A high performance humidity sensor based on dielectric detection with a novel coaxial nanostructure*

High throughput coaxial nanocavity arrays are developed by overlaying porous Al$_{2}$O$_{3}$ and Al layers on vertically aligned carbon nanotube arrays. The porosity of Al$_{2}$O$_{3}$ was electrochemically characterized. The dielectric properties of the nanocoax structure were measured by impedance spectroscopy, from 10 mHz to 1 MHz. The capacitance of the sensor responded to humidity applied to the chip, $i.e.$ soaking the array with water increased the capacitance by 130{\%}. The detection mechanism was established for sensing changes to the dielectric constant due to adsorbed moisture in the porous Al$_{2}$O$_{3}$ coax annulus, with theoretical calculations based on~the Clausius-Mossotti equation~in agreement with the measurements.~ Highly sensitive humidity detection was demonstrated by applying relative humidity between 0.1{\%} and 100{\%}, with a power-law response, \textit{RH}$\sim x^{\alpha }$. This nanocoaxial structure thus offers the possibility of unprecedented performance of porous Al$_{2}$O$_{3}$-mediated capacitancer sensing for humidity detection.