44481-G6
Electron Transport and Photoexcitation at Interfaces
The last decade has witnessed a remarkable minituarization of conventional microelectronic devices. If this trend is to continue, elements of microelectronic circuits will soon shrink to molecular size. Accurate and reproducible experimental measurements of quantum transport properties of single molecules have been recently reported. These experiments have revealed a wealth of interesting new physical phenomena such as Coulomb blockade, Kondo effects, negative differential resistance, vibronic effects and local heating, as well as switching and hysteresis. The understanding of the fundamental mechanisms of quantum transport in single molecule junctions as well as the interpretation of these experimental observations requires the development of theoretical methods for the description of non-equilibrium interacting many-particle quantum systems. We have developed several physically accurate theoretical models to obtain a fundamental understanding of non-equilibrium systems and to calculate electron transport properties of molecules and molecular nanoscale architecture. We have also performed modeling of specific molecular scale devices that can be compared directly to systems studied experimentally. The investigation of the role of metal-molecule interface in determining molecular junction transport properties has been the focus of our research for last two years The coupling to electrodes mixes the discrete molecular levels with the continuum of the metal electronic states such that the molecular orbitals protrude deep inside the electrode. The coupling also renormalizes the energies of the molecular orbitals. Therefore, it is no longer correct to talk about the transport properties of the molecule, but rather, only of the electrode-molecule-electrode heterojunction. If the strength of the molecule-electrode coupling is large, substantial perturbation of the molecular electronic structure can occur. In fact, it is expected that upon initial chemisorption, substantial charge transfer takes place between the metal electrode and the molecule even in the absence of an applied voltage bias. These effects are pivotal for molecular wire transport properties and they can be controlled by changing the interface geometry or by altering the anchoring groups, which provide the linkage between the molecule and the electrode. Therefore the study of the anchoring group chemistry in molecular electronics helped us not only to pin down the origin of significant discrepancy between experimental and theoretical molecular conductivities but also revealed new fascinating fundamental aspects of molecule-surface interactions. It has recently been shown experimentally that dithiocarbamate formation provides a reliable technology for conjugating secondary amines onto metal surfaces to form strongly absorbed molecular ligands, which are stable under various types of the environmental stress. This result suggests a possibility of connections molecular wires to gold electrodes via dithiocarbamate group instead of a thiol group. Our calculations showed that dithiocarbamate linking to the Au electrode leads to strong molecule-electrode coupling, which extends the conjugation from the molecule to the metal. The overlap of the peaks in the transmission spectrum for 4,4'-bipyridinium-1,1'-bis(carbodithioate) junction results in a transmission probability which is sufficiently large and does not change significantly with the variation of electron incident energies from -1 eV to 3 eV (relative to the electrode Fermi energy). Our calculations demonstrated that the conductance molecular junctions can be significantly improved by using dithiocarbamate anchoring group. We also studied computationally the electron transport properties of dithiocarboxylate terminated molecular junctions. On our calculations we established a microscopic origin of the experimentally observed current amplification by dithiocarboxylate anchoring groups. We predicted a new microscopic mechanism of rectification based on the electronic structure of asymmetrical anchoring groups. We have been developing the computational methodology to model the typical experiments in molecular electronics where the molecular junctions are formed by repeatedly crashing an Au STM tip into and pulling it out of the Au surface in a solution comprised of the molecules intended to form junction.To model these experimental conditions we developed and implemented into the state-of-the-art DFT computer program the computational method which calculates molecular conductance ”on-the-fly”. We explained by our calculation why amine-terminated molecules show well-behaved conductance in the scanning tunneling microscope breakjunction experimental measurements. We performed density functional theory based electron transport calculations to explain the nature of this phenomenon. We found that amines can be adsorbed only on the apex Au atom, while the thiolate group can be attached equally well to undercoordinated and clean Au surfaces. Our calculations showed that only one adsorption geometry is sterically and energetically possible for the amine anchored junction whereas three different adsorption geometries with very distinct transport properties are almost equally probable for the thiolate-anchored junction. We calculated the conductance as a function of the junction stretching when the molecules are pulled by the scanning tunneling microscope tip from the Au electrode. Our calculations demonstrated that the stretching of the thiolate-anchored junction during its formation is accompanied by significant electrode geometry distortion. The amine-anchored junctions exhibit very different behavior—the electrode remains intact when the scanning tunneling microscope tip stretches the junction.
