Theoretical Modelling and Characterisation of Concerted Tip and Surface Effects in NC-AFM
Rising interest in functionalising molecules and surface and building nano-devices, that is nanotechnology, has in uenced demand for techniques that image molecules and surfaces with atomic precision. One such technique that has answered this call is Non-Contact Atomic Force Microscopy (NC-AFM), which has a very successful history of directly probing many atomic properties on a wide variety of systems. Interpretation of measurements, however, is highly dependent on knowledge of the atomic probe tip, which is generally elusive and is the most problematic issue surrounding the eld. Another such issue is in understanding the atomic processes behind measurements, which to a large extent, also depends on knowledge of the atomic tip termination. Since its development, theorists have been attempting to ll in this knowledge gap, making signicant progress, but a substantial gap still remains. This thesis therefore addresses these two issues from a theoretical viewpoint. After describing the theoretical model used to simulate NC-AFM behaviour, it is used in conjunction with various theoretical techniques to study three important eects on NC-AFM measurements contributing to these issues: dierent tip terminations, chemical and structural; the relative stabilities of the surfaces and instabilities within the surface; surfaces defects. The systems studied were the p(2 1) and c( 6 1) reconstructions of the oxidised copper (110) surface and the buckled dimer c(4 2) reconstruction of the silicon (001) surface. The stabilities of the two Cu(110):O surfaces were studied thermodynamically, where conventional theoretical methods were found to unexpectedly fail to accurately describe them, and corrections for this were studied. Using this knowledge, NC-AFM simulations were performed using two dierent chemical tip terminations, and were compared to experimental results, where tip identication was found to be possible on the c( 6 1) surface and the p(2 1) surface if a common ad-atom defect is present. Instabilities in the third surface were studied using two dierent structural tip terminations, where dissipation signals were found to probe the tip-surface interactions in a specic way, due predominantly to soft modes in the surface. The role of defects in NC-AFM manipulations of this surface were then studied, which was found to be crucial to successful manipulation of the surface. As a result of these studies, a tip identication protocol whose successful implementation is novel as well as a novel atomic switching manipulation protocol have been demonstrated.This research demonstrates the important role of theory in addressing some of the most problematic issues limiting unambiguous interpretation of NC-AFM measurements. The studies have elucidated the atomic processes behind atomic NC-AFM dissipation signals when probing a surface with soft modes and have increase our ability to characterise dierent atomic tips.
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