Portable Ozone Detection Without the Heat

Lynn M. Savage

Too much ozone can cause breathing problems, especially among asthma sufferers, because it irritates the lungs. Concerns over ground-level emission of ozone, by common devices such as early-model photocopiers and laser printers, have driven the development of sensors for the gas. Metal-oxide thin films exist that could serve as ozone sensors, but they rely on thermal activation to cause reactions between their surfaces and atmospheric ozone. Unfortunately, generating enough heat requires high power consumption and causes high levels of localized heat, making it difficult to create more compact sensors that could be integrated into portable devices.

Shown here are a schematic cross section of an ozone sensor integrated onto the back side of a GaInN LED chip (a), and the front (b) and back (c) sides of the LED-integrated ozone sensor mounted into a ceramic-integrated chip package. Reprinted with permission of Applied Physics Letters.

Scientists at Technical University Ilmenau and at Fraunhofer Institut für Angewandte Festkörperphysik in Freiburg, both in Germany, have developed a technique that may enable detection of ozone at 40 ppb using a handheld device.

First, the researchers built an ultraviolet LED using GaInN/GaN on a sapphire substrate. On the opposite side of the substrate, they deposited a 15-nm layer of In₂O₃ nanoparticles. Chunyu Wang of the university said that In₂O₃ is the most promising of the ozone-sensing candidate materials because of its efficacy at room temperature. “Furthermore, In₂O₃ is very stable, even in harsh environments, (and) the deposition of the required In₂O₃ nanolayer is a cheap and relatively easy process.”

A high-resolution transmission electron microscope image shows a layer of In₂O₃ nanoparticles with a thickness of ~15 nm. The inset shows the size distribution of the In₂O₃ nanoparticles, indicating that their mean size is about 7 nm. Courtesy of Chunyu Wang, Technical University Ilmenau.

After depositing the In₂O₃, the investigators examined the material using a high-resolution transmission electron microscope made by FEI Co. of Hillsboro, Ore., and an x-ray diffraction device made by Bruker AXS Inc. of Madison, Wis. They found that the layers were composed of cubic In₂O₃ nanoparticles that were about 7 nm in diameter.

Using the 400-nm LED to irradiate the In₂O₃ layer, they alternated power to the diode: on for 2 min, then off for 2 min while they exposed the substrate to ozone gas. Cycling the LED/ozone exposure caused changes in the resistivity of the In₂O₃ layer that they could measure. They performed each cycle about 10 times before changing the ozone concentration and remeasuring. According to Wang, the sensor’s response to the ozone increased as a function of optical power, and 0.25 mW was required to activate the In₂O₃ layer.

The researchers plan to try to assemble the sensor by depositing the In₂O₃ layer on top of the LED, instead of on opposite sides of the substrate. According to Wang, this would ease the bonding technology required for the sensor’s contacts, although such a design would require an additional insulating layer between the LED and the In₂O₃.

Applied Physics Letters, Sept. 3, 2007, Vol. 91, 103509.