Time-dependent description of charge transport in molecular wires: bottom-up-approach

• Applicants: Professor Dr. Marcus Elstner; Professor Dr. Thomas Frauenheim; Professor Dr. Ulrich Kleinekathöfer
• Subject Area: Theoretical Chemistry: Molecules, Materials, Surfaces
• Term: from 2015 to 2019
• Project identifier: Deutsche Forschungsgemeinschaft (DFG) - Project number 260747833

• Final Report Year: 2020

Final Report Abstract

Charge transport through molecular wires has attracted a lot of attention over the last decade both experimentally as well as theoretically, on length scales ranging from a few nanometers for molecular junctions to micrometers for bulk polymers and organic crystals. Aim of this project was to improve the theoretical description of these charge transport processes through molecular wires in liquid environment. This improvement has been achieved on different levels, i.e., on the level of quantum transport theories as well as on the combination of classical molecular dynamics simulations together with quantum chemical calculations and the aforementioned quantum transport theories. Internal vibrations but also external fluctuations due to surrounding (liquid) molecules have a significant influence on the transport of charges through molecular junctions. On the level of quantum transport theories, two conceptually different ways on modeling these effects exist. On the one hand, the environmental effects can be described by so-called spectral densities and included in a time-independent non-equilibrium Green’s function theory. On the other hand, in a time-dependent description the environment leads to fluctuations in the orbital energies involved in the charge transport and thus to a significant influence on the transport properties. In the present project both conceptually different approaches were further developed and compared to each other. For the time-dependent quantum transport scheme, an approach was devised which easily can include molecule-lead couplings beyond the so-called wide-band limit and furthermore has no restrictions in the temperature range while keeping the full time dependence in the approach. At the same time, a physically grounded approach to dephasing in large molecules was introduced for the time-independent transport theory also to understand the length-dependent charge transport characteristics. To this end, the vibronic dephasing model was further developed and employed. Most interestingly, the results for the time-average current through a molecular function were numerically shown to perfectly match between the two schemes which is not obvious from looking at the quite different approaches. Furthermore, the time-dependent quantum transport approach was combined with molecular dynamics and electronic structure calculations in a multiscale fashion. To this end a previous approach was significantly improved to a more complete physical description of the process. In addition to the environmental fluctuations and their influence of the electronic structure of the wire, effects like 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 were included. This formalism being free of any model parameters and assumptions on the underlying transport mechanism was subsequently applied to two different systems. This enhanced scheme including a feedback loop from the quantum transport calculations to the molecular simulations was subsequently applied to the to charge transport through a double-stranded DNA as well as through a cyclic peptid between two metal leads. These simulations nicely displayed the influence of all the newly introduced effects.

Publications

• Microsecond Simulation of Electron Transfer in DNA: Bottom-up Parametrization of an Efficient Electron Transfer Model Based on Atomistic Details, J. Phys. Chem. B 121, 529 (2017)
  M. Wolter, M. Elstner, U. Kleinekathöfer, and T. Kubař
  
• Polaron Effects on Charge Transport through Molecular Wires: A Multi-Scale Approach, J. Chem. Theory Comput. 13, 286 (2017)
  T. Kubař, M. Elstner, B. Popescu, and U. Kleinekathöfer
  
• Non-Equilibrium Green’s Function Transport Theory for Molecular Junctions with General Molecule-Lead Coupling and Temperatures, J. Chem. Phys. 149, 234108 (2018)
  H. Rahman and U. Kleinekathöfer
  
• Vibronic Dephasing Model for Coherent- To-Incoherent Crossover in DNA, Phys. Rev. B 97, 195401 (2018)
  P. Karasch, D. A. Ryndyk und T. Frauenheim
  
• Chebyshev Hierarchical Equations of Motion for Systems with Arbitrary Spectral Densities and Temperatures, J. Chem. Phys. 150, 244104 (2019)
  H. Rahman and U. Kleinekathöfer
  
• Dephasing in a Molecular Junction Viewed from a Time-Dependent and a Time-Independent Perspective, J. Phys. Chem. C 123, 9590 (2019)
  H. Rahman, P. Karasch, D. A. Ryndyk, T. Frauenheim, and U. Kleinekathöfer