Distance- and Voltage-Dependence of Orbital Density Imaging Using a CO-Functionalized STM Tip

The appearance of frontier molecular ion resonances measured with scanning tunneling microscopy (STM) — often referred to as orbital density images — of single molecules was investigated using a CO-functionalized tip in dependence of bias voltage and tip-sample distance. As model systems we studied pentacene and naphthalocyanine on bilayer NaCl on Cu(111). Absolute tip-sample distances were determined by means of atomic force microscopy (AFM). STM imaging revealed a transition from predominant p- to s wave tip contrast upon increasing the tip-sample distance, but the contrast showed only small changes as a function of voltage. The distance-dependent contrast change is explained with the steeper decay of the tunneling matrix element for tunneling between two p wave centers, compared to tunneling between two s wave centers. In simulations with a fixed ratio of s- to p wave tip states we can reproduce the experimental data including the distance dependent transition from predominant p- to s-wave tunneling contribution.