Abstract: Graphene is an atomically thin zero band gap semiconductor or semimetal, with a strong ambipolar electric field effect, remarkably high carrier mobility at room temperature, high carrier density and high saturation velocity. Due to extraordinary electrical properties of graphene, it has been suggested as a possible candidate for beyond-CMOS field effect transistor (FET) with applications... read moremainly in RF and analog domain. Graphene is also mechanically strong and has good biocompatibility, is electrochemically stable, shows excellent broadband optical transparency and can be made very sensitive to the surrounding media, making it a promising candidate for sensing and bioelectronics applications. Although two dimensional graphene has shown remarkable electrical, physical and chemical properties, extending it in three dimensional form may enhance the functionality and performance of devices and enable new functions currently not possible. In this dissertation, the application of two and three dimensional graphene in electronics, sensing and bioelectronics is presented. A transparent graphene based microfluidic chip for dielectrophoretic cell trapping and lysis with graphene as electrode is introduced and it is has been shown that graphene behaves as an electrochemically stable electrode in the presence of high DC electric field in biological medium. Using graphene minimizes the Faradic reaction which would otherwise harm living cells and change the chemical and physical properties of electrolytes and electrodes. Next for the first time a three dimensional graphene field effect transistor is introduced and studied. The channel of this transistor is made of three dimensional graphene foam, which is gated using ionic liquid and ionogel to realize both liquid and semisolid state versions. Liquid and gel at the interface with the graphene forms a double layer capacitance (EDLs) of extremely large capacitance per unit surface area which provides an all-around electrostatic control of the transistor channel and leads to a lower operating voltages. Due to higher surface area of the foam, the transistors show up to 26.72 times higher current capacity than the equivalent conventional two dimensional graphene transistors. The network structure of the foam expanded in three dimensions leads to higher mechanical stability and results in mechanical fault-tolerance. Higher surface area of the foam and high mechanical strength of three dimensional graphene transistor make it interesting for sensing applications. In this dissertation, mechanical, chemical and biological sensors are realized based on a mono to double layer and few layers of the graphene foam. For chemical sensing, we demonstrate its application in pH sensing directly in biological fluids. For mechanical sensing, we present its application in sensing strain and for biological sensing, we show its ability to capture electrically activity from electrogenic cells. The pH sensor consists of a thin layer of HfO2 as a sensing surface was grown all-around of the three dimensional graphene foam serving as a transistor channel. The three dimensional graphene transistor shows higher pH sensitivity (79 mV/pH) than conventional two dimensional graphene based sensor even in high ionic strength medium and in body fluids. We attribute the high sensitivity and ability of sensing pH at high ionic strength media to the three dimensional structure of the channel and existence of sensing surface all-around the graphene channel. A strain sensor is based on few layers of the graphene foam. Due to the three dimensional network structure of the graphene foam, this sensor shows mechanical fault-tolerance and robustness, and also demonstrates high dynamic range compared to the two dimensional graphene based sensors. Graphene foam based device is also used as a scaffold for growing different types of cells and recording electrical signal from electrogenic cells. It is shown that graphene shows good biocompatibility and it can be used as an ideal electrode for recording the electrical activity of the cells. These applications indicate the promise of three dimensional graphene transistor as an all-in-one multimodal multifunctional transistor for smart biological interfaces.
Thesis (Ph.D.)--Tufts University, 2015.
Submitted to the Dept. of Electrical Engineering.
Advisor: Sameer Sonkusale.
Committee: Qiaobing Xu, Swastik Kar, and Mohammad Afsar.