Evolution of the Topological Energy Band in Graphene Nanoribbons.

Topological theory has been recently applied in graphene nanoribbons (GNRs) and predicts the existence of topological quantum states in junctions connecting GNRs of different topological classes. Through the periodic alignment of the topological states along a GNR backbone, frontier GNR electronic bands with tunable band gaps and band widths could be generated. In this work, we demonstrate the evolution of the topological band by fabricating GNR structures hosting a single topological junction, dimerized junctions, and multiple coupled junctions with on-surface synthesis, which guarantees the atomic precision of these nanostructures. Their structural and electronic properties are investigated by scanning tunneling microscopy and spectroscopy supported by tight-binding theory. The 1D superlattice of the topological junction states can be described by an effective two-band tight-binding Su-Schrieffer-Heeger (SSH) type model considering two alternating coupling motifs.