Theoretical and Experimental Adsorption Studies of Polyelectrolytes on an Oppositely Charged Surface

Using self-assembly techniques, x-ray reflectivity measurements, and computer simulations, we study the effective interaction between charged polymer rods and surfaces. Long-time Brownian dynamics simulations are used to measure the effective adhesion force acting on the rods in a model consisting of a planar array of uniformly positively charged, stiff rods and a negatively charged planar substrate in the presence of explicit monovalent counterions and added monovalent salt ions in a continuous, isotropic dielectric medium. This electrostatic model predicts an attractive polymer-surface adhesion force that is weakly dependent on the bulk salt concentration and that shows fair agreement with a Debye-Huckel approximation for the macroion interaction at salt concentrations near 0.1 M. Complementary x-ray reflectivity experiments on poly(diallyldimethyl ammonium) chloride (PDDA) monolayer films on the native oxide of silicon show that monolayer structure, electron density, and surface roughness are likewise independent of the bulk ionic strength of the solution.Comment: Revtex, prb format; uses amssym

Fiber optic long period grating sensors with a nanoassembled mesoporous film of SiO2 nanoparticles

A novel approach to chemical application of long period grating (LPG) optical fibers was demonstrated, which were modified with a film nanoassembled by the alternate deposition of SiO2 nanoparticles (SiO2 NPs) and poly(diallyldimethyl ammonium chloride) (PDDA). Nanopores of the sensor film could be used for sensitive adsorption of chemical species in water, which induced the changes in the refractive index (RI) of the light propagating in the cladding mode of the optical fiber, with a concomitant effect on the transmission spectrum in the LPG region. The prepared fiber sensor was highly sensitive to the change in the RI of the surrounding medium and the response time was very fast within 10 s. In addition, chemical infusion into the film was tested using a porphyrin compound, tetrakis-(4-sulfophenyl)porphine (TSPP), which could be saturated within a few min. The lowest detectable concentration of the TSPP analyte was 10 mu M. The TSPP infusion led to the development of well-pronounced dual resonance bands, indicating a large increase in the optical thickness of the film. The RI of the film was dramatically increased from 1.200 to ca. 1.540

Ultrafast fluorescent decay induced by metal-mediated dipole-dipole interaction in two-dimensional molecular aggregates

Two-dimensional molecular aggregate (2DMA), a thin sheet of strongly interacting dipole molecules self-assembled at close distance on an ordered lattice, is a fascinating fluorescent material. It is distinctively different from the single or colloidal dye molecules or quantum dots in most previous research. In this paper, we verify for the first time that when a 2DMA is placed at a nanometric distance from a metallic substrate, the strong and coherent interaction between the dipoles inside the 2DMA dominates its fluorescent decay at picosecond timescale. Our streak-camera lifetime measurement and interacting lattice-dipole calculation reveal that the metal-mediated dipole-dipole interaction shortens the fluorescent lifetime to about one half and increases the energy dissipation rate by ten times than expected from the noninteracting single-dipole picture. Our finding can enrich our understanding of nanoscale energy transfer in molecular excitonic systems and may designate a new direction for developing fast and efficient optoelectronic devices.Comment: 9 pages, 6 figure

Thermosensitive Cu2O-PNIPAM core-shell nanoreactors with tunable photocatalytic activity

We report a facile and novel method for the fabrication of Cu2O@PNIPAM core-shell nanoreactors using Cu2O nanocubes as the core. The PNIPAM shell not only effectively protects the Cu2O nanocubes from oxidation, but also improves the colloidal stability of the system. The Cu2O@PNIPAM core-shell microgels can work efficiently as photocatalyst for the decomposition of methyl orange under visible light. A significant enhancement in the catalytic activity has been observed for the core-shell microgels compared with the pure Cu2O nanocubes. Most importantly, the photocatalytic activity of the Cu2O nanocubes can be further tuned by the thermosensitive PNIPAM shell, as rationalized by our recent theory.Comment: 8 pages, 6 figures (Supporting Information included: 11 pages, 10 figures

Fabrication and characterization of red-emitting electroluminescent devices based on thiol-stabilized semiconductor nanocrystals

