Title: Impact of Electrode Properties on Charge Transport Dynamics of Molecular Devices
Description: This thesis aims to provide insights into two challenging problems in the field of molecular electronics: Understanding the role of the electronic and the mechanical properties of electrodes in determining the charge transport dynamics of molecular devices and achieving the optical control of charge transport through single-molecule junctions by exploiting the optical properties of electrodes. We start by investigating the impact of electrode band structure on the charge transport characteristics of molecular devices. To this end, we conduct two independent, yet highly related studies. In the first study, we demonstrate how the metallic band structure dictates the molecular orbital coupling at metal-molecule interfaces by studying charge transport through pyridine-based single-molecule junctions with Au and Ag electrodes using a newly developed scanning tunneling microscope-based spectroscopy technique and performing density functional theory calculations. We find that pyridine derivatives couple well to Au electrodes compared with Ag electrodes.

The density functional theory calculations show that the increase in the molecular orbital coupling to Au compared with Ag is due to an enhanced density of d-states near the Fermi level resulting from relativistic effects. Second, we study the interfacial charge transport properties of molecular devices with metal, semimetal and semiconductor electrodes using X-ray photoemission based spectroscopy techniques. In particular, we probe the hot electron dynamics of 4,4'-bipyrdine on Au (metal), epitaxial graphene (semimetal) and graphene nanoribbon (semiconductor) surfaces. We find that charge transfer from the molecule to the substrate is fastest on the metal surface and slowest on the semiconductor surface. We attribute this trend to a reduced electronic interaction between the molecule and the surface as a results of a decrease in the density of electronic states near the Fermi level as the metallic character of the substrate is reduced. Furthermore, we provide evidence for fast phase decoherence of hot electrons via an interaction with the substrate in these systems.

Third, we shed light onto the origin of flicker noise in single-molecule junctions, tunnel junctions and gold point-contacts at room temperature. We find that the switching of gold atoms between metastable sites in the electrodes due to the thermal energy leads to conductance fluctuations in these systems. We further demonstrate how the flicker noise characteristics of single-molecule junctions can be used to infer the nature of the electronic interaction at metal-molecule interfaces. Specifically, we find that flicker noise exhibits a power dependence on junction conductance that can distinguish between through-space and through-bond charge transport. This work demonstrates how the mechanical properties of electrodes affect charge transport through single-molecule junctions and how noise can be used to understand the electronic properties of metal-molecule interfaces. Lastly, we explore the possibility of driving currents through single-molecule junctions using electromagnetic radiation.

To this end, we perform photocurrent measurements on single-molecule junctions, tunnel junctions and gold point-contacts obtained using the scanning tunneling microscope-based break-junction technique. We find that the primary source of photocurrents in these systems is the laser induced local heating and the subsequent thermal expansion when probed using a lock-in type technique in which the light intensity is being modulated. We further develop an experimental method that differentiates between the photocurrents due to thermal expansion and the optical currents in single-molecule junctions, and provide evidence for optical currents due to electron-photon interaction during charge transport through single-molecule junctions. By using this method we estimate the plasmonic electric field enhancement factor in single-molecule junctions formed by 4,4'-bipyridine. Our estimate is in very good agreement with values inferred from tip enhanced Raman spectroscopy measurements and field emission measurements.

We believe that the results presented in this thesis provide original insights into the fundamentals of the physics that govern charge transport across metal-molecule interfaces. Furthermore, the new experimental techniques introduced in this thesis offer new ways for investigating the rich physics present in nanoscale systems.