In the modern age of gas sensing technologies for broad applications such as internet of things, the capability to make selective, small form factor, and highly responsive sensors for applications such as wearable devices and cell phones could revolutionize the fields of gas sensing systems and fabrications. This thesis aims at developing a millimeter-sized and microwatt-powered sensor prototype using graphene field effect transistors (FETs) by exploring various DC and AC modulation techniques to realize the critical gas sensing features of selectivity and fast recovery speed at room temperature for practical applications.
For the DC modulation, we applied the DC gate voltage on the graphene FET during the gas sensing events, and developed the concept of linear factor and the bandwidth enhanced noise method for improving graphene gas sensing performance at DC conditions. Specifically, we have experimentally demonstrated the label-free gas selectivity directly using a single graphene FET for NO2, NH3, H2O and CH3OH by measuring the linear factor parameter. Furthermore, we demonstrated the boosted sensing speed and linearity of the graphene resistance signal using the bandwidth-enhanced method to select the most gas sensitive frequency domain of the noise power density spectrum.
For the AC modulation, we applied a hybrid AC+DC gate voltage on graphene FET, and studied the scattering effect and the speed of charge transfer of the gas- graphene interaction for improving the gas selectivity and sensing recovery speed, respectively. Particularly, we measured the scattering strength of the gas adsorbents on graphene and directly resolve the scattering strength spectrum of water, methanol and ethanol adsorption on graphene, achieving the label-free gas sensing using a single graphene transistor. On the other hand, the studies of charge transfer speed between the gas and graphene inspired us to develop the phase- sensitive scheme on graphene FET, achieving the ultrafast baseline recovery speed (~10s) on a defect-rich, chemical vapor deposition (CVD) grown monolayer graphene FET - almost ten times faster than the previous report