Graphene produced with chemical vapor deposition (CVD) will form the cornerstone of future graphene-based chemical, biological, and other types of sensors. Graphene, however, is extremely sensitive to air, in particular to humidity. To avoid unwanted background coming from humidity and to calibrate future sensors, it is highly important to investigate the mechanisms by which water (in the form of environmental humidity) affects graphene sheets.
A research collaboration between the National Physical Laboratory (NPL, UK), the University of Surrey, and Graphenea, studied the effect of humidity on electrical transport and local surface potential of CVD graphene. By studying both single-layer graphene and bilayer graphene formed by transferring two layers on top of each other, the researchers managed to decouple effects of the substrate from interlayer coupling and atmospheric effects, in particular the effect of humidity.
It is found that mono-layer graphene on the standard SiO2 substrate is extremely sensitive to water vapour, with water molecules acting as physisorbed loosely bound p-dopants. The adsorbed water layer can be removed by placing the sample in vacuum. In the case of a bi-layer stack, produced by double transfer of two graphene layers, the layers are randomly oriented and decoupled with respect to each other. As a consequence, the bottom layer of the bi-layer stack is mostly affected by substrate charges, while the top graphene layer is externally doped by the water vapour. This finding clarifies earlier reports that monolayer and bilayer graphene act differently when exposed to humidity. It also explains why bilayer graphene has less p-dopants – because the bottom graphene layer is affected by the substrate.
This work indicates that electronic devices using CVD graphene on SiO2 must be properly encapsulated to ensure stable operation, and sensors must be appropriately calibrated to minimize the effect of ambient humidity. The work was published in the July issue of Carbon.