M. Rübhausen

Catalytic phenol hydroxylation with dioxygen: extension of the tyrosinase mechanism beyond the protein matrix.

A pinnacle of bio-inorganic chemistry is the ability to leverage insights gleaned from metalloenzymes toward the design of small analogs capable of effecting catalytic reactivity outside the context of the natural system.[1,2] Structural mimicry of active sites is an attempt to insert a synthetic catalyst into an enzymatic mechanism. Such a mechanism evolves by selection pressures for efficiency and traverses an energetic path with barriers and wells neither too high nor too deep in energy – a critical factor of catalytic turnover.[3] An advantage of metalloenzymes over small metal complexes is the site-isolation of the metal center in the protein matrix with its attendant ability to attenuate destructive decay processes – reaction sinks. This protection provides access to thermal regimes that allows barriers and wells to be traversed. Synthetic complexes too must avoid any deleterious reactions, often necessitating deliberate incorporation of protective superstructures.[4,5] Such limitations make reproducing enzymatic catalytic reactivity in a synthetic complex with native substrates a significant challenge, as evidenced by the dearth of good examples, despite decades of effort.

Silver substrates for surface enhanced Raman scattering: Correlation between nanostructure and Raman scattering enhancement

The fabrication of substrates for Surface Enhanced Raman Scattering (SERS) applications matching the needs for high sensitive and reproducible sensors remains a major scientific and technological issue. We correlate the morphological parameters of silver (Ag) nanostructured thin films prepared by sputter deposition on flat silicon (Si) substrates with their SERS activity. A maximum enhancement of the SERS signal has been found at the Ag percolation threshold, leading to the detection of thiophenol, a non-resonant Raman probe, at concentrations as low as 10−10M, which corresponds to enhancement factors higher than 7 orders of magnitude. To gain full control over the developed nanostructure, we employed the combination of in-situ time-resolved microfocus Grazing Incidence Small Angle X-ray Scattering with sputter deposition. This enables to achieve a deepened understanding of the different growth regimes of Ag. Thereby an improved tailoring of the thin film nanostructure for SERS applications can be realized.

Catching an Entatic State—A Pair of Copper Complexes

The structures of two types of guanidine-quinoline copper complexes have been investigated by single-crystal X-ray crystallography, K-edge X-ray absorption spectroscopy (XAS), resonance Raman and UV/Vis spectroscopy, cyclic voltammetry, and density functional theory (DFT). Independent of the oxidation state, the two structures, which are virtually identical for solids and complexes in solution, resemble each other strongly and are connected by a reversible electron transfer at 0.33 V. By resonant excitation of the two entatic copper complexes, the transition state of the electron transfer is accessible through vibrational modes, which are coupled to metal-ligand charge transfer (MLCT) and ligand-metal charge transfer (LMCT) states.

Stabilisation of a Highly Reactive Bis(μ-oxo)dicopper(III) Species at Room Temperature by Electronic and Steric Constraint of an Unconventional Nitrogen Donor Ligand

Herein we present an innovative combination of EXAFSspectroscopy and resonant Raman scattering for the charac-terisation of the ground state and structural dynamics of athermally stable binuclear bisACHTUNGRE(m-oxo) dicopperACHTUNGRE(III) species.Peralkylated bis(guanidine)-based ligands are used in thesynthesis of this compound.

A new route to graphene layers by selective laser ablation

Selectively creating regions of spatially varying thickness may enable the utilization of the electronic properties of N-layer (N=1 or more) graphene and other similar layered materials (e.g., topological insulators or layered superconductors) for novel devices and functionalities on a single chip. The ablation threshold energy density increases dramatically for decreasing layer numbers of graphene originating from the dimensional crossover of the specific heat. For the 2D regime of graphite (up to N≈7) the dominant flexural mode specific heat (due to its N-1 dependence) gives rise to a strong layer number-dependence on the pulsed laser ablation threshold energy density, while for 3D regime (N>>7) the ablation threshold saturates due to dominant acoustic mode specific heat. As a result, several energy density windows exist between the minimum energy densities that are required for ablating single, bi, or more layers of graphene, allowing layer number selectivity.

