Electrochemically etched tungsten STM tip apex

Joanna Millunchick

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.