The mechanisms of contrast formation in non-contact atomic force microscopy on insulating surfaces
This work considers the problem of the prediction and interpretation of atomic resolution in NC-AFM images of ionic surfaces and adsorbed molecules, and the relation between atomic resolution and electronic properties of the system. Several tip models were studied over the MgO(OOl) surface, where models giving the strongest contrast were selected. The oxide tip model and the model of a Si tip with a dangling bond were used in further investigation of clean surfaces, single adsorbed molecules, and molecular monolayers. Ab initio calculations based on density functional theory were used extensively for static structural optimisation and electronic structure analysis. Atomistic simulations with pair potentials were performed where possible. Interatomic potentials between organic molecules and oxide had to be fitted from the ab initio results. This study predicts that atomic and chemical resolution can be achieved on a clean surface of aluminium oxide for both tip models. It is demonstrated that using the Si tip with an apex dangling bond enables one to provide straightforward chemical interpretation of images on ionic insulating surfaces. The resolution, mechanisms of tip-surface interaction and contrast formation are explained by the study of HCOOH monolayers on a TiC>2(l 10) surface imaged using the Si tip model. An interpretation of previous experimental data on this system is suggested based on the result of this study. The resolution within charged formate ions on MgO(001) and the effect of the adsorbate on the substrate resolution is studied using the oxide tip model. Fundamental questions regarding limits of resolution in surface-adsorbate systems are addressed. Finally, the applicability of NC-AFM techniques for the detection of ionic filling inside single-wall nanotubes, and the interplay between electronic structures of the two systems are investigated.