Resistive random access memory (RRAM) has recently received considerable attention as one of the more promising technologies that could be used to highly miniaturized memory elements. This type of memory is based on materials that undergo reversible switching of their inherent resistivity. In several metal oxide structures, the application of a sufficiently high electric voltage generates a transition from a low-conductance state (off) to a high-conductance state (on). Such resistive switching behavior has also recently been observed for graphene oxide films embedded in specific types of structures, suggesting the possibility of producing RRAM elements over large areas on flexible materials. Sung-Yool Choi and colleagues from various institutions in Korea and the USA have now succeeded in fabrication highly efficient graphene oxide-based resistive switching elements on flexible substrates.1

Fig. 1: Photograph of a graphene oxide-based RRAM device on a flexible substrate.

The researchers spin-cast graphene oxide from solution onto a flexible polyethersulfone substrate bearing two aluminum contacts. The resulting devices showed a jump in conductance at an applied voltage of about 2.5 V, in either direction of current flow, with a ratio of about 100 between the on and off conductance states. “The memory performance of our device is comparable to that of an inorganic-based bipolar resistive memory element on a solid substrate,” says Choi. The bipolar resistive switching is highly reliable: the values of the conductance in the on and off states were maintained after several cycles of applied voltage.

Using high-resolution transmission electron microscopy and X-ray photoemission spectroscopy, the researchers investigated the mechanism behind this resistive switching. They established that applying a high voltage induces the migration of oxygen groups from the interface between the graphene oxide film and one of the aluminum electrodes toward the center of the film, creating conductive filaments between the graphene oxide film and the electrode. This scenario is in agreement with the sharp jump in conductance observed at 2.5 V — a signature of the formation of conductive filaments, according to the team.

“We have developed a very cheap and simple method to fabricate large-area, flexible nonvolatile memory devices, which could be readily applied in radio frequency identification tags and wearable electronics of the future,” says Choi.