Ionic current rectification, breakdown, and switching in heterogeneous oxide nanofluidic devices.

We investigate several ion transport behaviors in sub-20 nm nanofluidic channels consisting of heterogeneous oxide materials. By utilizing distinct isoelectric points of SiO2 and Al2O3 surfaces and photolithography to define the charge distribution, nanofluidic channels containing positively and negatively charged surfaces are created to form an abrupt junction. This method provides much more robust surface charges than previous approaches by surface chemical treatment. The fabricated nanofluidic diodes exhibit high rectification of ion current and achieve record-high rectification factors (ratio of forward current to reverse current) of over 300. The current-voltage property of the device follows the theoretical model quantitatively, except that at low ion concentrations the forward current degrades and the reverse current is greater than theoretical prediction, which can be attributed to access resistance and breakdown of water molecules. The breakdown effect characterized by a negative conductance followed by a rapid increase of current is observed in a double junction diode. The occurrence of the breakdown is found to be enhanced by the abruptness of the junction between the heterogeneous nanochannels. Finally, we demonstrate ionic switching in a three-terminal nanofluidic triode in which the ionic flow can be electrically regulated between different channel branches. The study provides insight into the ion transport behavior in nanofluidic devices containing heterogeneous surfaces.