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Chemical sensing study of reduced graphene oxide (rGO) devices (Master thesis)

Παπαματθαίου, Σωτήριος/ Papamatthaiou, Sotirios

Chemical sensor is a device that provides information about the chemical synthesis of its surrounding environment. Chemical sensors and particularly gas sensors are playing an increasingly important role in many fields including, national defense, environmental protection, workplace safety, medical diagnosis and industrial manufacture. In recent years, advances in sensor manufacturing technologies have been made directed by low-energy consumption need, improvements in miniaturization of semiconductor circuits and low-cost fabrication methods due to C-MOS implementation processes. One of the most promising applications of graphene-based materials is as sensitive materials for chemical sensors and particularly gas sensors. Graphene based sensors are characterized by their minimal dimensions, roomtemperature sensing operation, low energy consumption, low final cost due to simple device configuration (chemiresistor), low material cost (carbon is abundant in Earth), easier material modification due to their stable chemical nature and low limit of detection (ppb level vs 30 ppm for the best metal-oxide). In addition, graphene exhibits high surface-to-volume ratio and exceptional quality of crystal structure. These characteristics enable on one hand the exposure of all atoms to the target gas and on the other hand reduce the electrical noise. Graphene oxide (GO), a functionalized form of graphene, has largely improved sensing properties as its defects further enhance the chemical interaction with the gas molecules. However, its poor conductivity and mainly lack of long-term stability and repeatability are deterrent factors for reliable chemical sensing. Thus, reduced graphene oxide (rGO) is an appealing material for chemical sensing as it combines properties of graphene and graphene oxide. In this study, resistive chemical sensors were developed having as sensitive film GO spin-coated on glass substrates. Sensor electrical behavior to room temperature relative humidity was evaluated. High sensitivity characterizes GO sensor. However, it lacks repeatability and long term stability. To this end, we developed spin-coated GO sensors with different active layer thickness, which were subjected to subsequent thermal annealing steps. It was found that the resulting rGO sensors demonstrate excellent stability compromising, though, the sensitivity which mainly depends on the number of annealing steps and film thickness. The sensitivity was found to be 0.91 % (Ω/Ω/%RH) for one annealing step at 180 0C for 10 min. Response time for this 4 sensor was among the best reported in literature with similar fabrication processes as was as low as 4 s. Moreover, rGO-Pt (Platinum) composite was synthesized in order to fabricate a hydrogen (H2) sensor. Simultaneous chemical reduction of GO and K2PtCl6 was performed to acquire the rGO-Pt solution which was drop-casted on glass substrate. Room-temperature H2 detection was achieved and the best recorded sensitivity was 43 % (ΔΙ/Ι) for 2.25 % H2 at 70 0C. Finally, the rGO devices that were fabricated for humidity sensing were also tested for 200 ppm acetone with no satisfactory result even after drop-cast of selector for acetone.