Position-dependent spin-dynamics in cobalt-substituted graphene nanoflakes

Using ab initio quantum chemical calculations, we systematically investigate the ultrafast spin dynamics in a rhombic graphene nanoflake substitutionally doped with a Co atom (Co/GNF). We put particular emphasis on the position dependence of spin-flip processes. Our calculations reveal that when the Co dopant is placed near the zigzag boundary (N-type), spin-flip processes can be accomplished within 0.55 ps-about 40% faster than in centrally doped structures-with minimal Rabi oscillations and reduced decoherence. This enhanced efficiency arises from boundary-mediated exchange interactions, which facilitate a Co-centered spin-flip via hybridized molecular orbitals and energy-level restructuring. In contrast, central doping yields slower spin-flip processes, with reduced interaction between the dopant and edge-localized states. These results reveal how the zigzag-edge geometry governs spin dynamics in Co/GNF systems, establishing a broadly applicable design strategy-such as atomic-scale positioning. Although demonstrated in rhombic GNF-based structures, this approach may be extended to other nanostructures with engineered boundaries. Enabled by advances in the fabrication of substitutionally doped graphene, such atomically precise configurations are experimentally accessible, paving the way for next-generation spintronic device design.