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Determining the mechanism of charge transport through native DNA remains a challenge as different factors such as measuring conditions, molecule conformations, and choice of technique can significantly affect the final results. In this contribution, we have used a new approach to measure current flowing through isolated double-stranded DNA molecules, using fullerene groups to anchor the DNA to a gold substrate. Measurements were performed at room temperature in an inert environment using a conductive AFM technique. It is shown that the π-stacked B-DNA structure is conserved on depositing the DNA. As a result, currents in the nanoampere range were obtained for voltages ranging between ±1 V. These experimental results are supported by a theoretical model that suggests that a multistep hopping mechanism between delocalized domains is responsible for the long-range current flow through this specific type of DNA. This project was funded by the Special Research Funds (BOF) of Hasselt University. The authors thank Dr. Patricia Losada-Perez and Dr. Alexander Volodin for fruitful discussions and Profs. Marlies K. Van Bael and Hans-Gerd Boyen for providing access to their experimental equipment.

The sizes and shapes of single clay mineral layers are difficult to determine though they are important parameters which determine the final properties of clay polymer nanocomposites and of ultrathin clay mineral films. To determine these sizes and shapes, hybrid monolayers of clay minerals (saponite, hectorite, Wyoming bentonite, and Laponite) and Rhodamine B octadecyl ester perchlorate (RhB18) were prepared using the Langmuir-Blodgett (LB) technique and studied with atomic force microscopy (AFM). The AFM images reveal monolayers of elementary clay mineral layers, which are randomly oriented and have a wide range of sizes. The layers have typical shapes: lath-like for hectorite, plates for Wyoming bentonite, a mixture of laths and plates for saponite, and aggregates of very small layers of Laponite. Two types of layers were present in the LB films of saponite, Wyoming bentonite, and hectorite in a 40:60 ratio: (1) single layers 0.96 nm thick hybridized with RhB18; and (2) particles consisting of two clay layers with an intercalated monomolecular layer of water molecules and hybridized with RhB18. The Laponite particles in the hybrid LB films consist mainly of aggregates of two and three single layers.

The supramolecular organization of a new polychlorotriphenyl (PTM) radical bearing three long alkyl chains has been studied by scanning tunneling microscopy (STM) at the liquid-solid interface. This radical hierarchically self-assembles on graphite forming head-to-head dimers that organize in rows following an interesting spin-containing two-leg molecular ladder topology, in which the alkyl chains determine the space between the radical rows and act as diamagnetic barriers. In addition, these double-rows also self-assemble three-dimensionally, leading to a multilayer organization which is still influenced by the HOPG substrate symmetry. The observed nanostructures are sustained by different intermolecular interactions such as Cl...Cl, Cl...Ph, pi-pi, van der Waals, and CH...pi interactions. Theoretical calculations were used to model the observed assemblies, and the results were in complete agreement with the experimental data. Remarkably, atomic force microscopy (AFM) studies confirmed that this tendency to form double rows composed by the PTM magnetic heads surrounded by the alkyl chains is maintained after the complete evaporation of the solvent. The electrochemical and magnetic properties of these PTM nanostructures were also demonstrated.

We report on detailed low temperature scanning tunneling spectroscopy measurements performed on preformed nanometer sized gold clusters. The clusters are deposited on Au(1 1 1) films that are coated with a self-assembled monolayer of 1,4-benzenedimethanethiol molecules, thereby creating a double barrier tunnel junction. In this configuration electrons can only move between the scanning tunneling microscope tip and the substrate through tunneling via the cluster. Due to the very small cluster size this results in pronounced single-electron tunneling effects. Anomalies in the I(V) curves measured above a single cluster can be consistently described in terms of charging of the double barrier junction for the majority of the particles. Both Coulomb blockade and Coulomb staircase with step widths in the 200-600 meV range were observed on various individual clusters. The results agree with the commonly used semiclassical theory of charging effects in a double barrier tunnel junction. Furthermore, for a limited number of clusters, additional conductance peaks are observed which are attributed to strong electron confinement inside the nanoscale clusters. (C) 2007 Elsevier B.V. All rights reserved. ispartof: Surface science vol:602 issue:2 pages:552-558 status: published

