Unique Constant Phase Element Behavior of the Electrolyte-Graphene Interface.

We report a unique constant phase element (CPE) behavior (1Z=Q0(j omega)alpha) of the electrolyte-graphene interface with both Q0 and alpha showing dependence on the gate voltage. The frequency response of the electrolyte-graphene interface was studied using electrochemical impedance spectroscopy (EIS). The result suggests that (1) the electrolyte-graphene interface should be characterized as a CPE (alpha < 1), rather than an ideal capacitor; and (2) both Q0 and alpha show ambipolar dependence on the applied voltage. We speculate that the CPE behavior of the electrolyte-graphene interface arises from the charged impurities on the substrate and the defects in the graphene lattice, which could introduce inhomogeneity of local density of states (DOS). The low density of states of graphene makes alpha sensitive to these local DOS near the Dirac point, and thus showing dependence on the gate voltage. Measurement of the electrolyte-graphene interface capacitance based on multi-frequency capacitance-voltage (CV) profiling was demonstrated, and the extraction of the carrier mobility was performed. The study could lead to a more accurate understanding of the capacitive behavior of the electrolyte-graphene interface, which is instructive for the design and analysis of devices involving the electrolyte-graphene interface for nanoelectronics and bioelectronics applications.