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Graphene (G) is a two-dimensional material with exceptional sensingproperties. In general, graphene gas sensors are produced in field effecttransistor configuration on several substrates. The role of the substrates onthe sensor characteristics has not yet been entirely established. To providefurther insight on the interaction between ammonia molecules (NH3) and graphenedevices, we report experimental and theoretical studies of NH3 graphene sensorswith graphene supported on three substrates: SiO2, talc and hexagonal boronnitride (hBN). Our results indicate that the charge transfer from NH3 tographene depends not only on extrinsic parameters like temperature and gasconcentration, but also on the average distance between the graphene sheet andthe substrate. We find that the average distance between graphene and hBNcrystals is the smallest among the three substrates, and that graphene-ammoniagas sensors based on a G/hBN heterostructure exhibit the fastest recovery timesfor NH3 exposure and are slightly affected by wet or dry air environment.Moreover, the dependence of graphene-ammonia sensors on different substratesindicates that graphene sensors exhibit two different adsorption processes forNH3 molecules: one at the top of the graphene surface and another at its bottomside close to the substrate. Therefore, our findings show that substrateengineering is crucial to the development of graphene-based gas sensors andindicate additional routes for faster sensors.

Enhancing the response of NH3 graphene-sensors by using devices with different graphene-substrate distances