Capacitive Sensing of Intercalated H2O Molecules Using Graphene

TL;DR

This study demonstrates that graphene-based capacitive sensors can detect intercalated water molecules beneath graphene, with the process being reversible and influenced by humidity, confirmed through experiments and simulations.

Contribution

The paper introduces a method using a metal-oxide-graphene varactor to capacitively sense water intercalation, supported by experimental and computational validation.

Findings

Graphene capacitance changes with water intercalation.

Intercalation process is reversible and rapid.

Atomic force microscopy and simulations confirm water displacement.

Abstract

Understanding the interactions of ambient molecules with graphene and adjacent dielectrics is of fundamental importance for a range of graphene-based devices, particularly sensors, where such interactions could influence the operation of the device. It is well-known that water can be trapped underneath graphene and its host substrate, however, the electrical effect of water beneath graphene and the dynamics of how it changes with different ambient conditions has not been quantified. Here, using a metal-oxide-graphene variable-capacitor (varactor) structure, we show that graphene can be used to capacitively sense the intercalation of water between graphene and HfO2 and that this process is reversible on a fast time scale. Atomic force microscopy is used to confirm the intercalation and quantify the displacement of graphene as a function of humidity. Density functional theory simulations are used to quantify the displacement of graphene induced by intercalated water and also explain the observed Dirac point shifts as being due to the combined effect of water and oxygen on the carrier concentration in the graphene. Finally, molecular dynamics simulations indicate that a likely mechanism for the intercalation involves adsorption and lateral diffusion of water molecules beneath the graphene.