Research area C: Electronic properties of nanostructures

Nanostructuring of a three-dimensional material creates boundary conditions which can profoundly impact the electronic structure. Electric as well as optical and magnetic properties change with the dimensionality of a material, for instance. Present technology does not only allow for the fabrication of atomically thin films or the controlled positioning of single atoms and molecules, but also for the characterization of nanostructures targeted at applications. The electronic properties of such controllably produced nanostructures are in the focus of research area C.

The size of the building blocks in commercially available electronic circuits is reducing steadily and has reached the scale of several ten nanometers. Due to these developments there is a growing interest in charge transport at the smallest scales to explore the ultimate limit of miniaturization. Furthermore, new physical effects can arise, and there is hope for a cost-efficient fabrication of atomic and molecular nanostructures employing methods of chemical synthesis and self-organization. For these reasons, the study of nanostructures is of crucial scientific importance, particularly to realize functionality.

It is the goal of the research activities in area C to study the electronic properties of nanostructures using novel experimental techniques as well as to improve the present understanding of complex transport phenomena such as shot noise, thermopower, or superconductivity. The activities comprise chemical synthesis and device fabrication, the experimental characterization (including the development of new measurement approaches), and the theoretical description. In this way, novel functional nanosystems will be developed and ultimately small quantum-electronic circuits will be investigated. Research area C combines the competence of six geographic locations. The intense collaboration between synthetic chemistry, experimental physics, theoretical chemistry, and theoretical physics is of particular importance.

Project C2 joins experimental and theoretical efforts in Karlsruhe and Konstanz and explores the interplay between magnetism and superconductivity at the nanoscale. Superconductor-ferromagnet-hybrid structures are used to explain the origin of the long-range proximity effect and spin-dependent scattering at interfaces. Another goal of the project is the realization of functional spin-valves. Such superconducting electronics, based on spin information in the normal-state conductor, would be especially energy-saving.

Project C3, a Ulm-Karlsruhe collaboration between theory and experiment, physics and chemistry, is devoted to clarify the microscopic processes in electrochemically controlled atomic switches. Such quantum point contacts that could realize atomically small electronic building blocks with ultimately small energy consumption, are investigated comprehensively both experimentally and theoretically. In particular, the role of the electrolyte on charge transport will be studied in detail.

Project C4, a team of groups from Karlsruhe and Konstanz, works on “molecular electronics”. A major theme in that research area represents the realization of new electronic functionalities at molecular-sized volumes which would potentially be very resource-efficient. However, a crucial problem in molecular electronics is at the moment the limited comparability of results that are obtained with different contacting and measurement techniques. It is therefore the aim of project C4 to establish a molecular platform that is appropriate for measurements with the scanning electron microscope, mechanically controlled break junction, and nanopore. Through the direct comparison of the transport properties obtained with these various methods, intrinsic properties of the molecules will be separated from experimental artifacts.

An increasing importance is attributed to the field of thermoelectric devices due to their potential to convert temperature gradients into electrical energy. Project C5 represents a collaboration between groups in Konstanz and Karlsruhe. Similar to project C4, it is concerned with scientific challenges in the field of molecular electronics. Here, charge transport quantities will be studied that go beyond the measurement of the elastic conductance. In particular, inelastic transport in molecular contacts and thermoelectric nonequilibrium effects in hybrid metallic nanostructures are modeled at the atomic scale in the theoretical part by use of ab-initio electronic structure methods. In the experimental part they are analyzed through corresponding measurements of transport in optically generated nonequilibrium situations.

In project C9, between researchers in Karlsruhe and Tübingen, atomic quantum sensors are developed for the study of nanostructures. The force detection with ultracold atomic gases shows many similarities to scanning probe techniques. However, instead of a sold-state tip it uses the ultracold atomic clouds as probe head. The ultracold atomic gases will be positioned above different surfaces and will enable the ultrasensitive detection of electromagnetic forces, including dispersion forces.

In research area C, there exists an intense exchange between the different projects due to their common scientific focus on electronic properties of nanostructures and their expertise in related analysis methods. Thus, charge transport is measured in C2, C3, C4, and C5. Ab-initio approaches to electronic structure theory are applied in C3 as well as C5, and other projects such as C4 can collaborate with them, when needed. C9 develops new experimental characterization tools for electronic structure and transport properties that can be used in other projects after their demonstration and optimization. As a strength of research area C, various comparisons between measured and theoretically predicted physical processes are carried out.

Beyond research area C, there are many connections to the other areas in the research network. Thus, similar to C2, aspects of spintronics are investigated in research area D in projects D3, D5, and D7. Furthermore, the scanning tunneling microscope investigations of molecular magnets in D7 establish a link to the molecular contacts, which are the central object of study in C4 and C5. Furthermore, the molecular magnets represent interesting, future test objects for C9, and the optical excitation and detection mechanisms for heat pulses in C5 intensify the close relation to area D. Research areas A and B complement the studies of single-atom and single-molecule junctions in C. The analysis of biological systems and self-organization, carried out there, can be important for the integration of electronic and biological systems or for the industrially relevant, large scale production of controlled electrical circuits.

The research network of excellence “Functional Nanostructures” represents an ideal platform to conduct these sustainable, innovative research activities successfully and with international visibility.