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

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.