Graphene is a two-dimensional material with extremely favorable chemical sensor properties. Conventional nanolithography typically leaves a resist residue on the graphene surface, whose impact on the sensor characteristics has not yet been determined. Here we show that the contamination layer chemically dopes the graphene, enhances carrier scattering, and acts as an absorbent layer that concentrates analyte molecules at the graphene surface, thereby enhancing the sensor response. We demonstrate a cleaning process that verifiably removes the contamination on the device structure and allows the intrinsic chemical responses of the graphene monolayer to be measured. These intrinsic responses are surprisingly small, even upon exposure to strong analytes such as ammonia vapor.

Graphene oxide membranes up to 2000 microm(2) in size can be synthesized with 90% yield in bulk quantities through a microwave-assisted chemical method. Membranes are readily visualized on an oxidized silicon substrate, which enables efficient fabrication of electronic devices and sensors. Field effect transistors made of the membrane show ambipolar behavior, and their conductivity is significantly higher than previously reported values.

The growth of large-area graphene on catalytic metal substrates is a topic of both fundamental and technological interest. We have developed an atmospheric pressure chemical vapor deposition (CVD) method that is potentially more cost-effective and compatible with industrial production than approaches based on synthesis under high vacuum. Surface morphology of the catalytic Cu substrate and the concentration of carbon feedstock gas were found to be crucial factors in determining the homogeneity and electronic transport properties of the final graphene film. The use of an electropolished metal surface and low methane concentration enabled the growth of graphene samples with single layer content exceeding 95%. Field effect transistors fabricated from CVD graphene made with the optimized process had room temperature hole mobilities that are a factor of 2−5 larger than those measured for samples grown on as-purchased Cu foil with larger methane concentration. A kinetic model is proposed to explain the observed dependence of graphene growth on catalyst surface roughness and carbon source concentration.

Graphene is a true two dimensional material with exceptional electronic properties and enormous potential for practical applications. Graphene's promise as a chemical sensor material has been noted but there has been relatively little work on practical chemical sensing using graphene, and in particular how chemical functionalization may be used to sensitize graphene to chemical vapors. Here we show one route towards improving the ability of graphene to work as a chemical sensor by using single stranded DNA as a sensitizing agent. The resulting broad response devices show fast response times, complete and rapid recovery to baseline at room temperature, and discrimination between several similar vapor analytes.Comment: 7 pages, To appear in Applied Physics Letter