Ring-Boundary Engineering in Carbon Allotrope Nanoribbons: From Metallic Interconnects to Negative Differential Resistance and Gate-Tunable Transport

We report a first-principles investigation of the electronic structure and quantum transport properties of hydrogen-passivated X-graphene nanoribbons (XGNRs), a 2D carbon allotrope whose planar unit cell combines pentagonal, hexagonal, and heptagonal rings. Using density functional theory (DFT) coupled with the non-equilibrium Green’s function (NEGF) formalism, we systematically study four edge configurations: zigzag (zzXGR-H), two distinct armchair (armXGR-2P and armXGR-2Hep), and chiral (chXGR-Hep2H). We demonstrate that the ring type exposed at the nanoribbon boundary fully controls the electronic transport response. The zero-bias differential conductance spans more than one order of magnitude, from 2.8 μS (chXGR-Hep2H, semimetallic pseudogap) to 78.6 μS (armXGR-2Hep, heptagonal-edge resonance). armXGR-2P achieves 73.9 μS via a flat-subband pentagonal-edge mechanism, and armXGR-2Hep exhibits robust symmetric negative differential resistance (NDR) with onset near 0.77 V and dI/dV = −12.5 μS at the bias extremes, driven by voltage-induced misalignment of the heptagonal-edge transmission resonance. A systematic benchmarking against phagraphene, net-τ, C-57, PHOTH-G, and biphenylene establishes the unique versatility of XGNRs as all-carbon electronic material platforms.