The steady-state photoinduced absorption ͑PA͒, photoluminescence ͑PL͒, PL-detected magnetic resonance ͑PLDMR͒, and PA-detected magnetic resonance ͑PADMR͒ of poly-and oligo-͑para-phenylenes͒ films is described. In particular, the excitation density ͑laser power͒ N 0 dependence of the PA, PL, and PLDMR signals is analyzed by means of a rate equation model, which describes the dynamics of singlet excitons ͑SE's͒ and polarons in all three experiments quantitatively with the same set of parameters. The model is based on the observations that mobile SE's are quenched by trapped and free polarons and that the spin-1 2 magnetic resonance conditions reduce the total polaron population. Since the sublinear N 0 dependences of the positive ͑PL-enhancing͒ spin-1 2 PLDMR and the polaron PA band are essentially the same, we conclude that PLDMR is due to a reduced quenching of SE's by polarons. The agreement between the model, the current results, and results from other spectroscopic techniques provides strong evidence for this quenching mechanism. This also suggests that it is a very significant process in luminescent -conjugated materials and organic light-emitting devices. Consequently, the quenching mechanism needs to be taken into account, especially at high excitation densities, which is of great importance for the development of electrically pumped polymer laser diode structures.

wafers using a hydrothermal method. Electron microscopy shows the tubes to have outer diameters of 20-40 nm and wall thicknesses in the range 5-15 nm. HRTEM images and SAED analysis shows the tubes to be single-crystalline. Annealing the nanotube arrays in vacuum, at low (300°C) temperature, causes substantial enhancement of the UV PL following 325 nm excitation, and much reduced green-yellow emission. ExperimentalThin-ZnO-film-coated Si wafers were used as substrates for subsequent growth of nanorod and nanotube arrays. The ZnO thin films were formed via PLD (with the T sub in the range 25-450°C, and deposition times of 1-45 min). The PLD system has been described elsewhere [22]. 50 mL of 0.1 M aqueous solutions of zinc nitrate (Alfa Aesar, 99 %) and methenamine (Alfa Aesar, 99+ %) were each maintained at constant temperature (90°C) using a thermostatically controlled oil bath, and then mixed in a sealed glass bottle. For nanorod growth, and the growth of nanotubes on nanorods, the substrates were immersed in the reactive solution immediately after mixing. Typical growth times were t < 3 h for nanorod arrays, and t > 6 h for the growth of nanotubes on nanorods. The highest-density nanorod/nanotube arrays were grown on very thin ZnO template layers (1-5 min deposition time) on Si. These substrates were pre-heated (to ∼ 90°C) and immersed in the reactive solution that had been previously maintained at 90°C for a user-selected time s = 10-60 h after mixing. The as-grown samples were rinsed in deionized water, and then dried in air at room temperature. Deposited products were characterized and analyzed using SEM (JEOL 6300 LV), TEM (JEOL 1200EX), and HRTEM (JEOL 2010). PL spectra were measured at room temperature following excitation with a continuous-wave He-Cd laser (k = 325 nm, power ∼ 3 mW). Two-Polymer Microtransfer Molding for Highly Layered Microstructures**By Jae-Hwang Lee,* Chang-Hwan Kim, Kai-Ming Ho, and Kristen Constant* Microfabrication techniques for highly layered microstructures have attracted much attention, since highly layered microstructures permit fabrication of three-dimensional (3D) devices with functionality not possible in planar devices. With the growing demands of highly layered microstructures for a number of applications, including artificial bones, [1,2] integrated microfluidic systems, [3][4][5] polymer-based integrated optical circuits, [6,7] photonic crystals, [8,9] and catalytic reactors, [10,11] reliable low-cost microfabrication methods are desirable. Traditional fabrication methods using photolithography are usually slow and costly. Over the last few years, a number of new approaches for fabricating 3D microstructures have been reported as alternatives to conventional photolithography such as microtransfer molding (lTM), [12] two-photon polymerization, [13,14] holographic lithography, [15,16] and nanoimprinting.[17] Among these techniques, lTM showed a number of advantages, including low cost, capability for non-periodic 3D structures, a wide range of materials compatibil...

Arrays of ultraviolet–violet (indium tin oxide)/[copper phthalocyanine (CuPc)]/[4,4′-bis(9-carbazolyl)biphenyl (CBP)]/[2-(4-biphenylyl)-5-(4-tert-butylphenyl)-1,3,4oxadiazole (Bu-PBD)]/CsF/Al organic light-emitting devices, fabricated combinatorially using a sliding shutter technique, are described. Comparison of the OLED electroluminescence and CBP photoluminescence spectra indicates that the emission originates from the bulk of that layer. In arrays of devices in which the thickness of the CuPc and Bu–PBD were varied, but that of CBP was fixed at 50 nm, the optimal radiance R was obtained at CuPc and Bu–PBD thicknesses of 15 and 18 nm, respectively. At 10 mA/cm2, R was 0.38 mW/cm2, i.e., the external quantum efficiency was 1.25%; R increased to ∼1.2 mW/cm2 at 100 mA/cm2.

A fluorescent chemical sensor platform, integrating an organic light-emitting device ͑OLED͒ light-source with a fluorescent probe, is demonstrated for a subsecond-fast oxygen sensor. The integration results in strong light coupling and negligible heating of the sensor film or analyte. The potential in vivo operation of compact, stand-alone, battery-powered, OLED-based miniaturized sensor arrays for chemical and biological applications is discussed.

Described herein is the incorporation of nanocrystalline silicon nc-Si from porous silicon (PSi) in a silica matrix fabricated by the sol-gel technique that yields highly photoluminescent (PL) and optically transparent monoliths with uniformly distributed nc-Si inclusions or nanoclusters. The sample monoliths were prepared with PSi-derived nanoclusters (PSi-n) with average diameters of 14–45nm. Concentrated samples of PSi-n-exhibited blueshifted orange emission bands with maximum peaks between 600 and 750nm with PL emission intensities ten times stronger than those of the original PSi, while diluted samples exhibited UV to blue (350–450nm) emission bands. The PL quantum yield of the typical PSi-n monoliths was 44% higher than the native PSi. Light absorption measurements showed a linear response to laser powers before the saturation threshold at 80mW. PL bleaching following 3h of constant laser power exposure resulted in 90% reduction of the maximum initial PL. Mechanical and thermal stability properties of nc-Si were greatly improved within the silica matrix, demonstrating that PSi-n monoliths’ are more manageable materials that enable the fabrication of samples with high densities of nc-Si for semiconducting and optoelectronic purposes. No special chemical passivation of the nc-Si surfaces was used in the preparation of the PSi-n monoliths. A strong relation between the optical properties of this nanophase material and the size distribution and concentration of nc-Si in the sample is demonstrated.

The cw absorption, steady state photoluminescence (PL), photoinduced absorption (PA), PL-detected magnetic resonance (PLDMR), and the time resolved PL of a novel polyfluorene (PF) prepared with bulky polyphenylene dendrimer substituents are compared with those of (PF) with ethyl-hexyl substituents. We show that the dendronic sidechains suppress the contribution from unwanted low energetic emission, yielding a polymer with pure blue emission. The sidechains also strongly alter the dynamics of the excited entities. In particular, the time-resolved PL and temperature-dependence of the cw PL from 20-320 K reveal distinct singlet exciton (SE) dynamics in the polymer films, while the behavior in solution is essentially the same. However, the PA results show that the dynamics of polarons and triplet excitons (TEs) are similar, and the PLDMR shows that the interaction between the SEs and polarons are also similar.

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