Synthesis and characterization of silver—poly(methylmethacrylate) nanocomposites

The optical properties of silver nanoparticles embedded in poly(methylmethacrylate) (PMMA) was investigated as well as the influence of silver nanoparticles on the thermal properties of polymer matrix. The average size and particle size distribution of silver nanoparticles was determined using transmission electron microscopy. The obtained transparent nanocomposite films were optically characterized using UV-Vis and FTIR spectroscopy. Thermal stability of polymer matrix was improved upon incorporation of small amount of silver nanoparticles. Also, silver nanoparticles have pronounced effect on thermo-oxidative stability of PMMA matrix. The glass transition temperatures of nanocomposites are lower compared to the pure polymer.     

Photoluminescence of anatase and rutile TiO2 particles

Nonaqueous reactions between titanium(IV) chloride and alcohols (benzyl alcohol or n-butanol) were used for the synthesis of anatase TiO2 particles, while rutile TiO2 particles were synthesized in aqueous media by acidic hydrolysis of titanium(IV) chloride. The X-ray diffraction measurements proved the exclusive presence of either the anatase or the rutile phase in prepared samples. The photoluminescence of both kinds of particles (anatase and rutile) with several well-resolved peaks extending in the visible spectral region was observed, and the quantum yield at room temperature was found to be 0.25%. Photon energy up-conversion from colloidal anatase and rutile TiO2 particles was observed at low excitation intensities. The energy of up-converted photoluminescence spans the range of emission of normal photoluminescence. The explanation of photon energy up-conversion involves mid-gap energy levels originating from oxygen vacancies.     

Self-assembly of linear arrays of semiconductor nanoparticles on carbon single-walled nanotubes

Ligand-stabilized nanocrystals (NCs) were strongly bound to the nanotube surfaces by simple van der Waals forces. Linear arrays of CdSe and InP quantum dots were formed by self-assembly using the grooves in bundles of carbon single-walled nanotubes (SWNTs) as a one-dimensional template. A simple geometrical model explains the ordering in terms of the anisotropic properties of the nanotube surface. CdSe quantum rods were also observed to self-organize onto SWNTs with their long axis parallel to the nanotube axis. This approach offers a route to the formation of ordered NC/SWNT architectures that avoids problems associated with surface derivatization.     

Growth of InP nanostructures via reaction of indium droplets with phosphide ions: synthesis of InP quantum rods and InP-TiO2 composites

InP quantum rods were synthesized via the reaction of monodispersed colloidal indium droplets with phosphide ions. In(0) droplets, which do not act as a catalyst but rather a reactant, are completely consumed. The excess electrons that are produced in this reaction are most likely transferred to an oxide layer at the indium surface. For the synthesis of InP quantum rods with a narrow size distribution, a narrow size distribution of In(0) particles is also required because each indium droplet serves as a template to strictly limit the lateral growth of individual InP nanocrystals. Free-standing quantum rods, 60, 120, or 150 A in diameter, with aspect ratios of 1.6-3.5, and without the residual metallic catalyst at the rod tip, were synthesized from the diluted transparent solution of metallic indium particles. The same approach was used to synthesize InAs quantum rods. A photoactive InP-TiO(2) composite was also prepared by the same chemical procedure; InP nanocrystals grow as well-defined spherical or slightly elongated shapes on the TiO(2) surface.     

Spin-glass dynamics in interacting nanoparticle system La<Subscript>0.7</Subscript>Ca<Subscript>0.3</Subscript>MnO<Subscript>3</Subscript> obtained by mechanochemical milling

Spin dynamics in mechanochemically obtained nanoparticle manganite La_0.70Ca_0.30MnO₃ was investigated in this study by means of a series of AC and DC magnetic measurements. AC susceptibility indicates the presence of sizeable interparticle interactions, yielding collective magnetic behavior. The related properties were probed by experiments in weak DC field: memory effects were analyzed in both field-cooled (FC) and zero field-cooled (ZFC) regimes, while, after ZFC aging, magnetic relaxation was recorded. The system appears to be sensitive to magnetothermal history, in analogy with spin-glass-like compounds. The analysis of the data indicates the occurrence of slow dynamics in an ensemble of strongly interacting super spins.     

