Graphene's Latest Cousin: Plumbene Epitaxial Growth on a "Nano WaterCube".

While theoretical studies predicted the stability and exotic properties of plumbene, the last group-14 cousin of graphene, its realization has remained a challenging quest. Here, it is shown with compelling evidence that plumbene is epitaxially grown by segregation on a Pd1- x Pbx (111) alloy surface. In scanning tunneling microscopy (STM), it exhibits a unique surface morphology resembling the famous Weaire-Phelan bubble structure of the Olympic "WaterCube" in Beijing. The "soap bubbles" of this "Nano WaterCube" are adjustable with their average sizes (in-between 15 and 80 nm) related to the Pb concentration (x < 0.2) dependence of the lattice parameter of the Pd1- x Pbx (111) alloy surface. Angle-resolved core-level measurements demonstrate that a lead sheet overlays the Pd1- x Pbx (111) alloy. Atomic-scale STM images of this Pb sheet show a planar honeycomb structure with a unit cell ranging from 0.48 to 0.49 nm corresponding to that of the standalone 2D topological insulator plumbene.