Covariant field theory for self-dual strings

We give a gauge and manifestly SO(2,2) covariant formulation of the field theory of the self-dual string. The string fields are gauge connections that turn the super-Virasoro generators into covariant derivatives, © 1997 Elsevier Science B.V.

Caracterização e aplicação da Blenda PEDOT : PSS/PVA na construção de eletrodos transparentes e dispositivos eletroluminescentes

Coordenação de Aperfeiçoamento de Pessoal de Nível Superior (CAPES)

Self-assembly in aqueous solution of amphiphilic graft copolymers from oxidized carboxymethylcellulose

Coordenação de Aperfeiçoamento de Pessoal de Nível Superior (CAPES)

Al2O3 obtained through resistive evaporation for use as insulating layer in transparent field effect transistor

Alumina thin films have been obtained by resistive evaporation of Al layer, followed by thermal oxidation achieved by annealing in appropriate atmosphere (air or O-2-rich), with variation of annealing time and temperature. Optical and structural properties of the investigated films reveal that the temperature of 550 degrees C is responsible for fair oxidation. Results of surface electrical resistivity, Raman and infrared spectroscopies are in good agreement with this finding. X-ray and Raman data also suggest the crystallization of Si nuclei at glass substrate-alumina interface, which would come from the soda-lime glass used as substrate. The main goal in this work is the deposition of alumina on top of SnO2 to build a transparent field-effect transistor. Some microscopy results of the assembled SnO2/Al2O3 heterostructure are also shown.

The effect of self-adhesive and self-etching resin cements on the bond strength of nonmetallic posts in different root thirds

The purpose of this study was to evaluate the effect of self-adhesive and self-etching resin cements on the bond strength of nonmetallic posts in different root regions. Sixty single-rooted human teeth were decoronated, endodontically treated, post-space prepared, and divided into six groups. Glass-fiber (GF) posts (Exacto, Angelus) and fiber-reinforced composite (FRC) posts (EverStick, StickTeck) were cemented with self-adhesive resin cement (Breeze) (SA) (Pentral Clinical) and self-etching resin cement (Panavia-F) (SE) (Kuraray). Six 1-mm-thick rods were obtained from the cervical (C), middle (M), and apical (A) regions of the roots. The specimens were then subjected to microtensile testing in a special machine (BISCO; Schaumburg, IL, USA) at a crosshead speed of 0.5 mm/min. Microtensile bond strength data were analyzed with two-way ANOVA and Tukey's tests. Means (and SD) of the MPa were: GF/SA/C: 14.32 (2.84), GF/SA/M: 10.69 (2.72), GF/SA/A: 6.77 (2.17), GF/SE/C: 11.56 (4.13), GF/SE/M: 6.49 (2.54), GF/SE/A: 3.60 (1.29), FRC/SA/C: 16.89 (2.66), FRC/SA/M: 13.18 (2.19), FRC/SA/A: 8.45 (1.77), FRC/SE/C: 13.69 (3.26), FRC/SE/M: 9.58 (2.23), FRC/SE/A: 5.62 (2.12). The difference among the regions was statistically significant for all groups (p < 0.05). The self-adhesive resin cement showed better results than the self-etching resin cement when compared to each post (p < 0.05). No statistically significant differences in bond strengths of the resin cements when comparable to each post (p > 0.05). The bond strength values were significantly affected by the resin cement and the highest values were found for self-adhesive resin cement.

Al2O3 Obtained through resistive evaporation for use as insulating layer in transparent field effect transistor

Alumina thin films have been obtained by resistive evaporation of Al layer, followed by thermal oxidation achieved by annealing in appropriate atmosphere (air or O2-rich), with variation of annealing time and temperature. Optical and structural properties of the investigated films reveal that the temperature of 550°C is responsible for fair oxidation. Results of surface electrical resistivity, Raman and infrared spectroscopies are in good agreement with this finding. X-ray and Raman data also suggest the crystallization of Si nuclei at glass substrate-alumina interface, which would come from the soda-lime glass used as substrate. The main goal in this work is the deposition of alumina on top of SnO2 to build a transparent field-effect transistor. Some microscopy results of the assembled SnO2/Al2O3 heterostructure are also shown.

