CARBON NANOTUBE DEVICES FOR SENSING APPLICATIONS

Abstract

Semiconducting single-walled carbon nanotube (SWNT) field-effect transistor (FET) is one of the most promising applications at nanoscale dimensions due to their superior conductivity, short channel immunity and complementary metal-oxide-semiconductor (CMOS) compatibility. However, semiconducting CNTs are also highly promising for gas sensing applications due to the high surface area to volume ratio, thermal and chemical stability, ease of functionalizing CNTs’ surfaces. The SWNTs show higher response to the adsorbed gases molecules due to the semiconducting single shell. The MWNTs can have a mixture of metallic and semiconducting CNTs shells. However, the metallic CNTs do not show a significant change in the carrier concentration after the adsorption of the gas molecules. The CNT-based gas sensors are cost effective, consume low power, and can be operated at room temperature. Further, the devices with aligned CNT-based sensors have faster response and recovery times along with high sensitivity. Therefore, semiconducting CNTs can be used to replace conventional semiconductors in gas sensing and electronics applications. In this thesis, the devices with aligned semiconducting CNT channels (two and three terminals) are fabricated for gas sensing applications. The fabrication of semiconducting CNT-based devices involves the electrode fabrication, dispersion of CNTs in optimum solution, alignment between the electrodes, and decoration with nanoparticles (NPs) to tune their properties. The various types of electrode structures have been fabricated by conventional lithography process, which were then integrated with CNTs to realize sensing devices. The electrodes are fabricated on a thermally grown silicon dioxide (SiO2) on silicon (Si) substrate. The typical minimum finger width of interdigitated electrodes is 2.4 μm and the minimum separation between the two finger electrodes is 3 μm. The CNT suspension is drop-cast over the fabricated electrodes and the CNTs are then aligned between the electrodes by using the di-electrophoresis method. Thus, a channel of semiconducting CNTs is formed between the electrodes. The currentvoltage characteristics of the fabricated devices show characteristics like back-to-back connected Schottky diodes. The current transport between the electrodes is determined by the metal-CNT interface and conductivity of the channel (CNT).