Fabrication and ionic transportation characterization of funnel-shaped nanochannels

Solid-state nanochannels have attracted increasing interest because they exhibit similar properties to, but are more stable than, biological nanopores. However, existing solid-state nanochannels, such as conical-shaped nanochannels, possess a limited critical region at the tip side, and thus demonstrate poor controllability of the orientation and magnitude of ionic rectification. In this work, we demonstrate the fabrication of funnel-shaped nanochannels with a gradual structural transformation. The longer critical regions can be controlled by tuning the etching temperature. In addition, the rectification ratio can be increased from about 3 to 6 by introducing a longer critical region. Using finite-element analysis based on the Poisson-Nernst-Planck (PNP) equations, it can be found that the increased length of the critical region could contribute to the asymmetric ion transportation. The funnel-shaped nanochannels with enhanced asymmetric ion transportation may find applications in the fields of materials, electronics, and life sciences.