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

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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.