Study of two types of Ir or Rh covered single atom pyramidal W tips

The growth of a single-atom sharp pyramid tips on a tungsten substrate by depositing noble metals of the Pt family was investigated by field ion microscopy earlier. The Pd or Pt covered single-atom W pyramidal tips have been successfully prepared and established by our group. They are thermally and chemically stable and can easily be regenerated. In this study, we report the establishing and structural analysis of Rh and Ir covered single-atom W pyramidal tips. Two types of stable structures with bcc {2 1 1} facets are found for both metals. One is stacked by 1, 3, 10 atoms, and the other is stacked by 1, 6, 15 atoms for the top three layers and so on in series from the top to the deeper layers. The single atom tip is destroyed layer by layer after field evaporation to observe the structures of the different layers. However, the tip can be regenerated after it is annealed again and the two types of structures appear systematically depending on the annealing temperature. The regeneration process is investigated and the growth parameters of the two different types of Rh or Ir covered W tip are determined. The differences in the activation barrier and binding energy of these two types are calculated to be 0.08 eV and 0.064 eV for Rh covered single atom tips, and 0.03 eV and 0.14 eV for Ir covered single atom tips, respectively. Possible mechanisms and the relevance for application are discussed.