The memory resistor, or memristor, was theoretically predicted by Chua in 1971 as the forth fundamental circuit element needed to complete the set of six mathematical equations relating four basic electrical variables: charge, current, voltage and magnetic flux. This idea remained as a missing element until its experimental realization in 2008 as TiO2-based memories. Since then, many researchers have directed their attention and efforts not only to the fabrication of memristors, using different materials, but also to understand the physics behind the switching mechanism, in order to control the processing and structural parameters that allow overcoming some drawbacks to obtain reliable and reproducible memory devices. Two-terminal devices, with the switching material sandwiched between two electrodes, are promising structures for the next generation of non-volatile resistive random access memories, with high speed, low-power consumption and excellent scalability. The two-dimensionality of the insulating graphene oxide sheets makes this material ideal for use in low-dimensional structures in nanoscale devices.
In this work, the transport properties of graphene oxide-based Metal-Insulator-Metal (MIM) structures are addressed. The graphene oxide (GO) was synthesized using an eco-friendly modified Hummers method. The obtained GO sheets were placed between gold electrodes, and the cycling current-voltage curves were measured by applying successive forward and reverse voltage sweeps in a range between -3 and 3V. When the absolute value of the measured current is plotted, in logarithmic scale, versus the applied voltage the typical butterfly-shaped curves are observed, which are better interpreted in terms of their first derivative, or differential conductivity, in order to understand the transport mechanism. A simple model is proposed, whose voltage dependence helps identifying the underlying physics responsible for the bipolar switching mechanism, such as bulk or electrode effects, conducting filament formation or tunneling. The analytical expressions used for the differential conductance for the low resistance state and the high resistance state (LRS and HRS respectively) leads to simple expressions for the I-V curves. The symmetry of the curves after several cycles is lost when compared to those measured on pristine devices, which might indicate the occurrence of filament forming effect, affecting the bipolar switching.