The research in the group Molecular Nanostructures at Surfaces joins attractive research fields of molecular self-assembly and graphene. In particular, we utilize graphene as a substrate for molecular self-assembly and explore the possibility of tuning the graphene electronic properties to control the functional properties of supramolecular layers on the graphene surface.
Next, we offer our expertise in surface analysis (LEEM, LEED, STM, XPS) to our collaborators.
Building and efficient interfaces for organic electronics. Organic semiconductors (OSs) became an integral part of devices that aim at different applications employing transparent, flexible, and biocompatible materials and offer low-cost/high throughput processing. The interface between the OS and the metallic contacts defines the alignment of the molecular orbital levels of the OS with vacuum and Fermi levels of the metal. The alignment determines the electron and hole injection efficiency; a considerable contact resistance arises from energy level misalignment. The high contact resistance limits the operation frequency and restricts high-current devices such as organic field-effect transistors. One of the possibilities is to employ charge injection layers that reduce the energy level misalignment and, thus, increase the efficiency of OS-based devices. Our approach uses carboxylic acids as a tunable single-layer molecular CIL. When deprotonated, the carboxylate group possesses a partial negative charge, forming an interfacial dipole. We have demonstrated tuning the substate WF in the range of 0.8 eV by gradual deprotonation. Along with the WF change, the energy levels of moleculs in the second molecular layer shift accordingly.
A lattice of magnetic atoms with tunable magnetic coupling. Metal-organic networks self-assembled from metal atoms and small organic molecules present large arrays of equally-spaced magnetic centers in the same local environment. The magnetic centers can be employed for magnetic and spintronic applications but also as quantum bits, i.e., the functional units of a quantum computer. When multiple spin centers reside in close proximity, indirect magnetic interactions can cause the spins to arrange in specific patterns. The presence of these interactions is generally desirable. Still, the ideal strength of the interactions depends on the application: it should be dramatically larger in a spin waveguide than in a system intended for quantum computing. This motivates us to search for ways to control the properties of magnetic interactions by external parameters. An external electric field can tune the charge carrier concentration and polarity within graphene. The magnetic coupling of spin centers on surfaces can be mediated by superexchange via the negatively charged ligand. The electronic density of molecular states was shown to be controllable by a gate voltage applied on the graphene layer giving the promise of control of superexchange interaction strength via adjusting the Fermi level of the graphene layer.
Breaking Time-Reversal Symmetry for Good. Topological insulators have been attracting attention thanks to their fascinating properties and possess enormous potential for spintronics and quantum computing applications. Due to the strong spin-orbit coupling, the bandgap in TIs gets inverted; this gives rise to a special type of surface state. In these states, the electron's spin is locked to its momentum: they are topologically protected, i.e., robust against surface defects or disorder. This property leads to a nearly dissipaon-less current. Although the time-reversal symmetry topologically protects the electrons from backscattering, an interesting consequence arises upon its breaking, e.g., by the presence of ferromagnetic order. This potentially leads to the emergence of a quantum anomalous Hall effect. We have proposed to prepare a periodic array of magnetic atoms/ions embedded in the 2D metal-organic frameworks. The careful design of organic ligands and a proper selection of metal atoms allow fine-tuning of the MON properties, e.g., the type of lattice and its periodicity, molecule-substrate charge transfer, separation of the metal atom from the substrate. It is theoretically predicted that local magnetic moments of 3d atoms are not quenched in a metal-organic network on top of a topological insulator surface, and there is a significant exchange interaction between these atoms. Hence, the magnetic proximity effect can be achieved by properly designed MONs.
Metal-Organic Frameworks on Graphene
We have successfully synthesized metal-organic frameworks (MOFs) M-TCNQ (M = Ni, Fe, Mn) on epitaxial graphene/Ir(111), showing a single phase with M 1(TCNQ)1 stoichiometry. We demonstrate a remarkable chemical and thermal stability of TCNQ-based 2D MOFs: all the studied systems survive exposure to ambient conditions, with Ni-TCNQ doing so without any significant changes to its atomic-scale structure or chemical state. Thermally, the most stable system is Fe-TCNQ which remains stable above 500 °C, while all the tested MOFs survive heating to 250 °C. Overall, our system combines the atomic-scale definition required for fundamental studies with the robustness and stability needed for applications; thus, we consider it an ideal model for research in single-atom catalysis, spintronics, or high-density storage media.
Z. Jakub et al. : Remarkably stable metal-organic frameworks on an inert substrate: M-TCNQ on graphene (M = Mn, Fe, Ni). Nanoscale 14 (2022), 9507.
Tunable Dipolar Layers from Carboxylic Acids
We have demonstrate monolayer thick charge injection layers (CILs) based on aromatic carboxylic acids that can induce an energy level shift in the subsequent layers by up to 0.8 eV. By gradually transforming the as-deposited molecules, we achieve a highly tunable energy level shift in the range of 0.5 eV. We reveal that the work function and energy-level positions in the CIL increase linearly with the density of induced dipoles. The energy level position of the subsequent layers follows the changes in the CIL. Our results thus connect the energy alignment quantities, and the high tunability would allow precise tuning of the active layers deposited on the CIL, which marks a path towards efficient charge injection layers on metal electrodes, which are required to reduce contact resistance and enhance the efficiency of organic-semiconductor-based devices.
V. Stará et al.: Tunable Energy Level Alignment in the Multilayers of Carboxylic Acids on Silver.
Phys. Rev. Appl.
18 (2022), 044048.