Germar Hoffmann

Design of the local spin polarization at the organic-ferromagnetic interface.

By means of ab initio calculations and spin-polarized scanning tunneling microscopy experiments the creation of a complex energy dependent magnetic structure with a tailored spin-polarized interface is demonstrated. We show this novel effect by adsorbing organic molecules containing π(p(z)) electrons onto a magnetic surface. The hybridization of the out-of-plane p(z) atomic-type orbitals with the d states of the metal leads to the inversion of the spin polarization at the organic site due to a p(z)-d Zener exchange-type mechanism. As a key result, we demonstrate the possibility to selectively and efficiently inject spin-up and spin-down electrons from a ferromagnetic-organic interface, an effect which can be exploited in future spintronic devices.

Real-space observation of spin-split molecular orbitals of adsorbed single-molecule magnets

A key challenge in the field of molecular spintronics, and for the design of single-molecule magnet-based devices in particular, is the understanding and control of the molecular coupling at the electrode interfaces. It was demonstrated for the field of molecular electronics that the characterization of the molecule-metal-interface requires the precise knowledge of the atomic environment as well as the molecular orbitals being involved in electron transport. To extend the field of molecular electronics towards molecular spintronics, it is of utmost importance to resolve the spin character of molecular orbitals interacting with ferromagnetic leads. Here we present first direct real-space images of spin-split molecular orbitals of a single-molecule magnet adsorbed on a ferromagnetic nanostructure. Moreover, we are able to determine quantitatively the magnitude of the spin-splitting as well as the charge state of the adsorbed molecule.

Dynamics of molecular self-ordering in tetraphenyl porphyrin monolayers on metallic substrates

A molecular model system of tetraphenyl porphyrins (TPP) adsorbed on metallic substrates is systematically investigated within a joint scanning tunnelling microscopy/molecular modelling approach. The molecular conformation of TPP molecules, their adsorption on a gold surface and the growth of highly ordered TPP islands are modelled with a combination of density functional theory and dynamic force field methods. The results indicate a subtle interplay between different contributions. The molecule-substrate interaction causes a bending of the porphyrin core which also determines the relative orientations of phenyl legs attached to the core. A major consequence of this is a characteristic (and energetically most favourable) arrangement of molecules within self-assembled molecular clusters; the phenyl legs of adjacent molecules are not aligned parallel to each other (often denoted as pi-pi stacking) but perpendicularly in a T-shaped arrangement. The results of the simulations are fully consistent with the scanning tunnelling microscopy observations, in terms of the symmetries of individual molecules, orientation and relative alignment of molecules in the self-assembled clusters.

Molecular Kondo Chain

An important development in recent synthesis strategies is the formation of electronically coupled one and two-dimensional organic systems for potential applications in nanoscale molecule-based devices. Here, we assemble one-dimensional spin chains by covalently linking basic molecular building blocks on a Au(111) surface. Their structural properties are studied by scanning tunneling microscopy and the Kondo effect of the basic molecular blocks inside the chains is probed by scanning tunneling spectroscopy. Tunneling spectroscopic images reveal the existence of separate Kondo regions within the chains while density functional theory calculations unveil antiferromagnetic coupling between the spin centers.

Reversible chiral switching of bis(phthalocyaninato) terbium(III) on a metal surface.

We demonstrate a reversible chiral switching of bis(phthalocyaninato) terbium(III) molecules on an Ir(111) surface by low temperature scanning tunneling microscopy. With an azimuthal rotation of its upper phthalocyanine ligand, the molecule can be switched between a chiral and an achiral configuration actuated by respective inelastic electron tunneling and local current heating. Moreover, the molecular chiral configuration can be interchanged between left and right handedness during the switching manipulations, thereby opening up potential nanotechnological applications.

Two-electron photon emission from metallic quantum wells.

