Stochastic model for spontaneous formation of molecular wires

A stochastic model which may be used to analyze the formation of bonds connecting the interface formed by a substrate surface and a scanning tunneling microscope (STM) tip is presented. The model is tested by means of kinetic Monte Carlo simulations, and analytical predictions are given for some limiting cases. In the case of long time observations, the model may be applied to obtain information on the rate constants associated with the process and on the number of molecules trapped at the gap. In the case of short time observations, the results of the model for the probability of observing a given number of molecules bridging the gap are compared with the experimental results from the literature, showing a good predicting power.     

Multi-walled carbon nanotubes acting as free radical scavengers in gamma-irradiated ultrahigh molecular weight polyethylene composites

Multi-walled carbon nanotubes (MWCNTs) were incorporated in ultrahigh molecular weight polyethylene (UHMWPE), which is a polymer used in industrial and orthopedic applications. The composites were prepared by ball milling and thermo-compression processes at concentrations up to 3 wt.% and subsequently gamma irradiated at 90 kGy. Electrical conductivity measurements showed a low percolation threshold of 0.5 wt.%. Electron spin resonance detection of the radiation-induced radicals proved the radical scavenger behavior of MWCNTs: when the nanotube concentration increased, the number of radicals generated by the gamma irradiation process decreased. Allyl radicals seem to be the radicals most affected by the presence of nanotubes in this polymeric matrix. Fourier transformed infrared spectroscopy measurements and an accelerated ageing protocol were performed to ascertain the influence of the irradiation on the oxidation index. The results pointed to the positive contribution of the MWCNTs in increasing the oxidative stability of the composite compared to pure UHMWPE. Crosslinking density induced by gamma irradiation was obtained by swelling measurements. The findings showed that, despite the radical scavenger performance, MWCNTs are capable of maintaining the efficiency of the crosslinking density, unlike the other antioxidants, which inhibit radiation crosslinking.     

Acrylic microspheres as drug‐delivery systems: synthesis through in situ microemulsion photoinduced polymerization and characterization

Acrylic microspheres bearing a dense shell were successfully prepared by an in situ microemulsion photoinduced polymerization. Experiments following a fractional factorial design were performed for assessing the effect of the main reaction parameters (surfactant concentration in the aqueous phase, stirring rate, organic to water volume ratio, irradiation time) on the morphology of the microspheres and on their encapsulation yield. Environmental scanning electron microscopy and porosity measurements were carried out. Finally, the drug‐delivery features of the obtained products were investigated, evaluating the release of vanillin, chosen as a reference compound. The diffusion coefficients were found to be almost unvaried despite the increase in BET area. Such products show promising potentialities for home and personal care applications. © 2012 Society of Chemical Industry     

Organosilicon sulfides as co-initiators in photoinduced free radical polymerization

Summary Three sulfur-containing silanes (organosilicon sulfides) were investigated as electron donors in photoinduced free radical polymerization, in conjunction with xanthene dye (5,7-diiodo-3-butoxy-6-fluorone, DIBF) as sensitizer. The results were compared with the nonsulfur-containing silane, N-((trimethylsilyl)methyl)aniline (K1) and thiophenoxyacetic acid (TPAA). The kinetic studies clearly showed that the DIBF-organosilicon sulfides exhibit a significant increase in the efficiency of free radical polymerization of TMPTA compared to nonsulfur-containing silane and that the efficiency of all tested electron donors is only slightly dependent on the structure of the sulfur-containing co-initiators.     

