When a highly energetic heavy ion passes through a target material, the damaged region left in its wake often exhibits preferential chemical etching over the undamaged material. This etch-anisotropy can be used to create pores with nanometre sizes and different shapes such as cylindrical or conical in many materials. Track etched nanopores have been used for a wide range of applications such as ultrafiltration, bio- and medical sensing, nanofluidic and nanoelectronic devices. One of the major challenges in current nanopore technology is to combine the high throughput rates that nanopores in ultrathin membranes provide with the highly asymmetric transport properties characteristic of long, conical nanopores.

This work presents a method for fabrication of conical nanopores in thin amorphous SiO2 (a-SiO2) membranes using ion track etching. 1 μm thin a-SiO2 windows are irradiated with 2.2 GeV 197Au ions at the Universal Linear Accelerator UNILAC (GSI Helmholtz Centre for Heavy Ion Research, Darmstadt, Germany). Subsequently, the windows are exposed to hydrofluoric acid (HF) from one side leading to the formation of conical nanopores. This technique, most commonly used for polymers, enables the fabrication of single pores or multiple pores with adjustable pore density over large areas.

Structural characterisation is performed using a range of different techniques. Using atomic force microscopy and scanning electron microscopy, the cone base radius can be determined. The half-cone angle is measured using small-angle X-ray scattering, and the pore length using ellipsometry and surface profiling. Typical dimensions are a pore length of 600 to 800 nm, a half-cone angle of 10° to 20°, and a base radius of 150 to 200 nm.

The transport characteristics and surface properties are measured by performing conductometric measurements, where the ionic current of different electrolyte solutions such as KCl or MgCl2 is monitored across the membrane. This technique also provides an accurate estimate of the pore tip radius in the order of 5 to 20 nm. It is found that at neutral pH the surface is negatively charged due to SiO- groups present at the pore surface. The geometrical asymmetry of conical pores combined with a strongly charged surface that can be tuned by adjusting the pH and electrolyte concentration causes a large ionic current rectification (ICR), i.e. the ratio of positive to negative current for a given voltage, of up to 10. This indicates a very high selectivity towards cations (or anions in acidic environments). The surface charge and hence ICR changes in polarity when exposing the nanopores to acidic environments.

This highly versatile technology addresses some of the challenges that contemporary nanopore systems face and offers a platform to improve the performance of existing applications, including nanofluidic osmotic power generation and electroosmotic pumps.