New nanowire sensors are the next step in the Internet of Things

A new tiny nitrogen dioxide sensor could help protect the environment from lung disease-causing vehicle pollutants and acid rain.

Researchers from TMOS, the Australian Research Council Center of Excellence for Transformative Meta-Optical Systems, have developed a sensor made from an array of nanowires, one-fifth of a millimeter square on a side, meaning it can easily be incorporated into a silicon chip. .

In research published in the last issue of Advanced Materials, Ph.D. researcher at the Australian National University Center team and lead author Shiyu Wei describes the sensor as requiring no power source, as it runs on its own solar-powered generator.

Wei says: “As we integrate such devices into the sensor network for the Internet of Things technology, the low power consumption is a huge benefit in terms of system size and costs. The sensor can be installed in your car with an alarm alert and alerts are sent to your phone if it detects dangerous levels of nitrogen dioxide emitted from the exhaust.”

Co-author Dr. Zhe Li says: “This device is just the beginning. It can also be adapted to detect other gases, such as acetone, which can be used as a non-invasive breath test of ketosis, including diabetic ketosis, which can save countless lives.

Existing gas detectors are heavy and slow and require a trained operator. In contrast, the new device can quickly and easily measure less than 1 part per billion, and the TMOS prototype used a USB interface to connect to a computer.

Nitrogen dioxide is one of the NOx categories of pollutants. In addition to contributing to acid rain, it is dangerous to humans even in small concentrations. It is a common pollutant from cars, and is also produced indoors by gas stoves.

The key to the device is a PN junction – the motor of a solar cell – in the form of a nanowire (a small hexagonal pillar about 100 nanometers in diameter, 3 to 4 microns high) sitting on a base. An ordered array of thousands of nanowire solar cells spaced about 600 nanometers apart formed the sensor.

The entire device was made of indium phosphide, with the base doped with zinc to form the P section, and the N section on top of the nanowires, doped with silicon. The middle part of each nanowire was stripped (inner section, I) separating the P and N sections.

Light falling on the device causes a small current to flow between the N and P sections. However, if the inner middle section of the PN junction is touched by any nitrogen dioxide, which is a strong electron-absorbing oxidizer, this will cause a drop in current.

The size of the dip allows the concentration of nitrogen dioxide in the air to be calculated. Numerical modeling by Dr. Zhe Li, a postdoctoral student at EME, showed that the design and fabrication of the PN junction are crucial for signal maximization.

The properties of nitrogen dioxide – strong adsorption, strong oxidation – make it easy for indium phosphide to distinguish it from other gases. The sensor can also be optimized to detect other gases by functionalizing the surface of the nanowires with indium phosphide.

TMOS principal investigator Professor Lan Fu, leader of the research group, says: “The ultimate goal is to sense multiple gases on a small chip. As well as environmental pollutants, these sensors could be deployed for healthcare, for example, for tests of respiratory for disease biomarkers.

“The small gas sensor is easily integrable and scalable. This, combined with meta-optics, promises to achieve multisensors with high performance and multiple functionalities, which will enable them to adapt to networks of smart sensors.TMOS is a network of research groups across Australia dedicated to the advancement of this field.

“The technologies we develop will transform our lives and society in the coming years, with large-scale implementation of Internet of Things technology for real-time data collection and autonomous response in applications such as air pollution monitoring, industrial chemical hazard detection, smart cities, and personal healthcare.”