Session Chair: Christian Albertus Nijhuis, National University of Singapore
10:30am - 11:00am Keynote
Charge Transfer by Tunneling across Self-Assembled Monolayers
Harvard University, United States
This talk will describe the development and use of a junction (the “EGaIn” junction) based on insulating self-assembled monolayers (SAMs) sandwiched between two metal electrodes—one usually gold or silver, and the other a low-melting liquid eutectic alloy of gallium and indium—and the use of these systems to study mechanisms of electron transfer by tunneling across them. This junction enables the use of “physical-organic” strategies for studying these mechanisms, and is especially useful in separating the contributions to the junction from the organic material in the junction (the SAM, or other structures), from the contributions from interfaces. It also makes possible the identification of new phenomena (anomalous tunneling in oligomers for amino acids, ethylene glycol, and related materials attributable to a little studied class of orbitals: high-lying, filled, orbitals), and the study of tunneling in new environments (e.g., under, or in contact with, solvent).
11:00am - 11:30am Invited
Extracting Quantitative Information from Molecular Junction I-V Characteristics using a Compact Analytical Model
University of Minnesota, United States
One of the central challenges of molecular electronics is to establish clear connections between molecular structure, the ensuing electronic structure, and the current-voltage (I-V) characteristics of molecular junctions. In particular, the offset εh of the Fermi level relative to the appropriate frontier molecular orbital (HOMO or LUMO) and the electrode-molecule coupling strength Γ (level width) are recognized as two main factors that determine the electrical properties of a typical molecular junction. We show that a compact analytical model derived by Ioan Baldea from the Landauer formalism provides a quantitative fit to the I-V data for a broad spectrum of molecular tunnel junctions and yields values of εh and Γ that vary systematically with molecular structure and choice of electrode materials. Because of its simplicity, the model is readily accessible to the experimentalist and it is far superior to the commonly used Simmons model because it is based on appropriate physics (e.g., the correct electronic structure) and it provides far better fits to the I-V data. Furthermore, the model predicts the self-similarity of non-resonant tunnel junctions: upon appropriate renormalization, the I-V characteristics for a broad swath of molecular junctions collapse onto a universal curve, as could be expected. We will present transport data and theoretical analysis of tunnel junctions based on molecules such as oligophenylene dithiols, alkane dithiols, alkyl ether dithiols, and the corresponding monothiols – all with systematically varying lengths – and electrodes fabricated from Ag, Au and Pt metals. In all cases, we will emphasize that the compact analytical analysis facilitates structure-property correlations and a powerful physical organic chemistry approach to molecular electronics.
11:30am - 12:00pm Invited
Charge Transfer Dynamics in Self-Assembled Monolayers
Heidelberg University, Germany
Whereas static electric conductance properties of individual molecules and their assembles on solid supports have been extensively studied, information about the dynamics of the charge transfer (CT) in such systems is far less common. In this context, femtosecond CT dynamics in a variety of selected molecules, associated with specific "molecular wires" and assembled in monomolecular fashion on metal substrates, was addressed by resonant Auger electron spectroscopy, using the core hole clock approach. The CT pathway from a specific site to the conductive substrate was unambiguously defined by resonant excitation of a special marker group attached to the molecular backbone. In most cases, nitrile served as such as group but nitro and pyridine moieties were tried as well. The length of the backbone was varied to monitor the respective dependence of the CT time. Similar to the static conductance, this dependence could be coarsely described by an exponential function. The respective attenuation factors and CT time associated with the anchoring to the substrate were determined. These factors and characteristic CT times through molecular frameworks were found to depend strongly on the character of the molecular orbital which mediates the CT process. The individual orbitals could be addressed by either the energy or symmetry selection, depending on the character of the system studied. The CT direction could be controlled by the identity of the marker group. The efficiency of different molecular anchors in terms of CT dynamics was compared und the ultimate case of ultrafast CT for the marker group attached directly to the conducting substrate was tested.