Determining Adsorption Geometry, Bonding, and Translational Pathways of a Metal–Organic Complex on an Oxide Surface: Co-Salen on NiO(001)

Abstract

Individual molecules of Co-Salen, a small chiral paramagnetic metal–organic complex, deposited on NiO(001) were imaged with noncontact atomic force microscopy (NC-AFM) using metallic Cr coated tips. Experimentally, we simultaneously resolve both the molecule and the individual surface ions. Images recorded at low temperatures show that the Co-Salen molecules are aligned slightly away from the ⟨110⟩ directions of the surface and that the Co center of the molecule is located above a bright spot in atomically resolved images of the surface. Density functional theory calculations predict that the molecule adsorbs with the central Co atom on top of an oxygen ion and is in its lowest energy configuration aligned either + or −4° away from the ⟨110⟩ directions, dependent on the chirality of the molecule. Combining theoretical predictions and experimental data allows us to identify bright spots in NC-AFM images as oxygen sites on NiO(001) and hence determine the exact adsorption geometry and position of the molecule. Additionally, we observed tip-induced translations of the Co-Salen molecules along ⟨110⟩ directions on the substrate, which corresponds to the lowest energy pathway for diffusion. A comparison of these results with theoretical calculations and previously published experimental data for Co-Salen on the (001) surface of bulk NaCl highlights differences in the character of adsorption of individual molecules and the ensuing growth of Co-Salen thin films on these substrates.