Deconstruction of the Electronic Properties of a Topological Insulator with a Two-Dimensional Noble Metal-Organic Honeycomb-Kagome Band Structure

Some metal-organic (MO) lattices with a strong spin-orbit coupling (SOC) have been predicted to behave as two-dimensional topological insulators (2D-TIs). The polarization of metallic edge states with the opposite electron spin in 2D-TIs, in otherwise insulating 2D MO sheets, is interesting for spintronic technology. The 2D-TI character of MO lattices, however, has not been confirmed by experiment. The main challenge has been that MO lattices , usually self-organize on metal substrates, which can introduce interactions that modify and can even suppress the topological, character. We calculate the topological properties of 2D metal-organic honeycomb lattice composed of noble metal atom vertices and bidentate 1,4-phenylene diisocyanide (PDI) linkers, which form metal-organic honeycomb-Kagome lattices (MOHKLs) band structure, free and on metal substrates. The selection of vertices and linkers can independently tune the SOC and transport properties. Calculations predict that unsupported 2D MOHKLs indeed possess SOC-induced gaps between the Dirac bands at the K-points. Furthermore, nanoribbons of such MOHKLs are calculated to possess metallic spin-polarized edge states. Supporting MOHKLs on a metal substrate, however, introduces an electric potential, giving rise to Rashba SOC, which can collapse the band gaps. Molecule-resolved measurements by scanning tunneling microscopy and spectroscopy test the electronic properties of Ag-PDI MOHKL self-assembled on Ag(111) surface, but find no evidence of the 2D-TI electronic band structure. Releasing MOHKLs from the electronic and chemical influences of substrates to preserve their TI properties remains a challenge. The Rashba SOC provides an additional tool for designing 2D-TI band structures.