Indian Institute of Science researchers have developed a highly sensitive nanometre-scale carbon monoxide sensor by employing an innovative fabrication technique. It is known that carbon monoxide (CO) can have adverse effects on the health of people exposed to it. Hence, it becomes necessary to have good, low-cost carbon-monoxide sensors. The research is published in Sensors and Actuators B: Chemical.

Right size

Typically, a sensor would be a thin, current carrying plate whose resistance changes on exposure to carbon monoxide. This in turn changes the value of the current flowing through it. This change when measured indicates the level of carbon monoxide in the air. Most available sensors are in the micrometer range, a nanometer-sized detector would have a higher sensitivity, but the cost of manufacturing it goes up as the size decreases. This is where the work of C.S. Prajapati and coworkers of Indian Institute of Science comes in.

To build this zinc-oxide (ZnO) nanostructure on a silicon wafer substrate, the researchers first placed tiny polystyrene beads on the wafer. These beads arrange themselves into what is called a hexagonal close-packed structure on the oxidised silicon wafer.

Maintaining a reasonable level of vacuum, a high voltage is applied which etches away the surfaces of the beads until a gap of desired thickness is formed between adjacent beads. Then zinc oxide is deposited on the system.

This occupies the spaces between the beads, forming a honeycomb like nano-mesh that can function as a nanosenor.

Scaling down from 10 micrometer feature size to 10 nanometer feature (used in this work) can enhance the efficiency 1,000 times. However, the development cost of nanostructured gas sensors using existing lithography tools is really very high, which eventually impacts the overall cost of the device, explains Navakanta Bhat, Chair and Professor, Centre for Nano Science and Engineering, Indian Institute of Science, an author of the paper, in an email to The Hindu.

This device is also easy to scale for mass production. Nanostructure-based gas sensors are very promising in their performance due their high surface-to-volume ratio. The existing techniques to create honeycomb nanostructures using photolithography and e-beam lithography are expensive and time-consuming. The proposed technique can potentially reduce the cost by more than 50%, says Prof. Bhat.

Smart cities

The Ministry of Electronics and Information Technology has a vision of deploying such sensors in large scale for pollution monitoring in large cities like Delhi and Bengaluru. For instance, if the sensors are installed in all traffic intersections, we can do real time mapping of pollution hot-spots in a city. This would be an enabler in realizing smart cities, says Prof. Bhat.

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Low-cost, sensitive CO sensor from IISc - The Hindu