Unstrained islands with interface coincidence sites versus strained islands: X-ray measurements onAg∕ZnO

Abstract

The structure and shape of (111) Ag nanocrystals formed at room temperature on basal ZnO planes is followed in situ by synchrotron surface x-ray measurements. On well-ordered surfaces, fully relaxed islands are preferred with their in-plane lattice perfectly aligned with that of the substrate. These islands have their bulk lattice parameter from the earliest stages of the growth. At unit cell scale, the Ag lattice is incommensurate with that of ZnO ($\ensuremath{-}11%$ lattice mismatch), but interface coincidence sites occur every 2.6 nm. The islands grow with a plateau shape and the height-to-width ratio decreases rapidly with increasing coverage. These features are enhanced on the more ordered surfaces. In the low-coverage regime, small clusters rotated by $30\ifmmode\pm\else\textpm\fi{}3\ifmmode^\circ\else\textdegree\fi{}$ may be found. These minority clusters, in registry with ZnO, grow with the face-centered cubic stacking only on ill-crystallized surfaces. The 30\ifmmode^\circ\else\textdegree\fi{} epitaxial growth is rapidly inhibited due to the stress field generated by the compressive strain (up to $\ensuremath{-}2.6%$ with respect to bulk Ag) and the interface geometry. It is proposed that the different interface structures result from a subtle metal-oxide interaction potential possibly involving superperiodic lateral variations.