Sergei M. Bachilo

Advanced sorting of single-walled carbon nanotubes by nonlinear density-gradient ultracentrifugation

Existing methods for growing single-walled carbon nanotubes produce samples with a range of structures and electronic properties, but many potential applications require pure nanotube samples. Density-gradient ultracentrifugation has recently emerged as a technique for sorting as-grown mixtures of single-walled nanotubes into their distinct (n,m) structural forms, but to date this approach has been limited to samples containing only a small number of nanotube structures, and has often required repeated density-gradient ultracentrifugation processing. Here, we report that the use of tailored nonlinear density gradients can significantly improve density-gradient ultracentrifugation separations. We show that highly polydisperse samples of single-walled nanotubes grown by the HiPco method are readily sorted in a single step to give fractions enriched in any of ten different (n,m) species. Furthermore, minor variants of the method allow separation of the mirror-image isomers (enantiomers) of seven (n,m) species. Optimization of this approach was aided by the development of instrumentation that spectroscopically maps nanotube contents inside undisturbed centrifuge tubes.

Oxygen Doping Modifies Near-Infrared Band Gaps in Fluorescent Single-Walled Carbon Nanotubes

Better Imaging When Separated A fluorescent probe works better if its absorption and emission wavelengths are well separated; otherwise, the probe tends to reabsorb its own emission. Ghosh et al. (p. 1656, published online 25 November) found that oxygen doping of semiconducting single-wall carbon nanotubes (SWCNTs) improved the characteristics of these materials as imaging probes in the near-infrared. Exposure of SWCNTs to ozone and then to visible light caused the emission wavelength to be 10 to 15% longer than the absorption wavelength. They imaged these probes and untreated SWCNTs in cultured human cells and found an ∼20-fold improvement in contrast. Contrast can be improved in bioimaging applications by separating the emission and absorption wavelengths. Controlled chemical modifications of single-walled carbon nanotubes (SWCNTs) that tune their useful properties have been sought for multiple applications. We found that beneficial optical changes in SWCNTs resulted from introducing low concentrations of oxygen atoms. Stable covalently oxygen-doped nanotubes were prepared by exposure to ozone and then light. Treated samples showed distinct, structure-specific near-infrared fluorescence at wavelengths 10 to 15% longer than displayed by pristine semiconducting SWCNTs. Dopant sites harvest light energy absorbed in undoped nanotube regions by trapping mobile excitons. The oxygen-doped SWCNTs are much easier to detect and image than pristine SWCNTs because they give stronger near-infrared emission and do not absorb at the shifted emission wavelength.

Ultrafast carrier dynamics in single-walled carbon nanotubes probed by femtosecond spectroscopy

We present studies of the ultrafast carrier dynamics in single-walled carbon nanotubes using femtosecond fluorescence and transient absorption techniques. We find that the dynamics are dependent on excitation intensity and the electronic transitions initially excited

Structure-dependent fluorescence efficiencies of individual single-walled carbon nanotubes.

Single-nanotube photometry was used to measure the product of absorption cross section and fluorescence quantum yield for 12 (n,m) structural species of semiconducting single-walled carbon nanotubes in aqueous SDBS suspension. These products ranged from 1.7 to 4.5 x 10(-19) cm(2)/C atom, generally increasing with optical band gap as described by the energy gap law. The findings suggest fluorescent quantum yields of approximately 8% for the brightest, (10,2) species and introduce the empirical calibration factors needed to deduce quantitative (n,m) distributions from bulk fluorimetric intensities.

Analyzing Absorption Backgrounds in Single-Walled Carbon Nanotube Spectra

The sources of broad backgrounds in visible-near-IR absorption spectra of single-walled carbon nanotube (SWCNT) dispersions are studied through a series of controlled experiments. Chemical functionalization of nanotube sidewalls generates background absorption while broadening and red-shifting the resonant transitions. Extensive ultrasonic agitation induces a similar background component that may reflect unintended chemical changes to the SWCNTs. No major differences are found between spectral backgrounds in sample fractions with average lengths between 120 and 650 nm. Broad background absorption from amorphous carbon is observed and quantified. Overlapping resonant absorption bands lead to elevated backgrounds from spectral congestion in samples containing many SWCNT structural species. A spectral modeling method is described for separating the background contributions from spectral congestion and other sources. Nanotube aggregation increases congestion backgrounds by broadening the resonant peaks. Essentially no background is seen in sorted pristine samples enriched in a single semiconducting (n,m) species. By contrast, samples enriched in mixed metallic SWCNTs show broad intrinsic absorption backgrounds far from the resonant transitions. The shape of this metallic background component and its absorptivity coefficient are quantitatively assessed. The results obtained here suggest procedures for preparing SWCNT dispersions with minimal extrinsic background absorptions and for quantifying the remaining intrinsic components. These findings should allow improved characterization of SWCNT samples by absorption spectroscopy.

