Resolving chemical structures in scanning tunnelling microscopy

C. Weiss, C. Wagner, C. Kleimann, F. S. Tautz, R. Temirov

Published 2009-10-30Version 1

With the invention of scanning probe techniques, direct imaging of single atoms and molecules became possible. Today, scanning tunnelling microscopy (STM) routinely provides angstrom-scale image resolution. At the same time, however, STM images suffer from a serious drawback - the absence of chemical information. Here we demonstrate a modification of STM that resolves the chemical structure of molecular adsorbates. The key to the new STM mode is a combined force sensor and signal transducer that is formed within the tunnelling junction when a suitable gas condenses there. The method probes the repulsive branch of the surface adsorption potential and transforms the force signal into a current. Obtained images achieve the same resolution as state-of-the-art atomic force microscopy (AFM). In contrast to AFM, however, our (uncalibrated) force sensor is of nanoscale dimensions and therefore intrinsically insensitive to those long-range interactions that make atomic-resolution AFM so demanding.