Gas sensor based on single wall carbon nanotubes

Single-walled carbon nanotubes (SWNTs) are nowadays one of the most investigated materials and the realization of ordered SWNT structures is of fundamental importance for the improvement of many technological fields, from the non-linear optics to the realization of transistor, to the assembly of gas sensing devices. A SWNT is formed by rolling a graphene sheet into a seamless cylinder with a diameter on the nanometer scale. The individual SWNTs are joined each other and assembled into bundles by Van der Waals forces. Guest molecules can potentially interact with SWNTs via the outer surfaces of bundles, the inside of the tubes and /or the interstitial channels between the tubes in a bundle. These different situations are expected to play an important role in tuning the guest molecule/SWNT interaction during gas adsorption and/or desorption, and have been investigated theoretically and experimentally using different approaches. In particular, the interaction between gaseous molecules and SWNTs has been investigated from different point of view, including gas storage and gas detection through modification of electronic and thermal properties or through modification of the field emission properties. Compared with conventional solid-state sensors, that typically operate at temperatures over 200 °C, and conducting polymers-based sensors, that provide only limited sensitivity, sensing devices assembled with single-wall nanotubes can exhibit high sensitivity and fast response time at room temperature. Due to the high surface area of nanotubes, a little amount of nanotube material can provide many sites for gas interaction. The accessibility of these sites depends on the status of aggregation of the nanotubes. Our preliminary studies suggested that the sensitivity of a nanotube-based device can be optimized controlling the organization of the SWNTs. Ordered bundles of SWNTs exhibit indeed a sensitivity double with respect to that of a disordered deposit. This is likely due to the enhancement of surface area for organized SWNT systems with respect to randomly placed SWNT bundles. Hence, aligned nanotubes can serve as a very efficient material for use in gas detection. Directionality of SWNT can be obtained directly during the synthesis process, or after manipulation of dispersed nanotubes, by mean of several methods, such as filtration/deposition from suspension in strong magnetic fields, field emission, electrophoresis or dielectrophoretical processes. In particular the use of electric fields to move, position and align SWNTs has been reported in recent papers and the results indicate that both the electrophoresis (EP) and dielectrophoresis (DEP) routes have potential advantages for arranging nanotubes in controlled systems. Beyond the sensitivity, another severe constraint for gas detection is the time either for the reset of the sensor after exposure to the gas, either for the acceleration of the response itself. Since practical applications can be severely limited by slow absorption/desorption processes, we felt it worthwhile to investigate in a systematic way some physical parameters affecting the sensor response. In this thesis we present a study of NH3 ,NOx and H2 detection using organized SWNTs as sensing material and an innovative procedure to improve the time response of the sensor by applying a back gate voltage. Moreover study on gas detection and gas storage were done using QCM sensor.