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A single point defect surrounded on either side by quasi-ballistic, semimetallic carbon nanotube is a nearly ideal system for investigating disorder in one-dimensional (1D) conductors and comparing experiment to theory. Here, individual single-walled nanotubes (SWNTs) are investigated before and after the incorporation of single point defects. Transport and local Kelvin Probe force microscopy independently demonstrate high-resistance depletion regions over 1.0 μm wide surrounding one point defect in semimetallic SWNTs. Transport measurements show that conductance through such wide depletion regions occurs via a modified, 1D version of Poole-Frenkel field-assisted emission. Given the breadth of theory dedicated to the possible effects of disorder in 1D systems, it is surprising that a Poole-Frenkel mechanism appears to describe defect scattering and resistance in this semimetallic system.

A variation of scanning gate microscopy (SGM) is demonstrated in which this imaging mode is extended into an electrostatic spectroscopy. Continuous variation of the SGM probe's electrostatic potential is used to directly resolve the energy spectrum of localized electronic scattering in functioning, molecular scale devices. The technique is applied to the energy-dependent carrier scattering that occurs at defect sites in carbon nanotube transistors, and fitting energy-resolved experimental data to a simple transmission model determines the electronic character of each defect site. For example, a phenolic type of covalent defect is revealed to produce a tunnel barrier 0.1 eV high and 0.5 nm wide.

Single molecule experimental methods have provided new insights into biomolecular function, dynamic disorder, and transient states that are all invisible to conventional measurements. A novel, non-fluorescent single molecule technique involves attaching single molecules to single-walled carbon nanotube field-effective transistors (SWNT FETs). These ultrasensitive electronic devices provide long-duration, label-free monitoring of biomolecules and their dynamic motions. However, generalization of the SWNT FET technique first requires design rules that can predict the success and applicability of these devices. Here, we report on the transduction mechanism linking enzymatic processivity to electrical signal generation by a SWNT FET. The interaction between SWNT FETs and the enzyme lysozyme was systematically dissected using eight different lysozyme variants synthesized by protein engineering. The data prove that effective signal generation can be accomplished using a single charged amino acid, when appropriately located, providing a foundation to widely apply SWNT FET sensitivity to other biomolecular systems.

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.

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 (T880 K), thermal processing results in high resistance contacts with thermally activated barriers. Higher temperatures (T880 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.

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. © 2009 American Chemical Society.

Using point-functionalized carbon nanotube devices, we demonstrate continuous, multihour monitoring of a single carboxylate group interacting with its immediate environment. The conductance of the nanotube device directly transduces single-molecule attachments and detachments in the presence of the carboxylate-selective reagent 1-ethyl-3-(3-dimethylaminopropyl) carbodiimide hydrochloride (EDC). Because only one carboxylate is present in the device, it can be studied through hundreds of reactions, providing the statistical accuracy to directly determine a 12 s lifetime of the carboxy−EDC complex. An additional instability of the complex is transduced in real time and observed to have a median lifetime of 2 ms. By determining a turnover time in good agreement with bulk measurements and simultaneously illuminating additional dynamics, these results demonstrate this platform's potential for complementing optical methods in single-molecule research.

Transient grating measurements affirm the excitonic model for single-walled carbon nanotubes (SWNT) by identifying the dark exciton (D) as the population relaxation bottleneck in semiconducting-SWNT (S-SWNT). The data allow the reconstruction of the kinetics of excitonic cascade and cooling, from band continuum to vibrational cooling in the ground electronic state. In S-SWNT, the intraband relaxation occurs in 40 fs, localization into the 2 g exciton occurs in 50 fs, followed by the excitonic cascade: 2g f 1u f D f 1g with time constants of 175 fs, 3 ps, 300 ps, respectively. Fluorescence from the 1 u state is quenched by efficient population transfer to 1D dark exciton. In metallic tubes, cooling is completed on the time scale of 1 ps.

The temperature-dependent noise of individual, single-walled carbon nanotubes is measured here in a variety of different gases and liquids. The ambient environment is found to have only a weak relationship with device noise, even in cases where adsorption significantly changes the dc resistance. Correspondingly, a 450 K degassing procedure typically reduces the device noise by only 1 order of magnitude. An important exception to this finding is a pronounced, 100-fold increase in noise observed near gas -liquid phase transitions of the ambient. Wide-range temperature scans clearly identify the condensation of N 2 ,H 2, and CH4 onto metallic nanotubes, but not the sublimation of CO2. The observations suggest that nanotube devices can directly transduce ambient density fluctuations, though without an inherent gas specificity. Even so, the method is a particularly sensitive characterization of nanotube chemical interactions, one which is successful even for the extreme case of inert gases adsorbed on metallic nanotubes. Field-effect transistors, chemical sensors, and field emission sources and displays 1 are three carbon nanotube (CNT) research areas seeing preliminary commercialization efforts. In all three applications, and particularly in the first two, high levels of electronic noise threaten commercial viability. The noise levels of CNT devices are substantially higher than those in macroscopic films or materials of comparable resistance, 2 so controlling noise may be critical to the success of these applications. Despite this potential hurdle, noise sources in CNTs have remained relatively poorly characterized, particularly in comparison to the low temperature, quantum properties of these circuits. While the exact mechanisms have not yet been identified, the observed noise magnitude of CNTs 2-6 is compatible with the empirical Hooge observation 7 that 1/f noise scales inversely to the number of carriers in a system. Large amplitude noise is common in nanoscale circuits, 8,9 and a typical single-walled nanotube (SWNT) conductor 100 nm long incorporates less than 10 000 atoms and even fewer free carriers. The Hooge law predicts a 1/f noise singularity in very small circuits which is of interest from both fundamental and applied points of view, and SWNT circuits represent an ideal model system to further examine this issue. 9