Sharvil Desai

Adsorption of oxygen molecules on individual single-wall carbon nanotubes

Our study of the adsorption of oxygen molecules on individual semiconductiong single-walled carbon nanotubes at ambient conditions reveals that the adsorption is physisorption, the resistance without O2 increases by approximately two orders of magnitude as compared to that with O2, and the sensitive response is due to the pinning of the Fermi level near the top of the valence band of the tube, resulting from impurity states of O2 appearing above the valence band.

Morphological, structural, and chemical effects in response of novel carbide derived carbon sensor to NH3, N2O, and air.

The response of two carbide derived carbons (CDCs) films to NH(3), N(2)O, and room air is investigated by four probe resistance at room temperature and pressures up to 760 Torr. The two CDC films were synthesized at 600 (CDC-600) and 1000 degrees C (CDC-1000) to vary the carbon morphology from completely amorphous to more ordered, and determine the role of structure, surface area, and porosity on sensor response. Sensor response time followed kinetic diameter and indicated a more ordered carbon structure slowed response due to increased tortuosity caused by the formation of graphitic layers at the particle fringe. Steady state sensor response was greater for the less-ordered material, despite its decreased surface area, decreased micropore volume, and less favorable surface chemistry, suggesting carbon structure is a stronger predictor of sensor response than surface chemistry. The lack of correlation between adsorption of the probe gases and sensor response suggests chemical interaction (charge transfer) drive sensor response within the material; N(2)O response, in particular, did not follow simple adsorption behavior. Based on Raman and FTIR characterization, carbon morphology (disorder) appeared to be the determining factor in overall sensor response, likely due to increased charge transfer between gases and carbon defects of amorphous or disordered regions. The response of the amorphous CDC-600 film to NH(3) was 45% without prior oxidation, showing amorphous CDCs have promise as chemical sensors without additional pretreatment common to other carbon sensors.

Hypergolic fuel detection using individual single walled carbon nanotube networks

Accurate and reliable detection of hypergolic fuels such as hydrazine (N2H4) and its derivatives is vital to missile defense, aviation, homeland security, and the chemical industry. More importantly these sensors need to be capable of operation at low temperatures (below room temperature) as most of the widely used chemical sensors operate at high temperatures (above 300 °C). In this research a simple and highly sensitive single walled carbon nanotube (SWNT) network sensor was developed for real time monitoring of hydrazine leaks to concentrations at parts per million levels. Upon exposure to hydrazine vapor, the resistance of the air exposed nanotubes (p-type) is observed to increase rapidly while that of the vacuum-degassed nanotubes (n-type) is observed to decrease. It was found that the resistance of the sample can be recovered through vacuum pumping and exposure to ultraviolet light. The experimental results support the electrochemical charge transfer mechanism between the oxygen redox couple of the ...

Desorption kinetics of oxygen in plasma treated SWNTs by in situ thermoelectric power measurements.

The functionalization and defect formation of SWNTs caused by isotropic plasma treatments were studied using oxygen desorption/adsorption kinetics by measuring the time dependence of the in situ thermoelectric power (TEP). It is shown that the plasma treatments result in the formation of low binding energy sites for oxygen adsorption. Raman and x-ray photoelectron spectroscopy (XPS) data are in good agreement with the results.

Studies of electronic distribution in potassium-doped mats of single-walled carbon nanotubes, double-walled carbon nanotubes, and peapods

In this work, we have performed a systematic study of the electrical transport properties (both the resistivity and the thermoelectric power) on bundles of single-walled carbon nanotubes (SWCNTs), double-walled carbon nanotubes (DWCNTs), and peapods (derived from the same batch of initial SWNTs) during in situ doping with potassium (donor). The charge transfer effects in K-doping are similar in SWNTs and DWNTs, but drastically different in C60 at SWNTs. An impurity level associated with the chain of C60 in the band gap of the tube with the Fermi level pinned near the top of the valence band, leading to the p-type behavior for the peapod. Significant charge transfer is found to occur only in the case of very low coverage of these nanostructures.

Individual SWNT based thermoelectric power chemical sensors

Electrical properties of individual Single-Walled Carbon Nanotube/rope in the configurations of 2-probe resistance, field effect transistor (FET), and thermopower have been measured. It is shown that oxygen adsorption in SWNTs is indeed a physisorption process. The p-type behavior of SWNTs in the ambient is believed to be due to the Fermi level pinning at impurity states of O2- near the top of the valence band of the tube. Chemisorption processes involving ammonia and nitrous oxide have been explored by studying FET properties. The thermoelectric power of individual ropes of SWNTs is measured and related to the FET properties.

Alcohol Sensing using Individual Single-Walled Carbon Nanotubes

We have measured the 2 probe resistance of an individual Single Walled Carbon Nanotube (SWNT) device and investigated the change in the resistance due to the exposure to various organic vapors. These results are reversible and reproducible over many cycles of exposure of organic vapors.