Joanna Millunchick

Electrochemical etching of ultrasharp STM tips

Sponsor: National Science Foundation
In scanning tunnelling microscopy, sharper tips offer higher performance. The motivation for this project was to improve upon commercially available STM tips, which are relatively costly and have undesirable variability in sharpness. The etched tips are made from polycrystalline tungsten which is both readily available and inexpensive. Etching is done using the standard electrochemical process where sodium hydroxide reacts with the tungsten, etching the tungsten away. What enables us to etch the sharp tip is a fast (~40ns) electronic current shutoff device that senses when the etching process is complete. Dull tips result from current continuing to flow after etching is complete. Scanning electron microscopy (SEM) has revealed a radius of curvature at the tip apex on the order of 10-100nm. More accurate characterization is planned using TEM.
Highlights (Click an image for more information)
  • Electrochemical etching of ultrasharp tungsten STM tips

    This is a macro scale image of an etched tungsten scanning tunnelling microscopy (STM) tip. The ridges below etched region are the result of the die used in the extruding process. Uniform geometry contributes to a more stable tip.

  • Electrochemically etched tungsten STM tip apex

    This is a scanning electron microscope (SEM) of the tip apex of an etched tungsten STM tip. At this magnification (~650000X), the SEM image is not very crisp, but shows how sharp the tip is: the scale bar is 50 nm! This is much sharper than the commercially available tips that were looked at in the SEM and TEM.

  • TEM image STM tip

    A TEM image of a platinum coated iridium STM tip. The scale bar shows the length of 20nm. This image is part of a study in which we are attempting to make our own STM tips. This tip was manufactured and will be used as a comparison of our self-made STM tips. The apex of the tip is nicely curved. The protrusion shown is the point that interacts with the specimens.