Creating and characterizing a single molecule device for quantitative surface science

Molecular electronics is currently the focus of much research, since the minimization limit of conventional field effect transistors is gradually being reached. The ultimate goal of molecular electronics is to form functional electronic components from single molecules. There are two main challenges to reaching this goal: gaining control over the electronic properties of the molecule and gaining control over the geometric configuration of the molecule. Many experimental investigations of electron transport through molecules have been carried out in the past decades. However if the geometry of the molecule between the conducting electrodes is unknown, a deep understanding of the transport cannot be achieved. For this reason, knowledge of the geometry is essential. The first part of this work concerns a newly developed procedure for the controlled manipulation of single molecules, utilizing the example of 3,4,9,10-Perylenetetracarboxylic dianhydride (PTCDA) on the Ag(111) surface. This method is based on atomic force microscopy under ultrahigh vacuum and at low temperatures in combination with a 3D motion tracking system. Molecular self-assembly results in strong hydrogen bonds between the molecules on the surface and the new method provides an efficient way to search for the optimal trajectory for the extraction of single molecules from the molecular layer. The second part of this work builds on this method for the controlled functionalization of the scanning probe tip. A new method, scanning quantum dot microscopy (SQDM), was developed. SQDM is based on a surprisingly weak electronic coupling between the tip’s electronic bands and one or more molecular orbital(s), which exists in spite of the strong mechanical bond between the tip and the molecule. This results in the possibility to controllably modify the charge state of the tip-attached molecule by varying the bias voltage applied to the surface. SQDM made the quantitative measurement of the electrostatic potential of small surface objects possible, including the first measurement of the Smoluchowski dipole of a single silver atom on Ag(111). The first truly quantitative measurement of the work function change of Ag(111) after PTCDA adsorption with a scanning probe microscope was made possible with SQDM, where a value of (145±10) meV was obtained. A theoretical model of the experimental system was constructed, based on the orthodox theory of the Coulomb blockade. Unexpected discrepancies between theory and experiment were discovered, which contradicted the constant interaction model of quantum dots. These results are important for the future application of the theory on systems with similar characteristic length scales of ca. 1 nm