Polaron Effects on Charge Transport through Molecular Wires: A Multiscale Approach

Modeling charge transport
along molecular wires immersed in polarizable
environments poses a grand challenge due to the high dimensionality
of the problem and the various time scales involved. A previous multiscale
nonequilibrium Green’s function simulation scheme (Popescu,
B.; Woiczikowski, P. B.; Elstner, M.; Kleinekathöfer, U. Phys.
Rev. Lett. (2012), 109, 176802) has been extended significantly so
that the present approach provides a much more complete physical description
of the process. While the previous scheme involved the environmental
fluctuations and their influence of the electronic structure of the
wire, several previously neglected effects were added to the formalism:
the electric field between the leads, the polarization of the dielectric
environment in response to the charge present on the wire, and the
relaxation of the electronic structure of the wire. Still, the underlying
Hamiltonian of the wire is evaluated with electronic structure calculations,
and the dynamics of the molecular system are described using molecular
dynamics simulation so that (i) the formalism remains free of any
model parameters and (ii) no assumptions on the underlying transport
mechanism are being made. All the newly introduced details prove to
affect the charge transfer along the wire markedly, while interestingly,
their effects compensate each other partially. The new method is suitable
for application to charge transport in junctions composed of well-defined
molecular fragments, which is the case, e.g., in typical organic electronics
materials. In this work, the method has been applied to hole transport
through a double-stranded DNA, which nicely displays the influence
of all of the newly introduced effects.