Sensing devices are rapidly gaining importance in our society, across a range of applications including the Internet of Things and environmental monitoring. In particular, environmental sensing of different types of gases is highly relevant for security and safety applications, including detecting pollution. Efficient gas sensors that can reliably detect the presence of specific types of molecule are needed, and much effort is being dedicated to their development.
Working in collaboration with researchers at the Technical University of Berlin, researchers from the Graphene Flagship working at Chalmers University of Technology have proposed a new method of sensing gases based on a distinct optical fingerprint that arises in the presence of gas molecules. The research is published in Nature Communications, and demonstrates a new principle which could lead to efficient gas sensors based on few-atom-thick transition metal dichalcogenides (TMDs). "This could open up new possibilities for the detection of molecules. Our method is more robust than conventional sensors, which rely on small changes in optical properties," said Maja Feierabend, a researcher at Chalmers University of Technology and first author of the research.
The ideal sensor for environmental monitoring of gases will detect molecules adsorbed onto its surface, so the sensor material is unaltered and can be reused. The sensor should also be fast, so that changes in conditions can be detected in real-time. "Our method has promising potential that could pave the way for ultra-thin, fast, efficient and accurate sensors. In the future, this could hopefully lead to highly sensitive and selective sensors that can be used in environmental research," said Ermin Malic from Chalmers University of Technology, principal investigator of the work.
The Graphene Flagship is focused on exploiting the properties of graphene and related materials (GRMs) such as TMDs to develop new technologies. The thinness and strong surface response of GRMs are ideal for a variety of types of sensor. The strong optical response of TMDs is particularly promising for sensing using light. However, typical sensing methods rely on small shifts in frequency or intensity, which can be difficult to detect. The novel method proposed here should lead to new features in the light response, presenting an unambiguous detection signal.
The research team. Left to right: Ermin Malic, Maja Feierabend and Gunnar Berghäuser. Credit: Mia Halleröd Palmgren. Top image: a dark exciton peak is visible in the optical spectrum when a gas molecule is detected. Credit: Maja Feierabend and Ermin Malic
From dark to light
Typically, excitons are generated in a material when an electron absorbs a photon of light, and leaves behind an electron vacancy, or hole. The excited electron and the hole are bound together as a pair that can travel through the material. However, there are some exciton states – so-called "dark states" – that cannot be accessed by photons.
"In these sensors, the dark exciton states become bright, i.e. accessible by light, through the coupling with gas molecules," said Gunnar Berghäuser, postdoctoral researcher at Chalmers University of Technology and co-author of the work. Then, the TMD will absorb light that it previously could not, leading to a new, pronounced peak in the optical absorption spectrum– the optical fingerprint of the gas. The precise characteristics of the peak should depend on factors such as molecular coverage, polarity of the gas molecule, and distance between the molecule and the sensor. The response could also be tuned by using a different substrate for the sensor.
Proof of principle
This proof-of-principle result provides a solid foundation for the development of novel types of effective gas sensors. The researchers have applied for a patent for the new sensor mechanism, and are now working to demonstrate the proposal experimentally. Herre van der Zant, from Delft University of Technology, Netherlands, is the Leader of the Graphene Flagship's Sensors Work Package. "The potential for distinct optical fingerprints makes this new sensing concept appealing. With further investigation, this method might also have the promise to solve the specificity problem, at least for the detection of certain gases," he said.
M. Feierabend, G. Berghäuser, A. Knorr, E. Malic, Nature Communications 8, 14776 (2017)