A group of scientists from NPL, Chalmers University of Technology and the US Naval Research Laboratory have used a novel technique to examine the effects of ambient air on graphene in a controlled environment, in order to characterize its response. As graphene is sensitive to a wide variety of chemicals, it is vital for graphene-based sensors to differentiate between the changes that are caused by the target gas and those caused by the natural environment.
The researchers investigated the effects of nitrogen, oxygen, water vapor and nitrogen dioxide (in concentrations typically present in ambient air) on epitaxial graphene inside a controlled environmental chamber. All measurements were taken at NPL by applying Kelvin probe force microscopy whilst simultaneously performing transport (resistance) measurements. This novel combination gave researchers the unique ability to connect the local and global electronic properties together, a task that has proven to be difficult in the past.
The study experimentally showed that the combination of gases used does not fully replicate the effects of ambient air; even at concentrations higher than those found in the typical atmosphere, there is a large difference in graphene's response. This result contradicts past literature, which has mainly attributed the changes in graphene's electronic properties to these gases. This raises the question of what chemicals are actually causing this significant response.
It is clear that, while graphene-based gas sensors have great potential, there is still a lot of research to be done. Further exploration is needed to find the missing link between the effects seen in controlled laboratories and the effects seen in ambient air. Researchers are also interested in studying methods to optimize the devices by narrowing the sensitivity to specific target species, such as chemical functionalization.