Electrical and thermal transport through nanostructures

A large part of activities in the group belongs to the field of “molecular electronics”. In this field major themes are the construction, measurement and understanding of the electric response of circuits, in which molecular systems act as conducting elements. Advances in this area may help both to further reduce device dimensions in present microelectronic circuitry and to realize new logic functionalities, based on the properties of single atoms and molecules.

In my group, we try to better understand theoretically the charge transport mechanisms at the atomic scale. We are mainly interested in describing single-molecule contacts without any free parameters, and hence make use of quantum chemical ab-initio methods such as density functional theory and extensions. Beside applications of our existing programs to determine the conduction properties, we develop further the formalism and our methods to account, for example, for the influence of vibrations or light on the electric current.

Recently, research in molecular electronics evolves into the direction of thermoelectrics. Measurements of the thermopower of molecular junctions allow to distinguish electron from hole transport. This provides further crucial experimental information about level alignments at the molecule-electrode interface that can be directly compared with theory. In the context of thermoelectrics, we study beside the transport of charges through the molecular junctions also those of phonons. They are an important ingredient to make quantitative predictions on the value of the thermal conductance and of the thermoelectric figure of merit. It is the hope that through the control over chemical synthesis, it may be possible to tune the interrelated transport properties (electrical conductance, thermopower, thermal conductance) such that the figure of merit is strongly increased. In any case the research can be expected to lead to a better understanding of thermoelectricity at the atomic scale. Efficient thermoelectric devices may be used as nanorefrigerators without movable parts, to convert waste heat into electrical energy, i.e. for energy harvesting at the nanoscale, or nanoscale thermal heat management in general.

In addition to molecular and atomic junctions, i.e. metal-molecule-metal and metal-metal contacts, we study also other systems, including carbon nanotubes, graphene nanoribbons and other nanostructured materials such as stacks of two-dimensional materials or metal-organic frameworks. Spin-dependent transport phenomena constitute another aspect of future research directions of the group.

Excited states and light-matter interaction

Interesting phenomena arise, when materials are irradiated with laser light. Excited electronic states are accessible and plasmons, collective excitations of the electron fluid, may be excited at the surface. In the past we have considered how the conductance of molecular junctions changes in the presence of laser light. The theory based on the Tien-Gordon model shows that the electrons may now be transmitted under absorption or emission of photons, i.e. at energies that are multiples of the photon energy away from the Fermi energy. This may both enhance or decrease the conductance, depending on the precise shape of the energy-dependent transmission. Together with experimentalists we could also show how the field enhancement in a metallic nanogap can be determined through a combination of both optical and electronic measurement techniques.

We are presently extending our work along these lines and investigate plasmon-polaritons in crystals of metal nanoparticles. Additionally, with the help of density functional theory, quasiparticle methods (i.e. the GW approximation) and the Bethe-Salpeter equation, we study excitons in two-dimensional materials and hot carrier dynamics after light excitation. Our investigations may be relevant for optical nanoantennas in high-sensitivity chemical and biological sensors, for realizing active elements based on light-controlled electrical conduction, or for improved photovoltaic devices.

Ab-initio electronic structure theory

In many of our studies we use atomistic models. An efficient method to describe the electronic structure is density functional theory. We have recently implemented a method to determine coupling constants between electrons and vibrations into the electronic structure code TURBOMOLE. The method is based on analytical derivatives and density functional perturbation theory and thus constitutes an efficient and numerically accurate procedure. The electron-vibration couplings can be used to study the influence of vibrations on charge transport in molecular junctions, as we have shown recently.

Presently, we are working on extensions of TURBOMOLE to consider one-, two-, and three-dimensional periodic boundary conditions in collaboration with Prof. Marek Sierka (Friedrich Schiller University Jena) to genuinely describe systems of all kinds of dimensions within a single consistent method. This will allow us to improve our abilities to analyze solid-state and interface-related physics of nanostructured materials. But also other electronic structure codes such as QuantumEspresso, SIESTA or BerkeleyGW are routinely used in this research unit.

Since density functional theory is no quasiparticle method, electronic levels of insulating and semiconducting nanostructures are frequently not described accurately. Improvements are provided by quasiparticle methods that are based on many-body perturbation theory. Work of our group along these lines has recently started.