10 July 2015 ICFO in Science

Mid-infrared graphene molecule sensor

ICFO and EPFL develop a tunable, highly sensitive graphene-based infrared sensor Researchers Dr. Davide Janner, ICREA Professors Javier Garcia de Abajo and Valerio Pruneri, in a collaboration with EPFL’s Bionanophotonic Systems Laboratory, present a new device in which they have harnessed graphene’s unique optical and electronic properties to develop a reconfigurable highly sensitive molecule sensor. The results are described in an article appearing in the latest edition of the journal Science.

The researchers used graphene to improve on a well-known molecule-detection method: infrared absorption spectroscopy, which has important limitations when applied to molecules at the nano-scale. The wavelength of the infrared photon directed at a molecule is around 6 microns (6,000 nanometres), while the target measures only a few nanometres, making it very challenging to detect the vibration of such a small molecule.

This is where graphene comes in. If given the correct geometry, graphene is capable of focussing light on a precise spot on its surface and “hearing” the vibration of a nanometric molecule that is attached to it. In this study , researchers first pattern nanostructures on the graphene surface by bombarding it with electron beams and etching it with oxygen ions. When the light arrives, the electrons in graphene nanostructures begin to oscillate. This phenomenon concentrates light into tiny spots, which are comparable with the dimensions of the target molecules. It is then possible to detect nanometric compounds in proximity to the surface.

In addition to identifying the presence of nanometric molecules, this process can also reveal the nature of the bonds connecting the atoms that make up the molecule. Graphene is able to pick up the sound given off by each of the strings because it is able to identify a whole range of frequencies. Researchers “tuned” the graphene to different frequencies by applying voltage, which is not possible with current sensors. Making graphene\'s electrons oscillate in different ways makes it possible to “read” all the vibrations of the molecule on its surface.