Atomistic growth phenomena of reactively sputtered RuO₂ and MnO₂ thin films

We have synthesized RuO₂ and MnO₂ thin films under identical growth conditions using reactive DC sputtering. Strikingly different morphologies, namely, the formation of RuO₂ nanorods and faceted, nanocrystalline MnO₂, are observed. To identify the underlying mechanisms, we have carried out density functional theory based molecular dynamics simulations of the growth of one monolayer. Ru and O₂ molecules are preferentially adsorbed at their respective RuO₂ ideal surface sites. This is consistent with the close to defect free growth observed experimentally. In contrast, Mn penetrates the MnO₂ surface reaching the third subsurface layer and remains at this deep interstitial site 3.10 Å below the pristine surface, resulting in atomic scale decomposition of MnO₂. Due to this atomic scale decomposition, MnO₂ may have to be renucleated during growth, which is consistent with experiments.