Unusual emission of visible light is observed in scanning tunneling microscopy of the quantum well system Na on Cu(111). Photons are emitted at energies exceeding the energy of the tunneling electrons. Model calculations of two-electron processes which lead to quantum well transitions reproduce the experimental fluorescence spectra, the quantum yield, and the power-law variation of the intensity with the excitation current.

Color imaging with a low temperature scanning tunneling microscope

We report on an improved optical design for detecting light emitted from a scanning tunneling microscope (STM). Using a charge coupled device camera and a grating spectrometer a photon detection efficiency of ≈2.5% at 550 nm is achieved and count rates of up to 5×104 counts/nA/s are observed on a noble metal surface and a W tip. Statistically significant spectra from noble metal surfaces are detected in tens of milliseconds. Thus, new modes of measurement become available, which encompass spectroscopic imaging (acquisition of fluorescence spectra at each point of a STM image), and excitation spectroscopy (acquisition of fluorescence spectra while varying the tip–sample bias). Spectroscopic imaging is used to observe gradual changes of the emission spectra as the STM tip approaches a monoatomic step of Ag(111) on a nanometer scale. Excitation spectroscopy with high resolution in both wavelength and bias voltage is demonstrated for a Ag(111) surface.

Steering Two‐Dimensional Molecular Growth via Dipolar Interaction

The growth of self-organized molecular networks is recognized as a nature-given, potential bottom-up solution for further miniaturization of electronic devices into the nanometer regime. With self-assembling processes being fully parallelized rather than sequential as in conventional top-down approaches as e-beam writing, molecule-based devices gain their potential from a rational design of the fundamental molecular building blocks for controlled bond formation. Relevant aspects are bonding directions and bonding mechanisms. Several different bonding mechanisms are established for network formation as hydrogen bonding, covalent bonding, or even metal–organic coordination. Although the impact of dipole– dipole interaction on network formation is reported the design of molecular dipole fields is so far not further exploited. Here we report for the first time on such an approach and switch by synthetic means between dominating repulsive and attractive forces among metal–organic complexes. The effect is demonstrated for Co(5,5’-X2-Salen) complexes, X=H (1), Me (2), and Cl (3) locally monitored in ultra-high vacuum (UHV) by scanning tunneling microscopy (STM). Salen complexes of transition metals are versatile and easily modified to tailor electronic, magnetic, and structural bulk properties. Salen complexes are present in a variety of applications in material chemistry and catalysis. Moreover, most complexes are volatile in UHV which enable local studies in a well-defined nanoscopic environment. In Salen complexes the metal center is surrounded by two nitrogen and oxygen donor atoms, which can lead to complexes in quite high oxidization states. The hydrogen atoms at the 5,5’-positions can be substituted by CH3, F, Cl, Br, I, NO2, etc. via the respective salicylaldehyde precursor. Here, individual Co-salen complexes adsorbed on a surface are studied for the first time by STM and reveal an insight into the mechanism of intermolecular coupling. The experiments are performed in a variable temperature STM operated at ~25 K. Tips and Cu ACHTUNGTRENNUNG(111) surfaces are prepared by standard procedures with molecules sublimed from homebuilt Knudsen cells. Voltages refer to the potential of the tip relative to the sample. Positive voltages refer to tunneling into unoccupied sample states and negative voltages to tunneling out of occupied sample states. We focus on the analysis of STM images as acquired at 1.4 V, 0.1 V, and +1.4 V which are referred as ‘at large negative bias’, ‘at low bias’, and ‘at elevated positive bias’ throughout the text. Images at these biases show characteristic features as discussed below and are representative for images as acquired within a larger bias interval (~ 0.5 V) with only gradual changes in between. The chemical structures of the complexes used herein are presented in Figure 1 along with corresponding, equally scaled STM images at low bias of isolated molecules adsorbed on a Cu ACHTUNGTRENNUNG(111) surface. For visualization a scaled model of the structure is superimposed. For complex 1 the appearance of the complex in the STM image perfectly fits to the structure. Due to different substituents the apparent size of 2 and 3 varies in the topographic images. We will first focus on the appearance and adsorption of isolated molecules of complex 1, that is, after low temperature preparation which hinders thermally induced mobility and therefore self-assembling on Cu ACHTUNGTRENNUNG(111). Figures 2b–2d show the same isolated molecule of complex 1 but as imaged at different energies. Imaged at low bias (Figure 2c), the molecule nicely fits to the molecular structure and exhibits a maximum in the apparent height at a location between the cobalt center and the top C2H4 bridge. Imaged at a large negative energy a pronounced topographic maximum at the site of the Co ion dominates the overall molecular appearance (Figure 2b). We interpret this as resulting from occupied Co 3d orbitals perpendicular to the molecular plane similar to the case of other metal-organic complexes. A significant change can be observed when tunneling through unoccupied molecular states at elevated positive bias: molecules become asymmetric and the maximum in the apparent height is shifted towards one side of the C2H4 bridge (Figure 2d). A first interpretation is suggested by results of DFT calculations for the free complex. In Figure 2a an optimized structure of complex 1 is depicted. The side view clarifies a C2 symmetry due to a deformation of the C2H4-bridge which implies chirality. STM measurements identified complexes adsorbed in 12 different configurations on the Cu ACHTUNGTRENNUNG(111) surface. Figure 2e shows all 12 configurations as imaged at low bias. The observable asymmetry at elevated positive energies (Figure 2 f) unambiguously reveals two mirror symmetric (R vs L) sets of 6 molecules each rotated in steps of 608. With the exact crystallographic axes determined from atomically resolved images of the bare substrate, two mirror symmetric enantiomers can be attributed to each crystallographic direction. Molecules denoted with R are rotated clockwise by +118 38 relative to the substrate axes and with L anticlockwise by 88 38. This is schematically illustrated in Figure 2 g. [a] S. Kuck, Dr. S.-H. Chang, Dr. G. Hoffmann, Prof. Dr. R. Wiesendanger Institut f r Angewandte Physik Universit t Hamburg, Jungiusstrase 9, Hamburg (Germany) Fax: (+49)40-42838-2944 E-mail : skuck@physnet.uni-hamburg.de [b] J.-P. Klcckner, Prof. Dr. M. H. Prosenc Institut f r Anorganische und Angewandte Chemie Universit t Hamburg, Martin-Luther-King-Platz 6, Hamburg (Germany) Supporting information for this article is available on the WWW under http://dx.doi.org/10.1002/cphc.200900281

