Distance and Voltage Dependence of Orbital Density Imaging Using a CO-Functionalized Tip in Scanning Tunneling Microscopy

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 on 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.