A technique is reported for the combined growth of carbon nanotubes (CNTs) and carbon nanowalls (CNWs) by plasma-enhanced chemical vapor deposition. The observation serves as a direct proof of a close correlation between the growth of both materials because both are obtained in a single experiment without making any changes to the growth parameters. The growth of freestanding CNTs is driven by a nickel catalyst deposited on an oxidized silicon wafer. It is assumed that the remaining carbon radicals are inserted in the sidewalls and tips of the tubes after the saturation of the catalyst by abundant carbon, thereby forming a CNW layer on top of the CNTs. A possible growth scheme, based on qualitative analysis by electron microscopy, Raman spectroscopy and X-ray diffraction, is presented. It is further shown that the CNWs easily detach by dipping the sample into water, while the CNTs remain attached to the sample.

Phosphorylation of Tau protein and binding to microtubules is complex in neurons and was therefore studied in the less complicated model of humanized yeast. Human Tau was readily phosphorylated at pathological epitopes, but in opposite directions regulated by kinases Mds1 and Pho85, orthologues of glycogen synthase kinase-3beta and cdk5, respectively (1). We isolated recombinant Tau-4R and mutant Tau-P301L from wild type, Delta mds1 and Delta pho85 yeast strains and measured binding to Taxol-stabilized mammalian microtubules in relation to their phosphorylation patterns. Tau-4R isolated from yeast lacking mds1 was less phosphorylated and bound more to microtubules than Tau-4R isolated from wild type yeast. Paradoxically, phosphorylation of Tau-4R isolated from kinase Pho85-deficient yeast was dramatically increased resulting in very poor binding to microtubules. Dephosphorylation promoted binding to microtubules to uniform high levels, excluding other modifications. Isolated hyperphosphorylated, conformationally altered Tau-4R completely failed to bind microtubules. In parallel to Tau-4R, we expressed, isolated, and analyzed mutant Tau-P301L. Total dephosphorylated Tau-4R and Tau-P301L bound to microtubules very similarly. Surprisingly, Tau-P301L isolated from all yeast strains bound to microtubules more extensively than Tau-4R. Atomic force microscopy demonstrated, however, that the high apparent binding of Tau-P301L was due to aggregation on the microtubules, causing their deformation and bundling. Our data explain the pathological presence of granular Tau aggregates in neuronal processes in tauopathies.

Tapping mode atomic force microscopy (TM-AFM) was used to characterize individual gold clusters deposited on a 1,4-benzenedimethanethiol layer. Au n ( n =1–500) clusters were produced with a laser vaporization source and deposited with low kinetic energy (0.4 eV/atom). The free thiol group of the 1,4-benzenedimethanethiol layer gives rise to a large attractive force that inhibits reliable AFM probing of the sample with standard silicon tips. After etching the tip with hydrogen fluoride this interaction becomes repulsive and the image quality drastically improves. Tip–surface interaction measurements combined with an analysis of the cluster sizes reveal a configuration where clusters grow on top of the dithiol layer. The clusters are slightly deformed after impact and due to a strong sulfur–gold binding the cluster mobility on the layer is negligible.

Microtubules isolated from pig brains have been immobilized on an inorganic substrate for use in AFM studies. The method employs 4-aminobutyldimethylmethoxysilane and glutaraldehyde to activate a silicon wafer for binding the biopolymer. The covalent bond ensures the positional stability of the tubules on the substrate, and allows reproducible scanning probe experiments. Microtubules have been imaged both by atomic force and scanning tunneling microscopy, yielding results very similar to electron microscopy. The average apparent height of the tubules is smaller than observed with transmission electron microscopy (25 nm) and is smaller in buffer solution (10 nm) than in air (15 nm). The biopolymer surface is softer under buffer than in air. The highest resolution was obtained with the tapping mode where surface features as small as 10 nm in X and Y have been resolved. Gold-coated tubules bound on silicon have been successfully imaged by STM, while images of uncertain origin were generated for tubules deposited on graphite. It is shown that artefacts imaged on a blank graphite surface can easily be confounded with collapsed tubules.

