Exploiting Spin Manipulation and Stereo-Isomerization in Molecular Switches

The thesis deals with issues relating to the response of individual molecules to charge injection, investigated using a low-temperature Scanning Tunneling Microscope

Spin Manipulation by Creation of Single-Molecule Radical Cations

All-trans-retinoic acid (ReA), a closed-shell organic molecule comprising only C, H, and O atoms, is investigated on a Au(111) substrate using scanning tunneling microscopy and spectroscopy. In dense arrays single ReA molecules are switched to a number of states, three of which carry a localized spin as evidenced by conductance spectroscopy in high magnetic fields. The spin of a single molecule may be reversibly switched on and off without affecting its neighbors. We suggest that ReA on Au is readily converted to a radical by the abstraction of an electron.Comment: 5 pages, 3 figures, accepted for publication in Phys. Rev. Let

Interplay of boundary states of graphene nanoribbons with a Kondo impurity

We present and discuss the methodology for modeling 4f photoemission spectra, 4f photoelectron diffraction (PED) patterns, and magnetic dichroism effects for rare-earth-based materials. Using PED and magnetic dichroism in photoemission, we explore the electronic and magnetic properties of the near-surface region of the valence-fluctuating material EuIr2Si2. For the Eu-terminated surface, we found that the topmost Eu layer is divalent and exhibits a ferromagnetic order below 10 K. The valency of the next Eu layer, that is the fifth atomic layer, is about 2.8 at low temperature that is close to the valency in the bulk. The properties of the Si-terminated surface are drastically different. The first subsurface Eu layer (fourth atomic layer below the surface) behaves divalently and orders ferromagnetically below 48 K. Experimental data indicate, however, that there is an admixture of trivalent Eu in this layer, resulting in its valency of about 2.1. The next deeper lying Eu layer (eighth atomic layer below the surface) behaves mixed valently, but the estimated valency of 2.4 is notably lower than the value in the bulk. The presented approach and obtained results create a background for further studies of exotic surface properties of 4f-based materials, and allow us to derive information related to valency and magnetism of individual rare-earth layers in a rather extended area near the surface

Surface-supported supramolecular pentamers

Chiral pentamers of all-trans-retinoic acid molecules have been prepared on Au(111) surfaces and on a molecular monolayer. Over a range of coverages, pentamers are the building blocks of larger arrays that become increasingly enantiopure. The stability of pentamers is analyzed from experiments on an isomer and a more reactive substrate as well as from density functional theory. The linear shape of the molecule and suitable densities are crucial for the formation of pentamers, driven by cyclic hydrogen bonding between carboxylic acid moieties.Financial support by the Deutsche Forschungsgemeinschaft through SFB 677 and the Ministry of Science and Technology of China (2013CB933404) is acknowledged.Peer Reviewe

Data for "Gold-linked strings of donor–acceptor dyads: on-surface formation and mutual orientation"

Strings of gold-organic oligomers of polar units have been formed by on-surface synthesis and investigated with non-contact atomic force microscopy. The mutual alignment of dipoles within the strings is analyzed. While an alternating head-to-tail alignment might be expected from dipolar interactions, a more complicated alignment order is observed. The data suggests that coordination bonding to additional gold adatoms leads to stabilization of parallel pairs of molecules, suppressing a head-to-tail alignment order

Surface trapping and STM observation of conformational isomers of a bis(terpyridine) ligand from metallosupramolecular grids

cited By 1International audienceTetranuclear Co-grid complexes incorporating bis-tridentate ligands, namely 4,6-bis(2,2′-bipyrid-6-yl)-2-phenylpyrimidine, were electrosprayed onto a Au(111) substrate under an ultrahigh vacuum. Fragmentation occurs leaving the ligands in four different conformations. Most ligands are found to form H-bonded dimers. The most abundant conformer of the ligand on the Au surface is an asymmetric form, which has not been observed before. The present results indicate that the fragmentation of coordination compounds during the deposition process, in principle, allows for surface trapping, identification, and investigation of high energy, out-of-equilibrium conformations of the ligand molecules at low temperatures, which otherwise would not be observable

Surface-Supported Supramolecular Pentamers

Chiral pentamers of <i>all-trans</i>-retinoic acid molecules have been prepared on Au(111) surfaces and on a molecular monolayer. Over a range of coverages, pentamers are the building blocks of larger arrays that become increasingly enantiopure. The stability of pentamers is analyzed from experiments on an isomer and a more reactive substrate as well as from density functional theory. The linear shape of the molecule and suitable densities are crucial for the formation of pentamers, driven by cyclic hydrogen bonding between carboxylic acid moieties

Spin Control Induced by Molecular Charging in a Transport Junction

The ability of molecules to maintain magnetic multistability in nanoscale-junctions will determine their role in downsizing spintronic devices. While spin-injection from ferromagnetic leads gives rise to magnetoresistance in metallic nanocontacts, nonmagnetic leads probing the magnetic states of the junction itself have been considered as an alternative. Extending this experimental approach to molecular junctions, which are sensitive to chemical parameters, we demonstrate that the electron affinity of a molecule decisively influences its spin transport. We use a scanning tunneling microscope to trap a <i>meso</i>-substituted iron porphyrin, putting the iron center in an environment that provides control of its charge and spin states. A large electron affinity of peripheral ligands is shown to enable switching of the molecular <i>S</i> = 1 ground state found at low electron density to <i>S</i> = ¹/₂ at high density, while lower affinity keeps the molecule inactive to spin-state transition. These results pave the way for spin control using chemical design and electrical means