Development of Graphene Based Gas Sensors
Graphene is a two dimensional structure of sp2 hybridized carbon atoms arranged in a hexagonal structure. The use of graphene in any application begins with the challenge of producing high quality graphene. The quality, size, and structure of graphene films vary with the desired applications. While reduced graphene flakes are needed for composite materials or conductive paints, planar graphene is needed for high performance electronic devices. The purpose of this research was to study the sensing behavior of graphene. Therefore, a planar sheet of high quality graphene was essential for gas sensing applications. In this study, Chemical Vapor Deposition (CVD) was used to grow large scale graphene on copper foil. Graphene deposition was carried out on a copper foil and transferred to different substrates by wet etching of Cu foil by ferric chloride solution followed by the removal of polymer layer. After the synthesis and transfer of graphene to desired substrate, Raman spectroscopy was used to analyze the quality and number of layers of graphene. Reflectance and Transmittance spectroscopy measurements were also performed to analyze the quality and optical properties of graphene. After the characterization of graphene films, graphene based devices (two-electrode, multiple electrodes (cross-finger type) and top/back gated field effect transistors) were fabricated using photolithography and lift-off processes. A novel technique was developed to grow graphene directly on target substrate with the use of thin film of catalytic metals such as nickel and copper. For the gas sensing behavior, two different approaches (transistor based approach and resistance based approach) were employed to study the adsorption/desorption behavior of ammonia (NH3) on the graphene surfaces. When resistive based graphene sensors were used for the detection of different reducing gases (H2, NH3, CH4) in a practically important operating temperature range, the selectivity of NH3 detection was pronounced among NH3, H2 and CH4 at 150oC-200oC range in the air. Though high sensitivity was observed at 200oC for NH3 and H2, the most stable sensor characteristics was observed at 150oC. When the device was tested at room temperature and low operating temperature condition (RT-100oC), the sensors had reproducibility, reliability and repeatability issues. The issues with the sensing behavior of graphene at room temperature and low operating temperature conditions were resolved when the surfaces of graphene were functionalized with catalytic metallic nanoparticles such as platinum (Pt) and gold (Au). The sensing mechanism of graphene based gas sensors was also systematically demonstrated for three different organic vapors, namely acetone, ethanol and acetic acid vapors under the flow of air as a background gas. Out of three vapors, acetic acid vapor was the most sensitive, followed by ethanol and acetone vapors. The sensing mechanism of graphene based field effect transistors was also systematically demonstrated for NH3 molecules at different operating temperatures from RT to 100oC by in-situ FET measurements under the flow of dry air and continued vacuum condition. Graphene was synthesized by CVD method using the transfer-free approach to avoid mechanical difficulty during transfer. NH3 molecules acted as donors as the charge neutrality points progressed towards the lower gate bias voltages after the adsorption of NH3 molecules. The net charge transfer rate from NH3 to graphene channel was estimated to be around 0.04 electrons per NH3 molecule. When the device was tested from RT to 100oC, the response of the device was found to increase linearly with increasing concentrations. Average shifting rate of the Dirac peak was obtained as 0.49 V/ppm at 100oC. Shifting rate of around 0.5V/ppm demonstrates the high sensitivity of the device synthesized by this procedure.
Engineering, Mechanical Engineering, Materials Science, Electrical Engineering, Graphene, Synthesis, Gas Sensors, Field effect transistors, GFET devices, Ammonia sensing, Organic vapor sensing, Gas sensing mechanism
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