The research in the "Theory of Nanostructures Group" can be divided into several topics which are listed below. Many of these projects are carried out together with national and international partners. At the university of Konstanz, we collaborate closely with the groups of Wolfgang Belzig, Mikhail Fonin, Paul Leiderer, Peter Nielaba, and Elke Scheer.
The group presently receives funding from the collaborative research center 767 of the German science foundation, the research network of excellence "Functional Nanostructures" of the Baden-Württemberg foundation, the program for junior professors of the ministry of science, research, and art of Baden-Württemberg, the center for applied photonics at the University of Konstanz, and the Carl Zeiss foundation.
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 resorting to system-dependent parameters, and hence make use of quantum chemical ab-initio methods such as density functional theory. Beside applications of our existing programs to determine the conduction properties, we try to 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 electrode-molecule interface that can be compared with the theory. In the context of thermoelectrics, we study also the transport of phonons through the molecular junctions. It is an important ingredient to make quantitative predictions on the value of the thermal conductance and on the thermoelectric figure of merit. It is the hope that through the control over chemical synthesis it may be possible to tune to the interrelated transport properties (electrical conductance, thermopower, thermal conductance) such that the figure of merit is strongly increased. Such efficient thermoelectric devices may be used as nanorefrigerators without movable parts or to convert waste heat into electrical energy, i.e. for energy harvesting at the nanoscale.
In addition to molecular and atomic junctions, i.e. metal-molecule-metal and metal-metal contacts, we study also other systems including graphene nanoribbons and carbon nanotubes.
Interfaces and solid state physics
Interfaces play an important role in physics. Effects like the formation of a two-dimensional electron gas happen at the interface between two different materials. Also in molecular electronics, the description of the typical organic-inorganic interface between a metal and a molecule plays a critical role when electric transport properties are to be predicted, and density functional theory has kown deficits in that respect. We have recently implemented and applied a "self-energy-corrected" density functional scheme to correct for the underestimation of the molecular quasiparticle gap as well as to include the stabilization of charges on a molecule close to a metallic surface due to polarization effects.
Beside these interface-related physics, other parts of our work (for example the description of the electrodes in molecular electronics) require information about bulk properties, such as their electronic structure or density of states. We investigate them as well and are in the progress of extending the quantum chemistry software package TURBOMOLE to treat periodic systems (see further below).
Interesting phenomena arise, when nanostructures are irradiated with laser light. Excited electronic states are accessible and plasmons, collective excitations of the electron fluid, may be excited at the surface of the nanostructure. 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 plan to extent our work along these lines. Time-dependent electronic structure methods shall be used to study both metal-vacuum-metal and metal-molecule-metal nanostructures. They shall be applied to the quantum limit of plasmonics, where electrons can tunnel through the gap between the metal electrodes at both sides. By including electronic correlation effects, our studies ultimately aim at developing new tools for describing the light-matter interaction in extended systems. They 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 most of our studies, we use atomistic models of nanostructures. An efficient method to describe the electronic structure is by 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 consitutes an efficient and numerically accurate procedure. The electron-vibration couplings can be used to study the influence of vibrations on charge transport as we have recently shown.
Presently, we plan to implement one-, two-, and three-dimensional periodic boundary conditions into TURBOMOLE in collaboration with Prof. Marek Sierka (Friedrich Schiller University Jena). This will allow us to improve our abilities in analyzing solid state and interface-related physics in extended nanostructures.
Since density functional theory is no quasiparticle method, electronic levels of insulating and semiconducting nanostructures are frequently not described accurately. Improvement may be provided by quasiparticle methods that are based on many-body perturbation theory. The group plans work along these lines in the future.