Molecular 2D Kagome Lattices with Intrinsic Metallicity: Synthesis and Characterization

Fabricating new 2D materials from molecular components is desirable because of the extraordinary flexibility of molecular building blocks. This has driven much growth in the fields of covalent organic frameworks (COFs) and metal organic frameworks (MOFs). A common problem with molecular materials, however, is that they are typically insulators and very difficult to electrically dope. Here we describe a new synthesis strategy for growing periodic 2D and 1D molecular networks that exhibit intrinsic metallicity. We achieved this through a unique covalent bonding motif that occurs between N-heterocyclic carbenes (NHCs) and transition metal atoms. A carbene is a molecule with two unpaired electrons (i.e., radicals), and when two carbenes bond to a gold atom (i.e., NHC-Au-NHC) then this results in frontier molecular orbitals (FMOs) that each contain a single unpaired electron. Periodic structures built from such FMOs thus exhibit intrinsic metallicity. We have shown that it is possible to synthesize 1D linear chains of NHC-Au-NHC junctions as well as 2D Kagome lattices. The Kagome lattices were fabricated by using carbene precursors with trigonal symmetry that naturally bond with Au atoms on a Au(111) crystal surface. The NHC-Au-NHC FMO, however, has a relatively high energy and so electrons in FMO-derived bands transfer to high work function substrates like Au(111), thus quenching the metallicity on these substrates. We have used scanning tunneling microscopy to image such 1D chains and 2D Kagome lattices and to probe their local electronic structure. STM spectroscopy reveals that the properties of these networks are in reasonable agreement with ab initio theoretical simulations.