Local probing and manipulation of molecules and nanoscale contacts with scanning tunneling microscopy

Abstract

In this thesis single molecules, metal-organic complexes and single adatoms are characterized on a metal surface by means of scanning tunneling microscopy (STM). Specifically the organic molecule 4-[trans-2-(pyrid-4-yl-vinyl)] benzoic acid (PVBA) is primarily in the focus. The molecule has two functional groups, a pyridyl and a carboxylic group connected by a vinyl group giving rise to versatile electronic properties and interaction mechanisms. The PVBA molecules are deposited by molecular beam epitaxy on a Cu(111) substrate. Furthermore, Co is deposited on Cu(111) to study the properties of a single magnetic atom on the surface and in contact with the tip. The experimental studies are supported by calculations performed in collaboration with theoretical groups and are reported here to strengthen the interpretation of the experimental results allowing an enhanced view of the investigated structures from a different perspective.In Chapter 2 an overview of the theoretical background on scanning tunneling microscopy and spectroscopy is given. Other important spectroscopic techniques employed in this study, such as time resolved and point-contact spectroscopy are introduced. Besides an explanation of the experimental setup, a short introduction to density functional theory is given.In Chapter 3 the adsorption and electronic properties of single PVBA molecules and selfassembled supra-molecular structures are investigated by scanning tunneling microscopy and spectroscopy. Specific emphasis is given to the chiral nature of PVBA on the surface.In Chapter 4 the molecule-surface interface is investigated in detail. Measuring the current vs. tip-sample distance allows to unravel information about the potential barrier on and surrounding a molecule. Another aspect of this interface is the direct interaction of the molecules with the substrate. The adsorption geometry of PVBA serves to reveal information about the interaction with the substrate and the role of sub-surface atomic layers is explored. On surfaces such as Cu(111), a surface state exists, which lets electrons behave as a nearly free two-dimensional electron gas. Such electrons can scatter at adsorbates, such as single atoms or molecules. Due to the particle-wave duality this results in a characteristic interference pattern surrounding the adsorbate. The correlation between these patterns and the internal structure of adsorbed scatterers, such as molecules and metalorganic complexes, is presented, whereas specific emphasis is devoted to chiral scatterers.In Chapter 5 the use of the scanning tunneling microscope as a nanotool to manipulate adsorbates on the surface is reported. Two main techniques are employed to generate a displacement and modification of the adsorbates. Whereas in one case the adsorbate, such as a single adatom, is moved mechanically with the apex of the tip, in the other cases controlled voltage pulses are applied to manipulate and chemically modify single atoms and molecular structures. The formation of a metal-ligand bond of a single Cu atom and a PVBA molecule is explored. Furthermore, a transitory bond, where the bonding state varies as a function of time, is found for a self-assembled metal complex consisting of two PVBA molecules and one Cu atom. This is characterized spatially and temporally and the results are compared to the electronic structure measurements of the complex given in Chapter 3. By applying large voltages the molecule is subject to chemical changes. Several processes will be identified ranging from deprotonation, dehydrogentaion to a decomposition of PVBA. At the end of this chapter the formation of Co dimers is investigated by applying an electric field leading to an enhanced lateral movement of single atoms on the surface, which drives the system towards the formation of dimers.In the last Chapter investigations of operating the microscope in the point contact regime are reported. This regime is accessed by bringing the tip much closer to the surface than in the tunneling regime. As an example for single atom contacts, individual Co adatoms on Cu(111) are studied. Because Co atoms are single magnetic impurities with a localized spin, the itinerant electrons of the supporting non-magnetic metal surface can lead to a new many-body ground state in which the localized impurity spin is screened. This so called Kondo effect is sensitive to the geometry of the surface and serves to investigate the role of the atomic arrangement in the contact. The point contact experiments are expanded to single molecules and molecular structures. The contact of an STM tip to the adsorbates and the resulting conductance behavior is explored. Furthermore, a procedure is developed that allows to pick-up a molecule onto the tip and to measure the transport characteristics through the entire molecule.