Ultra-thin nanomaterials set to improve environmental sensing

22 Mar 2017

Chalmers University of Technology develops sensors based on dark excitons.

A project at Sweden's Chalmers University of Technology could lead to a new type of chemical nanosensor, thanks to research into atomically thin nanomaterials that are extremely sensitive to changes in their surroundings.

The sensors are based on transition metal dichalcogenides (TMDs), a class of substance known to be promising for novel sensor architectures thanks to their strong interactions with light, and an optimal surface-to-volume ratio when formed into thin layers. The project's findings were published in Nature Communications.

"Our method has promising potential, paving the way for ultra-thin, fast, efficient and accurate sensors," said Ermin Malic of Chalmers University. "In the future, this could hopefully lead to highly sensitive and selective sensors that can be used in environmental research."

TMDs are potentially effective as sensors thanks to the nature of the direct band gap in the material, which readily leads to the creation of excitons - bound states of electrons and electron holes - when they interact with light. These "bright" excitons are influenced by the environment of the material, and so offer a way to make TMDs respond to their surroundings.

Optical fingerprints
TMDs also show a variety of optically forbidden "dark" excitons, and the Chalmers project has found that in the presence of molecules with a dipole moment, those dark states can be transformed into bright excitons, leading to an additional pronounced peak in easily accessible optical spectra.

According to the project team, this effect offers a way to produce a clear optical fingerprint for the detection of certain molecules, in contrast to common sensing schemes relying on relatively minor peak shifts and alterations in intensity.

Tests with tungsten disulfide (WS₂), said by the team to be an exemplary TMD material, also showed a link between this detected optical effect and the extent of coverage of the dipolar molecules on the surface of the sensor.

As molecular coverage increased, the spectral position of the dark exciton peak changed relative to the bright exciton peak, moving from the higher energy side to the lower energy side. Such an effect may ultimately offer a way to directly measure the dispersion of dark excitons, and hence the distribution of the molecules influencing them.

"This could open up new possibilities for the detection of environmental gases," said Maja Feierabend of Chalmers. "Our method is more robust than conventional sensors, which rely on small changes in optical properties."