Junction Field Effect on the Retention Time for One-Transistor Floating-Body RAM

One-transistor floating-body random access memory retention time distribution is investigated on silicon-on-insulator UTBOX devices. It is shown that the average retention time can be improved by two to three orders of magnitude by reducing the body-junction electric field. However, the retention time distribution, which is mainly caused by the generation-recombination center density variation, remains similar.

Self assembly of human septin 2 into amyloid filaments

Septins are a conserved group of GTP-binding proteins that form hetero-oligomeric complexes which assemble into filaments. These are essential for septin function, including their role in cytokinesis, cell division, exocytosis and membrane trafficking. Septin 2 (SEPT2) is a member of the septin family and has been associated with neurofibrillary tangles and other pathological features of senile plaques in Alzheimer's disease. An in silico analysis of the amino acid sequence of SEPT2 identified regions with a significant tendency to aggregate and/or form amyloid. These were all observed within the GTP-binding domain. This was consistent with the experimental identification of a structure rich in beta-sheet during temperature induced unfolding transitions observed for both the full length protein and the GTP-binding domain alone. This intermediate state is characterized by irreversible aggregation and has the ability to bind Thioflavin-T, suggesting its amyloid nature. Under electron microscopy, fibers extending for several micrometers in length could be visualized. The results shown in this study support the hypothesis that single septins, when present in excess or with unbalanced stoichiometries, may be unstable and assemble into amyloid-like structures. (C) 2011 Elsevier Masson SAS. All rights reserved.

Self-assembly of Tetraurea Calix[4]arenes

We have elaborated a multistep strategy to synthesize ABAB-type tetraureas. There are overall nine steps but they involve very simple chemistry. The sequence starts with a 1,3-dialkylation and this is the step in which a difference between distal phenolic units is introduced. The selective ipso-nitration in the next step is based on the difference in reactivity between free phenolic units and alkylated ones. The direct reaction of tetraamino calixarene with tolylisocyanate appears not to be an appropriate method to synthesize 1,3-ditolylurea calixarenes but can be used to get tetraureas of ABBB- and AABB-types in two steps with yields of about 60%. A complete regioselective dimerization was obtained with mono-loop derivatives in which two adjacent urea residues are covalently connected. As predicted/expected the loop prevents the formation of one regioisomer, and only the dimer in which the open-chain residue slips through the loop is formed. To synthesize mono-loop tetraureas 1,2-diBoc protected tetraamino calixarene was acylated with activated di-urethanes under high dilution conditions. Di-loop compounds were synthesized by two different ways. In the reaction of tetraamine and di-urethanes the yield is about 30-40%. The second method is based on the metathesis reaction within a suitable heterodimer. For this strategy, tetraurea derivatives with residues which have terminal double bonds were prepared. The exclusive formation of the heterodimer with tetratosylurea as template is the key point in this strategy. Metathesis followed by hydrogenation give exceptionally good yields (> 80%) of the loop compounds. All the NMR data for di-loop compounds confirm that the loops prevent the interaction of the urea residues which are connected and thus, as expected, the di-loop derivatives do not form homodimers. The heterodimer between di-loop compounds and tetratolylurea (open-chain tetraureas) was the only species observed for a 1:1 mixture in benzene or chloroform. The rational synthesis of bis-[2]catenanes was a consequence of the selective formation of one regioisomer of mono-loop derivatives and the exclusive formation of heterodimers by di-loop derivatives. The formation of interlocking-ring in the synthesis of bis-[2]catenanes is an additional evidence that one open-chain residue slips through the loop in mono- or di-loop derivatives. Exceptionally good yields in the synthesis of bis-[2]catenanes are due to the high preorganization in the dimer which undergoes the metathesis. This preorganization decreases the number of the wrong connections and favors the new connections to be formed. Although the procedure for working up the reaction mixture should be still improved, these results are promising. A C2-symmetrical bis-[2]catenane was successfully resolved by column chromatography using a chiral stationary phase. Thus it should be possible to separate a larger amount to obtain pure enantiomers for further studies.

