Epitaxial Growth of Free-Standing Bismuth Film on Graphene Embedded with Nontrivial Properties

Topological insulators (TIs) have become one of the intensely pursed topics during the past few years due to their robust gapless edge states, among which bismuth (Bi) enjoys a good reputation because of its rich quantum states. However, bulk Bi itself is topologically trivial. While thin Bi film has long been theoretically perceived to be a two-dimensional topological insulator, it still remains a challenge to construct free-standing Bi(111) film experimentally. Herein, we investigate the epitaxial growth of bismuth films on graphene with the film facet ranging from (110) to (111) with increasing coverage and demonstrate the topologically nontrivial property embedded in Bi(111) layers by angle-resolved photoemission spectroscopy (ARPES) and density functional theory (DFT) calculations. In contrast to the bulk Bi with semimetallicity, a metallic surface state is revealed in the 7-bilayer Bi(111) with a circle-shape photoemission intensity plot, while the stacking of free-standing bismuth bilayers on graphene is rationalized to be responsible for the nontrivial characteristics of Bi(111) with an odd crossing of the Fermi level. Our report herein proposes an intriguing approach toward fabricating topologically nontrivial bismuth film, as well as advancing the fundamental understanding of Bi(111) bilayers. Importantly, the combination of the nontrivial Bi(111) film and graphene might also shed useful insights into the development of nanotronics and spintronics.