  Wide bandgap (WBG) semiconductors (Eg >2.0 eV) are integral to the
advancement of next generation electronics, optoelectronics, and power
industries, owing to their capability for high temperature operation, high
breakdown voltage and efficient light emission. Enhanced power efficiency and
functional performance can be attained through miniaturization, specifically
via the integration of device fabrication into two-dimensional (2D) structure
enabled by WBG 2D semiconductors. However, as an essential subgroup of WBG
semiconductors, 2D transition metal oxides (TMOs) remain largely underexplored
in terms of physical properties and applications in 2D opto-electronic devices,
primarily due to the scarcity of sufficiently large 2D crystals. Thus, our goal
is to develop synthesis pathways for 2D TMOs possessing large crystal domain
(e.g. >10 nm), expanding the 2D TMOs family and providing insights for future
engineering of 2D TMOs. Here, we demonstrate the synthesis of WBG 2D nickel
oxide (NiO) (Eg > 2.7 eV) thermally converted from 2D nickel hydroxide
(Ni(OH)2) with the lateral domain size larger than 10 um. Moreover, the
conversion process is investigated using various microscopic techniques such as
atomic force microscopy (AFM), Raman spectroscopy, transmission electron
microscopy (TEM) and X-ray photoelectron spectroscopy (XPS), providing
significant insights on the morphology and structure variation under different
oxidative conditions. The electronic structure of the converted NixOy is
further investigated using multiple soft X-ray spectroscopies, such as X-ray
absorption (XAS) and emission spectroscopies (XES).