Digitized Charge Transfer Magnitude Determined by Metal–Organic Coordination Number

Well-ordered metal-organic nanostructures of Fe-PTCDA (perylene-3,4,9,10-tetracarboxylic-3,4,9,10-dianhydride) chains and networks are grown on a Au(111) surface. These structures are investigated by high-resolution scanning tunneling microscopy. Digitized frontier orbital shifts are followed in scanning tunneling spectroscopy. By comparing the frontier energies with the molecular coordination environments, we conclude that the specific coordination affects the magnitude of charge transfer onto each PTCDA in the Fe-PTCDA hybridization system. A basic model is derived, which captures the essential underlying physics and correlates the observed energetic shift of the frontier orbital with the charge transfer.

Adsorption and conformation of porphyrins on metallic surfaces

Tetraphenyl porphyrins (TPP) belong to a highly interesting class of molecules with a large variety of electronic, magnetic, and structural properties. So far, local investigations by scanning probe techniques were primarily focused on larger agglomerates of TPP molecules. Here, experimental results of the observation and manipulation of isolated molecules adsorbed on cold metal substrates by means of low temperature scanning tunneling microscopy are presented. Depending on the surface geometry, i.e., Cu(111) vs Cu(100) three distinct deformations of the molecular structure are identified reflecting the interaction of the phenyl periphery with the substrate. In a second step, controlled manipulation in terms of deformation of the porphyrin core, ligand dissociation, and lateral displacement of the phenyl periphery are demonstrated.