Al2O3/W hetero-structured nanopore membranes: From native to tunable nanofluidic diodes

We present here Al₂O₃/W hetero-structured nanopore membranes which function as native and electrical field tunable nanofluidic diodes. A typical membrane is 100×100 μm² in size with pore density of ~20/μm². The nanopores are 26 nm in diameter and 400 nm in length. Owing to the opposite surface charge states of Al₂O₃ (positive) and W (negative with native oxide), the membrane exhibits clear rectification of ion current in electrolyte solutions. After thermal heating at 350°C for 2 hrs, approximately 10 nm WOx grows on the surface of W, forming a conformal and dense dielectric layer. The W layer allows the application of an electrical field to further modulate the ionic transport through the nanopores with low gate potentials and ultra low gate leakage current. We have demonstrated the control of rectifying factor from 2 to 11. Our experimental findings have a valuable potential for controllable high throughput molecular separation and chemical processors.