Tetraurea derivatives of calix[4]arenes and oligocalix[4]arenes, synthesis and self-assembly = Tetraharnstoff-Derivate von Calix[4 ]arenen und Oligocalix[4]arenen, Synthese und Selbstorganisierung

Wide rim tetraurea calix[4]arenes form hydrogen bonded dimeric capsules in apolar solvents in the presence of a suitable guest, which must be included in the cavity. The monomeric and dimeric form are never observed simultaneously under usual conditions. In general the combination of two different alkyl or aryl tetraurea derivatives results in the mixture of two homodimers and a heterodimer, however, only the heterodimeric species is observed in the 1:1 mixture of aryl and tosyl ureas. The (hetero)dimerization of oligourea calix[4]arenes (units) was used to construct larger structures via self-assembly of multiple calixarenes (building blocks) containing two (or more) covalently connected units. Among these self-assembled structures linear or branched polymers, cyclic oligomers and well-organized dendrimers were envisaged. The synthesis of the building blocks requires the preparation of calix[4]arene units possessing one (or more) functional group at the narrow or wide rim. Finally the oligourea units were covalently connected either directly or via suitable spacers within appropriate building blocks using amide bonds. Self-assembly properties of such building blocks were investigated.

Fluorescent ionene-dye nanoparticles by electrostatic self-assembly

It is investigated that the association of linear cationic model polyelectrolytes with oppositely charged pyrenetetrasulfonate (PY) in aqueous solution. For this purpose water soluble ionenes were prepared via Menschutkin reaction from 1-4-diazabicyclo [2.2.2] octane and e.g. 1,6-dibromohexane and 1,4 dibromotransbuten. The complex formation between dye molecules PY and oppositely charged ionenes (PD4, PD6, PD4-2 and PD4coPD6) of different chemical structures in aqueous solution was studied by light scattering (LS), small angle neutron scattering (SANS), UV-Vis, fluorescence spectroscopy and atomic force microscopy (AFM). Spectrophotometric titration results revealed that PY molecules were bind to ionenes cooperative process due to π-π interaction. Cooperative binding constant KD was determined as 6.4 x 10^6 M^-1 (+ or - 10^5 M^-1). It was found that binding mode and geometry of PY is predominantly depending on inter-charge distances of corresponding ionenes. Resultant complexes have exhibited size and structure variation as a function of charge ratio (L), ionic strength, inter-charge distances. Spherical dye-ionene complexes of which radius of gyration ranging between (RG) 50 and 190 nm have been observed in PD4-PY system while this was not possible with a different ionene (PD6) or either case ionene excess. It was found that most of the PD4-PY complexes had RG / RH ~ 0.78. Based on the AFM and LS results, spherical complexes have certain colloidal stability and their size can effectively controlled by changing the L.

Atomic-scale characterization of diamond surfaces and fullerene self-assembly

In this thesis, elemental research towards the implantation of a diamond-based molecular quantum computer is presented. The approach followed requires linear alignment of endohedral fullerenes on the diamond C(100) surface in the vicinity of subsurface NV-centers. From this, four fundamental experimental challenges arise: 1) The well-controlled deposition of endohedral fullerenes on a diamond surface. 2) The creation of NV-centers in diamond close to the surface. 3) Preparation and characterization of atomically-flat diamondsurfaces. 4) Assembly of linear chains of endohedral fullerenes. First steps to overcome all these challenges were taken in the framework of this thesis. Therefore, a so-called “pulse injection” technique was implemented and tested in a UHV chamber that was custom-designed for this and further tasks. Pulse injection in principle allows for the deposition of molecules from solution onto a substrate and can therefore be used to deposit molecular species that are not stable to sublimation under UHV conditions, such as the endohedral fullerenes needed for a quantum register. Regarding the targeted creation of NV-centers, FIB experiments were carried out in cooperation with the group of Prof. Schmidt-Kaler (AG Quantum, Physics Department, Johannes Gutenberg-Universität Mainz). As an entry into this challenging task, argon cations were implanted into (111) surface-oriented CaF2 crystals. The resulting implantation spots on the surface were imaged and characterized using AFM. In this context, general relations between the impact of the ions on the surface and their valency or kinetic energy, respectively, could be established. The main part of this thesis, however, is constituted by NCAFM studies on both, bare and hydrogen-terminated diamond C(100) surfaces. In cooperation with the group of Prof. Dujardin (Molecular Nanoscience Group, ISMO, Université de Paris XI), clean and atomically-flat diamond surfaces were prepared by exposure of the substrate to a microwave hydrogen plasma. Subsequently, both surface modifications were imaged in high resolution with NC-AFM. In the process, both hydrogen atoms in the unit cell of the hydrogenated surface were resolved individually, which was not achieved in previous STM studies of this surface. The NC-AFM images also reveal, for the first time, atomic-resolution contrast on the clean, insulating diamond surface and provide real-space experimental evidence for a (2×1) surface reconstruction. With regard to the quantum computing concept, high-resolution NC-AFM imaging was also used to study the adsorption and self-assembly potential of two different kinds of fullerenes (C60 and C60F48) on aforementioned diamond surfaces. In case of the hydrogenated surface, particular attention was paid to the influence of charge transfer doping on the fullerene-substrate interaction and the morphology emerging from self-assembly. Finally, self-assembled C60 islands on the hydrogen-terminated diamond surface were subject to active manipulation by an NC-AFM tip. Two different kinds of tip-induced island growth modes have been induced and were presented. In conclusion, the results obtained provide fundamental informations mandatory for the realization of a molecular quantum computer. In the process it was shown that NC-AFM is, under proper circumstances, a very capable tool for imaging diamond surfaces with highest resolution, surpassing even what has been achieved with STM up to now. Particular attention was paid to the influence of transfer doping on the morphology of fullerenes on the hydrogenated diamond surface, revealing new possibilities for tailoring the self-assembly of molecules that have a high electron affinity.

