A highly sensitive and durable electrical sensor for liquid ethanol using thermally-oxidized mesoporous silicon

Abstract

A capacitive detection of liquid ethanol using reactive, thermally oxidized films constructed from electrochemically synthesized porous silicon (PSi) is demonstrated. The sensor elements are fabricated as meso-PSi (pore sizes <50 nm, pore thickness ∼7 μm), with Ag metallic contacts made exclusively at the backside of bulk silicon, and ethanol sensing is achieved by measurement of the real-time capacitance and conductance. The thermally oxidized sensor displays excellent infiltration-evaporation behavior with a tremendous reversible capacitance change towards ethanol. The capacitance increased by forty-fold more than its margin which is the greatest capacitance response ever recorded compared to previously investigated devices. The control device of non-porous Si showed no response, while the sensor response using the original hydrophobic PSi surface exhibited almost a half sensitivity of the thermal oxide sensor. The response to water is achieved only at the oxidized surface and found to be ∼one quarter of the ethanol sensitivity, dependent on parameters such as vapor pressure and surface tension. The capacitance response retains ∼92% of its initial value after continuous nine cyclic runs and the sensors presumably keep long-term stability after three weeks storage, demonstrating excellent durability and storage stability. The observed behavior in current system is likely explained by the interface interaction due to dipole moment effect. The results suggest that the current sensor structure and design can be easily made to produce notably higher sensitivities for reversible detection of various analytes.