Chemnitz UT: Fakultät für Naturwissenschaften: Institut für Physik: Projects

SP1: Thin films of redox-active high-spin molecules

Single molecule magnets (SMMs) are currently considered for applications in spin electronics and information storage devices. In order to use such molecules in devices, it is essential to deposit and arrange SMMs on substrates for addressing purposes. In the present project novel high-spin molecules will be synthesized and the conditions under which they can be deposited onto surfaces without decomposition will be elucidated experimentally. Particular emphasis of the project will be put on: the targeted assembly of novel non-oxide based single molecule magnets using a modular approach, their deposition and arrangement on metallic (Au(111), Co), insulating (SiO₂), and semiconducting surfaces (Si, GaAs), and film characterization by various techniques such as SQUID magnetometry, near-field techniques (AFM, STM), variable angle spectroscopic ellipsometry, high-field ESR, IR, Raman and magneto-optical Kerr effect (MOKE) spectroscopy.

SP2: Preparation of spin coated thin films and self-assembled monolayers of magnetic transition metal complexes

A prerequisite for technological application of single molecule magnets (SMMs) is their conversion into ordered arrays on substrates. Magnetic trimetallic bis(oxamato) transition metal complexes as model systems of SMMs will be used for the formation of thin films by means of spin coating. By the functionalization of these complexes with terminal SH groups the formation of self-assembled monolayers (SAMs) will be performed as an alternative approach. The trimetallic complexes will be characterized by, for example, SQUID magnetometry, ESR, IR and Raman spectroscopy and thin layers by magneto-optical Kerr effect spectroscopy. It is essential to understand how a structural modification of the complexes (planarity, charge, functional groups such as, for example, the central N,N' bridging units or the terminal ligands) and the deposition conditions (solvent, substrate, and rotation speed for the spin coating process; SH-functionalized trimetallic species solely or in mixture with organic thiols for the SAM formation) will influence the molecular and the film properties. This knowledge will be used as a feed-back for the synthesis of new molecules with properties tailored to achieve desired magnetic, magneto-optic or spin polarized transport properties, which are processable for device fabrication.

SP3: Electronic structure, transport, and collective effects in molecular layered systems

Based on ab-initio density functional theory results, we will study charge and spin transport both in the scanning tunnelling microscopy geometry and in semiconductor/molecular layer/semiconductor structures. We will employ two complementary approaches: the non-equilibrium Green-function method and the quantum master equation. In the longer term, we plan to also consider collective effects in molecular layers and their relevance for magnetic and transport properties.

SP4: Electron spin resonance and magnetic studies

This project is dedicated to investigations of magnetic properties of magnetic molecular complexes, of molecular thin films and interfaces and eventually of spin electronic devices. For this purpose, tunable high-frequency high-field electron spin resonance and static magnetization measurement techniques ranging from experiments in high strength static and pulsed magnetic fields to studies employing micromagnetic sensors will be applied. The experiments will provide relevant information about the strength of intramolecular magnetic interactions, the size and the type of magnetic anisotropy, the spectrum of low-energy magnetic excitations and relaxation phenomena. By these means relationships between chemical structure, bonding topology and magnetism will be established, thus enabling to chemically engineer molecular complexes with predetermined magnetic properties. In addition, our experimental techniques will be extended towards strongly improved sensitivity, very small samples and the investigations of the magnetization under applied microwave irradiation, i.e. under precise control over the spin excitations. On the long term, the latter setup will be used for electrical studies on magnetic molecular spin electronic devices upon tuning/ switching their magnetic states. The project has strong links to other projects of the research unit. Specifically, we shall thoroughly characterize new magnetically active compounds which will be synthesized in the framework of the chemistry projects SP1, SP2 and external collaborations. The experimental results will be analyzed in a close cooperation with the theory project SP3. These studies will provide new and complementary information of high relevance to projects SP5, SP7, and SP8, and ensure an optimal selection of materials for the manufacturing of devices in the project SP9.

SP5: Spin dynamics in single molecules and thin films studied by nuclear probe spectroscopy

In this project we study the magnetic properties of new molecular magnets in bulk crystals and in thin films and monolayers on metallic and semiconducting substrates by means of local probe spectroscopy. For bulk crystals besides a determination of the ground state spin, the magnetic anisotropy, relaxation mechanisms, and the delocalization of the spin density on multinuclear transition metal complexes shall be determined to identify systems which may show molecule-substrate or inter-molecular interaction in thin films and monolayers. Using the high sensitive nuclear probes low energy muon spin relaxation, low-temperature high-field NMR and low-energy ?-NMR, the change of the magnetic properties in the deposited molecules and there interaction shall be studied. For systems containing Fe complexes also Mössbauer spectroscopy will be performed to study the magnetic properties and charge dynamics. The samples are synthesized in SP1 and SP2 and the results will be compared with DFT calculations in SP3 and ESR spectroscopy in SP4. The results on thin films are complementary to XMCD measurements in SP7 and the magneto-optical Kerr effect measurements performed in SP8.

