Graphene based field effect transistor for the detection of ammonia

Graphene synthesized by chemical vapor deposition has been used to fabricate the back-gated field effect transistor to study the sensing of ammonia (NH3) in ppm levels. Graphene has been synthesized directly on a target substrate using a thin Cu film as a catalyst, which has several advantages over deposition of graphene on Cu foil followed by a transferring process to another substrate. Raman spectroscopy was used to monitor the quality of the deposited graphene films on SiO2/Si substrates. The adsorption/desorption behavior of NH3 on graphene in dry air was analyzed from the progressive shift of the Dirac peak at smaller/larger gate voltages based on different time exposures to different concentrations of NH3. The relative change in the shift of the Dirac peak was consistent with a small charge transfer (0.039 +/- 0.001 electrons per molecule at room temperature). The response of the device was found to increase with increasing NH3 concentrations and operating temperatures. The dependence of device response on concentration indicated that the graphene sensors exhibited two different adsorption modes for NH3 close to room temperature, whereas only one adsorption mode was observed at higher temperatures close to 100 degrees C. The shift rate of the Dirac peak estimated with a simple model using the Langmuir approach indicated that the rate was increasing linearly with temperature within the range of temperature studied (25 degrees C-100 degrees C) in this work. (C) 2012 American Institute of Physics. [http://dx.doi.org/10.1063/1.4752272]