טכניון מכון טכנולוגי לישראל
הטכניון מכון טכנולוגי לישראל - בית הספר ללימודי מוסמכים  
Ph.D Thesis
Ph.D StudentLevine Ariel
SubjectCharge Transport through Molecular Bridges:
Beyond the Hopping Mechanism
DepartmentDepartment of Chemistry
Supervisor Professor Uri Peskin
Full Thesis textFull thesis text - English Version


Abstract

Both nano-electronics applications and natural processes, at their molecular level, require highly efficient charge transport through molecules. Much theoretical and experimental research has been devoted over the past decades to understanding the physical principles underlying charge transport through biomolecules in particular. However, this is not a simple task since one must take into account numerus parameters including the quantum mechanical nature of the molecules, interactions with the environment, the interplay between multiple conductance channels etc. In this work we explore new phenomena associated with the coherent and/or incoherent charge transport regimes. In both regimes the quantum energy eigenstates of the molecular bridge play a crucial role in determining the CT rate.

For the coherent transport regime, we present a new type of quantum interference effect arising from the different transport pathways through a molecule-electrode interface, which we termed, “contact interference”. Much research has focused on the conductance pathways through the molecule but little or no research has focused on the pathways along the molecule-surface interface. We present a detailed analysis of the contact interference phenomenon, and we formulate conditions for both the molecule and the contact, where the manifestations of the quantum interference can be observed in experiments. We also investigated the relevance of the common practice of replacing a complex three-dimensional electrode model with a reduced independent chains model, demonstrating the breakdown of this practice in the presence of contact interference.

Considering the incoherent transport regime, we chose to highlight the phenomenon of nearly length-independent charge transfer rates through molecular bridges. It is widely accepted that long range transport is facilitated through various sequential hopping-like mechanisms. Focusing on donor-bridge-acceptor molecules of biological relevance (poly-A DNA bridge, in particular), we propose a new type of transport mechanism, termed “quantum unfurling” which should dominate CT in such rigid molecular frameworks. According to this mechanism the charge transfer rate from the donor to the acceptor is limited by an incoherent transfer (thermally activated) from a localized donor orbital into a delocalized bridge orbital, extended over the entire molecular bridge. This implies that the rate is most sensitive to the energy gap between the donor and the bridge orbitals. Differing from hopping like processes through floppy bridges, unfurling does not necessitate transitions between bridge orbitals, and the rate becomes nearly independent on the bridge length.  Building on the different physics governing unfurling and hopping we propose an experimental test for differentiating between the two mechanisms in DNA models for different helix directionalities.