Fig. 1 : Histogram of the distance between islands and the step edges. About 30% of islands are in contact with sapphire step edges. One inset presents an AFM micrograph showing a typical morphology of islands. In the green ellipse, most of island are located in the
central part of the terrace. In the yellow one, islands are wetting the step edge.
Vicinal surfaces of (0001) sapphire surfaces annealed in air at 1400-1700K exhibit flat terraces and straight steps. The step height and terrace width depend on the initial miscut of the surface, the temperature and duration of annealing. After annealing, sapphire surfaces are nanostructured with typically 0.4-1 nm high steps separated by 50-500 nm flat terraces.
We have used these nanostructured surfaces as templates for the growth of metal nanowires. The aim was to decorate step edges with metal islands in order to create nanowires. Platinum, the chosen metal, is deposited under UHV conditions. In these experimental studies, we have explored the influence of different parameters on the morphology of the Pt islands.  In particular, the temperature of the substrate was varied between 300 and 600K. The influence of the substrate preparation, the conditions of annealing (air or oxygen saturated with water), were explored. We have also changed the incident flux of Pt atoms and its orientation with respect to the surface. Fig. 1. shows in the inset a typical AFM image of the morphology of Pt deposition on a substrate heated at 625K. Clearly, some islands are wetting sapphire steps.
The histogram of distances between the island and the step edges demonstrates that about 30% of the islands are in contact with step substrate. By increasing the Pt coverage, chaplets of islands are formed along the step edges. In some cases as shown in Fig. 2, nanowires are generated but they present defects, the nature of which remains unknown.