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.
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.
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.
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. KeywordsCarbon nanotube; scanning gate microscopy; defect; molecular electronics; electronic spectroscopy Scanning probe microscopies are enabling tools of nanoscience, providing a range of information to characterize topography as well as electronic, magnetic, chemical, and other physical properties. 1 In some modes, particularly tunneling current and magnetic force measurements, the standard implementation scans over an energy range in order to achieve spectroscopic detail. 1 Tunneling spectroscopy is not, however, particularly applicable to molecular scale electronic devices fabricated on gate oxides. For such devices, transport spectroscopy 2 can identify the presence of discrete electronic levels, but ideally one also desires spatial interrogation of these electronic features. Here, we combine the powerful principle of energy spectroscopy with the scanning probe technique of scanning gate microscopy (SGM). SGM is ideal for imaging local, electronic inhomogeneities, and converting this imaging mode into an electrostatic spectroscopy produces a tool well suited to examining functioning molecular devices and their local inhomogeneities.In conventional SGM, the scanning probe is a noncontact atomic force microscopy (AFM) cantilever, doped or coated to be conductive at its apex, that serves as a movable source of electric fields. 1 The long range interaction of these fields with a surface both probes and redistributes carriers and currents, making the technique particularly relevant to the characterization of operational electronic devices. An SGM image, which is usually acquired simultaneously with surface topography, represents changes in device conductance that accompany the probe movement for each location on a two-dimensional surface.
Uniform, conformal coating of MnO 2 onto single walled carbon nanotubes is achieved with precise thickness control and without the introduction or utilization of defect sites. The resulting composite electrodes enable electrochemical testing of novel carbon-metal oxide composites in which rate-enhancing, fast-electrontransfer defect sites are completely absent. Such sites are ubiquitous and believed to be enormously consequential to the electron transfer properties of graphitic carbon systems, and the techniques described here provide an experimental route to quantitatively study such effects. For example, we immediately discern the enhanced interfacial resistance that occurs in the defect-free system.
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