Characterization of the PtnGem/alumna samples is discussed in the SI, along with the XPS data used to quantify the Pt:Ge stoichiometry (Fig. ). The main conclusions from that analysis are as follows: For a Pt-free sample of the alumina thin film support, 5.9 x 10 13 Ge atoms/cm 2 were deposited, corresponding to ~2% of the atoms in the alumina surface layer. The low Ge/alumina coverage suggests that GeCl4 chemisorbed only at a small number of defects in the alumina film. Subtracting the amount of non-specific Ge deposition on alumina, resulted in estimation of the PtnGem cluster stoichiometries ranging from Pt4Ge1 to Pt7Ge2 to Pt11Ge4. Note that these stoichiometries were calculated assuming that the presence of ~0.1 ML equivalent of Pt on the Ptn/alumina samples did not significantly affect deposition of Ge at alumina sites.