Si Nanowire-Based Schottky Sensors for Selective Sensing of NH₃ and HCl via Impedance Spectroscopy

This work is aimed at the development of a highly sensitivesilicon(Si)-based sensor allowing for the selective detection and analysisof liquid solution compositions containing ammonia (NH3) and hydrochloric acid (HCl) in an indirect manner using electrochemicalimpedance spectroscopy (EIS). For optimization of the performance,we develop three types of sensors based on as-fabricated Si nanowires,nanowires treated with hydrofluoric acid (HF), and nanowires decoratedwith silver (Ag) nanoparticles. The fabricated sensors exhibit goodperformance governed by the sensitivity of the nanoscale Schottkybarriers at the interface between the golden pads and Si nanowires.The best results on sensitivity are obtained with untreated Si nanowiresproviding a detection limit at the level of 4 & mu;mol & BULL;L-1 and resistive sensitivities of 0.8% per & mu;mol & BULL;L-1 for HCl and 4 & mu;mol & BULL;L-1, -0.2% per & mu;mol & BULL;L-1 for NH3, correspondingly. Treatment with HF stimulates the surfaceoxidation providing higher density of the adsorption sites and foundpromising for the detection with the analyte content up to 1000 & mu;mol & BULL;L-1. In the end, we study the sensor response upon simultaneousexposure under NH3 and HCl vapors using developed approachfor the EIS data analysis involving characterization of the sensorresponse with two parameters Rresistance and EIS frequency correspondingto the change in the operation regime. Due to the use of two parameterssimultaneously, this approach is found as a pathway for qualitativeanalysis of the gas mixture composition using only one sensor. Theresults of the work shed light on the development of feasible highlysensitive sensors for health monitoring, allowing for selective mixedanalytes detection.