Thermal Conversion of Ultrathin Nickel Hydroxide for Wide Bandgap 2D Nickel Oxides

Wide bandgap (WBG) semiconductors in two-dimensional (2D) form have demonstrated great potential in 2D electronics, optoelectronics, and power industries.^1–3 However, as an essential group of WBG semiconductors, 2D transition metal oxides (TMOs) remain largely understudied in terms of physical properties and applications in 2D electronic devices due to the lack of sufficiently large 2D crystals. Various 2D TMOs nanosheets have been produced,^4–6 but the nanometer scale crystals are not desirable enough for 2D electronics study, which usually require larger lateral domains (&gt;10 µm) for device design.^7,8 And some common TMOs (e.g., Ni_xO_y) still lack of systematical study and knowledge to be designable for application studies. Therefore, we are inspired and driven to expand the 2D TMOs family from developing a universal synthesis pathway with desirably large lateral domains that will serve 2D electronics study well. Here, we present the WBG 2D nickel oxide (NiO) thermally converted from 2D nickel hydroxide (Ni(OH)₂) with largest ever reported lateral domain size (&gt;20 µm). A facile and scalable synthesis approach is employed firstly to produce 2D Ni(OH)₂ flakes, which are subsequently transformed into NiO through a simple and controlled thermal conversion process. The morphology and structure variation is investigated and the chemical reaction during the thermal conversion under different temperature zones are established. Optical bandgap of the thermally converted 2D NiO (E_g &gt; 3.7 eV) is higher than that of 2D α-Ni(OH)₂ (E_g &gt; 2.5 eV), showing even better potential to serve as gate dielectric layers. The oxidation process is further studied by X-ray absorption spectroscopy (XAS) and X-ray emission spectroscopy (XES), to provide more insights and understandings on the electronic structure and bandgap of 2D NiO. We believe our investigation on 2D NiO will project and inspire more profound studies on other 2D WBG semiconductors.<br/><br/><b>Reference</b><br/>1. Fujita, S. Wide-bandgap semiconductor materials: For their full bloom. <i>Jpn. J. Appl. Phys.</i> <b>54</b>, (2015).<br/>2. Varley, J. B., Shen, B. & Higashiwaki, M. Wide bandgap semiconductor materials and devices. <i>J. Appl. Phys.</i> <b>131</b>, 1633–1636 (2022).<br/>3. Neudeck, P. G., Okojie, R. S. & Chen, L. Y. High-temperature electronics - A role for wide bandgap semiconductors? <i>Proc. IEEE</i> <b>90</b>, 1065–1076 (2002).<br/>4. Kalantar-Zadeh, K. <i>et al.</i> Synthesis of nanometre-thick MoO3 sheets. <i>Nanoscale</i> <b>2</b>, 429–433 (2010).<br/>5. Boland, J. B. <i>et al.</i> Liquid phase exfoliation of MoO2 nanosheets for lithium ion battery applications. <i>Nanoscale Adv.</i> <b>1</b>, 1560–1570 (2019).<br/>6. Rui, X. <i>et al.</i> Ultrathin V2O5 nanosheet cathodes: Realizing ultrafast reversible lithium storage. <i>Nanoscale</i> <b>5</b>, 556–560 (2013).<br/>7. Gao, H. <i>et al.</i> Phase-Controllable Synthesis of Ultrathin Molybdenum Nitride Crystals Via Atomic Substitution of MoS2. <i>Chem. Mater.</i> <b>34</b>, 351–357 (2022).<br/>8. Cao, J. <i>et al.</i> Realization of 2D crystalline metal nitrides via selective atomic substitution. <i>Sci. Adv.</i> <b>6</b>, 1–9 (2020).