Anomalous length-independent frontier resonant transmission peaks in armchair graphene nanoribbon molecular wires

Molecular wires constitute the building blocks for nanoscale interconnects. However, the exponential decrease in conductance with wire length severely limits their applications. We predict, using first principles calculations, that armchair graphene nanoribbon (AGNR) wires, connected by transverse zigzag edges to wider AGNR electrodes, can exhibit anomalous resonant transmission peaks that are nearly independent of the wire length. We propose a new model to explain the unusual length independence of peak energies from the locally repeating property of the wavefunction in the middle-AGNR. We further uncover that this locally repeating pattern originates from states of a perfect AGNR with infinite length. The pattern can be well preserved when the AGNR is connected to wider AGNR leads because of the zigzag edges serving as electron sources and drains. The length independence of peak widths results from the zigzag edges absorbing most of the wavefunction renormalization as the length increases, so that the coupling strength to the electrodes does not change significantly. These anomalous properties arising from intrinsic wavefunction properties of the AGNRs are in sharp contrast to typical transmission properties of traditional molecular wire junctions, which suggests promising potential application as “molecular wire” interconnects in nano-electronics.     

A graphene sheet as an efficient electron acceptor and conductor for photoinduced charge separation

Tetrasulfonated zinc phthalocyanine (Pc) was bound to graphene (G) sheets by the π–π stacking supermolecular method. The few-layer graphene sheets were obtained by chemically reducing graphite oxide and characterized by AFM, IR and UV–vis absorption methods. Photoinduced electron transfer (PET) within the nano assembly is revealed by laser flash photolysis, time resolved and steady state fluorescence, as well as UV–vis absorption techniques. A graphene sheet can be attached by up to 52,000 Pc molecules to form a super molecule G(Pc)_52,000, in which many Pc molecules can be simultaneously photoexcited to the S₁ state. One graphene sheet can simultaneously quench thousands of excited Pcs with a large rate constant of the order of 10¹⁶ M⁻¹ s⁻¹ by PET. A graphene sheet not only accepts electrons from the excited Pcs on it but also delivers the captured electrons to its unexcited Pcs to form (Pc⁺)_nG(Pc⁻)_n, so that a large electron charge (i.e. n >> 1) is separated between unlinked Pc molecules with a small energy loss. These novel features of PET are explained by the following unique properties of graphene: (i) its excellent electron-transport and multi-electron-accepting ability, (ii) its multi-chromophore binding and concurrent multi-photon absorbing ability.     

Stable molecular orientations of a C60 dimer in a photoinduced dimer row

We present a theoretical study on stable orientations of photoinduced C₆₀ dimers in a C₆₀ monolayer film, on the basis of first-principles calculations within the framework of the density functional theory. Using Tersoff and Hamann’s tunnel current theory, current images of scanning tunneling microscopy (STM) for a C₆₀ dimer in several different orientations are reproduced from the spatial distribution of the wave functions of an isolated C₆₀ dimer. Comparing the resultant current images with experimentally obtained STM images, in consequence, two stable orientations of C₆₀ dimers are determined: one is that a C-C double bond shared by two adjacent hexagons faces to vacuum, and the other is that a C-C single bond shared by a hexagon and an adjacent pentagon faces to vacuum. These orientations are supported by empirical total-energy calculations.     

Phospholipid bilayers as vehicles of anthraquinone disperse dyes in wool dyeing

The use of multilamellar lipid vesicles as carriers of disperse dyes on wool fibres has been studied. Liposomes made from egg phosphatidylcholine containing the anthraquinone dye CI Disperse Violet 1 at different phospholipid/dye concentrations were used. The physical stability of these systems was assessed by measuring the mean vesicle size distribution of lipid vesicle suspensions after preparation and during dyeing. Kinetic aspects involving dye adsorption and bonding on untreated and chlorinated wool samples by means of the liposomes at different dye/lipid ratios were also investigated. This process led to the controlled dye exhaustion on wool, directly dependent on the relationship between the dye and lipid components, with a clear improvement in the dye-fibre bonding forces and in the dispersing efficiency, compared with conventional dispersing agents. The optimum dye exhaustion was reached for the dye/phospholipid concentrations 0.53 and 1.0 mmol/l respectively. The maximum amounts of dye bonded on untreated wool fibre were obtained for the same dye/lipid molar ratio. However, chlorinated wool samples showed a slight decrease in the total bonded dye as the chlorination level increased.     

