Iron-catalysed on-surface synthesis of substrate-decoupled graphdiyne monolayers

Graphdiynes constitute an emerging class of two-dimensional sp-sp2 carbon allotropes with tunable electronic properties not accessible by graphene. Although flawless monolayer growth of graphdiyne networks has been attempted by means of on-surface synthesis protocols, the experimental realization of fully covalent and structurally ordered graphdiyne networks remains challenging due to the persistence of metalated intermediates and reaction byproducts, limiting the structural self-organization required for long-range covalent order. Here, we demonstrate that adding minute amounts of Fe atoms on the surface during the growth can exceptionally improve the synthesis of a covalent 2D graphdiyne monolayer framework on Au(111). By means of low-temperature scanning tunneling microscopy and spectroscopy, combined with density functional theory, we unveil the atomic-scale surface dynamics of the reaction. We demonstrate the crucial role of Fe atoms that bond with Br by-products, thereby forming Fe-Br species that promote the removal of Au adatoms and drive the conversion from a metalated network to a purely covalent framework upon mild annealing conditions. Moreover, we show the covalent graphdiyne network to be structurally and electronically decoupled from the underlying metallic substrate, revealing a finite bandgap of about 1.6 eV defined by the position of p_z frontier orbitals. These results establish a viable route for the atomically precise on-surface synthesis of all-carbon graphdiynes, and open the way to semiconducting 2D carbon materials complementing graphene.