Simulation, Fabrication, and Characterisation of Ion Track-Etched Asymmetric Nanopores in Thin Amorphous SiO2 Membranes
Creators: name: Kiy, Alexander
Publisher: name: The Australian National University
Year: 2024

description: 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 in many materials with nanometre sizes and different shapes such as cylindrical or conical. 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 investigates conical nanopores in thin amorphous SiO2 (a-SiO2) membranes fabricated using ion track etching. 1 um thin a-SiO2 windows are irradiated with 2.2 GeV 197Au ions at the Universal Linear Accelerator (GSI Helmholtz Centre for Heavy Ion Research, Darmstadt, Germany). Subsequently, the windows are exposed to 2.5% hydrofluoric acid (HF) from one side leading to the formation of conical nanopores. This technique enables the fabrication of single or multiple pores with adjustable pore density over large areas. Structural characterisation is performed using a range of different techniques. Atomic force microscopy and scanning electron microscopy provide the cone base radius. The half-cone angle is measured by small angle X-ray scattering, and the pore length by ellipsometry and profilometry. 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 alkali and alkaline earth halide electrolyte solutions is monitored across the membrane. The experimental work is supported by a comprehensive analytical description of the ionic transport mechanisms. It provides an accurate estimate of the pore tip radius on the order of 2 to 20 nm. Both experimental measurements and the analytical description are further supported by finite element modelling. At neutral pH the surface is negatively charged due to SiO- groups present at the pore surface. The surface charge can be adjusted by changing the pH, leading to more SiO- groups and thus a higher negatively charged surface in basic conditions (increasing pH) and more dominant SiOH2+ groups in acidic conditions (decreasing pH), which render the surface positive. The geometrical asymmetry of the conical pores combined with a highly charged surface that can be tuned by adjusting the pH and electrolyte concentration causes a large ionic current rectification, i.e. the ratio of negative to positive current for a given voltage, of over 10. This indicates a high selectivity towards cations (or anions in acidic environments). Simulations reveal that by adjusting the nanopore geometry and surface properties, complete rejection of one ionic species is achieved, thus acting as a charge separator. Furthermore, simulations suggest that negatively charged nanopores with a small, positively charged orifice at the cone tip completely reject all ionic species for one voltage polarity, while allowing them to traverse the pore for the opposite polarity. Thus, the nanopore acts as a nanofluidic diode. This highly versatile technology addresses some of the challenges that contemporary nanopore systems face. Its potential for different applications, such as nanofluidic osmotic power generation and electroosmotic pumps is compared to existing systems, indicating that improved performance is possible. With an in-depth structural characterisation, investigation of the surface properties, and analysis of the ionic transport properties, supported by finite element analysis, this work lays the foundation for the development of future a-SiO2 based nanopore membrane technologies.