Browsing by Author "Kostadinova D."

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    Enhanced photoanisotropic response in azopolymer doped with elongated goethite nanoparticles
    (2016-04-07) Nedelchev L.; Nazarova D.; Berberova N.; Mateev G.; Kostadinova D.; Mariño-Fernández R.; Salgueiriño V.; Schmool D.
    We present a study of the photoinduced birefringence in nanocomposite films of an azopolymer (PAZO) doped with goethite (a-FeOOH, a characteristic antiferromagnetic material) nanoparticles (NPs). The NPs had an elongated shape with a size 15×150 nm, i.e., a ratio of 1:10. Samples were prepared with different concentrations of the NPs in the azopolymer varying from 0% (undoped azopolymer film) to 15 wt %. An unusual dependence of the birefringence on the concentration was observed - two peaks of enhancement at 1 % and at 10 % concentration. Our previous studies on ZnO and SiO2 NP have indicated only one peak of increase at low concentrations - 0.5 wt % and 2 wt %, respectively. This effect could be related to the elongated shape of the nanoparticles and the presence of two characteristic NPs sizes - 15 and 150 nm. Moreover, the birefringence increase for the samples with 10 wt % NPs concentration (compared with the non-doped samples) was rather significant - nearly 70%.
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    Photoinduced birefringence in azopolymer doped with Au nanoparticles
    (2019-10-08) Nazarova D.; Nedelchev L.; Stoykova E.; Blagoeva B.; Mateev G.; Karashanova D.; Georgieva B.; Kostadinova D.
    A study of the photoinduced birefringence in nanocomposite thin films of the azopolymer PAZO (poly[1-[4-(3-carboxy-4-hydroxyphenylazo)benzenesulfonamido]-1,2-ethanediyl, sodium salt]) doped with gold (Au) nanoparticles (NPs) is presented. The NPs are with a spherical shape and mean size 10 nm. Transmission electron microscopy (TEM) and Dynamic Light Scattering (DLS) analyses of the particles have also been made. Samples were prepared with five different concentrations of the NPs in the azopolymer films varying from 0 (non-doped azopolymer film) to 4 a.u. (1 a.u. = 0.015 wt %). Birefringence was induced by a He-Cd laser with a wavelength 442 nm and measured with probe Diode Pumped Solid State (DPSS) laser at 635 nm. An increase of the photoinduced birefringence in doped azopolymer nanocomposite films compared to samples of undoped azopolymers was observed. The maximal enhancement was achieved for films with 2 a.u. of Au NPs.
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    Photoinduced variation of the Stokes parameters of light passing through thin films of azopolymer-based hybrid organic/inorganic materials
    (2016-04-07) Berberova N.; Nazarova D.; Nedelchev L.; Blagoeva B.; Kostadinova D.; Marinova V.; Stoykova E.
    Hybrid materials synthesized by combining polymers and inorganic nanoparticles (NPs) have received considerable attention recently due to their advantageous electrical, optical, or mechanical properties. Here we present a polarimetric study of thin films prepared from photoanisotropic hybrid materials: azopolymer (poly [1-[4-(3-carboxy-4-hydroxyphenylazo)benzenesulfonamido]-1,2-ethanediyl, sodium salt]) doped with different concentrations of ZnO NPs with sizes below 50 nm. To obtain full information about the polarization response of these thin films, we determined the kinetics of the Stokes parameters of light passing through the films on illumination by linearly polarized light within the absorption band of the material, namely 473 nm. Using the Stokes parameters, the birefringence values were calculated as a function of time and compared for the films with different NP concentrations.