Molecular Nanostructures at Surfaces – Jan Čechal

Functional supramolecular nanostructures at surfaces: UHV preparation and in situ characterization.

Graphene: active material for control electronic structure of adsorbates.

Multimethod surface analysis: LEEM, STM, XPS and ARPES.

Core scientific activities in our group joins two attractive fields: field of molecular self-assembly and field of graphene. In particular, we prepare supramolecular nanostructures on metal and graphene substrates and study both self-assembly process itself and functional properties of supramolecular layers in connection with molecular spintronic and catalysis. 

Further, we offer our expertise on surface analysis, in particular, analysis of chemical composition by X-ray photoelectron spectroscopy to collaborating partners.

Molecular self-assembly of surface-confined architectures attracted significant attention for their promising applications in surface patterning, host-guest chemistry, molecular electronics and spintronics, and catalytic model systems. Here, the long-range ordered networks are formed on surfaces from elementary building blocks: organic molecules and metal atoms. The proper design of molecular building units and selection of metal atoms enables to engineer of extended 2D structures bearing the desired functionality.

Graphene, a zero-bandgap semiconductor, attracted much interest for its mechanical, optical, and electronic properties. From our point of view, the most exciting feature is the possibility of controlling its electronic properties, i.e., the type of charge carriers and their concentration, by the external electric field.

The research in the research group Molecular Nanostructures at Surfaces joins these attractive research fields. In particular, we utilize graphene as a substrate for molecular self-assembly and explore the possibility of tuning the graphene electronic properties to control both the self-assembly process and the functional properties of supramolecular layers on a graphene surface.

The research follows these principal directions:

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 the tuning of 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.