Tunable transport in carbyne-alkyne molecular wires: Length dependence, parity effects and resonant tunneling

We investigated coherent electronic transport in all-carbon two-terminal junctions formed by semi-infinite carbyne electrodes bridged by phenyl-terminated alkyne oligomers (n = 1–10 repeat units). Using self-consistent DFT (GGA-PBE) and NEGF as implemented in SIESTA/TranSIESTA (DZP-like localized basis, real-space mesh ≈200–300 Ry and dense k-sampling along the transport axis), we computed zero-bias transmission, I–V and G–V curves, projected DOS and full T(E,V) landscapes up to 1.0. V. At low bias all devices lie in an off-resonant tunneling regime: zero-bias conductance decreases strongly with bridge length, from conductances on the order of ∼20 μS for the shortest bridges to below ∼0.3 μS for the longest, corresponding to transmission at E F that falls from ∼0.26 G 0 to ∼4×10−3 G 0 (with G 0 = 2e2/h). At 1.0 V the current similarly spans microampere to sub-microampere ranges (≈22 μA → ≲0.3 μA) across the series. Superposed on this dominant exponential attenuation we find reproducible odd-even (parity) modulations of conductance (tens-of-percent scale) that reflect orbital- symmetry changes when a single repeat unit is added. Finite-bias behavior is governed by discrete resonant channels: T(E,V) maps show HOMO/LUMO-derived bands entering the bias window and producing pronounced nonlinearity and conductance peaks; resonance widths decrease and spacing narrows with increasing chain length, consistent with reduced level broadening and smaller HOMO-LUMO gaps. Taken together, these results demonstrate clear design knobs, bridge length, parity, and contact matching, that tune transport from deep tunneling to resonance-dominated regimes, suggesting applications in nanoscale interconnects, bias-triggered switches and chemically sensitive molecular sensors.