Solution‐Processed Diode‐Like ZnO Nanoparticle Device with Tunable Threshold Voltage and Super‐Nernstian Ion Sensitivity

This study introduces a fabrication method to produce a zinc oxide nanoparticles (ZnO NPs) diode-like interface device for sensing applications. This structure is achieved via the modulation of ionized oxygen molecules adsorbed on the surfaces of the ZnO NPs, distinguishing it from the conventional diode devices. The device exhibits an on/off ratio of 10⁵ and features a tunable threshold voltage contingent upon varying surface charge conditions, positioning it as a promising candidate for high-sensitivity chip-level pH sensing applications. A highly sensitive pH sensor based on this interface is successfully fabricated using a fully solution-based process, excluding any high-temperature steps. As the pH value of the test solution decreases, the sensor demonstrates an increase in threshold voltage, achieving a super-Nernstian sensitivity of 360 ± 11 mV pH^-1. The fabrication process reaches a maximum temperature of 120 °C and employs a UV-vacuum-heating (UVVH) technique. To maintain the electrical integrity of ZnO NPs, ethylene-vinyl alcohol (EVOH) is utilized to provide a protective, waterproof, and oxygen-barrier passivation layer. The operational behavior and diode-like characteristics of the sensor, attributes to ionized oxygen molecule adsorption on ZnO NPs, are accurately predicted using a combined adsorption isotherm and electrical model, aligning well with experimental results.