Modification of Spontaneous Emission from Periodic Array of Spherical Polystyrene Particles Containing Fluorescent Molec
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Mat. Res. Soc. Symp. Proc. Vol. 488 © 1998 Materials Research Society
structures consisted of microspheres are formed by evaporating water from the suspensions [17, 18]. The solid crystals are easy to handle and have higher refractive index contrast of 1.60 than the case of 1.20 for colloidal crystals in the water. We have started the study on the construction of the solid crystals, as an example of photonic crystals in visible regions, and tried to examine spontaneous emission properties by both optical and electrical pumping. EXPERIMENT Polystyrene particles doped with a fluorescent dye (YG) (average diameter of 213±8 nm) were used. The ordered arrays of polystyrene spheres were prepared on a glass substrate by controlling temperature and humidity during casting processes [18]. We have succeeded in preparing highly ordered solid films with the average thickness of 30 pm from aqueous suspensions of particle concentration of 2.5 wt %.The other film of dye doped polystyrene without an ordered structure was also prepared for reference by casting onto a glass substrate from the dichromethane solutions of the same particles. The average thickness of this film was 17 pm. The surface of ordered arrays were observed using an optical microscope and scanning electron microscope (SEM). Fig. 1 shows a typical SEM image from ordered film of dye-doped polystyrene micro- Fig.I SEM micrograph of the surface spheres used. The transmission and reflection spectra of polystyrene microsphere arrays; of the films were measured with Hitati 330 spectro- scale marker is 500 nm. photometer. Photoluminescence (PL) spectra of films were obtained with multichannel analyzer (PMA- 11, :i (4) Hamamatsu Photonics K. K.) from the films which i were fixed on a rotational stage and excited by the 3 light of xenon lamp through 250 mm monochromator. Self-organized 2D arrays of silica spheres (average
diameter of 550 nm) were prepared on ITO coated glass substrate using the improved method described eleswhere [19]. The device consisted of a glass substrate with a patterned ITO electrode, silica sphere layer, poly(9-vinylcarbazole) layer molecularly doped with 2,5-bis(4-naphtyl)-1,3,4-oxadiazole and Coumarine-6 for EML, an Alq layer for ETL and a top MgAg alloy cathode, in which EML and ETL represent emissive and electron-transporting layers, respectively. This device structure is similar to a standard-type polymer-dispersed dye LED's [20]. RESULTS microscope observations optical We confirmed forWthe surfacnfirmes ttfrom othe o imisofpeolystyerine the ordered films of polystyrene for the surfaces thatcomposed of aggregates of particles were microcrystals with specified orientations. The SEM image shown in Fig. 1 clearly demonstrates the presence of the regularly packed arrangement of polystyrene spheres corresponded to either (111) surface of the fcc structure or (001) surface of the we hexagonal close packed (hcp) one. At present, 772
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400 450 500 550 Wavelength I nm Fig. 2 Reflection and transmissi
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