Thermally induced evolution of on-surface-synthesized crown ether polymers on Cu(111): transition from semiconductor chains to metallic nanoflakes

We present a scanning tunneling microscopy study of on-surface synthesis using bromine-terminated crown ether (BrCR) molecules adsorbed on Cu(111) under ultra-high vacuum. Although theoretical models predict that high-temperature annealing yields energetically stable linear or trident-linked crown ether (CR) structures forming ordered two-dimensional (2D) networks, our experiments reveal alternative reaction pathways. A self-assembled BrCR monolayer prepared at 300 K with semiconducting characteristics transforms into one-dimensional polymer chains upon annealing at 450–500 K. In contrast, annealing at 600–700 K induces a drastic transition to 2D nanoflakes with metallic properties. This transformation is accompanied by a significant reduction in apparent molecular height and pronounced changes in the local density of states. These results demonstrate that annealing temperature enables controlled tuning of CR-derived nanocompounds and electronic properties from semiconducting to metallic.