The Moiré-type nm-scale patterns of graphene monolayers supported by metal single crystals were recently used for the fabrication of ordered arrays metal nanoclusters by metal vapor deposition under ultrahigh vacuum UHV conditions [1-3]. The corrugation within the adsorption potential of the graphene layer result in virtually monodisperse clusters. These are important for model (electro-)catalysis studies investigating the size dependency of Pt clusters on carbon support.

In this study we analyze the ripening behavior of Pt clusters (formed at room temperature) on monolayer graphene supported by Ru(0001) at temperatures above 450 K by means of STM. The size distribution change of the clusters gives insight into the rate determining step and into the mechanism of the ripening process. Two different annealing step methods (i: a single sample was successively heated to higher temperatures and ii: individual samples reproducibly prepared in the same manner annealed to certain temperatures; both methods using the same heating period of 10 min) bring us to the conclusion that Pt cluster ripening between 450 K and 725 K occurs via a mechanism proposed by M. Smoluchowski. Above 725 K we observe indications for a change in the ripening mechanism, where Smoluchowski ripening is likely in competition with Ostwald ripening. Also above 725 K, we observed an adverse influence of the Pt on the stability of the monolayer graphene. Single defects in the otherwise well ordered graphene appeared, which were absent after annealing to lower temperatures.

[1] A.T. N’Diaye et al., New J. Phys. 11, 2009, 103045. [2] Yi Pan et al., Appl. Phys. Lett. 95, 2009, 093106. [3] K. Donner and P. Jakob, J. Chem. Phys. 131, 2009, 164701.