Toxic nitrogen dioxide gas — an invisible and odorless air pollutant with the potential to cause health issues with long-term exposure — can be found in many places. 

However, an ultrathin sensor developed by researchers from UC Berkeley and the Lawrence Berkeley National Laboratory may soon allow people to check levels of nitrogen dioxide with just their smartphone. 

Only three atoms thick, the sensor is unique from others in that it is effective in real-world temperature and humidity conditions, according to campus postdoctoral researcher Mehmet Dogan. It can detect concentrations of nitrogen dioxide as minuscule as 50 parts per billion while ignoring other toxic gases present and has fast response and recovery times. 

“You can use the sensor in essentially any part of the world, any atmospheric condition,” Dogan said.

The sensor is notable since existing sensors can only be used in high temperatures, cannot selectively detect nitrogen dioxide well and recover slowly, according to campus postdoctoral scholar Amin Azizi. The sensor is made of a one-layer alloy of rhenium niobium disulfide that is flexible and looks transparent, making it suitable for use in devices such as wearable electronics, Azizi added.

The study was also special since it involved close collaboration between theory and experiment, with Dogan leading the theory work and Azizi leading the experimental work.

“Just running a lot of calculations on these materials without someone going in and trying it in the lab can feel kind of detached, and after some point, you’re not sure whether what you’re working on is actually going to work in real life,” Dogan said. “(The theory-experiment collaboration) is the kind of dialogue that leads to the real breakthroughs in this field.”

The collaboration involved constant back and forth between the theorists and experimentalists, according to Dogan. Experimentalists might observe a phenomenon with an experiment and go to the theorists, who do some modeling and come up with an explanation that is then tested by experimentalists.

The sensor is the culmination of years of research into the material and its potential as an effective nitrogen dioxide sensor, Dogan added.

“When I finally found the actual model that gave me the structure that I saw in the experiment, I was really happy,” Dogan said. “So when I saw this is the model, this is why this pattern is occurring, this is why the material looks so different from others. Finally, I was able to solve that puzzle, and that was really satisfying.”

While Dogan and Azizi have no further research planned for the particular material in the sensor, they are interested in working on some related materials. They are also open to hearing from industrial sectors that can incorporate the sensor in different applications, Dogan added.