Transient Dynamics in Single Molecule Junctions: Theoty and Measurable Phenomena

Physical and Analytical Chemistry

Lecturer: Maayan Kuperman

13 Oct 2015 @ 12:30

Location: Faculty Seminar Room

Abstract: The study of molecular junctions and the electronic transport through them is a wide field, combining theoretical and experimental efforts in aim to understand these complex non-equilibrium systems and to possibly utilize them in new technological applications. A lot of work has been done in the past decades, and yet there are some main obstacles standing in the way to broaden our current understanding of these systems. A main technical difficulty regards the dynamics time scale of the electronic and nuclear motion (typically pico-seconds) which is still far from the time scale in which standard tools for measuring the system operate. As soon as a new measurement tool would be able to probe the molecular dynamics we would probably observe the wealth of phenomena underlying non-equilibrium transport through molecules. One approach to such new measurements is to drive molecular junctions with external electromagnetic fields, where either the molecule or the leads plasmons, or both, are excited. From the theoretical point of view, there is still much work to be done in order to account for the full complexity of such processes. Some effects are well described by old theories such as Tien&Gordon’s formula for photo-assisted tunneling that was published in the 60’s. However, modern experiments require more elaborate account for the physical processes involved, and in particular account for non-adiabatic excitations of the leads or the molecule, pulse excitations, structured tunneling barriers, electronic and vibronic interactions, etc.
In this work we re-derive the theory of photo assisted transport through molecular junctions on the basis of the time-independent scattering theory for time-dependent Hamiltonians. General transport formulas for field driven leads are obtained beyond some of the limitations of Tien&Gordon’s formula. In particular, our formulas are not limited to ac-fields, or to structure-less tunneling barriers, nor to the wide band or weak molecule-lead coupling approximations. The new formulas are tested and compared to numerical simulations for model junctions, representing characteristic features of real molecules, including the discrete levels structure of the tunneling barrier, and coupling to leads beyond the wide-band approximation. The effect of the field’s parameters on the current is demonstrated and studied for ac-fields and for periodic (train of) pulses. In particular, we predict a “directional photo-electric effect” with promising applications, where not only the magnitude, but also the direction of photo-assisted currents can be controlled by the driving field at a given static bias voltage.
The rigorous derivation of formulas enables a detailed study of physical phenomena associated with photo-assisted tunneling. Some effects are still beyond the limitations of our formulas, requiring a treatment that accounts for incoherent transport, electronic/vibronic coupling, etc. However, a numerical approach based on the formulas developed here should allow, in future studies, to account for one of the most important aspects of driven molecular junctions, i.e., non-adiabatic excitations of the lead, the molecule, or both, which is missing in most treatments of photo-assisted tunneling through molecular junctions.