Multianalyte sensor based on Mg-doped ZnO nanorod in electrolyte-insulator-semiconductor structures

Abstract

The electrolyte–insulator– semiconductor (EIS) system is considered one of the most attractive chemical sensor and biosensor approaches. Due to its small size, feasible mass production at a low price, and uncomplicated comanufacture with other silicon devices. However, the fabrication of an array of capacitive EIS sensors for multiplexed detection, as well as the realisation of a stable, reliable, and compact electrode, is a major challenge. The presence of defects and lattice mismatches between the sensing membrane and the sensor substrate may affect the capacity of the sensor to detect chemical signals. Thus, choosing a suitable sensing layer is essential for producing very sensitive and accurate bio-sensors. Distinct nanostructures, particular material features, and superior crystallisation in sensing materials, on the other hand, could improve the device's sensing performance and capabilities. In this regard, this work reports the incorporation of magnesium (Mg2+) cation into ZnO nanorods to overcome its defect-rich nature and improve the feasibility of its role as a sensing membrane in EIS. The main objective of this research is to produce a room temperature multianalyte EIS based Mg doped ZnO nanorods sensors. The detailed morphology and structural evolution, optical behavior and defect states of ZnO nanorod were studied as a function of Mg incorporation concentration. Subsequently, the effect of doping on the EIS sensing performance based on ZnO nanorod toward detection several solutions such as pH, 2-methoxyethanol, ethanol, methanol, and calcium (Ca2+) was investigated. Mg doped ZnO nanorods samples have been synthesized via hydrothermal approach with different molar ratio of Mg/Zn (0 – 5%) on silicon substrates pre-coated with ZnO seed layer. A silver (Ag) thin film was coated via a thermal evaporation process on the back of the Si substrate as a contact. After doping, the crystalline quality of ZnO nanorod was improved up to 3% whereas the preferential growth of ZNR along c-axis remained unaffected. The photoluminescence (PL) emission related to oxygen defects (oxygen vacancy and interstitials) was suppressed by the incorporation of Mg with doping concentration up to 3 %. The dopant-related emission began to dominate by further increasing the doping concentration. Overall, the Mg(3%) sample with an optimum doping concentration of 3% demonstrated the best sensing performance, with the highest sensitivity of 83.77 mV/pH, 694 V/ppm, and 69 mV/decade toward detection of pH, 2-methoxyethanol, and calcium ions, respectively. The increase in sensitivity can be attributed to the suppression of oxygen defects, increased crystal grain formation, higher surface roughness, and increased surface site density. In addition, it exhibited the highest sensing stability (drift), selectivity, and low hysteresis rate, confirming that a suitable doping concentration can thereby enhance the sensing performance by improving the bond connections, filling in holes, and eliminating defects. The incorporation of Mg dopant at appropriate concentration has been shown to be a simple approach to obtain ZnO nanorods with large surface area while maintaining low surface defects, which is beneficial for electrochemical performance of EIS device.