Nanoscale Gas Sensors and Their Detection Mechanisms: Carbon Nanotubes and Beyond
Boyd, Anthony Karol
The research presented in this thesis focuses on the experimental investigation of nanoscale electronic devices for gas sensing applications. The majority of the experiments were conducted on carbon nanotube field-effect transistors (CNTFETs) made with variable density carbon nanotube networks. The carbon nanotube networks were grown using chemical vapor deposition on doped silicon wafers capped with silicon dioxide. Contact electrodes were attached to the networks with standard e-beam lithography and thin film deposition techniques. To better understand the sensing mechanism of CNTFETs, numerous samples were fabricated with varying densities of nanotubes and nanotube junctions. These samples were exposed to nitrogen dioxide and the change in conductance was recorded. Selected parts of the device were then passivated with a thick photoresist to determine whether they contribute to the sensing mechanism.Our previous work showed that for devices made with a single CNT, the response to nitrogen dioxide was mainly due to modifications at the contact interfaces rather than molecular adsorption on the nanotube sidewalls. However, here we show that when using CNT networks, both gas sensing mechanisms are involved. We will illustrate this through the comparison of the experimental response of high-density versus low-density CNT networks and show that the effect of adsorption is linked to the number of CNT junctions, or cross-over points, in the network. Adsorption plays a major role for high-density networks. Its effect is much weaker for low-density networks and not measureable in a single nanotube, where the response is mainly due to the electrodes.Experiments were also performed on field-effect transistor devices with molybedenum disulfide (MoS2) being substituted for the nanotubes. These devices were fabricated using the same substrates and metal deposition techniques used for carbon nanotube devices; however the MoS2 was mechanically exfoliated onto the wafers. Their gas sensing properties and majority carrier mobilities were explored. While testing these devices, we found that the choice of metals used for electrodes had a great impact on device behavior. In addition to their gas sensing properties, the potential use for optoelectronics was studied.
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Boyd, Anthony Karol (Georgetown University, 2013)The research presented in this thesis focuses on the experimental investigation of nanoscale electronic devices for gas sensing applications. The majority of the experiments were conducted on carbon nanotube field-effect ...
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