Atomically Precise Carbyne-Nanoribbon Devices: Transmission Engineering through Ring Topology and Contact-Site...

We present a first-principles density functional theory combined with the nonequilibrium Green's function formalism (DFT/NEGF) investigation of charge transport in eight molecular junction configurations formed by graphene and phagraphene nanoribbons with zigzag and armchair edge topologies connected to semi-infinite sp-hybridized carbyne chain electrodes at two geometrically distinct attachment sites. In the first contact geometry, the carbyne electrode bonds to an sp² carbon atom sharing its ring connectivity with one adjacent hexagonal ring (referred to as the edge-proximal site), whereas in the second contact geometry, the electrode bonds to an atom whose ring environment includes a pentagonal ring of the 5-6-7 phagraphene network or an equivalent laterally shifted hexagonal site (referred to as the extended-conjugation site). Systematic variation of ring topology, edge orientation, and contact geometry isolates their individual contributions to the quantum transport response. The electrode attachment site is the primary qualitative determinant: the edge-proximal attachment produces weakly conducting, rectifying junctions governed by sharp transmission resonances, while the extended-conjugation attachment opens high-transmission pathways. Ring topology provides the critical quantitative modulation: the zigzag phagraphene junction at the extended-conjugation site carries -27.5 to +28.3 μA at ±1.0 V with a rectification ratio of 1.03 and zero-bias conductance of 0.37 G₀, a 3.3-fold enhancement over its graphene counterpart. The armchair phagraphene device at the edge-proximal site exhibits voltage-controlled rectification reaching a ratio of 4.2 at ±0.5 V, arising from a near-resonant Fermi-level state with a density of states up to 356 eV^-1 created by the 5-6-7 ring topology. These results establish quantitative design rules linking ring topology, edge orientation, and contact geometry to conductance and rectification in sp-sp² carbon nanoelectronic junctions.