Synthesis of deoxy(thiosulfato)chitin as the precursor for noncatalytic photoinduced graft copolymerization

Photoinduced graft copolymerization of vinyl monomers onto deoxy(thiosulfato)chitin (S₂O₃–chitin) has been studied. Chitin was first tosylated and subsequently transformed into S₂O₃–chitin. S₂O₃–chitin has good solubility over tosyl–chitin. Graft copolymerization of S₂O₃–chitin proceeded very easily by ultraviolet irradiation without catalyst. Photolysis of S₂O₃ groups was confirmed by infrared spectra. But the photolysis occurred only in quartz, not in a Pyrex tube. Methyl methacrylate (MMA) and acrylonitrile showed good grafting activities. In the case of acrylic acid and acryl amide, homopolymer formation was predominant, and the degree of grafting was low. We chose the MMA monomer for further information. The grafting rate of S₂O₃–chitin using MMA was much faster than those of chitin and O‐acetyl–chitin. Under the appropriate conditions, the degree of grafting reached 600% only in 2 h, and the grafting efficiency was over 75% in any monomer concentration. But addition of DMSO into the polymerization system decreased the degree of grafting. © 1999 John Wiley & Sons, Inc. J Appl Polym Sci 71: 189–195, 1999     

Unusual polymethyl alkenes in tsetse flies acting as abstinon inGlossina morsitans

The major alkene of the male tsetse fly,Glossina morsitans morsitans, was isolated for characterization by thin-layer and gas chromatography (GC). The mass spectra of the alkene and the alkene DMDS derivative indicated one isomer, 19,23-dimethyltritriacont-1-ene. The material is present at 1–2g/male fly and is partially transferred to the female preparatory to or during mating. A dose-dependent antiaphrodisiac effect was seen with exposed male flies using the isolated natural product, with 2 and 4g causing 80% loss of copulatory attempts, and 10g extinguishing the attempts. This effect was increased by addition of male-produced alkane. This compound and a 31-carbon homolog also appear inG. m. submorsitans. Similar quantities of alkenes that are species-specific appear in all tsetse males. Structures of male-produced trimethylalkenes that appear in two other species,G. palpalis palpalis andG. fuscipes fuscipes, were investigated.     

Visible laser lights in photoinduced polymerization. VI. Thioxanthones and ketocoumarins as photoinitiators

The efficiency of thioxanthones and ketocoumarins as photoinitiators has been checked in visible laser light-induced polymerization reactions and discussed in terms of excited-state reactivity (as revealed by time-resolved laser spectroscopy). These compounds undergo fast electron transfer reaction in the presence of amines and onium salts. Transient absorption spectra and rate constants of the processes involved have been determined. The combination photoinitiator-amine-onium salt appears as very promising for the design of efficient photosensitive systems.     

Model calculations of electric forces acting on carbon nanotube tip in DC-plasma sheath

Numerous ·experimental investigations indicate the necessity of negative bias for low-pressure CVD of aligned carbon nanotubes. Based on the experimentally determined electron density in the dual hot filament/DC plasma deposition system the electrical field close to the substrate was calculated. Taking into account the field enhancement in the vicinity of the CNTs the force acting on their tips is determined. The calculated force has been found to exceed the weight of the droplet by four orders of magnitude. The computations have shown only a weak dependence between the electrical forces and the droplet shape.     

Adsorption of disulfide-modified polyacrylamides to gold and silver surfaces as cushions for polymer-supported lipid bilayers

Inclusion of a polymer cushion between a lipid bilayer membrane and solid surface has been suggested as a means to provide a soft, deformable layer that will allow for transmembrane protein insertion and mobility. In this study, we evaluate the properties of a disulfide- and lipid-modified polyacrylamide polymer cushion. High molecular weight random copolymers with various degrees of disulfide and lipid substitution were synthesized. X-ray photoelectron spectroscopy (XPS) was used to determine quantitatively the percentage of disulfide groups bound to gold and silver surfaces. A quartz crystal microbalance with dissipation (QCM-D) was used to study the adsorption process and resulting film properties in situ. The presence of backbone-surface interactions leads to a competition between physisorption of the acrylamide backbone and chemisorption of the disulfide side-chains. This competition limits the degree of chemisorption to gold and silver surfaces. For a polyacrylamide with 10 mol% disulfide side-chains, 78% of the side-chains covalently bind to silver and only 41% bind to gold. The undesired physisorption of the acrylamide backbone leads to adsorption of the homopolymer itself. In addition, film thicknesses, as indicated by both XPS and QCM-D, are limited to 15-30 Å. The QCM-D results for all films indicate the formation of relatively rigid films, rather than the soft, deformable films desired for a lipid membrane polymer cushion.     

