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Ionic Current Rectification of Porous Anodic Aluminum Oxide (AAO) with the Barrier Oxide Layer.

Synthetic nanofluidic diodes with highly nonlinear current-voltage characteristics are currently of particular interest because of their potential applications in biosensing, separation, energy harvesting, and nanofluidic electronics. We report the ionic current rectification (ICR) characteristics of porous anodic aluminum oxide membrane, whose one ends of nanochannels are closed by the barrier oxide layer. The membrane exhibits an intriguing pH-dependent ion transport characteristics, which cannot be explained by the conventional surface charge governed ionic transport mechanism. We reveal experimentally and theoretically that the space charge density gradient present across the 40-nm-thick barrier oxide is mainly responsible for the evolution of ICR. Based on our findings, we demonstrate for the first time the formation of a single 5 ~ 8 nm-sized pore in each hexagonal cell of the barrier oxide. The present work would provide viable information for the design and fabrication of future ultra-thin nanofluidic devices without being limited by the engineering of the nanochannel geometry or surface charge.