New Family of 2D Materials With Extended Metal-Metal Bonding

Since the discovery of graphene by Novoselov and Geim, many two-dimensional (2D) materials have been created and led to great scientific and technological developments. Finding more van der Waals (vdW) materials, and exfoliating them down to the 2D limit, are crucial steps towards even more exciting developments and finding new physical phenomena. However, the synthesis of an important class of 2D materials, composed of an extended network of metal-metal bonds, as in a classical metal, has been proven to be challenging due to the tendency of metal atoms to form a close-packed 3D structures. Here, we present the synthesis of a family of vdW materials with high-coordination extended metal-metal bonding. The general structure consists of alternate bilayers of hexagonally close-packed metal and non-metal atoms, so that the bonding between the metal atoms is metallic, extended and highly coordinated, the bonding between the metal and the non-metal is covalent/ionic, and the bonding between the non-metal atoms is a weak vdW interaction. This layered structure enables easy exfoliation into few-layer 2D materials, and their integration into electronic devices. We studied the properties of these new 2D materials by SEM, TEM, AFM, STM, ARPES, Raman, SQUID, and magneto-transport measurements. We show that different members of this family have a plethora of unique properties: some of them show metallic behavior, whereas others are topological insulators, and some are ferromagnetic in-plane or out-of-plane. Lastly, we show that these materials can be alloyed, marking the first steps in a new field of 2D metallurgy<b>.</b>