Real-Time Imaging of\nOn-Surface Ullmann Polymerization\nReveals an Inhibiting Effect of Adatoms

Ullmann coupling is a widely used reaction for the on-surface\ngrowth\nof low-dimensional carbon nanomaterials. The irreversible nature of\nthis reaction prevents the “self-healing” of defects,\nand a detailed knowledge of its mechanism is therefore essential to\nenable the growth of extended ordered structures. However, the dynamics\nof the Ullmann polymerization remain largely unexplored, as coupling\nevents occur on a timescale faster than conventional scanning probe\nmicroscopy imaging frequencies. Here, we reveal the dynamics of these\nsurface events using high-speed variable-temperature scanning tunneling\nmicroscopy (STM) (10 frames per second). Performing the measurements\nat the onset reaction temperatures provides an unprecedented description\nof the evolution of organometallic (OM) and covalent surface species\nduring the Ullmann polymerization of <i>para</i>-dibromobenzene\non Cu(110). Our results demonstrate the existence of an intermediate\nOM phase with Cu adatoms that inhibits the polymerization. These observations\nnow complete the picture of the pathways of on-surface Ullmann polymerization,\nwhich includes the complex interplay of the phenylene moieties and\nmetal atoms. Our work demonstrates the unique capability of high-speed\nSTM to capture the dynamics of molecular self-assembly and coupling.