Phosphonsäurehaltige polycyclische aromatische Kohlenwasserstoffe für Anwendungen in Brennstoffzellen

Die vorliegende Arbeit umfasst die Synthese und Charakterisierung phosphonsäurehaltiger, organischer Kristalle als ionenleitende Verbindungen in Brennstoffzellen-Anwendungen. Sie zielt dabei einerseits auf die Darstellung von protonenleitenden Polyphenylenverbindungen und deren Verwendung als Linker für den Aufbau protonenleitender Aluminium-Phosphonat-Netzwerke ab und behandelt andererseits die Einführung stark polarer Phosphonsäuregruppen in einen diskreten Nanographenkern sowie deren Einfluss auf die ionen- und elektronenleitenden Eigenschaften, um diese als gemischt-leitende Kompatibilisatoren an der isolierenden Elektrode/ Membran-Grenzfläche in einer Brennstoffzelle zu verwenden. Am Beispiel eines phosphonsäurefunktionalisierten, phenylenisch-expandierten Hexaphenylbenzols konnte ein solvothermisch stabiler Protonenleiter mit einer Selbstorganisation in kolumnare, supramolekulare Strukturen und hoher, temperaturunabhängiger Leitfähigkeit mit dominierendem Grotthuss-Anteil präsentiert werden. Durch einen Wechsel dieser 1D-radialen Phosphonsäureanordnung in der Molekülhülle hin zu 2D- und 3D-H2PO3-funktionalisierten, dendritischen Stäbchen- bzw. Kugelstrukturen konnte gezeigt werden, dass eine kolumnare Molekülanordnung jedoch kein notwendiges Kriterium für einen Grotthuss-artigen Protonentransport darstellt. Durch die mehrdimensionale Orientierung der Phosphonsäuren in der Außenhülle der Dendrimere garantieren die synthetisierten Strukturen hochaggregierte Phosphonsäurecluster, die als dichtes Säurekontinuum die eigentlichen protonenleitfähigen Kanäle darstellen und somit als entscheidendes Kriterium für das Auftreten eines Grotthuss-artigen Mechanismus definiert werden müssen. Eine signifikante Erhöhung der Leitfähigkeit konnte durch den Aufbau poröser, organisch-anorganischer Netzwerke (Al-HPB-NETs) über Komplexierung einer unterstöchiometrischen Menge an Aluminium-Kationen mit der Polyphosphonsäureverbindung Hexakis(p-phosphonatophenyl)benzol als Linkereinheit erfolgen, die anschließend mit kleinen intrinsischen Protonenleitern wie Phosphonsäure dotiert wurden. Diese dotierten Netzwerke wiesen außergewöhnliche Leitfähigkeit auf, da sie die σ-Werte des Referenzpolymers Nafion® bereits in einem Temperaturbereich oberhalb von 135°C übertrafen, aber gleichzeitig ein sehr gutes Säureretentionsverhalten von einem Gew.-% Säuredesorption über eine Immersionsdauer von 14 h gegenüber wässrigem Medium zeigten. Durch Mischen dieser Aluminiumphosphonate mit einer dotierten Polymermatrix wie PBI konnten synergistische Effekte durch zusätzliche attraktive H-Brückenbindungen zwischen molekular angebundener Phosphonsäure und mobiler H3PO4 an Hand eines signifikanten Leitfähigkeitsanstiegs für die resultierenden Membranen beobachtet werden. Die Protonenleitfähigkeit lag in diesen Materialien in dem gesamten untersuchten Temperaturbereich oberhalb von Nafion®. Durch das Einbringen der NETs in PBI konnte ebenfalls die Säureretention von PBI um etwa 9 % bei kurzen Immersionszeiten (bis 1 min) verbessert werden. Darüber hinaus wurde in der vorliegenden Arbeit die synthetische Kombination eines hydrophoben, elektronenleitenden Nanographenkerns mit einer, durch eine isolierende Peripherie getrennten, stark polaren, protonenleitenden Außenhülle realisiert. Am Beispiel von zwei phosphonsäurefunktionalisierten Triphenylenen, die sich in Länge und Planarität der gewählten Peripheriebausteine unterschieden, sollten polycyclische aromatische Kohlenwasserstoffe mit gemischt protonen- und elektronenleitenden Eigenschaften hergestellt werden, die über Impedanzspektroskopie und Vierpunktmessungen untersucht wurden. Da es sich bei der Anwendung solcher gemischtleitenden Verbindungen um grenz-flächenaktive Substanzen handelt, die das ohne verbesserte Anbindung bestehende Dielektrikum zwischen Elektrode und protonenleitender Membran überbrücken sollen, wurde die Untersuchung eines möglichen Elektronentransportes durch eine Molekülmonolage ebenfalls über kombinatorische STM- und STS-Technik durchgeführt.

