A. T. Johnson

Carbon nanotube composites for thermal management

Single-wall carbon nanotubes (SWNTs) were used to augment the thermal transport properties of industrial epoxy. Samples loaded with 1 wt % unpurified SWNT material showed a 70% increase in thermal conductivity at 40 K, rising to 125% at room temperature; the enhancement due to 1 wt % loading of vapor grown carbon fibers was three times smaller. Electrical conductivity data showed a percolation threshold between 0.1 and 0.2 wt % SWNT loading. The Vickers hardness rose monotonically with SWNT loading up to a factor of 3.5 at 2 wt %. These results suggest that the thermal and mechanical properties of SWNT-epoxy composites are improved, without the need to chemically functionalize the nanotubes.

Electrical and thermal transport properties of magnetically aligned single wall carbon nanotube films

Dense, thick films of aligned single wall carbon nanotubes and nanotube ropes have been produced by filtration/deposition from suspension in strong magnetic fields. Electrical resistivity exhibits moderate anisotropy with respect to the alignment axis, while the thermopower is the same when measured parallel or perpendicular to this axis. Both parameters have identical temperature dependencies in the two orientations. Thermal conductivity in the parallel direction exceeds 200 W/mK, within a decade of graphite.

High yield preparation of macroscopic graphene oxide membranes.

Graphene oxide membranes up to 2000 microm(2) in size can be synthesized with 90% yield in bulk quantities through a microwave-assisted chemical method. Membranes are readily visualized on an oxidized silicon substrate, which enables efficient fabrication of electronic devices and sensors. Field effect transistors made of the membrane show ambipolar behavior, and their conductivity is significantly higher than previously reported values.

Electrostatically-generated nanofibers of electronic polymers

Long nanofibers of conducting electronic polymers and their blends and also conventional polymers are conveniently fabricated in air by a non-mechanical electrostatic dispersion method. All fibers in a given preparation of certain polymers have diameters < 100nm. Fibers of 100% doped polyaniline as well as its blends in polymers such as polystyrene and polyethylene oxide have been prepared. Current/voltage in addition to conductivity/temperature relationships of single fibers as small as 419 nm have been obtained. Polyacrylonitrile and polystyrene fibers can be easily and uniformly coated from aqueous solution with conducting polypyrrole or with gold by electroless deposition. Polyacrylonitrile fibers can be thermally converted to conducting carbon nanofibers.

Electrospun polyaniline/polyethylene oxide nanofiber field-effect transistor

We report on the observation of field-effect transistor (FET) behavior in electrospun camphorsulfonic-acid-doped polyaniline/polyethylene oxide (PEO) nanofibers. Saturation channel currents are observed at surprisingly low source–drain voltages. The hole mobility in the depletion regime is 1.4×10−4 cm2/V s, while the one-dimensional (1-D) charge density (at zero gate bias) is calculated to be approximately 1 hole per 50 two-ring repeat units of polyaniline, consistent with the rather high channel conductivity (∼10−3 S/cm). Reducing or eliminating the PEO content in the fiber is expected to enhance device parameters. Electrospinning is thus proposed as a simple method of fabricating one-dimensional polymer FETs.

Nonvolatile Molecular Memory Elements Based on Ambipolar Nanotube Field Effect Transistors

We have fabricated air-stable n-type, ambipolar carbon nanotube field effect transistors (CNFETs) and used them in nanoscale memory cells. n-Type transistors are achieved by annealing nanotubes in hydrogen gas and contacting them by cobalt electrodes. Scanning gate microscopy reveals that the bulk response of these devices is similar to gold-contacted p-CNFETs, confirming that Schottky barrier formation at the contact interface determines accessibility of electron and hole transport regimes. The transfer characteristics and Coulomb blockade (CB) spectroscopy in ambipolar devices show strongly enhanced gate coupling, most likely due to reduction of defect density at the silicon/silicon-dioxide interface during hydrogen anneal. The CB data in the “on”-state indicates that these CNFETs are nearly ballistic conductors at high electrostatic doping. Due to their nanoscale capacitance, CNFETs are extremely sensitive to the presence of individual charges around the channel. We demonstrate that this property can b...

Controlled fabrication of nanogaps in ambient environment for molecular electronics

We have developed a controlled and highly reproducible method of making nanometer-spaced electrodes using electromigration in ambient lab conditions. This advance will make feasible single molecule measurements of macromolecules with tertiary and quaternary structures that do not survive the liquid-helium temperatures at which electromigration is typically performed. A second advance is that it yields gaps of desired tunneling resistance, as opposed to the random formation at liquid-helium temperatures. Nanogap formation occurs through three regimes: First it evolves through a bulk-neck regime where electromigration is triggered at constant temperature, then to a few-atom regime characterized by conductance quantum plateaus and jumps, and finally to a tunneling regime across the nanogap once the conductance falls below the conductance quantum.

Fabrication and electrical characterization of polyaniline-based nanofibers with diameter below 30 nm

We fabricate and electrically characterize electrospun nanofibers of doped polyaniline/polyethylene oxide (PAn/PEO) blend with sub-30 nm diameter. Fiber diameters near 5 nm are obtained for optimized process parameters. Scanning conductance microscopy (SCM) shows that fibers with diameter below 15 nm are electrically insulating; the small diameter may allow complete dedoping in air or be smaller than phase-separated grains of PAn and PEO. Electrical contacts to nanofibers are made by shadow mask evaporation with no chemical or thermal damage to the fibers. Single fiber I–V characteristics show that thin fibers conduct more poorly than thick ones, in agreement with SCM data. I–Vs of asymmetric fibers are rectifying, consistent with formation of Schottky barriers at the nanofiber-metal contacts.

DNA-Decorated Graphene Chemical Sensors

Graphene is a two-dimensional material with exceptional electronic properties and enormous potential for applications. Graphene’s promise as a chemical sensor material has been noted but there has been little work on practical chemical sensing using graphene, and in particular, how chemical functionalization may be used to sensitize graphene to chemical vapors. Here we show one route towards improving the ability of graphene to work as a chemical sensor by using single stranded DNA as a sensitizing agent. The resulting devices show fast response times, complete and rapid recovery to baseline at room temperature, and discrimination between several similar vapor analytes.

Controlled creation of a carbon nanotube diode by a scanned gate

We use scanning gate microscopy to precisely locate the gating response in field-effect transistors (FETs) made from semiconducting single-wall carbon nanotubes. A dramatic increase in transport current occurs when the device is electrostatically doped with holes near the positively biased electrode. We ascribe this behavior to the turn-on of a reverse biased Schottky barrier at the interface between the p-doped nanotube and the electrode. By positioning the gate near one of the contacts, we convert the nanotube FET into a rectifying nanotube diode. These experiments both clarify a longstanding debate over the gating mechanism for nanotube FETs and indicate a strategy for diode fabrication based on controlled placement of acceptor impurities near a contact.