III Nitride based Heavy Metal Ion Sensors for Smart Water Quality Monitoring

The group III nitride semiconductor devices like AlGaN/GaN high electron mobility transistors (HEMT) have become the most emerging candidates in the last two and half decades. The 2DEG forms at the AlGaN/GaN heterointerface without applying an external electric field, which makes AlGaN/GaN HEMTs normally ON devices. There are surface states on the AlGaN/ GaN High Electron Mobility Transistor due to the availability of dangling bonds at the surface. These surface states have a significant impact on the 2DEG. By using this phenomenon, the AlGaN/GaN HEMT are utilized for the detection of toxic heavy metal ions in water. The quick and easy monitoring of heavy metals in drinking water is utmost important due to their harmful effects on human health.<br/> AlGaN/GaN HEMTs with functionalized gate areas were fabricated for sensitive and selective detection of heavy metal ions such lead, and mercury. In order to study the presence of Pb²⁺ metal ions in water, the fabricated GaN HEMT sensor's gate region was functionalized using graphitic carbon nitride (g-C₃N₄) nanosheets. The limit of detection (LoD) of the g-C₃N₄functionalized sensor, obtained a sensitivity of about 0.46µA/ppb and 0.32 ppb as lowest limit which is substantially lower than the permissible limit as set by W.H.O. In another work on Pb²⁺ ion detection, the fabrication of AlGaN/GaN HEMT was carried out for the detection of toxic lead ions in the water by functionalization of the Au-gated region by the 2,5-dimercapto-1,3,4-thiadiazole (DMTD). The sensor reaches the lower detection limit of 0.018 ppb. It exhibits a rapid response time of ~ 4 seconds and high sensitivity of 0.607 µA/ppb. Moreover, a study for the selectivity analysis is performed, and it is found that the sensor is highly selective towards Pb²⁺ ions. In another interesting work, we developed a sensor for highly sensitive, selective, and rapid determination of the trace amount of Hg²⁺ ions using molybdenum disulfide (MoS₂) functionalized AlGaN/ GaN HEMT. The sensor showed an excellent sensitivity of 0.64 μA/ppb and an ultra-low-level detection limit of 0.01152 ppb or 11.52 ppt (parts per trillion) with a rapid response time of 1.8 s. Moreover, the sensor exhibits the linear range of detection from 0.1 ppb to 100 ppb and highly selective behavior towards Hg²⁺ ions. To further enhance the sensitivity, we have successfully demonstrated the detection of Hg²⁺ions using MoS₂ functionalized AlGaN/GaN HEMT under UV illumination which showed an extremely high sensitivity of 548.07uA/ppb with a lower detection limit of 6.15 ppt. In our another work, we observed that the fabricated sensor after functionalization with Ag-MoS₂ensured faster electron transfer kinetics and showed out remarkable sensitivity of 1.6 mA/ppb and limit of detection (LoD) of upto 20 ppt. Along with the detection on real time water samples from lake water, a proof of concept by enabling it with IoT interface was also demonstrated. Hence, the development and performance of the AlGaN/GaN HEMT based ion sensors suggest that these sensors have a huge potential for other heavy metal ion sensing applications