Photocurrent spectroscopy of dye-sensitized carbon nanotubes.

Monochiral (7,5) single walled carbon nanotubes (SWCNTs) are integrated into a field effect transistor device in which the built-in electric field at the nanotube/metal contact allows for exciton separation under illumination. Variable wavelength spectroscopy and 2D surface mapping of devices consisting of 10-20 nanotubes are performed in the visible region and a strong correlation between the nanotube's second optical transition (S22) and the photocurrent is found. After integration, the SWCNTs are non-covalently modified with three different fluorescent dye molecules with off-resonant absorption maxima at 532 nm, 565 nm, and 610 nm. The dyes extend the absorption properties of the nanotube and contribute to the photocurrent. This approach holds promise for the development of photo-detectors and for applications in photovoltaics and biosensing.

Fitting Single-Walled Carbon Nanotube Optical Spectra.

In this work, a comprehensive methodology for the fitting of single-walled carbon nanotube absorption spectra is presented. Different approaches to background subtraction, choice of line profile, and calculation of full width at half-maximum are discussed both in the context of previous literature and the contemporary understanding of carbon nanotube photophysics. The fitting is improved by the inclusion of exciton-phonon sidebands, and new techniques to improve the individualization of overlapped nanotube spectra by exploiting correlations between the first- and second-order optical transitions and the exciton-phonon sidebands are presented. Consideration of metallic nanotubes allows an analysis of the metallic/semiconducting content, and a process of constraining the fit of highly congested spectra of carbon nanotube solid films according to the spectral weights of each (n, m) species in solution is also presented, allowing for more reliable resolution of overlapping peaks into single (n, m) species contributions.

Aligned Carbon Nanotube Thin Films from Liquid Crystal Polyelectrolyte Inks.

Single walled carbon nanotube thin films are fabricated by solution shearing from high concentration sodium nanotubide polyelectrolyte inks. The solutions are produced by simple stirring of the nanotubes with elemental sodium in dimethylacetamide, and the nanotubes are thus not subject to any sonication-induced damage. At such elevated concentrations (∼4 mg mL(-1)), the solutions exist in the liquid crystal phase and during deposition this order is transferred to the films, which are well aligned in the direction of shear with a 2D nematic order parameter of ∼0.7 determined by polarized absorption measurements. Compared to similarly formed films made from superacids, the polyelectrolyte films contain smaller bundles and a much narrower distribution of bundle diameters. After p-doping with an organic oxidizer, the films exhibit a very high DC electrical to optical conductivity ratio of σ(DC)/σ(OP) ∼ 35, corresponding to a calculated DC conductivity of over 7000 S cm(-1). When very thin (T550 ∼ 96%), smooth (RMS roughness, R(q) ∼ 2.2 nm), and highly aligned films made via this new route are used as the front electrodes of carbon nanotube-silicon solar cells, the power conversion efficiency is almost an order of magnitude greater than that obtained when using the much rougher (R(q) ∼ 20-30 nm) and less conductive (peak σ(DC)/σ(OP) ∼ 2.5) films formed by common vacuum filtration of the same starting material, and having the same transmittance.

Sorting of Double-Walled Carbon Nanotubes According to Their Outer Wall Electronic Type via a Gel Permeation Method.

In this work, we demonstrate the application of the gel permeation technique to the sorting of double-walled carbon nanotubes (DWCNTs) according to their outer wall electronic type. Our method uses Sephacryl S-200 gel and yields sorted fractions of DWCNTs with impurities removed and highly enriched in nanotubes with either metallic (M) or semiconducting (S) outer walls. The prepared fractions are fully characterized using optical absorption spectroscopy, transmission electron microscopy, and atomic force microscopy, and the entire procedure is monitored in real time using process Raman analysis. The sorted DWCNTs are then integrated into single nanotube field effect transistors, allowing detailed electronic measurement of the transconductance properties of the four unique inner@outer wall combinations of S@S, S@M, M@S, and M@M.

Separation of double-walled carbon nanotubes by size exclusion column chromatography.

In this report we demonstrate the separation of raw carbon nanotube material into fractions of double-walled (DWCNTs) and single-walled carbon nanotubes (SWCNTs). Our method utilizes size exclusion chromatography with Sephacryl gel S-200 and yielded two distinct fractions of single- and double-walled nanotubes with average diameters of 0.93 ± 0.03 and 1.64 ± 0.15 nm, respectively. The presented technique is easily scalable and offers an alternative to traditional density gradient ultracentrifugation methods. CNT fractions were characterized by atomic force microscopy and Raman and absorption spectroscopy as well as transmission electron microscopy.

Separation of single-walled carbon nanotubes with a gel permeation chromatography system.

A gel permeation chromatography system is used to separate aqueous sodium dodecyl sulfate suspensions of single-walled carbon nanotubes (SWCNTs). This automated procedure requires no precentrifugation, is scalable, and is found to yield monochiral SWCNT fractions of semiconducting SWCNTs with a purity of 61-95%. Unsorted and resulting monochiral fractions are characterized using optical absorption and photoluminescence spectroscopy.

Carbon nanotubes as electronic interconnects in solid acid fuel cell electrodes.

Carbon nanotubes have been explored as interconnects in solid acid fuel cells to improve the link between nanoscale Pt catalyst particles and macroscale current collectors. The nanotubes were grown by chemical vapor deposition on carbon paper substrates, using nickel nanoparticles as the catalyst, and were characterized using scanning electron microscopy and Raman spectroscopy. The composite electrode material, consisting of CsH2PO4, platinum nanoparticles, and platinum on carbon-black nanoparticles, was deposited onto the nanotube-overgrown carbon paper by electrospraying, forming a highly porous, fractal structure. AC impedance spectroscopy in a symmetric cell configuration revealed a significant reduction of the electrode impedance as compared to similarly prepared electrodes without carbon nanotubes.