Individual single-walled carbon nanotubes (SWCNTs) become sensitive to H(2) gas when their surfaces are decorated with Pd metal, and previous reports measure typical chemoresistive increases to be approximately 2-fold. Here, thousand-fold increases in resistance are demonstrated in the specific case where a Pd cluster decorates a SWCNT sidewall defect site. Measurements on single SWCNTs, performed both before and after defect incorporation, prove that defects have extraordinary consequences on the chemoresistive response, especially in the case of SWCNTs with metallic band structure. Undecorated defects do not contribute to H(2) chemosensitivity, indicating that this amplification is due to a specific but complex interdependence between a defect site's electronic transmission and the chemistry of the defect-Pd-H(2) system. Dosage experiments suggest a primary role is played by spillover of atomic H onto the defect site.

Dual color four-wave-mixing (FWM) microscopy is used to spatially resolve the third-order optical response from individual carbon nanotubes. Good signal-to-noise is obtained from single-walled carbon nanotubes (SWNT) sitting on substrates, when the excitation beams are resonant with electronic transitions of the nanotube, by detecting the FWM response at the anti-Stokes frequency. Whereas the coherent anti-Stokes (CAS) signal is sensitive to both electronic and vibrational resonances of the material, it is shown that the signal from individual SWNTs is dominated by the electronic response. The CAS signal is strongly polarization dependent, with the highest signals found parallel with the enhanced electronic polarizibility along the long axis of the SWNT.

Using a model system of single, isolated carbon nanotubes loaded with high-capacitance metal-oxide films, we have quantitatively investigated electrochemical composites on the single-nanotube scale. Electrochemical charging and discharging of a model MnO2 storage material was used to probe interfacial charge transfer and surface impedances at the nanotube interface. We found that one single-walled carbon nanotube has an apparent surface resistivity of 30 mΩ cm(2), approximately 4 times smaller than for a multiwalled carbon nanotube and 50 times smaller than the 1.5 Ω cm(2) resistivity of Pt or graphite films. The improvement originates in the electrochemical-transport properties of microelectrodes shrunk to a nanotube's dimensions rather than any unique nanotube property like curvature, bandstructure, or surface chemistry. In explaining the enhanced performance of certain nanotube-containing composites, the results overturn widely held assumptions about nanotubes' roles while also providing guidelines for optimizing effective composites.

We investigate electronic devices consisting of individual, metallic, single-walled carbon nanotubes contacted by Pt electrodes in a field effect transistor configuration, focusing on improvements to the metal-nanotube contact resistance as the devices are annealed in inert environments including ultrahigh vacuum. At moderate temperatures (T < 880 K), thermal processing results in high resistance contacts with thermally activated barriers. Higher temperatures (T > 880 K) achieve nearly transparent contacts. In the latter case, analytical surface measurements reveal the catalytic decomposition of hydrocarbons into graphene layers on the Pt surface, suggesting that improved electronic behavior is primarily due to the formation of an all-carbon nanotube-graphite interface rather than to the improvement of the nanotube-Pt one.

We review an extensive study of the factors that influence the intensity of coherent, nonlinear four wave mixing (FWM) in carbon nanotubes, with particular attention to the variability inherent to single-walled carbon nanotubes (SWNTs). Through a combination of spatial imaging and spectroscopy applied to hundreds of individual SWNTs in optoelectronic devices, the FWM response is shown to vary systematically with free carrier concentration. This dependence is manifested both in the intrinsic SWNT bandstructure and also by extrinsic and environmental effects. We demonstrate the sensitivity of the SWNT FWM signal by investigating SWNTs transferred from one substrate to another, before and after the introduction of chemical damage, and with chemical and electrostatic doping. The results demonstrate FWM as a sensitive technique for interrogating SWNT optoelectronic properties. I. INTRODUCTION Carbon nanotubes (CNTs) display a rich set of optical and optoelectrical properties 1-3 that render them promising candidates for nanoscopic optoelectronic building blocks in circuits. This prospect motivates the need to develop sensitive tools for characterizing the optical properties of CNT devices. Since CNT fabrication methods generally produce heterogeneous mixtures of nanotubes, correlating the optical response to the electronic structure of CNTs is best carried out at the single nanotube level. In this regard, the sensitivity of optical microscopy techniques, employing either near-field or far-field detection, has proven sufficient for detailed examinations of the optical properties of individual nanotubes. 4 Whereas the linear optical properties of CNTs have been the topic of numerous studies, the nonlinear optical properties of nanotubes have received comparatively little attention. CNTs are known to exhibit high third-order nonlinear susceptibilities, 5, 6 a property that has propelled their use as saturable absorbers in laser cavities. 7, 8 The high optical nonlinearity also enables nonlinear spectroscopic investigations, which offer a closer look at the ultrafast carrier dynamics in nanotubes. Nonlinear optical measurements, and four-wave mixing (FWM) experiments in particular, can be optimized to reveal detailed information on the ultrafast evolution of optical excitations; information that remains hidden in linear spectroscopic measurements. The sensitivity of FWM techniques to both electronic and vibrational excitations makes them attractive probes for dissecting the nonlinear optical response of CNTs. Photon echo 9 and coherent Raman FWM 10 experiments, for instance, have enabled direct recordings of electronic and phonon dephasing, which hold important clues toward the extent of exciton-exciton and exciton-phonon interactions in CNTs. We have recently shown that the FWM technique can be extended to the level of individual nanotubes. 11 By making use of a dual-color picosecond excitation scheme, we collected coherent anti-Stokes Raman scattering (CARS) signals from both metallic and semi-conducting single-walled c...