Janus graphene nanoribbons with localized states on a single zigzag edge

Topological design of p-electrons in zigzag-edged graphene nanoribbons (ZGNRs) leads to a wealth of magnetic quantum phenomena and exotic quantum phases. Symmetric ZGNRs typically exhibit antiferromagnetically coupled spin-ordered edge states. Eliminating cross-edge magnetic coupling in ZGNRs not only enables the realization of a new class of ferromagnetic quantum spin chains, enabling the exploration of quantum spin physics and entanglement of multiple qubits in the 1D limit, but also establishes a long-sought carbon-based ferromagnetic transport channel, pivotal for ultimate scaling of GNR-based quantum electronics. Here, we report a general approach for designing and fabricating such ferromagnetic GNRs in the form of Janus GNRs (JGNRs) with two distinct edge configurations. Guided by Lieb's theorem and topological classification theory, we devised two JGNRs by asymmetrically introducing a topological defect array of benzene motifs to one zigzag edge, while keeping the opposing zigzag edge unchanged. This breaks structural symmetry and creates a sublattice imbalance within each unit cell, initiating a spin symmetry breaking. Three 'Z-shape' precursors are designed to fabricate one parent ZGNR and two JGNRs with an optimal lattice spacing of the defect array for a complete quench of the magnetic edge states at the “defective” edge. Characterization via scanning probe microscopy/spectroscopy and first-principles density functional theory confirms the successful fabrication of JGNRs with ferromagnetic ground state delocalised along the pristine zigzag edge.