Oxide and halide nanoclusters on ionic substrates: heterofilm formation and lattice mismatch

Abstract

We examine the form of clusters and islands formed by oxides and halides on ionic substrates where there is an appreciable lattice mismatch between substrate and adsorbate. Structures are optimised in the static limit using both periodic density functional theory and atomistic simulation techniques. Results are presented for three examples: (i) CaO on the (100) surface of BaO, (ii) CaF₂ on the (110) and (111) surfaces of BaF₂ ,(iii) Bi₂O₃ on the SrO-terminated (100) surface of SrTiO₃. In none of these do we observe direct overlay of groups of ions over substrate ions, accompanied by marked changes in bond lengths in the adsorbate cluster or layer. Rather, islands have a critical maximum domain size because larger islands would result in unfavourable electrostatic interactions between adsorbate and substrate. Instead loops or gaps or other defect regions form between individual islands. For Bi₂O₃ we propose the low temperature stabilization of the superionic δ-phase of Bi₂O₃ on deposition on (100) SrTiO₃ is not a consequence of epitaxial matching of Bi⋯Bi with Sr⋯Sr, as previously suggested, but rather due to the mismatch between the Bi–O and Sr–O bond lengths. This gives rise to individual Bi–O islands, which promote the formation of a disordered surface phase. We find no evidence for the stabilization of a symmetric cubic structure. The formation of the disordered Bi₂O₃ ultrathin film is also associated with a decrease in the band gap.