Growth of Transition-Metal Cobalt Nanoclusters on 2D Covalent Organic Frameworks

Abstract

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 bilayerstacking 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.

Designing 2D stripe winding network through crown-ether intermediate Ullmann coupling on Cu(111) surface

“…1(f) presents the STM topographic depiction acquired on the BrCR array structure prepared on Cu(111) at 300 K under UHV conditions, distinctly demonstrating the absence of on-surface synthesis at this temperature. This stands in stark contrast to observations with 4,4′′-dibromo- p -terphenyl 87 or TBB, 12,85,86 wherein Br deposition at 300 K on Cu(111) initiated on-surface synthesis, yielding a planar polymer chain. However, the BrCR arrangement on Cu(111) at 300 K did not manifest such synthesis, as evidenced in Fig.…”

“…This stands in stark contrast to polymer chains synthesized using precursors composed solely of benzene rings. 12,85–87 Such corrugations, reaching a maximum height of approximately 200 pm, may signify the existence of the intermediate state as anticipated in Fig. 2 (twist structure).…”

“…For example, 1,3,5-tris(4-bromophenyl)benzene (TBB) precursors, 12,85,86 known as a precursor to creating the honeycomb porous lattice (Fig. 1(a)), generate not only hexagons but also pentagons and heptagons, producing an incomplete 2D honeycomb network because bromine (Br) desorption, 12,85 thermal diffusion, and covalent bonding formation simultaneously occur.…”