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Turbostratic carbon nanotubes (CNTs) and nanofibers (CNFs) are synthesized by chemical vapor deposition using titania nanoparticle catalysts, and a quantitative lift-off model is developed to explain CNT and CNF growth. Micron-scale long turbostratic CNTs and CNFs were observed when acetylene is utilized as a carbon feedstock, and an alumina substrate was incorporated to improve the homogeneity of catalyst distribution. Turbostratic CNTs/CNFs are always found attached to nanoparticle corners, in the absence of the graphitic cage that is typically observed with metal nanoparticle-mediated growth. The observed morphology in turbostratic CNTs/CNFs supports a model in which several layers of graphene lift off from high-curvature corners of the titania nanoparticle catalysts. This model explains a key feature, which differentiates the growth of turbostratic CNTs/CNFs via non-metallic nanoparticles from growth using standard metal nanoparticle catalysts. The observed CNT/CNF growth and the accompanying model can impact the assessment of other metal-oxide nanoparticle catalysts, with the findings here contributing to a metal-free synthesis of turbostratic CNTs/CNFs. [ABSTRACT FROM AUTHOR]

We present results of wavelength-dependent ultra-fast pump–probe experiments on micelle-suspended single-walled carbon nanotubes. The linear absorption and photoluminescence spectra of the samples show a number of chirality-dependent peaks and, consequently, the pump–probe results sensitively depend on the wavelength. In the wavelength range corresponding to the second van Hove singularities (VHSs) we observe subpicosecond decays, as has been seen in previous pump–probe studies. We ascribe these ultra-fast decays to intraband carrier relaxation. On the other hand, in the wavelength range corresponding to the first VHSs, we observe two distinct regimes in ultra-fast carrier relaxation: fast (0.3–1.2 ps) and slow (5–20 ps). The slow component, which has not been observed previously, is resonantly enhanced whenever the pump photon energy resonates with an interband absorption peak, and we attribute it to interband carrier recombination. Finally, the slow component is dependent on the pH of the solution, which suggests an important role played by H[sup +] ions surrounding the nanotubes. [ABSTRACT FROM AUTHOR]

Raman excitation profiles were generated between 695 and 985 nm for individual carbon nanotubes dispersed in aqueous solution. We confirmed that previously published spectral assignments for semi-conducting and metallic carbon nanotubes are able to predict the location and resonant maxima of radial breathing mode features in the Raman spectrum. Three large diameter features were observed within the excitation space over the scan range and accurately predicted as metallic species. There was significant agreement between predicted and observed Raman modes. However, one discrepancy is noted with the (6,4) nanotubes. This species is not observed when excited at or near its absorption transition. We find that the Raman cross-sections in general, assuming a diameter-based distribution of nanotubes, are disproportionately smaller for mod(n-m,3)=1 semi-conducting nanotubes than their counterparts by at least an order of magnitude. These results have important implications for the use of Raman spectroscopy to effectively characterize the chirality distribution of carbon nanotube samples. [ABSTRACT FROM AUTHOR]

In the absence of standard single-wall carbon nanotube samples with a well-known (n,m) population, we provide both a photoluminescence excitation (PLE) and resonance Raman scattering (RRS) analysis that together can be used to check the calculations for PLE and RRS intensities for carbon nanotubes. We compare our results with available models and show that they describe well the chirality dependence of the intensity ratio, confirming the differences between type 1 and type 2 semiconducting tubes [(2n+m) mod 3]=1 and 2, respectively, and the existence of a node in the radial breathing mode intensity for type 2 carbon nanotubes with chiral angles between 20° and 25°. [ABSTRACT FROM AUTHOR]

We demonstrate an alternative path for achieving high-transconductance organic transistors by assembling bilayers of pentacene onto random arrays of single-walled carbon nanotubes. We show here that, by varying the connectivity of the underlying nanotube network, the channel length of a thin-film transistor can be reduced by nearly two orders of magnitude—thus, enabling the increase of the device transconductance without reduction the on/off ratio. These field-induced percolating networks enable assembling high-transconductance transistors that, with relatively large source drain distances, can be manufactured with available commercial printing techniques. [ABSTRACT FROM AUTHOR]

We demonstrate an alternative path for achieving high transconductance organic transistors in spite of relatively large source to drain distances. The improvement of the electronic characteristic of such a scheme is equivalent to a 60-fold increase in mobility of the underlying organic semiconductor. The method is based on percolating networks, which we create from a dispersion of individual single-wall carbon nanotubes and narrow ropes within an organic semiconducting host. The majority of current paths between source and drain follow the metallic nanotubes but require a short, switchable semiconducting link to complete the circuit. With these nanotube-semiconducting composites we achieve effectively a 60× reduction in source to drain distance, which is equivalent to a 60-fold increase of the “effective” mobility of the starting semiconducting material with a minor decrease of the on/off current ratio. These field-induced percolating networks allow for the fabrication of high-transconductance transistors having relatively large source to drain distances that can be manufactured inexpensively by commercially available printing techniques. [ABSTRACT FROM AUTHOR]