Artifact and fact of Si(111)7 x 7 surface images observed with a low temperature noncontact atomic force microscope (LT-NC-AFM)

We investigate Si(111)7x7 surface by using a low temperature noncontact ato mic force microscope (LT-NC-AFM). We obtain two types of AFM images with an d without an additional structure between the corner and center adatoms. Th e distance between the corner adatom and additional structure is 7.0 Angstr om +/- 0.2 Angstrom. which is not consistent with that (4.3 Angstrom) betwe en the corner adatom and the rest atom. This suggests that the additional s tructure observed is an artifact. We propose a model to explain these exper imental results. which is based on atomic arrangement of the Si tip apex wi th an asymmetric ad-dimer and the Si(111)7x7 surface.