Waterproof electronic circuits made of graphene

Water molecules typically distort the electrical resistance of graphene, but European researchers have discovered that when this two-dimensional material is integrated with the metal of a circuit, contact resistance is not impaired by humidity.

Published in the journal ACS Applied Materials & Interfaces, it is claimed that this finding will help to develop new sensors at a significantly reduced cost.

The many applications of graphene, an atomically thin sheet of carbon atoms with extraordinary conductivity and mechanical properties, include the manufacture of sensors. These transform environmental parameters into electrical signals that can be processed and measured with a computer.

Due to their two-dimensional structure, graphene-based sensors are extremely sensitive and promise good performance at low cost in the coming years. In order to achieve this, graphene needs to make efficient electrical contacts when integrated with a conventional electronic circuit. Proper contacts are crucial in any sensor and significantly affect its performance.

But graphene is sensitive to humidity; to the water molecules in the surrounding air that are adsorbed onto its surface. H₂O molecules change the electrical resistance of this carbon material, which introduces a false signal into the sensor.

Scientists have now found that when graphene binds to the metal of electronic circuits, the contact resistance (the part of a material’s total resistance which can be attributed to the contacting interfaces) is not affected by moisture. The study was carried out experimentally using graphene together with gold metallisation and silica substrates in transmission line model test structures, as well as computer simulations.

“This will make life easier for sensor designers, since they won’t have to worry about humidity influencing the contacts — just the influence on the graphene itself,” explained Arne Quellmalz, a PhD student at Sweden’s KTH Royal Institute of Technology and lead researcher on the study.

“By combining graphene with conventional electronics, you can take advantage of both the unique properties of graphene and the low cost of conventional integrated circuits. One way of combining these two technologies is to place the graphene on top of finished electronics, rather than depositing the metal on top the graphene sheet.”

As part of the European CO2-DETECT project, the study authors are using this new approach to create the first prototypes of graphene-based sensors, the purpose of which is to measure carbon dioxide (CO₂) by means of optical detection of mid-infrared light and at lower costs than with other technologies. Other participants in the project include Sweden’s Senseair, Germany research institute AMO and the Catalan Institute of Nanoscience and Nanotechnology, Spain.

Image caption: Schematic of a graphene device with a contact resistance that is not altered by the water molecules adsorbed on its surface. Image credit: Anderson D Smith.

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