[*] Dr. M. Gysemans, Prof. J. Snauwaert, Prof. C. Van Haesendonck Laboratory of Solid-State Physics and Magnetism Katholieke Universiteit Leuven Celestijnenlaan 200 D, BE-3001 Leuven (Belgium) E-mail: Chris.Van.Haesendonck@fys.kuleuven.be F. Leroux, Prof. S. Bals, Prof. G. Van Tendeloo EMAT, University of Antwerp Groenenborgerlaan 171, BE-2020 Antwerp (Belgium) E-mail: Sara.Bals@ua.ac.be Dr. K. J. Batenburg Vision Lab, University of Antwerp Universiteitsplein 1, BE-2020 Wilrijk (Belgium)

We investigate the magnetization reversal in Fe/MgO(001) films with fourfold in-plane magnetic anisotropy and an additional uniaxial anisotropy whose orientation and strength are altered using different growth geometries and postgrowth treatments. The previously adopted mechanism of $180\ifmmode^\circ\else\textdegree\fi{}$ domain-wall nucleation clearly fails to explain the observed $180\ifmmode^\circ\else\textdegree\fi{}$ magnetization reversal in Fe/MgO(001) films. We introduce a reversal mechanism with two successive domain-wall nucleations to consistently predict the switching fields of Fe/MgO(001) films for all field orientations with one set of values for domain-wall nucleation energies and uniaxial anisotropy.

The deposition of n-octylphosphonic acid on aluminum oxide was studied. The substrate was pretreated in order to achieve a root-mean-square roughness of

We performed polarized neutron reflectometry measurements on a 78 nm-thick Au97Fe3 film prepared on a thermally oxidized Si wafer. The measurements were done on the C5 spectrometer at Chalk River in reflectometry mode. We determined the magnetization of the specimen in a vertical magnetic field of 6 T in a temperature range from 300 to 1.5 K. We observed a cusp in the magnetization at about 10 K, close to the spin-glass freezing temperature of 16 K of a bulk Au97Fe3 alloy. From the fit to the reflectivity data we can infer a magnetic moment of 0.63 μ B per Fe atom at the cusp position which is a large reduction compared to the bulk value of 1.2 μ B per Fe atom.

Protein tau-3R/4R isoform ratio and phosphorylation regulates binding to microtubules and, when disturbed by aging or mutations, results in diverse tauopathies and in neurodegeneration. The underlying mechanisms were studied here in three transgenic mouse strains with identical genetic background, all expressing the tau-4R/2N isoform driven specifically in neurons by the thy1 gene promoter. Two strains, expressing human tau-4R/2N or mutant tau-4R/2N-P301L at similar, moderate levels, developed very different phenotypes. Tau-4R/2N mice became motor-impaired already around age 6–8 weeks, accompanied by axonopathy (dilatations, spheroids), but no tau aggregates, and surviving normally. In contrast, tau-P301L mice developed neurofibrillary tangles from age 6 months, without axonal dilatations and, despite only minor motor problems, all succumbing before the age of 13 months. The third strain, obtained by tau knock-out/knock-in (tau-KOKI), expressed normal levels of wild-type human tau-4R/2N replacing all mouse tau isoforms. Tau-KOKI mice survived normally with minor motor problems late in life and without any obvious pathology. Biochemically, a fraction of neuronal tau in aging tau-P301L mice was hyperphosphorylated concomitant with conformational changes and aggregation, but overall, tau-4R/2N was actually more phosphorylated than tau-P301L. Significantly, tau with changed conformation and with hyperphosphorylation colocalized in the same neurons in aging tau-P301L mice. Taken together, we conclude that excessive binding of tau-4R/2N as opposed to reduced binding of tau-P301L to microtubules is responsible for the development of axonopathy and tauopathy, respectively, in tau-4R/2N and tau-P301L mice and that the conformational change of tau-P301L is a major determinant in triggering the tauopathy.

