Stephan Rauschenbach

Spin and Orbital Magnetic Moment Anisotropies of Monodispersed Bis(Phthalocyaninato)Terbium on a Copper Surface

The magnetic properties of isolated TbPc(2) molecules supported on a Cu(100) surface are investigated by X-ray magnetic circular dichroism at 8 K in magnetic fields up to 5 T. The crystal field and magnetic properties of single molecules are found to be robust upon adsorption on a metal substrate. The Tb magnetic moment has Ising-like magnetization; XMCD spectra combined with multiplet calculations show that the saturation orbital and spin magnetic moment values reach 3 and 6 mu(B), respectively.

Polymer nanofibers via nozzle-free centrifugal spinning

We report on the unexpected finding of nanoscale fibers with a diameter down to 25 nm that emerge from a polymer solution during a standard spin-coating process. The fiber formation relies upon the Raleigh-Taylor instability of the spin-coated liquid film that arises due to a competition of the centrifugal force and the Laplace force induced by the surface curvature. This procedure offers an attractive alternative to electrospinning for the efficient, simple, and nozzle-free fabrication of nanoscale fibers from a variety of polymer solutions.

The Quantum Magnetism of Individual Manganese-12-Acetate Molecular Magnets Anchored at Surfaces

The high intrinsic spin and long spin relaxation time of manganese-12-acetate (Mn(12)) makes it an archetypical single molecular magnet. While these characteristics have been measured on bulk samples, questions remain whether the magnetic properties replicate themselves in surface supported isolated molecules, a prerequisite for any application. Here we demonstrate that electrospray ion beam deposition facilitates grafting of intact Mn(12) molecules on metal as well as ultrathin insulating surfaces enabling submolecular resolution imaging by scanning tunneling microscopy. Using scanning tunneling spectroscopy we detect spin excitations from the magnetic ground state of the molecule at an ultrathin boron nitride decoupling layer. Our results are supported by density functional theory based calculations and establish that individual Mn(12) molecules retain their intrinsic spin on a well chosen solid support.

A close look at proteins: submolecular resolution of two- and three-dimensionally folded cytochrome c at surfaces

Imaging of individual protein molecules at the single amino acid level has so far not been possible due to the incompatibility of proteins with the vacuum environment necessary for high-resolution scanning probe microscopy. Here we demonstrate electrospray ion beam deposition of selectively folded and unfolded cytochrome c protein ions on atomically defined solid surfaces in ultrahigh vacuum (10(-10) mbar) and achieve unprecedented resolution with scanning tunneling microscopy. On the surface folded proteins are found to retain their three-dimensional structure. Unfolded proteins are observed as extended polymer strands displaying submolecular features with resolution at the amino acid level. On weakly interacting surfaces, unfolded proteins refold into flat, irregular patches composed of individual molecules. This suggests the possibility of two-dimensionally confined folding of peptides of an appropriate sequence into regular two-dimensional structures as a new approach toward functional molecular surface coatings.

Electrospray Ion Beam Deposition: Soft-Landing and Fragmentation of Functional Molecules at Solid Surfaces

The ion beam deposition (IBD) of rhodamine dye molecules on solid surfaces in high vacuum is explored in order to characterize the possibility of fabricating molecular coatings or nanostructures from nonvolatile molecules. Molecular ion beams with a well-defined composition are deposited on silicon oxide surfaces with a controlled kinetic energy. Photoluminescence spectroscopy and time-of-flight secondary ion mass spectrometry (TOF-SIMS) are employed in order to characterize the sample with respect to coverage, homogeneity, and the fraction of intact landed ions (soft-landing ratio). We find that homogeneous rhodamine films of defined composition can be produced at energies of 2-100 eV. The coverage is found to be proportional to the ion dose. Soft-landing is observed for energies up to 35 eV.

