We report on employing molecular doping to enhance the sensitivity of graphene sensors synthesized via chemical vapor deposition to NH3 molecules at room temperature. We experimentally show that doping an as-fabricated graphene sensor with NO2 gas improves sensitivity of its electrical resistance to adsorption of NH3 molecules by about an order of magnitude. The detection limit of our NO2-doped graphene sensor is found to be ∼200 parts per billion (ppb), compared to ∼1400 ppb before doping. Electrical characterization and Raman spectroscopy measurements on graphene field-effect transistors show that adsorption of NO2 molecules significantly increases hole concentration in graphene, which results in the observed sensitivity enhancement.

Integration of a complementary metal-oxide semiconductor (CMOS) and monolayer graphene is a significant step toward realizing low-cost, low-power, heterogeneous nanoelectronic devices based on two-dimensional materials such as gas sensors capable of enabling future mobile sensor networks for the Internet of Things (IoT). But CMOS and post-CMOS process parameters such as temperature and material limits, and the low-power requirements of untethered sensors in general, pose considerable barriers to heterogeneous integration. We demonstrate the first monolithically integrated CMOS-monolayer graphene gas sensor, with a minimal number of post-CMOS processing steps, to realize a gas sensor platform that combines the superior gas sensitivity of monolayer graphene with the low power consumption and cost advantages of a silicon CMOS platform. Mature 0.18 µm CMOS technology provides the driving circuit for directly integrated graphene chemiresistive junctions in a radio frequency (RF) circuit platform. This work provides important advances in scalable and feasible RF gas sensors specifically, and toward monolithic heterogeneous graphene-CMOS integration generally. 1-3 In these cases, power and size requirements are not critical, and such sensors tend to be large and bulky. But with the rapid expansion and prevalence of newer technologies like smart phones, cloud computing and the Internet of Things (IoT), mobile sensors are recognized as an essential component in future ubiquitous sensor networks. 4,5 The goals of IoT sensor networks require mobile and untethered sensors in quantities that negate any realistic possibility of individual sensor maintenance or battery replacement. The expectation is that the sheer number of future sensor devices, the "things" of IoT, will preclude human maintenance of individual nodes within large sensor networks. The implications for future device production are then twofold: the sensors must operate at low-power, and the cost of each device should be low enough that the expected orders of magnitude increase in sensor nodes is feasible. Much of current gas sensor research is therefore directed at the need for low-cost, low-power portable gas sensors, as well as integration with the technology platform best suited to meet that need: silicon complementary metal-oxide semiconductor (CMOS).Solid-state gas sensors cover a wide range of technologies, from microelectromechanical thermal and mass sensors to optical and chemiresistive sensors. 6,7 Of these, one of the most common is the chemisresistive sensor, whose relatively simple design and operation make it a strong candidate for CMOS integration.

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