Reactive diffusion in nanostructures of spherical symmetry

To investigate reactive diffusion in nanosized spherical geometries, a clear model experiment has been designed. Thin film {Al/Cu/Al} and {Cu/Al/Cu} triple layers were deposited on tips of 25 nm apex radius and investigated by atom probe tomography (APT). At the interfaces within both samples, the growth of the reaction product proceeds parabolically from the very beginning but with remarkably different rates. Growth appears to be always faster if Cu is stacked to the outer side of Al. The complex quantitative analysis of reaction-induced stress, surface tensions and partial mobilities suggests that the different growth rates represent the Darken and the Nernst–Planck limits of interdiffusion. Since the curvature radius of the model samples ranges down to a few tens of nanometers, it is anticipated that an analogous effect may play a role in the oxidation of nanospheres or in chemical reactions of core–shell structures.