In recent years, two-dimensional (2D) materials have attracted a lot of research interest as they exhibit several fascinating properties. However, outside of 2D materials derived from van der Waals layered bulk materials only a few other such materials are realized, and it remains difficult to confirm their 2D freestanding structure. Despite that, many metals are predicted to exist as 2D systems. In this review, the authors summarize the recent progress made in the synthesis and characterization of these 2D metals, so called metallenes, and their oxide forms, metallene oxides as free standing 2D structures formed in situ through the use of transmission electron microscopy (TEM) and scanning TEM (STEM) to synthesize these materials. Two primary approaches for forming freestanding monoatomic metallic membranes are identified. In the first, graphene pores as a means to suspend the metallene or metallene oxide and in the second, electron-beam sputtering for the selective etching of metal alloys or thick complex initial materials is employed to obtain freestanding single-atom-thick 2D metal. The data show a growing number of 2D metals/metallenes and 2D metal/ metallene oxides having been confirmed and point to a bright future for further discoveries of these 2D materials.

Center for Energy and Environmental Technologies VSB Technical University of Ostrava 17 Listopadu 15 Ostrava 708 33 Czech Republic

Centre of Polymer and Carbon Materials Polish Academy of Sciences M Curie Sklodowskiej 34 Zabrze 41 819 Poland

College of Chemistry and Molecular Science Wuhan University Wuhan 430072 China

Institute for Complex Materials IFW Dresden P O Box D 01171 Dresden Germany

Material Science and Engineering Center Łukasiewicz Research Network PORT Polish Center for Technology Development Ul Stabłowicka 147 Wrocław 54 066 Poland

School of Energy and Power Engineering Xi'an Jiaotong University No 28 Xianning West Road Xi'an Shaanxi 710049 China

Soochow Institute for Energy and Materials Innovations College of Energy Collaborative Innovation Center of Suzhou Nano Science and Technology Key Laboratory of Advanced Carbon Materials Wearable Energy Technologies of Jiangsu Province Soochow University Suzhou 215006 China

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