Graphene-Based Gas Sensors with High Sensitivity and Minimal Sensor-to-Sensor Variation

Graphene as an atom-thick carbon material is promising for the detection of gaseous molecules owing to extremely high surface-to-volume ratio. However, the majority of graphene-based gas sensors, prepared by chemical vapor deposition (CVD), have suffered from non-uniformity in their responses. Such a high sensor-to-sensor variation in responses has not been systematically studied, limiting application of graphene gas sensors. Here we report processes that lead to a highly sensitive and uniform graphene gas sensor. We examined four types of graphene sensors by varying two conditions: (1) whether or not there is a carbon precursor while cooling down the reactor after graphene synthesis and (2) whether poly(methyl methacrylate) (PMMA), a polymer for transferring the graphene onto another substrate, is removed by annealing at high temperature or by rinsing with acetone. Using 5 ppm dimethyl methylphosphonate (DMMP), a nerve agent simulant, as a model analyte, we found that uniform responses are obtained by cooling down the reactor without carbon precursor and by removing PMMA by annealing. Additional heat treatment of the graphene in air greatly enhances the sensitivity, regardless of the synthesis conditions, by removing residual PMMA and impurities from the graphene surface. The uniform graphene sensors enabled us to find that the edge-to-surface ratio of graphene does not affect sensitivity, whereas noise increases at higher edge-to-surface ratio. Our study presents a design rule for fabricating sensitive and uniform graphene gas sensors, which may facilitate their applications in detecting a broad range of analytes.