Scanning gate spectroscopy and its application to carbon nanotube defects.

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.

Hydrogen sensing and sensitivity of palladium-decorated single-walled carbon nanotubes with defects.

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.

Atomistic oxidation mechanism of a carbon nanotube in nitric acid.

Motivated by recent experiments, we investigate how NO3-SWNT interactions become energetically favorable with varying oxidation state of a single-walled carbon nanotube (SWNT) using first-principles calculations. Chemisorption becomes less endothermic with respect to physisorption when the SWNT oxidation state is elevated. Importantly, the dissociative incorporation of an oxygen atom into the SWNT sidewall becomes highly favorable when the SWNT oxidation state is elevated from electron density depletion in the vicinity, as caused experimentally using electrochemical potential. The elevation of the SWNT oxidation state through accumulating local charge transfer from the surrounding molecules does not have the same effect. Our investigation reveals the crucial effects of the SWNT oxidation state in understanding the molecule-SWNT interaction.

Conductance-controlled point functionalization of single-walled carbon nanotubes.

We used covalent attachments to single-walled carbon nanotubes (SWNTs) to fabricate single-molecule electronic devices. The technique does not rely on submicrometer lithography or precision mechanical manipulation, but instead uses circuit conductance to monitor and control covalent attachment to an electrically connected SWNT. Discrete changes in the circuit conductance revealed chemical processes happening in real time and allowed the SWNT sidewalls to be deterministically broken, reformed, and conjugated to target species. By controlling the chemistry through electronically controlled electrochemical potentials, we were able to achieve single chemical attachments. We routinely functionalized pristine, defect-free SWNTs at one, two, or more sites and demonstrated three-terminal devices in which a single attachment controls the electronic response.