ment effect at the higher energy side more easily than quantum wires, we expect the SiNTs to be more useful for future optoelectronic devices. ExperimentalPorous alumina templates were prepared by an anodization process. An aluminum foil (99.99 %, 0.13 mm thick, Aldrich) was electropolished in a perchloric acid/ethanol solution (ethanol (95 vol.-%)/perchloric acid (70 vol.-%) = 5:1) at 9 C at a constant dc (direct current) voltage of 18 V for 1.5 min. This cleaned aluminum sheet was then anodized in a 0.3 M oxalic acid solution at 5 C at a constant applied voltage of 40 V for 6 h. The resultant aluminum oxide layer was removed by dipping into an aqueous mixture of phosphoric acid (6 wt.-%) and chromic acid (1.8 wt.-%) at 60 C. The second anodization was performed for 20 min at the same conditions in order to form a regular pore array of hexagons. After pore widening by dipping into an aqueous solution of 0.1 M phosphoric acid for 10 min, we performed heat treatment to enhance the crystallinity of the alumina templates at 500 C for 30 min under Ar atmosphere. A regular array of hexagonal porous alumina was formed on both surfaces of aluminum film. This template was brought into the MBE chamber to grow SiNTs, where the chamber was evacuated to a pressure of 5 10 ±10 torr. The Si atoms/clusters were supplied for 10 min by electron-beam evaporator with a growth rate of 0.07 s ±1. The substrate temperature was maintained at 400 C in order to prevent the aluminum layer from being melted. After growth, the sample was further heat treated at 600 C or 750 C under ambient conditions for oxidation.The morphology of the SiNTs was observed by the field-emission scanning electron microscope (FESEM, JEOL-JSM6770F). High-resolution transmission electron microscopy (HRTEM, JEOL-JEM3011, 300 keV) was also used to determine the atomic details of the SiNTs. For HRTEM observations, small pieces of SiNTs were peeled off from the alumina surface and suspended, followed by further dispersion in isopropanol simply by stirring and sonication. The solvent containing the SiNTs was dropped on a holey carbon micro-grid. The photoluminescence (PL) spectra were measured with a He±Cd laser (325 nm). The laser beam diameter was about 0.3 mm with a power of 10 mW.
A novel method for realizing selective growth of parylene-N and parylene-C synthesized by chemical vapor deposition is presented. Exposure of surfaces to transition metals, metal salts, and organometallic complexes, such as those of iron, ruthenium, platinum, palladium, copper, and silver, is found to inhibit polymer deposition on the substrate. The maximum thickness of the selectively grown polymer films is dependent on the monomer delivery rate to the surface and metal inhibitor used, and for lower growth rates on surfaces patterned with iron, structures 1.4 µm and 4.1 µm in thickness are realized for parylene-N and parylene-C, respectively. The selectively deposited polymer films show no overgrowth onto the metallized areas of the substrate and the slope of the feature sidewalls is steeper than 1.1.µm/ µm. Once polymer nucleation finally occurs on the metal films, the morphology of the deposited polymer layer reflects the effectiveness of the metal in preventing polymer deposition. For substrates with little effect on polymer deposition the film morphology consists of uniformly distributed small nodules reflecting multiple polymer nucleation sites on the surface. When the metal initially inhibits polymer growth, the morphology has significantly larger grains, indicating fewer nucleation sites. Possible mechanisms underlying the selective growth are discussed.
Fabrication of polymer light-emitting diodes based on emission from the phosphorescent molecule fac tris(2-phenylpyridine) iridium doped into a poly(vinyl carbazole) host are reported. Several spin-coating solvents were evaluated for deposition of the polymer layer; toluene and chlorobenzene were found to consistently produce device-quality films with sufficient incorporation of the dopant. For single-layered devices with Mg0.9Ag0.1 cathodes, the luminance efficiency at 20 mA/cm2 was measured to be 8.7 Cd/A for devices processed from chlorobenzene. This efficiency could be increased by over a factor of two with a trilayered device geometry consisting of the doped polymer layer, a hole-blocking layer, and electron transport layer. Further increases in efficiency, up to 30 Cd/A and 8.5% external quantum efficiency, were observed when a second dopant of 2-(4-biphenylyl)-5-(4-tert-butylphenyl)-1,3,4-oxadiazole was added to the polymer emitter layer.
A novel series of monodisperse conjugated oligomers were synthesized by inserting varied segments into blue-emitting oligofluorenes to complete the color gamut of light emission. Quantum mechanical calculations revealed that the electron transition dipoles lie largely parallel to the long molecular axes of the central segments responsible for light emission. The orientational order parameter characterizing molecular alignment in thermally processed glassy-nematic films was evaluated at 0.77 to 0.87 by UV-vis absorption dichroism. With an emission dichroic ratio ranging from 9.4 to 13.7, polarized photoluminescence provided further evidence that the long molecular axes are aligned with the nematic director. Polarized organic light-emitting diodes (OLEDs) comprising selected materials resulted in red and yellowish green light emission with dichroic ratios of 14.4 and 18.0 and luminance yields of 0.51 and 5.91 cd/A, respectively. These two sets of data represent the best performance to date of red and green polarized OLEDs.
A heptafluorene lightly doped with monodisperse conjugated oligomers is used for the first demonstration of organic polarized (see Figure) light‐emitting diodes utilizing Förster energy transfer. Emission of blue‐green, green, red, and white light is accomplished with a turn‐on voltage of < 4 V, peak polarization ratios of up to 26, integrated polarization ratios of up to 19, and luminance yields of up to 6.4 cd A–1.
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