Artifact and Fact of Si(111)7×7 Surface Images Observed with a Low Temperature Noncontact Atomic Force Microscope (LT-NC-AFM)

We investigate Si(111)7×7 surface by using a low temperature noncontact atomic force microscope (LT-NC-AFM). We obtain two types of AFM images with and without an additional structure between the corner and center adatoms. The distance between the corner adatom and additional structure is 7.0 ű0.2 Å, which is not consistent with that (4.3 Å) between the corner adatom and the rest atom. This suggests that the additional structure observed is an artifact. We propose a model to explain these experimental results, which is based on atomic arrangement of the Si tip apex with an asymmetric ad-dimer and the Si(111)7×7 surface.