The van der Waals atomic solids of noble gases on metals at cryogenic temperatures were the first experimental examples of two-dimensional systems. Recently, such structures have also been created on surfaces under encapsulation by graphene, allowing studies at elevated temperatures through scanning tunnelling microscopy. However, for this technique, the encapsulation layer often obscures the arrangement of the noble gas atoms. Here we create Kr and Xe clusters in between two suspended graphene layers, and uncover their atomic structure through transmission electron microscopy. We show that small crystals (N < 9) arrange on the basis of the simple non-directional van der Waals interaction. Larger crystals show some deviations, possibly enabled by deformations in the encapsulating graphene lattice. We further discuss the dynamics of the clusters within the graphene sandwich, and show that although all the Xe clusters with up to N ≈ 100 remain solid, Kr clusters with already N ≈ 16 turn occasionally fluid under our experimental conditions (under a pressure of ~0.3 GPa). This study opens a way for the so-far unexplored frontier of encapsulated two-dimensional van der Waals solids with exciting possibilities for fundamental condensed-matter physics research and possible applications in quantum information technology.

低温下金属上惰性气体的范德华原子固体是二维系统的第一个实验例子。最近，这种结构也在石墨烯封装的表面上创建，允许通过扫描隧道显微镜在高温下进行研究。然而，对于这种技术，封装层通常会掩盖惰性气体原子的排列。在这里，我们在两个悬浮石墨烯层之间创建了 Kr 和 Xe 簇，并通过透射电子显微镜揭示了它们的原子结构。我们证明小晶体（N  < 9）是根据简单的非方向范德华相互作用排列的。较大的晶体表现出一些偏差，这可能是由于封装石墨烯晶格的变形造成的。我们进一步讨论了石墨烯夹层内团簇的动力学，并表明尽管所有N  ≈ 100以内的 Xe 团簇仍保持固态，但在我们的实验条件下（在 ~ 压力下）， N  ≈ 16 的 Kr 团簇偶尔会变成流体。 0.3GPa）。这项研究为迄今为止尚未探索的封装二维范德华固体前沿开辟了一条道路，为基础凝聚态物理研究和量子信息技术中的可能应用提供了令人兴奋的可能性。