We are investigating inductive coupling optimization schemes and quantization effects for microscopic metal rings as a possible basis for a quantum bit (qbit). Faraday induction is proposed to provide electromagnetic coupling between the rings, therefore acting as an information carrier. Quantizing this information will produce distinguishable ring states that can be denoted by |0〉 and |1〉, representing the logic states of the qbit. We have set up simulation case studies with the aim of reducing signal loss between the rings. Further, different quantization mechanisms are investigated analytically. A combination of the two concepts can in theory be used to design qbits, consisting of metal rings with I/O facilities.

A promising method for the production of few-layer graphene (FLG) is microwave plasma-enhanced chemical vapour deposition (MW PECVD). However, the growth mechanism of PECVD-synthesized FLG is not completely understood. The aim of this work was to investigate the initial stages of the growth process of FLG deposited by MW PECVD on several substrates (quartz, silicon, platinum). The deposited thin films were characterized by angle-resolved x-ray photoelectron spectroscopy (ARXPS), electron backscattered diffraction (EBSD), scanning electron microscopy (SEM) and x-ray diffraction (XRD). It was found that the initial stages of the deposition were different for the three chosen substrate materials. However, the fully grown FLG layers were similar for all substrates.

Fe films with thickness varying between 5 and 100 nm were grown on flat MgO(001) substrates while rotating the substrates. Hysteresis loops with one step and two steps were observed and interpreted in terms of a magnetization reversal mechanism with either two successive or two separate 90° domain wall (DW) nucleations, respectively. This recently introduced, novel mechanism for 180° magnetic transitions was used to quantitatively evaluate both the uniaxial magnetic anisotropy (UMA), which accompanies the intrinsic fourfold in-plane magnetic anisotropy, and the DW nucleation energy. The strength of both the UMA and the DW nucleation energy turns out to be inversely proportional to the thickness of the Fe layers for Fe/MgO(001). This suggests that the extra UMA and the DW nucleation/propagation for Fe layers are pure interface related effects. By comparing six 15 nm thick Fe/MgO(001) films deposited under different conditions, it is found that these interface related effects originate from the presence of atomic steps due to the substrate miscut and from the presence of strain relaxation resulting from lattice mismatch.

The intrinsic domain wall resistance (DWR) of 180° Neel walls in a polycrystalline Fe film is determined by creating a periodic domain pattern, obtained by locally inducing exchange bias. After field cooling, the coercivity is spatially modulated, resulting in periodic 180° domain walls. To determine the intrinsic DWR, a rotating magnetic field is used to reversibly create and annihilate the domain walls. After correcting for the anisotropic magnetoresistance, the extracted DWR is positive.

We present the first measurements of both the morphology and electronic structure of preformed Co nanoclusters in the size range of a few up to several hundreds of atoms by means of low-temperature scanning tunneling microscopy (STM) and scanning tunneling spectroscopy (STS). Co clusters are produced in the gas phase and deposited under controlled ultra-high vacuum conditions onto clean Au(111) with low density, well below complete coverage of the Au(111) substrate. We find that smaller clusters, which typically contain less than 20 atoms, exhibit a significant surface mobility with subsequent aggregation, whereas larger clusters turn out to be immobile. From a systematic analysis of the cluster height distribution, we infer that the approximately spherical clusters experience only a restricted flattening and have a multilayered structure, which sometimes is observed to exhibit hexagonal facets, pointing to a truncated octahedral shape. Furthermore, detailed STS measurements on individual Co clusters reveal the presence of various size- and shape-dependent maxima with large energy spacings of the order of 100?meV, as well as an occupied state around ?200?meV that originates from the Co 3d band. Our findings provide direct evidence for the existence of strong electron confinement effects in the Co clusters stemming from delocalized Co valence electrons. Our results provide an original contribution to the understanding of cluster physics and in particular of the specific electronic structure.