A new processing aid for dry-pressing: A copolymer acting as dispersant and binder

PVX copolymers containing both carboxylate (COO⁻) and hydroxyl groups (vinyl alcohol) are synthesized with the objective to ensure the double function of dispersant and of binder for dry-pressed green parts.Carboxylate groups are responsible for strong adsorption of copolymers onto alumina surface and can promote sufficient electrostatic repulsive forces to achieve a good state of dispersion, similar to that obtained with a classical ammonium polymethacrylate (PMA-NH₄⁺) for a proportion of carboxylic groups in the copolymer larger than 35%. On the other hand, hydroxyl groups confer higher mechanical strength than PVA to the green pressed parts (4.2 MPa for 1.5 wt.% PV35 (35% COO⁻) versus 1.8 MPa for 1.5 wt.% PVA, with and without PEG as plasticizer, in the case of samples pressed at 120 MPa).The high density of the green parts obtained with soft PV35 copolymer and the good adsorption of this compound onto alumina surface lead to a high mechanical strength of the pressed compacts. After addition of 1.5 wt.% PV35, the amount adsorbed (0.7 wt.%) confers a high cohesion to the spray-dried granules and the non-adsorbed copolymer (0.8 wt.%) reinforces adhesion between granules through interdiffusion of the low T_g polymeric films.Then, synthesized PVX copolymers have proved to be efficient dispersants for aqueous suspensions of alumina and also efficient binders for dry-pressing, even for concentrations as low as 1.5 wt.%.     

Photochromic molecules as building blocks for molecular electronics

Energy and electron transfer processes can be easily induced by a photonic excitation of a donor metal complex ([Ru(bpy)3]2), which is connected via a wire-type molecular fragment to an acceptor metal complex ([Os(bpy)3]2+). The rate constant for the transfer process can be determined by emission measurements of the two connected metal complexes. The system can be modified by incorporation of a switching unit or an interrupter into the wire, influencing the transfer process. Such a molecular device corresponds to an interrupter, mimic the same function applied in molecular electronics. We have used organic switches, which show photochromic properties. By irradiation with light of different wavelengths, the switch changes its functionality by a photochemical reaction from an OFF- to an ON-state and vice versa. The ON- respectively OFF-state is manifested by a color change but also in different conductivity properties for energy and electron transfer processes. Therefore, the mentioned molecular device can work as a simple interrupter, controlling the rate of the transfer processes.     

Diamond-coated silicon wires for supercapacitor applications in ionic liquids

For a long time sp² carbon has been the dominating material for supercapacitor applications. In this paper a new concept of using boron-doped diamond for supercapacitors is proposed. Diamond surface enlargement is realized via bottom-up template-growth. In this method, silicon nanowire electrodes are coated with a thin (~ 100 nm) layer of nanocrystalline diamond (NCD) by microwave enhanced chemical vapor deposition (MWCVD). The quality of overgrowth is characterized by high resolution scanning electron microscopy which reveals a homogeneous coverage of diamond on Si nanowire surface. To enhance the potential window to 4 V, a room temperature ionic liquid is used as electrolyte. The dilution of the ionic liquid is investigated in terms of conductivity and specific capacitance. The capacitance as measured via cyclic voltammetry reaches 105 μF/cm². An energy density of 84 μJ/cm² and a high power density of 0.94 mW/cm² are obtained in combination with good stability of over 10,000 charging/discharging cycles.