Thiol-capped CdTe nanocrystals were used to fabricate light-emitting diodes, consisting of an emissive nanocrystal multilayer deposited via layer-by-layer, sandwiched between indium-tin-oxide and aluminum electrodes. The emissive and electrical properties of devices with different numbers of nanocrystal layers were studied. The improved structural homogeneity of the nanocrystal multilayer allowed for stable and repeatable current- and electroluminescence-voltage characteristics. These indicate that both current and electroluminescence are electric-field dependent. Devices were operated under ambient conditions and a clear red-light was detected. The best-performing device shows a peak external efficiency of 0.51% and was measured at 0.35mA/cm2 and 3.3V

Cathodic and Anodic Material Diffusion in Polymer/Semiconductor-Nanocrystal Composite Devices

In the present day, the information technologies and telecommunications sector continually increase their demand for low cost, low power consumption, high performance electroluminescent devices for display applications. Furthermore, general lighting applications, such as white light and large array colour displays, would also benefit from an increase in the overall efficiency. Several technologies are being investigated to fulfill these needs, such as organic light emitting diodes (OLED), polymeric light emitting diodes (PLED) and field effect emission devices. A new and promising technology is light emitting devices (LEDs) based on nanostructured materials. With organic LEDs (OLEDs) already making an impact on the market in an increasingly large number of applications, hybrid technologies based on organic/inorganic nano-composites are a potential the next step. The incorporation of highefficiency fluorescent semiconductor nanoparticles has been shown to have a beneficial effect on device performance, [1] modify the colour output from the device 2 and provide a simplified route to generation of LED type devices. [3

Whiter, brighter, and more stable cellulose paper coated with antibacterial carboxymethyl starch stabilized ZnO nanoparticles

Small, carboxymethyl-starch-stabilised zinc oxide nanoparticles with a defined shape, size and morphology were prepared in situ in water at relatively low reaction temperatures using soluble carboxymethyl starch (CMS) as a combined crystallising, stabilising and solubilising agent and triethanolamine as the reducing agent. Aqueous colloidal solutions of these CMS-stabilised ZnO nanoparticles were used to deposit a coating of ZnO nanoparticles on cellulose paper by a wet-chemistry, polyelectrolyte, layer-by-layer approach using water as the only solvent. Such cellulose paper samples, coated with these CMS-stabilised ZnO nanoparticles, show higher brightness and whiteness than that of blank reference paper and are more stable to UV-radiation than the paper reference as well as demonstrating good antibacterial activity against MRSA and A. baumannii

Thermal X-ray emission from shocked ejecta in Type Ia Supernova Remnants. Prospects for explosion mechanism identification

The explosion mechanism behind Type Ia supernovae is a matter of continuing debate. The diverse attempts to identify or at least constrain the physical processes involved in the explosion have been only partially successful so far. In this paper we propose to use the thermal X-ray emission from young supernova remnants originated in Type Ia events to extract relevant information concerning the explosions themselves. We have produced a grid of thermonuclear supernova models representative of the paradigms currently under debate: pure deflagrations, delayed detonations, pulsating delayed detonations and sub-Chandrasekhar explosions, using their density and chemical composition profiles to simulate the interaction with the surrounding ambient medium and the ensuing plasma heating, non-equilibrium ionization and thermal X-ray emission of the ejecta. Key observational parameters such as electron temperatures, emission measures and ionization time scales are presented and discussed. We find that not only is it possible to identify the explosion mechanism from the spectra of young Type Ia Supernova Remnants, it is in fact necessary to take the detailed ejecta structure into account if such spectra are to be modeled in a self-consistent way. Neither element line flux ratios nor element emission measures are good estimates of the true ratios of ejected masses, with differences of as much as two or three orders of magnitude for a given model. Comparison with observations of the Tycho SNR suggests a delayed detonation as the most probable explosion mechanism. Line strengths, line ratios, and the centroid of the Fe Kalpha line are reasonably well reproduced by a model of this kind.Comment: 11 pages, 8 figures (5 of them color), accepted for publication by the Ap