Atomistic simulations of 2D bicomponent self-assembly: from molecular recognition to self-healing

Supramolecular two-dimensional engineering epitomizes the design of complex molecular architectures through recognition events in multicomponent self-assembly. Despite being the subject of in-depth experimental studies, such articulated phenomena have not been yet elucidated in time and space with atomic precision. Here we use atomistic molecular dynamics to simulate the recognition of complementary hydrogen-bonding modules forming 2D porous networks on graphite. We describe the transition path from the melt to the crystalline hexagonal phase and show that self-assembly proceeds through a series of intermediate states featuring a plethora of polygonal types. Finally, we design a novel bicomponent system possessing kinetically improved self-healing ability in silico, thus demonstrating that a priori engineering of 2D self-assembly is possible.

Self-assembly of focal point oligo-catechol ethylene glycol dendrons on titanium oxide surfaces: adsorption kinetics, surface characterization, and nonfouling properties

This work covers the synthesis of second-generation, ethylene glycol dendrons covalently linked to a surface anchor that contains two, three, or four catechol groups, the molecular assembly in aqueous buffer on titanium oxide surfaces, and the evaluation of the resistance of the monomolecular adlayers against nonspecific protein adsorption in contact with full blood serum. The results were compared to those of a linear poly(ethylene glycol) (PEG) analogue with the same molecular weight. The adsorption kinetics as well as resulting surface coverages were monitored by ex situ spectroscopic ellipsometry (VASE), in situ optical waveguide lightmode spectroscopy (OWLS), and quartz crystal microbalance with dissipation (QCM-D) investigations. The expected compositions of the macromolecular films were verified by X-ray photoelectron spectroscopy (XPS). The results of the adsorption study, performed in a high ionic strength ("cloud-point") buffer at room temperature, demonstrate that the adsorption kinetics increase with increasing number of catechol binding moieties and exceed the values found for the linear PEG analogue. This is attributed to the comparatively smaller and more confined molecular volume of the dendritic macromolecules in solution, the improved presentation of the catechol anchor, and/or their much lower cloud-point in the chosen buffer (close to room temperature). Interestingly, in terms of mechanistic aspects of "nonfouling" surface properties, the dendron films were found to be much stiffer and considerably less hydrated in comparison to the linear PEG brush surface, closer in their physicochemical properties to oligo(ethylene glycol) alkanethiol self-assembled monolayers than to conventional brush surfaces. Despite these differences, both types of polymer architectures at saturation coverage proved to be highly resistant toward protein adsorption. Although associated with higher synthesis costs, dendritic macromolecules are considered to be an attractive alternative to linear polymers for surface (bio)functionalization in view of their spontaneous formation of ultrathin, confluent, and nonfouling monolayers at room temperature and their outstanding ability to present functional ligands (coupled to the termini of the dendritic structure) at high surface densities.