A Single-Atom-Thick TiO2 Nanomesh on an Insulating Oxide.

The electronic structures and macroscopic functionalities of two-dimensional (2D) materials are often controlled according to their size, atomic structures, and associated defects. This controllability is particularly important in ultrathin 2D nanosheets of transition-metal oxides because these materials exhibit extraordinary multifunctionalities that cannot be realized in their bulk constituents. To expand the variety of materials with exotic properties that can be used in 2D transition-metal-oxide nanosheets, it is essential to investigate fabrication processes for 2D materials. However, it remains challenging to fabricate such 2D nanosheets, as they are often forbidden because of the crystal structure and nature of their host oxides. In this study, we demonstrate the synthesis of a single-atom-thick TiO2 2D nanosheet with a periodic array of holes, that is, a TiO2 nanomesh, by depositing a LaAlO3 thin film on a SrTiO3(001)-(√13×√13)-R33.7° reconstructed substrate. In-depth investigations of the detailed structures, local density of states, and Ti valency of the TiO2 nanomesh using scanning tunneling microscopy/spectroscopy, scanning transmission electron microscopy, and density functional theory calculations reveal an unexpected upward migration of the Ti atoms of the substrate surface onto the LaAlO3 surface. These results indicate that the truncated TiO5 octahedra on the surface of perovskite oxides are very stable, leading to semiconducting TiO2 nanomesh formation. This nanomesh material can be potentially used to control the physical and chemical properties of the surfaces of perovskite oxides. Furthermore, this study provides an avenue for building functional atomic-scale oxide 2D structures and reveals the thin-film growth processes of complex oxides.