Non-contact AFM images measured on Si(111)√3 x √3-Ag and Ag(111) surfaces

We investigated the force interactions between a Si tip and a Si(111)root 3 x root 3-Ag surface, as well as between a Si tip and an Ag(111) surface, using non-contact atomic force microscopy (AFM) operating in ultrahigh vacuum (UHV), The AFM images on the Si(111)root 3 x root 3-Ag surface showed three types of contrast that depended on the distance between a tip and a sample surface: at a tip-sample distance of 0.1-0.3 nm the AFM image showed a honeycomb arrangement, at a tip-sample distance of 0-0.05 nm the image showed the periodic structure of a triangle, consisting of three bright spots with relatively strong contrast, but at a distance of 0.05-0.1 nm the image contrast seemed to be intermediary between the other two types of contrast. When the tip is far from the sample surface, the tip-sample interaction force is dominated by physical bonding interactions such as Coulomb and/or van der Waals forces between the tip apex Si atom and the Ag trimer on the sample surface. On the other hand, just before contact, the tip-sample interaction force is dominated by chemical bonding interaction due to the onset of hybridization between the dangling bond of the tip apex Si atom and the orbital of the Si-Ag covalent bond on the surface. Furthermore, atomic resolution imaging of a pure metallic surface of Ag(111) was achieved, suggesting that non-contact AFM has potential for the investigation of a pure metallic surface. Copyright (C) 1999 John Wiley & Sons, Ltd.