Transition-Metal Cobalt Nanocluster Formation on 2D Covalent Organic Frameworks

Abstract

Two-dimensional (2D) covalent organic frameworks (COFs) opened a new pathway to generate 2D nanosheets on a substrate using a single-molecule sublimation method in a vacuum. This led to nanoarchitecture with single-molecule accuracy. Such a growth process in a vacuum is essential for developing application device products. Primarily, those COFs are known to be used as nanoporous to trap guest atoms inside, activating the functionality for many applications such as spintronics, biomedicine, drug delivery, catalysts, and water purification. This study examined transition-metal cobalt (Co) atom deposition on the 2D COFs on an atomically flat and clean Cu(111) substrate in a vacuum at room temperature. Surface morphology was monitored using scanning tunneling microscopy (STM), and electronic structures were measured using photoelectron spectroscopy (PES). An initial growth process of Co on 2D COFs is crucial for further transition-metal nanocluster applications. We found that Co atoms have not stayed inside the nanoporous but prefer to be trapped by the edges of the 2D COFs. Once the Co was trapped, other atoms gathered, leading to nanocluster formation with a size of ~3 nm. Although the Co/Cu(111) formed bilayer 10-20 nm size triangular nano-islands, the height was restricted to one monolayer height for the Co nanocluster on 2D-COFs, indicating Co interaction below the COFs. No strong electronic hybridization between Co atoms and 2D COFs was confirmed; therefore, 2D-COFs were not disturbed by the transition metal adsorption. This is a clear contrast to the π-conjugated molecular case, where strong π-d hybridization destroyed the molecular structures.