Scanning Tunneling Spectroscopy of Molecules on Thin Insulating Films

J. Repp^a,c,P. Liljeroth^b,c, G.Meyer^c

^a Institute of Experimental and Applied Physics, University of Regensburg, 93040 Regensburg, Germany
^b Condensed Matter and Interfaces, Debye Institute, University of Utrecht, 3508 TA Utrecht, Netherlands
^c IBM Zurich Research Laboratory, Säumerstr. 4, CH-8803 Rüschlikon, Switzerland
email: jascha.repp@physik.uni-regensburg.de

Ultrathin insulating films on metal substrates facilitate the use of the scanning tunneling microscope to study the electronic properties of single atoms and molecules, which are electronically decoupled from the metallic substrate. In the case of molecules on ultrathin NaCl films the electronic decoupling allows the direct imaging of the unperturbed molecular orbitals, as will be shown in the cases of individual pentacene and oligothiophene molecules.
The bistability in the position of the two hydrogen atoms in the inner cavity of single free-base naphthalocyanine molecules constitutes a two-level system that was manipulated and probed by scanning tunneling microscopy. The molecules can be switched in a controlled fashion between the two states by excitation induced by the inelastic tunneling current. The tautomerization reaction can be probed by
resonant tunneling through the molecule and is expressed as considerable changes in the conductance through the molecule.We also demonstrated a coupling of the switching process so that the charge injection in one molecule induced tautomerization in an adjacent molecule.