Surfactant-Dependent Exciton Mobility in Single-Walled Carbon Nanotubes Studied by Single-Molecule Reactions

Measurements of stepwise photoluminescence quenching in individual, (n,m)-selected single-walled carbon nanotubes (SWCNTs) undergoing chemical reaction have been analyzed to deduce mobilities of optically generated excitons. For (7,5) nanotubes, the mean exciton range varies between approximately 140 and 240 nm for different surfactant coatings and correlates weakly with nanotube PL intensity. The results are consistent with a model of localized SWCNT excitons having substantial diffusional mobility along the nanotube axis.

S2 → S0 fluorescence and transient Sn ← S1 absorption of all-rans-β-carotene in solid and liquid solutions

Abstract The spectral and luminescence characteristics of all-rans-β-carotene* were studied at 77 and 4.2 K in isopentane. The fluorescence spectra exhibit a vibronic structure; their 00 band halfwidths are 770 cm−1 at 77 K and 380 cm−1 at 4.2 K. Fluorescence quantum yields ϱ at 4.2 and 77 K are (8 ± 3) × 10−5 and (4 ± 2) × 10−5 respectively. The lifetimes of the observed fluorescence, estimated from the fluoroescence quantum yields ϱ and the natural fluorescence lifetimes, appear to be (8 ± 3) × 10−14 s at 4.2 K and (4 ± 2) × 10−14 s at 77 K. The fluorescence observed originates from the S2(1 1B+u state; the extremely short lifetime of the latter is due to its electronic relaxation to the lowest S1(2 1A−g) state. The S1 state of β-carotene was studied at 293 K using picosecond laser spectroscopy (λexc = 528 and 352 nm). The Sn ← S1 absorption spectrum in n-hexane has a maximum at 555 nm (18 000 cm−1). In toluene and chinoline, bathochromic shifts of 400 ± 100 and 800 ± 100 cm−1 are observed relative to the maximum in n-hexane. The Sn ← S1 transition to the higher excited state Sn can take place; the spectrum of the Sn ← S0 absorption to this state has a maximum at 275 nm (36 500 cm−1 in n-hexane. The lifetime of the S1 state of β-carotene in solvents with different viscosities and polarities (n-hexane, toluene, chinoline and vaseline oil) is 10 ± 2 ps.

Strain Measurements on Individual Single-Walled Carbon Nanotubes in a Polymer Host: Structure-Dependent Spectral Shifts and Load Transfer

The fluorescence spectra of individual semiconducting single-walled carbon nanotubes embedded in polymer films were measured during the application of controlled stretching and compressive strains. Nanotube band gaps were found to shift in systematic patterns that depend on the (n,m) structural type and are in excellent agreement with the predictions of theoretical models. Loss of nanotube-host adhesion was revealed by abrupt irregularities in plots of spectral shift vs strain.

Translational and rotational dynamics of individual single-walled carbon nanotubes in aqueous suspension.

Near-infrared fluorescence videomicroscopy has been used to study simultaneously the translational and rotational diffusion of individual semiconducting single-walled carbon nanotubes (SWCNTs) in aqueous suspension. Analysis of translational trajectories revealed diffusion coefficient values from approximately 0.3 to 6 microm(2)/s. The nanotube lengths deduced from these values ranged between approximately 130 nm and 6 microm. From the minor bending motions observed in individual nanotubes several micrometers in length, we confirmed that the shorter SWCNTs of primary interest here can be considered to be rigid rods under normal conditions. Because the nanotubes act as highly rigid, photostable, steady, and anisotropic fluorophores, it was possible to monitor their rotational reorientations through fluctuations in emission intensity under linearly polarized excitation. The magnitudes of observed orientational fluctuations varied substantially among individual nanotubes. These magnitudes correlated strongly with translational diffusion coefficient, reflecting the length dependence of both types of motions. Combined translational and rotational measurements also revealed the influence of local environment on nanotube mobility.

Efficient photosensitized energy transfer and near-IR fluorescence from porphyrin–SWNT complexes

Energy transfer from photoexcited porphyrin molecules to single-walled carbon nanotubes (SWNTs) has been experimentally detected for samples in aqueous Triton X-100 micellar suspensions. Addition of SWNTs to micelle-suspended porphyrin results in strong quenching of porphyrin fluorescence. Measurements of concentration-dependent quenching and spectra suggest that this process arises from formation of ground state non-covalent complexes between porphyrins and SWNTs. Optical excitation of the porphyrin generates characteristic near-IR emission from the SWNTs, indicating efficient energy transfer within the complexes. This energy transfer is deduced to occur through a Dexter-type electron exchange mechanism. Complexation of SWNTs with organic photosensitizers provides a novel way of uniformly exciting a wide range of nanotube structural species in polydisperse samples using only a single excitation wavelength.