Two-dimensional
(2D) covalent organic frameworks (COFs) fabricated
through on-surface synthesis were investigated as a honeycomb nanopore
template for the growth of 3d-transition-metal nanoclusters
(NCs) with a size of 2 nm on a metallic substrate. The evolution of
these NCs and their electronic characteristics were studied employing
scanning tunneling microscopy/spectroscopy (STM/STS), angle-resolved
ultraviolet photoelectron spectroscopy (ARUPS), and X-ray photoelectron
spectroscopy (XPS) under an ultrahigh-vacuum (UHV) condition at room
temperature. The 2D COFs were synthesized on Cu(111) substrate utilizing
1,3,5-tris(4-bromophenyl)benzene (TBB) precursors, which engendered
a honeycomb nanopore array of approximately 2 nm in size. In contrast
to the behavior observed in the Co/Cu(111) system producing triangular-shaped
bilayer-stacking nanoclusters measuring 10–20 nm, STM imaging
of Co/COFs/Cu(111) revealed the growth of Co NCs of approximately
1.5 nm within a single COF nanopore. This growth occurred without
forming a monolayer film beneath the COFs, providing direct evidence
that the 2D COFs on Cu(111) can effectively entrap Co atoms within
the nanopore, giving rise to Co NCs. Spectroscopy measurements, including
STS/UPS/XPS, confirmed the different local densities of states for
Co NCs and COFs, corroborating the coexistence of Co NCs and COFs
on the surface.