Layered Hexagonal Metal Oxides Derived From the Metal-Gas Interface – Adding Oxides to 2D Toolkit

Metal oxides – one of the most abundant substances in nature, are rarely found to be naturally layered crystal. Such intrinsic property hinders their potential to form into two-dimension (2D), since the cleavage of the weak, interlayer van der Waals bonds in layered bulk crystal serves as the benchmark to obtain the atomically thin high-quality 2D layers such as ZnO. 2D-like non-layered metal oxides are usually created <i>via</i> a space-confined-growth or soft-chemically exfoliation of unilamellar bulk crystal, in which external ionic groups are necessary for crystal stabilization. Here we present our recent discovery of a series of the layered metal oxides with a unique planar hexagonal phase which can be formed across a wide variety metallic element, including transition metals, post-transition metals, lanthanides and metalloids. These oxides can naturally grow on the metallic surfaces under a controlled oxidation environment without involving sophisticated equipment or other chemicals. Subsequently, the hexagonal 2D metal oxide monolayers can be exfoliated on a substrate in a mechanical manner with ease, similar to the way of obtaining graphene and other layered metal chalcogenides. We have showcased the hexagonal TiO₂ as a representative to reveal the distinct properties of such hexagon coordinated oxides apart from their bulk counterparts. The monolayered and few-layered hexagonal TiO₂ shows p-typed semiconductor behavior with the hole mobility up to 950 cm²V^-1s^-1 at room temperature. This research findings can possibly expand the exploration of metal oxides in the 2D quantum regime and initiate numerous applications in the future.