Atomically-defined on-surface synthesis of multilayer metal-organic frameworks

On-surface synthesis allows the design and study of new materials with unrivaled resolution, but it is traditionally limited to materials of monolayer thickness. Here, we present on-surface-prepared multilayer Fe-TCNQ metal-organic framework (MOF), demonstrating that this limitation can be overcome. The multilayer Fe-TCNQ features both in-plane and out-of-plane chemical bonding, as proven by high-resolution Scanning Tunneling Microscopy and Density Functional Theory computations. The coordination geometry and electronic structure of the embedded Fe cations are affected by the interlayer chemical interaction, making the properties of the multilayer MOF distinct from the monolayer case. Furthermore, we demonstrate how the support defines the growth mode of the multilayer MOF: we observe Volmer-Weber growth mode on a graphene/Ir(111) support, but Stranski-Krastanov mode on Au(111). Our work presents new opportunities for on-surface approaches to 3D MOF synthesis and paves the way for atomic-scale characterization of these important materials.