Quantitative High-Resolution Atomic Force Microscopy of Organic Compounds with Copper-Oxide Functionalized Tips

点击翻译按钮获取中文摘要

In recent years, Noncontact atomic force microscopy (NC-AFM) has seen impressive developments by functionalizing the tip apex by picking up a single probe particle like CO or Xe from the surface under study. This tip functionalization procedure drastically increases the resolution and allows imaging the internal bonding structure of organic molecules by NC-AFM. While this has led to enormous progress in organic (surface-) chemistry, a major drawback of this approach is related to the flexibility of the probe particles during scanning. In particular, its dynamic deflection during the imaging process leads to image distortions, a systematic overestimation of bond lengths, and the appearance of virtual bonds (contrast features where no bonds exist) in the NC-AFM images. These issues have led to considerable uncertainties in data interpretation and associated controversies. Previously, we demonstrated that an oxygen-terminated Cu tip (CuO tip) can be created in a controlled way by slight indentations of an undefined tip in an oxidized Cu surface and subsequent analysis of the imaging contrasts. The terminating O atom of this tip is covalently bound in a rigid configuration and we were able to show that this leads to negligible deflection when imaging in the repulsive force regime. These encouraging results set the basis for this project, where we will explore the imaging properties of this tip with regard to the above mentioned tip flexibility issues. In particular, we will systematically push the limits of quantitative bond length determination within organic molecules. Also, by choosing dedicated molecular model systems, we will address the issue of virtual bonds. The crucial question is, whether such bond artefacts can be excluded for the CuO tip and if it is in general possible to unambiguously image intermolecular bonds such as hydrogen bonds within molecular assemblies by NC-AFM. Another major focus relies on experiments where single organic molecules are laterally manipulated by a CuO tip along specific crystallographic directions. By NC-AFM, it is possible to determine the quantitative thresholds of vertical and lateral forces within such manipulation processes. Most importantly, performing these experiments with CuO-functionalized tips, will drastically improve the comparability between different experiments and the correlation with theoretical models. Finally, we will investigate molecules with different inherent electric dipoles pointing perpendicular to the surface. Here we will map the (quantitative) dipole-dipole interaction force fields between the CuO tip and the polar molecules in three dimensions (3D NC-AFM), which will be compared to results obtained with conventional CO-functionalized tips. Our results will be highly relevant for a comprehensive understanding of NC-AFM contrast mechanisms within in the repulsive force regime and will establish the CuO tip as a benchmark probe for quantitative high-resolution NC-AFM experiments.