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IIT Jodhpur fabricates a highly sensitive device to detect lead
A team of researchers including Adarsh Nigam, led by Dr. Mahesh Kumar, Department of Electrical Engineering has fabricated a highly sensitive, portable device that can detect the presence of lead in water in a matter of 4 seconds, even when present in extremely small amount of 0.018 parts per billion (ppb). The sensor is highly specific to lead even in the presence of other metals such as mercury, copper, zinc, cadmium and chromium. The results were published in the journal IEEE Electron Device Letters. The research was done in collaboration with the Singapore-based Agency for Science, Technology and Research. 
The researchers had tested the sensor for 50 cycles but as the materials used in the sensor are stable, it can be used several times for real-time monitoring. The sensor is fabricated using silicon wafer as the base material. As the crystal structure and thermal expansion of silica and gallium nitride/aluminium gallium nitride are different, they experience stress when grown on silicon as the layers are grown at 1000 degree C. To produce a layer of gallium nitride that is stress-free, the researchers produced a stack of five layers of gallium nitride and aluminium gallium nitride on the silicon wafer base. A layer of AlGaN is then grown on the almost defect-free gallium nitride. The researchers then fabricated the source and drain terminals that allow the electrons to flow and then used a gate to regulate the flow of electrons. At the junction of the top layers of GaN and AlGaN, a two-dimensional electron gas is formed naturally, which is the conducting layer that moves the electrons from the source to the drain. When lead ions get adsorbed on the gate, the rate of flow of electrons increases leading to increased current flow. The amount of increase in current depends on the amount of lead ions that get adsorbed. There is a slight increase in current even when lead concentration in water is as low as 0.01 ppb. The increase is more at higher lead concentration. 0.5 volt was used to ensure a constant flow of current from the drain to the source. The sensor was tested using 15 samples each of tap and lake water. The sensor showed good agreement with the results from standard techniques used for lead detection. The sensor can be reused by washing with water. The sensor response time — the time taken to detect lead — is hardly affected when reused. An array of devices are being fabricated on a single chip to detect different heavy metal ions. A cadmium ion sensor is already developed that has higher sensitivity (0.255 ppb) than other electrochemical approaches and can detect cadmium in about 3 seconds. The results of cadmium sensor were published in April 2019 in the journal IEEE Electron Device Letters.
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