Distance dependence of noncontact-AFM image contrast on Si(111)√3 X √3-Ag structure

Atomic resolution imaging of the Si(111)root 3 x root 3 R30 degrees-Ag surface was investigated using a noncontact atomic force microscopy (NC-AFM) in ultrahigh vacuum. NC-AFM images showed three types of contrasts depending on the distance between an AFM tip and a sample surface. When the tip-sample distance was about 1-3 Angstrom. the images showed the honeycomb arrangement with weak contrast. When the tip-sample distance was about 0-0.5 Angstrom, the images showed the periodic structure composed of three bright spots with relatively strong contrast. On the other hand, the contrasts of images measured at the distance of 0.5-1 Angstrom seemed to be composed of the above-mentioned two types of contrasts. By comparing the site of bright spots in the AFM images with honeycomb-chained trimer (HCT) model, we suggested the following models: when the tip is far from the sample surface, tip-sample interaction force contributing to imaging is dominated by physical bonding interaction such as Coulomb force and/or van der Waals (vdW) force between the tip apex Si atoms and Ag trimer on the sample surface. On the other hand, just before the contact, tip-sample interaction force contributing to imaging is dominated by chemical bonding such as the force due to hybridization between the dangling bond out of the tip apex Si atom and the orbit of Si-Ag covalent bond on the sample surface. (C) 1999 Elsevier Science B.V. All rights reserved.