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)取得了令人印象深刻的发展，通过从所研究的表面拾取单个探针粒子如CO或Xe来功能化尖端。这种针尖功能化过程大大提高了分辨率，并允许通过NC-AFM成像有机分子的内部成键结构。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与探针颗粒在扫描过程中的灵活性有关。特别是，其在成像过程中的动态偏转导致图像失真、对键长的系统性高估以及在NC-AFM图像中出现虚键(不存在键的对比特征)。这些问题导致了数据解释中的相当大的不确定性和相关的争议。以前，我们证明了通过在氧化的铜表面上轻微压入未定义的尖端并随后分析成像对比度，可以以受控的方式产生端氧端铜尖端(CuO尖端)。末端的氧原子以刚性构型共价束缚，我们能够证明这导致在排斥力区域成像时可以忽略不计的偏转。这些令人鼓舞的结果为本项目奠定了基础，我们将在此项目中探索与上述尖端灵活性问题有关的该尖端的成像特性。特别是，我们将系统地推动有机分子中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，是否可以排除CuO尖端的这种键伪影，以及是否总体上可以通过NC-AFM明确地成像分子间的键，如分子组装内的氢键。另一个主要关注点依赖于实验，即单个有机分子被CuO针尖沿着特定的晶体方向横向操纵。通过NC-AFM，可以确定此类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，并与理论模型相关联。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，我们将在三维(3D NC-AFM)中映射CuO针尖和极性分子之间的(定量)偶极-偶极相互作用力场，并将其与传统的共官能化针尖的结果进行比较。我们的结果将对全面理解在排斥力区域内的NC-AFM对比机制具有很高的相关性，并将建立CuO针尖作为定量高分辨率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.