Helium scanning transmission ion microscopy and electrical characterization of glass nanocapillaries with reproducible tip geometries

Nanopores fabricated from glass microcapillaries are used in applications ranging from scanning ion conductance microscopy to single molecule detection. Still, evaluating the nanocapillary tip by a non-invasive means remains challenging. For instance, electron microscopy characterization techniques can charge, heat and contaminate the glass surface, and typically require conductive coatings that influence the final tip geometry. Per contra, electrical characterization by the means of ion current through the capillary lumen provides only indirect geometrical details of the tips. Here, we show that Helium scanning transmission ion microscopy provides a non-destructive and precise determination of glass nanocapillary tip geometries. This enables the reproducible fabrication of axially asymmetric blunt, bullet and hourglass-shaped tips with opening diameters from 20 nm to 400 nm by laser-assisted pulling. Accordingly, this allows for an evaluation of how tip shape, pore diameter and opening angle impacts on ionic current rectification behavior and the translocation of single molecules. Our analysis shows that current drops and translocation dwell times are dominated by the pore diameter and opening angles regardless of nanocapillary tip shape.