Detailed scanning probe microscopy tip models determined from simultaneous atom-resolved AFM and STM studies of the TiO_2(110) surface

摘要

The atomic-scale contrast in noncontact atomic force microscopy (nc-AFM) images is determined by the geometry and exact atomic structure of the tip apex. However, the tip state is an experimentally unknown parameter, and the lack of insight into the tip apex often limits the possibilities of extracting precise quantitative and qualitative atomistic information on the surface under inspection. From an interplay between simultaneously recorded nc-AFM and scanning tunneling microscopy (STM) data, and atomistic STM simulations based on multiple scattering theory, we demonstrate how the state of the scanning probe microscopy (SPM) tip in the experiments may be determined. The analysis of a large number of experimental SPM images recorded with different tips reveals that no general correlation exists between the contrast observed in the nc-AFM and the tunneling current (I_t) images on TiO_2(110) surface. The exact state of the SPM tip must, therefore, be determined for each specific case, which is normally a very difficult endeavor. However, our analysis of the AFM contrast on TiO_2(110) surface allows us to considerably reduce the number of tips to be considered in a full simulation. By carefully evaluating the contrast of a handpicked library of SPM tips, we manage to determine a very accurate model of the SPM tip used in an experiment for the first time. It is envisioned that the approach presented here may eventually be used in future studies to screen for and select a SPM tip with a special functionalization prior to imaging an unknown sample, and in that way facilitate precise modeling and chemical identification of surface species.