Wegner Group: Nanoscale Magnetism & Electronics

   How small can a magnet be? Can we reduce it down to just a few atoms and still use it for computation and as data storage? In the case of the “classical” inorganic magnets like iron, cobalt, or nickel, the magnetization of nanoscopic grains becomes unstable below a certain size due to thermal excitation. This so-called superparamagnetic limit poses a serious challenge for the computer industry. Current state-of-the-art hard drives are already working close to this limit. Ways to “tune” the properties of inorganic magnets are limited: the distance of metallic atoms is more or less fixed, and they prefer to arrange densely packed with only slight structural variations. Also the spin and orbital moments - the basis of magnetism - can hardly be altered in inorganic matter.

   To overcome the superparamagnetic limit and be able to create stable magnetic nanoparticles that consist of only a few atoms, we have to switch to a new class of materials. A very exciting strategy is to combine the knowledge of chemistry and physics. This surplus allows us to build hybrid organic/inorganic molecular nanomagnets, also called Single Molecule Magnets (SMMs).

   So far, most SMMs have been created and discovered more or less by serendipity. We do not yet understand how their magnetic properties are influenced by size, symmetry, and chemical reactivity of the ligands as well as the structure and size of the entire SMM. Even more, there is virtually no knowledge of SMM properties when they are in contact with a conducting surface (as would be the case in an electronic circuit).

  The goal of our research group is to understand how the magnetic coupling between spin centers in SMMs can be maximized while simultaneously limiting - or utilizing - the impact of the surface. For that purpose, we systematically study the influence of ligand properties and the surface on the magnetic interaction between spin centers in a molecular magnet. One strategy is to build magnetic molecules in a bottom-up fashion (i.e., atom by atom, molecule by molecule) by the use of a scanning tunneling microscope (STM). Therefore, the STM is not only used as for imaging but also as a tool to manipulate atoms and molecules with very high precision and to let them react with each other in a controlled way. In a second strategy, we try to deposit very large SMMs (that have been synthesized by chemists) onto clean surfaces using unconventional methods. More detailed properties (e.g. electronic structure, vibrational and spin excitations) can then be analyzed by various scanning tunneling spectroscopy (STS) techniques.

   For future applications, our research activities are a necessary precondition in order to tailor the properties of Single Molecule Magnets via rational design rather than serendipity.

   Pure organic OLEDs can only have a maximum conversion efficiency of 25% (luminsecence) due to the statistical distribution of electron and hole spins when an exciton is formed. 75% of the excitons are in the excited triplett state and cannot decay into the singlet ground state. In contrast, a heavy-metal complex shows large spin-orbit coupling, therefore also the triplet-singlet transition (phosphorescence) is possible, leading to an efficiency of up to 100%. For that reason, OLEDs based on so-called triplet emitters are the state of the art. The most intensively studied complexes are based on iridium.

   Alas, there are also drawbacks. It has been found that Ir(III) complexes only exhibit efficient light emission when they are kept separated via integration into a host material. The OLED structure then usually consists of several organic thin films, each consisting of different molecules. Each layer must be optimized for a specific task (charge injection at the electrode, charge transport toward the complexes, and host-guest interaction in the light emission layer). It is rather difficult to grow the mulitlayered structures well.

   Not as much research has been performed on Pt(II) complexes. Different from the Ir(III) brethren, they have an almost square-planar geometry, therefore the molecules can stack into aggregates that exhibit well-defined intermolecular interaction. Not only does this allow the chemists to tune the photophysical properties, but this also raises the question whether the aggregates can take over all tasks simultaneously in a single thin film as opposed to the many layers inside an Ir-based OLED.

   In order to test the feasibility of such a provocative idea, we fundamentally study the electronic properties of Pt(II) complexes when they are in contact with a metal electrode. We use STM to identify the different chemical groups within single molecules and then perform energy-resolved mapping of frontier orbitals using STS. The results give information on the molecule-substrate interaction and lateral as well as vertical molecule-molceule interactions, from which we can conclude on charge-injection and transport properties.

   Rashba systems are metals or alloys that use heavy-metal elements, leading to large spin-orbit coupling. At the surface, an electric-field gradient in conjunction with the spin-orbit coupling induces a spin splitting of surface bands. Under certain circumstances, this can lead to a situation where electrons with positive spin can only travel in one direction, while the electrons with negative spin always travel in the opposite direction. Surprisingly to the non-expert, these electrons do not interact with each other, i.e., they pass each other as if the other electrons weren‘t there. This is sometimes referred to a spin freeway (or, as we say in Germany, a spin autobahn) where the traffic in opposite direction is separated and does not interfer with each other.

   That‘s the theory. In practice, things can become more complicated. Therefore, we study spin-dependent scattering processes on Rashba-type surfaces. The major goal is to test if the theoretical predictions are true in real-life materials and, if not, to produce experimental evidence for the actual behavior of electron scattering.

   The method of choice for this study is called Fourier-transform scanning tunneling spectroscopy (FT-STS). Here, we take maps of standing waves around impurities and step edges of a Rashba-type surface alloy. This map is then fourier-transformed to identify the apparent scattering vectors in reciprocal space. With some careful thinking and systematic analysis it is possible to recover the entire surface band structure, including band offset, dispersion (i.e. effective mass), and the Rashba splitting.

   At a later stage, we plan to extend our studies to topological insulators.