(Invited) Free-Standing Two-Dimensional Nanomaterials from Functional Oxides

Abstract

Two-dimensional (2D) nanomaterials, particularly when their thickness is just one or a few atomic layers, exhibit physical properties dissimilar to those of their bulk counterparts and other forms of nanostructures. Nonetheless, 2D nanostructures so far have been largely limited to naturally layered materials, i.e. the van der Waals solids, synthesized either from top-down or bottom-up. A much larger and diverse portfolio of 2D materials including non-layered compounds are desirable to meet the specific requirements of individual components in various devices. We demonstrate that surfactant monolayers could serve as a soft template supporting the nucleation and growth of 2D nanomaterials in large area beyond the limitation of van der Waals solids. Through this approach, 1 to 2 nm thick, single-crystalline free-standing ZnO nanosheets with sizes up to tens of microns were synthesized at the water-air interface. This technique was denoted as the Adaptive Ionic Layer Epitaxy (AILE) – the first solution-based technique for growing large-area ultrathin nanosheets without the support of crystalline substrates. The nanosheet thickness was controlled by the initial ion concentrated zone (the Stern layer) formed underneath the negatively charged surfactant monolayer. While ZnO crystals are always an n-type semiconductor, the as-synthesized ZnO nanosheets exhibited a p-type semiconductor behavior with the highest hole mobility up to 10 cm2/Vs. Density functional theory (DFT) calculations revealed that absorption of the surfactant induces a p-type doping to the ZnO nanosheet and changes it to a direct bandgap p-type semiconductor. AILE is a major breakthrough in materials science. It vastly broadens the range of 2D nanomaterials from layered van der Waals solids to oxides, ceramics, and metals, opening up opportunities for discoveries of exciting transport, photonic, and catalytic properties. So far, in addition to ZnO, we have showed successful synthesis results from NiO, and CoO, MnO2. The new 2D nanomaterials enabled by AILE will lead to extraordinary performance gain or new functionalities in many scientific and technology disciplines, such as energy harvesting and storage, information systems, sensing, and optics.