Two-dimensional oxygen crystal with a honeycomb lattice on an ultraflat Cu(111)

Abstract Two-dimensional (2D) materials have served as key platforms for exploring novel phenomena and innovative applications, driven by their reduced dimensionality. The existing library of 2D materials includes carbon-based and transition metal-based systems. However, oxygen-based 2D materials are missing. Oxygen, the most abundant element on Earth with a higher electronegativity than carbon, holds the potential to introduce unprecedented functionalities in 2D materials. Here we report the experimental realization of a 2D oxygen crystal with a highly ordered honeycomb structure, termed “oxylene”, formed on an ultraflat Cu(111) surface with exceptional resistance to oxidation. Scanning tunneling microscopy and density functional theory studies reveal that oxylene consists of oxygen atom clusters occupying both face-centered cubic (fcc) and hexagonal closed-packed (hcp) sites with a 4×4 periodicity on the Cu(111) surface. Furthermore, oxylene exhibits distinct magnetic ordering due to the magnetic octupole moments of oxygen atoms. Symmetry analysis suggests that oxylene exhibits linear responses that are observed in the noncollinear antiferromagnetic Weyl semimetal Mn3Sn. Our discovery of oxylene provides intriguing opportunities for exploiting its unique electric and magnetic properties, as well as harnessing oxygen’s high reactivity in 2D form.