Ultrathin insulating films on metal substrates are unique systems to use the scanning tunneling / atomic force microscope to study the electronic and structural properties of single atoms and molecules, which are electronically decoupled from the metallic substrate. Individual gold atoms on an ultrathin insulating sodium chloride film supported by a copper surface exhibit two different charge states, which are stabilized by the large ionic polarizability of the film [1]. The charge state and associated physical and chemical properties such as diffusion can be controlled by adding or removing a single electron to or from the adatom with a scanning tunneling microscope tip. The simple physical mechanism behind the charge bistability in this case suggests that this is a common phenomenon for adsorbates on polar insulating films. Employing a low temperature tuning fork AFM the different charge states can be observed directly in the force signal[2]. In the case of STM of molecules on ultrathin insulating films the electronic decoupling allows the direct imaging and manipulation of molecular orbitals [3]. As we have recently demonstrated detailed structural information can be attained by Atomic Force Microscopy which leads to the direct imaging of the molecular geometry. I.e. the complete chemical structure of single molecules can be accessed in scanning probe microscopy [4].

 

1. J. Repp, G. Meyer, F.E. Olsson, M. Persson, ‘Controlling the Charge State of Individual Gold Adatoms’, Science 305, 493 (2004)

2. L. Gross, F. Mohn, P. Liljeroth, J. Repp, F. J. Giessibl , G. Meyer, “Measuring the Charge State of an Adatom with Noncontact Atomic Force Microscopy” Science, 324, 1428-1431 (2009)

3. P. Liljeroth, J. Repp, G. Meyer, Current-Induced Hydrogen Tautomerization and Conductance Switching of Naphthalocyanine Molecules Science, 317, 1203-1206 (2007)

4. L. Gross, F. Mohn, N. Moll, P. Liljeroth, G. Meyer ‘The Chemical Structure of a Molecule Resolved by Atomic Force Microscopy’, Science, 325, 1110-1114 (2009)