Decades of surface science studies on adsorbed molecules have shown the surface a molecule is adsorbed on can effect the molecule‘s electronic and geometric structure. However, the change in reactivity of a single molecule, induced by the presence of a surface, has not been rigorously investigated.
In this work, the influence of molecule-substrate bonding on the interactions experienced by a Scanning Probe Microscopy (SPM) tip were investigated both experimentally and computationally. Non-Contact Atomic Force Microscopy (NC-AFM) experiments were performed under Ultra-High Vacuum (UHV) conditions at 5 K, and reinforced with complementary atomistic ab initio Density Functional Theory (DFT) simulations of Simulated Non-Contact Atomic Force Microscopy (sNC-AFM) to gain a deeper understanding of the studied system.
A system comprised of C60 molecules adsorbed on the Cu(111) surface was selected for this work as it provides an ideal system for investigating the effects of molecule-substrate bonding. Previous work on this system has shown the C60-Cu(111) interface can be controlled to form two distinct structures, with quantifiable differences in the geometric and electronic properties of the interface. Furthermore, the molecules in both structures regularly adsorb in the same orientation. This allows any difference in the interaction between the tip and the adsorbed C60 molecules to be attributed to differences in the molecule-substrate bonding, rather than differences in the presenting face of the adsorbed C60 molecule. Previous work has also investigated the use of C60 functionalized tips. The studies have shown C60 functionalized tips to be stable and passivized relative to metallic tips. This provided the opportunity for this work to investigate the C60-Cu(111) interface using both reactive Copper tips and C60 functionalized tips.
In the computational side of this work, developments in Simulated Non-Contact Atomic Force Microscopy (sNC-AFM) were made. It was discovered multiple methods exist to calculate the force and potential spectra from the converged Fritz Haber Institute ab initio molecular simulations (FHI-aims) DFT simulations. Where the choice of method can have significant effects on the sNC-AFM spectrum, affecting how analogous the sNC-AFM is to the Experimental Non-Contact Atomic Force Microscopy (eNC-AFM). Furthermore, it was deduced the use of periodic boundary conditions to accurately represent a metallic surface can lead to systematic overestimation of the sNC-AFM spectra. Therefore theory for a periodicity correction was developed.
