Transition from monolayer-thick 2D to 3D nano-clusters on α-Al₂O₃(0001)

This paper reports on the long-standing puzzle of the atomic structure of the Ag/α-Al₂O₃(0001) interface by combining X-ray absorption spectroscopy, to determine Ag local environment [i.e. average Ag-Ag (d_Ag-Ag) and Ag-O (d_Ag-O) interatomic distances and Ag coordination numbers (CN)], and numerical simulations on nanometric-sized particles. The experimental key was the capability of a structural study of clusters involving only a few atoms. The concomitant decrease of d_Ag-Ag and CN with decreasing cluster size provides unambiguous fingerprints for the dimensionality of the Ag clusters in the subnanometric regime leading to a series of unexpected results regarding the size-dependent interface structures. At low coverage, Ag atoms sit on surface Al sites to form buckled monolayer-thick islands associated with a Ag-Ag distance (2.75 Å) which fits the alumina lattice. Upon increasing Ag coverage, as 3D clusters appear, the Ag interface atoms tend to leave Al sites to sit atop O atoms as d_Ag-Ag increases. The then highlighted size-dependent evolution, is built on structural models which seemed so far contradictory in a static vision of the interface. Theory generalizes the case as it predicts the existence of alumina-supported 2D clusters of Pd and Pt at small coverage and a similar 2D-3D transition upon increasing the size. The structural transformation from 2D Ag clusters to macroscopic 3D islands is accompanied by a noticeable reduction of adhesion energy at the Ag/α-Al₂O₃(0001) interface.