Synthesis of 2-Dimensional Metals via van der Waals (vdW) Compression

Since the discovery of graphene in 2004, two-dimensional (2D) materials have attracted considerable attention from the scientific community. To date, a wide variety of 2D materials are known, such as MXenes and transition metal dichalcogenides, as well as monolayers composed of a single type of atom from elements such as carbon (C), silicon (Si), germanium (Ge), and phosphorus (P). Most of these materials grow in three dimensions, forming structures stabilized by van der Waals (vdW) forces, which makes it relatively easy to exfoliate atomically thin layers.

However, this is not the case for metals, which grow three-dimensionally through strong chemical bonding. Until recently, it was believed that obtaining an atomically thin metal layer was practically impossible, as such structures would also be thermodynamically unstable.

Recently, a group of researchers in China succeeded in producing two-dimensional metals with thicknesses on the order of angstroms using a technique known as van der Waals compression. To carry out this process, they first grew a monolayer of molybdenum disulfide (MoS₂) on a sapphire substrate. This bilayer serves as a base or bottom anvil. A small amount of metal was then placed on the MoS₂ monolayer and heated until it formed a molten droplet. A second MoS₂/sapphire layer was placed on top, with the MoS₂ in direct contact with the molten metal. A pressure of 200 MPa was applied and maintained until both anvils returned to room temperature. The 2D metal was then obtained via a cleaving process that separated the MoS₂/2D-metal/MoS₂ sandwich from the sapphire substrates.

Using this simple and effective technique, two-dimensional metals have been synthesized from bismuth (Bi), tin (Sn), lead (Pb), indium (In), and gallium (Ga). Transport properties measured via Raman spectroscopy on 2D bismuth revealed enhanced electrical conductivity, improved field-effect performance, and increased conductivity due to a nonlinear Hall effect.

This paves the way to a new line of research focused on exploring metals, alloys, and non-layered materials at the 2D scale, along with the investigation of their properties and potential implementation in various technological devices.

For further information go to: NATURE