Semiconducting Single-walled Nanotubes Research Articles

Diameter-Specific Growth of Semiconducting SWNT Arrays Using Uniform Mo2C Solid Catalyst.

Semiconducting single-walled nanotube (s-SWNT) arrays with specific diameters are urgently demanded in the applications in nanoelectronic devices. Herein, we reported that by using uniform Mo2C solid catalyst, aligned s-SWNT (∼90%) arrays with narrow-diameter distribution (∼85% between 1.0 and 1.3 nm) on quartz substrate can be obtained. Mo2C nanoparticles with monodisperse sizes were prepared by using molybdenum oxide-based giant clusters, (NH4)42[Mo132O372(H3CCOO)30(H2O)72]·10H3CCOONH4·300H2O(Mo132), as the precursor that was carburized by a gas mixture of C2H5OH/H2 during a temperature-programmed reduction. In this approach, the formation of volatile MoO3 was inhibited due to the annealing and reduction at a low temperature. As a result, uniform Mo2C nanoparticles are formed, and their narrow size-dispersion strictly determines the diameter distribution of SWNTs. During the growth process, Mo2C selectively catalyzes the scission of C-O bonds of ethanol molecules, and the resultant absorbed oxygen (Oads) preferentially etches metallic SWNTs (m-SWNTs), leading to the high-yield of s-SWNTs. Raman spectroscopic analysis showed that most of the s-SWNTs can be identified as (14, 4), (13, 6), or (10, 9) tubes. Our findings open up the possibility of the chirality-controlled growth of aligned-SWNTs using uniform carbide nanoparticles as solid catalysts for practical nanoelectronics applications.

Triplet–triplet exciton dynamics in single-walled carbon nanotubes

Semiconducting single-walled carbon nanotubes (SWNTs) are considered as building blocks for novel optoelectronic and photonic devices. Energy transport, dissipation and nonlinear optical properties of such devices depend critically on the dynamics of singlet and triplet excitons. However, little is known about triplet excitons in SWNTs despite their important role in photovoltaic, photoelectric and other applications. We present pump–probe and spin-sensitive photoluminescence studies of semiconducting SWNTs that allow the determination of the quantum yield of triplet formation (5 ± 2%), the triplet lifetime (30 ± 10 µs) and the triplet exciton size (0.65 nm). Triplet–triplet annihilation is also found to induce delayed fluorescence. The power-law decay of pump–probe and time-resolved photoluminescence signals is characteristic of diffusion-limited annihilation in one-dimensional systems and allows an estimation of the triplet diffusion constant of 0.1 cm2 s−1. This work suggests that exciton annihilation in SWNTs is reduced by one-dimensional confinement of diffusive exciton motion. Little is known about triplet excitons in semiconducting single-walled nanotubes, despite their importance in various applications. The pump–probe and spin-sensitive photoluminescence of such nanotubes is studied, and the quantum yield of triplet formation, triplet lifetime and triplet exciton size are found to be 5 ± 2%, 30 ± 10 µs and 0.65 nm, respectively.

Solid-State 13C NMR Assignment of Carbon Resonances on Metallic and Semiconducting Single-Walled Carbon Nanotubes

Solid-state (13)C NMR spectroscopy was used to investigate the chemical shift of nanotube carbons on m- and s-SWNTs (metallic and semiconducting single-walled nanotubes) for samples with widely varying s-SWNT content, including samples highly enriched with nearly 100% m- and s-SWNTs. High-resolution (13)C NMR was found to be a sensitive probe for m- and s-SWNTs in mixed SWNT samples with diameters of approximately 1.3 nm. The two highly enriched m- and s-SWNT samples clearly exhibited features for m- and s-SNWT (13)C nuclei (approximately 123 and 122 ppm, respectively) and were successfully fit with a single Gaussian, while five mixed samples required two Gaussians for a satisfactory fit.

Near-infrared Fourier transform photoluminescence spectrometer with tunable excitation for the study of single-walled carbon nanotubes

A fast, sensitive, automated Fourier transform (FT) photoluminescence (PL) spectrometer with tunable excitation has been developed for analyzing carbon nanotube suspensions over a wide spectral range. A commercially available spectrometer was modified by the addition of a tunable excitation source, custom collection optics, and computer software to provide control and automated data collection. The apparatus enables excitation from 400to1100nm and detection from 825to1700nm, permitting the analysis of virtually all semiconducting single-walled nanotubes (SWNTs), including those produced by the high pressure carbon monoxide conversion and laser processes. The FT approach provides an excellent combination of high sensitivity and fast measurement. The speed advantage exists because the entire emission spectrum is collected simultaneously, while the sensitivity advantage stems from the high optical throughput. The high sensitivity is demonstrated in the measurement of very dilute SWNT suspensions and the observation of novel spectral features, and the speed is demonstrated by measuring the real-time changes in the SWNT PL during rebundling. This contribution describes the assembly of components, the methods for automating data collection, and the procedures for correcting the wavelength-dependent excitation intensity and the interferometer and detector responses.

Electric-Field-Dependent Charge-Carrier Velocity in Semiconducting Carbon Nanotubes

Charge transport in semiconducting single-walled nanotubes (SWNTs) with Schottky-barrier contacts has been studied at high bias. We observe nearly symmetric ambipolar transport with electron and hole currents significantly exceeding 25 microA, the reported current limit in metallic SWNTs due to optical phonon emission. Four simple models for the field-dependent velocity (ballistic, current saturation, velocity saturation, and constant mobility) are studied in the unipolar regime; the high-bias behavior is best explained by a velocity-saturation model with a saturation velocity of 2 x 10(7) cm/s.

Polarized raman study of single-wall semiconducting carbon nanotubes

Polarized Raman spectra were obtained from a rope of aligned semiconducting single-wall nanotubes (SWNTs) in the vicinity of the D band and the G band. Based on group theory analysis and related theoretical predictions, the G-band profile was deconvolved into four intrinsic SWNT components with the following symmetry assignments: 1549 cm(-1) [E2(E(2g))], 1567 cm(-1) [A(A(1g))+E1(E(1g))], 1590 cm(-1) [A(A(1g))+E1(E(1g))] and 1607 cm(-1) [E2(E(2g))]. The frequency shifts of the tangential G modes from the 2D graphitelike E(2g(2)) frequency are discussed in terms of the nanotube geometry.