¹Beijing National Laboratory for Condensed Matter Physics and Institute of Physics, Chinese Academy of Sciences, Beijing, 100190, China

²School of Physical Sciences, University of Chinese Academy of Sciences, Beijing, 100049, China

Two-dimensional (2D) materials, starting with the groundbreaking exfoliation of graphene in 2004, have initiated the “2D Age” and transformed the landscape of fundamental research and technological advances in condensed-matter physics, materials science, and beyond [1,2,3,4,5,6]. Over the past two decades, the 2D family has markedly expanded and now includes nearly 2000 theoretically predicted and hundreds of laboratory-accessible species with diverse properties [7]. However, these 2D materials are largely limited to layered crystals in which their weak van der Waals (vdW) interlayer interactions facilitate the isolation from their bulk, for example, by top-down exfoliation.

Atomically thin 2D metals stand as highly sought-after materials in condensed-matter physics, which can not only broaden the 2D family beyond vdW layered materials but also promote a variety of theoretical, experimental and technological advances, such as new quantum optical effects, topological edge states, superconductivity, metallic ferroelectricity, quantum Griffiths singularity, superior nonlinearity, catalysis, and all-metallic transistors [8,9,10,11,12,13,14,15,16,17,18,19,20,21]. In fact, if one looks back at the pioneering 2004 paper of Nobel laureates Andre Geim and Konstantin Novoselov that launched the 2D material field [1], you will find that its original goal, initially, was 2D metals and this is why graphene was highlighted as a metallic material. However, unlike the widely studied 2D vdW layered crystals, metals are highly symmetric and strongly bonded; thus, their 2D forms, especially at the atomically thin limit, tend to be thermodynamically unfavorable and are extremely challenging to realize.

Recently, Zhang’s group at the Institute of Physics, Chinese Academy of Sciences developed a convenient, universal method dubbed vdW squeezing and realized for the first time large-scale (> 100 µm), truly “all-vdW” 2D metals down to the ångström thickness limit [22, 23]. Figure 1a illustrates the vdW squeezing process. First, a certain amount of metal powder is placed on the bottom monolayer MoS₂/sapphire anvil and then heated until it melts to form a droplet (panel I in Fig. 1a). Then, another monolayer MoS₂/sapphire, which is utilized as the top anvil, is placed face to face to the bottom anvil, and they move towards each other until they make contact (panel II in Fig. 1a). Subsequently, the liquid metal is gradually squeezed by the two anvils at a high pressure of ~ 200 MPa, roughly double the pressure at the bottom of the Mariana Trench (panel III in Fig. 1a). The squeezing process is maintained until the two opposite anvils are cooled down to room temperature and 2D metals are eventually formed. Finally, MoS₂-encapsulated 2D metals on sapphire are realized for subsequent characterization by removing one of the two sapphire substrates (Fig. 1a, panel IV).

We highlight that a key trick to realizing 2D metals is the use of single crystalline monolayer MoS₂ epitaxially grown on sapphire as bottom and top anvils [24]. We envision that graphene, h-BN, and other 2D layered materials can also be used as the vdW anvils for preparation of atomically thin 2D metals [25].

With the vdW squeezing technique, diverse 2D metals (including Bi, Ga, In, Sn, and Pb) are realized (Figs. 1b-c). Remarkably, the thicknesses of these as-produced 2D metals can all reach to the ångström limit, e.g., ~ 6.3 Å for Bi, ~ 5.8 Å for Sn, ~ 7.5 Å for Pb, ~ 8.4 Å for In, and ~ 9.2 Å for Ga (Fig. 1d). The vdW squeezing represents a groundbreaking approach for producing 2D metals, featuring metrics unattainable by previous methods. First, 2D metals by vdW squeezing can be reachable in large sizes, e.g., ~ 100 µm for single crystalline Bi monolayer (Fig. 1b), two orders of magnitude larger than those reported previously [15, 18, 19, 26]. Second, the full-vdW encapsulated characteristics also ensure that the as-produced 2D metals present non-bonded interfaces (i.e., “all-vdW” nature), facilitating the access of intrinsic physical properties. By contrast, most 2D metals previously prepared are covalently bonded to substrates [13], and thus the measured properties are usually non-intrinsic. Third, the as-produced 2D metal films are full-vdW encapsulated by MoS₂ monolayers and therefore are extremely environmentally stable. Notably, the encapsulated monolayer Bi can last at least one year in an atmospheric environment without degradation or oxidation. Last but not least, the vdW squeezing methodology facilitates the realization of 2D metals with various thicknesses (such as one-, two- and three-unit-cell thick) by controlling the squeezing pressure, providing a firm basis for the exploration of fascinating layer-dependent physical properties.

Undoubtedly, this is just the beginning of 2D metal field, and there are plenty of new opportunities and emerging technological applications to be eagerly awaited. For example, the physical properties of 2D metals are a “new frontier” but have not yet been explored. Consequently, an important future research frontier is to explore and control the unique properties of 2D metals through different external degrees of freedom (such as ultrafast optical excitations, electric and magnetic fields, strain, twist angle, doping, pressure, and Floquet engineering). This will facilitate the opportunities of underpinning emerging quantum states of matter, addressing important fundamental problems in condensed-matter physics and materials science, and initiating original breakthroughs “from 0 to 1.” We envision that by integrating with other systems (e.g., optical cavities, resonances, silicon-based waveguides, and fibers), 2D metals will outline a bright vision for numerous emerging electronic, photonic and optoelectronic applications.

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[Source: https://link.springer.com/article/10.1007/s43673-025-00165-7]