Determination of Quantum Capacitance and Band Filling Potential in Graphene Transistors with Dual Electrochemical and Field-Effect Gates

We report here an investigation of graphene field-effect transistors (G-FETs) in which the graphene channel is in contact with an electrolyte phase. The electrolyte and the ultrathin nature of graphene allow direct measurement of the channel electrochemical potential versus a reference electrode also in contact with the electrolyte. In addition, the electrolyte can be used to gate the graphene; i.e., a dual-gate structure is realized. We employ this electrolyte modified G-FET architecture to (1) track the Fermi level of the graphene channel as a function of gate bias, (2) determine the density of states (i.e., the quantum capacitance C-Q) of graphene, and (3) separate the gate induced band filling potential delta from the electrochemical double-layer charging potential Delta phi(EDL). Additionally, we are able to determine the electric double-layer capacitance CEDL for the graphene/electrolyte interface, which is similar to 5 mu F/cm(2), the same order of magnitude as CQ. Overall, the electrolyte modified G-FETs provide an excellent model system for probing the electronic structure and transport properties of graphene and for understanding the differences between the two gating mechanisms.