The tip is perhaps the most crucial aspect of obtaining meaningful images through Scanning Tunneling Microscopy. An ideal tip would have at its point a single atom through which tunneling between the tip and surface may occur.
Picture taken from the Burleigh Instruction STM Workbook.
Having a dulled tip (more that a few atoms), it is possible that one would get tunneling from many different atoms at many different heights, and thus, obtain a rather fuzzy image. There are two methods for tip construction which were attempted during this experiment. They are cutting Platinum-Iridium wire and etching Tungsten wire.
Cutting a Platinum-Iridium (PtIr) tip:
Platinum-Iridium wire is a rather soft alloy which is excellent for STM. By merely using wire cutters, one may obtain a sharp tip which reveals quality images. To cut the wire, come in from approximately a 45 degree angle and cut. As you cut, pull the wire cutters away from the wire to obtain a sharp point. It takes some practice, but once you get the hang of it, it is a quick and easy way of making tips. One drawback, however, is that PtIr wire is rather expensive, so be frugal!
Etching Tungsten (W) wire using a DC source:
Etching is a more detailed process for creating a tip than the wire cutter method. But, if Tungsten is to be used as a tip (it is very hard, and frays when cut), it is really the only practical method. To etch a Tungsten tip on must place a 5 volt potential difference across the Tungsten wire and a Platinum electrode in approximately a 5M KOH solution.
The tungsten wire should be left in the solution with the power on until the end of the tip is observed to break off. The tip should then be removed from the solution, cleaned with distilled water and methanol, and immediately used.6 Care must be taken so that the tip is not dulled.
Unfortunately, we never were able to obtain a clear image using an etched
tip. We feel that the method does produce an excellent tip, yet in transport
to the microscope the tips had to be carried in the open air and a significant
distance thus probably dulling the ends.