Haile Takele

Plasmonic properties of Ag nanoclusters in various polymer matrices

Nanocomposite films containing Ag nanoparticles embedded in a polymer matrix of Teflon AF, poly(methyl methacrylate) (PMMA) and Nylon 6 were prepared by vapour phase co-deposition in high vacuum. A large variation of the particle plasmon resonance frequency in the visible region was obtained by increasing the Ag volume fraction from 4-80%. The metal volume fraction was measured by energy dispersive x-ray spectrometry (EDX) and the film thickness was measured by surface profilometry. The position, width and strength of the plasmon resonance depend strongly on the metal filling factor, cluster size and interparticle distance. The microstructure of the nanocomposites (shape, size, size distribution and interparticle separation of metal clusters) was determined by transmission electron microscopy. The effect of the surrounding dielectric medium on the optical properties of nanocomposites was investigated by comparing the Teflon AF/Ag, PMMA/Ag and Nylon/Ag composites.

Controlled syntheses of Ag?polytetrafluoroethylene nanocomposite thin films by co-sputtering from two magnetron sources

Co-sputtering from two independent magnetron sources was used to prepare polymer?metal nanocomposite films. Both gradient films with increasing metal fraction and homogeneous composite films were produced from polytetrafluoroethylene?(PTFE) and silver targets using a rotatable sample holder. The structure of the pure sputtered polymer as well as the composite structure was studied. Electrical properties of the composite material near the percolation threshold show the expected, sharp change in the resistivity from 107???cm atsmall silver content to 10?3???cm after percolation. The optical absorption in the visible region due to surface plasmon resonances also has a strong dependence on the metal content, showing a red shift of the absorption peak from 405?nm to more than 500?nm at higher silver content.

Nanostructured magnetic Fe–Ni–Co/Teflon multilayers for high-frequency applications in the gigahertz range

Thin multilayer films of sputtered polytetrafluoroethylene (Teflon) and Fe54Ni27Co19 with different layer thicknesses were prepared by vapor-phase tandem deposition. The films show ferromagnetic resonance frequencies from 3.0to4.7GHz and a high-frequency permeability in the range from 100 to 175, while having negligible losses up to 700MHz and a quality factor Q up to 12 at 1GHz. Thus these films could be promising candidates as high-frequency components used, for example, in mobile communication electronics.

Functional Polymer Nanocomposites

While extensive research has been carried out in the field of structural polymer-based nanocomposites much less investigations have been concerned with polymer nanocomposites for functional applications. Among the functional nanomaterials, nanocomposites consisting of metal nanoparticles dispersed in a dielectric matrix are of particular interest due to their novel functional properties offering hosts of new applications. Here, polymers are attractive as matrix, and several approaches have been reported to incorporate metal nanoparticles into polymers. The present review is concerned with the preparation of polymer-based nanocomposites by vapor phase co-and tandem deposition and the resulting functional properties. The techniques involve evaporation and sputtering, respectively, of metallic and organic components and inter alia allow the preparation of composites which contain alloy clusters of well defined composition. Emphasis is placed on soft-magnetic high frequency materials with cut-off frequencies well above 1 GHz and on optical composites with tuned plasmon resonances suitable for ultra thin color filters, Bragg reflectors, and other devices. In addition, antibacterial coatings and sensors for organic vapors are addressed. The latter take advantage of the steep drop of the electrical resistivity at the percolation threshold. First results are also reported on the incorporation of photo-switchable molecules into nanocomposites near the percolation threshold. Moreover, a novel approach to produce magnetic nanorods is presented.

Plasmonic properties of vapour-deposited polymer composites containing Ag nanoparticles and their changes upon annealing

The effect of temperature on microstructure and optical properties of nanocomposite films containing Ag nanoparticles embedded in a polymer matrix of Teflon AF was investigated in detail. Temperature effects were studied in two modes: the effect of temperature during preparation of the nanocomposites and post-deposition heat treatment. Substrate heating during deposition leads to a decrease in the condensation coefficient up to the glass transition temperature of the polymer and increases beyond this temperature. During heat treatment after deposition metal diffuses into the polymer and leads to a change in the microstructure of the nanocomposites resulting in larger metal cluster size, an increase in the interparticle distance and more spheroidal shaped clusters. An increase in the substrate temperature during deposition can lead to both blue shifts and red shifts of the plasmon peak position. Changes in the microstructure upon heat treatment above the glass transition temperature are caused by diffusion of metal clusters into the polymers. In this regime, the peak wavelength of the plasmon band shifts towards shorter wavelength with increasing temperature.

Improved effective medium approach: Application to metal nanocomposites

An improved effective medium approximation (EMA) is presented that accounts for higher order interactions between metal nanoparticles in metal-dielectric composite materials and compared to experimental results. The theoretical results of this formalism are applied to a composite material consisting of spherical gold nanoparticles randomly distributed in a dielectric matrix, which has been extensively characterized with respect to its structural and optical properties. The experimental results and theoretical predictions are compared and the results are discussed. It is shown that the modified theory expands the range to which EMA can be applied to a metal filling fraction of ∼20% at very little additional computational expenses. The improved theory also allows extracting more information from the optical characterization of the composite material such as the distribution of the interparticle distances in a composite.

Arrays of wirelike microstructures of Ag with visible wavelength transparent plasmonic response at near-ultraviolet and midinfrared regions

We present a simple masked thermal evaporation technique to fabricate arrays of wirelike Ag microstructures of width 8μm with an extremely high aspect ratio on either silicon or glass substrates. In accordance with the theory of periodic arrays of thin wires, the electromagnetic response of Ag microstructures has shown a characteristic low-frequency plasmonic behavior with a transparent visible region. Plasmon absorption is observed to be splitted largely into transverse and predominating longitudinal bands at near-ultraviolet (415nm) and midinfrared wavelengths (2867nm), corresponding to the oscillation of the free electrons perpendicular to and along the long axis of the wires.