SP6: Scanning tunneling microscopy and spectroscopy of magnetic molecules

An indispensable tool for studying single molecule magnets as the fundamental functional building blocks for future molecular spintronic devices is (spin-polarized) scanning tunneling microscopy and -spectroscopy (SP-) STM/STS. This method provides unique insight into the electronic and magnetic properties of molecular magnet systems. In this project we will use (SP-) STM/STS for the investigation of single molecular magnets adsorbed on well defined crystalline substrates, ordered aggregates, self-assembled monolayers and multilayer systems. Crucial aspects for the bottom-up development of molecular spintronic devices will be studied. These include the understanding of adsorption geometries, electronic charge transfer mechanisms, intra- and intermolecular magnetic exchange coupling mechanisms, the interaction of the molecular spin with the substrate and the tip, and in particular, the spin-dependent tunneling transport (magnetotransport) properties of the molecules.

SP7: Spectroscopic studies of magnetic molecular materials

The exploitation of the magnetic degrees of freedom of molecular materials is one of the possibilities to achieve future "spintronic" technologies. Progress in this direction requires the development of advanced functional materials and the thorough characterization of their magnetic and electronic properties. The achievement of a fundamental knowledge on the electronic and magnetic properties of novel molecular materials containing transition metal sites, as investigated within our research unit, is a prerequisite for any application and the central goal of this project. We apply complementary, spectroscopic methods in order to characterize the electronic and magnetic properties of the molecular materials and their interfaces to electrode materials. In particular, we determine the occupied and unoccupied electronic density of states, the electronic excitation spectrum, the alignment of the electronic molecular levels at contacts to electrode materials, the transition metal 3d occupation and spin and the changes induced by charging of the molecules. Our results and the close collaboration within this research unit will enable the further development of the materials and the understanding of their performance in the anticipated devices.

SP8: From the preparation of molecular layers and their (magneto-)optical investigation towards laterally stacked devices

The performance of devices based on molecular materials is not only determined by intrinsic properties of the molecular building blocks. The molecular orientation, the film morphology as well as the nature of the interfaces are crucial for the device performance. By probing vibrations and optical transitions, (magneto-)optical spectroscopic techniques provide excellent tools to explore molecular properties and film properties.

In this project thin films of magnetic molecules will be deposited by spin and dip coating as well as by organic molecular beam deposition onto flat substrates and their properties will be investigated by optical and magneto-optical spectroscopic methods. The knowledge on the film properties will be transferred to the fabrication of laterally stacked spintronic devices on substrates structured by semiconductor processing (lithography, Si patterning, oxide and metal deposition).

SP9: Transport through spin-polarized semiconductor/molecule/semiconductor tunnel junctions

This project aims at polarizable molecular spins in a tunneling barrier manipulated by an external magnetic field and operated as true spin valves, either for the generation or the control of highly spin-polarized currents. However, transport through magnetic self-assembled molecular layers (SAM) has been limited to metal/molecule/metal or - in much fewer cases - metal/molecule/semiconductor junctions. Ultimately, it would be desirable to create a metal free semiconductor/molecule/semiconductor (S/SAM/S) junction to make full use of the unique band structure tunability of intentionally doped semiconductors. In a first approximation, for semiconductor/molecule junctions, the corresponding barrier height is the energy difference between the edge of the conduction or valence band and the LUMO or HOMO, respectively, depending on the semiconductor doping type [A6]. Once the barrier can be tuned by changing the semiconductor doping type, S/SAM/S junctions would allow us to control the charge injection in the magnetic molecules by carefully setting the doping concentration of both semiconducting electrodes. This would for example allow studying transport through a p-type/molecule/n-type junction, which - for degenerate semiconductors - could reveal new features in the IV curve of an interband tunneling diode. Furthermore, a technique that allows large scale integration of S/SAM/S devices on a single chip would be highly desirable. While the assembly of molecules on a semiconductor (and therefore a bottom contact) is possible, the great challenge is to establish an equally good top semiconductor contact to a molecular layer, which has not been achieved, yet. The aim of this project is therefore to fabricate - for the first time - molecular magnet-based S/SAM/S spin-valves and the investigation of the transport properties of such vertical, but also horizontal devices.