Two-dimensional (2D) metals, similar to graphene, are one-atom-thick elemental metals that exhibit unique properties due to their ultrathin structure. Unlike graphene, metals have nondirectional metallic bonds, making exfoliation from 3D bulk metals difficult without a layered structure. The concept of 2D metals has been extended to include a few-layer thickness, akin to few-layer graphene. 2D metals, such as antimonene (Sb) and stanene (Sn), offer significant potential for applications that require large specific surface areas, such as in electronics, electrochemistry, and catalysis, due to their enhanced surface and edge conductivity, quantum optics, and plasmonic properties.

Background of 2D metals synthesis
Ultrathin metal films, down to a few nanometers, can be synthesized using conventional methods like sputtering, electron-beam evaporation, and physical vapor deposition (PVD), or more advanced techniques such as atomic layer deposition (ALD) and molecular beam epitaxy (MBE). These films often exhibit unique behavior, such as dewetting at thicknesses around 4 nm. For example, 2D stanene has been grown on Bi₂Te₃ substrates, showing well-aligned Sn atoms. However, 2D metals tend to suffer from oxidation in ambient conditions, which can be mitigated by encapsulating them in inert materials like h-BN or graphene. This encapsulation has been used to fabricate stable 2D metal films for device applications.

Five methods to prepare 2D metals
This perspective introduces five methods for fabricating 2D metals, categorized into top-down and bottom-up approaches. Top-down methods include van der Waals squeezing and selective extraction, while bottom-up methods involve electron beam-induced 2D metal growth, graphene-templated Au nanosheets, self-assembly, and CO reduction of Pd nanosheets. These methods are illustrated in the figure, which shows electron beam irradiation-induced 2D metal membranes inside graphene pores, extrusion of liquid metal between MoS₂ anvils, graphene-templated Au nanosheets, CO-reduced Pd nanosheets, and selective extraction of Au monolayers from Ti₃AuC₂ MXene sheets. Each method presents a unique approach to synthesizing 2D metals with distinct properties and applications. This paper briefly discusses these fabrication techniques in detail.

Method I: 2D metals grown in graphene nanopores
A group led by Mark H Rummeli observed a freestanding 2D iron membrane confined within a graphene nanopore using TEM. This 2D Fe membrane was formed from Fe adsorbents introduced during the graphene transfer process, which involved etching a Cu foil substrate with FeCl₃. This confinement technique can also be used to produce other 2D metals such as Cr, Mo, Au, and Zr, along with 2D compounds like Mo₂C and ZnO. The process is similar to injecting metal atoms or fullerenes into carbon nanotubes, where nanoparticle condensation occurs in confined spaces, a concept also applied to the synthesis of other 2D materials.

Method II: 2D metals encapsulated between two sapphire/MoS₂ anvils
Zhang and Du’s lab recently developed a squeezing strategy to produce 2D metals by applying high pressure between two MoS₂-coated sapphire substrates, known as van der Waals anvils. Metal pieces melted on a hot plate are squeezed into a thin planar membrane, with Ga being the most common liquid metal used due to its low melting point of around 30°C. Other metals, including In, Sn, Bi, and Pb, have also been successfully processed into 2D forms using this technique. The approach is inspired by the need for low-resistance metal/semiconductor contacts in 2D material-based transistors, which are essential for improving device performance. These 2D metals may also serve as alternatives to low-melting-point alloys in electronic packaging and solder materials for electrical interconnects.

Future opportunities
This perspective highlights emerging methods for synthesizing 2D metals, with applications in catalysis, superconductivity, and electronic interconnects. These metals, such as Bi, Pb, Sb, and Fe, show great potential as contact electrodes for 2D semiconductor devices, where van der Waals interactions can enable low-resistance ohmic contacts by reducing Schottky barriers. However, research on 2D metals is still in its early stages, with challenges in achieving thermodynamic stability, monolayer growth, and large-scale production. Advances like van der Waals squeezing and molecular beam epitaxy offer pathways for improving metal film quality and expanding the variety of 2D metals. Additionally, efforts are underway to regulate their intrinsic properties for use in devices, with artificial intelligence helping to discover new types of 2D metals. Future research will focus on their electronic, photonic, and catalytic properties.

The complete study is accessible via DOI: 10.34133/research.0790