The structure and dynamics of luminescent excitons in tetracene thin films and nanoaggregates are investigated using time-resolved spectroscopy and theoretical calculations. The orientation of tetracene's transition dipole moment along its short molecular axis leads to properties qualitatively different from those observed in aggregates of phenylene-vinylene and thiophene oligomers, despite similar crystal structures. The spectral shape, temperature dependence, and radiative lifetime are consistent with a short-lived superradiant exciton delocalized over approximately 10 tetracene molecules.

We apply a halo-based group finder to four large redshift surveys, the 2MRS, 6dFGS, SDSS and 2dFGRS, to construct group catalogs in the low-redshift Universe. The group finder is based on that of Yang et al. but with an improved halo mass assignment so that it can be applied uniformly to various redshift surveys of galaxies. Halo masses are assigned to groups according to proxies based on the stellar mass/luminosity of member galaxies. The performances of the group finder in grouping galaxies according to common halos and in halo mass assignments are tested using realistic mock samples constructed from hydrodynamical simulations and empirical models of galaxy occupation in dark matter halos. Our group finder finds ∼ 94% of the correct true member galaxies for 90 − 95% of the groups in the mock samples; the halo masses assigned by the group finder are un-biased with respect to the true halo masses, and have a typical uncertainty of ∼ 0.2 dex. The properties of group catalogs constructed from the observational samples are described and compared with other similar catalogs in the literature.

We study the dependence of galaxy clustering on Hi mass using ∼16,000 galaxies with redshift in the range of 0.0025 < z < 0.05 and Hi mass of M Hi > 10 8 M , drawn from the 70% complete sample of the Arecibo Legacy Fast ALFA survey. We construct subsamples of galaxies with M Hi above different thresholds, and make volume-limited clustering measurements in terms of three statistics: the projected two-point correlation function, the projected cross-correlation function with respect to a reference sample, and the redshift-space monopole moment. In contrast to previous studies, which found no/weak Hi-mass dependence, we find both the clustering amplitudes on scales above a few Mpc and the bias factors to increase significantly with increasing Hi mass for M Hi > 10 9 M . For Hi mass thresholds below ∼ 10 9 M , the inferred galaxy bias factors are systematically lower than the minimum halo bias from mass-selected halo samples. We extend the simple halo model, in which the galaxy content is only determined by halo mass, by including the halo formation time as an additional parameter. A model that puts Hi-rich galaxies into halos that formed late can reproduce the clustering measurements reasonably well. We present the implications of our best-fitting model on the correlation of Hi mass with halo mass and formation time, as well as the halo occupation distributions and Hi mass functions for central and satellite galaxies. These results are compared with the predictions from semi-analytic galaxy formation models and hydrodynamic galaxy formation simulations.

Metal-ferroelectric–insulator-semiconductor device structures with ferroelectric vinylidene fluoride-trifluoroethylene copolymer and SiO2 buffer layer integrated gate stack over n-Si are formed, and their potential for fabricating polymeric nonvolatile random access memory devices is demonstrated. Capacitance-voltage (C–V) studies show that switchable polarization in poled polyvinylidene fluoride PVDF copolymer film changes the Si-surface potential and causes modulation of the Si-surface conductance. The (C–V) hysteresis and bidirectional flatband voltage shift at −10 to +6V, depending on the polarization field direction and remnant polarization at the ferroelectric PVDF copolymer gate, presents a memory window. The space charge at n-Si and switchable polarization both reduce the field across the ferroelectric PVDF. The observed asymmetry of the negative flatband-voltage shifts in the negatively poled ferroelectric polymer state is the result of the depletion layer formation, which reduces the field across the polymeric gate. Internal field due to negative and positive bound charges within PVDF copolymer and SiO2, respectively, influences polarization switching by pinning of dipoles. Higher negative gate bias is needed to overcome the pinning effect and to switch the polarization field. @2004 American institute of Physics.

A colloidal suspension of nanoparticles of the conjugated organic molecule tetracene is made by injecting a small amount of concentrated tetracene/tetrahydrofuran solution into water. The resulting nanoparticles are characterized by atomic force microscopy, their optical absorption and emission spectra, and their fluorescence decay times and quantum yields. Their optical properties in room-temperature aqueous solution are similar to what is observed for bulk polycrystalline films of tetracene, indicating a substantial amount of crystallinity. Due to their high surface area, these organic semiconductor nanoparticles demonstrate photocatalytic activity toward several different organic dye molecules in aqueous solution. The kinetics and oxygen dependence of the reactions are consistent with a heterogeneous type I reaction, similar to what is observed for other semiconductor photocatalysts, and the quantum yields at high surface coverage and low light intensity are comparable to those of inorganic photocatalysts. LA026851X