Paper SS2-TuM11
Initial Bilayer-by-Bilayer Growth of Ag Islands on NiAl(110): DFT Analysis of underlying Quantum Size Effects

Tuesday, October 16, 2007, 11:20 am, Room 611

Growth of Ag on NiAl(110) is distinguished by an almost perfect match of the lateral unit cell of the substrate to that of Ag(110). Thus, Ag/NiAl(110) provides an ideal system in which to study morphological evolution during heteroepitaxy in the absence of lateral mismatch strain. Our STM studies reveal the nucleation and growth of large rectangular Ag islands for deposition of submonolayer amounts of Ag between 130K and 300K. Subsequent deposition produces smooth growth with large rectangular islands forming on top of coalesced clusters of such lower layer islands. However, the step height for the first layer islands is ~0.33 nm, and only slightly lower at ~0.29 nm for the next two layers. These values far exceed the step height of ~0.15 nm for Ag islands on Ag(110). Thus, we propose that islands have a predominantly bilayer Ag(110) structure for the first three (bi-) layers. This claim is supported by DFT calculations which reveal step heights for this structure in close agreement with experiment, and also indicate that the driving force for this bilayer growth is the presence of a quantum size effect perpendicular to the surface plane for electrons confined in the Ag film. Specifically, there is a strong oscillation of period 2 monolayers (ML) in the surface energy, and also in the binding energy of single Ag atoms on top of flat Ag films, favoring film heights equal to an even number of monolayers. For thicker films, step heights decrease so film structure must deviate from the perfect Ag(110) bilayer structure (a feature also reflected in a weak height modulation across the tops of Ag islands).