Vertically Aligned Carbon Nanotubes Capacitive Sensors

Capacitive sensors are key components in a wide range of sensing applications, such as gas detectors and safety systems. While the mainstream silicon sensors demonstrated good performances, vertically aligned carbon nanotubes (VA-CNTs) outperform them and, therefore, are an attractive candidate material for capacitive applications owing to their extremely high surface area. Specifically, their top (crust) layer is characterized by a porous and dense morphology that further enhances the capacitive area and, consequently, induces an electrostatic fringe field and an enhancement of the capacitance. In this paper, we study how the porosity of VA-CNTs determines their electrostatic behavior. We observed that the porous surface of the crust layer generates a significant enhancement of the capacitance comparing to parallel plate capacitors. The rough surface of the crust layer results in amplification of the VA-CNT effective capacitor area, which further increases with the electrostatic gap. As-grown VA-CNTs with lower porosity demonstrate higher capacitance; however, densified samples and samples reinforced with conductive nano-particles did not show an enhancement of the capacitance, due to a significant change in their CNT formation. Thus, this paper emphasizes the influence of the morphological structure of VA-CNTs on their capacitance and enriches the material library that can be used for capacitive sensing.