stranded DNA Depends on Nanotube Structure

Time-resolved fluorescence spectroscopy has been used to study the displacement of adsorbed sodium dodecyl sulfate (SDS) from the surface of single-wall carbon nanotubes (SWCNTs) by short strands of single-stranded DNA. Intensity changes in near-infrared emission peaks of various SWCNT structures were analyzed following the addition of six different (GT)n oligomers (n from 3 to 20) to SDS-coated nanotube samples. There is a strong kinetic dependence on the oligomer length, with (GT)3 giving an initial rate more than 300 times greater than that of (GT)20. For shorter oligos in the (GT)n series, we observe an inverse dependence of the displacement rate on the SWCNT diameter, with SDS displaced from (6,5) more than twice as fast as from (8,7). However, this diameter dependence is reversed for oligos with more than six (GT) units. There is also a systematic dependence of the displacement rate on the nanotube chiral angle that is strongest for (GT)5, leading to a factor of ∼3 initial rate difference between (9,1) and (6,5) despite their identical diameters. To account for these findings, we propose a simple two-step kinetic model in which disruption of the original SDS coating is followed by conformational relaxation of ssDNA on the nanotube surface. The relaxation is relatively fast for ssDNA oligos shorter than 12 bp, making the first step rate-determining. Conversely, relaxation of the longer oligomers is slow enough that the second step becomes rate-determining

Removing Aggregates from Single-Walled Carbon Nanotube Samples by Magnetic Purification

Purification of raw single-walled carbon nanotube (SWCNT) material remains an important challenge in nanotube research and applications. We describe here a simple but effective purification method that uses permanent magnets to remove many nanotube aggregates, as well as residual metallic catalyst, from aqueous suspensions of surfactant-coated SWCNTs. Samples have been characterized by optical absorption, fluorescence, and Raman spectroscopies; atomic force microscopy and near-infrared fluorescence microscopy; and thermogravimetric analysis. It is found that magnetic purification reduces absorption backgrounds and increases average fluorescence efficiencies to levels comparable to those in ultracentrifuged samples. The ratio of individualized SWCNTs to aggregates in magnetically processed HiPco samples is estimated to be approximately 4:1. As compared to ultracentrifugation, magnetic processing promises major advantages in cost, simplicity, energy consumption, and scalability

Synthesis and Characterization of the “Missing” Oxide of C₆₀: [5,6]-Open C₆₀O

Synthesis and Characterization of the “Missing” Oxide of C60:  [5,6]-Open C60

Diameter-Dependent Competitive Adsorption of Sodium Dodecyl Sulfate and Single-Stranded DNA on Carbon Nanotubes

The equilibrium compositions of coatings on single-wall carbon nanotubes were spectroscopically deduced for samples dispersed in dilute sodium dodecyl sulfate (SDS) and then exposed to low concentrations of ssDNA oligomers. With all studied oligomers, displacement of the SDS tended to occur at lower ssDNA concentrations for smaller diameter nanotubes than for larger diameter ones. However, the behavior varied significantly with oligomer. For example, the diameter dependence was steeper for (TAT)4 than for (ATT)4, suggesting that interstrand head-to-tail hydrogen bonding interactions play a role in SWCNT wrapping. Concentrations of ssDNA in the range of several μg/mL displace SDS from nanotubes dispersed in 1500 μg/mL SDS solutions. This effect allows the use of coating exchange to prepare ssDNA dispersions with minimal oligomer costs. Another demonstrated use exploits the structure-dependent relative coating affinities in a simple filtration method for the diameter enrichment of SWCNT mixtures

Ozonides, Epoxides, and Oxidoannulenes of C₇₀

Six new monoadducts of C70 with oxygen species have been prepared, isolated, and characterized following ozonation of C70 solutions. The initial products are two ozonide monoadducts, identified as a,b- and c,c-C70O3. These ozonides lose O2 through thermolysis or photolysis to form various isomers of C70O. The a,b-C70O3 isomer dissociates through thermolysis with a decay time of 14 min at 296 K to form the [6,6]-closed epoxide a,b-C70O. When photolyzed, it instead forms a [5,6]-open oxidoannulene identified as a,a-C70O. These reactions mimic those seen for C60O3. By contrast, the c,c-C70O3 isomer, which has a thermolysis lifetime of 650 min at 296 K, decays thermally only to an oxidoannulene deduced to be d,d-C70O. Photolysis of c,c-C70O3 produces a mixture of the oxidoannulenes b,c-C70O and c,d-C70O plus a minor amount of the c,c-epoxide. All four C70O oxidoannulene isomers undergo photoisomerization, giving eventually the a,b- and c,c-C70O epoxides