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Summary Atomically thin two-dimensional (2D) metal oxides exhibit unique optical, electrical, magnetic, and chemical properties, rendering them a bright application prospect in high-performance smart devices. Given the large variety of both layered and non-layered 2D metal oxides, the controllable synthesis is the critical prerequisite for enabling the exploration of their great potentials. In this review, recent progress in the synthesis of 2D metal oxides is summarized and categorized. Particularly, a brief overview of categories and crystal structures of 2D metal oxides is firstly introduced, followed by a critical discussion of various synthesis methods regarding the growth mechanisms, advantages, and limitations. Finally, the existing challenges are presented to provide possible future research directions regarding the synthesis of 2D metal oxides. This work can provide useful guidance on developing innovative approaches for producing both 2D layered and non-layered nanostructures and assist with the acceleration of the research of 2D metal oxides.

Heteroatom doping engineering is desirable in tuning crystal structures and electrical properties, which is considered an opportunity to further develop microwave absorption materials. However, the competition mechanism and priority among doped atoms have not been revealed, which are insufficient to guide the most reasonable dielectric coupling model and design highperformance absorbers. In this work, based on in situ N and O, ex situ S is introduced through external thermal driving, leading to fierce competition among anions. Specifically, S atoms replace pyrrole N, drive out lattice O, and create O vacancies, bringing more extensive local charge redistribution and stronger electron interaction, thus activating the defect-induced polarization (3-6 times higher than conduction loss) in the middle/high-frequency region. Therefore, the effective absorption bandwidth (EAB) of 9.03 GHz and the minimum reflection loss (RL min ) of −64.05 dB at a filling rate of 10 wt.% are obtained, which improves the record of carbon absorbers as reported. Through macro-designs, i.e., multi-layer gradient metamaterial, or utilizing other advantages, e.g., cost-effective, stable chemical properties and wideangle absorption, porous carbon may possess a great application prospect in the naval field.

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A lead-free, stable orange-red-emitting material (PEA)4Cu4I4 with a high photoluminescence quantum yield of 68% was successfully prepared by a facile strategy.

In this work, we designed and studied a feasible dual-layer binary metagrating, which can realize controllable asymmetric transmission and beam splitting with nearly perfect performance. Owing to ingenious geometry configuration, only one meta-atom is required to design for the metagrating system. By simply controlling air gap between dual-layer metagratings, high-efficiency beam splitting can be well switched from asymmetric transmission to symmetric transmission. The working principle lies on gap-induced diffraction channel transition for incident waves from opposite directions. The asymmetric/symmetric transmission can work in a certain frequency band and a wide incident range. Compared with previous methods using acoustic metasurfaces, our approach has the advantages of simple design and tunable property and shows promise for applications in wavefront manipulation, noise control and acoustic diode.

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