Substrate control of the electronic phase of low-buckled plumbene.

Low-buckled plumbene, the heaviest group-14 honeycomb monolayer with the strongest spin-orbit coupling of the family, is nevertheless a normal insulator rather than a quantum spin Hall insulator. Using first-principles calculations with a van der Waals density functional and spin-orbit coupling, we ask what a substrate does to it. On hexagonal boron nitride, whose bands stay far from the Fermi level, plumbene remains a 0.34 eV insulator. An adaptively converged first-principles Wilson loop gives Z 2 = 0 for this non-centrosymmetric interface, under the same occupied-subspace protocol that reproduces nontrivial stanene and trivial free plumbene. On graphene, independent full-SOC relaxations of the hollow, top, and bridge registries all give semimetallic indirect overlaps of 0.073-0.076 eV; hollow is lower by at least 3.03 meV per cell in every cutoff and k-mesh check. Stanene acts structurally, through the buckling its strong binding forces on the sheet, and produces a robust metal within the coherent 1 × 1 models tested: the metallisation persists however the lattice mismatch is partitioned, including with plumbene unstrained, and imposing that buckling on the free sheet closes its gap on its own. Fu-Kane parity analysis, validated against stanene as a known quantum spin Hall insulator, finds free plumbene trivial at every gapped strain; compression metallises it between -3.7% and -3.9% without inverting the bands. Thus these nonmagnetic supports select ordinary-insulating, semimetallic, or metallic phases through distinct electronic and structural mechanisms, but none induces a supported quantum spin Hall phase in the commensurate models tested.