Detecting vibronic excitations of individual pentagon defects with S = 1/2 spin states in graphene nanoribbons.

Non-hexagonal rings represent an important type of topological defects to tailor the electronic, magnetic, and vibrational properties in graphene-based nanomaterials. Despite recent advances of on-surface synthesis, there is still lack of an effective approach to create individual non-hexagonal defects with an open-shell feature in graphene nanoribbons (GNRs) and to decouple the non-hexagons from the metal surfaces for investigating the intrinsic properties. Here, we report an on-surface approach that combines thermally triggered reactions and tip-assisted manipulations to achieve decoupled individual pentagons in bilayer GNR crosses on Au(111) surface. By combining scanning tunneling microscopy/spectroscopy (STM/STS) with non-contact atomic force microscopy (nc-AFM), we can confirm the pentagonal structures with single-bond resolution and the open-shell character with S = 1/2 from the Kondo resonance in on-surface synthesized topological GNRs. By utilizing the tip-assisted manipulation, we construct the bilayer GNR cross with the methyl-group sandwiched individual pentagon on top of a pristine all-hexagonal GNR segment, which effectively decouples the pentagon from the metallic surface. The open-shell nature of the single pentagon defect can be directly confirmed by the presence of well-defined singly occupied and unoccupied molecular orbitals (SOMO and SUMO), supported by first-principles calculations. Benefiting from the decoupled nature, we also observe vibronic peaks associated with the resonant electron tunneling into SOMO and SUMO, which can be well attributed to the vibrational excitations of the local D and D' modes in defective graphene. These findings demonstrate a versatile manner to explore the intrinsic electronic, vibrational, and magnetic properties of individual defects in graphene nanostructures.