Charge separation in heterostructures of InP nanocrystals with metal particles

The optical and electron paramagnetic resonance (EPR) properties of InP nanocrystals, in which metallic gold or indium is present as an incorporated part of the nanocrystals, have been studied. A study of Au/InP quantum rods supports different carrier localization regimes compared to metal-free quantum rods, including the charge-separated state for which the electron and hole are located in different parts of the heterostructure. They also show that elongated semiconductors that grow on metallic catalysts have electronic properties that are different from those of pure semiconductor nanocrystals of the same shape. We have also developed a simple method for growing melted indium particles on the surface of colloidal spherical InP nanocrystals, and in these In/InP nanocrystals the emission is completely quenched while the absorption spectrum moves to red due to the strong mixing of the semiconductor and metal electronic states.     

Ultrafast photoinduced charge separation resulting from self-assembly of a green perylene-based dye into pi-stacked arrays

Condensation of 3,4,5-tris(n-dodecyloxy)aniline with the green chromophore 1,7-bis(N-pyrrolidinyl)perylene-3,4;9,10-tetracarboxylic dianhydride yields N,N'-bis(3,4,5-tris(n-dodecyloxy)phenyl)-1,7-bis(N-pyrrolidinyl)perylene-3,4;9,10-bis(dicarboximide), 5PDI-TAP, which absorbs light strongly from 550 to 750 nm. 5PDI-TAP dissolves readily in methylcyclohexane (MCH), resulting in self-assembly into H-aggregates. Small-angle X-ray scattering data obtained on 10(-4) M solutions of 5PDI-TAP in MCH show that the aggregates are pi-stacked monodisperse pentamers. Femtosecond transient absorption spectroscopy on solutions of (5PDI-TAP)5 in MCH shows evidence of charge separation occurring with tau < or = 150 fs between adjacent stacked members of 5PDI-TAP within the pentamer followed by charge recombination with tau = 860 ps. Transmission electron microscopy of 5PDI-TAP films cast from solution show isolated bundles of columnar aggregates. (5PDI-TAP)n is a potentially useful material for organic photovoltaics because efficient photoinduced charge generation is an intrinsic property of the assembly.     

Kinetics of radical polymerization of dimethylphenyl methacrylates

The kinetics of free radical polymerization of all six existing isomeric dimethylphenyl methacrylates were studied in bulk at 60°C, initiated by dilauroyl peroxide, using dilatometry. The different polymerization rates observed, resulting from the different monomer structures, are discussed in terms of steric and other effects, considering also the previous results for tolyl methacrylates. © 1993 John Wiley & Sons, Inc.     

Random algorithms for convex minimization problems

This paper deals with iterative gradient and subgradient methods with random feasibility steps for solving constrained convex minimization problems, where the constraint set is specified as the intersection of possibly infinitely many constraint sets. Each constraint set is assumed to be given as a level set of a convex but not necessarily differentiable function. The proposed algorithms are applicable to the situation where the whole constraint set of the problem is not known in advance, but it is rather learned in time through observations. Also, the algorithms are of interest for constrained optimization problems where the constraints are known but the number of constraints is either large or not finite. We analyze the proposed algorithm for the case when the objective function is differentiable with Lipschitz gradients and the case when the objective function is not necessarily differentiable. The behavior of the algorithm is investigated both for diminishing and non-diminishing stepsize values. The almost sure convergence to an optimal solution is established for diminishing stepsize. For non-diminishing stepsize, the error bounds are established for the expected distances of the weighted averages of the iterates from the constraint set, as well as for the expected sub-optimality of the function values along the weighted averages.     

Multifunctional PES fabrics modified with colloidal Ag and TiO2 nanoparticles

This study is aimed to highlight the possibility of engineering the multifunctional textile nanocomposite material based on the polyester (PES) fabric modified with colloidal Ag and TiO₂ nanoparticles (NPs). The effects of concentration of NPs as well as the order of Ag and TiO₂ NPs loading on antimicrobial, UV protective, and photocatalytic properties of PES fabrics were examined. The antimicrobial activity of differently modified PES fabrics was tested against Gram‐negative bacterium Escherichia coli, Gram‐positive bacterium Staphylococcus aureus, and fungus Candida albicans. The concentration of Ag colloid and the order of Ag and TiO₂ NPs loading considerably affected the antimicrobial efficiency of PES fabrics. The fabrics provided maximum UV protection upon surface modification with Ag and TiO₂ NPs. Ag NPs enhanced Ag NPs enhanced the photodegradation activity of TiO₂ NPs and total photodegradation of methylene blue was achieved after 24 hr of UV illumination. Copyright © 2010 John Wiley & Sons, Ltd.