Self-Ordering of Disorderly Arranged 2D Crystal Layers to 3D Regular Arrangement Using a Heat-Induced Chemical Reaction between 2D Crystal Layers

Abstract

Two-dimensional (2D) materials with a thickness of ∼1 nm are candidate nanobuilding blocks to fabricate electronic devices with a three-dimensional (3D) structure using a bottom-up technology. They can be stacked in a precisely controlled hierarchical structure with a controlled number of building layers. However, the atomic arrangements between individual stacked 2D crystal layers are generally not ordered as in a single crystal. The interface and the disordered atomic arrangements result in decrease in the performance of electronic devices prepared from 2D crystals, because the electron flow between 2D crystals is blocked by the interface and the disordered atomic arrangements. Therefore, ordered atomic arrangement of the stacked layers is one of the most critical challenges in the preparation of 3D electronic devices from 2D materials. Here, a successful example of self-ordering of disorderly arranged 2D crystal layers to 3D regular arrangement is described. The multilayer films of nickel hydroxide 2D crystal with a thickness of one NiO₆ octahedral unit was focused as the disorderly arranged 2D crystal layers. The 2D layered films deposited on a substrate were heated to 400 °C. This heat treatment converted the disordered 2D system to ordered 3D NiO with (111)-orientation. The heat-induced chemical reaction between 2D materials allowed the disordered layers to self-order to 3D regular arrangement. The NiO film exhibited a photocathodic current assigned to reduction of water, and then the photocurrent increased with increasing the number of layers. The improvement of the photocurrent property is due to the ordered atomic arrangements without interface