Towards the Isomer-Specific Synthesis of Higher Fullerenes and Buckybowls by the Surface-Catalyzed Cyclodehydrogenation of Aromatic Precursors†

Keywords: cyclodehydrogenation ; fullerenes ; heterogeneous catalysis ; nanotechnology ; surface chemistry ; Chemical-Synthesis ; C-60 Adsorption ; Transition Reference EPFL-ARTICLE-172115doi:10.1002/anie.201005000View record in Web of Science Record created on 2011-12-16, modified on 2017-05-12

The classical and quantum dynamics of molecular spins on graphene

Controlling the dynamics of spins on surfaces is pivotal to the design of spintronic1 and quantum computing2 devices. Proposed schemes involve the interaction of spins with graphene to enable surface-state spintronics3,4, and electrical spin-manipulation4-11. However, the influence of the graphene environment on the spin systems has yet to be unraveled12. Here we explore the spin-graphene interaction by studying the classical and quantum dynamics of molecular magnets13 on graphene. While the static spin response remains unaltered, the quantum spin dynamics and associated selection rules are profoundly modulated. The couplings to graphene phonons, to other spins, and to Dirac fermions are quantified using a newly-developed model. Coupling to Dirac electrons introduces a dominant quantum-relaxation channel that, by driving the spins over Villain’s threshold, gives rise to fully-coherent, resonant spin tunneling. Our findings provide fundamental insight into the interaction between spins and graphene, establishing the basis for electrical spin-manipulation in graphene nanodevices.

Atomic-scale observation of multiconformational binding and energy level alignment of ruthenium-based photosensitizers on TiO2 anatase

Dye-sensitized solar cells constitute a promising approach to sustainable and low-cost solar energy conversion. Their overall efficiency crucially depends on the effective coupling of the photosensitizers to the photoelectrode and the details of the dyes energy levels at the interface. Despite great efforts, the specific binding of prototypical ruthenium-based dyes to TiO2, their potential supramolecular interaction, and the interrelation between adsorption geometry and electron injection efficiency lack experimental evidence. Here we demonstrate multiconformational adsorption and energy level alignment of single N3 dyes on TiO2 anatase (101) revealed by scanning tunnelling microscopy and spectroscopy. The distinctly bound molecules show significant variations of their excited state levels associated with different driving forces for photoelectron injection. These findings emphasize the critical role of the interfacial coupling and suggest that further designs of dye-sensitized solar cells should target a higher selectivity in the dye-substrate binding conformations in order to ensure efficient electron injection from all photosensitizers.

Imaging proteins at the single-molecule level

Significance We report a method to image and reveal structural details of proteins on a truly single-molecule level. Low-energy electron holography is used to image individual proteins electrospray deposited on freestanding graphene. In contrast to the current state of the art in structural biology, we do away with the need for averaging over many molecules. This is crucial because proteins are flexible objects that can assume distinct conformations often associated with different functions. Proteins are also the targets of almost all the currently known and available drugs. The design of new and more effective drugs relies on the knowledge of the targeted proteins structure in all its biologically significant conformations at the best possible resolution. Imaging single proteins has been a long-standing ambition for advancing various fields in natural science, as for instance structural biology, biophysics, and molecular nanotechnology. In particular, revealing the distinct conformations of an individual protein is of utmost importance. Here, we show the imaging of individual proteins and protein complexes by low-energy electron holography. Samples of individual proteins and protein complexes on ultraclean freestanding graphene were prepared by soft-landing electrospray ion beam deposition, which allows chemical- and conformational-specific selection and gentle deposition. Low-energy electrons do not induce radiation damage, which enables acquiring subnanometer resolution images of individual proteins (cytochrome C and BSA) as well as of protein complexes (hemoglobin), which are not the result of an averaging process.

A hydrodynamically optimized nano-electrospray ionization source and vacuum interface

The coupling of atmospheric pressure ionization (API) sources like electrospray ionization (ESI) to vacuum based applications like mass spectrometry (MS) or ion beam deposition (IBD) is done by differential pumping, starting with a capillary or pinhole inlet. Because of its low ion transfer efficiency the inlet represents a major bottleneck for these applications. Here we present a nano-ESI vacuum interface optimized to exploit the hydrodynamic drag of the background gas for collimation and the reduction of space charge repulsion. Up to a space charge limit of 40 nA we observe 100% current transmission through a capillary with an inlet and show by MS and IBD experiments that the transmitted ion beams are well defined and free of additional contamination compared to a conventional interface. Based on computational fluid dynamics modelling and ion transport simulations, we show how the specific shape enhances the collimation of the ion cloud. Mass selected ion currents in the nanoampere range available further downstream in high vacuum open many perspectives for the efficient use of electrospray ion beam deposition (ES-IBD) as a surface coating method.