טכניון מכון טכנולוגי לישראל
הטכניון מכון טכנולוגי לישראל - בית הספר ללימודי מוסמכים  
Ph.D Thesis
Ph.D StudentVolkovich Roie
SubjectDissipative Molecular Electronic Switches
DepartmentDepartment of Chemistry
Supervisor Professor Uri Peskin
Full Thesis textFull thesis text - English Version


Abstract

In this thesis we introduce several theoretical models which suggest possibilities for new electronic molecular devices based on the interaction of a single molecule with its environment. Both nuclear and electronic external degrees of freedom were considered in this work, where the coupling between the molecule and the different reservoirs was treated within a reduced density matrix approach. In the first theoretical model we demonstrated the ability to control coherent electronic dynamics in complex molecular networks of multiple donor/acceptor sites by considering electronic coupling to molecular nuclear modes. The nuclear modes corresponded to the internal molecular degrees of freedom, and to the external modes of a dissipative molecular environment. We demonstrated that site-directed electronic tunneling prevails in the presence of dissipation, provided that the de-coherence time is longer than the time period for tunneling oscillations and that the strength of electronic coupling to the external reservoir of nuclear modes can be utilized to switch the short time propagation of an initial donor population into different acceptors. In the next step, we considered the possibility to measure the manifestation of intra-molecular coherent electronic transport in molecular junctions. The ability of using single-molecule junctions as electron pumps for energy conversion was raised, where we argued that the small dimensions of these systems allow the use of unique intra-molecular quantum coherences in order to pump electrons between two reservoirs and to overcome relaxation processes which tend to suppress the pumping efficiency.  In particular, we demonstrated that a selective transient excitation of one chromophore in a bichromophoric donor-bridge-acceptor molecular junction model yields currents which transfer charge unevenly to the two electrodes, thus converting the excitation energy into currents even in the absence of a bias potential. We then extended the concept of reservoir control in molecular junction to the molecular nuclear degrees of freedom. In particular, we demonstrated for generic asymmetric molecular junction models that the applied bias voltage controls the excitation of specific vibrational modes, and that by changing the bias polarity vibrational energy can be switched between two different nuclear modes regardless of the corresponding mode frequency. This Mode-Selective Vibrational Excitation (MSVE) was shown to result from selective vibrational cooling processes at the electrode surfaces which reflect the asymmetry of the molecule-electrode contacts.  In the broad context of mode-selective chemistry, studies of MSVE in single-molecule junctions may pave the way to control chemical processes in molecules adsorbed on surfaces.