Two-dimensional few-atom noble gas clusters in a graphene sandwich

TL;DR

This study visualizes and analyzes two-dimensional noble gas clusters encapsulated between graphene layers, revealing their atomic arrangements, stability, and dynamics, and opening new avenues for condensed matter physics and quantum technology research.

Contribution

First direct imaging of noble gas clusters in a graphene sandwich, revealing structural details and dynamic behavior at elevated pressures.

Findings

Small clusters (N<9) follow expected van der Waals arrangements.

Larger clusters show deviations due to graphene deformations.

Kr clusters remain solid up to N~100, Xe clusters show fluidity at N~16.

Abstract

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 tunneling microscopy. However, for this technique, the encapsulation layer often obscures the actual 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 direct imaging with transmission electron microscopy. We show that small crystals (N<9) arrange as expected based on the simple non-directional van der Waals interaction. Crystals larger than this show some deviations for the outermost atoms, possibly enabled by deformations in the encapsulating graphene lattice. We further discuss the dynamics of the clusters within the graphene sandwich, and show that while all Xe clusters with up to N~100 remain solid, Kr clusters with already N~16 turn occasionally fluid under our experimental conditions with an estimated pressure of ca. 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.