Jay Gaillard

Carbon-nanotube-based resonant-circuit sensor for ammonia

We present the design and development of highly sensitive and fast-responsive microwave resonant sensors for monitoring the presence of ammonia gas. The sensor consists of a circular disk electromagnetic resonant circuit coated with either single- or multiwalled carbon nanotubes that are highly sensitive to adsorbed gas molecules. Upon exposure to ammonia, the electrical resonant frequency of the sensor exhibits a dramatic downshift of 4.375 MHz. The recovery and response times of these sensors are nominally 10 min. This technology is suitable for designing remote sensor systems to monitor gases inside sealed opaque packages and environmental conditions that do not allow physical wire connections.

Photoinduced oxidation of carbon nanotubes

Photoinduced phenomena are of general interest for new materials. Recently, photoinduced molecular desorption of oxygen has been reported in carbon nanotubes. Here we present, using thermopower measurements, that carbon nanotubes when exposed simultaneously to UV light and oxygen exhibit photoinduced oxidation of the nanotubes. At least two plausible mechanisms for the experimentally observed photoinduced oxidation are proposed: (i) a lower energy barrier for the adsorption of photo-generated singlet oxygen, or (ii) due to the presence of defects in carbon nanotubes that may facilitate the formation of locally electron-deficient and electron-rich regions on the nanotubes which facilitate the adsorption of oxygen molecules on the nanotubes.

Mechanical properties of chemical vapor deposition-grown multiwalled carbon nanotubes

The bending modulus (Young’s modulus) of several chemical vapor deposition-grown multiwalled nanotubes (MWNTs) have been measured using a vibrating reed technique. Three different precursors were used to produce MWNTs with differing densities of defects in the tube walls. Individual MWNTs were electrostatically driven in air over a dark-field light microscope and the bending modulus of the nanotubes was determined from the frequency of the first vibrational resonance. A correlation between the defect density and the bending modulus was found which implies that the bending modulus is relatively more sensitive to wall defects than the nanotube diameter.

Tuning electrical and thermal connectivity in multiwalled carbon nanotube buckypaper

We find that the electrical and thermal connectivity in multiwalled carbon nanotube buckypaper can be tuned using a spark plasma sintering (SPS) technique. Elevated SPS temperatures promote the formation of inter-tube connections and consequently impact the electrical resistivity, thermoelectric power and thermal conductivity of the buckypaper. In particular, the electrical resistivity as a function of SPS temperature exhibits a percolation-type behavior while the low temperature lattice thermal conductivity shows a crossover behavior in the sample dimensionality. The results are discussed in terms of the quasi-one-dimensional metallic nature of multiwalled carbon nanotubes, the packing density and the electron-phonon coupling.

Electrical detection of oscillations in microcantilevers and nanocantilevers

Precise determination of the resonant frequency, phase, and quality factor in micromechanical and nanomechanical oscillators would permit, among other things, (i) the detection of trace amounts of adsorbed molecules through a shift in the resonant frequency, and (ii) pressure variations in the environment which affect the mechanical damping of the oscillator. The major difficulty in making these measurements in many cases is the ancillary equipment such as lasers or high magnetic fields that must be used. Being able to make precise measurements with a fully electrical actuation and detection method would greatly extend the usefulness of these oscillators. Detecting the oscillation through changes in the capacitance between the oscillator and a counter electrode is difficult because the static capacitance between them as well as the parasitic capacitance of the rest of the circuitry overwhelm the detection. We have found that the charge on a microcantilever or nanocantilever when driven by a nearby counter...

Three-way electrical gating characteristics of metallic Y-junction carbon nanotubes

Y-junction based carbon nanotube (CNT) transistors exhibit interesting switching behaviors, and have the structural advantage that the electrical gate for current modulation can be formed by any of the three constituent branches. In this letter, we report on the gating characteristics of metallic Y-CNT morphologies. By measuring the output conductance and transconductance we conclude that the efficiency and gain depend on the branch diameter and is electric field controlled. Based on these principles, we propose a design for a Y-junction based CNT switching device, with tunable electrical properties.

Development of RF carbon nanotube resonant circuit sensors for gas remote sensing applications

We present the design, development and analysis of highly sensitive and ultra-fast responsive electromagnetic microwave resonant sensors for monitoring the presence of gas. These novel sensors consist of circular electromagnetic resonant circuits coated with single and multi walled carbon nanotubes (SWNT & MWNT) that are highly sensitive to adsorbed gas molecules. Upon exposure to ammonia, the electrical resonant frequency of the sensor exhibits a frequency shift of as high as 6.25 MHz. The recovery and response times of these sensors is nominally 10 minutes when operating at room temperatures. This sensor technology is suitable for designing remote sensing systems to monitor gases inside sealed opaque packages and for environmental conditions that do not allow physical wire connections.

The role of γ-iron nanoparticulates in the growth of carbon nanotubes

Carbon nanotubes (∼200nm diameter) are grown by chemical vapor deposition using catalytic iron particles. Mossbauer spectroscopy enables differentiation among relatively large Fe3C, α-Fe, and nanosized superparamagnetic fcc γ-Fe particles. The antiferromagnetic configuration of γ-Fe nanoparticles yields a significant fraction of uncompensated spins, producing a weak ferromagnetism that allows estimation of size (2–3nm) via magnetization in zero field versus variable field cooling. This property of γ-Fe nanoparticles has not been previously employed. We propose that the surfaces of 200nm iron carbide particles are covered with nanosized γ-Fe and graphitized carbon that participate in the catalytic growth of nanotubes.

Growth and characterization of Bi/sub 2/Te/sub 3/ nanostructures

Bulk Bi/sub 2/Te/sub 3/ is one of the best known thermoelectric materials with a ZT /spl sim/1 at room temperature. Theoretical studies have suggested that low-dimensional materials may exhibit ZT values that exceed 1. In this study, we used the pulsed laser deposition method to prepare Bi/sub 2/Te/sub 3/ nanostructures by ablating a rotating Bi/sub 2/Te/sub 3/ target in an inert atmosphere. Silicon or quartz substrates are pretreated with poly-1-lysine to form an adhesion layer for 10, 20, and 30 nm colloidal Au particles which serve as catalyst seed particles for the growth of the nanostructures. Alternatively, we have also prepared Bi/sub 2/Te/sub 3/ nanostructures by subliming Bi/sub 2/Te/sub 3/ powder in the presence of gold coated substrates. Results from electron microscopy and vibrational spectroscopic studies are presented.