Application of Carbon Nanotube as Capacitive Sensor

Capacitive sensors can directly sense a variety of things such as motion, chemical composition, electric field and, indirectly, sense many other variables which can be converted into motion or dielectric constant, such as pressure, acceleration, fluid level, and fluid composition. Besides, carbon nanotubes because of their significant electrical and mechanical properties can be used as an essential component in different types of nano-electromechanical systems (NEMS). However, theoretical investigation of the capacitance in CNT-based systems have only just begun [1,2].This paper reports on the design of NEMS one-part variable capacitor by carbon nanotubes. Carbon nanotubes can be categorized as conductor and semiconductor. According to one of the recent studies, in a semiconducting nanotube with a energy gap, Eg, charge separation occurs as F exceeds the threshold value Fth=Eg/eh (here h is nanotube length and F is external electric field). For F>Fth, positively and negatively charged regions at the ends of the nanotube are separated by a neutral strip in the middle. Thus a semiconducting carbon nanotube can be simulated as a capacitor with conductor and dielectric parts. Torsional or axial strain in a carbon nanotube can change its band gap energy and the length of neutral strip which its magnitude depends on the nanotube chirality. Therefore, the charge sum and electrical capacitance of a semiconducting carbon nanotube under a given voltage change. In this way different parts of a conventional cantilever or bridge capacitor reduce to a one part nanotube capacitor. Numerical simulation is employed to investigate the behavior of this device to design variable capacitor. This nano-scale, one-part capacitor can be useful for future sensor and actuator applications.