Highly Rectifying Conical Nanopores in Amorphous SiO2

Nanopore membranes are a versatile platform for a wide range of applications ranging from medical sensing to filtration and clean energy generation. To attain high-flux rectifying ionic flow it is required to produce short channels exhibiting asymmetric surface charge distributions. This work reports on a system of track etched conical nanopores in amorphous SiO$_2$ membranes, fabricated using the scalable track etch technique. Pores are fabricated by irradiation of 1 $\mu$m thick SiO$_2$ windows with 2.2 GeV $^{197}$Au ions, and subsequent chemical etching. Structural characterisation is carried out using atomic force microscopy (AFM), scanning electron microscopy (SEM), small angle X-ray scattering (SAXS), ellipsometry and surface profiling. Conductometric characterisation of the pore surface is performed using a membrane containing 16 pores, including an in depth analysis of ionic transport characteristics. The pores have a tip radius of (5.7 $\pm$ 0.1) nm, a half-cone angle of (12.6 $\pm$ 0.1)$^{\circ}$, and a length of (710 $\pm$ 5) nm. The $pK_a$, $pK_b$ and $pI$ are determined to 7.6 $\pm$ 0.1, 1.5 $\pm$ 0.2 and 4.5 $\pm$ 0.1 respectively, enabling the fine tuning of the surface charge density between +100 to -300 mC $m^{-2}$, and allows achieving an ionic current rectification ratio of up to 10. This highly versatile technology addresses challenges that contemporary nanopore systems face, and offers a platform to improve the performance of existing applications.