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.

Using Metallic Noncontact Atomic Force Microscope Tips for Imaging Insulators and Polar Molecules: Tip Characterization and Imaging Mechanisms

“…NiO(001) with adsorbed Co-Salen and CO molecules was treated as previously described. 29 The B3LYP 64,65 hybrid functional was employed in order to describe the NiO surface and the adsorbed molecules to produce a NiO band gap of 3.6 eV compared to an experimental value of 4.3 eV 66 and negligible surface rumpling in agreement with experiment. 67 Additionally, prior calculations suggest that B3LYP is able to produce reasonable bond lengths for the CO molecule on NiO despite a very large basis set superposition error (BSSE).…”

“…When adsorbed to NiO(001), the Co-Salen molecule also acquires a 4.4 D vertical dipole moment due to distortion of the molecule and electronic polarization of the system. 29 When this vertical component is added to the horizontal component, the symmetry of the imaged dipole moment is broken. The bright attractive region should now be roughly twice as large as the dark repulsive one, as seen in Figure 11B.…”

“…26 Moreover, the conductivity of the coated tip can be checked to ensure that it is indeed metallic. 27 Recently, this method has been used to determine the adsorption site and geometry of single Co-Salen molecules on the NaCl(001) 28 and NiO(001) 29 surfaces with atomic accuracy. These metallic tips can be used to atomically resolve flat surfaces at large distances with minimal risk of contamination from the surface.…”

Calculating the Entropy Loss on Adsorption of Organic Molecules at Insulating Surfaces

“…[16][17][18][19][20][21][22][23][24] Adsorption geometries in particular can be investigated using scanning probe methods such as non-contact atomic force microscopy (NC-AFM). [25][26][27] However, achieving high resolution in NC-AFM images at room temperature is still a challenge 28 and often the interpretation of images down to an atomistic level requires a detailed understanding of the tip-sample interactions. [29][30][31] Theoretical calculations can provide a better understanding of adsorption geometries, film structures and nucleation sites.…”

Flexible and modular virtual scanning probe microscope

“…The instrument is, in principle, able to probe any kind of material: it allows detailed analysis of previously unexplored insulating surfaces [3], and thereby adsorbed molecular layers [4,5]. NC-AFM has the ability to operate in atmospheric and liquid environments, allowing characterisation of a wide variety of interactions in very different mediums [6][7][8].…”