Simultaneous AFM and STM measurements on the Si(111)-(7x7) surface

We perform simultaneous atomic force microscopy (AFM) and scanning tunneling microscopy (STM) measurements on the Si(111)-(7x7) surface. AFM/STM constant height images are obtained at various tip-surface distances. Force/current distance spectroscopy using the same tip apex allows us to estimate the relative tip-surface distance for each image as well as the short/long-range force and the tunneling current. We demonstrate that the tunneling current at tip-surface distances where AFM clearly resolves atoms is much larger than the typical values in conventional STM. In addition, at the tip-surface distances for conventional STM, the short-range force is too small to provide atomic contrast in AFM. We show that the differences in the signal-to-noise ratio of the constant height images between AFM and STM produce different optimal imaging distances. In addition, we note that the different imaging distance also influences images obtained using double tips.