Measurement Condition on True Atomic Resolution Imaging of Noncontact Atomic Force Microscope

本课题利用真空非接触式原子力显微镜（AFM），研究了Si针尖与Si（111）面之间的力相互作用<3><3>。利用Si针尖，在Si（111）面× Ag面上的AFM图像<3><3>显示出三种不同的对比度，这三种对比度取决于针尖与样品表面之间的距离。在针尖-样品距离约0.1 - 0.3nm处，AFM图像显示蜂窝状排列。在针尖-样品距离为0- 0.05nm处，图像呈现出由三个亮点组成的三角形周期结构，对比度较强。另一方面，在0.05- 0.1nm的距离处，图像对比度似乎是上述两种对比度的综合。当针尖远离样品表面时，针尖与样品的相互作用力主要由针尖顶端Si原子与样品表面Ag三聚体之间的库仑力和/或货车范德华力（vdW）等物理键合作用力所控制。另一方面，在接触之前，针尖-样品相互作用力由针尖顶端Si原子的悬挂键与表面上的Si-Ag共价键轨道之间的杂化引起的化学键相互作用主导。采用银吸附针尖，获得了不同的图像对比度，没有观察到其依赖性。首次以原子分辨率观察到表面上的单个Ag原子。这些实验结果表明，原子种类的识别是可能的，通过使用AFM。

In this project, we investigated the force interactions between a Si tip and Si(111)√<3>×√<3>-Ag surface, susing noncontact atomic force microscopy (AFM) operating in ultrahigh vacuum (UHV) by using Si and Ag-adsorped tips. By using Si tip, AFM images on Si(111)√<3>×√<3>-Ag surface showed three types of contrasts which depended on the distance between a tip and a sample surface. At the tip-sample distance of about 0.1 -0.3nm, AFM image showed the honeycomb arrangement. At the tip-sample distance of about 0- 0.05nm, the images showed the periodic structure of the triangle consisting of three bright spots with relatively strong contrast. On the other hand, at the distance of 0.05-0.lnm, the image contrast seemed to be the synthesis of the above two types of contrasts. When the tip is far from the sample surface, the tip-sample interaction force is dominated by physical bonding interaction such as the Coulomb force and/or the van der Waals (vdW) force between the tip apex Si atom and Ag trimer on the sample surface. On the other hand, just before the contact, the tip-sample interaction force is dominated by the chemical bonding interaction due to hybridization between the dangling bond out of the tip apex Si atom and the orbit of Si-Ag covalent bond on the surface. By using Ag-adsorbed tip, different image contrast was obtained and no its deitance dependence was observed. For the first time, individual Ag atoms on the surface was observed with atomic resolution. These experimental results suggest that the identification of the atom species is possible by using AFM.