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

High Resolution Image Download MS PowerPoint Slide 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.