Ozone is a harmful gas and is produced naturally by ultraviolet (UV) radiation from the Sun. It is also emitted by some old-style photocopiers and printers. Recently, researchers succeeded in making environmental sensors to detect humidity, temperature and UV radiation that could be integrated into mobile devices, such as mobile phones or personal digital assistants.

Now, Chunyu Wang and colleagues at the Ilmenau Technical University, and co-workers at the Fraunhofer Institute of Applied Solid State Physics, have made a similar device for detecting ozone for the first time. "If integrated into mobile devices, the sensor will allow us to monitor ozone gas at any time and in any place," Wang told nanotechweb.org.

The researchers fabricated their sensor by growing a 15 nm-thick indium oxide layer on the back of blue LEDs made from gallium indium nitride/gallium nitride. They did this using low-temperature metal organic chemical vapour deposition. They then bonded this structure onto a ceramic integrated circuit.

The sensor works in modulation mode – it is periodically illuminated by the LED light for two minutes (ozone off and LED-light on), followed by exposure to ozone gas for two minutes (LED-light off and ozone on). The resistivity changes of the indium oxide nanolayer, by a periodic switching between gas exposure and light illumination, were measured as the sensing signal.

Wang says that the new sensor works at room temperature, unlike existing thin-film ozone sensors that normally only function at high temperatures. Such devices need an integrated heating system, which means high power-consumption and high-cost. "Classical ozone sensors are therefore not suitable for integrating into mobile devices," he adds. "In contrast, our LED integrated sensor does not need a heating system and is thus energy-efficient, low-cost, compact and robust – and it can be integrated into mobile devices."

The sensor can detect ozone concentrations down to 40 ppb, or around 76 µg per cubic metre. Its performance is comparable to that of electrochemical or photometric ozone monitors already on the market, which are much larger and more expensive.

The team now plans to grow indium oxide nanolayers directly on top of the LED structure, which would allow for further miniaturization. "This should also lead to a decrease in the LED power and a further decrease in energy consumption," explains Wang.

The work was published in Appl. Phys. Lett..