On-surface growth of transition-metal cobalt nanoclusters using a 2D crown-ether array

Advanced methods of producing nanometer-size transition metal clusters have enhanced the functionality of nanoclusters (NCs), leading to single-atom catalysis, miniaturization of spintronics media, and biomedicine. Herein, on-surface growth of transition-metal cobalt (Co) NCs in a vacuum was demonstrated using a structurally flexible two-dimensional (2D) 4,4 ',5,5 '-tetrabromodibenzo[18]crown-6 ether (BrCR) ring molecular array, prepared on an atomically flat noble metal Cu(111) surface. Then, we successfully produced two different sizes (similar to 1.5 nm and similar to 3.6 nm) of Co NCs using the trapping ability of BrCR. The growth and electronic structures of Co NCs were investigated by low-temperature scanning tunneling microscopy/spectroscopy (STM/STS) and angle-resolved photoelectron spectroscopy (ARPES) in an ultrahigh vacuum. DFT calculations unveiled the energetically stable adsorption sites. The growth process of Co NCs is governed by the electronic pi-d hybridization between BrCR and Co. Once the Co atom was trapped, it became a nucleation center, around which other Co atoms gathered to form a nanocluster. In contrast to the standard behavior of crown ether molecules in solution, the stable position of the trapped metal atom is not at the center of the crown ring but at the BrCR edge, where the Co atom is located below the side substituents. We report an on-surface growth of transition-metal nanoclusters using a 2D self-assembled monolayer (SAM) crown ether ring molecular array prepared on Cu(111).