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

近年来，非接触原子力显微镜（NC-AFM）已经取得了令人印象深刻的进展，通过从被研究的表面拾取单个探针颗粒（如CO或Xe）来实现尖端的功能化。这种尖端功能化程序大大提高了分辨率，并允许通过NC-AFM成像有机分子的内部键合结构。虽然这导致了有机（表面）化学的巨大进步，但这种方法的一个主要缺点是与扫描过程中探针颗粒的灵活性有关。特别是，它在成像过程中的动态偏转会导致图像失真，对键长度的系统性高估，以及在NC-AFM图像中出现虚拟键（不存在键的对比特征）。这些问题导致了数据解释的相当大的不确定性和相关的争议。在此之前，我们证明了可以通过在氧化Cu表面上的未定义尖端的轻微压痕和随后的成像对比分析，以一种可控的方式创建氧端化铜尖端（CuO尖端）。该尖端的末端O原子在刚性构型中是共价结合的，我们能够证明，在排斥力区成像时，这导致了可以忽略不计的偏转。这些令人鼓舞的结果为这个项目奠定了基础，我们将在上面提到的尖端灵活性问题上探索这个尖端的成像特性。特别是，我们将系统地推动有机分子内定量键长测定的极限。此外，通过选择专用的分子模型系统，我们将解决虚拟键的问题。关键的问题是，这种键伪影是否可以被排除在CuO尖端之外，以及通常是否有可能通过NC-AFM明确地成像分子间键，如分子组装中的氢键。另一个主要的焦点依赖于单个有机分子沿着特定晶体学方向被CuO尖端横向操纵的实验。通过NC-AFM，可以确定这种操作过程中垂直和侧向力的定量阈值。最重要的是，用cuo功能化的尖端进行这些实验，将大大提高不同实验之间的可比性以及与理论模型的相关性。最后，我们将研究具有垂直于表面的不同固有电偶极子的分子。在这里，我们将绘制三维（3D NC-AFM） CuO尖端与极性分子之间的（定量）偶极子-偶极子相互作用力场，并将其与传统co功能化尖端获得的结果进行比较。我们的研究结果将与全面了解NC-AFM在排斥力范围内的对比机制高度相关，并将建立CuO尖端作为定量高分辨率NC-AFM实验的基准探针。

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.