Using metallic noncontact atomic force microscope tips for imaging insulators and polar molecules: tip characterization and imaging mechanisms

We demonstrate that using metallic tips for noncontact atomic force microscopy (NC-AFM) imaging at relatively large (>0.5 nm) tip-surface separations provides a reliable method for studying molecules on insulating surfaces with chemical resolution and greatly reduces the complexity of interpreting experimental data. The experimental NC-AFM imaging and theoretical simulations were carried out for the NiO(001) surface as well as adsorbed CO and Co-Salen molecules using Cr-coated Si tips. The experimental results and density functional theory calculations confirm that metallic tips possess a permanent electric dipole moment with its positive end oriented toward the sample. By analyzing the experimental data, we could directly determine the dipole moment of the Cr-coated tip. A model representing the metallic tip as a point dipole is described and shown to produce NC-AFM images of individual CO molecules adsorbed onto NiO(001) in good quantitative agreement with experimental results. Finally, we discuss methods for characterizing the structure of metal-coated tips and the application of these tips to imaging dipoles of large adsorbed molecules. © 2014 American Chemical Society

Determining adsorption geometry, bonding, and translational pathways of a metal-organic complex on an oxide surface: Co-salen on NiO(001)

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. © 2012 American Chemical Society