On-Surface Synthesis of Three-Dimensional Graphene Nanoribbons via a [2 + 2] Cycloaddition Reaction and Their Planarization

Novel carbon nanomaterials have garnered significant interest due to their unique structures and potential applications in molecular electronics. On-surface synthesis has emerged as a highly feasible method for the precise fabrication of these species, including zero-dimensional (0D), one-dimensional (1D), and two-dimensional (2D) carbon architectures. However, research on extending on-surface synthesis to three-dimensional (3D) molecular frameworks remains limited. Here, we investigate the self-assembly and [2 + 2] cycloaddition reaction of a three-dimensional molecule (C₂₀Br₆H₈) on a Au(111) substrate. Starting from a self-assembled structure in a side-lying configuration, thermal treatment first induces a structural transformation to a standing-up configuration, accompanied by molecular dehalogenation and metalation reactions. Further increasing the temperature triggers carbon–carbon coupling, leading to the formation of 3D graphene nanoribbons. These nanoribbons can be further planarized through additional thermal treatment or scanning probe manipulation. Our findings provide new insights into the fabrication of advanced carbon-based materials with complex architectures.