The Electric Field of CO Tips and Its Relevance for Atomic Force Microscopy
The Electric Field of CO Tips and Its Relevance for Atomic Force Microscopy
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DOI：
10.1021/acs.nanolett.5b05251
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发表时间：
2016-03-01
期刊：
影响因子：
10.8
通讯作者：
Perez, Ruben
中科院分区：
文献类型：
--
作者：
Ellner, Michael;Pavlicek, Niko;Perez, Ruben
用CO分子装饰的金属尖端为原子力显微镜（AFM）中令人印象深刻的高分辨率铺平了道路。虽然泡利排斥和相关的CO倾斜在短距离上起主导作用，但极性和金属系统的实验表明，静电相互作用对于理解所观察到的复杂对比及其距离演变是必要的。试图用单个静电偶极子取代尖端来描述这些相互作用，导致了对其性质和强度的矛盾陈述。在这里，我们通过对Cl空位的AFM对比进行全面的实验和理论表征来解决这个难题。我们的模型基于密度泛函理论（DFT）计算，重现了Na阳离子和Cl阴离子位点之间对比的复杂演变，以及带正电的空位作为尖端高度的函数，并强调了静电相互作用对尖端-样品距离大于500 pm的关键贡献。对于较小的分离，泡利排斥和相关的CO倾斜开始主导对比。CO-金属尖端的静电场可以用来自金属尖端和CO分子的电场叠加来表示。远程行为是由金属尖端定义的，该尖端贡献了偶极子的场，其正极位于顶端。在近距离内，CO表现出相反的磁场。这些场的相互作用，具有相反的符号和相当不同的空间扩展，对于描述对比度演变作为尖端高度的函数至关重要。
Metal tips decorated with CO molecules have paved the way for an impressively high resolution in atomic force microscopy (AFM). Although Pauli repulsion and the associated CO tilting play a dominant role at short distances, experiments on polar and metallic systems show that electrostatic interactions are necessary to understand the complex contrast observed and its distance evolution. Attempts to describe those interactions in terms of a single electrostatic dipole replacing the tip have led to contradictory statements about its nature and strength. Here, we solve this puzzle with a comprehensive experimental and theoretical characterization of the AFM contrast on Cl vacancies. Our model, based on density functional theory (DFT) calculations, reproduces the complex evolution of the contrast between both the Na cation and Cl anion sites, and the positively charged vacancy as a function of tip height, and highlights the key contribution of electrostatic interactions for tip-sample distances larger than 500 pm. For smaller separations, Pauli repulsion and the associated CO tilting start to dominate the contrast. The electrostatic field of the CO-metal tip can be represented by the superposition of the fields from the metal tip and the CO molecule. The long-range behavior is defined by the metal tip that contributes the field of a dipole with its positive pole at the apex. At short-range, the CO exhibits an opposite field that prevails. The interplay of these fields, with opposite sign and rather different spatial extension, is crucial to describe the contrast evolution as a function of the tip height.