Multiferroicity in the spin-1/2 quantum matter of LiCu2O2

Multiferroicity in LiCu2O2 single crystals is studied using resonant soft x-ray magnetic scattering, hard x-ray diffraction, heat capacity, magnetic susceptibility, and electrical polarization. Two magnetic transitions are found at 24.6 K (T1) and 23.2K (T2). Our data are consistent with a sinusoidal spin structure at T2<T<T1 and with a helicoidal spin structure at T<T2, giving rise to ferroelectricity. Surprisingly, above T2, the correlation lengths of the spin structures increase as the temperature increases with dramatic changes of ∼42% occurring along the c axis.

Temperature-dependent spectral generalized magneto-optical ellipsometry

We present a setup for temperature-dependent spectral generalized magneto-optical ellipsometry (SGME). This technique gives access to the electronic as well as the magnetic properties of ferromagnetic materials within one single magneto-optical measurement. It also allows the determination of the orientation of the magnetization. We show spectra of the real and the imaginary part of the refractive index N as well as the magneto-optical coupling parameter Q of permalloy and iron films for in-plane magnetization. Our findings demonstrate the relevance of SGME for the understanding of the interplay between electronic and magnetic properties of ferromagnetics.

Patterned Diblock Co-Polymer Thin Films as Templates for Advanced Anisotropic Metal Nanostructures

We demonstrate glancing-angle deposition of gold on a nanostructured diblock copolymer, namely polystyrene-block-poly(methyl methacrylate) thin film. Exploiting the selective wetting of gold on the polystyrene block, we are able to fabricate directional hierarchical structures. We prove the asymmetric growth of the gold nanoparticles and are able to extract the different growth laws by in situ scattering methods. The optical anisotropy of these hierarchical hybrid materials is further probed by angular resolved spectroscopic methods. This approach enables us to tailor functional hierarchical layers in nanodevices, such as nanoantennae arrays, organic photovoltaics, and sensor electronics.

Observation of room-temperature high-energy resonant excitonic effects in graphene

Using a combination of ultraviolet-vacuum ultraviolet reflectivity and spectroscopic ellipsometry, we observe a resonant exciton at an unusually high energy of 6.3 eV in epitaxial graphene. Surprisingly, the resonant exciton occurs at room temperature and for a very large number of graphene layers N≈75, thus suggesting a poor screening in graphene. The optical conductivity (σ1) of a resonant exciton scales linearly with the number of graphene layers (up to at least 8 layers), implying the quantum character of electrons in graphene. Furthermore, a prominent excitation at 5.4 eV, which is a mixture of interband transitions from π to π∗ at the M point and a π plasmonic excitation, is observed. In contrast, for graphite the resonant exciton is not observable but strong interband transitions are seen instead. Supported by theoretical calculations, for N⩽ 28 the σ1 is dominated by the resonant exciton, while for N> 28 it is a mixture between exitonic and interband transitions. The latter is characteristic for graphite, indicating a crossover in the electronic structure. Our study shows that important elementary excitations in graphene occur at high binding energies and elucidate the differences in the way electrons interact in graphene and graphite.

Temperature-dependent spectral generalized magneto-optical ellipsometry for ferromagnetic compounds

Spectral generalized magneto-optical ellipsometry is presented as an optical tool for the simultaneous measurement of the complex index of refraction n=n+ik, the complex magneto-optical coupling parameter Q=Qr+iQi (i.e., the Voigt-parameter), and the orientation of the saturation magnetization Ms of isotropic ferromagnetic bulk materials. For wavelengths between 220nm and 790nm and at temperatures between 4.2K and 800K measurements on iron and permalloy demonstrate the comfortable application of this technique in order to resolve the spectral response of spin-polarized carriers and bands, which can provide valuable insight about the formation of the ferromagnetic state.