  Nanopore sensing is a key technology for single-molecule detection and
analysis. Solid-state nanopores have emerged as a versatile platform, since
their fabrication allows to engineer their properties by controlling size,
shape, and chemical functionalization. However, lithography-based fabrication
approaches for non-planar nanopores-on-chip rely on polymers that have limits
with respect to hard- and robustness, durability, and refractive index. In this
respect, nanopores made of metal oxides with high dielectric constant would be
much more favourable and have the potential to extend the suitability of
solid-state nanopores towards optoelectronic technologies. Here, we present a
versatile method to fabricate three-dimensional nanopores of different
dielectric oxides with controlled shapes. Our approach uses photoresist only as
a template in the focused-ion-beam lithography to define the nanopore shape,
which is subsequently coated with different oxides (SiO2, Al2O3, TiO2 and HfO2)
by atomic-layer deposition. Then the photoresist is fully removed by
chemo-physical treatment, resulting in nanopores entirely made from dielectric
oxides on a thin solid-state membrane. Our methodology allows straightforward
fabrication of convex, straight, and concave nanopore shapes that can be
employed in various technologies and applications. We explored their
performance as ionic nanochannels and investigated the dependence of the ionic
current rectification on the nanopore geometry. We found hysteresis in the
ionic conductance that enables potential applications of the nanopores in
memristors. We also investigated the dielectric oxide nanopores for DNA sensing
by measuring both cis-trans and trans-cis translocations and support our data
with numerical simulations based on the Poisson-Nernst-Planck model.
