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.Keywords: lanthanum aluminate; scanning tunneling microscopy; strontium titanate; transition metal oxides; two-dimensional sheets;

Metallic conductivity observed in the heterostructure of LaAlO3/SrTiO3 has attracted great attention, triggering a debate over whether the origin is an intrinsic electronic effect or a defect-related phenomenon. One of the issues to be solved is the role of SrO layer, which turns the conductive interface into an insulator when inserted between LaAlO3 and SrTiO3. To understand the origins of this oxide interface phenomenon and to further explore unconventional functionalities, it is necessary to elucidate how SrO layers are formed during the initial growth process at the atomic level. Here, we atomically resolve growth processes of heteroepitaxial SrOx films on SrTiO3(001)-(√13 × √13)-R33.7° substrate using scanning tunneling microscopy/spectroscopy. On the sub-unit-cell SrOx film surface, no periodic structure was observed as a result of random Ti incorporation into the SrOx islands, indicating the importance of the control of excess Ti atoms on the substrate prior to deposition. This random arrangement of Ti atoms is a marked contrast to the homoepitaxy on SrTiO3(001)-(√13 × √13)-R33.7°. Furthermore, the formation of SrOx islands introduced defects in the surrounding SrTiO3 substrate surface. Such atom-by-atom engineering and characterizations of oxide heterostructures not only provide microscopic understanding of formation process of interfaces in metal–oxides, but also would lead to the creation of exotic electronic phenomena and novel functionalities at these interfaces.Keywords: epitaxial film; initial growth process; oxide thin films and interfaces; pulsed laser deposition; scanning tunneling microscopy; strontium titanate; surface reconstruction;

•External bias to substrates is significant to control the polarity of ZnO films.•Effect of polarity control depends on type of the substrates.•Difference of polarity in ZnO was critical to the film properties.Effects of the nature of substrates, either crystalline or non-crystalline, on the structure and properties of ZnO films deposited by sputtering were investigated. This study focuses mainly on the role of the external electric bias applied to substrates during magnetron sputtering deposition in controlling crystalline polarity, i.e., Zn-face or O-face, and the resulting film properties. It was found that polarity control was achieved on silica and silicon substrates but not on sapphire substrates. The substrate bias did influence the lattice parameters in the structural formation; however, the selection of the substrate type had a significant influence on the defect structures and the film properties.