Foster, Adam Stuart; (2000) Theoretical modelling of non-contact atomic force microscopy on insulators. Doctoral thesis (Ph.D.), University College London (United Kingdom).

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Abstract

The Non-contact Atomic Force Microscope (NC-AFM) now offers the surface scientist the ability to resolve individual atoms on the surfaces of insulators. Atomically resolved NC-AFM images have now been demonstrated on insulating surfaces such as sodium chloride, calcium difluoride and nickel oxide, as well as many metallic and semi-conducting surfaces. However, in nearly all experiments the amount of information that can be extracted from these images alone is limited, and usually the identity of the resolved features is unknown. A great deal more information and understanding can be achieved if experimental data is combined with theoretical modelling of NC-AFM. A theoretical model was developed to reproduce the real NC-AFM tip-surface interactions and to simulate the behaviour of the oscillating cantilever under the influence of these interactions. A general study of the components of the tip-surface interaction has been performed, with special regard to the complex electrostatic interactions which are relevant to NC-AFM both at the microscopic and macroscopic scale. This study was used to produce a physical model of a NC-AFM tip which could be used in further modelling. In an attempt to characterize some of the important processes in NC-AFM, the model was first used to analyze the role of the tip, atomic relaxation and image forces in NC-AFM imaging. The model was then applied to NC-AFM imaging of the 1x1 reconstruction of the (110) surface of titanium dioxide, a classic benchmark in surface science. Theoretical scanlines of the surface compared well with experimental results, but it was found that image forces were important in imaging of TiO2. The model, including image forces, was then applied to two characteristic systems which have been atomically resolved in NC-AFM: (i) a thin film of NaCl on a copper substrate and (ii) the (111) surface of CaF2. In both cases the theoretical results compared well with experiment and extracted a lot more information about the tip, surface and tip-surface interactions than was previously available. In particular, for the first time in NC-AFM imaging of insulators, comparison of theory and experiment on the CaF2 surface allowed the sublattice imaged in experiments to be identified. Finally, the possibility of detecting the exchange force on a magnetic surface with a metal coated NC-AFM tip has been studied theoretically and compared with experimental results on the antiferromagnetic NiO (001) surface. The importance of theoretical modelling of NC-AFM in improving understanding of experimental results has been demonstrated. The model developed here has proved successful in simulating a wide variety of surfaces and interactions, and has greatly increased the amount of physical information available on the tip and surface structure, and tip-surface interactions compared to experiment alone.

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