The training effect in polycrystalline exchange bias Co/CoO bilayers has been investigated with low-temperature magnetic force microscopy (MFM). After field cooling the bilayer to 13 K, no MFM contrast was detected but a clear MFM contrast related to the domain structure appears after the first magnetization reversal of the ferromagnetic Co layer. Once formed, the ferromagnetic domains survive even at very large fields and cannot be erased by the application of a magnetic field along the cooling field direction. On the other hand, it is possible to partially remove the magnetic domains by performing a hysteresis loop with a magnetic field perpendicular to the cooling field direction.

We report on detailed low temperature scanning tunneling spectroscopy measurements performed on nanoscale Co islands on Au(111) films. At low coverages, Co islands self-organize in arrays of mono- and bilayer nanoscale structures that often have an hexagonal shape. The process of self-organization is induced by the Au(111) 'herringbone' reconstruction. By means of mapping of the local density of states with lock-in detection, electron standing wave patterns are resolved on top of the atomically flat Co islands. The surface state electrons are observed to be strongly confined laterally inside the Co nanosized islands, with their wavefunctions reflecting the symmetry of the islands. To complement the experimental work, particle-in-a-box calculations were performed. The calculations are based on a newly developed variational method that can be applied to '2D boxes' of arbitrary polygonal shape. The experimental patterns are found to fit nicely to the calculated wavefunctions for a box having a symmetry corresponding to the experimental island symmetry. The small size of the Co islands under study (down to 7.7 nm2) is observed to induce a strong discretization of the energy levels, with very large energy separations between the eigenstates up to several 100 meV. The observed standing wave patterns are identified either as individual eigenstates or as a 'mixture' of two or more energetically close-lying eigenstates of the cobalt island. Additionally, the Co surface state appears not to be limited to mono- and bilayer islands, but this state remains observable for multilayered islands up to five monolayers of Co.

A two-step catalyst annealing process is developed in order to control the diameter of nickel catalyst particles for the growth of carbon nanotubes (CNTs) by microwave plasma-enhanced chemical vapour deposition (MW PECVD). Thermal annealing of a continuous nickel film in a hydrogen (H2) environment in a first step is found to be insufficient for the formation of nanometre-size, high-density catalyst particles. In a second step, a H2 MW plasma treatment decreases the catalyst diameter by a factor of two and increases the particle density by a factor of five. An x-ray photoelectron spectroscopy study of the catalyst after each step in the annealing process is presented. It is found that the catalyst particles interact with the substrate during thermal annealing, thereby forming a silicate, even if a buffer layer in between the catalyst and the substrate is intended to prevent silicate formation. The silicate formation and reduction is shown to be directly related to the CNT growth mechanism, determining whether the catalyst particles reside at the base or the tip of the growing CNTs. The catalyst particles are used for the growth of a high-density CNT coating by MW PECVD. CNTs are analysed with electron microscopy and Raman spectroscopy.

Grazing-incidence Ar+ ion sputtering has been used to produce nanoscale ripples on the surface of the Fe∕Mg(001) system. This way, a uniaxial anisotropy with both controllable strength and orientation can be superimposed on top of the cubic anisotropy, resulting in Fe∕MgO(001) films with unusual anisotropy symmetry. By combining longitudinal and transverse Kerr-effect measurements, different switching processes are revealed. Depending on the orientation of the external magnetic field, one-jump, two-jump, and “reverse” two-jump magnetization reversals can be observed. A simple model, which takes into account the relevant anisotropy energies, is developed to explain the experimentally observed switching fields and to evaluate the domain wall pinning energies of the sputtered sample.