Interactions between atoms or molecules are of various types, for example, ionic or metallic bonding that is characterized by electrostatic attractions, covalent bonding that involves electron pair sharing, or weak van der Waals (vdW) bonding that arises from temporary electron distribution fluctuations. These types of interaction dictate the distance and pathways for charge transfer between atoms or atomic planes. vdW crystals consist of two-dimensional (2D) atomic layers held together by vdW interactions. Such weak interlayer interactions allow guest atoms or molecules to squeeze in between the layers, while the robust intralayer covalent or ionic bonds ensure the structural integrity of the atomic sheets, creating a confinement space that can withstand collapse or degradation from chemical reactions or external events.

Noble gas atoms confined between graphene layers. Credit: E. Harriet Åhlgren

Inserting ions or molecules into layered materials, known as intercalation, has emerged as a versatile and powerful strategy for doping and engineering phases in electrochemical energy storage applications and electronics. This approach gains further momentum when applied to 2D materials with only a few atomic layers, amplifying the effects of quantum-level confinement and local distortion in the lattice and electronic structures1. Moving beyond simple intercalation, the migration and fusion of intercalated atoms can give rise to the formation of continuous matter, fostering the emergence of unconventional materials or phases stabilized in the interlayer space. For instance, 2D nitrides and metals, distinct from their bulk counterparts, have been synthesized through deposition-assisted confinement growth, employing atom intercalation and diffusion processes2,3.

In an Article in this issue of Nature Materials, Laisi Chen and colleagues present an alternative method for confinement synthesis, wherein bismuth flakes are directly processed in between hexagonal boron nitride (hBN) layers through melting and subsequent compression moulding. This technique yields ultraflat bismuth crystals with thicknesses of less than 10 nm, which is challenging to achieve with conventional growth methods. These ultrathin, ultraclean and ultraflat crystals enable investigations into their intrinsic transport properties and surface states, which are characterized by multi-carrier quantum oscillations and Landau level splitting. Although there is still room for improvement, namely, in reducing grain boundaries, better control over thickness and scaling up to larger crystals, this simple fabrication process complements conventional synthesis techniques such as molecular beam epitaxy, chemical vapour deposition and top-down stacking techniques, and holds promise for stimulating further explorations of similar 2D quantum materials.

Interlayer confinement stabilizes materials in unique structural configurations that are typically unattainable in ambient conditions. This enables the characterization and measurement of the dynamics or physics associated with these low-dimensional configurations. In another Article in this issue, Manuel Längle and colleagues report the observation of the solid and liquid-like dynamics of 2D noble gas atoms confined between graphene layers at room temperature using atomic-resolution scanning transmission electron microscopy. Through irradiation-assisted implantation, Kr or Xe atoms are introduced between the graphene interlayers. The deformation of the graphene sheets generates a pocket and exerts pressure of up to tens of gigapascals on the confined noble gas atoms, thereby maintaining them in a 2D cluster form with sufficiently long time for imaging (pictured). The authors observe a solid-to-liquid phase transition as the number of atoms in the clusters increases. Beyond the transition point, correlated with a critical cluster size, the clusters start to lose their solid characteristics, as the deformation of the graphene sheets and the corresponding pressure on the clusters becomes too low to retain the atoms.

Realizing 2D solid-state noble gas clusters at room temperature, along with the capability of direct atomic-scale imaging, is non-trivial. While graphene layers have often been used as a liquid container for transmission electron microscopy imaging owing to the flexibility and electron transparency of graphene4, the work of Längle and colleagues showcases the vast potential of the vdW interlayer space for characterizing and manipulating both solids and gases. This opens avenues for further studies of atomic ordering, lattice dynamics, symmetry breaking and phase transitions, as discussed in an accompanying News & Views article by Tao Xu and Litao Sun. Moreover, they highlight that the precise confinement of solid or gas atoms at desired locations could unlock potential applications in electronics, quantum computing and energy storage. This requires future efforts to design effective chemical or physical methodologies to achieve accurate positioning within interlayer spaces at a scalable level.

The interlayer space can actively shape materials properties. Materials engineering in terms of the constituent atoms and number of layers in vdW sheets, the type of inserted foreign atom and control of trapping positions can synergistically modulate atomic interactions in a locally confined area. This can facilitate the atomic-scale fabrication and exploration of innovative materials and hybrid structures.