Department of Mechanical Engineering, University of Maryland, College Park, Maryland 20742, United States.
Nano Lett. 2011 Nov 9;11(11):5026-33. doi: 10.1021/nl2030773. Epub 2011 Oct 20.
In 2004 Hembacher et al. (Science 2004, 305, 380-383) reported simultaneous higher-harmonics atomic force mocroscopy (AFM)/scanning tunneling microscopy (STM) images acquired while scanning a graphite surface with a tungsten tip. They interpreted the observed subatomic features in the AFM images as the signature of lobes of increased electron density at the tungsten tip apex. Although these intriguing images have stirred controversy, an in-depth theoretical feasibility study has not yet been produced. Here we report on the development of a method for simulating higher harmonics AFM images and its application to the same system. Our calculations suggest that four lobes of increased electron density are expected to be present at a W(001) tip apex atom and that the corresponding higher harmonics AFM images of graphite can exhibit 4-fold symmetry features. Despite these promising results, open questions remain since the calculated amplitudes of the higher harmonics generated by the short-range forces are on the order of hundredths of picometers, leading to very small corrugations in the theoretical images. Additionally, the complex, intermittent nature of the tip-sample interaction, which causes constant readjustment of the tip and sample orbitals as the tip approaches and retracts from the surface, prevents a direct quantitative connection between the electron density and the AFM image features.
2004 年，Hembacher 等人（Science 2004, 305, 380-383）报道了在使用钨针尖扫描石墨表面时同时获得的高次谐波原子力显微镜（AFM）/扫描隧道显微镜（STM）图像。他们将 AFM 图像中观察到的亚原子特征解释为钨针尖尖端电子密度增加的叶瓣的特征。尽管这些引人入胜的图像引起了争议，但尚未进行深入的理论可行性研究。在这里，我们报告了一种模拟高次谐波 AFM 图像的方法的开发及其在同一系统中的应用。我们的计算表明，在 W(001) 尖端原子上预计会存在四个电子密度增加的叶瓣，并且相应的石墨高次谐波 AFM 图像可以表现出 4 重对称特征。尽管有这些有希望的结果，但仍存在悬而未决的问题，因为短程力产生的高次谐波的计算幅度在百分之一到千分之一皮米之间，导致理论图像中的波纹非常小。此外，针尖-样品相互作用的复杂、间歇性性质，导致针尖在接近和远离表面时不断调整针尖和样品轨道，这使得电子密度与 AFM 图像特征之间无法直接进行定量连接。
