Mechanism and method of single atom pyramidal tip formation from a pd covered W tip

Single atom tips have many applications in nano-sciences, because of their unique properties (Crewe et al., 1970). They can be used as a point electron or ion source with coherent electrons and high spatial resolution. How to produce and regenerate single atom tips reliably and easily is important. Fink (1986) reported a method of creating a single atom W tip by using a thermal-induced tip-forming process to produce a three-atom island on the (111) facet of a W tip, followed by thermal deposition of a W atom on top of the island. This method requires very tedious procedures, great technical skills, and very high temperature. It is difficult to reproduce and impossible to regenerate, thus it is difficult to be routinely applied to where needed. We find single atom pyramidal tips with single atom sharp wedges of noble metal, Pd, can be routinely and repeatedly created on a W tip using a surface-science technique based on impurity and thermal induced faceting of a crystal face. Nieh et al. (1999) found and observed with STM that three-sided pyramids of a few nanometer size with either 112 or 011 facets can be formed on the W (111) surface by annealing the surface covered with a few monolayers of thermally deposited Pd. Fu et al. (2001) use the similar method to create a single atom pyramidal Pd tip on the (Ill) face of a W field emitter tip. The procedure is very simple and also the tip can be regenerated after the top atom is field evaporated. The mechanisms and the energetic of atomic processes involved in the formation of single atom tips and the thermal stability are also studied using field ion microscopy (FIM).