Here, I made an easy-to-use semi-automated machine that fabricates atomically sharp tips repeatably. It does so by submerging a tungsten wire into a solution of 2M NaOH, then applying approximately 12V between the wire, and the etchant. This etches away the tungsten wire, and produces tungsten trioxide (WO3), a solid, electrically insulating precipitate. As the wire is etched away, the WO3 coats the lower portion of the submerged wire. As a result, the wire is preferentially etched at the surface of the etchant, and forms a narrow constriction in the wire. Once the constriction is thin enough, the WO3 coated lower portion of the wire snaps off; at this point, the top portion of the wire is extremely sharp. To avoid blunting the tip, etching needs to be terminated as quickly as possible. Zyrus monitors the etching current, and automatically terminates the etching once the current falls below a threshold. The machine uses a well understood tip fabrication process that many STM groups use. However, each group seems to have their own special techniques for creating high quality tips. I first started development by exploring these group’s various techniques. Initially, I had difficulty producing consistent, symmetrical tips. I found that the key to producing good tips mainly comes from keeping the etchant free of insoluble particulates. Apart from this, there are really only two other important parameters, wire submersion distance, and the etching termination speed. I control the submersion distance by locating the surface of the etchant prior to etching, and then move the z-stage down a set distance before starting the etch. The termination is handled by the analog current monitoring electronics.
Zyrus can reliably produce tips with an effective radius of < 50nm, which is considered good geometry
Design files can be found at my Github page: https://github.com/bairdd/Zyrus-Etcher
If you would like to build one, or have questions about how this works, please reach out, I am more than happy to answer questions! I can assist in the building process, and help you getting sharp tips for your STM endeavors!
I did not include a bill of materials on the page here, but if there is interest, I can throw a BOM together. Total project cost is around $300.